ULTRA-FINE PARTICLES Exploratory Science and Technology
Edited by
Chikara Hayashi ULVAC Japan Ltd.
RyoziUyeda Nagoya University, Japan
Akira Tasaki University of Tsukuba, Japan
NOYES PUBLICATIONS Westwood, New Jersey, U.S.A.
Copyright © 1997 by Noyes Publications No part of this book may be reproduced or utilized in any form or by any means, electronic or mechanical, including photocopying, recording or by any information storage and retrieval system, without permission in writing from the Publisher. Library of Congress Catalog Card Number: 96-12584 ISBN: 0-8155·1404-2 Printed in the United States Published in the United States of America by Noyes Publications 369 Fairview Avenue, Westwood, New Jersey 07675 1098765432 I
Library of Congress Cataloging-in-Publication Data Ultra-fine particles: exploratory science and technology / edited by Chikara Hayashi, R. Uyeda, A. Tasaki. p. em. Includes bibliographical references and index. ISBN 0-8155-1404-2 1. Nanoparticles. 2. Nanostructure materials. 1. Hayashi, Chikara. II. Uyeda, Ryozi. III. Tasaki, A. TA418.78.UI5 1996 620'.43--dc20 96-12584 CIP
MATERIALS SCIENCE AND PROCESS TECHNOLOGY SERIES Series Editors
Rointan F. Bunshah, University of California, Los Angeles Gary E. McGuire, Microelectronics Center of North Carolina Stephen M. Rossnagel, IBM Thomas J. Watson Research Center
Electronic Materials and Process Technology HANDBOOK OF DEPOSITION TECHNOLOGIES FOR FILMS AND COATINGS, Second Edition: edited by Rointan F. Bunshah CHEMICAL VAPOR DEPOSITION FOR MICROELECTRONICS: by Arthur Sherman SEMICONDUCTOR MATERIALS AND PROCESS TECHNOLOGY HANDBOOK: edited by Gary E. McGuire HYBRID MICROCIRCUIT TECHNOLOGY HANDBOOK: by James J. Licari and Leonard R. Enlow HANDBOOK OF THIN FILM DEPOSITION PROCESSES AND TECHNIQUES: edited by Klaus K. Schuegraf IONIZED-CLUSTER BEAM DEPOSITION AND EPITAXY: by Toshinori Takagi DIFFUSION PHENOMENA IN THIN FILMS AND MICROELECTRONIC MATERIALS: edited by Devendra Gupta and Paul S. Ho HANDBOOK OF CONTAMINATION CONTROL IN MICROELECTRONICS: edited by Donald L. Tolliver HANDBOOK OF ION BEAM PROCESSING TECHNOLOGY: edited by Jerome J. Cuomo, Stephen M. Rossnagel, and Harold R. Kaufman CHARACTERIZATION OF SEMICONDUCTOR MATERIALS, Volume 1: edited by Gary E. McGuire HANDBOOK OF PLASMA PROCESSING TECHNOLOGY: edited by Stephen M. Rossnagel, Jerome J. Cuomo, and William D. Westwood HANDBOOK OF SEMICONDUCTOR SILICON TECHNOLOGY: edited by William C. O'Mara, Robert B. Herring, and Lee P. Hunt HANDBOOK OF POLYMER COATINGS FOR ELECTRONICS, 2nd Edition: by James Licari and Laura A. Hughes HANDBOOK OF SPUTTER DEPOSITION TECHNOLOGY: by Kiyotaka Wasa and Shigeru Hayakawa HANDBOOK OF VLSI MICROLITHOGRAPHY: edited by William B. Glendinning and John N. Helbert CHEMISTRY OF SUPERCONDUCTOR MATERIALS: edited by Terrell A. Vanderah CHEMICAL VAPOR DEPOSITION OF TUNGSTEN AND TUNGSTEN SII-ICIDES: by John E. J. Schmitz ELECTROCHEMISTRY OF SEMICONDUCTORS AND ELECTRONICS: edited by John McHardy and Frank Ludwig HANDBOOK OF CHEMICAL VAPOR DEPOSITION: by Hugh O. Pierson
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Series DIAMOND FILMS AND COATINGS: edited by Robert F. Davis ELECTRODEPOSITION: by Jack W. Dinl HANDBOOK OF SEMICONDUCTOR WAFER CLEANING TECHNOLOGY: edited by Werner Kern CONTACTS TO SEMICONDUCTORS: edited by Leonard J. Brillson HANDBOOK OF MULTILEVEL METALLIZATION FOR INTEGRATED CIRCUITS: edited by Syd R. Wilson, Clarence J. Tracy, and John L. Freeman, Jr. HANDBOOK OF CARBON, GRAPHITE, DIAMONDS AND FULLERENES: by Hugh O. Pierson MOLECULAR BEAM EPITAXY: edited by Robin F. C. Farrow HANDBOOK OF COMPOUND SEMICONDUCTORS: edited by Paul H. Holloway and Gary E. McGuire HANDBOOK OF VACUUM ARC SCIENCE AND TECHNOLOGY: edited by Raymond L. Boxman, Philip J. Martin, and David M. Sanders HIGH DENSITY PLASMA SOURCES: edited by Oleg A. Popov DIAMOND CHEMICAL VAPOR DEPOSITION: by Huimin Liu and David S. Dandy HANDBOOK OF MAGNETO-OPTICAL DATA RECORDING: edited by Terry McDaniel and Randall H. Victora HANDBOOK OF REFRACTORY CARBIDES AND NITRIDES: by Hugh O. Pierson ULTRA-FINE PARTICLES: edited by Chikara Hayashi, R. Ueda and A. Tasaki
Ceramic and Other Materials-Processing and Technology SOL-GEL TECHNOLOGY FOR THIN FILMS, FIBERS, PREFORMS, ELECTRONICS AND SPECIALTY SHAPES: edited by Lisa C. Klein FIBER REINFORCED CERAMIC COMPOSITES: edited by K. S. Mazdiyasni ADVANCED CERAMIC PROCESSING AND TECHNOLOGY, Volume 1: edited by Jon G. P. Binner FRICTION AND WEAR TRANSITIONS OF MATERIALS: by Peter J. Blau SHOCK WAVES FOR INDUSTRIAL APPLICATIONS: edited by Lawrence E. Murr SPECIAL MELTING AND PROCESSING TECHNOLOGIES: edited by G. K. Bhat CORROSION OF GLASS, CERAMICS AND CERAMIC SUPERCONDUCTORS: edited by David E. Clark and Bruce K. Zoitos HANDBOOK OF INDUSTRIAL REFRACTORIES TECHNOLOGY: by Stephen C. Carniglia and Gordon L. Barna CERAMIC FILMS AND COATINGS: edited by John B. Wachtman and Richard A. Haber CERAMIC CUTTING TOOLS: edited by E. Dow Whitney
Related Titles CODE COMPLIANCE FOR ADVANCED TECHNOLOGY FACILITIES: by William R. Acorn SEMICONDUCTOR INDUSTRIAL HYGIENE HANDBOOK: by Michael E. Williams and David G. Baldwin
Acknowledgment
This book is a translated and revised edition of the book "Ultra Fine Particles" published in Japanese. Dr. Robert Lewis at the Tsukuba Research Consortium corrected and did the preliminary editing of the entire manuscript and Professor Kanji Ono at International Christian University and University of California at Los Angeles provided the first translation into English. The Mita publishing house graciously gave the authors the freedom to enable the English edition to be available for publication. Ms. Reiko Ohya supplied most of the clerical service. The authors wish to express their sincere appreciation to them. Our many thanks also to Professor Rointan Bunshah and Mr. George Narita for their patience and continued interest in "Ultra Fine Particles."
vii
Contributors
GenyaChiba Research Development Corporation of Japan Kawaguchi, Saitama, Japan
Sumio lijima NEC Corporation Tsukuba, Ibaraki, Japan Kazuharu Iwasaki RMECompany Tagajo, Miyagi, Japan
Eiji Fuchita Vacuum Metallurgical Co., Ltd. Sanbu-gun, Chiba, Japan Chikara Hayashi ULVAC Japan Ltd. Chigasaki, Kanagawa, Japan
Akira Johgo Applied Materials Japan, Inc. Sinjuku-ku, Tokyo, Japan Hideo Kakuta Plant Ecochemicals Research Center Eniwa, Hokkaido, Japan
Toyoharu Hayashi Mitsui Toatsu Chemicals, Inc. Yokohama, Kanagawa, Japan
Seiichiro Kashu Vacuum Metallurgical Co., Ltd. Sanbu-gun, Chiba, Japan
Tsukasa Hirayama Japan Fine Ceramics Center Nagoya, Aichi, Japan
Akio Kato Kyushu University Fukuoka,Fukuoka,Japan
Toshinari Ichihashi NEC Corporation Tsukuba, Ibaraki, Japan viii
Contributors Michiko Kusunoki Japan Fine Ceramics Center ~agoya,AUchi, Japan
Akira Tasaki Institute of Applied Physics University of Tsukuba Tsukuba, Ibaraki, Japan
Takeshi Manabe Teisan K.K. Tsukuba, Ibaraki, Japan
Hideki Toyotama Stanley Electric Co., Lid. Tsukuba, Ibaraki, Japan
Tadashi Matsunaga Tokyo University ofAgriculture and Technology Koganei, Tokyo, Japan
Shunichi Tsuge Tsukuba, Ibaraki, Japan Akifumi Ueno Shizuoka University Hamamatsu, Shizuoka, Japan
Hiroshi Miyamoto Cellular Biophysics Laboratory ~ational Institute of Bioscience & Human Technology Tsukuba, Ibaraki, Japan Iku Nemoto Tokyo Denki University Hatoyama, Saitama, Japan
Shizuo Umemura Miyanodai Technology Development Center Fuji Photo Film Co., Ltd. Ashigarakami-gun, Kanagawa, Japan
Masaaki Oda Vacuum Metallurgical Co., Ltd. Sanbu-gun, Chiba, Japan
Ryozi Uyeda Nagoya University Nagoya, Aichi, Japan
Norio Saegusa Surnitomo 3M Ltd. Setagaya-ku, Tokyo, Japan
Nobuhiko Wada Nagoya University Nagoya, AUchi, Japan
Yasukazu Saito Science University of Tokyo Shinjuku-ku, Tokyo, Japan
Toyonobu Yoshida The University of Tokyo Bunkyo-ku' Tokyo, Japan
Shigetoshi Takahashi Nisshin, Aichi, Japan
Akinori Yoshizawa Yoshizawa Industry Inc. ~agaoka, Niigata, Japan
ix
NOTICE To the best of our knowledge the infonnation in this publication is accurate; however the Publisher does not assume any responsibility or liability for the accuracy or completeness of, or consequences arising from, such infonnation. This book is intended for infonnational purposes only. Mention oftrade names or commercial products does not constitute endorsement or recommendation for use by the Publisher. Final detennination ofthe suitability of any information or product for use contemplated by any user, and the manner of that use, is the sole responsibility of the user. We recommend that anyone intending to rely on any recommendation ofmaterials or procedures mentioned in this publication should satisfy himself as to such suitability, and that he can meet all applicable safety and health standards.
x
Contents Introduction Exploratory Research - Ultra-Fine Particle Research Chikara Hayashi
1
CHAPTER 1 ELECTRON MICROSCOPY STUDIES OF ULTRA-FINE PARTICLES 1.1 Introduction Ryozi Uyeda References 1.2 Ultra-Fine Particles and Electron Microscopy Sumio Iijima Ultra-Fine Particle Observation by Electron Microscopy - Background . . . . . . . . . . . . . . . . . . .. Atomic Scale Observation . . . . . . . . . . . . . . . . . . .. Microstructures of Ultra-Fine Particles References 1.3 Development of Electron Microscope Accessories Toshinari Ichihashi Sample Heating Device Video Imaging Systems Gas Bleeding Device Introducing Ion Beams . . . . . . . . . . . . . . . . . . . . . .. References 1.4 High Resolution Observation Methods . . . . . . . . . .. Sumio Iijima Construction of the Electron Microscope Electromagnetic Lens and Resolution . . . . . . . . . .. Imaging of Phase Objects xi
4 7 8
8 9 11 15
16 16 16 18 19 19 20 20 21 23
Contents Electron Diffraction and Crystal Structure Image . . . .. 27 Multi-Functional High Vacuum Electron Microscope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 28 Dynamic Observation with a High Resolution Electron Microscope. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 29 Observation of Ultra-Fine Particles 30 Observation of Crystal Surfaces 33 References 39 1.5 Evaporation Methods . . . . . . . . . . . . . . . . . . . . . . . . . . .. 40 Ryozi Uyeda Evaporation and Condensation in a Gas 40 Laboratory Evaporation Equipment . . . . . . . . . . . . . . .. 44 Heat Source and Coolant . . . . . . . . . . . . . . . . . . . . . . .. 47 References 49 1.6 Oxides " 50 Sumio Iijima Synthesis of Oxide Ultra-Fine Particles. . . . . . . . . . . .. 50 The Crystal Structure of y-Alumina Ultra-Fine Particles 52 References 57 1.7 Search for Industrial Applications of Spherical y -Alumina Ultra-Fine Particles 58 Tsukasa Hirayama Decrease of Specific Surface Area at High Temperatures and Transition to the a -Phase 58 Synthesis of Spherical Alumina Particles 60 References 63 1.8 Metal Catalysts 64 Sumio Iijima Alumina Carriers 65 Metal Complex Clusters 65 Observation of Metal Clusters . . . . . . . . . . . . . . . . . . .. 66 High Resolution Electron Microscopy of Metal Clusters 68 Interpretation of Electron Microscope Images . . . . . . .. 68
xii
Contents Electron Microscopy Observation of Other Metal Catalysts References 1.9 Crystal Growth of Silicon Ultra-Fine Particles Sumio Iijima Synthesis of Spherical Fine Particles of Silicon Growth Mechanisms of Spherical Silicon Ultra-Fine Particles References 1.10 Surface Oxidation of Silicon Ultra-Fine Particles Sumio Iijima Native Oxide Film of Spherical Silicon Ultra-Fine Particles Thermal Oxidation of Silicon Ultra-Fine Particles References 1.11 Surface Coverage of Ultra-Fine Particles . . . . . . . . . .. Sumio Iijima References I. 12 Non-Additive Sintering of Silicon Carbide Ultra-Fine Particles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. Shigetoshi Takahashi Preparation and Formation of UFP Samples " High Pressure Sintering . . . . . . . . . . . . . . . . . . . . . . . .. Characterization of Sintered Bodies References 1.13 Quenching of y-Iron UFPs to Room Temperature. .. Michiko Kusunoki Heat Treatment Apparatus for Ultra-Fine Particles . . .. Experimental Results . . . . . . . . . . . . . . . . . . . . . . . . . . . Discussion References 1.14 UFP Beam Experiments Toshinari Ichihashi Introduction Velocity Measurement and Ionization of Ultra-FineParticles xiii
71 73 74 74 78 81 82
82 85 87 88 91 92 92 94 95 97 98 98 100 101 103 104 104 107
Contents Introducing Samples into an Electron Microscope Heating UFP Beams References
1.15 Living Crystals Sumio Iijima Experimental Observation of Unstable Structures Crystal Habit of Ultra-Fine Particles Atomic Rearrangement Mechanisms Temperature Related Experiments for Ultra-Fine Particles Conclusions References
113 116 118 119 119 121 121 125 127 131 132
Chapter 2 SYNTHESIS AND CHARACTERIZATION OF ULTRAFINE PARTICLES 2.1 Synthesis of Compound and Individually Separated Ultra-Fine Particles by Gas Evaporation 133 MasaakiOda Synthesis of Ultra-Fine Particles 133 133 Features of the Gas Evaporation Method Production ofUFP Particles by the Gas Evaporation Method 135 Compound Ultra-Fine Particles 137 Synthesis Method 137 Results 138 Chemical Analysis 140 141 Catalytic Activity Measurements Formation Processes Leading to Double-Layer 141 Cu-Zn Ultra Fine-Particles Prospects for the Future . . . . . . . . . . . . . . . . . . . . . . 144 Individually Separated Ultra-Fine Particles 144
xiv
Contents Synthesis Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 144 Collection Method for Isolated Ultra-Fine Particles ... 147 Synthesis ofIsolated Ultra-Fine Particles of Iron Oxide 147 Results 148 Handling Techniques for Isolated Ultra-Fine Particles . 151 Results for the Handling Techniques 153 Prospects for the Future 158 References 159 2.2 Fluid Thermodynamics of UFP Synthesis 160
Shunichi Tsuge Equations for a Mixed-Flow of Vapor, Inert Gas, andUltra-Fine Particles Distribution of Metal Vapor in the Chamber and Evaporation Rate References 2.3 In-Flight Plasma Processes Toyonobu Yoshida Plasma Generation Method for In-Flight Plasma Processes UFP Synthesis Using the In-Flight Plasma Method Example of Fine Powder Synthesis by the In-Flight Plasma Method Synthesis of Si3N 4 and SiC Fine Particles Via Hybrid Plasma Apparatus and Procedures Primary Results Summary References 2.4 Gaseous Reaction Method AkioKato Manufacture of Ultra-Fine Particles by the Gaseous Reaction Method Characteristics of Ultra-Fine Particles Made by the Gaseous Reaction Method Requirements for Particle Synthesis xv
162 165 169 170
171 173 177 180 188 188 194 195 197
200 201 205
Contents
2.5
2.6
2.7
2.8
Reaction Process Synthesis of Oxide Ultra-Fine Particles Synthesis of Ultra-Fine Particles of Refractory Nitrides and Carbides SiC Particle Synthesis by RF Plasma Gaseous Method References Ultra-Fine Particle Synthesis by Chemical Methods Akinori Yoshizawa Path of Particle Formation Is Nucleus Formation Unnecessary? Role of Coalescence References Gas Evaporation Under Zero Gravity Nobuhiko Wada Experimental Fine Particle Observation Discussion Supplement References The Properties of Surface Oxide Layers of Metallic Ultra-Fine Particles Akira Johgo Experimental Results Surface Oxide Layer of Nickel Ultra-Fine Particles Application ofFT-IR/PAS Summary References Mossbauer Spectra of Iron Ultra-Fine Particles Norio Saegusa Methodology Surface Oxides of Iron Ultra-Fine Particles References
xvi
205 207 208 210 212 213 213 215 216 217 218 219 227 229 233 236 237 237 239 240 242 243 243 245 245 245 252
Contents 2.9 Preparation of Ultra-Fine Particle Alloy Catalysts by Alkoxide Methods Akifumi Ueno Alloy Catalysts Preparation of Alloy Ultra-Fine Particles Structure of Alloy Fine Particles Particle Size Control of Alloy Particles References
253 253 255 257 257 260
Chapter 3 ULTRA-FINE PARTICLES AND MICROBES 3.1 Phagocytosis of Ultra-Fine Particles by Cells 262 Hiroshi Miyamoto Slow-Speed Microscopic Photography of Ultra-Fine Particle Phagocytosis by Cells 264 Results 265 Rotational Movement of Consumed Particles in Hamster Kidney Cells 268 References 270 3.2 Application of Ultra-Fine Particles in the Detection of Cell Activity 271 Hideki Toyotama and Iku Nemoto Measurement of Magnetic Fields of Lungs and Cells .. 272 Intake ofInorganic Ultra-Fine Particles into Cells .... 276 Measurement of Cellular Magnetic Fields Using Cultured Cells 277 Modeling and Experimental Investigation of Relaxation 280 References 284 3.3 Organic Compound Ultra-Fine Particles 286 Hideki Toyotama UFP Formation by the Gas Evaporation Method 286 289 Properties of Organic Ultra-Fine Particles Possible Applications 290 References 292 xvii
Contents
3.4 Encapsulation of Magnetic Ultra-Fine Particles and Fixation of Antibodies and Enzymes 293 Hideo Kakuta Encapsulation of Magnetic Ultra-Fine Particles 294 Immobilization of Antibodies and Enzymes to Encapsulated Ultra-Fine Particles 297 References 299 3.5 Magnetic Ultra-Fine Particles Isolated from Bacteria .300 Tadashi Matsunaga Cultivation of Magnetotactic Bacteria 300 Characterization of Bacterial Magnetic Particles 302 Utilization of Bacterial Magnetic Ultra-Fine Particles . 305 311 References
Chapter 4 APPLICATION OF ULTRA-FINE PARTICLES 4. 1 Introduction 313 Akira Tasaki The Start of Applications for Evaporated Ultra-Fine Particles 315 Main Progress 317 Possibilities for Further Applications 319 References 323 4.2 Regular Arrangements of Ultra-Fine Particles and Super 324 High Density Recording Shizuo Umemura Regular Arrangements of Ultra-Fine Particles and Ultra-High Density Recording 325 Vapor Deposition and Particle Observation 327 Discussion 331 Applications to Electron Beam Recording 333 Regular Arrangements of Alkali Halide UltraFine Particles 334 Removal of Alkali Halide Ultra-Fine Particles by Electron Irradiation 335 xviii
Contents References 338 4.3 CobaltlPolymer Composite Thin Films 339 Kazuharu Iwasaki Experimental 340 Magnetic Properties of Cobalt-Polymer Composite Thin Films 342 Microstructure of Cobalt-Polymer Composite Thin Films 347 Summary 348 Prospects 352 References 353 4.4 Catalytic Applications of Gas Evaporated Ultra-Fine 355 Particles Toyoharu Hayashi Experimental 357 Hydrogenation Reaction 357 364 Synthesis of Methanol Prospects 368 References 368 4.5 Chemical Heat Pump 369 Yasukazu Saito A New Chemical Heat Pump System 369 Catalytic Activities of Metal Nickel Ultra-Fine Particles and Their Applications 377 References 379 4.6 Film Formation by the Gas Deposition Method 381 Seiichiro Kashu and Eiji Fuchita The Concept for Gas Deposition 381 Confirmation ofUFP Film Formation 381 Floatation and Transport of Ultra-Fine Particles in a Gas 385 Formation ofUFP Films by Gas Deposition 386 393 Characteristics of Gas Deposited UFP Films Uniformly Mixed Binary UFPFilms 397 Formation ofUFP Films and Applications 403 Prospects for the Gas Deposition Method 408 xix
Contents References 4.7 Surface Processing Using Solidified CO2 Ultra-Fine Particles. . Takeshi Manabe and Seiichiro Kashu Synthesis of Microscopic CO2 Particles Selective Stripping of Resist Films Removal of Plastics Applications of Solid Gas Ultra-Fine Particles References
ChapterS Prospects for the Future of Ultra-Fine Particles Chikara Hayashi Development of Applications Problems Unresolved by the Ultra-Fine Particle Project Problems with the Environment and the System General References
408 410 410 412 415 417 418
419 422 423 425
Appendix: Background on the Exploratory Research for Advanced Technology Program (ERATO) Genya Chiba ERATO Program Framework Research Projects Present Status Accomplishments Future Development
428 430 431 435 435
Index
437
xx
427
INTRODUCTION Exploratory Research - Ultra-Fine Particle Research (By Chikara Hayashi)
This book was written with several objectives in mind: ~To
share with as many scientists and engineers as possible the intriguing scientific aspects of ultra-fine particles (UFPs) and to show their potential as new materials. ~ Entice such researchers to participate in the development of this emerging field. ~To publicize the achievements of the Ultra-Fine Particle Project, which was carried out under the auspices ofthe Exploratory Research for Advanced Technology program (ERATO) during the period 1981-1986. In addition to the members of the Ultra-Fine Particle Project, contributions from other pioneers in this field are included. To achieve the first objective described above, the uniformity of the contents and focus on a single central theme have been sacrificed somewhat to provide a broad coverage. It is expected that the reader can discover an appropriate topic for further development of new materials and basic technology by reading selected sections of this book. Alternately, one may gain an overview of this new field by reviewing the entire book, which can potentially lead to new directions in the development ofUFPs. During the past few years, many symposia and workshops on UFPs have been held in Japan. In addition, a variety ofR&D projects on many aspects ofthese materials have been initiated. However, no noteworthy topics have emerged from industry. It should be emphasized that pioneering efforts in a new field cannot be easily 1
Ultra-Fine Particles evaluated based merely on economic factors. To lead the world economy, it is important to maintain a position in which new materials can be created and new, advanced industrial products can be developed. This can only become feasible through the efforts of ambitious and dedicated scientists and engineers who are motivated to meet the challenges. In the Ultra-Fine Particle Project, particles with diameters of 1 to 100 nm were referred to as UFPs. Each UFP is a collection of about 100 to 108 atoms. In comparison to the atomic scale, UFPs can be considered to range from large to giant bodies. However, their size is less than optical wavelengths and requires electron microscopy for morphological observations (i.e., an individual UFP cannot be observed optically). The UFPs that are visible to the naked eye are in fact coagulated bodies of UFPs. Traditional fine powders are generally agglomerates offine particles, with diameters ~2 j.lm. The UFPs behave as liquids or gases during mechanical treatments such as crushing and filtering. Particles smaller than UFPs (i.e., particles consisting of several atoms to hundreds of atoms) are usually called clusters. One generally possesses intuitive concepts for visible objects, such as metals, ceramics, plastics, and living matter, which are supracollections of atoms. Considering these materials, one can ask questions such as how many atoms are required for a collection of atoms to exhibit characteristics that are common to a metal? Or, how large a size is needed to show biological interactions and functions? Investigations of UFPs have such simple questions lurking in the background. However, such simple questions will be a key topic of science for some time into the future. That is, to understand and clarify such phenomena as changes in matter, phase transformations, and the science of synthesis and fracture, the smallest unit of a solid may be UFPs. The smallest unit in the world ofmicroorganisms today is thought to be the virus, a "particle" about the same size as that of UFPs. Considered from this perspective, UFPs may be thought of as giant molecules, but they cannot be described adequately by the molecular framework ofclassical chemistry in terms of the number of
2
Introduction atoms and their relative positions, especially the arrangement of atoms on the surface of the particles and at their interfaces. In industrial technology, one direction is to refine and control, as in "fme ceramics" and "fine technology," while the other is to extend the horizons, such as heat or radiation resistant materials or space technology. Applications for UFP technologies tend toward the style ofrefinement and control and are deeply related to the fields of electronics and bioengineering. The Ultra-Fine Particle Project included a group that investigated biological and chemical applications in anticipation ofsuch needs. Among the bacteria investigated, some were able to extract and concentrate ions from the surrounding solutions, creating within themselves fine particles of metal compounds. Non-biological UFPs with sizes on the same order as those of cells or microscopic bodies within cells are expected to interact with microscopic biological bodies in ways that are as yet unknown.
3
1 ELECTRON MICROSCOPY STUDIES OF ULTRA-FINE PARTICLES
INTRODUCTION (by Ryozi Uyeda)
In Sections 1.1 to 1.5 ofthis chapter, the methodology ofthe Basic Material Property Group is reviewed. From Section 1.6 to the end of this chapter, the achievements of the research are presented. The term "material properties" normally refers to electrical, thermal, optical, and other properties. Thus, the inclusion of electron microscopy in the title may create a strange impression at first. However, as the original organizer of this group, as well as S. Iijima who succeeded me, our training in the area of crystallography played an important role because electron microscopy is a key method for analyzing UFPs. Crystallography is concerned with the structure of crystals (atomic arrangements), lattice defects, morphology, structures (textures), and is the basis of materials research. Any measurement of properties requires support from crystallography. Sometimes crystallography can lead to a new understanding of the properties of materials and to new technologies. The objective of this chapter is to introduce our electron microscopy studies ofUFPs. Our group was formed from scratch for the present project, but has a long history dating back to the 1940s. This author used electron diffraction to study the epitaxial growth mechanism of evaporated films. The attempt failed in its main objective, but discovered the so-called island structure at the early stage of evaporated film formation [1]. The island was indeed a UFP in today's terminology; thus the author's link to UFPs, albeit
4
Electron Microscopy Studies subconscious, was a long one. Another link was the study of zinc black during World War II. This was done in conjunction with the development of an IR detector for heat-seeking bombs. Zinc black was then known to be the best IR absorber and it could be produced by evaporating zinc in a reduced nitrogen atmosphere. It was studied using electron diffraction and research confirmed that zinc black particles were UFPs of metallic zinc. Their diameters can be kept under 10 nm depending on the evaporation conditions. This work remained unpublished, but was the first Japanese work on UFPs. In 1962, the Kubo theory was published [2]. The author was inspired by the theory and resumed the study of metallic UFPs made by the evaporation method, recalling the zinc black research. By this time, electron microscopy became available. The beauty of UFP crystals was facsinating and research concentrated on the morphology of multi-faceted crystals [3]. At the suggestion ofN. Wada (Nagoya University), who was interested in material properties rather than crystallography, electron micrographs of ferromagnetic UFPs were presented at the first Kaya Conference in 1963. These are reproduced in Figures 1 and 2 and represent iron particles made in an argon atmosphere (30 torr) and strings of cobalt UFPs held together by ferromagnetism in a magnetic field, respectively. These attracted the attention of researchers investigating ferromagnetism. With the cooperation of A. Tasaki (Tokyo University, now at Tsukuba University), an industrial-scale development of high-performance magnetic recording tapes was done (1971-1977) under the sponsorship ofthe Research Development Corporation of Japan. This program did not succeed commercially, but it did contribute important clues that assisted later research and development programs. When the current project started, Dr. S. Iijima, who had been in the U. S. for over ten years, was asked to return to Japan to join this project. Dr. Iijima had obtained a doctorate under the guidance of Professor T. Hibi (Instrumentation Research Institute, Tohoku University) and then went to the u.s. to engage in high-resolution electron microscopy research under Professor 1. Cowley (University
5
•
'0. JO_ •••
•••
($> "
Ie:. ••••• •
...
Figure 1. The shape of each particle corresponds to the 12·sided diamond structure shown on the right. The particles indicated by A, B, and C in the photogrsphcorrespond to the (100), (110), and (Ill) projections shown on the right.
)1
~)
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i
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Figure 2. The particles align in chsins as a result of their strong magnetism. The chains align in the same direction due to the effects of an external magnetic field. 6
Electron Microscopy Studies of Arizona). There he achieved world-class accomplishments that brought him the Wollen Prize in crystallography. The success of our group owes much to his enthusiasm and outstanding experimental technique. For us, it is appropriate to use the term UFP because we examine individual particles by using electron microscopy. In many research and development projects, however, agglomerates ofUFPs are used. In powder technology [4], which has a long tradition, terms such as fine powders and ultra fine powders are used. The term ultrafine powders will be used in this chapter. The author began with an old story. Whether or not such a historical description was useful to this project remains to be seen. I am grateful that I was able to work with such an enthusiastic group for the past five years despite my advanced age [5]. References 1. Uyeda, R, Proc. Phys. Math. Soc. Jpn., 24, 809 (1942). 2. Kubo, R, Solid State Phys., UFP issue, p. 4, Agne Engr. Center (1984). 3. Uyeda, R, Parity, 2, 4-24 (1987). 4. Jinbo, M., Science ofPowders, Koudansha, p. 40 (1985). 5. Uyeda, R, Studies of Ultra-Fine Particles in Japan, Progress of Materials Science: vol. 35, no. 1, pp. 1-95 (1991).
7
Ultra-Fine Particles
1.2 mtra-Fine Particles and Electron Microscopy (by Sumio Iijima)
Imagine the surprise ofancient people when they encountered mysteriously shinny gemstones. When the initial surprise was over, they probably began to think about such materials in terms of their geometric shape. Such activities later developed into mineral collecting, classification by shape and color, and eventually led to mineralogy and crystallography. When man used only the naked eye, the microscopic world was that ofimagination. The Greeks did not have a means to see the microscopic world, but created the concept of atoms as the ultimate subdivided parts of matter. In the process of classifying atoms and comparing various atomic properties, Mendeleyev discovered the periodic table. The discovery of invisible bacteria was also the legacy of scientists who challenged the microscopic world. With the invention ofoptical microscopes, various types of bacteria were found and this became the basis of modern bacteriology. The basis ofscience begins with careful observations of matter and phenomena. In particular, the capturing of visible images has a direct and persuasive power. Thus, this section's title is a natural outcome ofthe study ofUFPs. The starting point is to observe UFPs with a super high resolution electron microscope to define the morphology and to then observe them at the atomic scale.
UFP Observation by Electron Microscopy - Background The resolution of an optical microscope is limited by the wavelength of the light used. This limit was well recognized, so the search for more sophisticated techniques began. In 1932, Ruska invented an electron microscope. Because the purpose of this invention was to observe microscopic matter, various small objects ranging from inorganic to biological materials were examined, including microscopic particles.
8
Electron Microscopy Studies One of the problems at the start was to prepare samples that allow the transmission of electrons. Early electron microscopes had low acceleration voltages and low penetrating power, so very thin samples were required. During the 1950s, techniques for preparing thin samples were inadequate and samples that could be observed were limited to cleaved mica samples and the like. At that time, microscopic particles were studied often, including needle crystals of zinc oxide, plate crystals ofmolybdenum oxide, and cubic crystals of magnesium oxide from the combustion of metallic magnesium. From electron diffraction patterns and electron micrographs, many important studies were done concerning the interaction of crystal and electron beams, namely, the electron diffraction phenomena. It was then well known that oxide UFPs can be synthesized by the combustion of metals, as discussed later for alumina UFPs. Subsequently, the preparation of thin metal films by vacuum evaporation methods and the development of techniques to prepare thin samples by electro- and chemical polishing shifted the center of electron microscopy from microscopic particles to thin films. The pioneering studies of metallic UFPs by Uyeda and others began with such a historical background [1].
Atomic Scale Observation The motivation for UFP research was to examine microscopic matter using electron microscopes with atomic scale resolution. Direct observation of atoms arouses our basic interests. It was a challenge to attempt this type of observation. Since the invention of the electron microscope, the direct observation ofindividual atoms has been a continual challenge both experimentally and theoretically. The design of electromagnetic lenses, electron beam generation and monochromatization, etc. were the problems faced in the development of electron optics. In addition, materials for electromagnetic lenses, precision machining technology, and electrical circuit design were also a part of electron microscope development. There were also 9
challenges associated with the physics of such systems, such as understanding the principles of imaging atoms or crystals using electron beams; specifically, the interaction of electron waves with matter.
When the present Ultra-Fine Particle Project began, it was possible to directly view atoms. In 1969, Crew [2] succeeded in imaging atoms for the first time. His group used scanning-
transmission electron microscopy, which used a different imaging method that was unlike orthodox transmission electron microscopy or optical microscopy. Atomic images using transmission electron microscopy (TEM) were first obtained by this research team in 1971 [3]. In this study, regularly arranged atomic images of niobium oxide were recorded
(Figure 1). The dark parts correspond to niobium atoms that were separated by a distance of 0.38 nm. This oxide has many similarities with high Tc supercondueting oxides.
Figure I. The first atomic image obtained. The image is ofniobiumtitanium oxide as obtained by using high~resolution transmission electron microscopy. The inserted figure is a model image of the crystal structure in which the squares represent metal atom tetrahedra surrounded by oxygen atoms. 10
Electron Microscopy Studies This technique spread to mineralogy, crystallography, metallurgy, solid state science, and materials science, with the development of high-resolution electron microscopy (BREM). This became a major trend in electron microscopy of the 1970s. Application ofthis technique to the examination of the microstructure of UFPs is one of the remaining research themes of interest in the HREMfield. Before joining the Intra-Fine Particle Project in 1982, a sample of UFPs arrived at Arizona State University where this author was doing research. This was a sample of iron sulfide UFPs. It was collected from the air in Pittsburgh. The challenge was to determine the crystal structure of these particles, which had diameters on the order ofseveral tens of nanometers. This was part of an air pollution abatement program. A post-doctoral research fellow worked on this problem, but the crystallographic analysis of a particle with a 100-nm diameter was very difficult and no definitive results were obtained. The best available electron microscope at the time was used (made by Phillips), but still failed. Because UFPs are crystalline, accurate control ofthe crystallographic orientation is essential for the analysis of crystal structure. This is an extremely difficult task in HREM. This example shows that in HREM the structural analysis of microscopic crystals is a research problem that remains. We need to develop an instrument that can quickly and simply adjust the crystallographic orientation of a UFP, an instrument that can be used to observe the morphology using a micro-electron beam, and one that can make high-resolution microscopy images. Another challenge was to observe the dynamic changes of a sample at the atomic scale. HREM is ordinarily used for the observation ofstatic samples. Dynamic electron microscopy has been routinely used to observe the annealing of crystals and to observe the growth process of thin films. However, few studies used HREM to observe atomic transport phenomena. Observing atomic-scale changes of the state of solids or chemical reactions should become a new theme in the field of HREM. Only HREM can provide information on the crystal structure and its time dependence in
11
Ultra-Fine Particles
localized regions such as at crystalline defects, surfaces, interfaces, and in UFPs. The background on UFP studies done with the aide ofHREM was presented in parallel with the development of electron microscopes. In the following section, we will summarize UFP research that was done with electron microscopy and describe work done by the Basic Material Properties Group in the mtra-Fine Particle Project. Microstructures of Ultra-Fine Particles UFPs are an example of an extreme state of matter and represent a region where concepts about bulk, molecular, or atomic materials cannot generally be adopted. A unique phenomenon associated with UFPs is known as the particle diameter effect. Problems related to this effect represent new research areas that have rich potential for both solid-state physics and industrial applications. The particle diameter effect appears either inherently for each ofthe particles or for collections of particles. In either case, we need to know the microstructures involved, including the particle morphology, crystal structure, surface structure, lattice defects, and stability, to understand the physics and chemistry of UFPs. In traditional powder evaluation, however, indirect methods have been used (e.g., measuring the specific surface area of a collection of particles to determine the average particle diameter). In the ceramics industry, microscopic particles are often used to make superior ceramics. Here, it is important to understand the sintering mechanisms ofpowders. Evaluation methods generally used in ceramics depend on macroscopic measurements of mechanical strength and hardness of the sintered body from which the raw materials are evaluated. An example is the relationship between sinterability and particle size. The processes involved when individual particles join and sinter are reactions involving mass transport. Thus, the microscopic structure, such as the shape, formation of surface layers, and crystal structures, are expected to influence the sintering of powders. 12
Electron Microscopy Studies The relationship between the microstructures of fine particles and the characteristics ofsintered bodies will become a new evaluation method for ceramics. The understanding ofthe microstructure of fine particles is also important in the study of catalysts in the chemical industry and in the evaluation of magnetic recording materials. Because of these viewpoints, the Basic Material Properties Group established observation techniques using electron microscopy for the evaluation ofUFPs having diameters ofless than several tens of nanometers. The microscopic structures of UFPs have been observed and, on the basis of the observations, their synthesis and modification have been studied. Research on UFPs is always involved with the search for the particle diameter effect, while the research for applications of UFPs is the use and control of such an effect. We constantly considered this aspect as we did our research. Our research methods and a summary of our UFP program are shown in Figure 2. The subject of the research can be roughly classified into four parts: microstructure evaluation using electron microscopy, synthesis of UFPs, UFP modification methods, and production and use of microscopic particle beams. The arrows in Figure 2 indicate the flow of the research. Let us explain the approach using oxide UFPs as an example. First, we developed a synthesis method for new oxide UFPs using gas evaporation with an electric arc as the heat source. UFPs thus produced were evaluated by electron microscopy for their particle size, crystal structure, surface structure, lattice defects, and stability. To evaluate the microstructure of each particle correctly, we had to develop an electron microscope with an ultra-high vacuum pumping system and appropriate observation methods for UFPs. Following the microstructure observation, the conditions for UFP synthesis were adjusted. This reiteration was repeated until the desired UFPs were obtained. To clarify the crystal growth mechanism ofUFPs produced by the electric arc method, we needed to consider basic problems such as the generation of metal vapor, oxidation of evaporated gas, microcrystal nucleation and growth, particle coagulation, and cooling
13
Ultra-Fine Particles
~
~
Heating in gas Heating in vacuum
Phase transformation
...] Modif~cation Oxide film
I~-----] UFPbea~ s I"" '"
Experiments on UFP beams
[I Ultra-Fine Particles I]
l" ~ Characterization byTEM
Production •
Crystal structures\ Cystalline defects \ Structural stability, v
Sintering Catalysts
Figure 2. Diagram of the UFP research done in the Basic Material Properties Group. effects. Information obtained from alumina UFP research to be presented later will prove valuable in evaluating ceramic raw materials or metal catalyst carriers. Once the characteristics of UFPs were understood, these particles were given to others for the study of ceramic sintering mechanisms and for use as catalyst carriers in further research. Modification methods for oxide UFPs were also studied. We addressed the synthesis of UFPs that have crystal structures unavailable at room temperature by using the formation of oxide and carbide surface coatings and the rapid cooling ofUFPs. As before, electron microscopy was used for the evaluation of the modified UFPs. 14
Electron Microscopy Studies A microscopic particle beam instrument was developed, which provided the team with a means for creating new types ofUFPs. Two methods for placing the UFPs in a vacuum and for flowing UFPs with a carrier gas were considered. The former was used for making UFP samples with clean surfaces needed for electron microscopy. As Figure 2 clearly shows, our research provided the starting point for understanding the microstructures ofUFPs.
References 1. "UFP Issue," Solid State Phys., Agune Engr. Center (1984), in
Japanese. 2. Crew, A. V., Wall, 1. and Langmore, J. P., Science, 168, 1138 (1970). 3. Iijima, S., J. Appl. Phys., 42, 5891 (1971).
15
Ultra-Fine Particles
1.3 Development of Electron Microscope Accessories (by Toshinari Ichihashi)
Sample Heating Device To remove surface oxide layers by heating in vacuum and to observe in situ the sintering phenomenon ofUFPs, a sample heating device for an electron microscope was developed. First, a CO2 laser was tried for heating (maximum output of lOW and output fluctuation of5%). The laser light was introduced into the electron microscope column through a ZnSe lens (focal distance, 5 in). The lens acted as a vacuum seal and its focal position was adjustable using bellows. The sample was held by graphite, which absorbed the laser light. When this device was used, the laser output fluctuation, which was only 5%, was enough to cause sample drift and vibration that prevented high resolution imaging. The cause appeared to be changes in the position of the laser irradiation due to anisotropic thermal expansion of the sample holder. The design of the sample holder was altered and improvements in the laser stability were requested, but these modifications were unsuccessful within the time limits of the project. A second attempt was made using direct Joule heating. The sample holder mesh was cut into a rectangular shape and was sandwiched between two silica plates (thickness 0.2 mm) with gold electrodes (see Figure 1). This was heated by passing direct current through the electrodes. The current source derived from live power was unusable due to power source frequency (60 Hz) noise, so a storage battery was used. This allowed the sample to be heated to I,300°C (the melting temperature of silicon) while maintaining the resolution of the Si (Ill) surface at 0.31 om.
Video Imaging Systems Recent electron microscopes are more frequently being equipped with devices for in situ observation and recording [1-3]. To
16
Electron Microscopy Studies
i
~ silicil
plate
ruby ball
electrode Figure 1. A direct Joule heating device used to heat samples during electron microscope observation.
observe the dynamic behavior of clusters (see Sec. 1.15), the camera chamber ofthe electron microscope was modified and a video camera connected to a video tape recorder was installed. As shown in Figure 2, a right angle prism was placed under the camera chamber, on which a fluorescent screen waS placed. The image on the screen was viewed horizontally by using a super-high sensitivity silicon intensifier target (SIT) video camera. The fluorescent screen and prism both had an opening through which electrons were directed to the electron energy loss spectroscopy (EELS) detector. The prism was movable from the outside so that the area for the EELS analysis could be selected while observing the image on the screen outside the opening. This system allowed for simultaneous examination ofEELS and video images. A video camera equipped with a SIT camera tube (Hamamatsu Photonics, C-I000) with sensitivity and contrast adjustability were used. This type ofcamera permitted real-time video recording, which was done on a U-matic video recording system (Sony, BVU-820).
17
Ultra-Fine Particle.~
t I ~ctron beam
! screen p. I
..
I
prr sm SIT
camera
EELS
Figure 2. Mechanism for moving a prism for simultaneous observation and EELS measurements in an electron microscope equipped with a video camera. Gas Bleeding Device The sample chamber ofthe electron microscope was equipped with a gas bleeding device to observe the movement of clusters in gaseous atmospheres (see Sec. 1.15) and to observe the reaction between solid surfaces and gases. Gases were introduced through a variable leak valve and a tantalum tube at a partial pressure of 10-9 torr and traveled to the sample, a distance of- 5 mm. This allowed for the observation ofsurface reactions (e.g., oxidation, reduction, catalysis, etc.). The gas tank was mounted on the electron microscope itself to prevent transmittance of outside vibration, other than those associated with the operation of the leak valve. Results from the gas bleeding experiments during the Ultra-Fine Particle Project were not obtained, however, tungsten clusters decomposed and precipitated from WF6 molecules adsorbed on the surface of silicon UFPs were recently observed using the device described above [4]. 18
Electron Microscopy Studies Introducing Ion Beams Samples were etched using an ion beam so that clean surfaces could be observed. An ion-beam gun for Ar+ or W with a maximum accelerating voltage of 5 keV was mounted at an angle to the sample chamber. This was used for the removal of oxide layers from silicon UFP surfaces. Because the irradiation ions were deflected by the magnetic field ofthe electron microscope, in-situ observation was not possible. Thus, ion irradiation was done at zero lens current and the sample was viewed immediately following etching. The incident angle of the beam, however, was only 20 0 and the ion beam was not focused to a spot (less than O.l-mm diameter) on the sample. This caused sputtering of the sample holder and sample mesh, which resulted in deposition on the sample surfaces and prevention of oxide removal. The procedures that the team developed are still incomplete, but it is expected that they will become fundamental techniques that will be essential for in situ observation of reactions between solid particles and electron beams, ions, and gas molecules. References 1. Iijima, S. and Ichihashi, T., Jpn. J. Appi. Phys. 24: L125 (1985). 2. Iijima, S. and Ichihashi, T., Phys. Rev. Lett. 56: 616 (1986). 3. Takayanagi, K. Tanishiro, Y, Kobayashi, K., Akiyama, K., and Yagi, K., Jpn. J. Appl. Phys. 26: L957 (1987). 4. Ichihashi, T. and Matsui, S., Extended Abstracts of the 19th Conference on Solid State Devices and Materials, p. 505 (1987).
19
Ultra-Fine Particles 1.4 High Resolution Observation Methods (by Sumio Iijima)
Crystal structural analysis methods using x-ray dim-action have been rather firmly established after the 70 some years since Bragg's analysis of the sodium chloride crystal structure. Corresponding methods with electron diflTaction originated from the discovery of electron beams by Davisson and Germer, as well as G. P. Thompson. This was developed in parallel with electron microscopy following WorId War II. Electron diflTaction methods, however, are not as popular as those using x-ray and neutron beams because quantitative interpretation of electron diflTaction intensities is difficult due to the strong interaction between the electron wave and the crystaL Despite this drawback, crystal structure analysis using electron microscopes has the unique capability of allowing a specific portion of a crystal or individual microscopic crystals to be analyzed. In recent years, the precise control of the microstructure of materials used in the semiconductor and ceramic industries has been required. To understand the physical and chemical properties ofthese materials, it is necessary to evaluate the local microstructures such as lattice defects, crystal surfaces, and interfaces. Another approach is to use focused electron diflTaction methods, which allow the space group ofa crystal to be determined based on the strong interaction of the electron beam with the crystal. In this section, the imaging and electron dim-action methods used in high resolution electron microscopy (HREM) for studies of the crystal structure, lattice defects, and crystal surfaces ofUFPs are discussed. Electron Microscope Construction
A transmission electron microscope consists of an electron gun, condensers, a sample to be examined, the objective, intermediate and projection lenses, and the screen. It has the capabilities of obtaining magnified images of a sample as a microscope and an electron diffiaction pattern. Magnified images are useful in studies of a specific part of the sample, such as lattice defects and surfaces, 20
Electron Microscopy Studies because the real space is examined. Electron diflTaction, on the other hand, allows for the observation of the reciprocal space and for evaluation ofthe average structure ofthe entire crystal. By switching the optical system, these two modes can be interchanged with ease, making this is a unique feature of electron microscopy that is unavailable in other diffraction methods. Each of the various parts of the microscope that was developed will be briefly explained below. Usually, the electron source uses a heated tungsten filament or LaB6 crystal chip that produces thermal electrons. High intensity electron sources using field emission electrons have also been developed. These have a narrow electron energy band width below 1 eV and are useful as the electron source for electron energy loss spectroscopy (EELS). Important aspects regarding the condenser are to have a high intensity beam and to produce parallel beams. To examine individual UFPs, we need a condenser design that is capable of focusing a high intensity electron beam upon the sample. This requirement is critical in high resolution observations using a video camera. The most important component of a high-resolution electron microscope is the objective lens. Ordinarily, the magnification ofthe objective lens is about 100 times, but its resolution determines the capability of the microscope. Magnified images are further magnified successively by the intermediate and projection lenses. Ultimately, a magnification of 106 times is obtained on the fluorescent screen.
Electromagnetic Lens and Resolution The resolution ofan optical microscope is proportional to the wavelength of the light used, and inversely proportional to the opening angle ofthe lens, but is inherently limited by the wavelength of the light. The shorter the wavelength, the better the resolution. This principle also applies to electron lenses. The wavelength of an electron beam is approximately given by A = v'(1501E) (A) where E is the accelerating voltage (V) (e.g., at E=120 kV, A = 0.035 A. The wavelengths ofhigh energy electrons are much shorter than the optical wavelength, thus making higher resolution imaging possible. 21
Ultra-Fine Particles The path of electrons moving within an electromagnetic lens is governed by the Lorentz equation. Electromagnetic lenses are designed on the basis of this equation. Lens characteristics are expressed by the spherical aberration coefficient, C.. For the observation of UFPs, a goniometer was required to adjust the crystallographic orientation. The goniometer was inserted into the polepiece ofthe objective lens, which was an important consideration during the design phase. Even with a perfectly designed lens, asymmetry of the lens geometry is unavoidable due to the limited accuracy of the machining processes. Thus, a lens always has astigmatic aberration, which can be corrected by a stigmater device in practical electron microscopes. This correction is especially important in high resolution work. Another problem is chromatic aberration, which appears as changes in the lens focusing distances due to variations in the lens magnetization and accelerating voltage or due to changes in the wavelength of the electron wave. The chromatic aberration is also caused by the energy loss ofthe electron beam as it passes through the sample. An electron microscope with as small an aberration as possible was needed to obtain high resolution images.Theoretical resolution, determined by the spherical aberration and the beam opening angle, is given approximately by the following equation.
(1) The microscope used had an accelerating voltage of 120 kV (A = 0.035A) and an objective lens with C.=0.3 mm; that is, the theoretical resolution, ax was 0.23 nm. Equation (1) indicates that a x can be reduced more effectively by reducing A rather than C.. This led to the construction of high voltage microscopes with accelerating voltages of 500 kV or 1 MY, providing resolution approaching 0.1 nm.
22
Electron Microscopy Studies Imaging of Phase Objects To understand the imaging mechanisms that are involved in HREM, it is best to consider a single, isolated atom. Two mechanisms exist. One is the Coulomb interaction when an electron beam passes near the atom. When a sample is a crystal, this can be treated using the theory of electron ditrraction. The other is the interaction due to the Lorentz force acting on the electron beam due to the electromagnetic field of the objective lens. An electron beam is scattered by the atomic nucleus and by the electro-static potential, <1>(x), due to the electrical charges of electrons that surround the nucleus. The strength of this interaction is inversely proportional to the electron beam energy, E, and the wavelength, A , and is written as CJ = Tt/A E. The scattering effect of the electron beam is expressed with respect to the refractive index of the beam. When an electron passes through a potential field, it is accelerated and the phase of the electron beam changes. Figure 1 illustrates the scattering of an electron by an atom. Electron scattering by an isolated light atom affects only the phase, but does not affect the amplitude. Such an object is called "a phase object" and is expressed by the following equation.
q(x)
= exp( -iCJ
(2)
Electron beams are focused by a lens after passing through an object. Imaging by the objective lens can be described by the imaging theory for thin lenses as described by Appe. Electron beams ditrracted by an object are collected at the rear focusing plane (Figure 2) and produce an amplitude distribution lJ1 (u), which is given by the Fourier transform of q(x) as shown in the following equation.
'feu) = Flexp( -iCJ
23
(3)
Ultra-Fine Particles
e. f\N\N
~.Yv Figure 1. Accelerating effect of electron beam caused by atoms. Electron bealT'
q(x)
Specimen
~-r-....,r-----,I---....p.-
Objective lens
\jI (u)
q(Mx)
Image plane
Figure 2. Focusing of an electron beam by the objective lens. The sample distribution, q(u), and the amplitude distribution, lJ1 (u), at the rear focusing plane are related by the Fourier transform and the image amplitude on the focusing plane is the Fourier transform oflJ1(u). 24
Electron Microscopy Studies Here, F is the Fourier transform operation, u= 2 sinO/A, and 0 is onehalf of the scattering angle. The electro-static potential
(x) for an isolated light atom is small. Here, a(x)«l and equation (3) can be simplified by expanding the exponential function as follows: lj1(u)
= l>(u)
- ia(u)
(4)
Here, l>(u) is the amplitude distribution ofthe non-scattered wave and (u) is that ofthe scattered wave. The amplitude distribution of the image on the imaging plane of the objective lens is given by the Fourier transform. For an ideal lens, the distribution becomes q(Mx) and an image magnified M times forms on the imaging plane. This image is similarly enlarged by the intermediate and projection lenses. In a real electron microscope, the scattered wave (u) suffers phase changes due to the spherical aberration and focusing adjustment. The resultant amplitude distribution is given as follows: 'I'(u)
=F
[exp(-ia(x»] . exp( -X(u»
(5)
Here, X(u) is called the phase contrast transfer function and is approximated by the following equation:
(6)
Here, E is the shift from the focal distance ofthe objective lens and Cs is the spherical aberration coefficient. When the value of E, the magnitude of defocusing, is properly selected, sin X (u) varies as shown in Figure 3. This corresponds to C. = 0.3 mm, E = -64.5 nm, and A = 0.037A. The curve in Figure 3 includes the effects of chromatic aberration and other effects (for details, see Ref 1). As shown in Figure 3, in a certain region of u, sin X(u) is approximately unity and the corresponding value of E is called the
25
Ultra-Fine Particles
A CS
=
=
.037 A
4
.75 mm
a
-
=
80A 1.2 mred
£ = -645A u.. ... 0 ~---,--------.--------j"""--"r---:lPc:::II""'T"""",,"=_----,r-- .625 A" .25
u
-I
Figure 3. The phase contrast transfer function. "optimum focus." The resolution of the microscope is defined by U o when sinX (u) = 0 and is equal to !l x (Eqn. (1)). At the optimum focus, the phase change of the scattered wave can be regarded as constant for u < U o ' The amplitude distribution on the lens imaging plane,I(x), due to an object is then equal to the Fourier transform of tIr(u), or can be described as follows:
[(x) = 1 + 20(x)
(7)
This equation implies that the image intensity of HREM is proportional to the electrostatic potential distribution of the sample. A heavy atom gives a darker image than a light atom. Based on the theory of image formation for phase objects, a computer simulated image of a tungsten atom is shown in Figure 4. Figure 4a shows the electrostatic potential distribution and Figures 4b-f are through focus images for € from +425 to -2,275 A in steps of 635A. The image in Figure 4d is at the optimum focus and is closest to the original potential distribution. In other images, the center appears light [2]. Electron microscopy images of clusters of a few atoms that are close to an isolated atom have often been observed, as will be presented later. 26
Electron Microscopy Studies
Figure 4. Computer simulated images of an isolated tungsten atom in HREM. a: The electrostatic potential distribution of the atoms and bf: through focus images. The image of d corresponds to the optimum focus and agrees with the potential distribution.
Electron Diffraction and Crystal Structure Image The theory of image formation of a simple phase object, presentetiin the-pr-eeed-ingseetion; br-eaks-do~'il- -in- -a-thick- crystal-,where multiple scattering effects due to the crystal must be taken into account. The interaction of an electron beam with a thick crystal is treated by the dynamic theory of electron diffraction. Such calculations use Bethels method of wave mechanics [3] and the Cowley-Moodie method of physical optics [4]. These are used to obtain the amplitude distribution, W(u), and the phase ofthe electron beam passing through a crystal, with the thickness of the crystal and the crystallographic orientation ofthe incident electron as parameters. Methods for these calculations were established by the late 1960s and descriptions can be found elsewhere [5,6]. In 1970, Uyeda et al. published an electron micrograph showing the two dimensional molecular arrangement in a crystal [7]. In the following year, a photograph showing the metallic atom arrangements (resolution 0.38 nm) in a niobium oxide crystal was 27
Ultra-Fine Particles obtained, followed by direct crystal structure analysis from the electron microscopy images [8]. The electron micrograph that was obtained has a one-to-one correspondence to the arrangement of metallic atoms and, because it directly reflects the crystal structure, it was called a crystal structure image [9]. In the 1970s, computation techniques using computers advanced and many studies appeared in which the simulated and experimentally obtained electron diffraction pattern intensities and images ofhigh resolution (HR.) electron micrographs were compared. In the simulations, numerical calculations ofljl(u) and X;(u) were done. The results showed that HR electron micrographs of relatively thin crystals correspond well to the atomic arrangements in the crystal, similar to that found for phase objects [10, 11]. However, when the thickness of a crystal exceeds several tens of nanometers, the experimental and theoretical images typically do not match. Experimentally, it is difficult to adjust the crystallographic orientation of a thick sample crystal, thus lowering the chances of obtaining reliable images. Understanding ofelectron beam absorption processes is also inadequate.
Multi-Functional High Vacuum Electron Microscope For crystal structure analyses of UFPs, the x-ray powder diffraction method is generally used. This method mvolves the use of an electron microscope to examine a single microscopic particle with a diameter less than several tens of nanometers, thus allowing for analysis ofthe crystal structure. It is important to use various modes of observation (crystal structure images, microscopic area crystal structure images, selected area electron diffraction patterns, focused electron diffraction patterns, microscopic area electron diffraction patterns, and observation ofparticle behavior) and to be able to switch from one mode to another rapidly and easily. Similarly, the lattice defects, surface structures, etc. can be obtained for a particular spot on a sample. The point to point resolution of the electron microscope used in this study (Akashi EM-002A, accelerating voltage of 120 kV) was
28
Electron Microscopy Studies approximately 0.23 om. With equipment ofthis level, we can directly observe the crystal structure of metallic UFPs (atomic positions are directly visible). High-resolution electron microscopy and electron diffraction using a micro-focused beam are effective for the observation of the surface and lattice defects of UFPs as well as for observing the effects of sample temperature and charging. It is necessary, however, to focus the electron beam to a 3-om diameter simply and quickly. It is also important to have the capability ofbeing able to return to the conventional high resolution observation mode after the micro-beam mode. A commercial electron microscope with such functions was not available at the time of this research [12]. A technically important problem in observations using highresolution electron microscopes is the contamination of the sample. The vacuum system of the microscope used in this study was upgraded with an ultra-high vacuum pumping unit to minimize the contamination problem. The electron gun and sample chambers were pumped with two ion pumps (40 Vs, 150 Vs) and a turbo-molecular pump (330 Vs), while the camera chamber was pumped separately with a turbomolecular pump (50 Vs). All the movable parts, such as movable apertures, were sealed with bellows and the microscope column used greaseless O-rings. In the sample chamber, the vacuum reached 2 x 10,8 torr.
Dynamic Observation with a High Resolution Electron Microscope Previously, high resolution electron microscopy done in conjunction with the use of a video camera was primarily used to record static images on-line for image processing purposes. Dislocations in a crystal and movement of grain boundaries have been examined, but few studies used high resolution electron microscopy. A video camera was used by chance during the observation of a metal catalyst. When this catalyst was observed at one million magnification through the attached magnifier, rhodium clusters were observed moving. To monitor such movement with atomic resolution, a high-
29
Ultra-Fine Particles
sensitivity video camera was installed on the electron microscope so that images at two million magnification on the fluorescent screen could be monitored and simultaneously recorded on a video recorder. To obtain an adequate signal-to-noise ratio, a current density of200 Ncm2 (107 electron /nrrr) was required. The spot resolution on the CRT was 0.04 nm/line at 512 scan lines. If the electron beam was focused to increase the current density, the opening angle ofthe beam at the sample became large, which reduced the resolution. Obtaining a bright beam without losing the resolution was a technical challenge [14]. Static electron microscope images reproduced from a video recorder had 0.23 nm spot resolution without any image processing. The exposure time of each image was 1160 sec, so two orders of magnitude reduction in the exposure time was achieved in comparison to conventional high resolution electron microscopy, which took 2-3 sec. It was remarkable that the image quality was not degraded. This technique will allow dynamic atomic examination of phase transformations, crystal growth, and lattice defects during heating. These have been difficult to observe due to sample drift. All the reproduced images in this article were from a video recorder with 1160 sec exposure, but the image tube ofthe video camera had a frequency of one image every 1130 sec, which was the actual limit in temporal resolution. In the following paragraphs as well as in Section 1.15, examples of dynamic observations of atomic movement on crystal surfaces and metal atoms will be described. Observation of Ultra-Fine Particles
One feature of crystal structure characterization using high resolution electron microscopy is the capability of observing regions of non-periodicity, such as lattice defects, surfaces, and interfaces. The structural examination ofUFPs indeed benefits from this feature. Detailed findings will be given in Section 1.6 and later. Sintering of UFPs is described, by which the capability of this new technique is demonstrated. 30
Electron Microscopy Studies In the study of advanced ceramics, the physical and chemical characterization of grain boundaries is most important and the understanding ofthe sintering mechanism of powders forms the basis ofmuch research. In the past, macroscopic evaluation methods such as the hardness and mechanical strength of sintered materials were predominant. Examination ofmicrostructures of grain boundaries and lattice defects via electron microscopy and the correlation of these features with the macroscopic behavior are relatively new research methods. The UFPs can be readily imaged by electron microscopy without using any preparation because oftheir small size. This feature was exploited and the team studied the initial stages of sintering of UFPs. When spherical UFPs ofalumina, titania, and silicon that were produced by gas evaporation methods were heated, the individual UFPs fused together. The initial processes of sintering was observed at the atomic level. Figure 5 shows an electron micrograph of y-alumina UFPs heated to 1250 0 C. The neck or the connection (shown by an arrow) between two UFPs is surrounded by crystallographic planes. The conventional explanation of the neck geometry at the initial stage has been that the neck is covered by a curved surface minimizing the surface area. This description, however, ignores the growth of crystallographic planes. The above observation appears to require a new theory for describing sintering. The two particles have a common horizontal crystallographic image, indicating that these particles are joined with a single crystal axis in common. These micrographs were obtained after the crystallographic orientation was adjusted with an accuracy of 1 mrad. It should be noted that this requires very advanced techniques. The team also examined the neck that was formed between sintering silicon UFPs. Figure 6 shows the neck when the crystallographic orientation is accurately set to [110]. Images ofthe {Ill} lattice can be seen to be running in two directions. The entire image ofthese particles is shown in Figure 1 ofSection 1.10. Because the thickness of the crystal at the neck is 23 nm, the conditions for obtaining crystal structure images given in the previous
31
Figure 5. An electron micrograph of y-a1umina UFPs showing the initial processes of sintering. Two spherical UFPs are fused together and fonn a neck.
Figure 6. A high resolution electron micrograph of the neck of two silicon UFPs. The entire image is shown in Fig. I ofSec. 1.10. 32
Electron Microscopy Studies section are not met. Thus, the lattice images do not represent accurate atomic positions. However, one can clearly recognize a twin and a high angle r,9 boundary on the right side of the figure. The grain boundary on the left side is complex. The study of silicon grain boundary structures is important in the semiconductor industry in terms of its relation to the recrystallization of amorphous silicon. Silicon UFP joint structures are similar to those of silicon grain boundaries and their study should benefit silicon device technology. Next, an example ofthe high-resolution electron microscopy of the fusion processes ofUFPs as recorded using a video camera is described. When the platinum clusters formed on silicon UFP surfaces by vapor deposition were examined, the clusters were seen to move around and coagulate. Details of this observation will be given in Section 1.15. Here, the image quality and resolution using the video technique will be presented. Figure 7a shows two platinum clusters migrating on the surface of a silicon particle. This image was reproduced from the recorded video image. The vertical stripes are the (111) lattice image of silicon, with spacings of 0.32 DID. This indicates that the size ofthe platinum clusters is about 0.8 DID. These clusters collided a few minutes later resulting in the larger cluster shown in Fig. 7b [17]. The smallest platinum cluster had a diameter of 0.5 - 0.6 DID and contained several atoms in the cluster.
Observation of Crystal Surfaces The UFPs have large exposed surface areas and are suited for the study of crystal surface structures and surface phenomena. Figure 8 shows a schematic diagram ofthe surface ofa spherical particle with a stepped structure. By viewing from the direction of the arrow, the profile of the steps can be obtained at the atomic level by using the crystal structure imaging method [18]. Figure 9 shows a part of the crystal structure image of a 6alumina UFP with an 80-DID diameter as viewed from the <110> direction. Its surface appears spherical at low magnification (see Figure 10), but it is composed of many microscopic crystallographic planes. The horizontal straight section is the (Ill) surface with
33
Figure 7. Electron micrographs ofplatinum clusters formed on silicon UFP surfaces by vapor deposition. The vertical stripes are the image ofthe silicon (111) surface, showing a lattice spacing of0.32 run. a: Two platinum clusters having sizes ofshout 0.8 nm. b: These clusters became a single large cluster by fusing.
Figure 8. Method ofviewing the surface steps of a spherical particle. By viewing from the direction of the arrow, the projection of the steps can be obtained.
34
w
V>
!!1
Figure 9. Sequential electron micrographs showmg the step structure on the (Ill) surface ofa 8]Malumina OFP. The mono-layer of aluminum atom clusters on the (111) surface in image a appear as a result of crystal growth under the electron irradiation.
i f
~ ~
~
~
[ion 10nm
>-'--'-l
Figure 10. Entire image of the B-a1umina UFP viewed from the <110> direction. The rectangular area at the top is shown in Figure 9 at high resolution.
exposed oxygen atoms. An image representing the structure several tenths of a nanometer below the surface indicates a surface defect (shown by the arrow). Several rows of black spots on the top surface appear to be monolayer aluminum atom clusters on the (III) surface. These clusters grew during the observation in the electron microscope as can be seen in Figures 9a-c. This implies that the aluminum atoms move while under electron irradiation [12]. Oxygen atoms have a smaller electron scattering cross section than that of aluminum and cannot be seen. The structure of alumina particles carrying rhodium clusters as metal catalysts is discussed in Section 1.8. Similarly, the surface structures ofgold single crystal particles having lO-om diameters and a truncated octahedral shape were also examined. Figures Iia and b show the changes with time of the particle obsetved from the <110> direction. The profile of the 36
'.
Figure 11. Sequential electron micrographs reproduced from a video recorder showing the (100) surface atoms of a gold UFP having 10nm diameter and the movement of the steps. particle is seen as the projection of the {I OO} and {Ill} lattices (refer to Figure 12). Within the particle were seen lattice images corresponding to the {OO I} and {Ill} lattices, The step height of the atomic layer on the (001) surface is 0.2
om (= d200 lattice spacing) and the black spots near the top are the images of rows of several gold atoms parallel to the beam direction. The black spot contrast varied with time and the steps disappeared and reappeared, showing the movement of gold atoms on the (001) surface [13]. The movement could be analyzed from the video images taken with the video camera. One such example is shown in Figure 13, which shows the top right corneT of the particle as a function of time. The time interval is shown at the bottom right. The motion ofgold atoms was visible in real time. The comer atoms were more mobile than the face atoms, which indicated clear differences in the bonding characteristics. It was observed that amorphous matter appear like a cloud on the (001) surface of a gold particle. It was not clear whether this cloud consists only of gold atoms, a Au-Si alloy. or carbon contamination. It was also not clear why such matter appears only on the (001) surface and not on the (Ill) surface. !fthese were due to
contamination or residual gases, one should be able to eliminate them by using an ultra-high-vacuum electron microscope. 37
Figure 12. Method of viewing the surface of the gold UFP in Figure II. Observed from the [110) direction (the direction of the arrow).
Figure 13. Sequential electron micrographs showing the top right comer of the gold UFP in Figure 11. The time interval is shown at the bottom right. 38
Electron Microscopy Studies
References Frontiers of Physics 3, . Ohtsuki, Kyoritsu, Tokyo (1983). Iijima, S., Optik, 48, 193 (1977). Bethe, H. A.,Ann d. Phys., 87, 55 (1928). Cowley, J. M. and Moodie, A. F., Acta Cryst. 10,609 (1957). Cowley, J. M., Diffraction Physics, North-Holland (1975). Ishizuka, K., Nippon Kessho Gakkaishi (J Jpn Cryst. Soc.), 29, 209 (1987). 7. Uyeda, N., Kobayashi, T., Saito, E., Harada, Y. and Watanabe, M., Microscopie Electronique, (Favard, P., ed), vol. 4, p. 23 (1970). 8. Iijima, S., J Appl. Phys., 42, 5891 (1971). 9. Buseck, P. R. and Iijima, S., Amer. Mineral., 59, 1 (1974). 10. Skamulis, A. J., Iijima, S., and Cowley, J M, Acta Cryst. 32, 799 (1976). 11. O'Keefe, M.A., Buseck, P.R. and Iijima, S., Nature 274, 322 (1978). 12. Iijima, S., Surface Science 139, 1003 (1985). 13. Iijima, S. and Ichihashi, T., Jpn. J Appl. Phys., 24, L125 (1985). 14. Iijima, S. and Ichihashi, T., Phys. Rev. Lett., 56, 616 (1986). 15. Iijima, S., J Electron Microscopy, 34,249 (1985). 16. Iijima, S., Jpn. J Appl. Phys., 23, L349 (1984). 17. Iijima, S. and Ichihashi, T., in Proc. XI Int'l Congo on EM., Kyoto, p. 1439 (1986). 18. Marks, L. D. and Smith, D. J., Nature, 303,316 (1983).
1. 2. 3. 4. 5. 6.
39
Ultra-Fine Particles
1.5 Evaporation Methods (by Ryozi Uyeda)
Conventional fine powders, such as cement and flour, are manufactured by crushing, but this method cannot be used to make UFPs. The reason for this is an interesting problem in itself, but here evaporation methods that can be used to make UFPs will be discussed [1 J. In evaporation methods, a substance is heated and vaporized. The vapor is then cooled by some method and condensed into UFPs. A representative method is evaporation in an inert gas atmosphere. The smoke shown in Figure 1 is formed when metal vapor is cooled in a gas and the vapor coalesces. This author refers to this method as "gas evaporation" following the convention of the term "vacuum evaporation," but this process is often referred to as "evaporation in a gas." In Europe, the term "gas condensation" is also used to refer to processes in which a metal or compounds are vaporized and then condensed. The SiC UFPs were also formed by evaporating silicon in argon gas containing methane. This may be broadly included as one of the evaporation methods. Evaporation and Condensation in a Gas
When a solid or liquid is in thermal equilibrium with its vapor, its pressure is the saturated vapor pressure, Ps' The value of Ps increases with temperature. The pressure for evaporation methods is generally not at 1 atm, so the boiling temperature has no significance. Evaporation occurs when the vapor pressure, p, above the solid or liquid is lower than Ps' The key point of interest here is the rate of evaporation, m. According to the kinetic theory for gases, m (evaporated mass per unit area and unit time) is given at p = 0 as follows: m
=P
s
(21tR)-1/2 (M/T)l/2
40
(1)
Electron Microscopy Studies
Figure 1. Metal smoke. Evaporation in an inert gas atmosphere. Here, M is the molecular weight, T is the absolute temperature, and R is the gas constant. It is not known if this equation accurately fits experimental data, but it is helpful in estimating the approximate value. While no tlueshold temperature of vaporization exists, sufficient evaporation begins when P. reaches 1 torr. For most metals, this temperature, T» is 10 to 50% above the absolute melting temperature. However, T 1 for tin is three times the melting temperature and that for chromiwn is below the melting temperature
(see Table I in Ref. I). For evaporation in an inert gas, the evaporation rate is less
than 10% of the value from equation (I) beeause P does not equal 0 at the evaporating surface. When p. reaches pressures around 0.1 - 1 torr, however, the fonnation of smoke becomes visible. Naturally, the lower the gas pressure, the higher the rate of evaporation. In an attempt to increase the evaporation rate under constant gas pressure, 41
Ultra-Fine Particles a stream of gas was directed at the evaporation surface. This was a failure, however, due to cooling of the surface. Increasing the gas temperatures did not improve the situation. Vapor from evaporation is gradually cooled as it diffuses through a gas and reaches a supersaturated state. This leads to nucleation. As shown in Figure 2, a region where the vapor extends above the evaporating surface (vapor region) is formed. Outside this region, nucleation occurs and causes almost immediate condensation of most of the vapor (vapor growth). The particles formed here can be either solid or liquid. The state of these particles cannot be observed directly, but if they are spherical, the particles are presumed to be liquid at the vapor growth stage. Platelets and needles are probably solid upon condensation, but multi-faceted particles can be either solid or liquid [2]. These particles are carried upward by convection as smoke. The smoke appears white because the radiated light from the heated evaporating surface is scattered by the particles. Thus, the vapor region without particles appears darker. The smoke is cooled as it ascends, but vapor growth after the condensation of much of the vapor can be ignored. Yet, significant particle growth was observed, indicating the occurrence of coalescence by particle collisions (coalescence growth). This is expected when the particles are liquid. Even solid particles are known to fuse as demonstrated by electron microscopy studies. This depends on the temperature. In low temperature regions at the upper parts of the smoke, the particles attach to each other upon collision, but no fusion occurs. These findings imply that there are three steps involved in these processes: (1) evaporation and supersaturation, (2) vapor growth, and (3) fusion growth. Step (1) involves fluid mechanics including diffusion and thermal conduction, Step (2) resembles cloud and snow formation from supersaturated water vapor, and Step (3) involves Brownian motion within an aerosol until collision occurs, after which it involves crystal growth including the surface energy of fusion. Experimentally, the parameters controlling the particle diameter are the temperature of the evaporation source and the type (molecular weight) and pressure of the atmospheric gas. Roughly 42
10<'"
a ,, ,
, ,,
• Figure 2. Diagram of evaporation and condensation in an inert gas
atmosphere.
speaking, the higher the temperature and pressure. the larger the particle diameter. There is an example of varying the diameter of
aluminum from 1 to 10 ~m by controlling these parameters [1]. When the type of gas is changed, nearly the same result can be obtained by keeping the product of the molecular weight and pressure of the gas constant. In addition. the size and shape of an evaporation chamber as well as the size and shape of the furnace used affect the diffusion and convection of the particles, and consequently the particle diameter. Generally, the particle shapes are not constant throughout the smoke. It is not unusual that, depending on the conditions, one part yields small multi~faced particles while another part yields large
multi-edged plates [2]. 43
Ultra-Fine Particles Impurities also affect the diameter and morphology of UFPs. For example, 10-3 torr of oxygen in 10 torr of argon reduces particle size and produces polyhedra with rounded comers. Smoke particles are generally good absorbers of impurities so that effects due to impurities tend to decrease with increasing evaporation time if the equipment is well baked. However, in ordinary equipment, the amount of desorbed gases due to heat from the evaporation source exceed the impurity absorbing ability of the particles.
Laboratory Evaporation Equipment The large scale evaporation equipment used in our laboratory will be described next. This equipment is intended to make 10 to 100 g of high purity material, using mainly an electric arc as the heating source. For easy-to-evaporate materials such as silver, direct resistance heating was also used. The vacuum chamber (Figure 3a) is a stainless steel cylinder 1 m in diameter and 1 m high. It was made larger to reduce gas emission from the inner wall due to temperature increases during evaporation. For seals where temperature increases are large, copper gaskets were used. The pumping system consists of a rotary pump (500 lis), a mechanical booster pump (600 m 3/h), and a turbo molecular pump (500 lis). The system is capable of obtaining a chamber pressure of 3 x 10-6 torr after 1 hour of pumping. The upper and lower arcing electrodes were supported by water-cooled copper components (Figure 3b). The sample to be evaporated was placed on the lower electrode (the sample holder) and a rod electrode was attached to the upper support. The sample holder is a cylinder (50-mm diameter, 50-mm height) that can be moved from the outside along the three (x, y, and z) axes. The rod electrode can be moved linearly along its axis. The movement mechanism used rotary UHV seals. The power source used for the arc was a power source for an argon arc welder (80 V no load, 35 V full load). The UFPs of iron, Si, y-A120 3, SiC, etc. have been made by using this equipment. Table 1 summarizes the key data for these UFPs. Argon gas was used and mixed in oxygen for making oxides 44
Vocuum chamber
Filter assembly
Carbon
Ie
Cu elec t rode
Electric power
s,~u~p~ptly;1~~~~~/
Cu electrode
b
Figure 3. Large-scale evaporation apparatus used in the project. Outside view (a) and diagram of the inside (b).
45
Ultra-Fine Particles and methane for carbides. For iron UFPs, hydrogen gas was mixed into the argon to prevent oxidation. After trial and error, the optimum electrode material and geometry were determined, as shown in Table 1. Silicon was preheated to increase its electrical conductivity, after which it was arced. A surface oxide was made on the surface of aluminum and the sample was arced through the oxide layer. A silicon sheet was placed between a SiC sample and the sample holder. When an arc was struck, the silicon melted and the liquide silicon climbed up to the top of the SiC block, evaporating the SiC. During arc heating, molten droplets of the sample also move along with the vapor. Larger droplets drop immediately, but those smaller than 1 ~m in diameter mix with the UFPs in the smoke and cannot be separated. The researchers tried to increase the evaporation efficiency while attempting to prevent the mixing of UFPs and molten droplets. This resulted in the process shown in Table 1. To use arc discharge, it is important to use one's intuition based on observation of the physical phenomena. Table 1. Key Data for the Large-Scale Evaporation Equipment. Materials
Gas Pressure (torr)
Gas (torr)
Upper Electrode mm 3
Lower Electrode mm 3
Current A
Yield
Si
1-500
--
Si (l5x30xI6)
Si
dc 30-70
(<\> 60x10)
80 mglmin
ac 20-25
100 mglmin
SiO z SiC
100
395
02 (100)
Si
Si
(<\> 60x36)
(<\> 60x30)
CH4 (5)
C
C (D33x25)
dc 70150
10g/h
(<\> 13xIOO)
Al z0 3
660
02 (100)
AI (D30xI50)
AI (D25x25)
ac 50-70
10gih
TiO z
390
(02)
Ti
Ti
dc 5-10
--
(10)
(<\> 5x90)
(<\> 5x90)
46
Electron Microscopy Studies
Various means were used to capture the smoke particles. Ultimately, the capturing device shown in Figure 3b was found to be suitable. This is a stainless steel cylinder (inner diameter 250 mm, length 400 mm) with a 25 stainless steel mesh having an opening density of 50%. A Sirroco fan was used to pull the smoke through the device, where the mesh captured the particles that entered. For SiC, the device became clogged after one hour. The capture efficiency for particles during this period was about 50%. If two such devices were built into the chamber and used alternately, continuous operation would be possible and the capture efficiency would be improved. UFPs made with the equipment described above were not always uniform. Electron micrographs ofthe various UFPs represent typical UFPs that were produced. Heat Source and Coolant
The evaporation method requires a heat source and coolant. When an inert gas is used as coolant, resistance heating was used initially, but it is limited to easily evaporated materials and is not amenable to continuous operation. Various heat sources have been developed since that time, the first of which was the plasma flame method [3]. This started the development in UFP technology, but it is not used today because of a number of difficulties. The next method used was induction heating developed by Vacuum Metallurgy Ltd. before this project was started. This method was a technical success. An even less expensive process was required, however, so the arc method was developed by Uda [3], the hybrid plasma method was developed by Yoshida, the chemical method was developed by Yoshizawa, and other methods have been developed. The first two methods involve evaporation in a gas. Uda's arc method is based on a unique theory for which the scientific validity is unclear; however, it is a practical technological method. Yoshida's method will be described later in this book. These methods, including the induction
47
Ultra-Fine Particles
method, possess certain features for which the applicability to various final products should soon become clear. Electron and laser beams were also examined as heat sources. Because electrons are scattered by gases, they can only be used, if ever, under vacuum conditions. Laser beams are appropriate for materials that are especially difficult to evaporate. While it is a little different from the above methods, microclusters produced by pulsed lasers are also under investigation [4]. The coolant that was used is not limited to a gas. Island particles, as discussed in Section 1.1, were made at an early stage in the development of vacuum evaporation by using solid surfaces as a coolant. The surface of a low vapor pressure liquid can similarly be used. In either case, UFPs are formed in a random manner over the surface (less than one layer). Here, appropriate collection methods must be devised. The first of these is vacuum evaporation on a running oil surface (VEROS), where particles are continuously collected on a fresh oil surface and the oil is later distilled. The next method is alternate evaporation of a metal and a solvent (e.g., acetone) [5]. Here, the solvent is evaporated onto a cooled substrate, and then the metal is deposited over the fresh surface. After repeating these steps several times, the solvent is dissolved and distilled. In these methods, evaporation is done in vacuum and the evaporation efficiency is higher than in a gas. Here, electron beam heating can also be used. These methods are suited for the formation of UFPs of platinum and other materials, producing particles with diameters of less than 10 nm. These methods, however, still need much improvement. It is not clear whether these are better than chemical methods. Takagi [6] has developed a method to make cluster beams by cooling metal vapors by means of adiabatic expansion without using a coolant. This method efficiently forms metallic films having good crystallinity. Although the nature ofthe films is different, the Takagi method is, in a broad sense, one of the coating methods and parallels that of the gas deposition method used in this project.
48
Electron Microscopy Studies
References 1. Uyeda, R., Powder Technology, edited by the Chemical Engineering Society, Maki Publ., Tokyo, p. 24-35 (1985). 2. Uyeda, R., Parity, 2, 4-24 (1987). 3. Gashu, S., Applications ofUFPs, edited by the Japan Society of Powder Engineering, Nikkan Kogyo, Tokyo, p. 11-40 (1986). 4. Yamauchi, K., Nihon Butsuri Gakkai Shi (J. Phys. Soc. Japan), 11, 912-915 (1986). 5. Hayashi, T., unpublished. 6. Takagi, T., Oyo-Butsuri (Applied Physics), 55, 746-763 (1986).
49
Ultra-Fine Particles 1.6 Oxides (by Sumio Iijima)
Synthesis of Oxide Ultra-Fine Particles In nature, UFPs are produced via gas evaporation on a global scale, although this fact is not widely known. When a volcano erupts, black smoke is evolved and consists partially of spherical glassy UFPs. When the molten lava is blown out from the volcano, there are no chemical reactions and a process equivalent to the gas evaporation method for forming metallic UFPs occurs. The gas evaporation methods are classified depending on the heating method used, such as resistance heating and others. There are several methods to synthesize oxide UFPs. One method involves oxide melting followed by evaporation. A second method produces UFPs by heating and evaporating metals and semi-metals in an oxygen atmosphere causing oxidation. This generates heat that can be used for heating. In the past, photographers burned magnesium powders to produce a flash, which instantly formed magnesia UFPs. Because metals and semimetals originally exist as oxides and are obtained by reduction, oxidizing metals to remake oxides does not appear to be the most efficient or economical method for making the oxide UFPs. When a piece of metal is heated in a mixture of inert gas containing oxygen and evaporated, metallic oxide UFPs are obtained. To heat the metal, resistance or electric arc heating can be used. The former has problems due to reaction between the molten metal and the heating element and oxidation of the heating element itself. In the latter, the electrode itself is evaporated, so it does not have the drawbacks of the resistance heating method. In comparison to wet methods for making UFPs, the gas evaporation method allows for the synthesis of high purity samples because the process is simple and contamination by impurities is minimal. The purity of UFPs depends on that of the metal and the gas atmosphere. For alumina UFPs, 99.9999 % purity was achieved.
50
The general features ofoxide UFPs made by the arc discharge
method are that the UFPs are nearly spherical in shape and that they are metastable crystal structures, which is probably due to the growth of the UFPs in non-equilibrium conditions. This method has yielded
oxide UFPs of iron, aluminum, titanium, silicon~ etc. UFPs of y-alumina, which are discussed later, were obtained under the following conditions: total gas pressure of 100·400 torr and gas ratio (O,lA.) of 1110 - 1/4. The particles produced were spherical, with diameters from 5 to 70 run and BET specific surface areas 000 - 100 m'/g (Figure 1).
Figure 1. Electron micrographs of y-alumina UFPs. 51
Ultra-Fine Particles
The Crystal Structure of y-Alumina Ultra-Fine Particles Powders of y-alumina are usually synthesized by the decomposition of aluminum hydroxide by heating and dehydration, which results mainly in a spinel-like structure. It is known, however, that some variations in crystal structure occur depending on the raw material used. Alumina from the dehydration of hydroxides is porous and lacks the crystallinity that would normally generate x-ray diffraction patterns typical of single crystals. Consequently, the reported crystallographic data has much uncertainty. In contrast, the yalumina produced by the gas evaporation method consists ofUFPs with diameters less than several tens of nanometers and with good crystallinity. The crystal structure of y-alumina was determined using these UFPs [1,2]. For the measurements, a single UFP of y-alumina was selected and its orientation was set to the [110] direction with an accuracy of 10-4 radian while observing its focused electron diffraction pattern. The electron micrograph of this particle (25-nm diameter) was then photographed (see Figure 2a). The white area on the photograph is the double-exposed image of the electron beam probe. Its position and size (5-nm diameter) can be seen. The micro area electron diffraction pattern from this part is shown in Figure 2b. The HREM image of the particle was simultaneously obtained (Figure 2c). Next, using diffraction conditions that produce no multiple reflections, the particle was tilted and the extinction characteristics of the diffraction patterns were examined. From this, the crystal structure was determined. It was found that three crystal structures (8 1, 8 2 , and 0' ) are present in a mixed form in the y-alumina particles (see Table 1). Each structure is modeled on the spinel structure, in which cation vacancies and tetrahedrally coordinated cations are present in orthogonal arrays of oxygen atoms. The 8 2, and o'phases are nearly identical to the already reported 8 and 0 phases. This investigation led to the first experimental determination of the crystallographic space groups. 52
'"
w
r f
Figure 2. Electron micrographs of y-aiumina UFPs. a) Electron micrographs of a a '-alumina UFP (25-nm diameter) fonned by the gas evaporation method. The electron beam is along the [001] direction. The white
,§
portion is due to double exposure and indicates the size of the microbeam probe (5-run diameter). b) The micro
'"~
area electron diffraction pattern from the white part in a. c) HREM image of a part of the particle in a. A twin
... ~.
appears in the center.
Ultra-Fine Particles
Table 1. Crystal Structure Data for the Three Types of Alumina Produced by Gas Evaporation Using the Arc-Discharge Method.
Type
0' El] El 2
System
Orthorhombic Monoclinic Monoclinic
a (A)
Lattice Parameters b(A) c(A)
PC)
16.4 II.1 12.1
11.8 12.1 5.6
103 103
Space Group
P2 12 12 B2tb B2tb
8.2 17.7 2.9
One interesting aspect of the UFP studies is to examine the anomalies in the crystal structures due to the particle diameter effect. As will be noted later, multiple twinned particles are observed in metallic UFPs as the diameter decreases. Recent studies indicate that anomalous crystal structures in UFPs arise due to quenching during crystallization because UFPs have large specific surface areas, making them easily affected by cooling. Each phase of y-alumina found in UFPs from the gas evaporation method is believed to be metastable and transforms to (Xalumina by heating to over 1200°C. Both the 8, and 82 crystal structures are monoclinic, but they share an axis that corresponds to the [110] direction in spinel. The crystallographic relation between the 8, and 82 crystal structures was also obtained. In the electron diffraction pattern shown in Figure 3c, some of the 8 2 diffraction spots indicate random scattering [3]. The electron micrographs from these spots show wavy lattice images not found in normal lattice images (see Figure 3d). This is believed to show an intermediate phase between the 8 I and 8 2 phases. Because the oxygen atoms in a spinel have an fcc structure, aluminum atoms may form a short-range ordered structure. Among the oxides similar to y-alumina is y-Fe203, which is a well known material used as a magnetic recording media. This material exists only in a single crystal form, but the actual crystal
54
~
~
l!l
i
a~~
~
Figure 3. Selected area electron diffraction patterns. a) aI-phase; b) 6 z-phase. c) Diffuse scattering diffraction pattern thought to be an intermediate phase between the I and 6 2 phases.
e
"\':> ~
~
Figure 3 continued, d) Electron micrographs of UFPs corresponding to C (the magnified image is of the top left part of the lower magnification image shown in the insert). There are horizontal wavy stripes visible in the micrograph.
56
Electron Microscopy Studies form is not known. The HREM studies ofthe structure of y-Fe 20 3 are therefore desirable. References 1. Iijima, S., Jpn. J. Appl. Phys., 23, L347 (1984). 2. Iijima, S., Surface Science, 139, 1003 (1985). 3. Iijima, S., unpublished work.
57
Ultra-Fine Particles 1.7 Search for Industrial Applications of Spherical y-Alumina Ultra-Fine Particles (by Tsukasa Hirayama)
The feasibility of using spherical y-alumina UFPs [1] for industrial applications, especially for catalysis, was examined. Here, all the aluminas with a spinel structure will be referred to as yalumina. Commercially available y-alumina powders are produced by dehydration of aluminum hydroxide and have large specific surface areas. This was the main reason for their use as a catalyst carrier. However, the specific surface area was reduced drastically at elevated temperatures [2]. This is undesirable for some applications [3]. Several commercial y-aluminas were studied for their thermal stability and one was selected that had a high surface area after thermal treatment. Using this as a control, its properties were compared to those of the spherical UFPs that were produced during this study.
Specific Surface Area Decreases at High Temperatures and Transition to the a-Phase. Figure 1 compares electron micrographs of our spherical UFPs and the control (representative commercial y-alumina). The differences are clear. After heating these samples in an argon atmosphere, the specific surface areas were measured, the results are given in Figure 2. The heating time was always set at one hour and the maximum heating temperature was 1360° C. As seen in the micrographs shown in Figure 1, the control sample had a much larger surface area initially. At high temperatures, however, the surface area of the control drops rapidly and becomes less than that of spherical UFPs at temperatures above 1260°C. The spherical UFPs also show a similar decrease, but there is a shift of about 100 ° C. Because this difference can be significant in practical applications, this was studied in more detail.
58
Figure I. Electron micrographs of y-alumina UFPs. a) Spherical particles made in the present project; b) Commercial particles.
-'"
•E
u
150
•
Control sample
100
•
50
Spherical UFPs \
\,---.:, \
, Raw
•
1000 1200 1400 Heating temperature (OC)
Figure 2. Specific surface areas of samples after heating for 1 hour at each temperature.
59
Ultra-Fine Particles Based on x-ray diffraction analysis, the control changed completely to the a-phase when heated to 1260°C, while the spherical UFPs barely showed any peaks due to the a-phase. These materials were examined by electron microscopy. The two photographs in Figure 3 show the spherical particles after heating to 1260°C for 1 hr. Figure 3a shows typical images found in most viewing areas. These appear to differ little from untreated particles. Upon closer examination, however, two particles can be seen to be fused together (as indicated by the arrows) and these all consist ofthe y-phase. In the areas observed, particles such as those shown in Figure 3b were found. Here, the particles exhibited fusion growth and are of the a-phase as determined by the electron diffraction data. In the control sample heat treated to 1260°C, the entire area observed had undergone fusion and grown to form the a-phase as shown in Figure 3b. These results imply that the transformation from the yphase to a-phase proceeds by nucleation and growth. It is not clear whether the nuclei are impurities or broadly defined lattice defects, but few nuclei exist in the spherical particles.
Synthesis of Spherical Alumina Particles Spherical alumina particles were synthesized initially by the arc method (Sec. 1.5), which used an electrical arc discharge between two aluminum electrodes in an argon atmosphere containing oxygen. This method can be used to make a small amount of material for electron microscopy, but it requires a much more extensive effort to make the several grams of material that are needed for the specific surface area measurements. After a number of modifications and considerable development time, a new method for making kilogram quantities of samples appeared feasible [5]. Figure 4 shows a schematic of the new device developed. It consisted ofa powder container, a glass blowing torch, quartz tubing, and a collector. Aluminum powder (several microns in diameter) was placed in the container. The powder was blown out by combustible
60
Electron Microscopy Studies
a I
O.l/olrn
I
Figure 3. Electron micrographs ofspherical particles after heating at 1260°C for 1 hr. a: Typical images seen in most viewing areas, which differed little from untreated material. b: A few particles exhibiting fusion growth (transformed to a-phase).
61
~
~I
~ ::: ~
,Stainless steel mesh
~
d..
tt
~
~
Quartz tube
I;oj
02g as
-
~Fuel
gas
0\
tv
•
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.
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Aluminum powder
_,
';', '. '::' '4/,,'!;
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Figure 4, A diagram of the apparatus used to synthesize spherical alumina particles according to the burner method.
Electron Microscopy Studies gas through a nozzle inserted into the powder and sent to the torch along with the gas where the mixture was mixed with oxygen and burned. A flame propagated along the wall of the quartz tubing and forms a white smoke at the tip. The smoke was pulled to the collector by exhausting the system with a fan. There were many stainless steel meshes in the collection chamber that served as collectors for the particles as the smoke passed though the chamber. Electron microscopy of these particles indicated that they were spherical yalumina, similar to that produced by the arc method. The yield increased with increasing amounts of aluminum powder going into the system, but beyond a certain limit unburned aluminum powder contaminated the alumina particles. Some special techniques were needed to optimize the amount of aluminum powders added. The tubing was vibrated so that the aluminum powder would not accumulate within the tubing leading to the torch. This new method is less expensive and easier to use than the arc method thus making it a promising method for producing various oxide UFPs on an industrial level. The high temperature properties of the synthesized particles were similar to those of UFPs formed by the arc method. This method is applicable to any metal that can be oxidized, such as zirconium. Zirconia UFPs that were made using this method had particle diameters less than one-half of those of the alumina UFPs. The zirconia UFPs tended to have crystal habits and were tetragonal at room temperature, although no additives such as Y203 were used.
References 1. Iijima, S., Jpn. J Appl. Phys., 23, L347 (1984). 2. Her, R. K., JAm. Ceram. Soc. ,44[12], 618 (1961). 3. Wanke, S. E. and Flynn P. C., Catal. Rev. Sci. Eng., 12,93 (1975). 4. Hirayama, T., JAm. Ceram. Soc., 70[6], c122 (1987). 5. Hirayama, T., J Ceram. Soc. Jpn., 95[2], 253 (1987).
63
Ultra-Fine Particles 1.8 Metal Catalysts (by Sumio Iijima)
In studies of heterogeneous catalysis, it is important to understand the interaction at the interfaces between the oxide carrier and the fine metal particles. The catalyst activity depends on the shape of metal particles, the interaction between the metal and the carrier, the sintering of the metal particles, and the chemical activity of the surface of the oxide carrier. Consequently, the dependence of the catalyst activity and selectivity on the morphology and particle diameter of fine particles was studied extensively. In metal catalysts formed by impregnation and coprecipitation methods, it is difficult to control the size and shape of the metal clusters and ambiguity remains in the evaluation of the catalytic reactions. A newly developed method for making catalysts that uses metal complexes to place catalysts on the surface of inorganic metallic oxides such as A1 2 0 3 , Ti02 , and Si0 2 is a promising and potentially superior method for forming ultra-fine particles of metals or alloys with uniform size and shape. Infrared (lR) absorption spectroscopy, x-ray photo emission spectroscopy (XPS), and EXAFS are used to characterize the composition and surface condition of UFPs on oxide surfaces. A number of studies using these methods have been done. However, the oxide carriers themselves, especially the physical characteristics of the oxides, have not been studied much. For example, y -alumina particles, which are the most common carrier used for catalysts, are known to have many irregular depressions and holes on their surface. However, these have been evaluated only through the measurement ofthe specific surface area. For UFPs, this only provides information on the average structure. In the laboratory, the dependence of the catalyst activity on the crystal surface has been studied in terms of Miller indices. The microstructure of oxide surfaces has been related to catalyst activity, but these results cannot yet be applied to practical investigations of catalysts.
64
Electron Microscopy Studies In this section, high-resolution electron microscopy observations of a new type of alumina UFP with a well known chemical and crystallographic structure is reported. The work on using these UFPs as supports for metal clusters is also described.
Alumina Carriers The alumina UFPs described here were made by using the gas evaporation method (see Sec. 1.5). The UFPs with diameters of 5 80 nm were made by arc discharge of aluminum electrodes in an inert gas atmosphere containing a small amount of oxygen. The purity of the gases used were 99.99% and it was found that adjustment of the gas pressure permitted control ofthe particle diameter to some extent. When very high purity aluminum electrodes were used, very high purity UFPs (99.9999%) were obtained. The gas evaporation chamber had an inner volume of 1 m 3 and was able to supply enough UFPs for laboratory scale catalyst experiments. The synthesis of yalumina UFPs was also discussed in Sec. 1.6. The UFPs have a specific surface area of 90 m 2/g as determined by the BET method. This value is similar to the value for the y-alumina generally used for catalyst carriers [1].
Metal Complex Clusters The metal carbonyl clusters, ~(CO)16 , discussed here were made by Ichikawa [2] based on the synthesis method of Chini and Martinengo. These clusters were recrystallized in dichloromethane to purify them. When alumina UFPs were mixed into dichloromethane solutions containing Rh6(CO)16 the blue solution immediately becomes transparent, indicating the adsorption of Rh6(CO)16 particles to the surface of the alumina particles. The excess Rh6(CO)16 was removed by washing with a fresh solution of dichloromethane. For electron microscopy observation, a liquid suspension of alumina UFPs carrying ~(CO)16 was dispersed by ultrasonic agitation and scooped from the suspension by using a copper mesh
65
Ultra-Fine Particles covered with a porous carbon film. This mesh was placed in a vacuum (10-5 torr) for 15 minutes at 50-100 ac to remove the solution and to avoid contamination of the electron microscope. Subsequently, the sample was treated in vacuum (10-7 torr) at 150 a C for 1 hour, which removed carbonyl radicals and fixed the rhodium clusters on the alumina particle surfaces. The end ofthis reaction can be determined based on IR absorption measurements, which also indicated that the y-alumina UFPs produced in this study had a lower amount of hydroxyl groups than the y-alumina gels made by dehydration. Observation of Metal Clusters The crystallographic direction of UFPs was oriented while monitoring the focused electron diffraction pattern. In HREM, the orientation must be accurately matched to the beam axis. These delicate adjustments were made by using a side-entry sample tilting holder. The electron microscope used was an Akashi EM-002A microscope with an accelerating voltage of 120 kV. Figure 1 shows an electron micrograph of spherical y-alumina UFPs carrying the metal carbonyl molecule Rh6(CO)16. Small spots about I-nm in diameter can be seen covering the entire surface of the particles. Judging from the size, these are believed to be isolated Rh6 clusters. These clusters can be observed better on alumina UFPs that have a lower contrast (see the particle on the left). As can be seen from the photograph, the surfaces of the alumina particles are very smooth, which is suitable for the observation of very small clusters. If the surface is rough, phase contrasts appear, causing disturbance of the cluster images. Conventional alumina carriers are formed by dehydration of hydroxides and have very rough surfaces. Such structures make the detection of clusters difficult. The dark particle in Figure 1 (indicated by the arrow) has its [110] axis matched to the beam. This causes strong Bragg reflections within the particle, making it difficult to observe the images of the clusters. According to the electron diffraction pattern of this particle, the crystal structure is 8 1 (monoclinic: B2/b, a = 1.11 nm, b = 1.21
66
Figure I. Electron micrograph of y·alumina UFPs with adsorbed Rh,(CO)16 clusters. nm, c - 1. 77 nm, and p = 103 0 ). This particle appears to be spherical, but has some crystallographic planes on the surface after the orientation was matched. This indicates the existence of crystal habits within the particle. The plane that is easy to fonn is the (Ill} plane, over which the oxygen atoms are in a close packed hexagonal arrangement. The (Ill} planes are terminated by oxygen, which cannot maintain charge neutrality. Thus. some additional atomic surface structures are expected to occur. In fact, an electron micrograph indicating such an effect has been obtained [3].
67
Ultra-Fine Particles High Resolution Electron Microscopy Observation of Metal Clusters The particle with its {Ill} plane visible in Figure 1 (indicated by the arrow) was further magnified as shown in Figure 2. Figure 3 shows this part schematically and in a perspective view. The broad, black arrow indicates the direction of observation in the [111] direction. In the photograph, two types of clusters, A and B, with different diameters, can be seen. The former has a diameter of several tenths of a nanometer and shows low contrast. This size is close to that of a six atom cluster from a ~(CO)16 carbonyl cluster. The latter has a larger diameter of 0.8 - 1 nm and can be considered to be a fused cluster consisting of one to three Rh6 clusters. All the clusters were found to be hemispherical and the rhodium clusters on the {Ill} planes do not have the so-called raft structure. Figure 2 shows that the surface steps of a y-alumina UFP consists of {Ill} and {I OO} planes, but the {Ill} planes grow to larger sizes. Next, the location of cluster nucleation on the particle surface was examined. It is known that deposited metals tend to coagulate selectively on the steps of substrate crystals when thin films of metals are formed by vapor deposition in vacuum. As far as can be determined by Figure 2, the clusters were not formed preferentially on the steps of the alumina particle, although there is a Rh6 cluster formed at one of the steps (See Figure 2, location c). This may be due to the tendency of ~(CO)16' with carbonyl radicals to uniformly adsorb on the alumina surface prior to the heat treatment. After prolonged heat treatment, the clusters tend to coalesce and gather at steps.
Interpretation of Electron Microscope Images The black lines on the {Ill} plane are formed by rows of dark spots separated by a distance of 0.24 nm. This corresponds to the distance of neighboring aluminum atoms when the spinel structure is projected along the [110] direction. The black spots, however, do not necessarily indicate the position of aluminum atoms.
68
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Figure 2. Magnified image of the particle indicated by the arrow in Figure l. The orientation of this alumina particle is in the [110] direction, where the horizontal lines are edge-on views of the {Ill} planes. The small objects on the top planes afe rhodium clusters, the smallest of which has a diameter of several tenths of a nanometer. This size corresponds to that of a six atom cluster of adsorbed ~(CO)16 carbonyl cluster.
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Ultra-Fine Particles
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Figure 3. A perspective view of the surface ofthe particle shown in Figure 2. A cluster on the uppermost {Ill} plane is also shown. The arrow indicates the direction of observation for the image shown in Figure 2.
The height of the top-most terrace is 0.83 nm, which is smaller than the height expected for the spinel structure of 0.92 nm. The abnormally dark contrast of the {Ill} surface layer is probably due to the ghost image that arises during the process of electron microscope imaging. These electron micrographs were obtained with great care at the optimal focal distance. The images depend strongly, however, on the tilting of the crystal, the electron beam angle, imperfections in the lenses, etc., so that perfect adjustment of all the parameters is very difficult. To discuss the images of HREM in detail, we would require computer simulations based on theories of electron diffraction and lens imaging, as discussed in Section 1.4. 70
Electron Microscopy Studies
The rhodium clusters shown here were found to fuse under electron irradiation and to grow to large sizes during the course of electron microscopy observation. Recrystallization of the alumina particles during electron irradiation was also observed. Electron Microscopy Observation of Other Metal Catalysts
The electron microscopy observations of metal clusters in the previous section using the example of Rh6 clusters were discussed earlier. Other metal carbonyl clusters were also examined. When the number of metal atoms is reduced further (e.g., OS3 clusters) their observation becomes more difficult. Another example of observation is shown in Figure 4, which shows the image of a platinum cluster on a silicon UFP covered by Si02 and adsorbed using the cluster Pt 1sCCO)30' The lower part is that ofthe silicon UFP, where the lattice image shows the {Ill} planes with the characteristic spacing of 0.31 nm. In reference to this lattice image, the elongated black body marked by the arrow is 0.3 x 0.7 urn. Judging from the size of this body, this is probably a single Pt 15 cluster. Larger clusters than this one probably result from the fusion of several Pt 15 clusters. In this section, emphasis was placed on the finding that alumina UFPs based on a new type of spinel structure are useful for the study of metal catalysts. Using an oxide UFP with good crystallinity and well-known morphology, one can directly photograph metal clusters on the surface ofthe particles. Such HREM observations are expected to provide a new characterization method that will provide new insights into the shape, size, and location ·of metal clusters on oxide support surfaces. Finally, the team members acknowledge the guidance provided by Professor M. Ichikawa ofthe Catalysis Research Institute at Hokkaido University and thank Dr. M. Haruta of the Osaka Government Industrial Research Institute for the electron microscope observations of the metal catalysts.
71
;::; ~ ~ ~
~ ~.
if
i::J
Figure 4. An electron micrograph ofPt 1S(CO)30 clusters on a silicon UFP. The lattice image shows the {Ill} planes for which the spacing is 0.31 run. The fuzzy images on the surface are amorphous SiOz. The arrow indicates a Pt lS cluster.
Electron Microscopy Studies References 1. Iijima, S., Jpn. J Appl. Phys., 23, L347 (1984). 2. Iijima, S. and Ichikawa, M., J Catalysis, 94, 313 (1985). 3. Iijima, S., Surface Science, 139, 1003 (1985).
73
Ultra-Fine Particles
1.9 Crystal Growth of Silicon Ultra-Fine Particles (by Sumio Iijima)
The synthesis ofUFPs via the gas evaporation method is often used in the laboratory [1]. Various metal UFPs have been produced and the microcrystal structures and crystal growth processes have been investigated [2]. The growth process for microcrystals with well-developed structures can be described using the Curie-Wulff theory of crystalline equilibrium. The UFPs of metals that melt at relatively low temperatures (e.g., lead and magnesium) are known to grow into spherical shapes without a well-defined crystal habit. Along with these metals, UFPs of silicon and oxides such as yA1 2 0 3 , Ti0 2 , Si0 2 , etc., synthesized by gas evaporation using an electric arc as the heat source, have been found to have spherical shapes [3]. These particles grow in free space, emulating crystal growth experiments done at zero gravity. This section discusses planar defects within silicon fine particles froni the gas evaporation method and describes a possible growth mechanism for spherical particles that occurs via a process that goes from the gas to the liquid and finally to the solid phase [4,5]. Synthesis of Spherical Fine Particles of Silicon
Silicon fine particles were synthesized using the gas evaporation method, in which pieces of silicon were used as arcing electrodes with argon gas at a pressure of 300 torr. A direct current arc was used for this melt evaporation process. The temperature of the electrodes reached 2500°C, which exceeds that obtainable with resistance heating. Fine particles are produced as smoke and are absorbed on the wall ofthe evaporator as a yellow brown powder. To synthesize a mixture of silicon and silicon carbide fine particles, a small piece of silicon carbide was placed on the negative silicon electrode and an arc was generated between the silicon and the silicon carbide.
74
Figure 1 shows an electron micrograph of spherical silicon fine particles having diameters of 20 - 200 JUn. The dark lines or bands within most of the particles are planar defects. A question arises as to why the particles are formed with planar defects. Figure
2 shows electron micrographs of the typical planar defects seen. Each of the particles was accurately oriented along the [110] direction. Lines crossing the particles and those tenninating near the center are
planar defects that run parallel to the [110] direction. These are mostly twins or stacking faults on the (111) surface. There is a tendency for several planar defects to intersect near the center. The geometrical arrangements of the planar defects within a particle can be classified into about six types. As an example, pentagonal decahedron particles with multiple twins will be described. Figure 3 shows an electron micrograph of such a particle, which is spherical, not polyhedral. The crystal structure of particles with 5-fold symmetry axis has been reported for UFPs of fcc metals [6] and for the diamond form of germanium and carbon [7]. This structure is formed by joining five ideal tetrahedra with a common < 110> ridge and {Ill} twin planes, leaving a lattice mismatch of 7.5 0 (Figure 4). The mismatch is relaxed by introducing
100 nm
Figure 1. Electron micrograph of silicon UFPs synthesized by the gas evaporation method.
75
Figure 2. Electron micrographs of spherical particles with twins and stacking faults. Every planar fault grows parallel to the [11 OJ surface, so translation symmetry OCCillS. The bar represents 10 run.
FigUte 3. Electron micrograph of a pentagonal decahedron particle with multiple twins. The direction of the observation is along the 5fold symmetry axis. A small angle grain boundary can be seen for the tetrahedron indicated by the arrow.
76
Electron Microscopy Studies
Figure 4. A model of an ideal pentagonal decahedron with multiple twins consisting of five tetrahedra having the diamond structure.
a small degree of tilt in one of the tetrahedra at a grain boundary (shown with an arrow in Figure 4). The tilt boundary consists of a stack of edge dislocations, which are absent from the central region where twin boundaries join, which leaves the center elastically strained. As stated above, the planar defect arrangements have translational symmetry. This is an important point in considering mechanisms for the crystal growth of particles. Silicon UFPs prepared by the gas evaporation method are cooled at about 104 DC/sec. The particles are believed to crystallize without reaching thermal equilibrium. This assumption is verified by heat treatment in which a sample is sealed in an evacuated quartz tube and heated. When this is done, the particles change from a spherical shape to a polyhedral shape. Here, atomic rearrangements occur that lower the surface energy ofthe particles and approach the equilibrium Curie-Wulff forms. This observation indicates that the spherical particles are formed in a non-equilibrium state.
77
Ultra-Fine Particles Growth Mechanisms of Spherical Silicon Ultra-Fine Particles
Figure 5 shows electron micrographs of Si-SiC composite particles. An electron energy loss spectrum of the dark section with a developed crystal habit shows an absorption peak corresponding to the K-electron excitation of a carbon atom, which indicates that this section is silicon carbide. On the other hand, the round part on the (111) surface ofthe silicon carbide shows only an absorption peak for silicon. Thus, this part is shown to be silicon. The mechanism for the synthesis of Si-SiC composite particles is schematically illustrated in Figure 6. Here, a small portion of silicon carbide is represented by a small square. Zone A reached the highest temperature, because it was in contact with the molten silicon, and the vapors of the silicon and silicon carbide coexist. Because the vapor pressure of silicon carbide was lower than that of silicon, silicon carbide vapor rose by convection and cooled, initiating the condensation of silicon carbide particles in Zone B. The growth of silicon carbide particles occurred via crystal growth due to homogeneous nucleation within the supercooled vapor. The higher vapor pressure of the silicon allows silicon vapor to be present over a larger range of Zones A, B, and C. Because the temperature in Zone C was lower than in A and B, silicon particles began to condense in this zone, where silicon carbide fine particles were already present. Consequently, supercooled silicon vapor began to grow on the silicon carbide particles floating in the argon atmosphere. The rounded part of a composite particle suggested that it solidified from a melt. Silicon that condensed as a liquid over a silicon carbide particle was rapidly cooled upon entering Zone D, keeping the spherical shape intact. Silicon that grows over Si-SiC composite particles can be explained by crystal growth from heterogeneous nucleation. However, it is believed that silicon particles nucleate homogeneously in Zone C (Figure 6) and grow. Consider the growth of pentagonal decahedron with multiple twins, which is a representative planar defect with the translational symmetry that is produced within silicon particles. Because the 5fold symmetry axis is [110], the growth direction must be parallel or 78
Figure 5. Electron micrographs ofSi-SiC composite particles. The dark part is P-SiC and the round part is silicon. The shape of the silicon part is like a droplet, indicating that the silicon part was crystallized from the liquid state. The bar represents 10 run. ~
~
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"
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L -Si V-Si V-SiC S-SiC S-Si
Figure 6. Schematic diagram of the crystal growth of a composite particle near the evaporation source. The temperature decreases in the orderA -B-C -D.
79
Ultra-Fine Particles normal to the [110] axis. Assuming a situation in which we have crystallization of a liquid droplet, the crystal growth site can be on the [110] axis or at the ends of the axis. Assuming that surface nucleation occurs due to cooling from the surface, it appears that the decahedron grows from one end of a [110] axis. Figure 7 shows a diagram of this crystallization process. A liquid droplet forms first, on which a decahedral nucleus is formed. The nucleus grows and the entire particle solidifies.
Figure 7. Crystal growth process for a pentagonal decahedron with multiple twins. The above mechanism can be applied to the growth of spherical particles having planar defects with translational symmetry. Nuclei with triple, quadruple, or pentaple intersections between grain boundaries in a particle are formed on the surface of a liquid droplet at the beginning stage of crystallization. Thereafter, growth takes place inward maintaining the translational symmetry of planar defects. A diagram of the growth of a silicon particle from the liquid phase is shown in Figure 8. Crystallization begins at the surface of a liquid droplet and spreads rapidly over the surface. As the temperature of the particle drops, the particle diameter increases and a solid cap (radius of rs) is formed. As mentioned previously, the nucleus of a planar defect is already present within this cap. As the crystallization progresses toward the final phase, the crystal shell is 80
Electron Microscopy Studies
Liquid
Solid
Figure 8. Growth model for a spherical silicon particle. nearly complete, but one part remains in the form of a liquid. This shell is slightly larger than the liquid droplet (radius of r1), so there is sufficient material to maintain a spherical shape at the end of the solidification process. Because the liquid silicon phase is surrounded by solid silicon, strain develops during the final crystallization. Particles with one part missing and particles with complex stacking faults and dislocation structures are often observed. These are probably formed by this mechanism. The mechanism for crystal growth due to rapid cooling of silicon liquid droplets into UFPs is similar to the mechanism of laser annealing of silicon thin films as used in the silicon microelectronics industry. It is hoped that the research presented here will evolve into basic studies that will be useful for such industries.
References 1. Kimoto, Y, Kamiya, Y, Nonoyama, M. and Uyeda, R., Jpn. J Appl. Phys., 2, 702 (1963). 2. Yatsuya, S., Uyeda, R. and Fukano, Y, Jpn. J Appl. Phys., 11,408 (1972). 3. Iijima, S., Jpn. J Appl. Phys., 23, L347 (1984). 4. Iijima, S., Jpn. J Appl. Phys., 26, 357 (1987). 5. Iijima, S., Jpn. J Appl. Phys., 23, 365 (1987). 6. Ino, S. and Ogawa, S., J Phys. Soc. Jpn., 22, 1365 (1967). 7. Saito, Y, J Cryst. Growth., 47, 61 (1976). 81
Ultra-Fine Particles
1.10 Surface Oxidation of Silicon Ultra-Fine Particles (by Sumio Iijima)
The magnetic properties of ferromagnetic metallic UFPs of iron and cobalt are widely used for magnetic recording media, but these degrade upon oxidation. The UFPs used for this purpose are coated with an antioxidant. To control the condition of the surface, one needs to understand the surface oxidation phenomena. For the first and second surface layers of a metal particle, various spectroscopic methods can be used to evaluate the process of the oxidation reaction. The characterization of surface oxide layers with thicknesses of up to several nanometers can use Auger electron spectroscopy and secondary ion mass spectroscopy (SIMS). These surface analysis techniques reveal the average compositions of the surface. In contrast, high resolution electron microscopy (HREM) allows the surface microstructures to be observed on the atomic scale. Electron microscopy also allows for the observation of specific parts of a particle, such as ridges and comers. Because of these features, the oxidation phenomenon of silicon UFPs were examined using HREM techniques. Native Oxide Films on Spherical Silicon Ultra-Fine Particles
One of the general features of UFPs is the increase in surface activity with decreasing particle diameter. Thus, one can expect to see differences in oxidation reactivity with changes in the particle diameter for metal and semiconductor UFPs. The spherical silicon UFPs discussed in Section 1.9 were convenient samples for studying surface oxidation phenomenon. A particle from Figure 1 in Section 1.9 was selected and its profile obtained by HREM as shown in Figure 1. This particle was heated to 1150°C for 1 hour in an evacuated quartz tube. When silicon UFPs are formed by gas evaporation with
82
Figure I. Electron micrograph of a silicon UFP synthesized by the gas evaporation method after heating in vacuum. The crystallographic orientation was accurately adjusted to the [110] direction. The particle exhibits {Ill} and {1 OO} surfaces. Refer to
Sec. 1.4 for the neck structure (indicated by the arrow) in which particles become joined. an electric arc. they are nearly perfectly spherical. When they are heated, the (111} and (tOO} planes grow and the crystal habit
becomes apparent.
Surface atoms become mobile by heating
probably due to the reduction of the surface free energy of the particle
(i.e., higher indexed planes disappear and low indexed planes develop). This teclmique can provide a means for measuring the surface energy. It is also expected that unique information can be gained concerning UFP sintering mechanisms, especially at the early stages of sintering. An enlarged view ofthe area around a (111) plane of a heated
particle is shown in Figure 2. This particle is precisely oriented along the [101] direction and the micrograph shows the (111), (111), and (001) planes. A fuzzy layer about 1 run thick can be seen on the surface of this crystal. This is the native oxide film that is known to
cover normal silicon surfaces. The MOS devices of silicon semiconductors use Si02 fonned by thermal oxidation. The Si-Si02 interfaces influences the characteristics ofthe devices. Many studies of the interfaces have been done, but the transition in the atomic 83
84
Electron Microscopy Studies structure from silicon to Si02 is not well understood [1]. According to the profile image, a monoatomic transition layer can be observed at the interfaces. A detailed interpretation of this image requires a computer simulation of the lattice image, for which we need correct information on experimental details such as the thickness of the sample crystal. Spherical silicon UFPs are ideally suited for this study because their shape easily yields the necessary thickness information. Thermal Oxidation of Silicon Ultra-Fine Particles
Electron micrographs of spherical silicon UFPs after heating at 900°C for 3 hours in air are shown in Figure 3. Each particle shows a concentric core region, which is crystalline silicon and the external Si02 layer. The core has dark and light contrast because this region is crystalline, which affects the diffraction conditions. The micrographs indicate a wide range of particle sizes and the thickness of the Si0 2 layer depends on the particle diameter. An interesting point is the cause for the difference in oxide thickness in spite of all particles being heated under the same conditions. To study the formation ofthe Si02 layer, thermal oxidation experiments were done in which the time of heating was varied while keeping the temperature at 900°C [2]. Heating times of 0.25, 0.5, 2, and 3 hours were used. From electron micrographs of the samples thus prepared, the Si02 layer thickness was measured as a function of the particle diameter (see Figure 4). The plots indicate that the oxide thickness depends on the particle diameter. The smaller particles have less tendency to oxidize, indicating that smaller particles are more stable. This observation was contrary to the generally understood behavior of UFPs. It was necessary to consider the gas-solid reaction of a spherical particle to explain the above results. The problem itself was well-known and several rate equations for the reaction have been proposed [3]. To obtain a rate equation for an oxidation reaction, the diffusion of oxygen gas to the particle surface, the diffusion of
85
Figure 3. Electron micrographs of spherical silicon UFPs after heating at 900°C for 3 hours in air. Unreacted silicon remains in the center of each particle. The left image is the bright field image and the right image is the dark field image. 900'C
"
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Figure 4. Increase in the Si02 layer thickness when silicon particles are thermally oxidized. The layer thickness is plotted against the particle diameter.
86
Electron Microscopy Studies oxygen within the oxide layer, and reaction at the surface of the unreacted silicon core must be taken into account. It is interesting to determining whether or not these approaches are valid for very small particles. In addition, the reaction product (SiO z) from the oxidation of silicon experiences an increase in its volume, so extra space for the formation of SiOz is needed at the silicon surface. This should lead to the formation of internal stress at the silicon surface as the oxide layer is formed. In the silicon microelectronics industry, the control of the oxidation reaction of silicon at microscopic dimensions is an important process. The above data has not been analyzed in detail, but may lead to new insight into microelectronic materials processmg. References 1. Ourmazd, A., Tayor, K.W. and Rentschler, J. A., Phys. Rev. Lett., 59,213 (1987). 2. Okada, R. and Iijima, S., Appl. Phys Lett., 58, 1662 (1991). 3. For example, see Powders, eds. S. Hayakawa et aI., Maruzen, Tokyo (1965).
87
Ultra-Fine Particles 1.11 Surface Coverage of Ultra-Fine Particles (by Sumio Iijima)
One feature ofUFPs is their large specific surface areas. The material properties of UFPs may be affected by the structure and conditions of their surfaces. When the particle surface is covered with foreign matter and contaminates, the characteristics of UFPs, which may be observable in the ideal state, can be lost. The true characteristics can only be revealed by maintaining a clean particle surface. Large specific surface areas imply high surface activities, which are important in industrial applications of UFPs. Gas sensors using UFPs take advantage of this feature. On the other hand, it is necessary to protect UFPs because of their high surface activities. For example, when iron UFPs are exposed to air, they are rapidly oxidized to cx-Fe 20 3 • Here, a reduced oxidation treatment is used to form a stable oxide layer on the particle surface. To control and/or maintain the surface features of UFPs, an evaluation of UFP surfaces is essential. In particular, atomic scale microstructural characteristics are important. Many studies have been done on the surface modification ofUFPs [1]. Surface treatments of powders use chemical, physical, physio-chemical, and mechanical methods, but few techniques are applicable to UFPs for controlling coating thicknesses below 10 nm, and allowing for characterization of the layers formed. Thus, it was decided to study the surface treatment of UFPs by coating UFPs with carbon films and then examining the UFPs using electron microscopy. It is known that by flowing a hydrocarbon gas over heated UFPs of iron, carbon whiskers will be formed as the hydrocarbon decomposes on the surface. It is believed that carbon atoms build up not from simple physical accumulation, but via catalytic decomposition of the gas at the iron UFP surface. The simplest method for forming a carbon coating is to heat the sample to be coated in a hydrocarbon gas atmosphere. Electron
88
Electron Microscopy Studies
micrographs of a y-A1 20 3 UFP thus treated are shown in Figure 1. The treatment conditions were as follows: benzene gas pressure of 10 torr, argon gas pressure of 290 torr, temperature of 1000°C, and reaction time of 30 minutes. A graphitized carbon film about 5 nm thick was formed on the particle surface. The lattice fringes parallel to the surface correspond to the basal plane of graphite crystal (<1002=0.34 nm); that is, the graphite grows with its basal plane parallel to the particle surface. The carbon atoms on the basal plane are bonded by Sp2 electrons, which makes them very stable. Thus, this coated surface may be inappropriate for the adsorption or modification of certain organic materials. Alumina particles are normally white, but when they have a 5-nm carbon coating they become black. Optically, the characteristics of the alumina are completely lost and the property of absorbing visible light becomes dominant. UFPs of Ti02, Si02, and F~ G.3 can similarly be coated with graphite films. For Ti02 particles, hollow, spherical UFPs of TiC were obtained at a treatment temperature of 1100°C, as shown in Figure 2. This resulted from the reduction of carbon. The reduction of W03 UFPs was due to the hydrocarbon gas and was caused by the partial loss of oxygen. This produced numerous planar defects in the interior of the particle due to crystallographic shear [2]. To examine the coating of metals, graphite films were coated on nickel UFPs. The optimal treatment temperature was 400 500°C. Carbon films are amorphous below this temperature, while at higher temperatures Ni3C cementite is obtained. For iron UFPs, a graphite layer grows on the particle surface, but the entire particle transforms into cementite. No appropriate treatment condition was found where graphite coatings could be grown on iron UFPs. Observation of the graphite layers and carbide particles due to carbonizing treatment of iron and nickel UFPs implies that one should study the carbon behavior associated with the carbonizing reaction. With only slight variation in the treatment conditions, the formation of carbon coatings was greatly affected. The present study is related to basic research on the carburizing of bulk iron and nickel. Carburization and decarburization of iron and nickel are a key aspect in steel making processes and an enormous amount of accumulated
89
~l
90
Electron Microscopy Studies
Figure 2. Hollow, spherical UFPs of titaniwn carbide obtained by carburizing Ti02 particles. research results exist. However, such studies using UFPs can provide a new approach for research on surface treatments. The purpose for the surface modification of UFPs and for the synthesis of compound UFPs are many fold. Some examples include the reduction of the surface activity ofUFPs. the prevention ofJong-
tenn degradation, the conversion of insulator UFPs into conductive particles by coating with graphite, and the fonnation of carbon
coatings that enhance polymerization reactions so that UFPs of phannaceuticals can be encapsulated. References 1. Japan Chern. Soc., Chemistry Review, No. 44, 1985, Surface Modification, Gakkai Pub!., Tokyo (1984). 2. Iijima, S., J. Solid State Chern., 14, 52 (1975).
91
Ultra-Fine Particles
1.12 Non-Additive Sintering of Silicon Carbide Ultra-Fine Particles (by Shigetoshi Takahashi)
While the sintering ofUFPs is said to generate products with superior mechanical properties, few examples have been obtained [1]. There has been an attempt to sinter conventional silicon carbide powders without additives [2], but adequate sintering has not been confirmed even under ultra-high pressure sintering (5 OPa, 2500°C), which produced only local sintering [3]. Ultra-high pressure sintering was attempted (6 OPa, about 1650°C) with the silicon carbide UFPs prepared during this project without the use of additives. After the completion of the investigation, Honma et al. [4] reported nonadditive sintering of silicon carbide using hot isostatic pressing (HIP) under sintering conditions of 0.15 OPa and 1800 - 2000°C, which were different than the conditions used in the experiments reported here.
Preparation and Formation of UFP Samples By evaporating silicon in an argon atmosphere containing about 1.2% methane, silicon carbide smoke was obtained and collected (see Section 1.5). By using electron microscopy, the sample was shown to consist ofUFPs having an average diameter of 100 nm. The specific surface area as determined by the BET method was 49 m2/g, and x-ray diffraction indicated that the structure was that of pSiC mixed with a small amount of unreacted silicon carbide. Combustion infrared absorption analysis indicated about 12% amorphous carbon. This sample was referred to as sample-O. First, sample-O was heated in vacuum at 1300° C for 1 hour. By using this treatment, the unreacted silicon was reacted with amorphous carbon until silicon was no longer detected by x-ray diffraction analyses. This sample (sample-A) still contained amorphous carbon, which was removed by heating in air at 600°C for 2 hours. This treatment removed amorphous carbon, but produced 92
Pyrophilite .Pyrophilite Pressure medium
we anvil \0 W
I'-
Pyrophilite Graphite
Thermocouple
~
~ ~
~
::z
~ ~
a
b
Figure 1. Ultra-high pressure system using anvils and a pressure medium. a) Diagram of the pressure system showing the eight anvils. b) Cmss-section of the pressure medium.
~ ~
~
'-e
~
~
&: ~
Ultra-Fine Particles about 5 wt% of Si02 according to molybdenum blue light absorption spectroscopy. Next, the sample was heated to 500 c e for 30 minutes in an atmosphere containing a few torr of benzene, and subsequently heated in vacuum at 1300 c e for I hour. This sample (sample-B) showed essentially no Si02 • To compress the powder samples, a die with a 2-mm I.D. by lO-mm L was used. Samples were compressed under a pressure of 500 kg/cm2 for several minutes. The compressed samples were 3 mm in diameter and 3.2 mm in length and had a relative density of 0.62%. These samples were heated in vacuum at 1300 c e for 1 hour to expel absorbed gases and were then sealed in an argon atmosphere until final sintering.
High Pressure Sintering The pressure of a 2000 ton uniaxial hydraulic press was used to compress eight we anvils in six directions as shown in Figure 1a. Each anvil had one comer ground off to form an octahedral cavity when the eight anvils were pressed together. An octahedral pyrophilite pressure medium was put into the cavity and the high pressure was generated. As shown in Figure 1b, the octahedron has a 4-mm diameter hole in it in which two compacted silicon carbide samples were inserted and between which a thermocouple was placed. When sample-A was pressed at room temperature, the electrical resistance varied as shown in Figure 2 and it became nearly saturated at 6 GPa (press load of 400 tons). While a higher pressure was possible with this press system, 400 tons was selected as the maximum pressure for these experiments because it was anticipated that larger samples may be desired in future studies. Heating was accomplished by both direct- and indirectheating methods. For sample-A, which contained amorphous carbon and had a low electrical resistance, direct heating was used, while indirect heating was used for sample-B, which had a high electrical resistance. For direct heating, a current was passed through the sample by using graphite electrodes as shown in Figure lb. The 94
Electron Microscopy Studies
100
50 '. -" a •••••••••••••••••
OL--..-----..-----...----..----100 200 300 400 Press Load (tons) (40G-ton load
=6 GPa pressure)
Figure 2. Electrical resistance versus applied pressure.
boron nitride ring shown in the figure was used for insulation. For indirect heating, the boron nitride ring was replaced with a graphite ring that acted as a heater. Based on preliminary experiments, the sintering temperature was set to suppress the grain growth (see Table 1). For Sample-A, this temperature was 1600°C. For Sample-B, this temperature was in the range of 1650 - 1700°C. Pressures of approximately 6 GPa were reached in about one hour, after which the pre-set sinter temperature was reached in less than 30 minutes, at which time the temperature was held for 10 minutes. The cooling and depressurization steps were the reverse of the above: first the temperature was lowered, then the pressure was removed.
Characterization of the Sintered Bodies About a dozen sintered samples were polished and then examined by optical and scanning electron microscopy. Their density, Vickers hardness Hv (0.5 and 5 kg load), and K 1C via the 1M method were also measured. Typical results are given in Table 1. 95
Ultra-Fine Particles
Direct heating of sintered bodies (Sample-A) produced excess concentrations of carbon at places and the samples were heterogeneous, giving different hardness values depending on where they were measured. It is noteworthy that a maximum Hy value of 3700 was obtained. The value ofK,c could not be determined due to Table 1. Hardness and K,c of SiC Sintered Bodies. Heating Method
Sintering T,cC
Density (g/cm 3)
H Y2 * (kg/cm 2)
K IC
001
Direct
1600
3.14
3700
--
102
Direct
1600
3.22
3650
--
p-random
Direct
1600
3.12
2600
--
202
Indirect
1700
3.11
2740
4.67
202
Indirect
1750
3.17
2610
4.91
p-random
Indirect
1650
3.20
3100
3.92
Sample
(MPaem)
* Value under a 500-g load. Note: Samples 001 and 102 were prepared by direct heating (Sample-A) and sample 202 was prepared by indirect heating (Sample-B), the heterogeneity. For comparison, "P random" SiC powders from Ibigawa Denko Ltd. (average diameter of 270~m, specific surface area of 17.1 m2/g) were sintered under the same conditions. This sample was homogeneous but had a low Hy value of 2600 (Table 1). According to the SEM micrographs, the grain sizes of the sintered UFP samples were about 0.3 ~m while that of the "p-random" sintered sample was 2 ~m. Samples prepared by indirect heating (Sample-B) were homogeneous, but their hardness values were lower than the similarly sintered "P -random" sample. Their K,c values were fairly high (see Table 1).
96
Electron Microscopy Studies During experimentation there was insufficient time to systematically determine optimum sample treatment conditions. Therefore, intuition and trial and error methods served to determine treatment conditions. The results given in Table 1 have not been repeated. It was confirmed that SiC sinters at 6 GPa and 1600°C and that high hardness and high K 1C materials could be prepared. In particular, the materials produced from Sample-A by the direct heating method had a low electrical resistance, which allowed them to be used in electrical discharge machining to produce arbitrary shapes with dimensional accuracy of several microns. Honma et al. [4] obtained SiC samples having a maximum H v value of3240 and a maximum K1C value of5.95 MPaJm starting with eight types of commercially available SiC powders with submicron diameters. Their results follow the same trends as those reported in Table 1.
References 1. Ando, Y., Pressureless Sintering of SiC UFPs by Gas Evaporation, Yogyo Kyokaisha, 94(1), 2634 (1986). 2. Kinoshita, M., High Temperature Ceramic Materials, ed. Suzuki. H., Nikkan Kogyo, p. 102 (1985). 3. Nadeau,1. S., Am. Ceram. Soc. Bull., 52, 170-174 (1974). 4. Honma, K., Yamamoto, F., and Okamoto, K., HIP Sintering of SiC without Additives, Yogyo Kyokaishi, 95, 223-228 (1987).
97
Ultra-Fine Particles
1.13 Quenching of y-Iron UFPs to Room Temperature (by Michiko Kusunoki)
Iron has the bcc structure (a-phase) at room temperature, but transforms to the fcc structure (o-phase) at 911°C and again to the bcc structure (o-phase) at 1392°C. In bulk materials, the y-form of iron cannot be retained at room temperature. Yoshizaki et al. [1] and Tanaka et al. [2] showed that y-iron particles could be precipitated in MgO matrices when heat treated. Fukano [3] observed the y-form in UFPs. He found that a small amount of y-iron UFPs are present in UFPs of iron made by the evaporation method. This finding was interesting, but the amount of y-iron was too small for the study of the properties of the material. A sufficient quantity of y-iron UFPs was obtained and quenched to room temperature [4] so that the properties could be measured. After this study, Hayakawa and Iwama [5] reported an efficient method for preparing y-iron UFPs by using microwave plasma processing.
Heat Treatment Apparatus for Ultra-Fine Particles The team attempted to make a-iron smoke by using the gas evaporation method, transforming this to y-iron by heating, and then quenching the material to room temperature. The apparatus [2] is shown in Figure 1 and was comprised of a UFP synthesis chamber, a heating chamber, and a quenching chamber. The synthesis chamber was the large evaporation apparatus that was described in Section 1.5, and it used the synthesis conditions given there for UFPs of iron. The particles formed were all a-iron UFPs, which were drawn into the heating chamber and the quenching chamber by continuous gas pumping. The pressure of the synthesis chamber was kept constant by bleeding gas into the chamber. The heating chamber was heated externally to about lOOO°C so that the smoke passing into it was heated to above 911°C, which transformed the material into y-iron while maintaining individual particle character. After the particles
98
I.
Thennal Treatment
UFP Fonnation ---.:
Heating
I
I'''''
../ Rapid Cooling I
. 'I .
,
-II
,
I
r Liq. N2
I
'-' Rotation
Tungsten Electrode /
Heater Molten High-Purity Iron
\0 \0
~ ~
Arc Power
~
""u u u
~
-
-
-
--
I
~lkPi\lPa I
I
I
~
'i-r=t- Observation Window
::
UFP Adsorption
~
J
~ ~
Water-Cooled
Vacuum Exhaust
Vacuum Exhaust
Copper Base
Figure 1. Experimental apparatus for heat treatment and quenching of iron smoke. This consists of a UFP synthesis chamber, a heating chamber, and a quenching chamber.
~
~ ~
~ ~ ~
passed through the heating chamber, the gas was "removed by differential pumping and the beam ofUFPs (see Section 1.14) was directed toward a rotating liquid nitrogen-cooled copper drum. The particles attached to the surface, where they were rapidly quenched.
Experimental Results Electron micrographs of iron UFPs treated as described above were obtained. As shown in Figure 2, the particles, which range from several nanometers to several tens of nanometers in diameter are well dispersed. Figure 2a shows particles that were not treated with the furnace heating, while Figure 2b shows those treated at lOOQ"C. The particles in Figure 2a are rhombohedral dodecahedra typical of a iron particles. The diameters of the particles range from 50 to 100 om. Because of ferromagnetism, the particles are held together in a
(0)
(b) 1000'C
•
Figure 2. Electron micrographs of iron UFPs collected on the surface of a rotating drum. a) particles fanned without using furnace heating; b) particles formed with heating at 1000"C. 100
Electron Microscopy Studies chain. The particles observed in Figure 2b have rounded comers, which are believed to be due to the surface tension at high temperatures. The particles have larger diameters with increased heating, which results from collisions during heating and growth by fusion (refer to Section 1.5). Electron diffraction rings are shown in Figure 3. The left side is from the sample of Figure 2a and the right side is from that of Figure 2b. Except for the rings due to Fe304 (shown with arrows and indices), the rings shown on the left correspond to those of a-iron (bcc). The Fe30 4 was produced by surface oxidation. The electron diffraction pattern on the right also shows rings due to y-iron (fcc), indicating that the y-iron, which has not been formed in bulk, was quenched to room temperature in the form of UFPs. Only a part of the iron UFPs, however, became y-iron. The indices in Figure 3 have subscripts of b and f, which indicate the bcc and fcc phases, respectively. There are some overlapping rings, but 311 f and 211 bare independent and their respective multiplicity is the same. Thus, the ratio of their intensities corresponds to the mass ratio of y- and airon. From the right side of Figure 3, the intensity ratio is found to be approximately 10%, so this amount of symbol y-iron was quenched to room temperature. Magnetic measurements were done using about 10 g of this sample (see Sec. 2.9). When the limited area electron diffraction method is used to examine individual particles, the y-iron particles are limited to those with a diameters less than 40 nm. Thus, only smaller particles were quenched. Many a-iron particles have (110)b twins repeated at intervals of several nanometers. These appear to be transformation twins due to martensitic transformation, which is often observed in carbon steels. The twins were not present in samples that are not heat treated, indicating that martensitic transformation occurs due to a slower cooling speed after heating. Discussion
When this study was planned, the team had hoped to obtained 100% y-iron as a result of the high cooling rate afforded by this 101
Figure 3. Electron diffraction patterns of iron UFPs. Left:: sample
not heat treated; right: sample heat treated at IOOO·C and quenched. The rings due to Fe304 are indicated with arrows. Note the fcc-rings in the pattern on the right. experimental method. However, only 10% y-iron was obtained. It is not clear what cooling rate is required to obtain 100% y-iron. but the factors that Jed to the reduced cooling rate were considered. The first factor is insufficient differential pumping, which induces heating in particles attached to the cooling surface by the bombardment of gas molecules arriving later. In the future, plans have been made to make use of two stages of differential pumping and to use a cooled and flowing liquid surface in lieu of the rotating copper drum. The second factor is cooling due to adiabatic expansion when the smoke moves out of the heating chamber. It is a challenge both theoretically and experimentally. to clarify this process in which the gas is cooled by the adiabatic expansion, which causes the particles to be cooled. The speed of the evolution of the smoke, however, was 102
Electron Microscopy Studies on the order of 100 m/sec so that it took about 1 ms for the particles to arrive at the cooling surface. This could be the factor that caused the reduction in the cooling rates. To eliminate this possibility, one could make a beam of iron UFPs in good vacuum, heat it above the transformation temperature, and direct the particles into a cooled liquid. See Sec. 1.15 for details on this point. Finally, it is possible that y-iron particles captured at liquid nitrogen temperature become a-iron when they are warmed to room temperature for observation. References 1. Yoshizaki, F., Tanaka, N., and Mihama, K., J Electron Microscopy, 39, 255-259 (1990). 2. Tanaka, N., Yoshizaki, F., Katuda, K., and Mihama, K., Acta Metal!. et Mater., 40, S275-280 (1992). 3. Fukano, Y, Bull. Jpn. Inst. Metals, 15, 639-641 (1976). 4. Kusunoki, M. and Ichihashi, T., Jpn. J Appl. Phys., 25, L219 (1986). 5. Hayakawa, K. and Iwama, S., J Cryst. Growth, 99, 188-191 (1990).
103
Ultra-Fine Particles 1.14 UFP Beam Experiments (by Toshinari Ichihashi)
Introduction A UFP beam was obtained by directing UFPs in a nearly parallel flight path in a vacuum [1]. A schematic diagram of an apparatus is shown in Figure 1. The device consisted of an evaporation chamber, a nozzle chamber, and a beam chamber. In the evaporation chamber, a metal smoke was synthesized using the gas evaporation method [2]. For example, silver UFPs about 10 nm in diameter were formed as smoke by evaporating silver from a tantalum boat in a helium gas atmosphere at about 10 torr. The smoke was passed though a nozzle (I-mm diameter) into the nozzle chamber along with helium gas. The flow of the gas with respect to the nozzle is viscous flow and could be directed in an arbitrary direction by bending the nozzle. The helium gas was rapidly pumped out of the nozzle chamber using a mechanical booster pump. The pressure of the nozzle chamber was kept at 0.1 torr. The UFPs blown from the nozzle passed through an aperture (I-mm diameter) and entered the beam chamber where a vacuum ( 1 x 10-4 torr) was created using a diffusion pump. The UFP beam, which had a slight distribution in its direction (Figure 1), was formed in the chamber and struck a glass plate 1.7 m away from the aperture. When a shutter placed behind the aperture was opened, a silvery disk about 5 mm in diameter was observed on the glass plate after several seconds elapsed. For electron microscopy studies, a copper grid used in conventional electron microscopy was placed on the glass plate and UFPs were collected on the grid. Figure 2 shows an electron micrograph of silver UFPs obtained by this procedure. According to the beam exposure time and the number of UFPs per unit area, the beam intensity was about 1011 particles/cm2 ·sec. The particles were nearly uniform with about 10-nm diameters. Most were icosahedral, multi-twinned particles [3]. The morphology and size ofthe particles
104
Electron Microscopy Studies
et/
Aperture MKhanical _ booster PI6\"IP
.J
Movable shutter
of
Glass plat.
Na..l.
I
Evaporation source
Diffusion pump
Figure 1. Diagram ofthe UFP beam generating apparatus,
.. • • •• •• • • , • • •• • • ... • C.. • •• • ~.
•
~
AI
•
••
.III•
.
4
"•
•• :. < .. -
~ • • III
.. •
~
'.~. ,~Onmll
Figure 2. Electron micrographs of silver UFPs obtained from a UFP beam, 105
Ultra-Fine Particles
can be controlled by adjusting the evaporation conditions (temperature of the evaporation source, atmospheric gas type, and pressure) and the location of the particle collection [4,5]. Once the smoke was drawn into the nozzle, it was expected that the particle growth terminates, which allowed for the study of UFP growth processes via the UFP beam method. The UFP beam can be formed using semiconductors and oxides by changing the evaporation source and atmosphere. The first objective in making UFP beams was to transfer UFPs synthesized by the gas evaporation method directly into an electron microscope without exposing them to air atmosphere. This makes the observation of easily oxidized UFPs possible and allows for the study ofUFPs with clean surfaces as will be discussed later. The second objective was to heat UFP beams and to synthesize new materials by subsequent quenching of the heated UFPs. Two methods were used to heat the beam. One was to heat the smoke and to then make a UFP beam, as described in Section 1.13 [6]. The other method involved generating a UFP beam and heating it by passing it through a pipe heated to high temperatures. The team experimented with silver and magnesium UFP beams and confirmed, based on the morphological changes observed, that it was possible to heat the beam. The measurement of the temperature of UFP beams remains unsolved. The initial concept for this experiment was that the radiated heat from the pipe walls may heat the UFPs, but the UFPs may have come in contact with the pipe walls. In either case, the desired heating effect was accomplished. The highest cooling speed was expected to be obtained by directing the heated UFP beam into a cooled liquid. The third objective was to accelerate UFPs by charging them and to cause collisions with various surfaces. Electron microscopy could be used after this to observe the changes to the UFPs and their surfaces. The composition ofthe UFPs is broad, ranging from metals to SiC and the temperature can range from low to high temperatures. There are infinite variations that are possible for the experimental conditions, including varying the surface being treated. The reflection of metallic UFPs from a high temperature surface is one of 106
Electron Microscopy Studies the interesting problems that can be considered. This is of interest as a collision phenomenon of mesoscopic particles (region between microscopic and macroscopic sizes), but it is also of interest because it may help clarify individual processes in future work on mesoscopic surface coating and polishing. Plastic coating is macroscopic, while vacuum vapor deposition is microscopic. The use of UFPs falls between these two processes. These results may be of use in understanding what happens when dust hits the outer surface of a spacecraft.
Velocity Measurement and Ionization of Ultra-Fine Particles The UFP beams directed horizontally are deflected due to gravitational effects. The beam velocity was determined using this information. Figure 3 shows a diagram of the velocity measurement method that was used. After the beam passes through the aperture, it then passes through two slits (hole diameters of 0.5 mm). The center of the aperture and those of the slits were aligned using a He-Ne laser beam, which extends to Point on the glass plate. After passing the slits, SI and S2' the beam reaches Point B on the glass plate. At Point B, a silvery spot about 1 mm in diameter forms after several seconds of exposure. The beam path is expressed by a parabola and the deflection, BO, is given by the following equation:
°
(1/2) g[(1I2) 1 + 12f
BO =
1 -------=----=-2
Vo
(1)
Here, g is the gravitational acceleration, I] and 1 2 are the distances between S] and S2 and between S2 and 0, respectively, and V o is the beam velocity. Measurements yielded values for 11 = 0.4 m and 12 = 1.0 m and BO was determined to be 0.5 mm, yielding a beam velocity of 120 m/sec. To allow for the acceleration or deceleration of UFPs, the particles brought into the beam chamber were ionized by bombarding 107
Ultra-Fine Particles
I 5Ur
Aperture
51i152
Glass plate
o
I I I-- I,
_ _ _----..B
of·
Figure 3. The trajectory of a horizontal UFP beam (not to scale). them with electrons (Figure 4). The experimental conditions were as follows: electron energy of 50 to 160 V and current of 1-5 rnA. When the ionized beam was deflected by the electric field, the beam was found to have neutral, negatively, and positively charged particles. Because the particle size and velocity were non-uniform and the charge was different from particle to particle, the beam intensity distribution shown in Figure 4 was obtained. The distribution chang,ed depending on the electron energy. On the other hand, no deflection due to the electric field occurred when electron bombardment was absent, so the particles were not initially charged. From the intensity distribution shown in Figure 4, the positively charged particles have charges of 1 - 4 e/particle and the negatively charged particles have charges of 1 e/particle, assuming a diameter of 10 nm for the silver UFPs and a velocity of 200 rn/sec (to be discussed later). The negative charge on some particles results from the addition of an electron to the particles and the positive charge on some particles results from secondary electron emissions from the particles. Using a Faraday cage, the total beam current was measured after the electron bombardment. This naturally depends on the electron current, but the results for bombardment with 1 rnA of 108
Electron Microscopy Studies
Deflector
UFP
Iii ~
O~O~---Tee==-=~'----------e-~-----+---------7intensity
Figure 4. The trajectory and intensity distribution ofthe ionized UFP beams deflected by the electric field. electrons is shown in Figure 5. As the electron energy is increased, the total current changes from negative to positive and reaches a maximum at 150 V, above which the beam current decreases again. The beam current is negative at low voltages because the electrons stick to the particles. The change to positive values is due to secondary electron emission. The maximum in the beam current probably corresponds to the maximum in the efficiency of secondary electron emissions. The value of 150 V is much lower than 800 V, which gives the maximum efficiency in secondary electron emissions in a bulk material [7]. This difference is due to the fact that only secondary electrons excited near the surface can escape from the solid; that is, the lower value can be explained by considering the diameter ofUFPs and the energy dependence of the penetration depth of primary electrons, the excitation efficiency of secondary electrons, and the escape distances [8]. Using the particles ionized by electron bombardment, the beam velocity was measured using the time of-flight method. Figure 6 shows a diagram of the apparatus. The ionized particles were chopped by a deflector and could pass through the slit only for a fixed 109
Ultra-Fine Particles
•
-10
Figure 5. The total current of the ionized UFP beams. The electron bombardment current was fixed at 1 rnA.
Bombarder
Deflector
Beam detector
Slit
I
Irr=
r!
I IL-...JI I--O.4m----t
Diffusion pump Figure 6. Diagram of the apparatus used to measure the beam velocity. 110
Electron Microscopy Studies
period of time. The chopped beam induced a voltage in the detector (a cylindrical capacitor). Figure 7 shows the signal (calculated) when a single charged particle passed through the detector. In actual practice, the measured signals are the superposition of these signals and are observed on a high sensitivity oscilloscope. Figure 8 shows the voltage applied to the deflector and that observed on the detector (no acceleration voltage applied). The signal indicates that the particles passing through the deflector arrive at the detector after 2 ms. The deflector-detector distance of 0.4 m yields a beam velocity of 200 m/sec.
Figure 7. Voltage induced by a single charged particle passing through a cylindrical capacitor.
111
Ultra-Fine Particles
01
1:1
+J
<5 >
...
V
10
0
+J
U
....
0 J-lV
400
4 I·
8
ms
2ms I
-400 Figure 8. Voltage applied to the deflector (top) and detector signals (bottom). The measured value of 120 m/sec due to gravitational effects was much lower than the value reported above. This may be due to residual gas in the beam chamber, which would lower the beam path compared to the theoretical value (Point B in Figure 3 became lower). In the last experiment, the vacuum level was higher and the flight distance was shorter, so the measurement was closer to the theoretical value. A potential relative to the ground can be imposed on the anode used in the electron bombardment and this can be used to accelerate the ionized particles. By accelerating the particles with a 0.4 kV potential, the particle velocity increased to 220 m/sec. By taking the diameter of the silver UFPs as 10 nm, this velocity change corresponds to an average positive charge on the particles of about 2 e/particle.
112
Electron Microscopy Studies Introducing Samples into an Electron Microscope Figure 9 shows a diagram of the apparatus used. It consists of an evaporation chamber, a nozzle chamber, and a beam chamber as shown in Figure 1. In addition, it has intermediate primary and secondary chambers to improve the vacuum of the beam chamber. The nozzle chamber was evacuated by a mechanical booster pump, while the intermediate chambers and beam chamber were evacuated by independent and dedicated turbomolecular pumps. The nozzle and skimmers that separate these chambers can be move from outside the apparatus. It was inconvenient to align the beam axis in the apparatus of Figure 1 because the nozzle and aperture were not movable from outside. An optical system for axial alignment was not included, however, for the nozzle and skimmers for the UFP beam generation apparatus. This caused a delay in matching the beam to the sample position in the electron microscope after the alignment had been completed. Because UFPs were sampled from an arbitrary position within the smoke, the location of the smoke generator was made adjustable from outside. The distance between the nozzle and the sample stage of the electron microscope was about 2 m. The beam diameter was 2 mm at the sample stage. UFPs were directed at a small angle of less than 10 0 onto a carbon film on a mesh at the sample stage (Figure 9b). On the fluorescent screen of the electron microscope, one could observe newly arriving particles in situ. Observations were made at 10,000 X , and the silicon UFPs arrived at a rate of one particle per second. The first problem that had to be resolved in this experiment was the elimination of the vibrations transferred from the UFP beam generator to the sample stage of the electron microscope. By connecting the generator to the sample chamber, the vibration modes ofthe laboratory floor and electron microscope changed and degraded the image quality. To prevent this problem, the connection was made using welded bellows wrapped with anti-vibration rubber. Rotary vacuum pumps were placed outside the room and connected via vinyl hoses with vibration damping weights. These counter measures
113
Ultra-Fine Particles
(a)
(b) Figure 9. a) Diagram of the UFP beam generating apparatus for injecting UFP samples into an electron microscope; b) expanded view of the sample chamber shown in (a). allowed us to observe the lattice fringes of the silicon (111) surface to be 0.31 nm. To begin the observation ofUFPs with clean surfaces, silicon UFPs were studied. Silicon UFPs exposed to air are covered with a 2 nm thick layer of amorphous Si02 • By directly bringing silicon UFPs into the electron microscope, the amorphous layer thickness was reduced to 1 nm, but it could not be reduced further. The team next experimented with the metal calcium, which oxidizes rapidly. Calcium UFPs made by the gas evaporation are a brown color just after their synthesis. This color changes to white as the UFP change to CaO after exposure to air for 5 min. Thus, no experiments directly using calcium UFPs were done, but by using the direct introduction method their structure was observed for the first time. Figure 10 114
shows electron micrographs of calcium UFPs. Figme lOa shows a calciwn UFP directly introduced into the electron microscope and viewed from the [Ill] direction. This shows the octahedral structure for an fcc metal (i.e., octahedron). In the diffraction pattern the diffraction spots of epitaxially grown CaO extended into an arc and can be seen in addition to those of calcium. Figure lOb shows the same particle after being exposed to air for 5 min. This indicates a completely different morphology due to severe oxidation. The diffraction pattern changed almost completely to that of CaO.
•
•
, 0 0 "'"
Figure 10. Electron micrographs and diffraction patterns of calcium UFPs viewed from the [Ill] direction. a) Calcium UFPs before exposure to air; b) Calcium UFPs after exposure to air for 5 min. 115
Ultra-Fine Particles To observe particles with clean surfaces, it is important to improve the purity of the atmospheric gas in the evaporation chamber during the synthesis. In addition to the use of a high-purity gas, the evaporation chamber should be baked. Furthermore, the electron microscope should be heated to above 150°C and the entire apparatus needs to be converted to an ultra-high vacuum system. This remains a problem for the future. Heating UFP Beams When a UFP beam can be heated to high temperatures, it can be rapidly quenched by hitting a cooled solid surface or liquid. In the first method, the particles were heated in the smoke state and then converted into a beam (Section 1.15). In this method, cooling due to adiabatic expansion during expansion of the smoke was unavoidable. Therefore, a second method of radiative heating of a UFP beam in vacuum was tried. Figure 11 shows a diagram of the experimental apparatus. The heating furnace had a length of 240 - 500 mm and was operated at temperatures of 1600 - 1800 o e. Assuming a particle velocity of200 m/sec, the UFPs take 1.2 - 2.5 ms to pass through the furnace and are expected to be heated by radiative heating. Actually, the particle diameters are smaller than the wavelength of thermal radiation so it is questionable whether radiative heating takes place. Nevertheless, this method for the heating ofUFPs was examined. To measure the temperature ofthe particles that emerge from the furnace, the usual method was to measure the thermal radiation from the particles. However, because it was not possible within the time frame of the project to develop a suitable apparatus for doing this, one simply observed the changes in the particle morphology of magnesium UFPs, which have a clean crystal structure, when they were heated by the furnace. Later, silver particles with 30-om diameters were used in lieu of magnesium UFPs. Silver UFPs have a uniform diameter and nearly all particles have a multi-twinned icosahedral structure (see Figure 2). The results from the experiments show that about 20% of the particles were converted to spherical single crystal particles upon emerging from the furnace. 116
Electron Microscopy Studies
! h
r--~'1'--::=:i'!,.f
9
I
.j-
3
:i:
1----lXJ-
,.
DP
I~
~
. i
. i· I.
2
b
Ar
a
Figure 11. Diagram of the UFP beam generating apparatus. 1: Evaporation chamber (10 torr), 2: differential pressure chamber (0.1 torr), and 3: heating chamber (10-4 torr). a: evaporation boat, b: extraction nozzle, c: blowout nozzle, d: skimmer, e: slits, f: furnace, g: reflector, and h: mesh holder for TEM observations. In the experiments described here, the parallelness of the beam was not adequate and the particles may have contacted the furnace wall. Thus, heating may not have been limited to radiation. If the particles were heated by bouncing on the wall, this is an interesting phenomenon and gives a new means of particle heating. Because of these complications, these experiments did not produce a clear-cut view of the processes studied, but they did raise the following new prospects: 1) Theoretical and experimental study of radiative heat transfer of UFPs (when the diameter is less than the wavelength of thermal radiation), 2) Development of temperature measurement methods for UFP beams based on analysis of the 117
Ultra-Fine Particles thermal radiation, and 3) Experimental study ofUFP beam reflection at solid or liquid surfaces (especially when the surface is at a high temperature).
References 1. Ichihashi, T., Jpn. J Appl. Phys., 25, 1247 (1986). 2. Kimoto, Y., Kamiya, Y., Nonoyama, M. and Uyeda, R., Jpn. J Appl. Phys., 2, 702 (1963). 3. Ino, S., J Phys. Soc. Jpn., 27, 941 (1969). 4. Yatsuya, S., Kasukabe, S. and Uyeda, R, Jpn. J Appl. Phys., 12, 1675 (1973). 5. Kasukabe, S., Yatsuya, S. and Uyeda, R, Jpn J Appl. Phys., 13, 1714 (1974). 6. Kusanoki, M. and Ichihashi, T., Jpn. J Appl. Phys., 25, L219 (1986). 7. Weast, R C., Handbook of Chemistry and Physics, 57th ed., CRC Press, Cleveland (1976). 8. Dekker, A., Solid State Physics, Prentice-Hall, Englewood Cliffs, NJ (1957).
118
Electron Microscopy Studies 1.15 Living Crystals (by Sumio Iijima)
As a part of the electron microscopy study of the microstructures of UFPs, rhodium and platinum catalysts on oxide supports were examined [1]. These UFPs were found to move about like living matter when they were observed by electron microscopy at a magnification of one million times. The particles always existed in a morphology with comers and they continued to move without signs of being in either a molten state or undergoing evaporation. Conventional HREM generally uses 500,000 magnification, but because of the higher magnification used in this study, the UFPs received a stronger dose of electron irradiation. This resulted in abnormal crystal structures [2]. When the higher magnification was used, the image intensity decreased. When this was compensated for by adjusting the condenser to increase the beam intensity, the resolution was degraded. Thus, it is contrary to normal practice to magnify to one million times and attempt to obtain high-resolution images. In the following, the results on the dynamic behavior of metallic UFPs are presented. In particular, the experiments on temperature increases and charging experienced by the samples and the investigation into the cause for the instability of metallic UFP structures are discussed. The peculiar morphologies of UFPs, such as multi-twinned particles, [3-5] are discussed by Ino [6] and Marks et al. [7] in terms of the Curie-Wulff crystalline equilibrium theory. Experimental
Metallic UFPs were prepared by vacuum evaporation. As supports for the UFPs, spherical silicon was used, y-alumina, (XFe Z0 3, and graphitized carbon UFPs, as well as sputtered carbon films. Silicon UFPs were made by the gas evaporation method (see Section 1.9) and had particle diameters of less than several tens of nanometers with a SiO z surface coating 1-3 nm thick [8]. It was important to use an insulator as the substrate. Metallic UFPs initially had a diameter of about 1 nm but grew under electron irradiation. 119
Using this particle growth, the particle size was adjusted to I to 10 nm. Figure 1 shows an electron micrograph of gold clusters with diameters of 1-5 nm. which were produced on silicon particles. Metal carbonyl UFPs on silicon or y-Al 20) UFPs were examined, but no significant differences were found due to changes in the fonnation
method. For electron microscopy observations, selected UFPs were attached to the edge ofspherical particles and their profiles obtained. An Akashi 002A electron microscope was used with a point resolution of 0.23 run and an accelerating voltage of 120 kV. This
system was equipped with a video camera for recording the images, With the level of resolution provided by this system, the atomic positions of a single gold crystal can be viewed directly. To reduce the contamination on the sample, the vacuum pumping system was specially designed and incorporated three turbo-molecular pumps and two ion pumps, which allowed a sample chamber vacuum of 3 x 10- 1 torr to be achieved. The following electron micrographs were reproduced from individual frames of the video tapes (time resolution of 1/60 sec) using a Sony BVU-820 video recorder and a Hamamatsu Photonics SIT video camera.
Figure I. Electron micrograph of gold clusters (1-5 nm dial that were vapor deposited on a spherical silicon particle. 120
Electron Microscopy Studies Observation of Unstable Structures Figure 2 shows electron micrographs of a moving gold UFP. These show the images of a single individual particle over a fiveminute period. Analysis of the video recording indicates that the particles continually change shape about every 1/10 sec. Along with such deformation of the shape, the particles rotated and shifted their centers of gravity by as much as 3 to 6 nm. The action of moving particles that collide and fuse together was also observed. As will be mentioned later, the deformation is accompanied by changes in the internal structures and the particles become single crystals or twinned particles. As the particle size grows, the deformation is slowed and no deformation occurred in particles larger than 10 nm in diameter. Even in these large particles, however, the surface atoms were found to move around. It is worth pointing out that the melting point of gold is 1064 0 C. The motion of the UFPs increased as the electron beam intensity was increased (the electron beam intensity at the sample position was 1.3 x 107 electrons/nmz/sec). When the intensity of the beam was less than 105 electrons/nmz/sec, the changes in the particles stopped. When the motion of particles with the same diameter are compared, particles with a smaller contact area with the substrate changed faster. The contact points of the UFPs can be seen at the bottom of each photograph. In insulator-type substrates such as Al z0 3 and silicon particles covered with SiOz, the UFPs moved in an active manner, but such activity ofthe UFPs did not occur with the a-FeZ0 3 and graphite substrates. From these observations, the instability of metallic UFPs can be thought to arise from the increase in the particle temperature or the charging of the particles in the vicinity of the UFPs.
Crystal Habit of Ultra-Fine Particles The HREM video images of the gold UFP shown in Figure 2c, e, f, and k show truncated octahedra with well-developed crystallographic planes. Figure 3 shows a diagram of the various states of the particle. In the image shown in Figure 2k, the lattice 121
-l::l
Figme 2. Sequential electron micrographs ofa gold UFP consisting of 460 atoms. Truncated octahedron (c, e, f, and k), twinned (a, d, and i, and pentagonal icosahedral multi-twinned particles (b, It, and I) are visible. No multi-twinned
pentagonal decahedral particles were observed.
Electron Microscopy Studies
a
c
b
d
Figure 3. Diagrams ofmetal UFPs with various shapes. a: Truncated octahedron, b: twinned truncated octahedron, c: pentagonal icosahedral multi-twinned particle, and d: pentagonal decahedral multi-twinned particle images (dill = 0.235 urn) can be seen in two directions. From the number of fringes, the size of this truncated octahedron is about 1.9 urn in the vertical direction. An ideal truncated octahedron corresponding to this particle consists of 459 atoms and its {100} planes (indicated by the shaded spheres in Figure 4) are 3x3 arrays of gold atoms.
Figure 4. Model of the truncated octahedron shown in Fig. 2k and consisting of 459 atoms. 123
Ultra-Fine Particles The obtuse angle seen on the top left of the image of the UFP in Figure 2a is due to a typical fcc twin enclosed by {I OO} and {Ill} planes. Other images oftwinned states ofthe particle (Figures 2d and I) were also found. The changes observed in the shape of particles is found to accompany the generation and destruction of twin planes and stacking faults and to exhibit cooperative atomic movement throughout the entire particle. The twins will be discussed in more detail later. The images of the hexagonal state of the particle as seen in Figures 2b and h are a pentagonal icosahedral multi-twinned particle viewed from its three-fold symmetry axis (Figure 3c). The image of the UFP seen in Figure 2h appeared 20 sec after the appearance of the image of the particle shown in Figure 2g. According to a calculation by Ino [6], an ideal pentagonal icosahedral multi-twinned particle consists of a close-packing of twenty tetrahedra. The number of atoms is given by the following equation. N =
r
1
(10n
3
-
15n 2 + lIn - 3)
(1)
Here, n is an integer and N becomes 1, 13,55, 147,309,561, etc. [9] The UFP discussed above is close to N = 561. In relatively large particles, truncated multi-twinned pentagonal decahedra were often observed (Figure 5). These particles are formed by the growth of five twin planes by sharing one <110> ridge of five tetrahedra (Figure 3d). The fcc metal particles with diameters ofless than 10 nm have been reported [6] to become less stable in the following order: multitwinned pentagonal icosahedron> a single crystal> a multi-twinned pentagonal decahedron. The less frequent appearance of decahedra agrees qualitatively with this ordering of stabilities. It could not be confirmed from the video image observations which particle shape was most likely to be more common for a given particle diameter. Structural instabilities similar to those mentioned above have also been observed for UFPs of platinum (m.p.: 1769°C), rhodium (l963°C), nickel (l455°C), and silver (962°C).
124
Electron Microscopy Studies
Figure 5. Gold pentagonal decahedral multi-twinned particle. The surfaces are covered by {IOO} and {Ill} lattices. Atomic Rearrangement Mecbanisms
Figure 6 shows a series of photographs ofa gold UFP similar to that ofFigure 2 [10]. The last image (Figure 6d) was recorded 1 sec after the first image (Figure 6a). This UFP is accurately oriented to the [110] direction and black dots corresponding to rows of gold atoms along the [110] direction can be seen. The crystal structures deduced from Figures 6a, b. and d are shown in Figures 7a, b, and c, respectively. For each particle. two twin planes can be seen to cross the particle horizontally (indicated by arrows). The twin plane seen in Figure 6c is shifted by one atomic layer. while one twin plane is missing in Figure 6d. The atomic arrangements near the edges ofthe UFP are also seen to change. After the disappearance of the twin plane, the crystallographic orientation of the upper part of the UFP rotated clockwise as shown in Figures 6c and d. Such a cooperative 125
Figure 6. Sequential electron micrographs of a gold UFP viewed from the [110] direction. a: The two twin planes are separated by only 3 x dill' b: After 0.2 sec the separation between the twin planes becomes 4 x dill (the abnormality in the crystal array indicated by the circle is probably due to a twin dislocation, c: After 0.5 sec, one of the twin planes has disappeared. Shear deformation occurred in the upper portion of the UFP. 126
Electron Microscopy Studies
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• • • • • • • • • • • • • ••
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..
. .
" I , ' , ' , I, I, " ' I " "
•
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I'
a
I, I, I, I, .. I, II I.
b
.
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•
.
......••.•.... . ............... · . . .. .......•........ ................ ... I.··. . .....•......... ................ ....... ...... ................•. . .•.•.......•• ............. •.....•..... ••••••••••••••• A
~
•• ••• •
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•• I
~
•
' . 1••••••••1 •••••••• 1• •1 •• I•••••
•
I
•
•
•
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•
•
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............ ...........
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•• I
c
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d
Figure 7. Models ofthe atomic arrays of the UFPs shown in Figure 6. Arrows A and B indicate twin planes. atomic movement can be explained by the shear deformation mechanism for crystals. To produce shear deformation, twin dislocations must cross the UFP. The fuzzy lattice image seen in Figure 6b (circled) appears to indicate this twin dislocation. A similar lattice anomaly can also be found in the UFP shown in Figure 2f. A magnified image is shown in Figure 8, where the circled portion has the hcp atomic arrangement and may indicate a twinning deformation process. The driving force for twinning is likely to be the Coulomb force as discussed later. Temperature-Related Experiments for Ultra-Fine Particles Let us consider here two experiments related to temperature effects for UFPs. The first experiment involves the observation of a bismuth UFP using the same conditions as that used previously for
127
•••••••• • •• •••• • •• • •• •••••• •• •• •• ••••••••••• • • • • • •• ••• •••
~
\~\\\\\\\\\\\~\~'\ b
Figure 8. Abnonnallattice arrays of the gold UFP shown in Fig. 2f. The hcp atomic array structure can be seen in localized areas (see circled portion). the gold UFPs. The bismuth UFP showed clear lattice images, which implies that the temperature of the substrate particle under electron microscopy observation is below the melting temperature of bismuth (271°C). The second experiment concerns the differences in the instability of the structure of gold UFPs when the beam diameter of the irradiating electrons is varied. The images shown in Figure 2 were obtained using the conventional observation method with a beam diameter of several hundred nanometers. Thus, the entire silicon particle supporting the metallic UFPs was also irradiated with electrons (see Figure 9). Some of the incident electrons were inelastically scattered in the silicon particle and lost energy. which resulted in an increase in the sample temperature. The energy loss
depends on the size (thickness) of the silicon particle. To prevent the rise in the temperature of the substrate, a narrowly focused electron
beam was used and single UFPs were observed by irradiating only the particle, not the substrate (Figure 9b). 128
Electron Microscopy Studies
a
b
Figure 9. Two electron microscopy observation methods for a UFP supported on a spherical silicon particle. a: Conventional observation method with a beam diameter of 200 nm. b: Microbeam observation method with a beam diameter of 5.5 nm (indicated by r).
The images shown in Figures lOa and b were recorded using the conventional method and indicate a multi-twinned pentagonal decahedron. After confirming via observation ofthe video image that the structure ofthe particle was unstable, the electron microscope was switched to the microbeam mode. On the video screen, shape changes in the microbeam mode continued to be observed. Some of the images are shown in Figures lOb-f. The size of the electron beam probe was 5.5 nm. Note that only a portion of a particle is irradiated by the electrons. In these images, one can see the overall shape ofthe particles and the {Ill} lattice images with the planar spacings of 0.235 nm. It is apparent that the images are different from each other. When a probe with a smaller diameter of 3.5 nm is used, the UFP moved more slowly but it continued to move. Using a model describing the observation of UFPs under conditions similar to those described above, an attempt to estimate the sample temperature during observation was made. The sample temperature was controlled by heat conduction. The difference 129
Figure 10. Comparison of electron micrographs using the conventional and microbeam methods. a and b: a multi-twinned pentagonal decahedron observed by using conventional highresolution electron microscopy. c·f: Electron micrographs of the same particle using the microbeam method. The external shape and interior atomic arrangement can be seen to change. The lattice images correspond to the lattice plane, dill = 0.235 nIn.
between the temperatures of the particle and the substrate, liT, is given by the following equation:
130
Electron Microscopy Studies Here, t is the thickness of the UFP, Jb is the total beam current, K is a constant that is dependent on the geometries and thermal conductivities of the sample and substrate. The factor (dE/dZ)o is Bethe's stopping coefficient for fast electrons, which is dependent on the materials involved. Substituting the size of 2 nm for a gold UFP and other experimental parameters, the particle temperature was calculated for localized irradiation. The result indicates negligible temperature difference; that is, the particle temperature is close to ambient [11]. Conclusions The morphology of crystals in thermal equilibrium can be described in terms of the Curie-Wulff theory in which the surface energy ( fy(n)dS) is minimized. Here, yen) is the specific surface energy of a given crystallographic plane. This concept ignores the roles of potentially influential parts of UFPs, such as ridges and apexes. When the particle diameter becomes small, one can no longer ignore the presence of the atoms in these parts. In this respect, the present study provides guidelines to determine the limit of applicability of the classic theory of crystallographic habit. That is, gold UFPs have well-developed crystal habit even when their diameters are as small as 2 nm. This indicates that the classical theory is applicable down to this size. The observation of even smaller particles remains a challenge for the future. The research team found that the temperature had little effect on the deformation of UFPs. In addition to thermal effects, particle charging was another possible factor leading to deformation. When a UFP was irradiated with an electron beam, plasmons and inner core electrons were excited. The excitation was accompanied by secondary electron emission, so that the UFP became positively charged if it was not grounded adequately. In the process of relaxation following the excitation of an inner core electron having high energy, Auger electrons were emitted. When metallic UFPs were irradiated with an intense electron beam and were simultaneously placed on a substrate of poor electrical conductivity, 131
Ultra-Fine Particles the positive holes created by the secondary electron emission could not be neutralized rapidly. This gives rise to the possibility that the UFPs will become charged for a very short duration [12]. If the surface of the metal clusters contained impurities or was otherwise contaminated, it was likely that the sites containing such contamination would become locally ionized. Instantaneous charging of the UFPs may lead to shear deformation of positively charged particles by Coulomb forces. References
Iijima, S. and Ichikawa, M., J Catal., 94, 313 (1985). Iijima, S. and Ichihashi, T., Phys. Rev. Lett., 56, 616 (1986). Mihama, K. and Yatsuda, Y., J Phys. Soc. Jpn., 21, 1166 (1966). Allpress, J. B. and Sanders, J. V., Surf Sci., 7, 1 (1967). Ogawa, S., Ino, S., Kato, T. and Ota, H., J Phys. Soc. Jpn., 21, 1963 (1966). 6. Ino, S., J Phys. Soc. Jpn., 27, 941 (1967). 7. Marks, L., Phil. Mag., A49, 81 (1984). 8. Iijima, S. and Ichihashi, T., Jpn. J Appl. Phys., 24, L125 (1985). 9. Mackay, A. L.,Acta. Cryst., 15,916 (1962). 10. Iijima, S., Materials Transactions, JIM, 31, 582 (1990). 11. Iijima, S., Microclusters (Sugano et aI., eds), Springer-Verlag, p. 186 (1987). 12. Howie, A., Nature, 320, 684 (1986).
1. 2. 3. 4. 5.
132
2 SYNTHESIS AND CHARACTERIZATION OF ULTRA-FINE PARTICLES 2.1 Synthesis of Compound and Individually Separated UltraFine Particles by the Gas Evaporation Method (by Masaaki Oda)
SYNTHESIS OF ULTRA-FINE PARTICLES Features of the Gas Evaporation Method
Ultra-fine particles can be produced by chemical reaction or by physical processes, as indicated in Table 1. The gas evaporation method is a physical processes and creates UFPs by evaporating metals in a gaseous atmosphere. There are several heat sources for the evaporation process such as arc, laser or resistance heating. Studies have shown that induction heating is the most appropriate method for making a large amount of UFPs with a uniform particle diameter [13]. UFPs produced via the gas evaporation method have the following features. 1. They are of high purity because they are synthesized in a high purity inert gas atmosphere via condensation phenomenon. 2. The particles are produced under conditions of quasithermal equilibrium and have good crystallinity.
133
S :::2'
Tabie i. Methods for Producing UFPs. I
I
.
Method Type
I
Physical Methods
;5 I
I
Method Name
I
I
Principles and Features of Method
.
~.
[
Evaporate metals in a.."1 inert atmosphere a.."1d produce tJFPs by cooling a..."ld condensing "ria collision 'Nit.'I gas atoms. Evaporation is done by induction, resistance, and for high melting point materials, laser, arc and electron beam heating. For hybrid plasma methods, a plasma gun and RF induction heating are combined to generate a high temperature region, into which raw powders are introduced and vaporized. This method is suitable for compound synthesis.
Gas evaporation [1-4]
~
~
~ ~
...... W
~
I
II ~~e~i~al
Sputtering [5]
Evaporation is done by sputtering. This method is suitable for UFPs made from materials that melt at high temperatures. When the gas pressure is low, films are produced.
ivietal vapor synthesis [6]
A metal is evaporated li.ilder vaCtiwu conditions (below 10-3 torr). r,,1.etal and organic solvent vapors are co-deposited on a cooled suhstrate below the condensation temperature of the solvent. The UFPs are made in the matrix of condensed organic solvent.
Vacuum deposition on flowing oil [7}
Metals are deposited on oil films. UFPs with very small diameters «5nm) and sharp particle size distributions can be fanned
Colloid method [8]
Deoxidation ofnoble metal salts with polymer surfactants in an agitated alcohol medium produces colloidal UFPs covered with the surfactants. Tne size distribution is sharp.
MethOdS (Liquid Phase) Alkoxide method [9]
I"'. __:__. I Methods (Vapor Phase)
I
I
I'
of organometallic compounds (10]
.:.:~"
I
Aqueous decomposition of metal alkoxides produces oxide UFPs. Applicable to most elements and suitable for compound particle synthesis. I'
Hydrogen reduction of metallic chlorides [II]
Reduce metal chlorides in flowing hydrogen gas.
Hydrogen reduction of oxides and hydroxides [12]
Reduce a-FeOOH in hydrogen gas at several hundred degrees C. Most current commercial metal UFPs used in magnetic tapes are produced by this method.
I
Synthesis and Characterization 3. The size of the particles can be controlled by adjusting the evaporation source temperature and the helium gas pressure. Furthermore, the particle size distribution is narrow. 4. Any element that can be vaporized can be made into UFPs. To further expand the range of applications ofUFPs produced via the gas evaporation method, we investigated the following areas. • The synthesis of UFPs with compositions that had been thought to be difficult to form via the gas evaporation method because of large differences in the vapor pressures (more than a ratio of 105) of the components used. For example, to successfully make binary systems such as Cu-Zn in which there is little variation in the composition of various particles (compound particle synthesis). • The production of individually separated particles rather than particles that are sintered or aggregated. This is important because in spite of the small size of the UFPs produced by the gas evaporation method, their unique characteristics are lost when they undergo sintering or aggregation. The methods for producing UFPs are discussed and the morphology and catalytic properties of Cu-Zn compound UFPs as well as the morphology of isolated UFPs and their applications are described.
Production of Ultra-Fine Particles by the Gas Evaporation Method Figure 1 shows a schematic diagram of a gas vaporization apparatus for producing UFPs. The equipment consisted of an evaporation chamber, a UFP collection chamber, a vacuum pumping system, and an RF induction heating unit. A few kilograms of metal were melted in a crucible (lOO-mm OD x 100-mm height) in the evaporation chamber. The temperature of the sample was raised 135
Ultra-Fine Particles
collection
system
Figure 1. Diagram of the gas evaporation synthesis apparatus.
further and, as the metal evaporated, helium gas was pumped through a guide tube and introduced into the chamber via the orifice at the bottom. The helium gas pressure in the chamber was maintained at a pre-determined value by adjusting the feed and pumping rates. The atoms that evaporated from the metal source collided with· the gas atoms in the vicinity of the metal source and were cooled and condensed into particles. These particles in tum, collided with each other and grew into larger particles that were swept by the gas into the collection chamber where they were collected on the cooled surface. 136
Synthesis and Characterization The particle diameter of the UFPs could be controlled by the metal source temperature and the gas pressure in the chamber. The diameter could be increased by raising the temperature or by increasing the pressure. It has been shown that when an alloy having elements with similar vapor pressures is evaporated from a single crucible, alloy UFPs with uniform compositions can be obtained [13]. COMPOUND ULTRA-FINE PARTICLES [14] Synthesis Method
Synthesis Conditions. It was anticipated that the large vapor pressure differences between copper and zinc would lead to difficulties in producing alloy UFPs by the gas evaporation method. At an evaporation source temperature of 1500°C, the vapor pressure of copper is 2 torr, whereas that of zinc is 105 torr. When both copper and zinc were melted in the same crucible under such conditions, copper and zinc particles were produced separately [15]. To make alloy UFPs, the low vapor pressure metal, copper, was melted in the crucible and a zinc rod was slowly fed into the crucible by using a special feeder mechanism. The atmosphere consisted of 99.99% pure helium gas at a pressure of 2.4 torr. The evaporation source temperature was 1500-1600°C. Control of the Cu-Zn Composition. Composition was controlled by changing the temperature of the molten copper in the crucible and the zinc rod feeding rate. The diameter of the zinc rod used was 3.2 mm. The rod was covered with a copper pipe (lO-mm OD, 4-mm ID) to prevent melting of the zinc rod due to radiative heating from the melt. By lowering the melt temperature and increasing the feeding rate of the zinc rod, the zinc content of the UFPs could be increased. The UFP aggregates were quantitatively analyzed by using fluorescent x-ray analysis. An electron energy loss spectroscopy (EELS) analyzer attached to the transmission election microscope was used to determine the changes in the composition of individual UFPs. The EELS analysis was useful for identifying atomic elements. 137
Results
Particle morphology_ The UFPs aggregated to form black powders, similar to the UFPs obtained by the conventional gas evaporation method. The UFP components of the powder were observed by using a transmission electron microscope. The images of these particles are shown in Figure 2. The diameters of the individual particles ranged from 30 to 70 om, and the nearly spherical particles were fused together due to sintering. There were no rectangular platelets characteristic of zinc particles present among the particles. These particles, however, have numerous protrusions of2 to 3 nm in diameter on their surfaces.
Cu - Zn Complex Particles
Figure 2. The relationship between the average particle diameter and the collection position of isolated nickel UFPs.
138
~nfflaU.ndC~toiumon
Figure 3 shows the electron diffraction patterns of the samples. These patterns exhibit diffraction rings arising from copper and ZnO crystals. The diffraction rings due to ZnO appear to be broader than those due to the copper, indicating that the ZOO crystals
are smaller than those ofcopper. The EELS analyses indicated that the composition of individual particles differ little from each other. These results suggest that the Cu-Zn particles obtained here were double-layer particles consisting of copper particles with diameters of several tens of nanometers that are covered with ZoO particles of 2 to 3 om in diameter.
Figure 3. Electron diffraction pattern of compound UFPs of copper and zinc.
139
Ultra-Fine Particles Chemical Analyses Figure 4 shows the results from quantitative analyses of CuZn particles using the fluorescent x-ray analyzer. The y-axis indicates the zinc content of the Cu-Zn particle aggregates and the xaxis indicates the feed rate ofthe zinc rod. For a constant copper melt temperature of 1500°C, the zinc content increases linearly with the feed rate. At the higher melt temperature of 1560°C, the amount of vaporized copper increases and the zinc content decreases for a given zinc rod feed rate. Feed Speed of Cu-Zn Rod (mm/min)
10 20 30 100.------.-----,-----,--,----y----r-----,n feed bar size Cu - ~10x c.64 Zn - ~3.2
80
o
-~
60
....... ~
40 ~
1560°C
/~.-----
20
'"o§
. •
o
L.-....L-_----L_------l_ _...l....-_---I....._---'-_ _L.1
0·6
1·2
1·8
Feed of Zn (g/min)
Figure 4. Zinc content of Cu-Zn particle aggregates as a function of the feed rate of the zinc rod (fluorescent x-ray analysis). 140
Synthesis and Characterization Catalytic Activity Measurements [16] The catalytic activity ofthe Cu-Zn particles was measured for the synthesis of methanol from carbon monoxide and hydrogen. The results are given in Table 2 and can be summarized as follows. 1. The Cu-Zn UFPs produced by the present gas evaporation method exhibit equivalent or slightly better catalytic activity and selectivity than catalysts prepared by the conventional coprecipitation method. 2. Samples aged in air show no degradation in their catalytic
activity. 3. No large difference was found in the reactivity between the Cu-Zn UFPs with or without a reduction treatment in hydrogen.
Formation Processes Leading to Double-Layer Cu-Zn Ultra-Fine Particles Here the processes involved in the formation of the Cu-Zn UFPs with a double-layer structure are described. Fundamentally, the UFPs in this study were formed by the condensation of metal vapor due to collision with helium gas atoms, similar to the normal process that occurs in the conventional gas evaporation process. The vapor pressures of the two elements, copper and zinc, however, differ substantially as shown in Figure 5. For example, at 1500°C, the vapor pressure of copper is about 2 torr while that of zinc is 105 torr. This difference increases at lower temperatures. Thus, when copper atoms are cooled by the collisions with helium gas atoms and begin to form particles, the vapor pressure of the zinc is still more than several torr. The vapor pressure of the zinc is well above that of the helium in the chamber, so the zinc exists as a vapor in the chamber. Consequently, the copper particles pass through the zinc vapor as they are being carried by the flow of the helium gas. The zinc vapor 141
s
Table 2. Methanol Synthesis by Catalytic Reaction of Cu-Zn Particles. Catalyst
co
COz
CH30H
CH4
Cz
C3
C4
Conv. (%)
Selec. (%)
~
~
::: ~
...... ~
tv
1. UFP (64: 18)'Rb
17.68
1.06
8.22
0.04
0.08
0.01
0.02
35
98
~
2.NR
12.31
2.11
10.83
0.16
0.05
0.01
0.00
52
98
!")
3. UFP (57:24)R
14.02
0.53
15.38
0.09
0.15
0.02
0.00
54
98
4. NR
13.74
0.82
15.58
0.04
0.16
0.02
0.00
55
99
5. UFP (64: 18)"NR
8.08
0.16
11.47
0.03
0.09
0.01
0.00
60
99
6. Repeat #4
11.89
2.00
14.95
0.18
0.22
0.00
0.00
60
97
7. UFP (57:24)dR
12.60
2.31
17.53
0.03
0.31
0.08
0.05
62
97
8.NR
12.82
2.74
15.64
0.15
0.22
0.03
0.04
60
97
9. UFP"
14.88
0.67
17.56
0.00
0.27
0.08
0.05
56
98
10. Repeat #9
15.43
0.77
16.63
0.00
0.33
0.11
0.07
55
97
a: Cu:Zn wt%; b: R indicates that the catalyst was reduced by hydrogen gas before the reaction and NR indicates that the catalyst was directly used without reduction; c:H2/CO = 3; d: UFPs used about one month after their preparation; e: co-precipitated.
a.. ~
r..!i
Synthesis and Characterization
oMelting Point: I I
I
I I
Zn
o
500
1000
1500 2000
Temperature (OC) Figure 5. Vapor pressures of copper and zinc.
can be assumed to behave as a gas even within the helium atmosphere, so the zinc atoms will collide with the copper particles and repeatedly adsorb and desorb on their surface. The zinc vapor is also cooled by collisions with the helium gas. In the presence of the copper particles, however, it is more likely to have heterogeneous nucleation on the surface of the copper UFPs rather than homogeneous nucleation of the zinc in the helium gas. Thus, a layer of zinc particles is formed on the surface of the copper particles. The zinc particles thus produced are presumably converted to ZnO during the slow oxidation treatment that is used to stabilize the UFPs. Metal UFPs will combust upon contact with air, so they are stabilized by slowly introducing oxygen into the evaporation chamber to form surface oxide layers.
143
Ultra-Fine Particles Prospects for the Future By modifying the conventional gas evaporation method, one can produce Cu-Zn UFPs with little inhomogeneity in their compositions. Such materials are normally difficult to produce due to the large difference in their vapor pressures. These UFPs have a complex morphology and exhibit superior catalytic activity for methanol synthesis. In the future, the present method will be used to produce other binary UFPs in forms that cannot be produced by conventional co-precipitation methods. This should allow for the study of catalysts that have been difficult to study because of effects due to impurities and complex morphologies associated with UFPs produced by the co-precipitation method. It has been demonstrated that one can produce uniform compound UFPs with little segregation, even for the Cu-Zn system, which has a large difference of 105 between the vapor pressures of the two metals. The same method can now be applied to the production of other binary particles consititing of different combinations of elements. For example, other low vapor pressure elements in addition to copper include platinum, iron, cobalt, nickel, chromium, manganese, silver, and others. High vapor pressure elements in addition to zinc include lead, magnesium, barium, and rare earth elements like samarium and cesium.
INDIVIDUALLY SEPARATED ULTRA-FINE PARTICLES [17]
Synthesis Method
Features of Synthesis Method. In the gas evaporation method, individual UFPs produced in a helium gas atmosphere collide with each other while they are being transported by the flow of the helium ~s, When_the_ c.nllisinns- OCCllT_ at_temp.er.ature..,,-hel.ow the particle coalescence temperature, the particles form secondary particles, in which multiple particles are sintered together. These UFPs aggregate further during the collection processes in the 144
Synthesis and Characterization
collection system. Thus, a special technique must be used to collect isolated UFPs before they collide and undergo sintering or aggregation. For this study, a method was selected that allowed the withdrawal of UFPs directly from the gas atmosphere in the region; where the particles were being formed. The method used a sampling tube that extended from a chamber under ultra-high vacuum into the UFP generation region thus, withdrawing the UFPs due to the pressure difference applied across the tube. This method allowed for the collection of individually isolated UFPs that had a sharper size distribution when compared with those produced by the conventional gas evaporation method. Synthesis Apparatus and Conditions. A diagram of the UFP synthesis apparatus used in this study is shown in Figure 6. It consists of an evaporation chamber (1200-mm diameter, 1000-mm height), a collection system placed on top, a vacuum pumping system, and an RF induction heater for the evaporation source. The collection system was comprised of a system for collecting isolated UFPs and one for collecting other UFPs. The isolated-UFP collection system had a reaction chamber, a collection chamber, a sampling tube connecting the evaporation and reaction chambers, and an orifice between the reaction and collection chambers. The sampling tube had an inner diameter of 7 mm, a length of 600 mm, and the orifice diameter was 15 mm. About 99% of the UFPs formed in the vicinity of the evaporation source were transported by the flow of helium gas and collected as aggregated UFPs in the collection system. The remaining particles passed through the sampling tube and were injected into the reaction chamber as isolated UFPs. Isolated UFPs of any desired particle size can be collected by varying the distance between the evaporation source and the end of the sampling tube. The region in which UFPs were formed was a dome-shaped region that extends 100 mm above the evaporation source. The top of this dome was defined as the starting reference point (L=O). The region of sampling was defined as the distance from the reference point in units of millimeters, where the values were expressed as positive integers when the sampling tube was move up from this reference 145
Ultra-Fine Particles
L-N2
Orifice
SoLvent
Figure 6. Schematic of apparatus for producing isolated UFPs. point and as negative integers when the sample tube was move down from the reference point. Isolated UFPs drawn into the sampling tube were accelerated by the flow ofthe helium gas through the tube and attained a velocity of several hundred m/sec at the outlet of the tube. The particles thus entered the reaction chamber as a particle beam. The isolated particles in the beam passed through the orifice into the collection chamber and were collected on a substrate. The reaction and collection chambers were pumped individually to 10-2 and 10-4 torr, respectively. An attempt was made to obtain isolated UFPs of nickel, iron, and iron oxides, among others. In addition to adjustment of the source-sampling tube distance, the temperature of the evaporation, which was varied between 1780 - 2000°C, was used to control the particle diameter. The helium gas pressure in the evaporation chamber was 1.6 torr. 146
Synthesis and Characterization
Collection Method for Isolated Ultra-Fine Particles. The following two methods were used to collect isolated UFPs in the collection chamber.
Collection ofSamples for Transmission Electron Microscope A sampling rod with an attached electron microscopy mesh coated with a carbon film (about 30 nm thick) was positioned such that the UFPs passed by. The sampling rod had a shutter so that UFPs could be collected for a set period of time.
Collection ofSamples in an Organic Solvent A copper substrate cooled to liquid nitrogen temperatures was placed in the collection chamber. The UFPs collided with the substrate and were attached along with an organic solvent, such as ethyl alcohol, which was injected as vapor from a nozzle located below the substrate. Because the substrate was cooled to the temperature of liquid nitrogen, the solvent immediately froze upon contact and the UFPs were trapped within the frozen matrix. Thus, a mixture UFPs and ethyl alcohol was accumulated on the substrate. Synthesis of Isolated Ultra-Fine Particles of Iron Oxide Oxygen gas was introduced into the reaction chamber during production of iron UFPs. While the iron UFPs were moving through the chamber, they reacted with the oxygen to produce iron oxide UFPs. The oxygen pressure was set at 1.9 x 10-2 torr. Through a monitoring window on the reaction chamber, it was possible to observe a red glow due to the oxidation of the UFPs. The red glow was analyzed using an optical multi-channel analyzer, from which the emission spectrum indicated that the particle temperature was about 1000°C [18]. The oxidized UFPs were collected in the collection chamber after passing through an orifice. 147
Results
Isolated Nickel Ultra-Fine Particles. Figure 7 shows an electron micrograph of isolated nickel UFPs produced under the following conditions: evaporation source temperature of 2000°C, heliwn gas pressure of2.4 torr, and collection position ofL=O for the sampling tube. The particles obtained were clearly individually isolated particles with an average diameter of 16.5 run and a half· width of 7 om. This half-width is ahout one-half of the half-widths of UFPs produced by the conventional gas evaporation method [13]. Figure 8 shows the relationship between the average particle diameter and collection position. As the sampling tube approaches the evaporation source, the particle diameter decreases, reaching an average of about 4 nIn. When the particles were collected in ethyl alcohol, the UFPs were found to be well dispersed in the solvent. Similar results were also observed for iron UFPs.
Figure 7. Transmission electron micrograph of isolated nickel UFPs collected on carbon film. 148
Synthesis and Characterization
Ni Isolated Particles
P: He 1·6 torr o : T 1930·C
-
A :
T 1780·C
o
'-
~ 80 CII
E c o c
40
C
(1)
~
-40
-20
Position
o
20
L(mm)
Figure 8. The relationship between the average particle diameter and the collection position of isolated nickel UFPs.
Isolated Iron Oxide Ultra-Fine Particles. Figure 9a shows an electron micrograph of isolated iron oxide UFPs collected after allowing isolated iron UFPs to react with oxygen. The particles have an average diameter of 24 nm and appear to be well isolated as was found for nickel and iron UFPs. These also have well defined crystal habits. A representative particle was examined by using a transmission electron microscope at a high magnification (see Figure 9b). The lattice image of this particle exhibits no lattice defects, implying that this particle was a perfect single crysta1. Analysis of the electron diffraction patterns of the particles indicated that these were magnetite (Fe30 4). By changing the collection position, we were able to obtain smaller particles having an average diameter of 12 nm and with a similar crystal habit. The Mossbauer spectra of these particles were analyzed and have been reported [19]. 149
•
. • -
•
..
•• • • "of• • • •• •• • •• • • •• • • ••• •• • • •• • • • •
•
(a)· a)
b)
~
~
•
10001.
(b) 501.
Figme 9. a) Transmission electron micrograph of isolated iron oxide (Fe30 4) UFPs obtained after reacting isolated UFPs with oxygen (1.9 x 10-2 torr). b) The lattice image ofa single particle.
150
Synthesis and Characterization Handling Techniques for Isolated Ultra-Fine Particles [20]
Ionization Apparatus ofIsolated Ultra-Fine Particles. To use isolated UFPs, handling techniques that permit the control the kinetic energy stored within them and their directionality need to be developed. In many respects, a control technique that ionizes UFPs and uses static electric fields to control them is superior to other methods that one might envision. The most efficient method for ionizing UFPs is to introduce them into a glow arc discharge under low pressure. In addition to obtaining a high ionization efficiency, the use of reactive gases allows for the synthesis of compound particles. Figure 10 shows a diagram ofthe apparatus used in this study to produce ionized particles. It is comprised of a system to produce individually isolated particles, a glow discharge device for producing a region of ionization in the reaction chamber, an extraction electrode with a 4-mm diameter orifice, a three-stage electrostatic lens system, deflection electrodes, and a film formation chamber. The ionization region is produced by an antenna located near the tip of the sampling tube. The antenna is fed lOO MHZ signals (100 W) from outside through a port. In addition to the helium gas that enters the ionization region from the sampling chamber, hydrogen gas was fed directly to the glow discharge. This region was differentially pumped by a mechanical booster pump to maintain a pressure of 10-2 torr, which is optimum for producing a stable glow discharge. Ionization Method and Film Formation. When 100 MHZ RF was supplied to the antenna in the ionization region, a glow discharge region was generated in the vicinity of the tip of the sampling tube. By applying a voltage of -7.5 kV to the extraction electrode, the glow discharge region was extended to the electrode. This created a cylindrical glow discharge region of about 10 mm in diameter between the sampling tube tip and the orifice located at the center ofthe extraction electrode. The UFPs produced in the reaction chamber have a velocity of over several hundred m/sec upon exiting the sampling tube. So, these pass through the glow discharge region and are positively or negatively ionized. Because the extraction 151
Ultra-Fine Particles
•
=O--50KV
Lens
Orifice
RF ---il-------:;I Pipe
Figure 10. Schematic of apparatus for producing isolated, ionized particles. electrode had an applied potential of -7.5 kV, only positively charged particles and gas ions are accelerated toward the electrode. Except for those particles that attach to the electrode, most of the particles and ions pass through the orifice and reach the electrostatic lens system, where the particles are converged. The particles and ions are further accelerated by the electric field due to the high voltage applied to the substrate. After that, the particles collide with the substrate.
152
Synthesis and Characterization In the film formation chamber, there is a substrate holder that can hold up to four substrates, to which a dc voltage of 0 to 50 kV can be applied through a metal clip. A -10kV dc voltage was applied to the first and third stages ofthe electrostatic lens system. By changing the potential applied to the second stage between 0 and -10 kV, the convergence of the particle beam was controlled. A potential of ±1.5 kV was applied to the deflection electrodes to change the direction of the beam. For the ionization experiment, isolated iron UFPs were mainly used.
Results for the Handling Techniques
Particle Ionization Efficiency and Rate ofFilm Formation. By deflecting the particle beam, it was found that over 99% of particles were ionized. The rate of film formation was about 100 nm/min. Dependence of Coercivity and Electrical Resistivity of the Films on Acceleration Voltage. Figure 11 shows the B-H curves for the films formed from isolated iron UFPs. Figure 12a shows the relation between the coercivity and the accelerating voltage. Using particles having an average diameter of 13.9 nm, the results indicate that the coercivity decreases from Hc=325 Oe at zero acceleration voltage as the voltage increases. In particular, the decrease becomes large beyond -30 kV. At -40 kV, the coercivity decreased to 10 Oe, 1/30 of that at zero potential. Similar behavior was seen for particles with a smaller diameter of 11.1 nm, but a drastic reduction in the coercivity appeared to occur at lower voltages. The changes in the electrical resistivity measured at 4.2 and 274K with acceleration voltage are given in Figure 12a. Again, a drastic decrease occurred with increasing acceleration voltages. Film Morphology. Figures 13a and b show scanning electron micrographs of the films that were formed. Figure 13a shows an image of a film produced at -10kV and exhibits aggregated particles, which still retain the morphology of individual particles, as well as rough surfaces. These findings coincide with the black appearance of the UFP films to the naked eye. Figure 13b shows a micrograph 153
Ultra-Fine Particles
B
(0)
B
(b
Voce =1 OkV
Voce =0 kV He =3250e
He
=3000e
2.0 L5 2.0 H(KOe)
Voce =20kV He =1250e
20 1.5 1.0
J} B
2.0 l.5
LO
(d)
B
Voce=30kV He =450e
7 05 1.0
2.0 15
H(KOe)
(c)
B
0.5
Vocc=40kV He =130e
1.5 2.0
0.5
1.5 2.0 H(KOe)
0.5
l.0 1.5 2.0 H(KOe)
1. (e)
1.0 05
2.0 1.5 1.0 0.5 LO 15 2.0 H(KOe)
B
Voee = 50kV He =IOOe
(f )
05 0.5 1.0 1.5 2.0 H(KOe)
Figure 11. The B-H curves of films fonned from isolated iron UFPs. The average diameter of the particles was 13.9 nm and the acceleration voltage was a: 0 kV, b: -10 kV, c: -20 kV, d: -30 kV, e: -40 kV, and f: -50 kV. 154
Synthesis and Characterization
(a)
400~
ell
o
(
......
't)300:I:
o
.... ....
,
o 139 A o A 111 A
"0, \
\ \
\ \
,,
\
,,
-
I
I
,, ,
I
ell
> .u
"eli
o
U
Q \
\
100~
-
\
\
\
\ \
,,
, 0....
o
I
I
10
20
-
-
-
A
...... ..
"9-----J"lIy'
~_ _ _..LA
30
40
50
Accelerati on Voltage Vac(KV) Figure 12. The change in the coercivity and electrical resistivity with acceleration voltage. a) Coercivity vs. acceleration voltage for films formed from isolated iron UFPs. 155
Ultra-Fine Particles
(b) ~~
~
"" " \\
\\
'\
'\
.
\, '\ '\
o 274K 6. 4.2 K
'\
"
""'\
"'b
JiI,
"'\
",\ \,
,,\
\\
\,,\
'\
"\,,, \, \\
",',,0-
.~
_ _ ----0-
h-
U
(J)
- - - --6_
0.
(/)
o
"
10
20
30
"
40
50
Acceleration Voltage Vac(kV)
Figure 12, continued. b) Electrical resistivity vs. acceleration voltage. 156
Synthesis and Charaderization
Fig. 13 Scanning electron micrographs of the films formed from isolated ionized iron. Acceleration voltage of a: -10 kV and b:-40 kV.
157
Ultra-Fine Particles
of a film produced at -40 kV. Here the particles were sintered together and the grain diameters grew to 50 to 60 nm. The film surface appears smooth, which is in agreement with the metallic luster of the material as observed by the naked eye. This indicates that the higher kinetic energy ofthe particles leads to their conversion to bulk metal. Prospects for the Future [21,22]
The particle collection mechanisms of the gas evaporation apparatus were modified, which allowed the production of high purity UFPs with good crystallinity. The modification also allowed the production of isolated UFPs that were not sintered or aggregated. Isolated UFPs can be made from any material that can be used in a gas evaporation apparatus that uses induction heating. During this study Fe 30 4 particles were produced by reacting iron UFPs with oxygen gas in the reaction chamber. By using discharge phenomena in the reaction chamber, it should become possible to produce isolated non-oxide compound UFPs such as nitrides. Isolated UFPs may be passed through a metal vapor in the reaction chamber, allowing for the synthesis of complex isolated particles with unique surface coatings. Biological and medical applications of isolated iron UFPs collected in an organic solvent have been examined [23]. A technique for coating UFPs dispersed in an organic solvent with a polymeric film was developed [24]. Using this technique, antibodies can be supported on the polymeric coating. By combining the magnetic UFPs with a particular antigen, such as a cancer cell, via an antigen-antibody reaction, the cell may be separated and purified using an external magnetic field. It is also possible to accelerate ionized particles for the study of collision processes at a solid surface or for applications such as surface polishing. For future UFP investigations, it is important to obtain particles having a uniform particle size. Fractionization by size may be feasible by the use of static electric fields.
158
Synthesis and Characterization References Yatsuya, S., Oyo Buturi 41:604 (1972). Iwama, S., Jpn. J. Appl. Phys. 12:1531 (1973). Uda, M., Jpn Soc. Met. Meeting Abstract, No. 88:185 (1981). Yoshida, T., J. Appl. Phys. 54:640 (1983). Yatsuya, S., Powder Met. Meeting Abstract, p. 618 (1985 Spr.). Matsuo, K. and Klabunde, KJ., J. Catal. 73:216 (1982). Yatsuya, S., Jpn. J. Appl. Phys. 13:749 (1974). Hirai, E., High polymer complex catalysts (ed. Koubunshisakutai Kenkyukai, Koubunshisakutai --Kinou to Ouyou, No.2), Gakkai Shuppan Center (1982). 9. Ozaki, Y., Kogyo Zairyo 29:85 (1981). 10. Thomas, J. R., J. Appl. Phys. 37:2914 (1966). 11. Yoshizawa, A.: Jpn Soc. Met. Meeting Abstract, p. 62 (1981 Spr.). 12. Imaoka,T., Oyo Jikigakkai Kenkyukai Shiryo, MSJ18-9 (1981). 13. Oda, M., Kotai Butsuri, Tokushugou Chobiryushi, p. 103 (1984). 14. Oda, M., Oyo Butsuri 56(3):395 (1987). 15. Oda, M., Powder Met. Meeting Abstract,P164 (1985 spr.). 16. Hayashi, T., Appl. of Gas Evaporation UFPs to catalysts, this book. 17. Oda, M., Jpn J. Appl. Phys. 24:L702 (1985). 18. Johgo, A., Proc. Int. Conf. Photochem. p. 361 (1985). 19. Saegusa, N., Mossbauer Effect of Fe UFPs, this book. 20. Oda, M., Exploratory Science, Hayashi Ultra-Fine Particle Project Final Report (1987). 21. Oda, M., Proceedings of the 8th International MicroElectronics Conference (lMC94), Omiya, Japan, April, 2022 (1994). 22. Oda, M., Denshizairyou (Electronic Materials), Vol. 10 (1994). 23. Hayashi, C., Kagaku Sosetsu UFPs (Jpn Chern. Soc.) 48:10 (1985). 24. Kakuta, H.: 37th Colloid Interface Chemistry Meeting Abstract (Oct. 1984).
1. 2. 3. 4. 5. 6. 7. 8.
159
Ultra-Fine Particles
2.2 Aerothermodynamics of UFP Synthesis (by Shunichi Tsuge)
Developments in aeronautics and rocket technology hastened the advances of aerothermodynamics, a term coined by T. Von Karman, a pioneer in the field of modem aeronautics. For example, combustion, a closely related area dealing with the flow within engines and other systems was not long ago thought to be an empirical science, but can now be constructed from first principles. In terms of fluid mechanics, the phenomena that occur within a UFP synthesis chamber during gas evaporation are far simpler than those occurring within a diesel engine, where such processes as the production of soot (a form ofUFPs) and combustion occur. Thus, the processes associated with UFP production are suitable subjects for study in terms ofthe aerothermodynamics that are involved. Ultra-fine particles are situated between the molecular (microscopic) and particle (macroscopic) levels and exhibit unique properties. It is known that parameters controlling the synthesis of UFPs are basic thermodynamic factors. That is, the pressure in the synthesis chamber and the surface temperature of molten metal control the particle size and the rate of synthesis. This implies that the phenomena inside the evaporation chamber can be described in terms of macroscopic thermodynamic quantities even though the chamber is evacuated. This is because the chamber pressure is typically 10-2 atm and the mean free path is around 10 Ilm based on extrapolation from the atmospheric value of 10- 1 !-tm (the mean free path is inversely proportional to the pressure). This value is still much smaller than the characteristic distance controlling the synthesis (e.g., the size of a crucible), thus the principles of fluid mechanics are still applicable. A diagram of the flow within the synthesis chamber is shown in Figure 1. The metal is melted in a crucible at the center and the metal evaporates, creating an upward flow of metal vapor. This induces a forced flow in the surrounding inert gas (generally helium, but on occasion, argon) that has a similar flow velocity. The major 160
Synthesis and Characterization
y
UFP formation area
Inert gas flow
Inert gas flow
Figure 1. Diagram of the flow inside a reaction chamber. roles played by the inert gas are as follows. •
The gas cools the metal vapor at high temperatures and forms metallic droplets or UFPs.
•
The gas carries the UFPs thus created to the collector on top.
•
The gas diffuses into the metal vapor and dilutes the metal vapor above the liquid surface, thereby promoting further vaporization and increasing the yield of UFPs.
From both practical demands and academic interests of aerothermodynamics, there are two central problems to be solved. Which parameter controls and determines (qualitatively) 1) the rate ofUFP synthesis and 2) the particle size of the UFPs. The first point is of interest in terms of mass-producing UFPs and the second is the most important factor related to the quality of the UFPs. Next, the equations governing these factors are considered. 161
Ultra-Fine Particles
Governing Equations of a Mixed-Flow of Vapor, Inert Gas and Ultra-Fine Particles The mechanical state of a single component fluid at a point, x, at time, t, is determined by the velocity, u. In principle, the thermodynamic state of the fluid is defined by two thermodynamic quantities, such as the pressure, P, (or density, p) and temperature, T. In aerothermodynamics, mechanical and thermodynamic quantities are not independent of each other, and the physical quantities take a form of simultaneous equations in terms of (u, P (p ), and T). These quantities are obtained by solving one vector equation and two scalar equations. These are the conservation of mass, as shown in Equation 1. ap/at + div (pu) = 0
(1)
The conservation of momentum (Navier-Stokes) equation, as shown in Equation 2. apulat + div (puu
+ P) = 0
(2)
And the conservation of energy (the first law of thermodynamics) equation, as shown in Equation 3.
(alat)p (u 2/2
+ e) + div [pu (u 2/2 + e) + Pu + Q] = 0
(3)
Here, P is the pressure tensor, e is the internal energy per unit mass, and Q is the heat flux density. Assuming ideal gas behavior, we have the following relationships. P
= pRT
(R
= gas
constant per unit mass)
162
(4)
Synthesis and Characterization
e = c vT
(5)
The pressure tensor is given by the sum of the scalar pressure (POjj) and the viscous stress expressed by the velocity gradient. The heat flux density is given by the Fourier law, -Agrad T (where A is the coefficient of the thermal conductivity). Taking u, p, and T as dependent variables, by taking Equations 4 and 5 into consideration, Equations 1-3 constitute a closed system for the five unknowns. When one deals with a multi-component gas, such as a mixture of metal vapor and inert gas, the average values of the thermodynamic quantities are used in these equations. In addition, equations governing the mass fractions of the constituents (Ya) must be added. That is, one has the following equation. (6) Here, Va, is the diffusion velocity of the a-component fluid and is generally given by Fick's law as - DgradYa (D: diffusion coefficient). Wa is the amount of the a-component fluid produced per unit volume and time. Physically, Wa*O implies the existence ofphase change. This term plays an important role in the flow of mixed phases of liquids and gases that contain growing groups of droplets with a particle size distribution. To describe this new degree of freedom, we need an additional dependent variable, Y(v ), the number of molecules in a droplet, with v» 1. This can be treated as a continuous variable. Y(v) also obeys Equation 6. Here, the diffusion coefficient D(v) can be given as follows when v is not too large and it can be treated as a molecule.
Dv =
3 8n(OUl)2
[kT(m 1 +m u) 21tm 1m U
163
]112
(7)
Ultra-Fine Particles For particles with very large values of v, which should be treated as Brownian particles, the Einstein relation may be used.
(8) It should be noted that Tl is the density of all the particles (number per milliliter), k is the Boltzmann's constant, m v (= mv), r v (= r l V 113 ) are the mass and radius of v-particles, 0vl (= rl + ~ ) is the collision radius between a molecule and a v-particle and !l is the viscosity coefficient of the surrounding gas. According to Taniguchi [1], the rate of synthesis of vparticles, W(v ), is given by the following equation.
u
m u- W(u) = 1/2!P(U /,U-u)n(u/)n(u -u/)du l 1
o - !B(u,u/)nUn(U/)dul + (Q/Qu)[b(u)n(u)]
(9)
n
Here, n(v) is the density ofthe v -particle, P(v, Vi) is the number of collisions between v and v I -particles per second, and b(v) is the number of molecules that vaporize from the surface of a single vparticle per second. The first term expresses the formation of vparticles and (v, v')-particles. The second term is the annihilation of v-particles by collision with other particles and the third term is the difference (or differential) between the synthesis of v-particles due to the evaporation of one molecule from (v+1)-particles and the annihilation due to the evaporation of one molecule from v-particles. By substituting Y a with Y(v ) in Equation 6 and combining this with Equation 7 [or 8] and 9, one obtains a differential integral equation in five-dimensional space (x, t; v).
164
Synthesis and Characterization The above system of equations describe the macroscopic behavior of UFPs. This system constitutes a new area in aerothermodynamics, and contains a potential approach to unresolved problems in combustion such as soot formation, which is important in diesel engine design.
Distribution of Metal Vapor in the Chamber and Evaporation Rate The formulation in the previous section is logically complete but it is impractical for solving the problem of this section, as expressed in the title. To treat this problem analytically, one must use a two-dimensional (u = (u, v, 0», steady state flow ( %t=O) approximation and assume the streamlines to be nearly vertical, or that u/v =dx/dy «1. Thus, a solution to Equation 1; op/ot + div (pu) = 0 is obtained as follows. pu = m :constant
(10)
Here, m is the mass flux density of the metal vapor at the surface where vaporization takes place. Next, we assume that the Mach number of the flow is small and that p »pu2 , and Equation 2 becomes the same as if the pressure is constant. Using these approximations in Equations 3 and 6 and ignoring the condensation of vapor (W=O), the governing equation for temperature, T, and mass fraction, Yet. (ex = 1, 2), is as follows. (11)
Here Z is either T or Yet. and E is )J cpp or D, respectively. Taking the index for metal vapor as ex =1, the boundary condition for Y=O is as follows.
165
Ultra-Fine Particles
Y l = EOY/Oy = m (metal surface; Ixl ~ a) o (for others; Ixl > a) T=
(12)
Tb (metal surface) Too (for others)
When y goes to infinity, one has the following relationships (13)
This equation can be solved analytically. Figure 2 shows isothermal curves as solutions to the equation. The shape agrees qualitatively with the actual data observed [2].
y50
4.0
3.0
.yo
~,
0
2.0
~
1.0
~/
C)G ~
1.0
2.0
Figure 2. Isothermal curves in a furnace (8 166
30 =
40x
(T - To)/(Tb - To)) [1].
Synthesis and Characterization Figure 3 shows the distribution of the partial vapor pressure PI near the evaporating surface of the iron in helium and argon atmospheres. Due to the low molecular weight of the helium, the metal vapor diffuses rapidly into the surrounding gas, leaving the vapor pressure at low levels near the metal surface and promoting further evaporation. The amount ofevaporation is given by the following equation.
m
(14)
Here, Ps = P (T) is the saturation vapor pressure given by the Clapeyron-Clausius equation, PI is the metal vapor pressure on the evaporating p plane, and is equal to the following. (15)
Here, p is the pressure in the chamber and M o is the molecular weight of the inert gas. Because the mass fraction of the metal vapor Y I is itself dependent on m, Equation 14 is an Eigen value equation involving m. The actual amount of evaporation, m, is much lower than that obtained by setting k = 1 in this expression (the Langmuir equation), which may be due to gravity. According to a microgravity experiment [4], evaporation is very vigorous under microgravity, which indicates that the Langmuir equation is valid in zero gravity. In ordinary treatments of evaporation, gravitational suppression effects are minor. Thus, a new concept must be introduced to explain the observations described here. Still, it should be noted that Eq. (14) with an evaporation coefficient of k = 5 x 10-3 provides results in good agreement with the experimental data (see Figure 4).
167
Ultra-Fine Particles
Fe vapor in He
-1
o
x/o
Fe vapor in Ar
-1
o
x/a
Figure 3. Partial vapor pressure above the surface of a metal evaporating in an inert atmosphere [1]. 168
Synthesis and Characterization m.------....-----....,....------,-r-~
(kg/m2 'sec)
.004
I
I
I
I
I
I
I
(ada, )985)3)
I
____ --..L - - - - ---+-- - - - -
o : Exp. I I I I
I
I I
I I
I
I
I
I
I
I I
1
.003 ------T-------+- --
.002
I ------,----r
I I
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I
.001 - - - ---t-O
I
I
()
I I
___ 1I
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-+-I I
I
I I
____ ..i __ I
I I I I
+- __ I
I
I I
I I I
2000
2100
2200
2300 T(k)
Figure 4. Dependence of the amount of evaporation on the temperature of the evaporating surface [1]. References
1. M. Taniguchi, M.S. Thesis, Tsukuba University (1986). 2. M. Oda, Kotai Butsuri, UFP Issue, p. 104 (1984). 3. M. Oda, Doctoral thesis, Nagoya University (1986). 4. N. Wada, Gas Evaporation in Non-Gravity, this book.
169
Ultra-Fine Particles
2.3 In-Flight Plasma Processes (by Toyonobu Yoshida)
Processes used to manufacture materials by injecting a solid, liquid, or gas into a thermal plasma to use its high enthalpy and reactivity are referred to as in-flight plasma processes (IFP processes). An example of an industrial application using this method is the thermal decomposition of zircon sand (ZrSi04) into a mixture of zirconia and silica [1]. When zircon sand powders under 44 !lm are treated in a kiln furnace at 2000°C for 24 hours, only 50% decomposition occurs. Using an IFP process with a plasma furnace, 75-150 flm zircon sand can be decomposed completely in 3 ms. The relative reaction rates differ by a ratio of about 107 • The size of the reaction chamber can also be reduced by the same ratio from a furnace 3 m in diameter and 10m long to a plasma furnace 30 mm in diameter and 100 mm long. Also, batch processes can be converted to continuous processes, and the simpler processing scenario reduces the scale of facilities and number of operating personnel. If a suitable production application were developed, it would be possible to commercialize the IFP method. The IFP processes have been applied to the synthesis of fine particles for about 20 years, but more demanding reqirements for the raw materials used to make ceramic powders have drawn renewed attention to the processes in recent years. In the US, Alcoa has constructed a 10 kg/hr pilot plant for the production of fine particles of TiB 2 , SiC, and ij C as raw materials for sintering based on a process using a pure hydrogen plasma reactor [2]. This field is expected to develop in Japan in the near future. This section describes the features and problems associated with using IFP processes to produce fine particles. For an introduction to IFP processes, see References 3-6.
170
Synthesis and Characterization Plasma Generation Method For In-Flight Plasma Processes Figure 1 shows four types of plasma torches used for IFP processes of up to 100 kW in power. The direct current (dc) jet torch shown in Figure 1A is widely used as a plasma spraying torch. It produces an arc discharge between two water-cooled electrodes and the plasma jet is ejected from the anode nozzle. An argon plasma of about 80 kW that has been generated in air reportedly produces a temperature of about 11600K and a velocity of about 410 m/sec at a distance of 25 mm from the outlet, but the temperature reduces to about 20% of its maximum value at a distance of 80 mm from the outlet. The reaction time for reactants injected near the anode at high temperatures is about 400 IlS and the gaseous raw materials are mainly used for the synthesis of fine powders. Notably, the mixing of atmospheric gas reached 80% at a distance of 80 mm from the inlet [7]. This phenomenon was used to produce nitride fine powders by flowing NH3 gas from a tube outside the plasma jet [8]. The installation of the tubing aids in effectively mixing the reactants and in extending the high temperature region of the plasma jet. The reaction time of the reactants is estimated to be 1 ms. A radio frequency (RF) plasma torch is shown in Figure lB. The plasma generation principle is similar to induction heating of metals. The injected gas is heated to 8,000-10000K by an induction coil and forms a low speed arc of about 30 m/sec, which is ejected from the torch outlet. The reaction time is 5 to 10 times that of a dc plasma. Thus, by using this method, the evaporation of solid particles is possible. Because RF plasmas are generated without electrodes, reactive gases can be used. In particular, oxygen plasma generation becomes important in connection with high temperature superconductor synthesis. Numerous torch designs have been proposed and 1 MW torches are commercially available. Basically, these were developed based on the torch built in 1961 by Reed [9]. Figure 1C shows an RF-DC coupled plasma torch, where the RF field is directly coupled to a dc plasma jet, thus adding power. According to spectroscopic measurements [10] of the temperature at the center axis, the temperature is higher at the exit than at the inlet 171
Sl ~ I
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,
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t I
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Figure 1. Four types of plasma torches used for the IFP process. A) dc plasma torch; B) RF plasma torch; C) RF-DC coupled plasma torch and D) RF-DC hybrid plasma torch.
~
~' ~ t"l
Synthesis and Characterization at the coil section, implying that the RF power heats the outer region of the dc plasma jet, which in turn heats the tail section via thermal convection. In this study, the effect appeared to extend the length of the jet by threefold to about 200 mm. This type of torch has not been applied to the IFP process, but is likely to be more effective for the control of fine powder synthesis processes. Figure ID shows a hybrid RF-DC plasma torch, where the RF and dc plasmas are superimposed [11]. This was developed based on the need to supply energy to maintain the plasma when the eddycurrent in the RF plasma was disturbed by the injection of material in the axial direction. This approach was intended to stabilize the RF plasma. Indeed, one can increase the amount of reactant injected into the plasma by a factor of 20 for this torch compared to the RF plasma torch. The different plasma torches have been briefly described above. Note that the type shown in Figure ID produces all of the other types of plasmas. Figure 2 shows the details of the hybrid plasma torch that was developed. Figure 3 shows the results from numerical calculations of the flow within RF and hybrid plasma torches as well as the temperature distributions. The differences in temperature distributions between the plasma torches exist only in the dc arc jet portion, but the flow characteristics are distinctly different. These may explain the differences in the torch behavior.
UFP Synthesis Using The In-Flight Plasma Method
Problems Associated with the In-Flight Plasma Method. Figure 4 illustrates the problems associated with the IFP method and points out the areas in need of change for improved efficiency. The IFP process can be separated into three steps or regions as follows: 1. Heating and decomposition of the injected material, 2. Reaction in the tail flame, and 3. Product recovery.
173
Ultra-Fine Particles
(SiCI 4 +Ar)
Water out o
en
(Y)
------
o o o
.........
~ ~
o
Induction coil
-
Waterin
NH3 or CH 4 + H.'--.---
Figure 2. Structure of the hybrid plasma torch. In the first region, it is important to uniformly inject materials into the high temperature part of the plasma. A number of problems remain due to the presence of a steep temperature gradient at the plasma boundary and because of the high viscosity of the plasma. Both of these factors act to prevent injection of the material into the plasma. Material injection and plasma generation require conflicting conditions. Regarding the RF plasma, it is desirable to development a power supply that is capable of effectively providing power even when the impedance changes due to the injection of material. In the second region, it is necessary to produce reactions uniformly and 174
0 1
2
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Figure 3. Results from numerical calculations of the flow and temperature distribution within RF and hybrid plasma torches.
Q ~ ~
~
~.
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;::
Ultra-Fine Particles
Penetrati on disturbance cooling
t
Injection zone Plasma generation controll efficiencies Input power
Heatin! dissociation zone
t
Thermal histories Trajectories Efficiencies ~ ,
Reaction zone
t
Deposition or collection zone
~
-----+--...
Reaction histories Quenching Mixing Temp. controll Collection Handling
1 Figure 4. Problems and areas in need of improved efficiency for the IFP method. under control. In particular, the synthesis ofUFPs requires advanced control of nucleation and growth processes using high efficiency quenching techniques. In the third region where recovery takes place, filters and cyclones are usually used, but these are not ideal and electro-static and liquid collection methods have been considered. 176
Synthesis and Characterization Recent advances in plasma diagnostic methods are substantial and process design based on the modeling of the reactor has become feasible. With the IFP method, industrial production of fine powders is currently possible. Problems have shifted to areas such as highlevel compositional control, crystal structure, particle diameter and size distribution.
Example of Fine Powder Synthesis by the In-Flight Plasma Method
In-Flight Plasma Evaporation Method. Figure 5 shows a diagram of the IFP evaporation method, which consists of the stage where the evaporation of a solid particle or a liquid droplet is used to generate a high temperature vapor and the fine powder synthesis stage where fine powders nucleate and grow in the cooling region. Calculations show about 8 ms are required to completely evaporate iron particles that are 30 !lm in diameter in an argon plasma at 9000K. Thus, conventional dc plasmas (Figure1A) cannot be used and RF plasmas are generally used. Experimental results indicate that 3 !lm diameter powders can be vaporized at a rate of 20 g/min using an apparatus that generates a 100 kW RF plasma [13]. This implies that fine powders such as iron, nickel, cobalt, titanium, etc. can be produced at a rate of 1 kg/hr. The main feature of this method is that it can be used to produce UFPs of alloys [14,15]. This method allows the generation of high temperature vapors of multi-component systems within the same space and enables the synthesis of alloy fine powders of high temperature phases and non-equilibrium phases. In systems with vastly different vapor pressures, special fine powders can be produced. For example, refractory metal fine powders with a surface coating of low melting metal layers and semiconductor fine powders supporting metals on their surface can be synthesized. These can be used in sintering as well as for catalysis and electronic applications. Reactive In-Flight Plasma Evaporation Method. This method is a modification of the IFP method discussed above and involves the introduction of chemical reactions at the fine-powder 177
Ultra-Fine Particles
Mixed powder
Plasma heating and evaporation
Mixed vapor
Rapid quenching
Alloy powder
Figure 5. Flowchart for the IFP evaporation method. synthesis stage. To produce oxides, carbides, and nitrides, gases such as oxygen, hydrocarbons, nitrogen or ammonia, respectively, are mixed into the plasma gas or injected into the tail flame. This method appears to be suited to the formation of high-purity compound fine powders and produces no by-products. Until recently, this method was considered too costly for scale-up in comparison to the IFP-CVD method. The use of a 100 kW plasma torch, however, has been found to allow 1 kg/hr production rates of TiN and SiC, therefore, commercialization is feasible. This method can also be applied to the production of fine powders having no suitable precursor gas for CVD based production methods. For instance, in producing fine powders of a high-temperature superconductor ceramics of the Y-Ba-Cu-O system, the CVD method cannot be used because appropriate and inexpensive precursor gases are not available. With the present method, it is possible to make superconducting fine powders by mixing the desired amounts ofY20 3 , BaC03 , and CuO and injecting them into an oxygen plasma while controlling the cooling process. 178
Synthesis and Characterization IFP-CVD Method. This is a variation of the CVD method used to make thin films. Most of materials that can be used in CVD processes can be made into UFPs by this method. In particular, the PPG process is said to have a production capability of 10 kg/hr for TiB 2, SiC, and B 4C UFPs. Alcoa licensed this process and has built a pilot plant. The process has been disclosed [16] and is described below. Figure 6 is schematic illustration of an apparatus using a 100 kW plasma torch with multiple raw material inlets. The operating gas is pure hydrogen and is introduced near the cathode via a high speed rotating component that produces better plasma jet stability and increased electrode life. The rotating flow is diminished by hydrogen that is injected just below the anode. This extends the reaction time and increases the production yield by increasing the axial component ratios. In the production of TiB 2 fine powders, a mixture of hydrogen chloride and hydrogen is injected from the top reactant inlet, which prevents the accumulation of powders at the reactant inlet, and allows for extended operation. The TiCl4 and BCl3 are injected through the second and third injection inlets, respectively. The total amount of gas is about 500 Vmin and the tail flame is not forcibly cooled. Thus, the particle size depends strongly on the amount of injected reactants. When several hundred grams per minute of reactant are injected, the particles produced have diameters of about O.3~m and possess good sintering characteristics. Similar techniques with appropriate starting materials can be used for the synthesis of SiC and B4C. This example demonstrates the feasibility of using the IFP-CVD method for commercial production of ceramic fine powders. However, the reaction processes are difficult to adequately control and the particle size is strongly affected by several parameters, in particular, the reactant injection rates. It is necessary to develop a quenching method at the tail flame to obtain particles with the desired particle size distribution. A description of the synthesis of Si 3N 4 and SiC follows.
179
Ultra-Fine Particles
Figure 6. Diagram of the apparatus for the PPG process.
Synthesis of Si3N4 and SiC Fine Particles Via Hybrid Plasma
Thermodynamic Considerations. Fine particle synthesis using plasmas relies on non-equilibrium processes. Thermodynamic equilibrium theory cannot be applied directly, but is useful in obtaining some guidelines for these processes. The tail flame temperature is estimated to be 3500-4000K therefore, the calculations were done using a temperature of 3500K. Here the Ar-H2-SiCI 4-NH3 system is referred to as SN and the Ar-H2 -SiCI 4 -CH4 system as SC. The method of calculation is given in ref. 5. In the following figures, the ordinate indicates the amounts synthesized in moles per minute at each temperature. The range covers the experimental conditions. 180
Synthesis and Characterization Figure 7 shows an example using the SN system. Within the assumed temperature range, the main species are Hb N2 , and HCI, which change little. At temperatures below 500K, these species decrease rapidly as NH3 becomes stable and condensation ofNH4CI begins. That is, when the particles are collected above 500K, NH4CI is unlikely to contaminate the product. In regard to the fraction that reacts, the concentration of SiCl2 becomes comparable to that of Si N 4 at around 2000K. In the conventional CVD method, where the reaction occurs at 1800-2000K, a large amount ofH2 is required to reduce the partial pressure ofSiCl2• In the plasma, most of the SiCl2 is decomposed to Si(g) and CI at high temperatures and the reaction rates for Si+2CI-+ SiCl 2 are negligibly slow compared to the reverse reaction. Thus, once decomposed, the SiCl2 concentration is considerably less than the equilibrium value. It is possible that liquid droplets of silicon are formed in the temperature range of 1974-2325K. Here, the maximum supersaturation ratio of Si(g) is about 3. According to the Lothe-Pound theory of nucleation, this does not satisfy the condition of homogeneous nucleation. However, at high injection rates, this may occur. Because the temperature range for the formation of Si(l) is narrow, the appearance of Si(l) can be minimized by non-equilibrium processes. When the formation of Si(l) is suppressed by some means, the main reaction for Si3N 4 synthesis is Si(g)+ N 2• Using the reactive IFP vaporization method, silicon powder is injected into an N 2 plasma in an attempt to produce Si3N 4 • However, the nitrogen concentration of the powder reaches at most only several percent [20]. This is because the reactivity ofN2 is very low and one needs to make use of the higher reactivities ofN, NH, and NH 2• Figure 8 shows an example ofthe SC system. The molar ratio of CH4 and SiCl4 is assumed to be 1: 1. The main species are H2 and HCI and are only slightly altered. At the precipitation temperature for P-SiC, the amounts of SiC1 2 and SiCI are from 1/3 to 1/5 of the amount of Si(g). To increase the fraction that reacts, these species should be reduced. However, Si(l) and C(graphite) are absent in contrast to the SN system and P-SiC is apparently easier to synthesize than Si 3N 4 • 181
~~
Total mole number ,1
Ar
[
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I
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w
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10- 5 10- 6 10- 7
500
1000
1500
2000
2500
3000
3500
Temperature (K)
Figure 7. Equilibrium composition diagram ofthe Ar-H2-SiCI4 -NH3 system (SN system).
10 1
I
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,
,
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.
.
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Total mole number
Ar 10°
C
~
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Ultra-Fine Particles
The precipitation temperature of SiC is 2850K and fine particles are likely to grow by the Vapor-Solid (V-S) mechanism. Because SiC 2 and Si2C exist in amounts about 400 times more than that of SiC(g), these are expected to play important roles in the nucleation processes. Furthermore, it should be noted that C2H2 is stable at elevated temperatures, so that careful consideration of the CH4 injection method is needed. From these considerations, the following requirements exist for effective production of Si3N 4 and SiC. 1. The SiCl4 injected into the plasma is thermally decomposed into Si + 4CI rather than by H 2 reduction. That is, the hydrogen concentration in the plasma has little chemical effect on the amount of Si produced. In an Ar-H 2 plasma, the H2 concentration does not need to be high in terms of the chemistry of the gas. 2. To minimize the recombination reaction of Si(g) + CI(g) during cooling, it is effective to quench from above 3000K using H2 • 3. In the SN system, N2 starts to form at about 6000 Keven when NH3 is used as the N source for Si3N 4 synthesis. [21].Thus the injection ofNH3 into the high temperature region of the plasma has little effect. Instead, for effective reaction one should use NH x (x = 1.2) radicals that form during the decomposition ofNH3 • Similarly, in the SC system using CH4 as the carbon source, C2H2 starts to form at about 4000K. Instead of injecting CH4 into the high temperature region, one should use CHx (x = 1 - 3) radicals that arise from the decomposition of CH4 for reaction. 4. In the SC system, excess CH4 relative to SiC1 4 is likely to be exhausted as C2H2 • The mole ratio ofSiCl 4 and CH 4 does not need to be 1:1. Based on these requirements, the synthesis experiments used a plasma carrier gas with less than several percent of H2 due to electrical power supply limitations and requirement (l). The reactant, SiCI4 , was injected into the arc-jet region of the hybrid plasma (see Figure 1, requirement 1), and NH3 and CH4 were mixed with H 2 and injected into the tail flame (requirements 2 and 3). This method is referred to as the reactive quenching method. 184
Synthesis and Characterization The Reactive Quenching Method. As noted above, this method can be used to control the particle sizes in addition to the effective use of radicals in the reaction. That is, it is effective in reaction process control and as a method of simultaneously controlling the cooling speed and reaction space. Considering an RF or hybrid plasma with a 10 kW output power level, the energy flowing out of the tail flame is about 3.3 kW. In our experiments, a visible light from the tail region disappeared completely when NH 3 gas was radially injected into the region at a rate of 20 l/min from slits about 0.1 mm in size. Under these conditions, the reaction appears to be complete within ~ 10 mm above and below the slits. In terms of the heat balance, the energy to cool the plasma carrier gas, argon, from 5700 to 2000K at 40 Vmin is almost equal to that needed for the decomposition ofNH3 , converting it to a mixture ofN2 and H 2 gas at 2000K (2.3 kW). That is, the large injection ofNH3 gas is for both reaction and cooling. This method is therefore called the reactive quenching method. To obtain comparable cooling by using N 2 or H 2 gas, a gas injection rate of 55 60 lImin is required. To study this method more quantitatively, a water cooled ring slit was developed to control the quenching as shown in Figure 9. Using this slit, a mixture of CH4 and H2 was inj ected into the plasma tail and the decomposition and condensation processes were examined. The slit width (W) can be continuously varied between 5 and 400 /lm (± 3 /lm accuracy). Figure 10 shows the conversion efficiencies to C2H 2, when the CH4 flow rate was set to II/min and the & flow rate was varied by adjusting the slit width between 30 and 250 /lm. The gas mixture was injected into the plasma tail and the gas composition was determined using a gas chromatograph. The second curve corresponds to the undecomposed CH4 and the difference between the two curves represents the precipitated carbon particles. The amount of undecomposed CH4 increased as the 2H flow was increased because of the decrease in the temperature of the plasma tail. This effect is more significant for the 30 /lm slit width. When the H2 flow rates are low, the CH4 decomposes completely, being partly 185
Ultra-Fine Particles
,I I
Plasma
I I
I I I
Quenching gas
Water
Figure 9. A water-cooled ring slit for controlling quenching. converted to C2H 2 and producing carbon particles. Thermodynamically, ideal quenching from 2500-3000K should completely convert CH4 into C2H2 • In the plasma tail where natural cooling occurs, a part of the C2H2 appears to precipitate as carbon particles. At hydrogen flow rates below 10 l/min, the effects of the slit width are absent and quenching is obviously ineffective. At an H2 flow rate of 50 l/min, all of the CH4 that was available for reaction was converted to C2H2 and almost no carbon precipitation occurred. This is most significant at a slit width of 30j.lm, indicating that the 186
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Ultra-Fine Particles quenching is effective. The curve for C2H2 has a peak around an H2 rate of 20 lImin, possibly due to a decrease in the plasma tail temperature and the balance with quenching efficiency. The comparison of the two figures indicates that inhomogeneous cooling occurs in the flame tail for a slit width of 250!lm. Figure 11 shows the results from a calculation of the temperature and flow at the plasma tail flame assuming a slit width of 300 !lm and an argon gas injection rate of 90 lImin. At a value of z equal to 16 cm (z is the distance from the torch head), the 4000K isotherm is nearly horizontal at the slit level, whereas the 3000K isotherm stretches to the lower part at the center. This condition appears to correspond to a case where the slit width equals 250 !lm. The present method is important for the precise control of plasma processes and needs more study in terms of the control of the particle size of fine powders.
Apparatus and Procedures Figure 12 shows a diagram of the plasma reactor. This apparatus consists of three parts: 1) a hybrid plasma torch (Figure 2 shows more detail), 2) a ring slit for reactive quenching, and 3) the powder recovery section. In the synthesis of Si3N 4 , it is necessary to recover on high temperature walls of 400-500°C to avoid mixing with NH 4Cl. For this purpose, a borosilicate glass tube is used. In the synthesis of SiC, a water-cooled double-walled collector is used to improve the recovery efficiency. Table 1 summarizes the main experimental conditions. Primary Results
Sifi4 Synthesis. Figure 13 shows the relationship between the composition ratio of the reactants, namely, the mole ratio of [NH3]/[SiC4], which is referred to as R[N/S], and the nitrogen
188
Synthesis and Characterization
2
mOOK 18
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1
2
r(cm)
r(cm)
Figure 11. Results from the calculation of temperature and flow at the plasma tail flame. (Assume a slit width of 300llm and an argon injection rate of 90l/min). 189
Ultra-Fine Particles
DC Power supply
8------tl. ffi
Waterin
_-----~I
Pyrex tube
Pump Exhaust
Thermocouples
::r;]
·1Z'2'.W=S)3JgUr~LJ~I[5~~-out ~ N __ Water Waterin
Figure 12. Diagram of the phase reactor. concentration [N] in the particles that are produced. The filled and open points correspond to a total flow rate (QT) of 20 l/min and approximately 50 l/min injected into the tail flame. Under identical conditions, [N] increases with increasing R[N/S]. It is clear,
190
Synthesis and Characterization Table 1. Experimental Conditions.
(l) Gas flow rate: DC arc jet gas: RF sheath gas: Carrier gas: Quenching gas:
1.67 X 10-4 m 3/s (Ar) 5 x 10-4 m 3/s (Ar) + 3.33 x 10-5 m 3/s (Hz) 3.33 x 10-5 m 3/s (Ar) 3.33 x 10-4 m 3/s or 8.33 x 10-4 m 3/s (Hz)
(2) Reactants injection rate:
CH4 :
1.00 X 1O-z -- 7.75 1.67 X 10-5 -- 3.33 1.67 x 10-6 -- 1.33
(3) DC power supply:
5kW
(4) RF plate power input:
20kW
SiCI4 : NH3 :
X X X
1O-z gls 10-4 m 3/s 10-5 m 3/s
however, that RD'J/S] alone does not control D'J]. For example, at an RD'J/S] value of about 5, particles having an [N] value of 25 mass percent are obtained at a QT value of 20 l/min, whereas the value of [N] increases to 37 mass percent at a QT value of 50 l/min. That is, the value of [N] strongly depends not only on the vlllue of R[N/S] , but on the cooling rate as well. In either case, the particles synthesized were essentially amorphous. At RD'J/S] values of less than three, small amounts of (X- and P-Si3N 4 and silicon were present in the products. The presence of these materials indicates insufficient reaction as estimated from the equilibrium phase diagram. The value ofRD'J/S] must be over five to avoid insufficient reactions. SiC Synthesis. Figure 14 shows the relationship between the composition ratio of R[C/S], which equals [CH4]/[SiC14], and the mole ratio [C/Si], which equals [C]/[Si], in the particles synthesized. Again, the filled and open points correspond to QT=20 l/min and 50 191
Ultra-Fine Particles
NH)
H2
Ds
w
Vs
(Lfmin) (Lfmin) (mm) ().1m) (m/sJ
• •
20 20-x
0 x
52 90
60 800
o
1-5 1-5
50 50
52 90
60 80
100
200
o '2
3
5
10
20
R[N/S]
(molar ratio)
30
50
35 15 85 40
500
Figure 13. Relationship between the composition ratio of the reactants, R[N/S], and the nitrogen concentration, [N], in the particles synthesized.
2
• H2 3 33 X lO-'mJjs
.. ° III U
::l 'tl
e
Q.
H2 8.33X1O-'mlfs
---
• gO
~C6
I
.5 l:'
0 0
0
~
§: 2
0
R[C/SI]
3
4
(molar ratio)
Figure 14. Relationship between the composition ratio of the reactants, R[C/S], and the ratio of the carbon and silicon concentrations, [C/Si], in the particles synthesized. 192
Synthesis and Characterization L/min, respectively, of injected hydrogen and CH4 • The value of [C/Si] increases along with R[C/Si]. At QT=20 l/min, for R[C/S] equal to about 1, the value of [C/Si] is about unity, indicating the homogeneous nature of the reaction. Further increases in the value of R[C/S] tend to saturate the value of [C/Si], which decreases with higher QT values. These findings are compatible with the decomposition-condensation processes of CH4 discussed previously. The particles synthesized were primarily ~-SiC and free carbon or free silicon was present depending on the value of [C/Si]. The amount of free carbon is adjustable via QT' which is important in applying this method to the preparation of reaction products for sintering. Particle Size Control. While composition control can be achieved, as stated above, it is especially important to control the particle size to generalize the applicability of the IFP process. The particle size is naturally affected by the reactant concentration and cooling speed. At a constant cooling speed, it is easy to reduce the particle size by decreasing the amount of injected reactants [22,23]. However, this approach is inappropriate for production level processes, which means that other advanced size-control methods are needed. Control was obtained by changing the cooling speed, as discussed below. Figure 15 shows TEM electron micrographs of the SiC particles that were synthesized. These images indicate that particle size control is feasible by gas quenching. The average particle diameter is 200 nm with a QT of 20 l/min. Particle size is reduced to 30 nm with a QT of SOl/min. With the increased value of QT' the particle size distribution was clearly narrowed, indicating that control of the cooling speed is an effective way to control the particle size distribution. The present method allows for refinement ofthe particle size. To obtain particles of an arbitrary size, the method still needs more development, especially to provide uniform particle growth.
193
Figure 15. TEM micrographs of the SiC particles synthesized and the effects of gas quenching on the particle size.
Summary
Processes to synthesize Si)N 4 and SiC fme particles using hybrid plasmas from NH, or CII. and SiCl, were described. It was
shown that reactive quenching, in which the injected reactants are reacted and rapidly cooled simultaneously, is effective in controlling
the composition and particle size of the particles that are synthesized. This method can also be applied to the production of A1N and
BN particles. Better controlling techniques. however. need to be introduced to the IFP process. In addition, development of particle handling techniques is required because the particles synthesized are very active. In this section, the synthesis of fine particle via IFP processes was briefly introduced, including the results from our studies. The 194
Synthesis and Characterization main feature of this process is its applicability to a wide variety of materials and nearly perfected capability for producing particles. Further developments are needed in 1) commercialization (e.g., increased plasma power, long-term stable operation, etc.) and in 2) basic process research (e.g., advanced plasma control, nucleation and growth within the plasma, and particle characterization). For commercialization, advances in related scientific areas are essential. With such developments, this method has the capability of being able to produce particles that other methods cannot produce. The author has avoided the use of the term UFPs because the term should not be used just for particles having small sizes, but to indicate particles that have properties that do not overlap with those of bulk materials. In this sense, at present, few UFPs have been produced. References Wilks, P. H., Pure and Appl. Chern., 48: 195 (1976). Meyer, T. N., private communication (1987). Yoshida, T., Tetsu to Hagane 10: 1498 (1982). Yoshida, T. Sci. & Tech. ofJapan 25: 34 (1984). Yoshida, T., Processing Methods of Materials, Univ. Tokyo Press (1986). 6. Yoshida, T. and Akashi, K, Photo. & Plasma Processing, Nikkan Kogyo (1986). 7. Lewis, J. A. and Gauvin, W. H., A.ICh.E.J.19: 982 (1973) 8. Futaki, S., Kubo, N., and Shiraishi, K., ISPC-8, paper B VIII-07, p. 2040 (1987). 9. Reed,T. B., J Appl. Phys. 32: 821 (1961). 10. Vermulen, P. J ., Boddie, W. L., and Wierum, F. A., AIAA J. 5: 1015 (1967) . 11. Yoshida, T., Tani, T., Nishimura, H., and Akashi, K., J Appl. Phys. 54: 640 (1983). 12. Yoshida,. T. and Akashi, K, J Appl. Phys. 48: 2252 (1977). 13. Yoshida, T., unpublished.
1. 2. 3. 4. 5.
195
Ultra-Fine Particles 14. Harada, T., Yoshida, T., Ozeki,T., and Akashi, K. J. Japan Inst. Metals 45: 1138 (1981). 15. Anekawa,Y., Ozeki, T., Yoshida, T., Akashi, K. J. Japan Inst. Metals 49: 451 (1985). 16. Meyer, T. N., Becker, A. J., Edd, J., F., Smith, F. N., and Lio, J., ISPC-8, paper B VIII-Ol, p. 2006 (1987). 17. Tani, T., Yoshida, T., and Akashi, K., Yogyo Kyoukaishi 94: 1 (1986). 18. Tarnao, Y, Yoshida, T., and Akashi, K., J. Japan Inst. Metals 51: 737 (1987). 19. Abraham, F. F., Homogeneous Nucleation Theory, Academic Press, New York (1974). 20. Yoshida T., unpublished. 21. Yoshida, T., Kawasaki, A., Nakagawa, K., and Akashi, K., J. Mater. Sci.14: 1624 (1979). 22. Yoshida, T. and Akashi. K., Trans. Jap. Inst. Metals 22: 371 (1981). 23. Girshick, S. L., Chiu, C.P., and McMurry, P. H., ISPC-8, paper B VIII-09, p. 2052 (1987).
196
Synthesis and Characterization
2.4 Gaseous Reaction Method (by Aida Kata)
The applications ofUFPs of metals and inorganic compounds can be classified into two types. The first type of application is to use UFPs as they are or to disperse them in another medium. Representative applications include: 1) pigments; 2) fillers for rubber, plastics, and paper; 3) additives for medicines and cosmetics; 4) abrasives; 5) solid lubricants; 6) various pastes; 7) magnetic particles for recording tapes, and 8) catalysts and their supports. The second type of application is to use UFPs as raw materials for sintering. Sintered bodies are formed by pressing powders and heating the pressed parts to high temperatures (below the melting point of the materials used). The morphology ofthe powder changes significantly during the fabrication process, but the raw powder characteristics affect the process and the final properties of the sintered bodies. The important characteristics of powders used for these applications are listed in Table 1. These characteristics must be controlled during the production of the powders so that they are suitable to specific applications. Some examples are given below. Table 1. Important Characteristics of Powders. Chemical Composition
Purity, stoichiometry, unifonnity of chemical composition and composition of adsorbed surface layer
Phase Composition
Crystalline modification and amorphous phase
Particle Morphology
Particle size distribution, particle shape, presence of pores, surface topology and surface area
Crystallinity
Single- and polycrystalline particles, type and amount of lattice defects, surface and internal strains
Agglomeration
Size, strength and structure of agglomerated particles.
197
Ultra-Fine Particles A representative white pigment, Ti02, has three crystalline modifications: rutile, anatase, and brookite. The first two are normally obtained artificially. The refractive index of rutile is 2.76 and that of anatase is 2.52. A white pigment should have low absorption in the visible region and strong scattering of incident light. The scattering increases with increasing refractive index. Thus, rutile is preferred as a pigment. The scattering intensity is also dependent on the particles size. The optimum size for Ti02 is 0.2-0.4~m. Particles of y-Fe20 3 are used for magnetic recording tapes, for which fine needles 0.3 - 0.5 !-tm in length and 0.05 - 0.07!-tm in diameter are suited for high density recording and improved signal-to-noise ratios. Consider next the applications of fine powders for sintering. In a sintered body, crystalline grains, grain boundaries, and voids are present. The properties depend not only on the structure ofthe crystal itself, but on microstructures including grain boundaries and voids (see Table 2). To obtain the highest and most reproducible functionality from materials, the control of their microstructures is crucial. In Table 2, the first factor (1), the primary composition, is defined by a material itself, but the other factors (2-6) depend on the processing used. To obtain materials with good microstructures, every manufacturing process associated with the production of the materials must be precisely controlled. Table 2. Parameters Controlling the Properties of Sintered Bodies. Microstructure (Depends on manufacturing)
Chemical Composition (Determined by material used) (1) Primary components (2) Stoichiometry (3) Impurity (type and amount)
(4) Grain size (5) Size and amount of pores (6) Grain boundary - glassy phase, impurity precipitation
198
Synthesis and Characterization The production of materials with uniform microstructures is far more difficult to accomplish with hard to melt ceramics than with metals or polymers. In general, the properties of sintered materials are better when the grain size is small. Thus, it is important to suppress grain growth during sintering. Both sintering and grain growth occur via mass transport and their rates increase with temperature. Generally, the grain growth becomes significant at temperatures higher than that used for sintering. Pressure sintering, which has been used in recent years, raises the driving force for sintering by applying pressure. This is done to make materials denser at lower temperatures and to obtain finer microstructures. Improvements in pressureless sintering are needed to make this process more useful for mass production. By using powders with good sinterability, sintering at atmospheric pressure, at lower temperatures, and for shorter times eases sintering operations and suppresses grain growth. The desirable characteristics of powders used in sintering are as follows. 1. Small particles and narrow particle size distributions. When particles become smaller, the driving force for sintering increases and the mass transport distance for sintering decreases, thereby increasing the rate of sintering. Narrow particle size distributions are needed to homogenize the microstructure of sintered bodies. 2. Spherical particles. 3. Low agglomeration. To homogenize the green structure by improving flowability and to increase the packing density, particles need to be spherical and to have low agglomeration. 4. Controlled chemical composition. It is important to control the purity and homogeneity when using multi-component systems.
199
Ultra-Fine Particles High temperature materials such as Si 3N 4 and SiC, have strong binding forces between constituent atoms and poor sintering characteristics. The sinterability of powders depends on many factors, but the fineness of the powders used is especially important for improving the sinterability. In making high performance ceramics, each step of the manufacturing processes must be controlled. However, it is essential that precursor green bodies have uniformly packed structures to obtain sintered bodies with homogeneous microstructures. This requires raw powders with the characteristics stated above, with the control of the powder characteristics being one the most important factors. From this view point, powder preparation has been actively studied since the mid1970s.
Manufacture of Ultra-Fine Particles By The Gaseous Reaction Method [1-6] There are two manufacturing methods for producing powders: (1) breakdown and (2) build-up. Method (1) is used to obtain UFPs from coarse powders and relies primarily on mechanical crushing. It is difficult, however, to efficiently produce fine particles < 1 !lm in size and contamination often occurs during the crushing processes. Method (2) is a process where particles are made from ions or atoms by nucleation and growth. Ultra-fine particles with diameters less than 1 !lm can be obtained with ease using this approach. For preparing raw materials for high performance electronic materials and advanced ceramics, Method (2) is suitable because high purity UFPs can be produced. Method (2) can be subdivided into solid-, liquid-, or gas-phase methods. The starting materials for the solid-phase method are synthesized using the liquid-phase method and crushing is generally needed. The author will focus on describing the gaseous reaction method and the characteristics of the powders produced.
200
Synthesis and Characterization
Characteristics of Ultra-Fine Particles Made by the Gaseous Reaction Method Ultra-fine particles can be produced from the gas phase either by physical vapor deposition (PVD) or by chemical vapor deposition (CVD). In the PVD method, a starting material is vaporized by heating and the vapor is condensed by quenching. UFPs with particle diameters of less than 0.1 f..lm can be produced for mono or complex oxides, carbides, and metals. However, this method is not suited for making ceramic powders with low vapor pressures. The gaseous reaction method uses the chemical reaction of vapors of metallic compounds. Reactions can involve thermal decomposition (A(g) -+ B(s) + C(g)) or reaction between more than two chemical species (A(g) + B(g) -+ C(s) + D(g)). For the thermal decomposition approach, suitable compounds are required. For the chemical reaction approach, many combinations of chemicals are possible. The CVD method has the following features. 1. Volatile metallic compounds can be purified with ease. The powders require no crushing and are of high purity. 2. Agglomeration of the powders produced is suppressed. 3. A narrow size distribution can be obtained depending on the reaction conditions. 4. The atmosphere can be easily controlled. Particles of metals, nitrides, carbides, and borides that cannot easily be made directly into UFPs by other methods can be prepared. Metal chlorides are widely used as starting materials in the CYD JY.Ie.t.bDil. Jv.1e.t.aJJjc. y.a.p£lYS SJ.1i'JJ .as D.¥?,i'JJJ£lyj.di"s,. •MD.J~"J'I1~: alkoxides, M(OR) n; alkyl compounds, M(R)n; etc. are also used. The CVD methods so far commercialized are for the manufacture of carbon blacks by the~al decomposition ofhydrocarbons, ZnO by the oxidation of Zn vapor, and Ti0 2, Si02, A1 20 3, etc. by the oxidation or 201
hydrolysis of the respective metal chloride vapors. In addition, the CVD method is being considered for the production of high purity UFP refractory carbides and nitrides such as SiC and SiJN•. Figure 1 shows photographs ofUFPs ofCVD-TiD, fanned by the chloride process (TiCl.(g) + 0,(&) -+TiO,(s) +2Cl~. The particles are single crystals with no agglomeration apparent. Because of this feature, CVD-Ti02 is used widely as a white pigment.
•• •• • • •• • •. •
•
-. -
2pm
'" .1'Y~: ~~~, • • •• • •... :-'tt ". ... • •
.... . , .. \ . • . •
,
•• -
2Jjrn
.~
.- ,
' .;'
~
-
•.1..
O.2)Jm
Figure I. TiD, particles (Anatase fonn) fanned from TiD, (s) + 2CI,(g).
-+
202
TiC~(g)
+ 0,
Synthesis and Characterization The same property is required of powders to be used for sintering. Figure 2 shows the sintering behavior of CVD-MgO as a function of the particle size [7]. The CVD process is used to prepare MgO by reacting magnesium vapor with oxygen to produce singlecrystal (cubic) particles similar to CVD-Ti02 as shown in Figure I, No agglomeration is apparent for this material. The sinterability improves with decreasing particle size as shown in Figure 2.
100
>,
80
+-' If)
c
Q.)
"'0 Q.)
> +-'
60
1
<0
2
Q.)
0::::
3 4
120 A180 A400 A530 A-
~t·-'\r-!L...---...L.....-....l......----L...----I....-----'--I Green 1000 compact F"Iring
1200 1400 t emp. (OC )
Figure 2. Sintering characteristics ofMgO produced by CVD (in air for 6 hours). Average particle diameters are given in the figure. Figure 3 compares the sintering behavior ofCVD-MgO and MgO particles produced via a solid state process [7]. For the same particle size, the CVD-MgO sinters well because the particles produced by the solid state process are agglomerated. The MgO particles prepared by the thermal decomposition of magnesium compounds generally form skeleton particles of the precursor compounds. Within the skeleton particles, MgO crystallites are 203
Ultra-Fine Particles
100
_~>-3-=..:::..:::..:..~ 12
..-...
~ 90 80
70
60 50
a
8
•
500 1000 Crystallite size (A)
Figure 3. Comparison of the sintering characteristics of MgO produced by CVD and solid state MgO. 0 = CVD; • = solid state reactions. Sintering temperature = 1400°C. aligned and compressed together. Heating leads to easy grain growth within the skeleton particles. Thus, the sintering of MgO produced by the solid state process is preceded by sintering and grain growth inside the skeleton structure, which reduces the final density of the sintered bodies. Until now, the effects of particle sizes under 0.1 /lm on the sinterability have not been understood well as a result of the use of agglomerated particles. The agglomeration obscures the effect of the fineness of the powders on the sintering characteristics. Particles with no agglomeration, such as CVD particles, are appropriate for studying particle size effects on sintering. An important aspect in the study of the effect of particle size on the reactivity of solids is to use powders that have not undergone agglomeration so that principles representative of the particles themselves can be determined. The effects of the particle size and 204
Synthesis and Characterization crystal structure ofTi02 in the solid state reaction of Ti0 2 and BaC0 3 by using CVD-Ti02 have been clarified as shown in Figure 1. Requirements for Particle Synthesis The morphology of solids formed from the gas phase varies widely depending on the type and reaction conditions of the reaction system. Particle synthesis from gas phase reactions consists of homogeneous nucleation and growth in the gaseous phase. The rate of nucleation is sensitive to the supersaturation ratio (actual vapor pressure/equilibrium vapor pressure: P/Po). To obtain UFPs, numerous nuclei must be produced by obtaining a high degree of supersaturation. The supersaturation ratio when precipitating solids by a gas phase reaction is proportional to the equilibrium constant of the overall reaction. Thus, reaction systems with large equilibrium constants are required for particle synthesis. In fact, particle synthesis of oxides, nitrides, and carbides by gas phase reactions requires reaction systems having equilibrium constants above 102 - 10 3 (per one mole ofthe metal source). Because chemical reactions are involved, a large equilibrium constant is not a sufficient condition due to the existence of reaction rate restrictions. In particle synthesis by gas phase reactions, particle size can be controlled by the reaction temperature and the composition of the reacting gases. Such examples are shown in Figures 4 and 5. In gas phase reaction processes, homogeneous nucleation mainly occurs in the beginning and particles with a narrow size distribution can be obtained. Reaction Process The simplest reaction process is to pass reactive gases through an externally heated reactor. Other processes include the chemical flame process and, what have become popular processes as of late, the plasma and laser processes. In these various processes, an advantage is that the reactor walls can be kept at low temperatures. Oxide UFPs have been produced from volatile metal halides using 205
Ultra-Fine Particles 1.0
E
30 0 .5
a
'"
c'"
c
~ 01-0--'---9.....0 -0-.l.----,-1.....L- 10 0
Reaction temp.
........ O
(·C)
Figure 4. Temperature dependence oftlie size (median diameter, Dsof of oxide particles formed by gaseous reaction of metallic hallides with oxygen. Reaction systems: o = TiC14 + O2 ~ Ti0 2 • = AlBr3 + O2 ~ A1 2 0 3 ... = ZrC14 + O2 ~ Zr0 2 ~ = FeClz + Oz ~ Fe z0 3 1 0 0 ----------===-"-~-~::::=-"--, en
Q) ()
.... I-
Q)
50
>
.....
E :J
()
.....- -
oa.-.::;...........:::~.::::;;..------I.-----
0.10
0.05
0.15
Particle size (,lAm)
Figure 5. Particle size distribution of TiN formed by gas phase reaction of the TiC14-NH3 system using Process 2. The reaction temperatures are indicated in the figure. 206
Synthesis and Characterization H2-02 and CxHy-02 flames. In the plasma process, a metallic source is vaporized and reacted in a reactive gas atmosphere or a reactive gas containing metal is fed into the plasma flame. In the chemical flame and plasma processes, spherical particles can be produced when their oxides in the molten state are stable and have a low vapor pressure. When the particles are melted in a sufficiently hot flame, they become spherical and produce spherical particles. The properties ofparticles synthesized by the gaseous reaction method depend on the physio-chemical characteristics of the reaction system. The properties are also affected by the construction of the reactor, the heating method, the temperature gradient, preheating of the reactive gases, and the method of introducing the gas into the reactor.
Synthesis of Oxide Ultra-Fine Particles There are three basic types ofprocesses for synthesizing oxide UFPs via gas phase reaction methods. 1. Oxidation or hydrolysis of volatile metal chlorides. TiCI4 (g) + 02(g) -+Ti02 (s) + 2Clig) SiCI4 (g) + 2H20(g) ---+ Si0 2(s) + 4HCI(g) 2. Oxidation of metal vapor. Zn(g) + ~ Oig) ---+ ZnO(s) 3. Thermal decomposition of metallic compounds. Zr(OR)ig) ---+Zr0 2(s) + ROH(g) + Olefin(g) To supply water in the hydrolysis process, one can use either a direct method or an indirect method using such reactions as: cO z + Hz --->CO + HzO, Hz + Oz ---> HzO, and CxHy + Oz ---> HzO +CO z·
Hydrolysis of metal chloride vapors proceeds faster than oxidation and produces finer oxide particles. Hydrolysis is generally done by feeding metal chlorides into a flame. On the other hand, most metal 207
Ultra-Fine Particles
chlorides react with oxygen at around 1000°C to generate oxide powders. The reaction of metal halides with oxygen start at temperatures in the range from 300-400°C (FeCI3, AlBr3 ) to about 800°C (SiCI4) and yields of 100% are obtained at temperatures in the range from about 700°C (FeCI 3, AlBr3) to I 100°C (SiCI 4 , AICI 3 ). Hydrolysis or oxidation of metal halides can be used to synthesize single or multi-component metal oxides from several tens of .nanometers to 0.5 !lm. -
Synthesis of Ultra-Fine Particles of Refractory Nitrides and Carbides Technologically, non-oxide powders such as nitrides and carbides have been synthesized by solid state reactions mainly from metals and metal oxide~ Gaseous-l'eaction and similar methods are gradually being introduced. The reactions of metal chloride with NH3 to produce nitrides have large equilibrium constants and nitride powders can be produced at relatively low temperatures. For carbide syntheses using vapor phase reactions of metal compounds, carbides can be produced at temperatures below 1500°C in systems having large equilibrium constants. However, the equilibrium constants are generally small at low temperatures, where the reaction rates are also slow. Thus, high temperatures are needed for carbide synthesis, so plasma processes are often used. Table 3 shows examples of the ranges of the particle size of nitrides and carbides that can be produced by gas phase chemical reaction methods. This indicates that the gaseous reaction methods can be used to synthesize UFPs. The synthesis processes can be classified into three types.
Process 1. Formation of adduct particles between reactants and their thermal decomposition into nitrides or carbides. Process 2. Nucleation and growth of nitrides or carbides.
208
Synthesis and Characterization Process 3. -Formation of metallic particles and their nitridization or carburization. Even in a reaction system, the particle synthesis process is affected by the mixing of reacting gases or by the introduction into the reaction zone. The particle synthesis process influences the characteristics of the particles synthesized. An example is given below. Table 3. Examples of the Particle Size of Nitrides and Carbides Formed by Gaseous Reactions. System
Temperature
Product
Particle Size (J.lm)
ee) SiCI4-NH3
100-1500
SiNxHy
0.01-0.15
SiH4 -NH3
500-900
SiNxHy
<0.2
TiCI4.NH3
600-1500
TiN
0.01-0.4
ZrCl4.NH3
1000-1500
ZrN
<0.01
VCl 4-NH3
700-1200
VN
0.01-0.1
Si(CH3)4
900-1400
SiC
0.01-0.2
Si(CH3)Cl3
plasma
SiC
<0.03
SiH4 -CH4
1300-1400
SiC
0.01-0.1
TiCI4-CH4
plasma
TiC
0.01-0.2
TiI4 -CH4
1200-1400
TiC
0.01-0.15
NbCl s-CH4
plasma
NbC
0.01-0.1
MoCI 4-CH4
1200-1400
Mo 2C
0.02-0.4
WCI6-CH4
1300-1400
WC
0.02-0.3
209
Ultra-Fine Particles
In the synthesis of TiN fine particles by gaseous reaction of TiC4 and NH3 , the particle synthesis process depends on the mixing temperature of the TiCl4 and Nfl gases. When the mixing temperature is below 250°C, the adduct particles of TiCl4 and NH3 form first, then they thermally decompose into TiN in the high temperature region above 500°C (Process 1). On the other hand, when the mixing temperature is above 600°C, the gaseous reaction ofTiCl4 and NH 3 results in the formation of TiN nuclei. The nuclei then grow into TiN particles (Process 2). In Process 1, the TiN particles from the TiCl4-NH3 system are porous, spherical, polycrystalline particles and have a broad particle size distribution of 0.01 - 0.4 Ilm. In Process 2, the particles are single crystals and, depending on the reaction conditions, the particle size can be reduced to less than 0.1 Ilm (see Figure 5). SiC Particle Synthesis by RF Plasma Gaseous Reaction
Table 4 lists gas phase reaction systems for producing SiC particles. By using Si(CH3)4 and SiI-L. as silicon sources, very fine SiC particles can be obtained by gas phase reactions below 1500°C. This approach is well suited for the production of CVD-SiC particles for research purposes. These silicon compounds are expensive as raw materials, so they are not attractive for commercial scale production of SiC particles. When the cheaper SiCl4 and CH3 SiCl3 are used as silicon sources, higher reaction temperatures are needed due to their lower reactivities. Reflecting these constraints, many patented processes for the gas phase synthesis of SiC particles use plasmas. The use of plasmas for the gas phase synthesis of SiC has attracted renewed attention in recent years. In the following section, experiments on the synthesis of SiC by gas-phase RF plasma reactions are described. By feeding the reacting gases ofthe CH3 SiCI3-H2 and SiCl4 CH4-H2 systems into an RF plasma flame, single crystal SiC particles of less than O.lllm in diameter were obtained. In these reactions, the position for injection of the reacting gas into the plasma flame and control of the reaction temperature were important. When the gases 210
Synthesis and Characterization were injected into the center of the flame, the reaction temperature was too high and excessive free carbon was formed. When the gases were injected into the flame tail, the ratio of carbon to silicon in the reaction product could be made equal to unity. It is possible that SiC particles can be synthesized using the RF plasma method with Si02 fine particles and the gases methane and hydrogen [10]. In the manufacture of powders, the requirements for specific chemical composition and specific powder characteristics must be Table 4. Gaseous Reaction Systems for SiC Synthesis. H2 1. Si(CH3)4 - SiC (0.01-0.2/lm) 900-1400°C H2 2. SiH4 + CH4 - SiC (0.01-0.1/lm) 1200-1400°C H2 3. CH3 SiC13 - SiC (0.01-0.1/lm) Plasma H2 4. SiCI 4 + CH4 - SiC (0.01-0.1 /lm) Plasma
satisfied simultaneously. For many oxides and non-oxides, we can produce high purity and very fine particles by the CVD method. The particles produced by CVD in this manner are useful for research on the characteristics of powders. However, at present the particles produced by CVD are expensive for commercial uses and the handling ofUFPs is also difficult. As the cost for production ofUFPs is reduced and the handling techniques become more advanced, the applications for high-purity UFPs should begin to expand.
211
Ultra-Fine Particles References Kato, A, Ceramics, 13: 625 (1978), 19: 478 (1984). Kato, A, J Powder Engr. 18: 36 (1981). Kato, A, J Powder Engr., 21: 65 (1983). Kato, A, Chemistry Review No. 48: 13 (1985). Kato, A, High Tech Ceramics (ed. by Vincenzini, P.), p. 459, Elsevier Science (1987). 6. Kato, A, Ceramic Powder Science (Advances in Ceramics, vol. 21) Am. Ceram. Soc., 181 (1987). 7. Kato, A and Toda, Y, Mem. Faculty Eng. Kyushu Univ. 47. 135 (1987). 8. Suyama, Y and Kato, A, Ceramurgia Int 1: 5 (1975); 9. Suyama, Y and Kato, A, Bull. Chem. Soc., Jpn. 50: 1361 (1977). 10. Asakami, 0., Hokazono, S., and Kato, A, J Ceram. Soc. Jpn. Inter. Ed., vol. 96, 1173 (1988).
1. 2. 3. 4. 5.
212
Synthesis and Characterization 2.5 UFP Synthesis by Chemical Methods (by Akinori Yoshizawa)
The subject of this section is particle formation in the synthesis ofdense aerosols. Except for the area ofcluster physics, we know very little about the basic mechanisms of particle formation, particularly those that occur by chemical reactions, and only a qualitative discussion can be presented. Path of Particle Formation
The following processes appear to apply for purely physical gas evaporation. 1. High-temperature, metallic mono-atoms evaporated from the surface of molten metal lose their kinetic energy by colliding with inert gas atoms. 2. The collisions ofcooled metal atoms produce metal dimers, trimers, etc. This may involve simple condensation reactions, but the most difficult step is the formation of dimers. Once the size of the particle grows to a point at which the excess energy arising from condensation can be converted to internal vibration, the addition of more atoms becomes easier. 3. When the smoke is in a steady state, the density and temperature distribution ofmetallic atoms determines the final particle size. The gas evaporation system is simple and the temperature difference between the source and atmosphere is sufficiently large. Thus, the range of operating conditions is wide. The chemical methods, however, have generally limited ranges of operation. Consider particle formation due to the reaction of iron chloride, FeCI 2, and hydrogen (the use of italics denotes hypothetical and intermediate species). 213
Ultra-Fine Particles FeC12(g) + H2(g)
-4
FeClzHz
-4
FeClH + HCl(g)
-4
Fe(g) + 2HCl(g)
This is a plausible reaction, however, the compound FeCl(g) is also known to occur, so chlorine mono-atoms should be present as a result of the equilibrium of FeClz(g) ~ FeCl(g) + Cl(g). With the occurrence of chain reactions, it is possible to have the following reactions. Cl + Hz ---. HCl + H, H + FeClz ---. FeCI + Hcl FeCI + Hz ---. FeH + HCl, FeH + FeCl z ---. Fe 2Cl + HCl Although the reaction path is not clear, plausible reactions may lead to the following condensation reactions.
Finally, the following reaction will take place.
This means that the intermediate particles should be detected as compounds of FenCl y (n»Y). In the thermal decomposition of alkoxides, the container wall is known to act as a catalyst, but here only the gaseous reactions will be considered [1]. Consequently, the particle size depends on the distributions of the concentrations of all the existing chemical species and the temperature. It is also possible to have reverse reactions when the chemical saturation decreases, which results in the loss of particles. As seen above, the chemical methods are more complex than the evaporation method. At present, the data necessary to verify the hypothesis is unavailable.
214
Synthesis and Characterization Is Nucleus Formation Unnecessary?
In the discussion in the previous paragraphs, the nucleation and growth of nuclei was not considered. The so-called classical theory of nucleus formation was constructed for liquid droplets of more than 104 atoms that can be regarded thermodynamically as a phase. This theory cannot be applied when the degree of supersaturation is so high that the critical nucleus size is only two to three times the atomic diameter. As pointed out by Gibbs, surface atoms in the first and second layers of a liquid droplet or a particle cannot be regarded as being a condensed phase. It is appropriate to consider a cluster often atoms as a molecule, for example, Fe\O. Only when the surface atoms become less than half of the total number of atoms does it become proper to treat a cluster as a particle. Rusmussen [2], assuming that only the first surface layer is not a condensed phase, obtained 8G for particle formation as a function of the degree of supersaturation and the particle diameter as shown in Figure 1. Although this result may not be entirely correct, the barrier for the nucleus formation clearly vanishes beyond a certain degree of supersaturation. The condensation process can proceed according to this thermodynamic mode. This is the reason for neglecting the nucleation in the previous section. The degree of supersaturation diminishes rapidly once the particle formation/condensation begins. In gas evaporation, it is difficult to re-vaporize the clusters once they are formed because the atoms exist in a condensed state at ambient temperatures. In chemical methods, the clusters that are formed can degrade and disappear via reverse reactions due to a decrease in the reactant concentrations and temperature. For chemical methods, there is a region where 8G becomes negative (i.e., the condition where even a large particle reverts to the original gaseous state). The particles are presumably obtained because of slow reverse reaction rates. When the system that is synthesized is more stable than the original one at ambient temperature (as is the case for some oxides), the above consideration is not needed. For hydrogen-reduction or thermal decomposition systems, unconventional phenomena are y
y
215
Ultra-Fine Particles
Figure 1. Barrier for the nucleus formation versus the degree of supersaturation and the particle diameter. The diameter is represented by 0 and the tension at the plane interface is represented by a oo • likely to occur unless. this. effect is. taken into account- For instance~ particle formation does not occur under some operating conditions, although the particulars will not be presented here.
Role of Coalescence When clusters or other small units that can hardly be called particles are formed, collisions occur and particles are produced as an observable consequence. The processes of collision and coalescence 216
Synthesis and Characterization have long been studied in the fields of aerosols and colloids [3]. Equations of population balance describe these processes. These can only be solved numerically, but it is known [4], that the resultant particle size distribution is a log-normal distribution with a The geometrical standard deviation of approximately 1.5. information on the initial cluster size distribution is lost in the final equilibrium distribution. The particle size distribution of UFPs obtained by chemical methods can be explained in general, by the above processes [5]. In the field of particle synthesis via gas-to-particle processes, there is still much left to be learned. Rapid developments in the field of cluster physics indicate a strong interest in this area. However, problems, including those related to the chemical reactions, leave many questions unanswered. It is expected that experimental data will be accumulated, additional experiments will be devised, and that the processes involved will be modeled in an effort to resolve the many complex phenomena associated with the synthesis of UFPs by chemical methods. References 1. Kanai, Komiyama, Inoue, Kagaku Kogaku Ronbushu, 11: 317 323 (1985). 2. Rasmussen, D. H., J Crystal Growth 56: 45 - 55 (1982). 3. Friedlander, S. K., Smoke, Dust and Haze-Fundamentals of Aerosol Behavior, 7. John Wiley & Sons, New York (1977). 4. Otsuka, Yamamoto, Yoshizawa, Nihon Kagakkai-shi 6: 869 - 878 (1984). 5. Kim, K., Oxide UFP Synthesis by Vapor Phase Reaction, Ph.D. Dissertation, Tokyo University (1987).
217
Ultra-Fine Particles
2.6 Gas Evaporation Under Zero Gravity (by Nobuhiko Wada)
When matter is evaporated by heating in a gas, the material vapor spreads into the gas and condenses into fine particles upon cooling. Gas evaporation is an attempt to intentionally synthesize fine particles. The synthesis processes are not well understood. For example, a theoretical expression for the rate of vaporization in a gas is not available. Only empirical formulas based on engineering experience are known. The lack of theoretical expressions is, in part, due to the complexities introduced by the presence of convection arising from gravity. Theoretical treatments of evaporation in a gas without convection consider the diffusion of vapors in the gas [1-3]. Because convection occurs in real 'systems, the theory has not been verified. In fact, any inconsistencies that have been found, have been attributed to the effects of convection. Diffusion can be described by the diffusion equations. To solve the equations, the boundary conditions must be known. When the equations are applied to evaporation phenomena, the vapor pressure at the surface of a material has been taken as the saturation vapor pressure of the material since the time of Langmuir [4]. However, this hypotheses may not be correct. This hypothesis was probably adequate to prove, for example, that the higher the gas pressure in a light bulb, the lower the amount of evaporation from the tungsten filament. This originated from Langmuir who invented a light bulb with a gas that reduced the amount of evaporation from the tungsten filament, which resulted in a lower light intensity. The development of high intensity xenon lamps that have a high pressure of xenon gas came about due to the lower diffusion coefficient of tungsten vapor in the heavy xenon gas. If a layer at the saturation vapor pressure is present on the evaporation surface, however, no vaporization is expected. Yet the flow of vapors in such a situation actually occurs. Thus, the above 218
Synthesis and Characterization hypothesis is contradictory. It appears reasonable to consider the vapor pressure at the evaporation surface to be not a unique material parameter (i.e., the saturation vapor pressure), but to be dependent on the type and density of the surrounding gas and on the speed of vaporization. Considering the vaporization source to be like a toll gate on a highway, this phenomenon corresponds to the change in the traffic density near the gate due to the width and length of the road near the gate.
Experimental To clarify the theoretical contradiction described above, it is important to do experiments in a zero gravity environment without complications due to convection. Experiments in a spacecraft can last for extended periods, but are expensive and labor intensive, and are limited in number. In contrast, free-fall capsules and ballistic flights provide short, but accessible zero gravity environments. Many experiments can be done using these methods and are suitable for the study of gas evaporation processes. The following two methods were used.
Free-fall experiments These experiments use a free fall capsule equipped with a gas evaporation device as shown in Figure 1. The capsule was dropped in a 14 m high tower, as shown in Figure 2. The fall took 1 - 1.4 sec, during which the filament of the evaporation bulb ignited producing gas evaporation. The state of the smoke generation during the experiments was recorded by using a video camera in the capsule. The video images were sent via a transmitter to a video recorder on the ground. The capsules landed in a box of sand. The evaporation bulbs, which had a diameter of approximately 100 mm (see Figure 3), were recovered from the device and cut open. Fine particles were found attached to the inner wall of the bulbs. These particles were examined by electron microscopy.
219
t.:l
o
Figure I. A free-fall experimental capsule.
Figure 2. A 14 m high tower fOl' free-fall experiments.
2
EVAPORAnON BUtB
3 4
1 ) Protection Cover (poly.corbonole)
N N
2) Evaporation Bulb (glass 80t;6,seo(ed He or Xe)
5
3) Filament (W, Toj with
6
Metal sample (Ag or AI)
7
4) Thermo-couple (VVRe4W)
5} Thermo-couple {PtRh.Pt}
8
6) Pressure Detector (QUdrtz)
9
7) Stem
...
~
..... ;;~
(W·cod 2¢ )
8) Getter
~ ~
10
9) Thermol reflector (quartz disk)
10) Electrode-Bose (poly-carbooote)
Figure 3. A gas evaporation experiment bulb.
!:l ...~
~ If
~. ~
Ultra-Fine Particles Figure 4 shows fine silver particles from a zero gravity gas evaporation experiment in which a helium gas atmosphere at 150 torr was used. Explosive vapor evaporation was observed [5]. The maximum filament temperature was BOO°e. Because the saturation vapor pressure was below 1 torr, the explosive evaporation observed in a 150 torr gas atmosphere cannot be accounted for in terms of the vapor pressure. The particles that were obtained were larger in size than those from a similar experiment done on the ground. Ballistic flight When an aircraft follows a flight pattern such as that shown in Figure 5, a zero gravity condition is obtained that lasts for about 20 seconds. A twin engine jet (Mitsubishi MU-300, 12 passenger) was used for these experiments. Experiments similar to those described above for the free-fall experiments were carried out, but at lower evaporation temperatures « 1100 °C) and for periods that were longer by an order of magnitude. In the free-fall experiments, rapid heating rates were required to reach a certain temperature due to the short duration of the experiments. By extending the experimental time to 20 seconds, the temperature could be increased to a predetermined value and held for fixed periods. The experimental setup is shown in Figure 6. Multiple experimental bulbs were used during a single flight. The gas used in the bulbs was one of the following: helium, argon, or xenon. For each experiment the smoke generation varied depending on the gas pressure although the limiting values were different for each gas. Figure 7 shows examples of silver particles obtained using xenon gas. Three key findings were obtained as follows (corresponding to the nomenclature of the figure). A. When the gas pressure was low, the smoke erupted radially from the evaporation source. Fine particles deposited inside the wall of the bulb in a manner similar to what is observed for vacuum evaporation.
222
~
~ lio
"~
~.
l
Q
about 12WC
about 1300·C
Figure 4. Explosive behavior of the smoke during a gas evaporation experiment (silver particles in a helium gas atmosphere of 150 torr).
I
§. ~.
s
40.000
~
~
S·
Parabolic flight ~
~
~
~
OG maneuver (Power down) Recovering maneuver
30.000
1:bO
~
.t"'~
.OJ
1.>oo~
::c
Max speed Max power
T
20000.-t l'-J l'-J ..j::.
.1.
_ 2G accelerate
1
~
3
1010- G micro__~O_ '" 20 sec
l~---'" 3G pull up
Flight time Apparatus
~ ~.
G - meter
Figure 5. A flight pattern producing zero gravity conditions and the position of experimental devices within the aircraft.
SynthDis and Characteri:atioll
225
f
f i.... ~ ~
'"~
(J)Xo-5 Torr
!%lXe-JO Torr
(1IXe- 300 Torr
Figure 7. Examples of silver particle evaporation in aircraft experiments using a xenon gas atmosphere at different pressures (evaporation temperatures of 1100 °C).
Synthesis and Characterization B. When the pressure was moderately high (several tens of torr of xenon), the smoke becomes spherical in shape, gradually expanding and attaching uniformly to the wall [6]. C. When the pressure was increased further (several hundred torr of xenon), the spherical shaped smoke partially broke up in an explosive manner. The limiting values for the first process described above were 3 torr for xenon, 5 torr for argon, and 150 torr for helium. Those for the second process were 30 torr for xenon, 50 torr for argon, and more than 300 torr for helium. For the third process, the limiting value for xenon was over 300 torr and that for argon was over 600 torr. When helium was used, we could not observe the explosive behavior of the smoke even at 700 torr, although the outline becomes nonspherical and the smoke spread in the container in a blob-like manner. Fine Particle Observation After each experiment, the experiment bulbs were cut open and the fine particles on the wall were first examined using a scanning electron microscope. A portion of the particles were examined in more detail using transmission electron microscopy. In general, compared to particles produced in experiments done on the ground, the particles produced in zero gravity were larger, but the shapes and sizes were similar. Images of the particles produced in zero gravity are shown in Figure 8. The smaller particles that were formed were fused into chains (i.e., the silver UFPs obtained under 300 torr in a helium atmosphere, Figure 9). When the atomic weight of the gas was larger and its pressure was higher, we obtained larger particles. This trend is similar to that of gas evaporation done on the ground. The explosive behavior observed at high pressures is a phenomenon unique to zero gravity environments.
227
Ag UFP Xe-5 Torr
Ag UFP Xe-30 Torr Figure 8. Large silver UFPs obtained by gas evaporation under zero gravity conditions. The particle crystal habits and sizes are uniform.
228
Synthesis and Characterit.aJion
Figure 9. Small silver UFPs obtained by gas evaporation under zero gravity conditions. The small particles are fused into chains. Discussion The explosive behavior of smoke was found in both free-fall and aircraft experiments. It is a phenomenon that is unique to zero gravity conditions. The values of the saturation vapor pressure of
silver at 1100-1300·C range fonn Hy' to I torr. These are low compared to the sealed gas pressure ofseveral tens to hundreds oftorr and the explosive behavior cannot be explained by the existing theory. In particular, it is strange to find explosive behavior when the gas pressure in the sealed vessels is increased. When the saturation vapor pressure is lower than the atmospheric pressure, the vapor expands by pushing the atmospheric gas. This is the percolation phenomenon. Under zero gravity, explosive behavior of the smoke occurs or percolation occurs even when the saturation vapor pressure is lower than the surrounding gas pressure. 229
Ultra-Fine Particles The assumption of the existence of a layer of supersaturated vapor on the surface of an evaporation source was introduced as a boundary condition merely to solve the diffusion equation. This assumption contains the contradiction mentioned previously. Thus, the author has proposed a model [5,6] based on the spontaneous emission ofvapor molecules from the source, in addition to diffusion, instead of simply treating the gas evaporation as a diffusion phenomenon. The expressions are given below, but the details will not be presented here. DV2p(r) - VJ(r)
Op(rt)/Ot
(1)
Here, p (r) is the density of the vapor at radial distance r, 1(r) is the vapor flux emitted from the source, and D is the diffusion coefficient. Without the second term on the left side of equation (1), the equation is reduced to the usual diffusion equation. In place of a layer of saturated vapor on the source surface, a boundary condition is chosen in which the vapor flux emitted from the source surface 10 is constant as follows. (2)
Here, Pc is the saturation vapor pressure, M is the molecular weight of the evaporating molecules, R is the gas constant, and T is the temperature of the evaporation source. This is identical to the situation for vacuum evaporation. When vapor is emitted from a spherical evaporation source of radius, ro, in a spherically symmetrical manner as in the present experiment, the vapor flux 1 from the source is scattered by gaseous molecules and loses its directionality, which changes the flow direction. Thus, 1 is reduced in proportion to the distance of flow. Using the rate of reduction per unit distance, 1-1, 1 is given by the following equation.
230
Synthesis and Characterization
J = J o (r/r 0 )exp[ -!!(r-r0 )]
(3)
By substituting this expression into Equation (l), we obtain the spatial distribution of the vapor density in the r direction, per), with '"'" as a parameter (Figure 10). The value of,"", is determined by the types of evaporating molecules and gas molecules, relative velocity at collision, etc., but its value is believed to increase as the gas density increases with increasing pressure. Solving Equation (l), a maximum in per) at a certain value of r is found, as shown in Figure 10. With a larger ,""" or with a higher pressure, the maximum is greater and it occurs at a smaller r. When,"", is extremely small or for vacuum, the maximum pressure nearly vanishes and the position of the maximum is far from the source. That is, per) itself vanishes as well. The above results show that, as the gas pressure becomes higher, the evaporated molecules are trapped in the vicinity of the source and produce a high density. The explosive behavior observed at higher gas pressures is indicative of this fact. When gas flow is present as when convection occurs, the evaporated molecules are blown away as they are emitted, so high pressures are not produced and no explosive behavior is observed. This concept assumes the presence of vapor clouds having higher gas pressures than the saturation vapor pressure in the vicinity of the source. This appears rather contradictory, but the clouds and source are not in direct contact because a buffer layer of gas (helium, argon, etc.) exists between them and the feedback of evaporated molecules occurs via diffusion. From the source, on the other hand, vapor molecules are fed into the clouds by spontaneous emissions due to the thermal energy and the vapor pressure of the clouds are maintained. When the diffusion coefficient of vapor into the gas, D, is small, the supply of vapor molecules to the clouds exceeds the amount lost by diffusion. This raises the vapor pressures of the clouds. When the vapor pressures increase, the amount lost by
231
Ultra-Fine Particles OBr------r----------------, )1=5 ::J
~ 07
06 05
1 03
02 01
oooL!C=S==========:3 o r=O.1
- - - . . r(em)
Figure 10. Calculated radial distribution of the vapor pressure. diffusion also increases. Thus, the cloud vapor pressure reaches a fixed value. When this value exceeds the saturation vapor pressure, a ball of smoke is formed and expands gradually as shown in Figure 7-2 (upper right). Alternately, when the vapor pressure exceeds that of the environment gas, the smoke breaks apart and erupts as shown in Figure 7-3 (lower right). In this way, the experimental observations can be described. The situation corresponding to Figure 7-1 (upper left) has the lowest gas pressure, a low Il value, and a large D. This is similar to the conditions for vacuum evaporation. When Il is small, the density distribution in Figure 10 has no maximum and the pressure of the vapor clouds is nearly zero over the entire volume. The vapor flux from the source may be scattered by the atmospheric gas, but expands radially without changing direction. Figure 7 shows this behavior. The smoke spreads radially to the left and right as if to indicate the vapor flow. The non-symmetrical expansion ofthe smoke reflects the non-spherical nature of the source. 232
Synthesis and Characterization Using the model for the gas evaporation and Equation (1), most evaporation phenomena in the absence of convection can be explained. Supplement During preparation of the manuscript ofthis paper, the author had the opportunity to make low gravity experiments on the orbiter Endeavor under the joint program between NASA (USA) and NASDA (Japan). These tests, which were referred to as the "First Material Tests," were done during the period of September 12-20, 1992. The equipment used in these tests was based on earlier apparatus designed by the author's research team. The results from the orbiter were successful. The four experimental bulbs contained filament tips coated with 50 mg of silver and were filled with 50 torr of argon gas (A), 300 torr of argon gas (B), 5 torr of xenon gas (C), and 100 torr ofxenon gas (D) on the ground. These were ignited oneby-one in the low-gravity environment provided by the orbiter. The experimental set-up and procedure were the same as those used for the ballistic flights, but the level of the low gravity was much smaller (less than 10-4 G) compared to that of the ballistic experiments and the duration of the low gravity was essentially infinite. Although the actual time of the experiments was limited for each bulb (3 minutes), the level of the low-gravity before and after the experiments was stable, which was important for stable and precise experiments in low gravity [9, 10]. The evaporation temperatures were maintained at 1150°C, a temperature at which the smoke plumes were barely detectable in all of the ground experiments. No smoke was observed for sample (A), but it was observed for samples (B) and (C) and a burst of smoke that extended in various directions from the ball of smoke was observed for sample (D) (Figure 11). These results were almost the same as those obtained in the ballistic experiments, but the burst of smoke that extended in various directions was unlike the unidirectional burst observed in the ballistic experiments (Figure 12). This effect was due to the low level of the gravity. For the ballistic experiments, the 233
A
Ar50
B
Ar300
..
c
Xo.
D
Xe100lorr
Figure 11.· Smoke due to silver gas evaporation inspace a low gravity; A) argon at 50 torr; B) argon at 300 torr; C) xenon at 5 torr; and D) zenon at 100 torr.
234
Synthesis and Charactuization
Figure 12. Burst of silver smoke in xenon at 100 torr (sample D in Figure 11).
235
Ultra-Fine Particles residual gravity may have promoted a weak unidirectional force due to convection, which caused a unidirectional extension of the burst of smoke. It was confirmed that high temperature material vapor or gas can be confined in a specific area or spot in an inert gas atmosphere of high pressure under the low gravity of space. This fact suggests that it should be possible to more easily do various applications in space that are difficult to do on the ground, such as CVD or nuclear fusion.
References Fonda, G. R., Phys. Rev. 31 :260 (1928). Bryant, W. A., J. Vac. Sci. Tech. 8:561 (1971). Kawamura, K., Jpn. J. Appl. Phys. 12:1685 (1973). Dashman, S., Vacuum Technology, John Wiley & Sons, p. 78. Wada, N., Proc. 13th Int. Symp. Space Tech. Sci. p. 1661 (1982);14th Int. Symp. Space Tech. Sci., p. 1599 (1984). 6. Wada, N., Kato, M., Doi, M., Sato., T., and Goto, T. Proc. 15th Int. Symp. Space Tech. Sci., p. 2173 (1986). 7. Wada, N., Kotai Butsuri, UFP Special Issue No. 85 (1984). 8. Dohi, M., Sawai, S., and Kato, M.. Jpn. Appl. Phys., 31, 39573962 (1992). 9. Wada, N., Kato, M., Dohi, M., Sawai, S., Tani, M., Sengoku, M., Goto, T., Sato, T., and Noda, T., Jpn. J. Appl. Phys. 33,66486653 (1994). 10. Wada, N.,Evaporation of Materials in Gas Atmosphere on Earth Orbit (FMPT-M14), Science & Technology in Japan, 12, No. 48,42-48 (1993).
1. 2. 3. 4. 5.
236
Synthesis and Characterization 2.7 The Properties of Surface Oxide Layers of Metallic Ultra-Fine Particles (by Akira Johgo)
In general, metals become oxidation resistant when an oxide layer forms on their surface. This phenomenon can be seen in many examples that surround our daily life, such as bridges, towers, and many other structures can stand for many years. This study was started to investigate the corresponding behavior of surface oxide layers on UFPs. Experimental UFP samples The surface oxide layer of nickel UFPs was studied. Many studies have been done on the oxidation processes and resultant oxides that are formed on nickel. The only stable oxide of nickel is NiO, which makes it one of the simplest metal oxidation systems. Thus, it was anticipated from the outset that much useful information would be obtained by comparing the results from UFP studies. Sample preparation Nickel UFPs were formed and then oxidized. Nickel UFPs were made by gas evaporation, which can generate high purity particles with small diameters and good crystallinity. Raw nickel (99.99% pure) was evaporated at 2100K in high-purity helium gas (99.99%). Nickel UFPs thus prepared were slowly oxidized in air diluted with nitrogen gas over a two-week period. The average diameter of the UFP samples was 21 nm, which was determined by electron mIcroscopy.
237
Ultra-Fine Particles Experimental procedures The morphology and composition of UFP samples were examined using x-ray diffraction, x-ray photoelectron spectroscopy (XPS), thermal analysis, and Fourier-transform infrared photoacoustic spectroscopy (FT-IR/PAS). Thermal analysis was also used to investigate the reactivity of the samples towards oxygen. Emission spectroscopy was used to determine the dynamic chemical properties of the samples. The data gathered by these methods are not for individual UFPs but for ultrafine powders or coagulated UFPs. X-ray diffraction was used to analyze the crystalline components and the size of the crystallites for each component. Xray photoelectron spectroscopy [1,2] a method that is useful for surface analysis, was used to determine the chemical composition of the surface of the samples. For thermal analysis the following methods were used: thermogravimetric analysis, thermally stimulated dehydration analysis, differential thermal analysis, and thermally stimulated oxygen absorption analysis. In terms of thermal analyses, thermogravimetric and differential thermal analysis methods are conventional methods. In the research described here, the thermally stimulated dehydration and thermally stimulated oxygen absorption analyses were done by combining hygrometer and oxygen analyzers, respectively, with a thermogravimetric analyzer. Fourier transform infrared photoacoustic spectroscopy can be used to analyze materials in a manner similar to what can be done with infrared absorption spectroscopy and is especially effective as an optical analysis method for materials that are opaque to infrared radiation, such as metallic UFPs. Emission spectroscopy detects the intensity and spectrum of photon energy emitted from sample surfaces due to chemical reactions at the surface. This is used to examine dynamic chemical reaction processes.
238
Synthesis and Characterization Results Results from x-ray diffraction analyses [4]
The x-ray diffraction patterns of the UFP samples in this study showed the presence of microcrystals of nickel along with microcrystals of NiO and Ni(OH)2' To determine the size of the crystallites from the width of the diffraction peaks, Sherrer's formula was used, as follows. D = KA./pcos8
(1)
Here D is the size of crystallites, K is a constant (usually taken to be 0.9), A. is the wavelength of incident x-ray, and p is a constant for a given diffractometer. The size of the nickel crystallites were found to be 16 nm and the NiO crystallites were found to be 2 mm. Results of x-ray photoelectron spectroscopy [4]
The x-ray photoelectron spectra ofUFP samples were obtained after each of the following treatments: 1) evacuation at room temperature, 2) heat treatment at 723K in vacuo, and 3) sputter-etching of the surface using argon ions. The spectrum after evacuation indicated that the surface of the UFPs was covered with NiO and Ni(OH)2' Following heat treatment, the fraction ofNiO increased and that of Ni(OH)2 decreased. After etching, NiO and Ni(OH)2 decreased and nickel appeared. Results from thermal analyses [4]
Thermogravimetric analysis ofUFP samples showed peaks in the weight loss curve at 360 and 51 OK. The dehydration curve showed similar peaks at 350 and 51OK. In the differential thermal analyses, endothermic peaks were observed at 350,510, and 570K.
239
Ultra-Fine Particles In the thermally stimulated oxygen absorption analyses of UFP samples, the oxygen concentration downstream from the sample chamber decreased sharply at 400 and 600K. This indicates that the stepwise absorption of oxygen begins at these temperatures.
FT-IR/PAS results [5] The FT-IRIPAS spectra of the UFP samples in the region from 400 to 4000 cm- I showed the strongest absorption band at 520 cm- I along with absorption bands at 430, 750, and 875 cm- I .
Results from emission spectroscopy [6] After the UFP samples were heated in vacuo at 700K and cooled to room temperature, a fixed amount of oxygen was introduced into the sample chamber. With each exposure to oxygen, the samples emitted visible light, but the light intensity was not constant for each exposure to oxygen. As the number of exposures increased, the intensity showed clear oscillations. In the 400 - 1000 nm region of the light emission spectra there was no fine structure and a gradual increase occurred in the longer wavelength region.
Surface Oxide Layer of Nickel Ultra-Fine Particles Morphology and composition Results from x-ray diffraction studies provided interesting findings on the morphology of the surface oxide layer of nickel UFPs that were slowly oxidized by air. The surface oxide consists ofNiO crystallite flakes in a mosaic tile structure, with a layer thickness the size of one crystallite (about 2 nm) [4]. On the surface of the UFPs, nickel hydroxides are formed in addition to the oxide. Results from x-ray photoelectron spectroscopy give direct evidence of the presence of the hydroxides, but the exact characterization is difficult with this method alone. Although it was concluded that the hydroxide was Ni(OH)2' P-NiO(OH) also 240
Synthesis and Characterization produced a similar spectrum [7]. These two hydroxides were formed on the surface when nickel was exposed to air [8-9]. Consequently, the hydroxide was characterized by using thermal analysis. The differential thermal analyses showed three endothermic processes. The endothermic peak at 570K corresponds to the thermal decomposition ofNi(OH)2' [10,11] while the peaks at 350 and 510K agree with those due to P-NiO(OH) [7]. Thermogravimetric analyses and thermal stimulated dehydration analyses also lead to the independent identification of the two hydroxides [4]. From the above results it was concluded that NiO and two hydroxides, Ni(OH)2 and P-NiO(OH), exist on the surface of nickel UFPs that are slowly oxidized in air.
Formation processes The presence ofNi(OH) 2and P-NiO(OH) in the surface oxide layer of nickel UFPs that are slowly oxidized in air suggests that oxides other than NiO and intermediate oxidation products were formed during the oxidation process. Ni(OH)2 is formed by reaction between water and the surface where NiO and Ni20 3 coexist [8]. The P-NiO(OH) is formed by reaction between water and the nickel surface, which absorbs atomic oxygen [12]. These intermediate oxide products during slow oxidation processes are thermodynamically unstable compared to NiO. In high temperature oxidation, they become NiO, which is the only stable oxide. When UFPs are slowly oxidized in oxygen at high temperatures, the entire particle is completely oxidized, not just the surface region. During slow oxidation in moist air at room temperature, however, nickel forms hydroxides and becomes chemically stable.
Thermal stability Slow oxidation treatment in air is an effective way to chemically stabilize metallic UFPs that will spontaneously combust. In fact, nickel UFPs slowly oxidized in air show no evidence of
241
Ultra-Fine Particles
further oxidation for at least a few years. In slowly oxidized UFPs of other metals, similar results have been reported [13]. Such results provide empirical proof that slow oxidation is an effective treatment for preventing further oxidation of UFPs. The thermal stability of surface oxide layers is still unknown. Thus, oxygen absorption analyses were done while increasing the temperature of the samples. The analyses showed that further oxidation ofUFPs occurs with increasing temperature. The amount of reaction increased in conjunction with the thermal decomposition of the double-hydroxides that exist in the surface oxide layer [4]. These results imply that the ability of surface oxide layers to prevent oxidation is limited and is lost when the double-hydroxide decomposes. This result does not necessarily mean that the use of air for slow oxidation treatments does not have merit. When the atmosphere for slow oxidation treatment is changed from wet air to dry air, the stability of UFPs in air deteriorates. Application of FT-IRIPAS
The FT-IR/PAS was used to analyze the composition of the surface oxide layers. The absorption bands at 430 and 520 cm'! were identified as absorptions due to the surface phonon mode of NiO UFPs [5]. The bands at 750 and 875 cm'! were found to originate from oxides other than NiO,[5] although we have no IR absorption data that allows us to identify the oxides. There is no other report of the use of FT-IR/PAS to analyze the oxidation of metallic UFPs. Investigations have shown, however, that this method is easy to use (no prior sample preparation needed), it is nondestructive, and it is sensitive enough to be used as a surface analysis technique. The results from FT-IR/PS analyses provided information on the morphology of the NiO particles. These results indicated that the microcrystallites ofNiO are nearly spherical and that they are bound together in densely packed structures [5]. Further investigations are needed to firmly establish these aspects.
242
Synthesis and Characterization
Oxidative luminescence Interesting results from the oxidative luminescence study of the oxidation of nickel UFPs were obtained. The observed spectra were mainly due to thermal emissions resulting from the formation of oxides [6]. The oscillations in the light intensity that were observed, however, suggest that oxidation occurs by complex mechanisms. In general, chemical oscillation phenomena are observed when there is a cyclic series of reactions. There are a few oscillation ph"enomena have been found for chemical reactions when the shift from equilibrium is large.
Summary The surface of nickel UFPs slowly oxidized in air had surprisingly complex morphology and composition. It was also found that suppression of the oxidation of the surface layer is due to the formation of hydroxides. These phenomenon are, however, not unique to the UFPs, but are instead in common with those of bulk materials. The use ofFT-IRIPAS for the analysis ofthe surface oxide layer was successful and demonstrated the potential for this method in surface studies. The luminescence method was similarly promlSlng.
References 1. Kimura ed., Kagaku Sosetsu 16, Electron Spectroscopy, Gakkai Shuppan Center (1977) 2. Aihara, Kyouritsu Chern. Library 16, Electron Spectroscopy, Kyouritsu (1978). 3. Sawadaed., Japan Spectrographic Soc. Meas. Methods 1, PAS and its Applications, Gakkai Shuppan Center (1982). 4. Johgo, A. and Ozawa, E., submitted to Appl. Surface Sci. 5. Johgo, A., Ozawa, E., Ishida, H., and Shoda, K., J Mater. Sci. Lett. 6:429 (1987).
243
Ultra-Fine Particles Johgo, A., Hayashi Ultra-Fine Particle Project Research Report, Research Development Corporation of Japan, p. 91 (1986). 7. Moroney, M. L., Smart, R. St. C., and Roberts, M. W., J Chern. Soc. Faraday Trans. I, 79: 1769 (1983). 8. Kim, K. S. and Winograd, N., Surface Sci. 43: 625 (1974). 9. Linn, J. H. and Swartz, W. E. Jr., Appl. Surface Sci. 20: 154 (1984). 10. Hazell, 1. F. and Irving, R. J., J Chern. Soc. A669 (1966). 11. Gravelle, P. C. and Teichner, S. J., Adv. Catalysis 20: 167 (1969). 12. Carley, A. F., Rassias, S., and Roberts, M. W., Surface Sci, 135: 35 (1983). 13. Haneda, K. and Morrish, A. H., Surface Sci. 77: 584 (1978); Nature (London) 282:186 (1979).
6.
244
Synthesis and Characterization 2.8 Mossbauer Spectra of Iron Ultra-Fine Particles (by Norio Saegusa)
Methodology When metal UFPs are handled in air, the formation of surface oxide layers is unavoidable. Such layers make it difficult to study the physical properties of metallic UFPs. It is very difficult to prevent oxidation, so one may use a microscopic method of analysis to individually observe and analyze the metallic and oxide parts of the UFPs. Another method to analyze UFPs is to use Mossbauer spectroscopy [1]. This effect can be used to measure the electronic states and the binding state of Mossbauer atoms by determining their nuclear energy levels through the interaction between Mossbauer nuclei (e.g., 57Fe, 119 Sn, 155 Gd, etc.) and the surrounding electric charges. For example, metallic iron and iron oxides have different Mossbauer spectra because 57Fe nuclei have different electrical charge distributions. This allows one to determine the properties of the metallic and oxide parts ofUFPs independently. Qualitative analysis of the two parts may be possible. Because there is only one nuclei that is measurable for anyone x-ray source, specific nuclei can be selectively measured. For instance, using a 57CO source, 57Fe is the only nuclei that can be measured. When UFPs that include Mossbauer atoms are dispersed in a non-ferrous medium, the particle properties can be determined without the interference of the medium.
Surface Oxides of Iron Ultra-Fine Particles When a clean surface of metallic iron is exposed to air, it is rapidly oxidized. For small iron UFPs, the surface area is large relative to the volume. Thus, they burn when the UFPs have exposed surfaces that are in the metallic state. This is suppressed by slow oxidation treatments, which form thin oxide layers on the surface.
245
Ultra-Fine Particles After such a stabilization treatment, the UFPs can be taken out into the atmosphere. Another method is to disperse UFPs in organic solvents or oil, [2] preventing direct contact with air. However, if oxygen is dissolved in the medium, the surface is gradually oxidized [3]. There are still many aspects of the surface oxidation processes that are still unknown. The formation of surface layers on metallic UFPs was examined. To elucidate the processes associated with the formation of surface oxides, UFPs were collected (diameters of about 20 nm) on polyimide films and measured their Mossbauer spectra. Figure 1 shows a typical Mossbauer spectrum of iron UFPs. Metallic iron (M) and iron oxides (0) can be distinguished based on the magnitude of the internal magnetic field (Hhi) and the magnitude of the shift of the spectral center (isomer shift: IS). To estimate the ratio of the metallic and oxide parts of metallic UFPs, we compared the areas of the respective Mossbauer sub-spectra. It should be noted that the effective Debye temperatures of iron atoms in metallic and oxide parts are generally different, and may contribute to a large error when basing determinations on room temperature measurements alone. That is, room temperature measurements often underestimate the amount of the oxide layer in the types of samples used in this study. The samples used had been subjected to two oxidation processes [4]. During synthesis, the UFPs were treated by an in-flight oxidation process (the particles pass through a treatment chamber with a controlled oxygen partial pressure). The UFPs were subsequently oxidized when they were removed from the synthesis equipment. Oxides are formed during both treatments. When the samples were slowly oxidized, UFPs with both metallic and oxide parts were produced for P02 ~ 0.008 torr. For P0 2 = 0.013 torr, oxide UFPs were formed in which the entire particle was oxidized. Figure 2 shows Mossbauer spectra ofUFPs formed at P02 = 0.013 torr and measured at 4.2 and 293K. These spectra match those of hypermagnetite [5]. The ratio of tetrahedral and octahedral sites in the spinal structure is 1:2, but the estimated ratio for this oxide UFP sample was about 1: 1, which suggests that there is a deficiency of 246
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Synthesis and Characterization higher oxygen partial pressures. Figure 3 shows Mossbauer spectra (measured at 293K) of UFPs obtained at various oxygen partial pressures. Figures 4 and 5 show the dependencies Hhf and IS on P0 2 at 293K. Both values approach within error the respective values of magnetite as P02 increases over 0.013 torr. Thus, the ratio of Fe2+ and Fe3+in octahedral sites is expected to be 1: 1 as in bulk magnetite. When air is introduced rapidly during the oxidation treatment, the oxide generated was hypermagnetite when P02 ~ 0.017 torr for in-flight treatment. When P02 was below 0.007 torr, the particles were completely oxidized. Considering the results for slowly oxidized samples, the particles reacted due to the heat of reaction from the rapid exposure to air. Oxides produced under these conditions were poorly crystallized. Consequently, their Mossbauer spectra had broad line widths. When the particles were oxidized to hypermagnetite, they were relatively stable and the conversion of Fe2+ to Fe3+did not proceed even under rapid oxygen introduction. On the other hand, particles having metallic interiors and surface oxide layers were completely oxidize when suddenly exposed to air. For iron UFPs with diameters of about 20 nm, the oxide phase stabilized as hypermagnetite in which both Fe2+ and Fe'+ were present. This was true for completely oxidized particles as well as for surface oxidized particles. In-flight oxidation could be considered similar to the initial stage of slow oxidation. Oxides of iron UFPs resulting from slow oxidation treatment appear to depend on the initial partial pressure of the oxygen used during the oxidation treatment. The ratio of iron in tetrahedral and octahedral sites seems to have some regularity, but a definite conclusion was not obtained using the current data. Although it was not discussed in this section, the metallic parts of UFPs behave in the same way as bulk ex -iron in terms of their Mossbauer spectra. This section presented examples of the application of Mossbauer spectroscopy to the analysis of iron UFPs and their oxidation processes. Details of the spectroscopy methods used can be found in the literature.
249
Ultra-Fine Particles
Fe UFP
T = 293K
Figure 3. Mossbauer spectra at 293K of iron UFPs prepared at various values ofPo2• The + marks indicate data points and the solid lines represent calculated spectra based on the least-squares fit of the data. 250
Synthesis and Characterization
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Figure 4. Dependence of the internal magnetic field Hhf on P02 at 293K. 251
Ultra-Fine Particles
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References 1. For a general reference, see Greenwood, N. N. and Gibb, T. C. Mossbauer Spectroscopy, Chapman & Hall (1971). On Mossbauer effects on UFPs, M0rup, S., Dumesic, J. A and T
Synthesis and Characterization 2.9 Preparation of UFP Alloy Catalysts by Alkoxide Methods (by Akifumi Vena)
Metal catalysts are used widely in the chemical industry. Most catalysts are formed by the dispersion of fine metal particles on refractory oxides, such as silica and alumina. Catalytic reactions proceed on the metal surfaces and the apparent reaction rates are higher when the surfaces areas are larger. Thus, many studies have focused on the reduction of the metal particle size [1]. When the particle size becomes extremely small, the properties start to change and affect the catalytic reactions as well. Reaction selectivity has often been found to vary with the particle size of the catalyst [2]. Such qualitative changes in catalytic reactions may originate from electronic effects due to the size reduction or from changes in the geometric arrangements of the metal atoms. Alloying is also effective in controlling the properties ofmetal particle catalysts, but few reports are available on methods for preparing alloy UFPs. Because numerous studies have been published [3] on the production of catalysts using dispersions of metallic UFPs, the preparation of catalysts of alloy UFPs is discussed in this section. Alloy Catalysts There are not many alloy catalysts used in practical applications. Examples that exist include Pt-Re and Pt-Ir catalysts for steam reforming. However, many basic studies have been done on such catalyst systems. Balandin [4] was the first to use an alloy as a catalyst. He found that small additions of nickel to copper metal particles, which are inactive in the hydrogenation of benzene, make them active hydrogenation catalysts. Balandin thought that this phenomenon is related to the geometrical arrangement of the metal atoms and benzene molecules on the catalyst surface. He studied the catalyst activity as a function of the lattice parameter of copper nickel 253
Ultra-Fine Particles
alloys. Copper nickel alloys have fcc structures in which the (111) planes have hexagonal atomic arrangements that match the size ofthe benzene molecule. Long [5] advanced the Balandin concept further and applied it to alloy catalyst systems of Fe-Ni, Ni-Co, and Fe-Co. With the subsequent developments in the band theory of metals, this theory was applied to alloy catalysts. Schwab [6] measured the activities of Cu-Sn, Au-Cd, and Cu-Zn catalysts in the decomposition offormic acid and compared the results to the electron band structures of these alloys. Dowden [7] started to use the vacant states of the 3d electron orbital as a parameter in the activity of catalysts. He thought that, in the hydrogenation reaction of styrene, styrene and hydrogen molecules give up electrons upon being absorbed on the catalyst surface and become cations. This requires the presence of vacant states in the 3d orbit of the metal atom. Takeuchi [8] and Reinaker [9] applied this concept successfully to a model for the hydrogenation of ethylene on alloy catalysts of the CuNi, Pd-Cu, and Pt-Cu systems. In the 1970s, electron spectroscopic methods such as XPS and AES became popular as surface analysis techniques. Alloy catalysts were also examined using these new techniques. First, the electronic states of the alloys were studied to determine if the electrons of an alloy exist in bands that are common to constituent metal elements (rigid band model) or if the characteristic electron bands for each element exist independently without shared electrons (virtual bound state). According to XPS results, [10] there is very little electron transfer from copper to nickel in Cu-Ni alloys, in which the Cu and Ni atoms retain their identities. At the same time, it became clear that the alloy compositions differ between the surface and the interior. That is, the catalytic activity changes not from theorized electron effects of electron transfer between different elements, but from the changes in the surface alloy composition. Sachtler [11] treated the surface alloy composition as an ensemble of structures where an atom of one element is surrounded by atoms of another element and used the size of such ensembles as a parameter that was indicative of the catalytic activity. The current thinking in this area is that the effect of alloying 254
Synthesis and Characterization on catalytic activity is chiefly controlled by the geometric arrangement of the metal atoms (ensemble effect), but the hypothesis that electronic effects control catalytic activity (ligand effect) still remains.
Preparation of Alloy Ultra-Fine Particles Because the catalytic activity of metal particles changes with the particle size, a similar effect is expected with alloy catalysts. To examine this behavior, we examined methods for varying the particle size. Because the catalytic activity is strongly affected by impurities, the preparation must avoid contamination. Aqueous decomposition of metal alkoxides was used in this study, because it was suited for the preparation of high purity oxide powders. Iron-nickel and and iron-cobal catalysts supported on Si0 2 were produced. The method is shown in Figure 1.
1Dissolve Ni and Fe nitrates in ethylene glycol
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Figure 1. Flowchart showing catalyst preparation steps using the alkoxide method. 255
By dissolving nitrates of iron, nickel, and cobalt in ethylene glycol (EG), one obtained metal-ethylene g1ycolates [12]. By adding tetraethyl orthosilicate and continuing to stir, chemical species having (-Si-O-NiO-Si-) and (-Si-O-Fe-O-Si-) structures were formed. Gels were produced by adding water to these solutions, but the structures just mentioned are retained in the gels. In the powders obtained by drying the gels, nickel and iron ions exist in a highly dispersed state. By heating the powder at 500°C, the iron and nickel ions combine to form oxide particles (it is possible that nickel fenites may be partially formed, although these were not detected spectroscopically). By heating the mixture in flowing hydrogen at 900°C, the oxides were reduced to metal UFPs that reacted instantly to fonn alloy UFPs. As shown in Figure 2, these alloy particles have unifonn diameters. A sharp particle size distribution was found (average diameter of 16.8 run) as shown in the lower part of Figure 2. As a comparison, an alloy catalyst of the same composition was prepared using the
60
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Figure 2. TEM micrographs and particle size distributions of alloy fine particles of Fe-Ni/Si02 catalysts. a) Alkoxide method, and b) immersion method.
256
Synthesis and Characterization
conventional impregnation method. An electron micrograph of this material and its particle size distribution are shown in Figure 2b. In the impregnation method, silica powders were added to an aqueous solution of iron and nickel nitrates, dried, and reduced under the same condition as used for the alkoxide method.
Structure of Alloy Fine Particles Extended x-ray absorption fine structure spectroscopy (EXAFS) was used to examine the structure of the alloy fine particles prepared by the alkoxide method. The EXAFS is an effective method for doing structural analyses of fine particles and amorphous samples that are hard to examine using x-ray diffraction. Figure 3 shows EXAFS Fourier patterns of standard iron and nickel samples and those of the alloy catalyst near the iron and nickel absorption edges. The standard patterns from iron and nickel show the bcc structure for iron and fcc for nickel. However, the iron pattern for the alloy catalyst (lower left) strongly resembles the fcc pattern of nickel. This implies that alloy particles of the catalyst are formed by dissolving iron into the nickel lattice. For Fe-Co alloys, cobalt dissolved into the iron lattice and the particles formed the bcc structure. It is not known, however, why iron dissolves in the nickel of Fe-Ni alloys whereas cobalt dissolves in the iron of Fe-Co alloys. Figure 4 shows EXAFS spectra near the iron and nickel absorption edges of the alloy catalyst as it was heated in air. These show that the iron and nickel in the alloy are oxidized progressively, forming iron and nickel oxides. However, iron oxidizes before nickel, implying that iron is concentrated on the surface of the alloy particles. That is, the alloy particles have a high concentration of iron on the surface.
Particle Size Control of Alloy Particles It was found that alloy fine particles having uniform particle sizes can be prepared by the alkoxide method. To control the particle
257
Ultra-Fine Particles
6
8
8
Distribution from center atom I A
Figure 3. TheEXAFS Fourierpattems ofiron, nickel and Fe-Ni/SiOs catalysts (arrows indicate characteristic peaks of fcc lattice).
Ni EXAFS
Fe EXAFS
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Figure 4. The EXAFS analysis ofthe oxidation process of Fe-Ni/SiOs catalysts. With increasing temperatures, the first peak of the Fe absorption edge grows indicating the formation of iron oxide. 258
Synthesis and Characterization
size, several catalytsts were prepared each with different alloy concentrations while keeping the ratio of iron to nickel constant. For this purpose, gels were produced by varying the amount of ethylene glycol that was used to dissolve the iroti and nickel nitrates and tetraethyl orthosilicates in the steps shown in Figure 1. For all ofthe concentrations, EXAFS analyses confirmed that alloys were formed. The results shown in Figure 5 indicate that the particle size varies with the alloy concentration. This figure also includes the results for Fe-Co particles.
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Concentration ofalloy / wt% Fe-Ni alloy Fe-Co alloy Figure 5. Control of the particle size of alloy particles by varying the alloy concentration. 259
Ultra-Fine Particles The preparation of supported metal catalysts via the alkoxide method allows for control of the metal particle diameter by varying the metal concentration in the catalyst [12]. This was extended to binary alloy catalysts in the present study. Currently, the decomposition reaction of formic acid by preparing several silica supported catalysts with Fe-Ni alloy particles with diameters of 4 to 20 nm is being examined. In decomposing formic acid, the formation of CO 2 by dehydrogenation reactions and the formation of CO by dehydration reactions proceed in parallel. Thus, by determining the amounts of CO and CO 2 formed, one can measure the selectivity of the catalysts. It was confirmed that the selectivity for dehydration increases as the alloy particle size increases. This will be reported elsewhere.
References 1.
2.
3.
4. 5.
Boudart, M., Adv. Catal. 39: 125 (1969); Anderson, J. R., Structure of Metallic Catalyst, Academic Press (1975); Anpo, M., Matsuoka, M., Shioya, Y, Yamashita, H., Giamello, E., Che, M., and Fox, M. A., J Phys, Chern. 98, 5744 (1994). Nakamura, M., Yamada, M., and Amano, A., J Catal. 39:125 (1975); Veno, A, Suzuki, H., and Kotera, Y, JCS Faraday Trans. 1(79): 127 (1983); Brunelle, J. P., Sugier, A., and Page, J. F., J Catal. 43:273 (1976); Takahashi, N., Mizushima, T., Kakuta, N., and Veno, A., JCS Faraday Trans., (1) 88: 2551 (1992). Arai, H., Surface 17: 675 and 746 (1979); Aika, K., Surface 19:495 (1981); Veno, A., Surface 22: 18 (1984); Furusawa, K., Tsutsumi, K., Matsumoto, A., Kaneko, K., and Veno, A., Jikkenn Kagaku Kouza, Maruzen (Tokyo), vol. 13, p. 85 (1993). Balandin, A. A., Z. Physik. Chern. B.2: 289 (1929); Adv. Catal. 10: 96 (1958). Long, J. H., Frazer, J. C. W., and Ott, E., J Am. Chern. Soc. 56:1101 (1943). 260
Synthesis and Characterization 6. Schwab, G. M. and Holz, G. z., Anorg. Chern. 252:205 (1944): Schwab, G.M., Trans. Faraday Soc. 42: 689 (1946). 7. Dowden, D. A and Reynolds, P. W., J. Chern. Soc. 242: 255 ( 1950). 8. Takeuchi,T., Sakaguchi, M., Miyoshi, I., and Takaba, T., Bull. Chern. Soc. Jpn. 35:1390 (1962). 9. Reinacker, G. and Bommer, E.A, Z. Anorg. Chern. 242:302 (1939); Reinacker, G. and Vormum, G.:Ibid. 283:287 (1956). 10. Hufner, S., Wertheim, G. K., Cohen, R. L., and Wernic, 1. H., Phys. Rev. Letters 28:488 (1972). 11. Sachtler, M. W. H. and van Santen, R. A, Adv. Catal. 26:69 (1977). 12. Tohji, K., Udagawa, Y, Tanabe, S., and Ueno, A, J Am. Chern. Soc.l06: 512 (1984); Ueno, A, Trends in Organic Chemistry, 1, 103 (1991).
261
3 tTL TRA-FINE MICROBES
PARTICLES
AND
3.1 Phagocytosis of Ultra-Fine Particles by Cells (by Hiroshi Miyamoto)
One of the basic interactions between UFPs and cells is the phagocytosis of UFPs by cells [1]. Phagocytosis is the process whereby a cell absorbs particulate matter from the exterior. This is observed widely from protozoans to mammals. This action is important in nutrition intake and in the removal of extraneous substances. In mammals, including humans, this activity is highly important as a self-defense function in immunocytes such as white blood cells and macrophages [2]. UFPs and organisms meet at various organs such as breathing organs, skin, digestive organs, etc. There is a tremendous amount of UFPs in the environment on the earth, and these interact with living organs on a regular basis. In the atmosphere, UFPs take the form of dust, fumes, smoke, and mist, or, more broadly speaking, as aerosols. In the aqueous environment of rivers, seas, and tap water, UFPs exist as colloidal particles. When these UFPs enter a body, phagocytosis acts as a defense mechanism to eliminate such UFPs. When UFPs enter the lungs, alveolar macrophages treat them, [3] while those in the bloodstream are processed by white cells and monocytes in the blood and by Kupffer's stellate cells in the liver. In addition to natural UFPs, UFPs are introduced artificially. For example, medicines are often encapsulated in a film of fatty acid 262
Ultra-Fine Particles and Microbes and synthetic polymer to form microcapsules that gradually release the medicine so that it acts for an extended period. Similar methods are used in "bullet treatments" that target specific parts that are affected and in treatments in which drugs need to be protected from the decomposing action of enzymes. However, when UFP-sized medicines enter the bloodstream, they can be consumooas extraneous matter by white cells and macrophages. This ultimately prevents the medicine from acting as intended. The problem is that phagocytosic cells, such as white cells and macrophages, recognize all UFPs from outside as being extraneous and consume them. These cells recognize cells like themselves and do not exhibit cannibalism. It is thus important to understand how these cells recognize and discriminate other cells and particles that they encounter. Depending on the first step of phagocytosis (i.e., recognition and discrimination), phagocytosis can be classified into two types. One type is receptor mediated phagocytosis, which is called specific phagocytosis. The other type is phagocytosis without receptors, which is called non-specific phagocytosis. The receptor mediated phagocytosis that is involved in the uptake of UFP-like matter can involve recognition of the Fc part of antibodies, recognition of the C3b complement, recognition of proteins that specifically bind the sugar known as lectin, or that recognize various complex sugar chains [4]. When a bacterium enters a body from the outside, the antibody, if it exists for that bacterium, is coupled. The antibody is recognized by white cells or macrophages, which then consume the antibody and bacterium. When a complement body is coupled, the same process occurs. When these will not couple, phagocytosis occurs by recognizing the superficial sugar chains on the bacterium. Such phagocytosis via receptors is efficient and the rate of uptake is high. In contrast, the intake mechanisms of non-specific phagocytosis are not clearly understood [5]. It is unlikely that specific receptors exist for latex beads, metal or ceramic UFPs, and aerosols like soot or smoke. When such dusts enter the lungs, this kind of phagocytosis cleanses the lungs in a manner similar to the undesired consumption of microcapsulized medicines as mentioned 263
Ultra-Fine Particles above. Thus, the study of recognition mechanisms in this type of phagocytosis is quite important in terms of the consumption of undesirable waste particles, while inhibiting the undesired consumption ofdesirable particles. However, the related mechanisms are not well understood. So far, it is speculated that surface charges and hydrophobicity may playa role, but this has not been confirmed. One reason for the lack ofprogress is the difficulty of getting samples with uniform particle sizes and surface characteristics. To overcome this problem, UFPs produced by gas evaporation were used as phagocytosis samples, which were of uniform size and possessed clean surface states. By selecting silver UFPs, one can further limit the variables involved because no other elements are present. Optical microscopy was used to study the phagocytosis by cells of silver and other UFPs.
Slow-Speed Microscopic Photography of UFP Phagocytosis by Cells Experimental procedures Two kinds of cells were used in the experiments during this study. One was a macrophage-like cell, J774.2, derived from mouse lymphoblast and the other was a fibroblast-like cell, BHK-21, from hamster kidney. Both were cultured in a Dulbecco modified MEM culture medium with 10% calf fetal serum as the culture fluid. These were cultured in plastic dishes in a CO2 incubator and sub-cultured every 3 days to 1/10 and 1/40 each. The macrophages were withdrawn from the dishes by pipetting and the BHK cells were removed using trypsin. For slow speed recording with an inverted microscope, a warming apparatus was installed and phase contrast images were obtained using a Newvicon high-resolution camera and recorded on a U-matic video tape recorder. For slow speed photography, intermittent operation with synchronized lighting was used to record 4 to 5 frames every 6 seconds. This produced about a 40-fold
264
Ultra-Fine Particles and Microbes decrease in the speed, which allowed us to record 40 hours of continuous observation on a 60 min tape. Macrophage and BHK cells were transferred to flat-bottom flasks a day before the beginning of video recording and were kept in an incubator. The UFPs that were fed to the cells were suspended in pure water at a concentration of2 mg/ml immediately before feeding. For 8 ml of culture fluid, 100 III ofUFP dispersion was added. When Zymosan particles were used, 100 III of a suspension containing 1 mg/ml was added. Results
From the slow speed recorded video, the following information, which had not been noticed before in the usual microscope observations, was found. Macrophage-like J774.2 cell
1- The cultured cells have two types of morphologies. One of the morphologies is round and the other is flat and stretched out. The long, flat cells are more active in phagocytosis and have faster consumption rates than the round cells. 2- Phagocytosis is done by extending a pseudo-pod (I-11m diameter protrusion) in search of UFPs or other extraneous matter. Upon coming into contact with a UFP, the cell attaches to the UFP, brings it closer to the cell, and absorbs it into the cell membrane. In another method, the cell extends a flat protrusion of more than 5 11m in width over a UFP, capturing it and bringing it closer by contraction. This process is characterized by the large size of the protrusion and the active "ruffling motion" that can be seen. The round cells capture particles by using thin pseudo-pods, while the elongated cells use a ruffling motion. 3- About 10 min before the cell division, the round cells retract the pseudo-pods and the elongated cells stop the ruffling motion and 265
Ultra-Fine Particles contract, both becoming spherical cells without protrusions. In this state, no phagocytosis occurs. Phagocytosis starts again after the cell division is completed and the two cells are almost separated. 4- The particles that were consumed by cells and internalized by a membrane move toward the nucleus of the cells. 5- When the phagocytosis of silver UFPs was completed and the cells were continuously observed for 10 more hours, no cell damage was apparent, but the occluded particles tended to gather at the cleavage furrows when the cells split. Sometimes the region joining two daughter cells that have not separated are found to collect silver UFPs. This means that the coagulated UFP region is rigid and the last step ofcell division or the separation ofthe cell membrane cannot proceed to completion (see Figure 1). Baby hamster kidney cell BHK-21
1- Unlike macrophages, all the BHK-21 cells are basically stretched flat. The overall mobility is less than that of macrophages. The phagocytosis of particles by BHK-21 cells involves only motion at the leading edge. 2- The dynamic type of phagocytosis that would correspond to the ruffling of macrophages is weak in comparison and it is limited to only a small movement around the periphery of the BHK-21 cells. 3- Before cell division, the cell shape changes from a stretched shape to a round shape and no phagocytosis occurs until the completion of splitting, as was observed for macrophages. 4- The particles that entered a cell by phagocytosis were enclosed in a membrane and moved within the cell toward the nucleus. For BHK-2 I cells, a strange phenomenon was observed in this process. After phagocytosis at a relatively early period, the particles within the
266
Ultra.Fine Particles and Microbes
Figure I. Effects of phagocytosis of silver UFPs on cell splitting. Micrograph 10 hr after the phagocytosis of silver UFPs by a macrophage.like cell. Left side: phase contrast images. Right side: bright field images of the same fields. The silver UFPs appear bright
on the phase contrast images and dark on the bright field images.
267
Ultra-Fine Particles cells rotated in a certain direction for as long as 30 min. This will be described in detail in the next section. Rotational Movement of Consumed Particles in Hamster Kidney Cells Analysis of the rotational movement of fine particles
Silver UFPs were given to BHK cells using the previously described method and the cells were left to complete phagocytosis. For recording, a 16-mm movie camera was used and photographing was done at a rate of one frame every three seconds with the slow speed attachment. A video camera could not be used because the resolution was inadequate, even with a high resolution unit, to record the rotation ofthe particles. From the film, it was possible to identify rod-shaped particles and to use the film to analyze the rotation. Not all of the particles exhibited rotation, which was limited to a certain period after their phagocytosis. When the particles were gathered near the nucleus, no rotation was observed. Thus, it was difficult to determine the ratio of rotating and non-rotating particles. This is in part due to the small size and spherical shape of the particles and because the rotation was difficult to detect by using an optical microscope. The present analysis is based on the chance observation of a rotating particle. Using an editing viewer to analyze the 16 mm film, the degree of rotation was directly measured on each frame using a protractor. The results are shown in Figures 2a and 2b. The particle in Figure 2a started to rotate counter clockwise for about 1 min, then clockwise for the next minute, then counter clockwise for the following 2 min, and finally clockwise for an extended period. The last rotation occurred for about 22 min during which time the angular velocity was almost constant at an average value of 400 D Imin. This is about the speed of a second-hand on a clock. The particle in Figure 2balwaysrotated in one direction for about 32 min with an average angular velocity of 110 D/min. The major diameters of the particles in Figures 2a and b were 2.4 and 3.9/lm, respectively. Assuming that there is a
268
Ultra-Fine Particles ami Microbes Rotation
(radian)
)
I
a.
18" f-----+-----t-------Ir---40 " I - - - - - i ' - - - 16"
~-__+_---+--~~H_/--38"
/
I
/
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~-------t--_+__-I+__/- t - - - , , "
IOn
/
he 1
46,,-
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I - 30" 40,,-··-'t--/--II'-t------+--32" /
"
8" 1--------1----t---:t-
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4 ,.1-----+------1+-/-----26 "
/
1
:~I----~--If----A-I-f--+----241"---f/'---+----t---
-2 n
Rotation
(radian)
v \f /
+10
~201t/
2tI
210
I
,..,.......
Time (min)
b.
20"
V
--
18· \6"
,/ ,../
14"
12" lOr
V
8"
4" /'
o
/'"
~
---
V
/
/
/
~ 5
10
15
20
25
30
Time (min)
Figure 2. Rotational motion of occluded particles within a BHK cell. After silver UFPs were consumed by a BHK cells, the rotation within the cell is shown. The degree of rotation in radians is shown as a function of time.
269
Ultra-Fine Particles membrane surrounding each particle, one can calculate the speed at the periphery. The peripheral speed was 8.4 !-un/min for the particle shown in Figure 2a and 3.6 Ilm/min for that in Figure 2b. These speeds are much slower than the rate of muscle contraction, and are even slower -than- -the- -pa..'1:icle- -motion--within: ·flerve-IDwns· {6; 7]. -Howevcr; -the speed is comparable or a little slower than the parallel movement of granules inside a fibroblast. Following the rotational motion, each particle moved toward the nucleus. This speed was slower than the rotational speed. In any event, the rotation at a fixed location for 20 30 min is a very novel phenomenon. In the biological field, the only rotational movement known to date is the flagellum motor of a bacterium [8]. The rotational movement of the particles has not been clarified, but this discovery originated from the phagocytosis of nonisotropic particles (with morphological anisotropy). In this sense, this is an example ofUFPs providing insight into the motion of cells.
References 1. Miyamoto, H., Application Techniques ofUFPs, Japan Powder Ind. Assoc., p. 115 - 131, Nikkan Kogyo (1986). 2. Minakami, S., White Cells and Phagocytosis ( Minakami, S. and K. Kakinuma, eds) p. 1 - 8, Kodansha (1979). 3. Brain,1. D., Environmental Health Perspectives, 97: 5-10 (1992). 4. Sharon, N., Immunology Today, 5: 143 - 147 (1984). 5. Costa, M., Ann. Rev. Pharmacol. Toxicol., 31: 321-337 (1991). 6. Yanagida, T. et aI., Nature 316: 366 (1985). 7. Kurokawa, M., Protein Nuc. Acid Enzyme, 28: 450-468 (1983). 8. Macnab, R. and Aizawa, S., Ann. Review Biophysics Bioeng., 13: 51-83 (1984).
270
Ultra-Fine Particles and Microbes
3.2 Application of Ultra-Fine Particles to the Detection of Cell Activity (by Hideki Toyotama and Iku Nemoto)
Work on the measurement of pulmonary magnetic fields was started in 1973 by D. Cohen and co-workers at MIT [1]. This study was aimed at evaluating lung function by putting magnetic UFPs into the lungs and measuring the weak magnetic fields associated with the UFPs. After volunteers were aspirated with aerosols of magnetite (Fe30 4), a direct magnetic field of about 100 gauss was applied to magnetize the magnetic UFPs. Immediately afterward, volunteers were placed in a magnetically shielded room and the very weak residual magnetic fields formed outside the lungs were measured using a very sensitive superconducting quantum interference device (SQUID) magnetometer. From the results, the amount of particles that settled in various parts of the lungs were estimated [2, 3]. When the results of pulmonary magnetic fields are examined, the magnitude of the observed fields decreased with time after the magnetization [4]. Because the magnetic fields of the UFPs cannot decrease at these high rates, the origin of the relaxation phenomenon must be belated to an irregular rotation of the particles. The UFPs in the lungs are expected to be taken in by wandering cells that have a high capacity for phagocytosis, known as alveolar macrophages. The magnetic relaxation can be attributed to the rotation of UFPs within the alveolar macrophages. If such a process actually occurs, analysis of the relaxation will allow for determination of the microscopic viscosity within cells. The type of cells that can be used is not limited only to alveoli because UFPs can be introduced into other cells as well. Furthermore, other type of measurements are also possible. In the past, optical microscopy has been the only means of quantitatively examining the movement inside a cell. The use of UFPs may provide a new approach for this kind of investigation.
271
Ultra-Fine Particles
Measurement of Magnetic Fields of Lungs and Cells
First, it was confirmed that the magnetic relaxation is also observed in alveolar macrophages taken out of the lung. Hamsters were selected as the experimental animal because they had been used earlier in the study of pulmonary magnetic fields. For the magnetic UFPs, magnetite (Fe30 4) and iron oxide (y-Fe 20 3) of several tens to hundreds of nanometers in diameter were used. Metallic UFPs of iron and nickel were also used, but the iron oxides were the least damaging to the living systems. Figure 1 shows a diagram of the apparatus used to measure the magnetic field of cells. Alveolar macrophage cells removed from hamster lung were attached to the bottom of a glass flask. The cells were magnetized to several thousands gauss by using a direct field from a coil and the weak magnetic fields from the cells were immediately measured inside a magnetically shielded area using a flux-gate magnetometer. To avoid the effect of baseline drift, the sample was moved back and forth over the magnetometer probe and the difference in the magnetic field measured when the sample was over the probe and when it was not was used as the basis for the measurement. The sample temperature was controlled using a water jacket. When alveolar macrophages were taken from the lung of a hamster that had aspirated magnetic UFPs (20 Ilg/hamster) and were then observed under an optical microscope, the cells contained the magnetic UFPs. An example of magnetic measurement using such cells is shown in Figure 2. The results indicate that the relaxation is almost identical to that observed in the magnetic fields of the lungs and that the effects seen are due to the rotation of the magnetic UFPs in the cells. Figure 2b shows that the relaxation stops when a few drops of formalin are added to the culture fluid during the measurement. This shows that the magnetic relaxation phenomenon is observed when cells are in an active state, but that this relaxation phenomenon disappears when cells become inactive. 272
Magnetic shield
r------------I I -
tv
7
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7',
/
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.
,,7/
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(
t
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~
I
\
' 5 '' I I i: . Ii
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\ \
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I
'l' \\
Probes for flux \ gate magnetometer \
'------ --- - - ----y Figure 1. Diagram of the apparatus used to measure the magnetic field of cells.
I / /
I
~
:::~
~
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~ ~
l::l
:::
l::l..
~
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Ultra-Fine Particles
(0 )
(bl
\. ~
r
0< t < 1 min
+10% formalin
Figure 2. An example of the measurements of magnetic fields from cells. The coefficients Ao and Bo are obtained when the relaxation curve for the first 1 minute is approximated by B oe-.1.ol. 274
Ultra-Fine Particles and Microbes Next, the effects of temperature on the rate of relaxation were examined. Because the relaxation curves cannot be represented by a cells single exponential function, several rate parameters were used (i.e., the time to reduce to half strength (T1/2), the coefficient ~ derived from the relation Bo-Aot for the first I-min period, A obtained from the data from 2 to 5 min, etc.). Actual relaxation curves as a function of the temperature are shown in Figure 3. The activation energy determined from Ao was found to be 6.3 kcal/mol, while that determined from A was 8.0 kcal/mol.
100
~
~ ~
"C
Cii
.
u: 0
ell
C Cl
ns
== "iii
22
:::l "C
·iii. ell
27 32
0::
T~---,---'--------L-----L.---L-----'-02468 Time After Magnetization (min)
Figure 3. Actual relaxation curves as a function of temperature. The dashed curves indicate the limit of repeatability of the measurements done at 3rc. 275
Ultra-Fine Particles Intake of Inorganic Ultra-Fine Particles into Cells
The experimental system using alveolar macrophages removed from lungs allowed for direct comparison to the results obtained from a live body in terms of pulmonary magnetic fields and those of cells. A similar principle of measurement was used by a Harvard group, which reported on the magnetic relaxation presumed to originate from the phagocytosis by liver cells that had taken in magnetic UFPs in the bloodstream [9]. To study microscopic cellular information, it is advantageous to use cultured cells rather than those from animals that have differences between individuals. Thus, we also evaluated the relaxation phenomenon for inorganic UFPs using cultured cells. The cells were inclined to take in magnetic iron oxide UFPs of Fe30 4 and y-Fe2 0 3 and they suffered essentially no damage. Table 1 lists the phagocytosis by 1.774 cultured cells fed with various gas evaporated inorganic UFPs. The zeta potentials of these UFPs in aqueous solutions are also given for comparison [10]. For cells to recognize inorganic UFPs, the surface charges and hydrophobicity may playa role [11]. The present results indicate that the cells recognize the negative charges on the particle surfaces and take in these particles. Other reports indicated opposite results, so the recognition mechanisms are still inconclusive. To evaluate the nonspecific recognition of cells, the characteristics of particles to be taken in must be uniform with respect to their size, shape, and synthesis conditions. The UFPs used in other studies to date have had problems, but the gas evaporated UFPs used in our study should meet these requirements. Figure 4 shows a phase contrast optical micrograph of a J.774 cell that was fed silver UFPs (by gas evaporation, 70-nm diameter) and held for 3 hrs. The white spots on the photograph are secondary particles of coagulated silver UFPs. These were found to be uniformly distributed over some areas, but other areas lacked the spots entirely. The particles that were in regions were the spots were not seen were taken in by the moving cells.
276
Ultra-Fine Particles and Microbes
Table 1. The Zeta Potentials and Phagocytosis of Inorganic UFPs. UFP Type
Mobility (l-l/secN/cm)
Zeta Potential (mV)
Phagocytosis
Comments
Cu
+3.12
+44.9
-
UFP
Ni
+2.94
+42.3
-
UFP
Fe
+2.36
+34.0
-
UFP
yAl 20 3
+1.87
+26.9
+
UFP
Fe30 4
+0.81
+11.6
+
Chemical
y-Fe20 3
-0.34
-4.9
+
UFP
ZnS
-0.46
-6.6
+
UFP
y-Fe 20 3
-0.90
-12.9
+
Chemical
Ti0 2
-2.23
-32.1
+
UFP
Ag
-2.95
-42.5
+
UFP
Si02
-3.69
-53.1
+
UFP
The zeta potentials were measure in 2 mM KCI with 2 mM imidazol buffer solution at pH 7.2. Phagocytosis was judged by optical microscopyjive hours after feeding UFPs. "UFP" in the Comments section indicates that these experiments used gas evaporated UFPs and "Chemical" indicates that these experiments used chemically produced particles. Measurement of Cellular Magnetic Fields Using Cultured Cells [8]
Because magnetic UFPs can be put inside ordinary cultured cells of animals, the movement in these cells was studied in more detail. Figure 5 shows the results of magnetic measurements that indicate the processes by which cells take in magnetic UFPs as a 277
Figure 4. A phase contrast optical micrograph of a 1774 cell 3 hours after feeding silver UFPs. 100
.-,•
• ~
80
~ ~
~
~c a
• ;',"; ~
":c o
,
3
•
TIme After Magnetization (min)
Figure 5. Results from magnetic measurements that indicate the processes by which cells take in magnetic UFPs as a function of time (8). Lines A,B,C and D indicate the data from measurement of the magnetics fields IS, 45, 60 and 150 min., respectively, after UFPs were added to the cells. Line D' indicates the data from measurement 150 min after giving metabolism inhibitant along with the UFPs.
278
Ultra-Fine Particles and Microbes
function of time [8]. When magnetic UFPs are added to the cultured fluid, they sink to the bottom of the dish as chains of particles due to magnetic attraction. The magnetic measurements were made in a direction normal to the bottom of the dish and are sensitive only to particles that have been taken into cells and that have become free to rotate. As the number of particles taken into the cells increased with increasing time, the fraction of particles that relax increased. At the same time, the fraction that relax at a rapid rate also increased. The particles were taken in by pseudo-pods on the periphery of the cells and gradually moved to the central part of the cells. Within the cells, a filament structure known as a cytoskeleton developed, which is related to the intracellular motion. This structure is also known to be different at different parts within a cell. The changes in the rate of relaxation appear to reflect microscopic environmental variations surrounding the magnetic UFPs. What causes the rotation of particles in cells? A possibility is thermally induced rotational Brownian motion. Another possibility is motion induced by the intracellular filament structure (cytoskeleton), which would require metabolic energy. The first possibility may be discarded based on the following observation. The relaxation immediately stopped (Figure 2) when formalin was added to the culture fluid. The rates of relaxation were strongly affected by temperature (if it is a simple thermal motion, the energy is proportional to the temperature). Furthermore, the particles were associated with the cytoskeleton. Thus we presume that the rotation is of a biological origin that is accompanied by the consumption of ATP to produce energy. Next, the concentrations of anti-metabolites such as KCN, FCCP, monoiodine acetate, etc were changed. When these were added to the culture fluid, the relaxation rates slowed down after several min, and no relaxation occurred after an hour [12]. The decreasing concentration of ATP in the cells also correlated well with the decrease in the relaxation rate. When drugs that inhibit the action of cytoskeleton (i.e., actin, microtubule, and 10 nm filament) or destroy it were given, there was an effect on the rate of relaxation [13]. The effects observed were not
279
Ultra-Fine Particles uniform, with some showing faster Ao, some showing slower A, and others in which all parts became slower, but these results indicate that each component forming the cytoskeleton has complex interactions with other parts inside the cell. By using a cellular model [8], one can directly demonstrate that intracellular relaxation phenomena are active, energy consuming processes that producing energy by the reaction of ATP -+ ADP + Pi + 7 kcal. The cellular model used was a dead cell that had part of its surface membrane dissolved using a surfactant, which made it permeable to low molecular weight materials. Such models have been used to show that the energy source for the contraction of muscle and for the bending motion of sperm flagellum is the decomposition of ATP. Figure 6 shows the results in which the relaxation phenomena with a cellular model was reproduced by feeding an ATP solution from outside the cell. This model was made by treating cultured cells of animal BHK cells with detergents such as Triton X-IOO. When the ATP concentration was zero, no relaxation occurred, while at an ATP concentration of 0.1 mM, the rate of relaxation was comparable to that observed in a live cell.
Modeling and Experimental Investigation of Relaxation [14,15] An attempt was made to develop a mathematical model for the phenomenon ofrandomizing the particle orientations inside a cell. In terms of polar coordinates, the initial magnetization direction is Z, the azimuth angle of the magnetization direction from Z is e, and the longitude is <1>, as shown in Figure 7. When a magnetic UFP undergoes Brownian-like motion under irregular forces, the distribution density of e (per unit dimensional angle) at time t can be obtained by Equation 1 (where: Y= 8n 3,,): 1 sine
a. e(Er ap ae y ae
----SID
280
+
JlH .
P-SID
y
e
)
(1)
Ultra-Fine Particles and Microbes
,
.
"C
a;
u:u
.... ~\
".''\'..,', "";~\. .
~
.
c:
01
co-
:=;;: ii
.g
..
~. \.~.
:\ ..:
~'\/""~"".'''''''''\''''.' . ./\~'''/'J'\''\/_'_'' D
50 1'0
'iii 0:
Figure 6. Results producing the relaxation phenomena with a cellular model. The data indicated by A is a control (living cells with an ATP concentration ~ 0.3mM). The data indicated by B, C, and D are measurements with the cellular model with Mg ATP concentrations 0[0,0.1, and 3.0 mM, respectively.
z
y
x Figure 7. The polar coordinate system used in the model. 281
Ultra-Fine Particles
Here, Er is the energy to randomize the particle orientation, r is the particle radius, "is the apparent viscosity surrounding the particle, fl is the magnitude of the magnetic moment of the particle, and H is the external magnetic field strength. Given the initial value of Po as obtained by Equation 2 below, Equation 1 can be solved analytically. P (8) = Ce o
acos8
(2)
The distribution of Equation 2 represents the equilibrium distribution under the external magnetic field Ho. Thus, the solution of Equation 1 for H=O with the initial value given by Equation 2 corresponds to finding the expression for relaxation under zero external field after reaching a state of equilibrium under H = Ho • The solution is as shown in Equation 3.
Here, P n (cos8) is the Legendre polynomial and I n+1I2 (a) is the modified Bessel function of the first order. The measured magnetic field strength is proportional to M, as shown in Equation 4. 1t
fcos8sin8P t(8)d8 M
0
(4)
1t
f sin8P t(8)d8 0
282
Ultra-Fine Particles and Microbes From Equations 3 and 4, M takes the form shown in Equation 5. E,
M(t) = e
-2-t 1 v [coth(a)--]
(5)
a
Setting t in Equation 5 to zero, one obtains Equation 6. 1
L(a) = coth(a) - -
a
(6)
This is a Langevin function that gives the strength of cellular magnetic field in the equilibrium state under a field ofHo = aEill When a weak field H o was applied during the relaxation, the cellular magnetic field approached L(a). This is the condition where the force to randomize the particles and that to align by Ho are in balance. The rate of relaxation is given by Er / 41tr3 11, which is a function ofEr and 11. In comparison, Er is the only cellular parameter a depends on. Thus, Er can be estimated. By combining this with the rate of relaxation, the apparent viscosity 11 can be estimated. As an example, the ratio of the energy needed to randomize within a cell, Er , at 20°C was obtained. At 3rC, E r was about 2 to 3 times its value at 20°C (the ratio of the thermal energy, kT, at 20 and 37°C was 1.06). The following is a summary of the above results that support our hypothesis that the rotation of magnetic UFPs within a cell is not merely due to thermal energy, but that it is a result of active motion within the cell. 1) There is a relation between the temperature and the rate of relaxation, 2) there is an effect caused by anti-metabolites, 3) there is an effect caused by cytoskeleton inhibiting agents, 4) the relaxation phenomenon could be reproduced in a cellular model without a membrane, and 5) the mathematical modeling of relaxation and the experimental determination of the energy involved support the model. 283
Ultra-Fine Particles In this section, studies of living organs were described, especially the motion within cells. These studies involved the introduction of magnetic UFPs into living organs and subsequent measurement of the magnetic fields produced by the UFPs. The magnetic UFPs tended to coagulate, which was a problem in using this method. If a suitable coating technique can be developed, this method may find wider application in such studies. The analytical results concerning the reduction in intracellular mobility due to changes in physiological parameters were presented. The rate of relaxation was also affected by changes in the morphology of the cells. The structure and composition of the cytoskeletons also changed when the cells became cancerous. Such changes may be detectable by the present method. It appears that the motion of magnetic UFPs taken into cells is controlled by active processes involving intracellular structures. The results from this study and further use of the methods described here can lead to a deeper understanding of cellular motion [16].
References 1. Cohen, D., Science 180: 745 (1973). 2. Cohen, D., IEEE Trans. Magn. MAG-II: 694 (1975). 3. Kalliomaki, P. L., Kohonen, 0., Vaaranen, V., Kalliomaki, K., and Koponen, M., Proc. Int. Arch. Occup. Environ. Health 42: 83 (1978). 4. Nemoto,1. and Toyotama, H., Densi Tsuushin Gakkai Res. Group, MBE84-24 (1984). 5. Toyotama, H. and Nemoto, 1., Biomagnetism Applications & Theory (ed. by Weinberg, H.) p. 401, Pergamon Press (1984). 6. Nemoto, 1., Toyotama, H., Gehr, P., and Brain, J. D., ibid p. 433 (1984). 7. Nemoto, 1., Ogura, K., and Toyotama, H., Transactions of the IEeE of Japan, 69(11): 1231 (1986). 8. Toyotama, H. and Nemoto, 1., Surfaces 25(5): 299 (1987). 9. Gerh, P. and Brain, J. D., Nature 302: 336 (1983).
284
Ultra-Fine Particles and Microbes 10. Toyotama, H., 39th Colloid. Interfacial Chern. Meeting Abst., p. 394 (1986). 11. Van Furth, R., Mononuclear Phagocytes Functional Aspect, Part II., p. 895, Martinus NijhoffPublishers (1980). 12. Toyotama H., Surface Chern., 8(5) (1987). 13. Toyotama, H., Pharmacia 23: 901 (1987). 14. Ogura, K., Nemoto, 1., and Toyotama, H., 6th Inter. Conf. of Biornag Proc. (1987). 15. Nemoto, 1., Ogura, K., and Toyotama, H., IEEE Transactions on Biomedical Engineering: 26, 598, (1989). 16. Nemoto, 1., Toyotama, H., Brain, J. D., and Gehr, P., Frontiers Med Biol. Engng., 1, 193 (1989).
285
Ultra-Fine Particles
3.3 Organic Compound Ultra-Fine Particles (by Hideki Toyotama)
Organic UFPs rather than inorganic UFPs have a closer connection with the biological sciences. In particular, there are a variety ofUFP-size «0.1 pm) materials that surround us in our daily lives and which have attracted great interest in recent times. For example, there are UFP-sized food additives, foods, and medicines that enter our bodies through our mouths and there are aerosols such as cigarette smoke, factory and automobile exhaust, and other such materials that enter through our respiratory track. Let us next examine living matter by dividing it into constituents such as individuals, structures, cells, etc. Within cells, there are small organs like mitochondria (about 1 ,urn) and endoplasmic reticula (about 0.1 ,urn), and materials that form cell structures such as lipids that form membranes (thickness about 10 nm) and biopolymers like proteins (several nm), and their aggregates (supramolecules). The basic constituents of cells can be considered to be organic UFPs consisting of C, H, 0, S, and N. A typical example of a UFPs that equals living matter is a virus, which is composed of proteins and nucleic acids. In this section, a description ofthe investigation that was done on the production of UFPs of organic compounds (including biosynthesized matter), which consists of C, H, 0, S, N, and other elements is provided. This study used the gas evaporation method as described elsewhere in this book. This work was a variation of the study on cell-UFP interactions, which covered the biophysical aspect in the Ultra-Fine Particle Project. UFP Formation by the Gas Evaporation Method In this study, the gas evaporation method that was developed for metal and ceramic UFP synthesis was used [1-3]. Organic UFP production was started by placing an organic compound in a crucible in the vacuum chamber. Following evacuation, an inert gas (argon or
286
Ultra-Fine Particles and Microbes
helium) was back-filled into the chamber to a pressure of 0.01 to several tens of torr and the cmcible was heated. The organic molecules that evaporated were then collected in the form ofUFPs on a substrate placed above the cmcible. Although the mechanisms involved in this production process are not known in detail, the evaporated molecules collide repeatedly with inert gas atoms and with other organic molecules, thereby leading to coagulation and growth that yields UFPs. This is based on analogy to the synthesis of metal and ceramic UFPs. The recovered UFPs have larger diameters when the flight-time of the evaporated molecules is long, such as when the inert gas pressure is high or the distance between the cmcible and the substrate is large. Unlike metal UFPs, organic molecules have lower crystallinity, yet they form UFPs with ease. This may mean that the heat and energy transfer processes involved in the collisions between the organic molecules and the inert gas atoms are different from those in the metallic systems. Using the present method, it was confirmed that organic UFPs can be made from a large number of organic compounds number. Several examples are discussed below. Initially, the use of low molecular weight compounds such as anthracene, pyrene, phthalocyanine, and carbazole is described. This will be followed by a description of the use of high polymers such as polyvinyl chloride, polyvinyl alcohol, polyethylene, and polystyrene, as well as biosynthesized materials such as p-carotene, cortisone acetate, and chloramphenicol. As is known for the main gas evaporation method, if the starting raw materials (in this case, organic compounds), are fairly stable to heating in vacuum or inert atmosphere and have a certain level of vapor pressure, then they can be made into UFPs. Generally, the temperature of evaporation is low for organic compounds, so the resistance heating method is adequate. However, some materials tend to sublimate, so accurate temperature control is required. The shape and yield of the organic UFPs are sensitive to the stmcture of the cmcible used, the substrate temperature, and the material used. Figure 1 shows a fluorescent micrograph of pyrene (C I6 H IO) UFPs. The UFPs shown in Figure 1a were obtained by using a 287
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Figure I. Fluorescent micrograph of pyrene (C 16H IO) UFPs. a: UFPs obtained with a helium pressure of 0.1 torr and h: with a helium pressure of5 torr. For both experiments the crucible temperature was at 150°C and the substrate was glass.
288
Ultra-Fine Particles and Microbes crucible temperature of 150 a C, a distance to the substrate of20 cm, and a helium gas pressure of 0.1 torr. Those shown in Figure lb were produced with a helium pressure of 5 torr. The average particle diameters were about 100 nm and 3 ,urn, respectively. The UFPs that were produced had sharp particle size distributions compared to particles produced by conventional physical crushing methods. They also show good dispersion. Metal UFPs produced by gas evaporation sometime exhibited chain-like structures, but the organic UFPs showed no such behavior. Properties of Organic Ultra-Fine Particles [4-6]
Many organic compounds can be made into UFPs by using the gas evaporation method. The properties of organic UFPs will be described. The main asset is the availability of submicron particles having a uniform size, which is difficult to achieve by any other means. In addition, a filtering and refining effect of the materials exists. When high polymer compounds are used as raw materials, the molecular weight distribution of the UFPs is shifted to lower molecular weight compared to the starting material and the distribution becomes sharper. At the same time, the material becomes purer due to the removal of impurities during UFP formation. This is due to the higher vapor pressure of the shorter molecules, and to the difference between the vapor pressure of impurities and that of the raw material. Such behavior was expected considering the processes involved in gas evaporation, but some of the findings were unexpected. This study was started in an effort to examine the interaction between cells and UFPs and to examine the nature and effect of the UFP surfaces. In particular, the behavior ofUFPs in culture media was of interest. Thus, the hydrophilicity and hydrophobicity of various UFPs formed by the gas evaporation method were examined. For example, all of the organic materials mentioned above are hydrophobic in their bulk form and they float on water even when they are finely divided by mechanical means. However, when these compounds are made into UFPs, they disperse well in aqueous
289
Ultra-Fine Particles
solutions and behave like colloidal solutions. To evaluate the degree of dispersion, we measured the zeta potential (the surface charge in a solution) of all the UFPs. The results showed that the values are in excess of -30 mV (in deionized water) and confirmed the good dispersivity of the organic UFPs in water. It is unknown why the hydrophilicity ofthe UFP surfaces increases. The increase, however, was found for even large (micron order) particles formed by use of the gas evaporation method. Thus, this change may be a result of the inert gas evaporation process itself, rather than being due to an increase in the surface area. In regard to metallic UFPs, the Kubo theory and others predict the existence of a size effect on the electronic state ofUFPs and some experimental confirmation of these predictions have been made. For organic UFPs, however, no such theory is available. For compounds containing 1t-electrons, the electronic states of the surface and the molecules in the interior can be probed using spectroscopic means. This should be an interesting aspect of organic UFPs to study. Possible Applications
The significant increases in the affinity of organic compounds to various solvents by forming UFPs by the gas evaporation method can be used in many fields that are concerned with the dispersion of fine particles. For example, drugs that are insoluble or hard to dissolve in water require complex treatments to make them dispersible and stable. Such treatments require changing the chemical compositions and adding surfactants and emulsion agents. If these treatments can be eliminated or simplified by making the drugs in the form ofUFPs, the side-effects associated with forms of the drugs modified for better dispersion may be eliminated and their applications broadened. Similar advantages can be expected for foods, cosmetics, printing, agricultural chemicals, and pigments. When the production of organic UFPs by the gas evaporation method becomes an extension of conventional powder production methods, the amount needed to be produced will increase, so the development of mass production techniques will become an issue. In
290
Ultra-Fine Particles and Microbes the current study, a laboratory evaporator was used, which allowed for the production of only 1 g per run. Continuous feeding of raw materials to the crucible and continuous methods to recover the UFPs must be developed [7]. There is another perspective in which one may view the organic UFPs as a form of functional materials [8]. The special surface effects and other characteristics of the UFPs can be used to advantage in devices and sensors. For example, Figure 2 shows the fluorescent photoemission spectrum of anthracene UFPs produced by the gas evaporation method as a function of the pH. Anthracene is insoluble in water, but UFPs of anthracene can be dispersed in water. By changing the pH from 4 to 10, the relative emission intensity (assuming a value of 100 at pH 6.0) changes from 78 to 132. The pH sensitivity appears to reflect changes in the quantum yields of the fluorescence from anthracene molecules near the surface. As a result,
~pH
10.0
100
pH 4.0 ~50 OJ
()
l/l
OJ
H
o
::J rl
r...
300
350 400 450 Wave Length (nm)
500
Figure 2. Dependence of the fluorescent photoemission spectrum of anthracene UFPs on pH. Anthracene UFPs (0.5,um average diameter) produced by gas evaporation in helium were dispersed in water and excited at 380 nm. The effect of the pH level was reversible. 291
Ultra-Fine Particles the increase in the surface area has caused a large change in the UFPs. Although the molecules that are exposed to the surfaces represent less than 1% of the total, the intensity changes by almost a factor of two from 78 to 132, implying that the surface effects are penetrating well into the particle interior. These results are preliminary and further studies by organic chemists are needed to verify the results. This section has presented the methods for making organic UFPs via the gas evaporation method and has described the characteristics of these new materials. This project ended before the application of these materials to cell studies could begin, but some studies in this area are underway.
References 1. Uyeda, R. and Kimoto, K., Oyo Butsuri 18: 76 (1949). 2. Hayashi, C., Oyo Butsuri 50: 178 (1981). 3. Solid State Phys. Special Issue on UFPs, Agne Tech. Center (1984). 4. Toyotama, H., 39th Meeting Colloid. & Interfacial Chern., p. 394 (1986). 5. Toyotama, H., High Polymer Soc. Abst. (1986). 6. Toyotama, H., 2nd Spec. Meeting Colloid. & Interfacial Chern., p. 95 (1987). 7. Toyotama, H. and Oda, M., Funtai to Kogyo, 20: 43 (1988). 8. Toyotama, H., Functional Materials 6: 44 (1987).
292
Ultra-Fine Particles and Microbes
3.4 Encapsulation of Magnetic Ultra-Fine Particles and Immobilization of Antibodies and Enzymes (by Hideo Kakuta)
Bioreactors are the core of biotechnology, which has seen substantial advances in recent years. Bioreactors are used in the manufacture of amino acids and isomerized sugars, in the removal of environmental contaminants, and in analytical instruments used for clinical diagnosis. In a bioreactor, a raw material undergoes changes caused by enzymatic action and the resultant product is continually removed. Here, processes for the separation and recovery of the raw material, the product, and the enzyme are required. Commercial technology has been developed in which the enzyme is separated from the reaction solution by immobilizing the enzyme on a carrier and using it repeatedly. In most processes, the support particles are poured into a column and a reaction fluid is passed through the column. This process uses what are called plugflow type bioreactors, which allows for easy separation of the various components of the reaction medium. Another approach is to mix carrier particles and a reaction medium for continuous reaction (complete mixing type bioreactors). For such systems, when the carrier particle size is smaller, the diffusion resistance becomes lower and the cumulative reaction rate increases (for diffusion-limited reactions). Thus, it is possible that if the size of the carrier particles is comparable to that of the enzyme, the cumulative reaction rate of the bioreactor will increase and the efficiency can be improved. However, the separation and recovery would be difficult if colloidal particles are used as the carrier in complete mixing type bioreactors. This separation problem could be resolved by using magnetic UFPs, which would improve the reaction rate while allowing for easy separation and recovery. The technology for the separation and recovery of magnetic particles has been established and used widely in mining as magnetic ore enrichment techniques. Though UFPs have not been used in this
293
Ultra-Fine Particles way, the use of magnetic particles as carriers has been proposed to make separation easier [1]. In this study, an attempt was made to encapsulate magnetic UFPs in a polymer so that, 1) the ease of separation and manipulation by magnetic fields and 2) small particle size and large specific surface area could be used as the immobilization carriers for antibodies and enzymes. Here, the method of encapsulation and the evaluation of encapsulated UFPs is discussed. Antibodies and enzymes were also immobilized on the encapsulated UFPs and these materials were evaluated as well. Encapsulation of Magnetic Ultra-Fine Particles Chain-like iron UFPs (Vacuum Metallurgy, Ltd.) were used with a minor diameter of about 30 nm and major diameter of about 500 nm. These were made by the gas evaporation method. The specific surface area was 50 m 2/g. A two-step process was used for the encapsulation. The first step used a condensation reaction between the hydroxyl groups on the surface of the iron UFPs and a silane coupling agent (vinyltrimethoxysilane, VTS). Using this reaction, vinyl groups were attached to the surface of the UFPs, which allowed for selective reaction at these sites in the next step as described in the following paragraph. The silane coupling reaction was done by heating at reflux for 40 - 50 min a suspension of iron UFPs in an organic solvent containing the silane coupling reagent. When a silane coupling agent concentration of 0.2% was used, a surface density of vinyl groups of about 3 x 10 14 per cm 2 was obtained (calculated from the carbon analysis). In the second step, an encapsulating film was formed on each UFP by vinyl polymerization between the vinyl groups on the surface of the iron UFPs and various vinyl monomers. For this reaction, the UFPs were suspended in an organic solvent, to which surfactants such as SDS and vinyl monomers were added. Using azobis (isobutyronitrile) (AIBN) as the initiator, the reaction continued for 2 hrs at 60 - 70°C in flowing nitrogen. As vinyl monomers, styrene 294
Ultra-Fine Particles and Microbes
(Sn, 2-hydroxyethyl methacrylate (HEMA), and acrylaJdehyde (AA) were used. The organic solvents, initiators, and monomers were purified by conventional means. By copolymerizing with AA or HEMA, the surfaces of the encapsulated UFPs could be covered with aldehyde or hydroxyl groups, respectively. These reactive functional groups can be used to immobilize enzymes. From infrared absorption spectra measurements (by diffuse reflection methods) of the encapsulated UFPs, the absorption spectra of copolymerized encapsulating films were obtained. An example of encapsulated iron UFPs is shown in Figure 1. This electron micrograph shows uniform. encapsulation with a film thickness of about 10 run. By changing the amount of monomer loading in the copolymerization process, the thickness of the encapsulation film can be varied between 5 and 20 run.
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100 nm
Figure I. Electron micrograph of encapsulated iron UFPs.
295
Ultra-Fine Particles
The charge states of the UFP surface are expected to vary greatly depending on the VTS treatment and subsequent encapsulation by synthetic polymers. Therefore, the zeta potential of UFPs was measured before and after the VTS treatment and after encapsulation. Figure 2 shows the results from this comparison. Before the VTS treatment, the iron UFPs had a positive zeta potential. This changed to a slightly negative potential after the VTS treatment. Following encapsulation, the potential became strongly negative. The distributions of the zeta potentials showed a single peak both before and after the encapsulation. These values are well separated, so it can be concluded that most of the iron UFPs are encapsulated.
No treatment
After encapsulation
After VTS treatment
Zeta potenti 1 +33mV
-1.1 V
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+4
+3
+2
+1
o
-1
-2
-3
-4
-5
ELECTROPHORETIC MOBILITY
[ pm /
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Figure 2. Changes in the zeta potential of iron UFPs before and after encapsulation.
296
Ultra-Fine Particles and Microbes Dispersion of magnetic UFPs in an aqueous solution is strongly affected by magnetic and electrostatic interactions. The electrostatic interactions become dominant following encapsulation, which enhances the stability of the dispersion. This was confirmed by experiment by comparing the sedimentation rates before and after encapsulation. As carriers for bioreactors, superior dispersion is essential and the encapsulation is one method of promoting the stability of dispersions.
Immobilization of Antibodies and Enzymes on Encapsulated Ultra-Fine Particles To promote chemical bonding of antibodies and enzymes to UFPs, we encapsulated iron UFPs by using monomers containing an aldehyde group (acrylaldehyde). The aldehyde groups on the surface of capsule films react with amino groups in a protein by Schiff salt formation reaction. This allows for the chemical bonding of antibodies and enzymes on the encapsulated UFPs (Figure 3).
_
..
-CHO + NH 2-prote in
Figure 3. Bonding antibodies or enzymes to encapsulated iron UFPs.
To test the enzyme binding to these modified UFPs, an antirabbit mouse antibody, labeled with a fluorescent dye, bovine serum albumin, and glucose oxidase (an example of an oxidizing enzyme) were selected as bonding materials. The binding was done by suspending encapsulated UFPs in a buffer solution, adding the antibody or other materials, and stirring at room temperature. After reaction, the UFP samples were washed repeatedly under weak 297
Ultra-Fine Particles
ultrasonic agitation to eliminate adsorbed antibodies. When the UFPs were treated with antibodies labeled with fluorescent dye and were then examined under a fluorescent microscope, clear fluorescence from the bonded antibody was observed. These UFPs were also found to orient under an applied external magnetic field. Next, a similar experiment was done with bovine serum albumin. The result indicated that 140 mg of bovine serum albumin could be bonded to 1 g of encapsulated iron UFPs. Bonding experiments were also done using glucose oxidase (GOD), which is an oxidizing enzyme. A 200-mg amount of GOD was bonded to 1 g of encapsulated iron UFPs [2-4]. To compare these results with other carrier particles, a comparison of the specific activities of various carriers is shown in Figure 4. Here, GOD was bonded to a porous inorganic carrier (specific surface area of 125 m2 /g), latex, and porous glass. This shows that the magnetic UFPs have a specific activity (per unit weight) that is 30 times higher than the previously reported value for a porous inorganic carrier [5]. 10000
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Figure 4. Comparison ofglucose oxidase immobilized on UFPs and other carriers.
298
Ultra-Fine Particles and Microbes
From these results, it was found that much larger amounts of antibodies and enzymes could be immobilized on the surface of encapsulated iron UFPs per unit weight. The encapsulation also improved the stability of dispersions of the carrier particles. Furthermore, the magnetic character ofthe UFP carriers allowed them to be isolated and recovered via magnetic separation techniques. These advantages of the magnetic UFP carriers should find use in new bioprocesses using bioreactors and in the purification of living matter. Recently, magnetic fine particles were used for the biolistic delivery of Gus-gene into suspension cultured cells oftobacco [6] and timothy [7]. The efficiencies of Gus-expression for these cells were much larger than those of non-magnetic fine gold particles after separation by using a magnet. Although further investigation is in progress, magnetic fine particles are novel microprojectiles that are useful for the separation and concentration of small fractions of cells into which foreign genes have been successfully introduced.
References 1. Robinson, P. J. et aI., Biotechnology, 15: 603 (1973). 2. Kakuta, H., International Symposium on Immobilized and Cells, Abstracts Session VI (1986). 3. Kakuta, H., 39th Meeting on Colloidal and Interfacial Chemistry, p. 126 (1986). 4. Kakuta, H., Chemical Instrument 29: 2, 94 (1987). 5. Markey, P. E. et aI., Biotechnol. Bioengr., 17: 285 (1975). 6. Kakuta, H., Chemical Regulation ofPlants, 28: 98 (1993). 7. Horikawa, H., Plant Ecochemical News, 2 (1995).
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Ultra-Fine Particles
3.5 Magnetic Ultra-Fine Particles Isolated from Bacteria (by Tadashi Matsunaga)
New characteristics of magnetic UFPs have recently attracted attention. However, magnetic UFP synthesis is difficult and careful control of the temperature and pressure, as well as the vapor, liquid and solid phases are required to produce uniform particle sizes. Some organisms synthesize magnetic UFPs under ambient temperature and pressure. The first magnetic particle in a biological system was discovered in the teeth of chiton [1,2]. In the mid-1970s, Blakemore discovered that magnetotactic bacteria synthesize magnetic UFPs within their cells [3]. The existence of magnetic particles in honey bees, carrier pigeons, and tuna has also been confirmed. Generally, a multi-domain structure exists in larger particles where the particle's energy is lowered by anti-spin alignment among the domains. On the contrary, internal spin is aligned in ultrafine particles, which results in a single domain structure with a large magnetic moment. The magnetic UFPs synthesized by magnetotactic bacteria possess large magnetic moments due to the single domain structure. These particles can move through very small spaces because the particle size is only about 50 - 100 nm. Hence, magnetic UFPs may be used as new drug carriers immobilized with physiologically active substances and as microsensors. Magnetic UFPs have excellent properties and promising applications are expected. This section describes the isolation and cultivation of magnetotactic bacteria, the separation and characterization of magnetic UFPs, the immobilization of enzymes and antibodies on the particles, and their incorporation into animal cells.
Cultivation of Magnetotactic Bacteria Blakemore reported the isolation and cultivation of the fresh water helical magnetotactic bacterium, Aquaspirillurn magetotacticum, strain MS-l [4]. A sample of mud was collected from a
300
Ultra-Fine Particles and Microbes
swamp and left in a dark room at room temperature for several days. The MS-I was collected with a magnet and transferred onto a culture after washing it with filtered and sterilized swamp water. Separation of the magnetotactic bacteria was carried out in a semi-solid culture containing about 10% filtered sterilized swamp water, organic acids, vitamin, inorganic salts, and agar. The culture used to cultivate the bacteria contained succinic acid, sodium acetate, sodium thioglycolate, sodium nitrate, calcium phosphate, and vitamin (pH 6.7) and the atmosphere consisted of nitrogen with 0.6 - 1.0% oxygen. Proliferations of the bacteria have been confirmed, but a problem with an increase in bacteria not containing magnetic UFPs was also seen for such conditions [5]. A. magnetotacticum is a Gram-negative bacteria with a GC content of 64.9% [6]. Approximately seven species of magnetotactic bacteria were isolated from the waters of ponds, swamps, lakes, and beaches in Japan [7- 9]. An example is shown in Figure 1. Most bacteria were spherical but there were also rod and spiral-shaped bacteria. These spherical bacteria possess two magnetosomes each consisting of a chain of 5-15 magnetic UFPs. As a result of Gram staining, these bacteria were also found to be Gram-negative. Next, the magnetotactic bacteria were mass-cultivated in mud. The living conditions and environment of magnetotactic bacteria were analyzed to establish optimal cultivating conditions in mud. At a depth of 0 50 cm from the surface of the mud, the oxygen concentration decreased as the depth increased and the number of magnetotactic bacteria decreased with increasing depth. About 90% of the bacteria were distributed near the surface (0 - 2 cm) of the mud. The residual dissolved oxygen concentration in this region was 0.047 ppm. The optimal conditions for cultivating magnetotactic bacteria were obtained through a series of experiments in which the conditions were systematically varied. The optimum conditions include a large surface area, a mud depth of3 cm, and a temperature around 24°C. The effects of carbon and nitrogen sources were also evaluated to increase the growth rate of the bacteria. Substances suitable for promoting proliferation were glucose, maltose, succinic acid, and trypsin for the carbon source and ammonium chloride and sodium 301
'.0 ~m Figure 1. TEM image of magnetotactic bacteria.
nitrate for the nitrogen source. The growth was inhibited, however. when the combined carbon and nitrogen sources exceeded 0.1 gil. Hence, a value of 0.05 gil for the combined carbon and nitrogen sources was added to the mud and the cultivation was carried out in air. As a result, the magnetotactic bacteria grew at a rate that was about twice that without any additives and the average generation time was about 12 hours.
Characterization of Bacterial Magnetic Particles There are a nwnber of reports on the characterization and separation of bacterial magnetic particles. Towe et aI. concluded that the magnetic fine particles were pure magnetite based on electron diffraction analysis [10]. Furthermore, it was shown by highresolution TEM analysis that the particles were hexagonal columns
302
Ultra-Fine Particles and Microbes
with a length of 99.3 ± 8.7 nm and that they had a diameter of 62.3 ± 6.1 nm. Mann et al. and Matsuda et al. analyzed the particles by high-resolution TEM and determined that the particles were single domain hexagonal single crystals [11,12]. Frenkel et al. did Mossbauer spectroscopy analyses and also found that the magnetic particles synthesized by the bacteria are magnetite [13,14]. For this study, the magnetic fine particles collected from magnetotactic bacteria cultivated in mud were characterized. The collected bacteria were further concentrated and separated using a centrifuge. The cell walls of the bacteria were dissolved by treating the cells with a 0.2% lysozyme solution for one hour at 37°C. The particles were separated using a centrifuge and were then washed. Identical processes were repeated using lipase and trypsin. The protein and other parts coating the particles were removed by treating them with 5M sodium hydroxide for 12 hours. The resulting magnetic UFPs were used for characterization. The TEM observations confirmed that the size and shape of the particles are very uniform (Figure 2). Iron and oxygen were identified to be the major constituents of the particles by using an energy dispersion analyzer. Fourier transform infrared absorption spectra and electron diffraction indicated that the particles were magnetite (Fe30 4). The extent of magnetization of the bacteria containing magnetic particles was measured using a vibrating sample magnetometer and was found to be 1 emu per gram of the dry weight of the cells. This would be equivalent to about 50 emu/g of the resulting particles and is roughly equal in value to artificial magnetite [15]. The coercive force was found to be 230 Oe and the it was confirmed that the particle consists of single domains based on Butler-Banerjee diagram analysis. The lattice parameters of the isolated particle were measured by highresolution TEM and found to be 0.485, 0.302, 0.208, 0.242, and 0.253 nm for the (111), (220), (400), (222), and (311) planes, respectively. These values are equivalent to those of pure magnetite. Hence, the particles synthesized by the bacteria were confirmed to be magnetite [16]. The bacterial magnetic particles are covered with a uniform organic membrane and exist within bacteria in the form of chains. 303
2000
A
Figure 2. Magnetic UFPs separated from magnetotactic bacteria. The organic membrane was investigated next. The particles in the cells were covered with a 10-15 nm thick organic membrane. The
thickness of the membrane after each treaunent with lysozyme, lipase, and trypsin was measured by TEM and high-resolution TEM. The initial 10-15 run thick layer became 2-7 ron thick after the lysozyme treatment, clearly indicating that the membrane became thinner by this treatment. The lipase treatment reduced the thickness of the layer to 1-2 om. The layer that was removed by the treatment with lipase is thought to contain much fatty-like residues. The results from the enzyme treatments are swnmarized in Table 1. The energy dispersion analysis also indicated the presence of carbon, sodium, silicon, and chromium. in addition to the constituents of magnetite, 304
Ultra-Fine Particles and Microbes
Table 1. Organic Membrane on Bacterial Magnetic Particles After Various Enzyme Treatments. Treatment
Thickness of Organic (nm)
Untreated
10-15
Lysozyme
2-7
Lysozyme, Lipase
1-2
iron and oxygen. Carbon is believed to arise from the organic layer, while sodium and silicon are thought to arise from the soil. The magnetic particles treated with lysozyme, lipase, and trypsin were further dispersed by ultrasonic agitation and were then boiled in a 5M sodium hydroxide solution. Boiling in the alkaline solution reduced the organic membrane to a thickness of 0.5-2 nm. Repeated ultrasonic treatment after boiling in the alkaline solution was found to result in considerable damage to the surface of the particles and etching was observed. Utilization of Bacterial Magnetic Ultra-Fine Particles Enzymes
Glucose oxidase and uricase were immobilized on bacterial magnetic particles. The particles were allowed to react with yaminopropyltriethoxysilane for 10 minutes. After washing, the particles were then allowed to react for one hour with a phosphate buffer solution containing 2.5% glutaraldehyde. The particles were then incubated for 12 hours at 4 DC in a solution containing the enzyme. Glucose oxidase was also immobilized on artificially synthesized magnetic UFPs, zinc ferrite (500 nm), and magnetite (100 nm). As a result, 200 .ug/mg of glucose oxidase was immobilized on bacterial magnetic UFPs, while zinc ferrite particles immobilized 305
Ultra-Fine Particles
only 1.8 ,ug/mg. The surface area of bacterial magnetic particles is large because its diameter is about 1/10 that of zinc ferrite particles. Hence, the bacterial particles were able to immobilize over 100 times more glucose oxidase than the zinc ferrite particles. The amount immobilized on artificial magnetite was about the same as that on zinc ferrite due to the formation of secondary fine particles. Similar results were also obtained with uricase. Extremely high enzyme activity was noted with bacterial particles due to their larger surface area and due to the larger quantity of enzyme that could be fixed on the particles. That is, about a 30-40 fold increase in enzyme activity was observed compared to that of materials using zinc ferrite and magnetite (see Table 2) [17].
Table 2. Enzyme Immobilization on Bacterial Magnetic UFPs and Artifical Magnetic UFPs. Enzyme
Bacterial Magnetic Particles
Magnetite
Zn Ferrite
Glucose
Immobilized amount
200 j..lg/mg
2.5 j..lglmg
1.8j..lglmg
Oxidase
Enzyme activity
59 U/mg
1.8 U/mg
1.5 U/mg
Uricase
Immobilized amount
196 j..lg/mg
7.6 j..lg/mg
5.9 j..lg/mg
Uricase
Enzyme amount
0.59 Ug/mg
0.020 U/mg
0.015 U/mg
Next, a new glucose measurement system using enzyme immobilized on bacterial magnetic particles and an optical fiber was developed. This system, illustrated in Figure 3, used a reflective probe on an optical fiber sensor with enzyme immobilized on bacterial magnetic particles. Analysis using this probe is based on
306
Ultra-Fine Particles and Microbes
A B
11
Figure 3. Diagram of the reflection-type probe. A. Incident Beam B. Detection (Signal Beam) C. Epoxy Resin D. Dialysis Membrane E. Membrane Filter F. O-Ring
measuring the change in the absorption peak at 436 nm, which results from oxidation of o-dianisidine by H20 2 that is generated by the reaction between glucose and glucose oxidase. The enzyme immobilized bacterial magnetic particles were dispersed within the probe and the optical absorption change was monitored by observing the changes in the electrical potential of the photodiode. Glucose oxidase immobilized magnetic UFPs were magnetically stirred by a 307
Ultra-Fine Particles
magnetic stirrer under the probe. The reflective probe consisted of a separated light emitting layer and an area containing the glucose oxidase immobilized particles. A linear relation between the absorption and the glucose concentration was found in the concentration range of 0.1 - 4.0 mg/ml. These results demonstrate the utility of this apparatus for measuring glucose content. External stirring of the oxidase immobilized particles accelerated the enzymatic activity, providing a two-fold increase compared to when the system was not stirred [18]. Immobilization of Antibodies The bacterial magnetic particles with antibodies can be recovered and reused by using a magnetic field [19]. The reaction can be done with small amounts of samples. Various antibodies were immobilized on bacterial magnetic particles and artificial magnetic UFPs, and measurements of the bacterial count and carcino embryonic antigens (CEA) were done. Escherichia coli was separated magnetically by using the antigen/antibody reaction with anti-E. coli antibodies immobilized on bacterial magnetic particles. In contrast to the reduction seen in the number ofE. coli in the medium, no reduction in the number oflactic acid bacterium and yeast was observed because they do not combine with the anti-E. coli antibody particles. Also, no change in the E. coli collected in this manner was observed. These results indicate that anti-E. coli antibody immobilized particles can selectively discriminate E. coli. Use of antibody bacterial magnetic particles allowed for selectively recognition of bacteria at a level of 106 [20]. Next, CEA detection was done using CEA antibodies immobilized on bacterial magnetic particles. Strong aggregates were formed after a few minutes due to antigen/antibody reactions when CEA was added to ultrasonically dispersed CEA antibodies immobilized on magnetic particles. On the other hand, when albumin was added, the aggregation was weak because it was due solely to magnetic interactions. This made it possible to distinguish the aggregates by observation with an optical microscope. It was shown 308
Ultra-Fine Particles and Microbes that by using antibodies immobilized on magnetic bacteria particles, a minute quantity of antigen (about 100 pg/ml) can be detected very quickly [21]. Incorporation of Bacterial Magnetic Particles into Animal Cells Ordinary animal cells and microorganisms do not contain magnetic particles. However, if magnetic particles can be introduced into these cells, it would be possible to magnetically move the cells, which would make it possible to treat the cells in a variety of ways. Magnetotactic bacteria were suspended in a 0.1 M phosphate buffer solution (pH 7.0) containing 1 mg/ml of lysozyme and 1 mg/mlofEDTA. Three hours later, more than 70% of the bacteria became spheroplasts. The spheroplasts were combined with sheep red blood cells in the presence of 40% poly(ethylene glycol) (MW 6,000) to introduce the particles into the red blood cells. Transmission electron microscopy observations confirmed that a few chains of magnetic UFPs were introduced into the red blood cells in this manner (see Figure 4). These cells became sensitive to magnetic fields and the cells rotated when a Sm-Co magnet was rotated near them. The incorporation rate of magnetic bacteria particles increased as the poly(ethylene glycol) treatment time increased when a mixture of3.8 x 106 cells/ml of red blood cells was treated with 3.2 x 10 7 cells/ml of magnetotactic bacteria at 25 DC. According to optical microscope observations, 16% of the red blood cells incorporated bacterial magnetic particles and became magnetically sensitive. When red blood cells were treated with a magnetic field, only the cells containing magnetic particles were affected [22]. Next, the introduction of bacterial magnetic particles into white blood cells was done by taking advantage of their phagocytosis process. It was found that 60% of the white blood cells became magnetically sensitive after 90 minutes of cultivation in blood plasma at 3JCC (the ratio ofwhite blood cells and magnetotactic bacteria was 1:25). The phagocytic activity and NBT reduction capability of the white blood cells was then measured. After one hour of mixing 309
1.0J.l.rn
Figure 4. Magnetosomes introduced into sheep red blood cells. bacteria with the white blood cells, their phagocytic ability was reduced to 50% and their NBT reduction capability was about 70% of their original value. Furthermore, it was found that the magnetosensitive white blood cells could be moved at a speed of 6 J,<m1see by applying a 1.3 kGauss magnetic field. These results showed that
bacterial magnetic particles can be introduced into white blood cells and that the cells maintain their activitY. In general, blood consists of red and white blood cells. The white blood cells are made of granulocytes, lymphocytes, and monocytes. Of these, granulocytes and monocytes show phagocytic activity. Nonnally, 70% of the white blood cells are granulocytes and monocytes~
thus, when treated with magnetic particles, 50% of the
white blood cells becoming magnetic, imply that most cells with
phagocytosis have incorporated the magnetic UFPs. The white blood cells with magnetic UFPs inside can be moved magnetically. An example of a practical application of this
310
Ultra-Fine Particles and Microbes phenomenon would include the separation of granulocytes and monocytes from lymphocytes. Furthermore, it is possible to target cells, such as Killer T cells, with cancer attacking agents only in the affected area. The processing, separation, characterization, and application of magnetic UFPs produced by microorganisms was described. Each of the bacterial magnetic particles is coated with an extremely thin organic membrane and their shapes and sizes are very well controlled. Artificial synthesis of such ultrafine particles is very difficult. At present, however, mass cultivation of magnetotactic bacteria is extremely difficult and large quantities of magnetic UFPs cannot be produced. This will limit further developments in this area. Hence, an important topic is to investigate the mass cultivation of these bacteria. Progress in research on the mechanism of bacterial magnetic particle formation and genetic research on these bacteria is also important. References 1. Lowenstam, H. A., Oeo!. Soc. Am. Bull. 73: 435 (1962).
2. Lowenstam, B.A., Science 156: 1373 (1967). 3. Blakemore, R.P., Science 190: 377 (1975). 4. Blakemore, RP., Maratea, D. and Wolfe, R S., J Bacteriol. 140: 720 (1979). 5. Bazylinsky, D. A. and Blakemore, RP., App!. Environ. Microbiol. 46: 1118 (1983). 6. Frankel, RB., Blakemore, RP., Torres de Araujo, F.F., Esquivel. D.M.S. and Dannon, J., Science 212: 1269 (1981). 7. Matsunaga, T., Sakaguchi, T. and Tadokoro, F., Appl. Microbiol. Biotechnol. 35: 651 (1991). 8. Sakaguchi, T., Burgess, J. G. and Matsunaga, T., Nature 365: 47 (1993). 9. Thornhill, RH., Burgess, 1. G. and Matsunaga, T., App!. Environ. Microbiol. 61: 495 (1995). 10. Towe, K.M. and Moench, T.T., Earth Planet. Sci. Lett. 52: 213 (1981). 311
Ultra-Fine Particles 11. Mann, S., Frankel, RB. and Blakemore, RP., Nature 310: 405 (1984). 12. Matsuda, T., Endo, 1., Osakabe, N. and Tonomura, A., Nature 302: 411 (1983). 13. Frankel, R.B., Blakemore, R P., and Wolfe, RS., Science 203: 1355 (1979). 14. Frankel, R.B., Papaefthymiou, G.C., Blakemore, RP. and O'Brien, W.D., Biochim. Biophys. Acta 763: 147 (1983). 15. Matsunaga, T.,JapanAppl. Mag. Soc. 1. 10: 488 (1986). 16. Matsunaga, T., Chemical Ind. 263: 63 (1987). 17. Matsunaga, T. and Kamiya, S., Appl. Microbiol. Biotechnol. 36: 328 (1987). 18. Matsunaga, T., Mol. 303: 41 (1987). 19. Nakamura, N., Hashimoto, K., and Matsunaga, T., Anal. Chem. 63: 268 (1991). 20. Nakamura, N., Burgess, 1. G., Yagiuda, K., Kudo, S., Sakaguchi, T.and Matsunaga, T., Anal. Chem. 65: 2036 (1993). 21. Matsunaga, T. and Kamiya, S., Digests ofIntermag Conference, HE-04 (198,). 22. Matsunaga, T. and Kamiya, S., Abstracts of 6th International Conference on Biomagnetism, p. 50 (1987).
312
4 APPLICATIONS PARTICLES
FOR
ULTRA-FINE
4.1 Introduction (by Akira Tasaki)
The study ofUFPs is an interesting subject for physicists. In particular, UFPs are not merely smaller in size, but they exhibit completely different properties below a critical diameter, which results in their appearing to be different from their bulk counterparts. The critical size depends on the particular property being observed, but it is roughly < 111m where individual characteristics become significant. For example, UFPs of gold are black and they do not exhibit a gold color, while magnetic UFPs become a single domain structure with sudden increased coercivity that gives them characteristics that are suitable for magnetic recording material. It is natural to explore the applications of the unique properties of UFPs. Examples include magnetic recording media with single domain structures, catalysts with large surface areas, and light absorbers with optical properties. In addition to the direct use of these unique properties of UFPs, UFPs can be heat treated and sintered to produce new materials. While UFPs are said to sinter at very low temperatures, few quantitative studies exist. In this chapter, results of experiments involving UFPs with magnetic properties that can be used as a probe of their unique features are presented. The applications of UFPs originally extended to broad areas centering around chemistry, such as cements and foods. This chapter, however, will focus on the studies involved with the use of UFPs produced by the gas evaporation method.
313
Ultra-Fine Particles The topics to be discussed are briefly described below. 1. In the section on the lattice structure ofUFPs, the project's work on pursuing the limits ofrecording by current techniques is described. The read-out processes used an electron microscope, so the feasibility of obtaining information storage in terms of units of 10 nm was sought.
2. The section on cobalt-polymer composite films describes the project's study on making magnetic tapes by directly depositing UFPs on base films. These materials appear as films, but microscopically the cobalt UFPs are aligned on the film and separated by the polymer. Magnetic recording materials use these cobalt UFPs. 3. The section on catalysts is an orthodox study among the UFP studies. The key feature of the gas evaporation method is the ability to vary the composition of alloy UFPs. The objective of this work was to produce catalysts with high selectivity. 4. The section on chemical heat pumps is an example of the use of UFPs as catalysts. 5. The section on gas deposited films is the centerpiece of this project and involves the formation of films and circuits by using a carrier gas to spray UFPs made by the gas evaporation method. 6. The section on the solid gas spraying apparatus describes work that is similar to the gas deposition method, but this method uses UFP beams cooled to low temperatures for etching the surface of materials. These techniques originated from the free and creative environment that exists in the various ERATO projects and are expected to spawn new applications in various fields.
314
Applications The Start of Applications Using Evaporated Ultra-Fine Particles
The use of evaporated UFPs has now advanced to the industrial scale and the current emphasis is on their development. In the beginning studies ofthese materials focused on obtaining samples for material properties research. Kubo suggested that different electronic properties are expected for metallic UFPs and that such small bodies should possess unique properties. This was the driving force for our UFP research. To review the progress made from this beginning may be helpful in planning future R&D programs. It was commonly thought that when metallic UFPs are collected that they immediately became oxidized throughout their structure. However, x-ray analysis indicated the presence of metallic absorption lines, which means that it is unlikely that the interior is oxidized. The starting point of the project's study was to estimate the amount the of surface oxide. Ferromagnetic metals such as iron exhibit ferromagnetism, which arises from their fundamental nature. When more than 1000 iron atoms are gathered together, such ferromagnetic behavior appears. When the surface atoms are oxidized, the overall magnetization decreases. Thus, weighing UFPs and measuring their magnetization should allow one to determine the ratio of ferromagnetic iron atoms to oxidized non-magnetic iron atoms. Using ferromagnetic measurements, it was found that about 30% of the iron in iron UFPs having diameters of20 urn is oxidized, while less than 10% of the nickel in nickel UFPs is composed of oxides. This agrees with the common knowledge that nickel is more difficult to oxidize that iron. Interestingly, the starting point for application of these materials was accidentally discovered. It was found that oxidation did not proceed beyond an initial level even after exposure ofUFPs to air for several months. Figure 1 shows the data from measurement of the saturation magnetization of Fe-Co alloy UFPs before and after an accelerated aging test. No change was observed in the specific saturation magnetization, indicating no progression in the oxidation of these materials. This discovery was
315
Ultra-Fine Particles
--
200
C) ~
E Q)
III
0
+J~
c: Q)
E
100
0
:!E Fe-Co (80: 20)
0
+:i Q)
c:
C)
as :!E
0 0
5
10
Magnetic Field (kOe) Figure 1. Fe-Co UFPs that show no change in the magnetization when placed under an accelerated weathering test (40°C, 95% RH, 10 days). quite surprising and suggested the possibility of using UFPs as magnetic materials. The principal application ofUFPs as magnetic materials is as a recording medium. Magnetic recording started for audio frequencies of up to 15 kHz. This technology has been extended to video recording and computer data storage using the same basic concept. In video recording, the frequency extends to 5 MHZ and high-density magnetic recording technology has rapidly developed. From the view point of basic properties, a medium with high residual magnetism and high coercivity is needed for high-density magnetic recording. The need for high coercivity is generally 316
Applications
difficult to understand, but a magnetized material under zero external field is subjected to its own field. To withstand this reverse field and to leave a large residual magnetization, it is essential to have a high coercivity. Since the beginning of magnetic recording, oxide UFPs have been primarily used. The oxides have excellent weatherability and their particle size can be controlled chemically. The magnetization of metals is far greater than that of oxides and the coercivity can be better controlled using alloys of iron, nickel, and cobalt. Tl1e common sense expectation regarding metal particles being oxidized was the major barrier to the development of applications that use them. The UFPs that were produced showed that after the initial oxidation of the surface, the UFPs resist oxidation and exhibit good weatherability. This pointed to the feasibility of using UFPs as magnetic materials and led us to start developmental efforts on this use. Main Progress It is difficult to disperse UFPs into a liquid once they have been collected. As the synthesis mechanism of the evaporation method suggests, individually grown particles are in the state of aerosols. Ferromagnetic UFPs are magnetically coupled and are recovered in the form of chains. This inspired us to extend the chains along the magnetized direction by applying a magnetic field while the UFPs are still in the form of aerosols. It was thought that when UFPs grew and the temperature decreased to below the Curie temperature, the magnetic field can be applied to effectively extend the chains. Figure 2 shows the relation between the field strength during the withdrawal of the UFPs and the magnetic anisotropy of the samples. This indicates that a small field aligns the chains completely along the magnetization direction and that the control of the magnetic properties requires a treatment during particle growth. The biggest advantage associated with the use of magnetic UFPs is the conservation of the alloy composition when an alloy of iron, nickel, and cobalt is evaporated because of the similarity of the 317
Ultra-Fine Particles ~
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u I
o C >-
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~
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Figure 2. The relation between the field strength during the withdrawal of ferromagnetic Fe:Co:Ni alloy UFPs and the magnetic anisotropy ofthe samples. The arrow indicates the value for an oxide magnetic tape three metals. The control of the alloy composition enables one to design the required coercivity and saturation magnetization. So far, particles with coercivities of about 2500 De in the Fe-Co system have been produced. An attempt has also been made to increase this value further using Co-Sm particles. For this material, however, the rare earth element evaporated first because its properties are vastly different from the iron group metals. This prevents effective use of the evaporation method for this system. 318
Applications In the laboratory, suitable UFPs with the desired characteristics have been obtained. On a larger scale, various problems arise and industrial application of evaporated UFPs have not yet started to take off. In part, this is because stable metallic UFPs have been established via chemical methods and magnetic tapes with painted magnetic powders have become commercially available. Possibilities for Further Applications [1-5]
The application of UFPs has another important aspect. This aspect takes advantage of the nature of UFPs to sinter at very low temperatures and has wide possibilities. Conventionally, metal powders start to sinter rapidly above about two thirds oftheir absolute melting temperature. In UFPs, the sintering temperatures are quite low. For example, gold UFPs start to sinter at room temperature to produce gold aggregates. This is actually one of the problems associated with producing gold UFPs. At present, there is no quantitative means to evaluate the sintering of UFPs, except for the volume changes of greens or electron microscope observations. Magnetism was used as a way to probe sintering and the results from this study are presented here. Nickel is ferromagnetic and solid solutions with copper are completely miscible. By mixing a small amount of nickel UFPs with copper UFPs and compacting the mixture into pellets, it was possible to examine the diffusion of nickel into copper by the loss of ferromagnetism. Strictly speaking, this was a diffusion experiment, not a sintering experiment, but it provided quantification of atomic movement. To mix the nickel and copper UFPs, the UFPs were mixed in the presence of alcohol in a ball mill for 72 hours. After vacuum drying, the mixture was pelletized in a press at a pressure of7.5 t/cm2 • The state of mixing is difficult to evaluate. An x-ray microanalyzer was used to scan the surface of the pellets for their nickel and copper contents. Figure 3 shows the results for the composition versus the scan distance. This shows that the metals are homogeneously distributed, but the x-ray beam diameter of 5 Ilm is much larger than 319
Ultra-Fine Particles
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Copper Nickel
o
L--
500 A 100 A
90 wt. % 10 wt. %
---L1
a
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....I1_ - - '
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Measurement Position (mm)
Figure 3. The compositional distribution oftwo types of metallic UFP mixtures. The data was obtained by scanning with a 5 .urn diameter x-ray beam. the size of the UFPs and the uniformity at the UFP level cannot be determined. The pellets used for magnetic measurements contained 0.2% nickel and showed no magnetism once the nickel diffused into the copper. A pellet was heat treated as follows: the temperature was raised, held for I hr, and then quenched to room temperature. The pellet was weighed and its magnetic properties were measured. Next, the temperature was raised to a slightly higher value and the measurements repeated. Both of these measurements, weighing and determination of the magnetization, were done with the sample on a magnetic balance in vacuum. The results are shown in Figure 4. The dashed line indicates the results for copper UFPs with a diameter of 40 11m. Clearly, the UFP results show magnetization losses starting at much lower temperatures. Under a hydrogen atmosphere of 21 320
Applications
_ 100 ~
-
r-e----=~~
"'
\
" \
~
\
.He 3.5Torr OH 2 21Torr
100 1 4 -...- ~................~....-
a
200
__
400
Sintering Temperature ('C)
Figure 4. Decrease in the magnetization of nickel UFPs mixed with copper UFPs due to heat treatment.
torr, the magnetization loss occurs at an even lower temperature than that under a helium atmosphere of35 torr. The bottom part of Figure 4 shows that there is a large weight loss when the pellet is treated under the hydrogen atmosphere. Assuming this reduction to be from the reaction CuO .... Cu+02 , half of the copper UFPs were oxidized. Normally, only 10% is believed to be oxidized, so the difference may be due to oxidation during the mixing in alcohol.
321
Ultra-Fine Particles The magnetization loss results due to diffusion of nickel into the copper, as well as to sintering of the copper. In the hydrogen atmosphere, changes in the magnetic properties started at 100 °C and changed rapidly at temperatures above 200°C. This may be due to reduction of the surface oxides of copper UFPs by the hydrogen, which allows nickel to diffuse easily. This experiment confirms that UFPs start to sinter at temperatures above 200°C. In practical applications, low sintering temperatures are desirable. If 200°C sintering is feasible, heat resistant plastic substrates, in place of glass substrates, can be used. By printing UFPs dispersed in a solvent on a film, circuits can be formed by heating with a laser beam or information can be burned into surfaces. Sintering of UFPs has a wide range of applications, but UFPs are not suitable for general sintering because they are too small. The density of green UFPs cannot be raised even with pressing, so the density of the sintered bodies that can be produced is limited. In addition, UFPs are expensive in comparison to other metal powders. The best uses of UFPs may lie in improving the sinterability of other materials by judicious mixing. In developing new magnetic materials, there is a large area between the technologies of evaporation and vacuum deposition. In vacuum depositing metal alloys onto a polymer base, the use of an inert gas atmosphere allows for the creation of changes in the film structure. If the metal magnetic film structure is uniform, the magnetic domain boundaries can move easily and only low coercive forces can be attained. For magnetic recording films, columnar structures are best. Each column can be regarded as a UFP, but the surface shows metallic luster. Contemporary technology is used to form columns with a small tilt and leaves spaces of about 30% between the columns. These recording media still lack weatherability, which may be improved by ion nitriding, which have been recently explored. This, however, will not be discussed here because it is not related to UFPs. Cobalt/polymer composite films (see Section 4.3) are related to this area because columnar UFP structures are formed within a polymer film, thereby improving the weatherability. 322
Applications
References 1. T., Mishima, K., Kita, E. and Tasaki, A., J Magn. & Magn. Mater. 104-107, 1589-1590 (1992). 2. Sawada, Y, Kageyama, Y, Iwata, M. and Tasaki, A, Jpn. J Appl. Phys., 31 3858-3861 (1992). 3. Tagawa, K., Utuno, N., Umehara, H. and Tasaki, A, Jpn. J Appl. Phys. 33, 1320-1322 (1994). 4. Sasaki, Y., Iwamoto, Y., Erata, T., Kita, E. and Tasaki, A., Proceedings of the First Magneto-Electronics International Symposium, Nagano, Shinshu Univ. Press, 147-150 (1994). 5. Y, Sasaki, Y, Kita, E. and Tasaki, A, Proceedings of the First Magneto-Electronics International Symposium, Nagano, Shinshu Univ. Press, 173-176 (1994).
323
Ultra-Fine Particles
4.2 Regular Arrangements of Ultra-Fine Particles and UltraHigh Density Recording (by Shizuo Umemura)
Optical disc systems are the most promising commercialized high-density recording method. These systems use a finely focused laser beam for reading/writing and achieve recording densities of about 108 bits/cm2 • Future demands for higher density recording are expected to push the limit to 1010 bits/cm2 or more. One method to increase the recording density is to use frequency domain optical recording, where multiple bits of information are recorded at a single location in the recording medium by a frequency tunable laser beam [1]. This is a form ofthree-dimensional recording. To increase the density to 1010 bits/cm2 , one can use UFPs and store one bit of information for each UFP (particle diameters less than 100 nm). Here, it is appropriate to use an electron beam for reading/writing, because it can be easily focused to a diameter of less than 10 nm and stable beams can be produced. The combination of UFPs and electron beams for creating a high-density recording system were examined and the results are reported in this section. To record one bit of information for each UFP, one must overcome two obstacles. 1. Create regular arrays of UFPs on a substrate. 2. Irradiate individual UFPs with an electron beam and achieve reading/writing. The first aspect is an absolute requirement before one can address the topic of the reading/writing of individual UFPs. In addition, the attainment ofregularly arranged UFPs can also be of use in applications other than high density recording.
324
Applications Regular Arrangements of Ultra-Fine Particles It is known [2] that when a minute amount of a particular material is deposited on a substrate, it does not form a film on the surface but instead forms island structures of dispersed fine particles. The island structures reflect the microstructure of the surface of the substrate [3]. When a substrate that is easily damaged by irradiation (e.g., rock salt) is used, the island structure is affected by prior electron irradiation. When deposition is done on a specially treated substrate that has been irradiated with an electron beam, the growth of vapor deposited films is changed substantially by the effect of the electron beam [4]. Considering these phenomena, the following method was used to create regular arrangements ofUFPs. By using a finely focused electron beam, sites were formed where deposited particles (islands) preferably grow at sizes on the scale of UFPs. Vapor deposition is then done on the substrate in an attempt to grow a regular arrangement of UFPs.
Experimental method Following the basic premise described above, the following three-step experiments were performed as follows. 1. Electron beam irradiation of a substrate in a regular pattern. 2. Vapor deposition of a UFP forming material on the irradiated substrate. 3. Heating of the deposited substrate and examination of its surface using scanning electron microscopy (SEM).
325
Ultra-Fine Particles Substrate and electron irradiation
The substrate used for this study was a commercially available silicon wafer cut into 5 mm squares. Silicon wafers have some electrical conductivity, so there is no charge-up ofthe substrate as a result of the electron irradiation. A further advantage of commercial silicon substrates is that their surfaces are very well polished. Electron irradiation was done by the apparatus shown in Figure 1. This is basically a scanning electron microscope, that is equipped with an electrode for beam blanking and an external electron beam scanner. The sample chamber was evacuated using an oil diffusion pump. This microscope was also used for the observation of the UFP systems that were produced.
B
1111--+-..... ~ ~
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Figure 1. Schematic of the electron beam irradiation apparatus. A) LaB 6 electron gun; B) ion pump; C) blanking electrode; D) pulse generator; E) signal generator; F) sample chamber; G) rotary oil pump; H) oil diffusion pump; and I) liquid nitrogen trap. 326
Applications
A silicon wafer was fixed in the sample chamber and irradiated with a focused electron beam according to a regular pattern. The accelerating voltage was 30 kV, the beam current was 10 - 12 A, and the beam diameter was 5 run. The scanning patterns used were of two types as shown in Figure 2a and b. The pattern shown in Figure 2b is a combination of two of the patterns shown in Figure 2a rotated 90° with respect to each other. The irradiation dose was changed by changing the scanning speed. Vapor Deposition and Particle Observation
The substrate irradiated by the electron beam was transferred to a vacuum evaporation apparatus and coated with 3 - 6 run average film thickness of a UFP forming material. The substrate temperature was room temperature and the chamber pressure was 10-6 torr. Various materials including gold, silver, cobalt, nickel, potassium chloride, etc. were deposited, but only the results for gold are reported.
.•.......................••.••..
.•....•..•...•...•.•..............
.......•...•••••••.•••••.........• B
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Figure 2. Electron beam scanning patterns. A) stripe pattern; and B) grid pattern. 327
Following the vaporization, the sample was transferred to the apparatus shown in Figure 1 and heated to 250°C. The morphology ofthe UFPs was examined using the secondary electron imaging. For gold, the heat treatment promoted the growth ofUFPs.
Experimental results
In this section, the effects of prior electron irradiation on the morphology of gold UFPs formed on silicon substrates are reported. The most important finding is shown in Figure 3, which resulted from electron irradiation using the stripe scanning pattern shown in Figure 2a with a stripe distance of 100 om. This was followed by gold deposition (average thickness 4 nm) and heating to 250°C. The white spots shown in Figure 3 are gold UFPs, which are aligned in rows. Several dark lines can be seen, which correspond to the tracks of the electron irradiation. That is, the irradiated areas show very low levels ofUFP deposition and the UFPs tend to foom along, not on, the dark
lines. This means that one can control the location of the UFP deposition.
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Applications
Figures 4-6 show the results of experiments aimed at controlling the size ofUFPs on the substrate. In Figure 4, the stripe distance of Figure 3 was reduced to the same size as that of the UFPs. Here, one-dimensionally aligned arrays of UFPs were obtained. The phenomenon observed was basically identical to that in Figure 3, except that the fonnation of UFPs was limited to narrow bands between the irradiated tracks, which produced the linear arrays seen
in the figure.
Figure 4. One-dimensionally aligned gold UFPs.
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"
• :
, , ::·~:; ;.::·:::::::~~;·~~r ,~
~ to
~~: ~:
.
Figure 5. Two-dimensional square array of gold UFPs (beam line separation of 40 run). 329
Figure 6, Two-dimensional square array of gold UFPs (beam line separation of25 nm),
Figure 5 shows the results from electron beam irradiation
using a square grid pattern (line separation of 40 nm), This indicates that two-dimensional square lattice arrays of UFPs can be obtttined, The UFPs Conn in the regions that were not irradiated, inside the grid pattern. Figure 6 shows the result of a narrower grid pattern with a spacing of 25 ron. A smaller scale lattice pattern of UFPs was produced by changing the scale of the irradiation grid pattern. Another interesting result is that it is possible to control the size of
the UFPs by changing the petiod ofthe irradiation pattern, in addition to the size orUFP lattice itself. By prior irradiation of the substrate, one can control the position and size of the UFPs. When the dose of the electron irradiation was changed, it was found that there is an optimum dose. An excessive or insufficient dose leads to poor control. The optimum dose depends on the material that is vapor
deposited and on the substrate temperature. For ambient temperature deposition of gold. the optimum electron irradiation dose was 0.03 0,04 Clem',
330
Applications
Discussion It became clear that prior irradiation of a finely focused electron beam on a smooth substrate causes the UFP deposition to occur away from the irradiated tracks. One can also align the deposited UFPs linearly in a lattice pattern. The origin of these observations will be discussed next. In the electron irradiation apparatus used, the sample chamber was evacuated to a pressure of 2 x 10-5 torr using an oil diffusion pump. When a substrate was irradiated under such conditions, the following changes on the substrate surface are possible. 1. Hydrocarbons in the vacuum chamber adsorbed on the surface are polymerized, carbonized, and ultimately converted to carbides on the irradiated tracks. 2. Lattice defects are produced on the substrate by radiation induced damage. Of these two possibilities, the second is not important. This is because an electron energy in excess of 1 MeV is required both theoretically and experimentally to produce radiation damage on silicon. Only 30 keV electrons were used; thus no lattice defects should have been produced. Furthermore, the first effect was likely to affect UFP formation. In the vacuum system used, oil molecules from the oil diffusion pump were the major source of hydrocarbons. The carbides formed during the electron irradiation can affect the formation of vapor deposited particles in two possible ways. One is due to the presence of a geometrical pattern on the substrate surface as a result of electron irradiation and carbide formation. The other is the chemical inhomogeneity of the surface (i.e., one part is covered with carbide while the rest has a regular silicon substrate surface). To clarify which of these factors is most important, the entire substrate surface was coated with carbide by uniformly irradiating it with electrons. Next, electron irradiation in a grid pattern was done to 331
Ultra-Fine Particles
produce a carbide grid over the uniform carbide layer. On this substrate, a geometrically patterned surface was formed without any chemical inhomogeneity. When gold was deposited on this substrate and heated, a controlled gold UFP arrangement according to the irradiation pattern was obtained, similar to that produced on the normal substrate as described earlier. This fact indicated that the microscopic geometrical surface pattern due to the carbide formation is the important factor in controlling the formation ofUFPs. The relationship between the electron dose and the height of the carbide was measured. The height was roughly proportional to the electron dose and was 7 - 10 nm when the optimum electron dose for UFP control was used (0.03 - 0.04 C/cm2). This result in combination with previous findings implied that the UFPs selectively form at the edges between the 7 - 10 nm high carbide layer and the substrate surface as shown in Figure 7. This phenomenon is essentially the same as the selective formation of deposited metal particles at atomic steps on a cleaved rock salt surface when certain metals are vapor deposited in a minute amount. For gold UFPs, this occurred because the interfacial energy with the carbide or silicon surfaces was lower than the surface energy of gold in vacuum or in a gas. Electron Beam
Au
Carbon
I
/
~
Figure 7. Model of the UFPs produced on the substrate. 332
Applications
Why is an excessive electron dose less effective in controlling vapor deposited particles? When the electron dose was increased to an extremely high level, the carbide layer became very thick. This was visualized by secondary electron imaging using scanning electron microscopy. The images indicated a broadening of the carbide stripes as well. When the carbide stripes became so broad, the UFPs formed even on top of the carbide stripes. These phenomena suggest that the carbide stripes became smoother than those shown in Figure 7. This reduced the angle between the carbide and the substrate surfaces, lowering the difference in the interfacial energy and the stability of the system as shown in Figure 7. Thus, high electron doses diminish the irradiation effects that can be used to control the formation of the UFPs and their patterns. The key aspect of this technique was to produce tall, sharp carbide lines. Applications to Electron Beam Recording
A description of the attempt to use an electron beam as a probe to make ultra-high density recording based on regular arrays of UFPs follows. To record information by irradiating an electron beam on a UFP, the UFP needs to be changed by the irradiation. The change must be detectable using an electron beam probe. The most primitive method satisfying these conditions is to selectively eliminate a particular UFP from among the UFP array. The process of reading involves the detection of the presence of a UFP and can employ the secondary electron emission used in the scanning electron mIcroscope. For example, alkali halide UFPs were used. Alkali halide crystals are known to easily form lattice defects via irradiation, and are being considered as a high-resolution electron beam resist material [6]. Alkali halide UFPs were expected to be easily damaged and were selected for this study.
333
Regular Arrangements of Alkali Halide Ultra-Fine Particles
To evaluate the feasibility of using alkali halide UFPs as an electron beam recording medium, an attempt was made to produce a regular arrangement of UFPs as discussed in the previous section. The same method was employed, but the heating process used for the gold UFPs was not needed. When alkali halide was vapor deposited on the silicon substrate at room temperature, UFPs observable by SEM were fonned, so the growth stage was unnecessary. An example ofa KCl UFP arrangement is shown in Figure 8, which was obtained by scanning an electron beam in a square grid pattern. The control was better than in gold and excellent regularity was achieved using KCI.
100nm
Figure 8. Two-dimensional square array ofKCl UFPs. 334
Applications Removal of Alkali Halide Ultra-Fine Particles by Electron Irradiation It was possible to obtain regular arrays of alkali halide UFPs using the process described above. To use these arrays for electron beam recording, the changes in the UFPs due to electron irradiation had to be determined.
Experimental method 1. Vapor deposition of an alkali halide to an average film thickness of 3 - 5 nm using resistance heating on a room temperature silicon substrate. 2. The vapor deposited substrate was set in the sample chamber of a scanning electron microscope, heated up to 280°C, and held at that temperature. 3. A part of the deposited substrate (3 mm square) was irradiated uniformly with electrons and the changes in the UFP system were observed. The last step was the scanning mode used in a limited area observation by the SEM, which allowed monitoring of the secondary electron images to determine morphological changes in the alkali halide UFPs. The response of the UFPs to electron irradiation was thus examined using an electron beam with an accelerating voltage of 30 kV and a beam current density of20 Afcm2 • Experimental results and discussion By using the first step described in the previous section, UFPs of alkali halide were formed on the substrate. When the morphology of these UFPs was examined while irradiating with electrons as per the third step, the removal of the UFPs was observed, which resulted from vaporization of the alkali halide. If the electron irradiation is 335
Ultra-Fine Particles done at room temperature, carbides accumulated on the surface of the UFPs and the UFPs no longer vaporized away. The heating of the substrate is used to prevent the accumulation of carbides. To record data by removing UFPs by electron irradiation, the most critical parameter is the time needed to evaporate the UFPs. It is difficult to measure time using the current apparatus, therefore, the following procedure was adopted. Taking the electron beam current as I, the irradiated area as S, and time to remove the UFPs as to, the electron dose per unit area Q is given by Q = I to / S. When one UFP is irradiated and removed, the time required, t, is given by t = Q / q, where q is the beam current density. From the experiments described in the previous section, it is possible to obtain Q, so by determining q, one can estimate the time required to remove one UFP. Here, the current density, q, of the SEM used in these experiments (20 A/cm2) was used, and the value oft determined. The results are given in Table 1. The amount of material evaporated affects the size of the UFPs, such that larger UFPs are produced as the amount of material that is evaporated is increased. When a layer thickness of 3 nm is evaporated on the substrate, the particle diameter is 40-50 nm, while at a layer thickness of 5 nm, the particle diameter is 50-60 nm. The frequency represents the number ofUFPs that can be removed per second and corresponds to the data transfer rate (bit/sec) in recording systems. According to Table 1, KI was the easiest material to remove among the three alkali halides studied and NaCI was the most difficult. For the same material, smaller UFPs can be removed more quickly, but the time needed is on the order of 0.1-1 ms, making the data transfer rate at most 10kHz. For practical applications, the rate must be 1-10 MHz at the least, so the present system is three orders ofmagnitude too slow. A higher irradiation intensity can improve the data transfer rate by ten to one hundred times, but such a speed is still inadequate. For the present concept to be workable, a UFP system having much faster responses must be found.
336
Applications Table 1. Time to Remove Alkali Halides by Electron Beam Irradiation. Material
Amount Evaporated (A)
Removal Time (ms)
Frequency (kHz)
KCI
35
1.0-1.5
~1
KI
50
0.3-0.6
~3
KI
30
0.2-0.5
~5
NaCI
50
2.0-3.0
~0.5
NaCI
30
1.0-1.5
~1
This section has introduced one method for forming regular arrays of UFPs on a substrate. An earlier attempt at forming regular arrays ofUFPs on a substrate used electron beam lithography with a high resolution resists [7]. In this method, a continuous film was deposited on a substrate. The film was then etched into the shape of UFPs using complex processes. Because the etching process is used, radiation-sensitive materials are inappropriate, so the applications of this method are limited. Thus, the present method of directly forming regular arrays of UFPs on a substrate is a better candidate for commercial applications. For ultra-high-density recording, the photochemical hole burning (PHB) system has recently attracted much attention [1]. This has the potential for attaining molecular level high-density recording. However. this method requires cryogenic temperatures to attain molecular level high-density recording, and many kinds of associated optical technologies need to be developed. Therefore, the present attempt to produce two-dimensional recording by the use of electron beams may have potential considering the significant advances that are taking place in related electronics technologies.
337
Ultra-Fine Particles References Moemer, W. E., J. Molecular Electronics 1: 55 (1985). Pashley, D. W., Advanced Phys. 14: 327 (1965). Bassett, G. A., Phil. Mag. 3: 1042 (1958). Kaspaul, A. F. and Kaspaul, E. E., Trans. 10th Natl. Vac. Symp., p. 422, (1963). 5. Coad, J. P., Bishop, H. E., and Riviere, J. C., Surface Sci. 21: 253 (1970). 6. Murray, A. and Isaacson, M., J Vac. Sci. Technol., Bl (4): 1091(1983). 7. Croighead, H. G. and Niklasson, G. A., Appl. Phys. Lett.: 44: 1134 (1984).
1. 2. 3. 4.
338
Applications 4.3 Cobalt-Polymer Composite Thin Films (by Kazuharu Iwasaki)
Recent demand for higher density magnetic recording has necessitated improvements in recording medium and their manufacturing processes. Naturally, the role of the recording media used in such materials is important. In this section, a brief review of the technical developments in this area is provided in order to lay a foundation for understanding the current studies carried out by this research team. In the area of conventional recording media, coating type _re~Qrding media are widely used. Needle-shap-eQmagnetic particles(e.g., y-Fez0 3 , Fe30 4 , CrOz, Co-modified y-Fe z03, etc.) and organic binders are mixed in an organic solvent to form a well-dispersed magnetic paint that is then coated on polymer films. The coated films are subjected to magnetic orientation, dried, and calendared. To increase the recording density, accicular magnetic metal particles have been used for high performance media. Because oftheir limited saturation magnetization and reading output it is difficult to reduce the thickness of the medium, which clearly limits further improvements in their recording density. Thus, it has become clear that particulate recording media using magnetic metal particles are inadequate for high density recording. In contrast to the media described above that use longitudinal recording, perpendicular magnetic recording is used in particulate recording medium made with hexagonal barium ferrite particles. Perpendicular recording is inherently well suited for high density recording, which has lead to the development of particulate media based on barium ferrite [1-4]. Drastic increases in the recording densities over those of metallic particulate media, however, cannot be expected. For any of the coated media, the manufacturing processes are complex and enormous investment is required for facilities to recover the organic solvents, for safe handling of the magnetic powders and organic chemicals, and for pollution abatement.
339
Ultra-Fine Particles There is another type of recording media has not yet reached the stage of commercialization. This is based on metallic thin film media that use polymer-based films and ferromagnetic metallic thin films produced by wet plating, vacuum deposition, ion plating, sputtering, etc. Representative metal thin film media are the longitudinal magnetic recording media based on cobalt, iron, and nickel vapor deposited films and perpendicular magnetic recording media based on Co-Cr films produced by sputtering and vapor deposition methods. In longitudinal magnetic recording media [5-9], the saturation magnetic flux density and the reading output can be increased by eliminating non-magnetic material in the thin film. This allows for an increase in the magnetic density. By taking advantage of the perpendicular recording method that can be used with perpendicular recording media based on Co-Cr thin films [10-18], it is possible to obtain higher output in the shorter wavelength region, which makes ultra-high density recording possible. These metallic thin film media are not practical in terms of their mechanical endurance and chemical stability. Because of the low adhesive strength between the base and thin film, the action of the magnetic heads and drums during high speed operation cause wear, scratches, and delamination of the thin films. Metallic thin films also lack environmental stability and drop-out occurs from rust. To improve the mechanical durability, attempts were made to form a protective layer on a thin film medium [19-22], but this produces spacing loss during reading and reduces the recording density. The study described here was aimed at making a high density recording medium that has high perpendicular magnetic anisotropy, with improved mechanical durability and chemical stability. Experimental Methods Cobalt-polymer composite thin films [23-28] can be produced in a conventional high vacuum evaporator by simultaneous deposition using two different sources, one for cobalt and the other 340
Applications for the polymer. The cobalt source was heated by electron-beam heating and the polymer was heated by crucible resistance heating. The deposition rates were independently controlled, and the vacuum was maintained in the range of 4.0 - 8.0 x 10-6 torr. Figure 1 shows a diagram of the apparatus used for simultaneous vapor deposition. The incident beams of the polymer and cobalt were coplanar in the xz plane, but in opposing directions with incident angles of WI' 1J1z respectively (measured relative to the z-axis, which is normal to the film plane). The polymer used was poly(ethylene terephthalate). The substrate was cooled to lOOK. To characterize the cobalt-polymer composite films, the following equipment was used: a vibrating sample magnetometer (VSM) and torque magnetometer to measure the magnetic properties, transmission and scanning electron microscopy (TEM and SEM) to observe the cross-sectional microstructure, and an x-ray microprobe analyzer to measure the cobalt composition.
z Co vapor beam
Incidence plane
Polymer vapor beam
---j'-------->l'::'=------r----x
Film plane
y
Figure 1. Schematic of the alignment used for the simultaneous deposition of cobalt and polymer. 341
Ultra-Fine Particles Magnetic Properties of Cobalt-Polymer Composite Thin Films The magnetization curves of cobalt-polymer composite films as a function of the incidental angles of the polymer beam are shown in Figure 2. Here, the incident angle of the cobalt beam was kept constant at = 0 (i.e., it was deposited from a direction normal to the film plane).
"'2
(b)
(a)
'f2 = o· (Const)
Ml Q.u.J
Mla.u.}
'1l =
20·
H
H
Co=23 vol%
10
5
10 KOe
10
5
5
10 KOe
MIQ.u.1
~=42·
Co=16 vol%
H 10
5
Co =11 vol%
10 KOe
Mla.u.l
Mla.u.)
~=60·
5
y H ---<_---,;-_--{-;;----;!;----:-=--::-H. 10 10 KOe
10
5
10
5
10 .KOe
~=80· Co=14vol%
H
H 10
10 KOe
5
10 KDe
Figure 2. Dependence of the magnetization curves of cobalt-polymer composite thin films on the polymer incident angle. a) The in-plane magnetization curves; and b) the perpendicular magnetization curves. 342
Applications The cobalt content was held within the range of 11 - 23 vol%. The incident angle of the polymer beam (WI) was set to the following angles: 20°,42°,60°, and 80°. The magnetization curves measured in a direction parallel to the film plane (in the X and Y directions) and the magnetization curves measured in a direction normal to the film plane are given in Figure 2. There was no magnetic hysteresis observed when $1 = 20 ° or 42 0. With increasing W\> some hysteresis was observed at 60 ° and a pronounced hysteresis was seen when WI was equal to 80°. In particular, the magnetization curve normal to the film plane showed strong hysteresis when $1 was equal to 80 °, indicating that this film exhibits perpendicular magnetic anisotropy. Figure 3 shows the dependence ofthe perpendicular magnetic anisotropy constant of the composite thin film on the cobalt composition. The value ofKi (equal to Ku - 2n M S2) was calculated from the magnetic torque curve in the plane parallel to the polymer incident plane. While holding the polymer incident angle $ I constant at 80°, the effects of varying the cobalt incident angle (W2) was studied by setting $2 to 0°, 33°, and 51°. The dependence ofK.l on the cobalt composition for these three angles is shown in the figure. For all three angles, K.l increased with decreasing cobalt content. The sign of K.l changed from negative to positive at about 30 vol% cobalt and K.l reached a maximum at a cobalt content of 15 vol%. Below 15 vol% cobalt, K.l converged for the three angles to nearly the same value. The positive and negative values of Ki indicate that the easy axis coincides with a direction perpendicular to the film plane and in the in plane direction, respectively. Thus, composite thin films having perpendicular magnetic anisotropy were formed below 30 vol% cobalt. The value of K.l reached its highest values in all ofthe composition ranges when $2 was equal to 33 ° and the maximum value obtained was 5 x 105 erg/cc. Figure 4 shows the dependence of coercivity Hc.l on the cobalt composition measured in a direction normal to the film plane. The value of Hc.l increases remarkably for all samples when the cobalt content was decreased and it reached a maximum value at about 15 vol% cobalt, but it decreased below 10% cobalt. When $2 was equal to 33 0, Hc.l reached its highest value, exceeding 1000 Oe. 343
Ultra-Fine Particles 10 ,-----,----~--~--~-----.-------,
z
Co
o 0"
~=80· (Const.)
-10
-...:.---
~
---0----
~ = 33·
- - -0---
)",
= O·
5"
=
- 20
o
w
~
Co
ro
(vol%)
Figure 3. Dependence of the perpendicular magnetic anisotropy constant of cobalt-polymer composite thin films on the cobalt composition.
When the cobalt content was decreased, certain changes occurred in the microstructure of the composite thin films, which produced monotonic increases in KJ.. and HcJ... When the cobalt content was 30 vol% or less, the perpendicular magnetic anisotropy increased, reaching a peak at 15 vol%. At concentrations below 10 vol% cobalt the dispersion of the magnetic anisotropy reduced KJ.. andHcJ.. . 344
Applications
1250 r------~----~----____, He.!.
C
o.."t
~
o
1000
--'
o
- -~---
,
Polymer
!
Ql
~ 750 -i
u
::I:
500
Yo =80' ---t;r-.-
250
--0--
---0----
(C onst . )
~= 0'.
'I. =33 'I. =5(
OL-_--'-_ _-'--_---''--_--'-_ _-'--_----J
o
20
40
60
Co (vol%)
Figure 4. Dependence of coercivity in the direction normal to the film plane, Hc.L, of cobalt-polymer composite thin films on the cobalt composition.
Figure 5 shows the temperature dependence of the coercivity in the direction normal to the film plane, Hc.L , and the saturation magnetization Ms. In the cobalt-polymer composite thin films, the coercivity Hc.L increases monotonically with decreasing temperature, irrespective of the cobalt incident angle W2 and the cobalt content. In contrast, such temperature dependence is essentially absent in ironpolymer composite thin films. In either film, M s shows no temperature dependence. The perpendicular magnetic anisotropy of iron-polymer composite films is primarily due to shape magnetic anisotropy that originates from geometrical structures within the thin films. This is because Hc.L and Ms show no temperature dependence, implying that 345
Ultra-Fine Particles 500,---,.--,-----,-----,-----,-----..---..------,
:; 400 u
":J
~ 300
----------------- --------------
200
':f. = 80·
100
---------
1500
I Co nst.l
%=0· Co=30vol% S". =33· Co: 28 vol % S".=33· Co=16 vol% ~=33· Fe=13vol%
~ 1000
...
u I
500
o
100
200 300 Temperature (K)
400
Figure 5. Dependence of the coercivity, Hc.L, and the saturation magnetization, Ms, of cobalt-polymer composite thin films on the temperature.
the magneto crystalline anisotropy of a-Fe is small and contributes very little to the magnetic anisotropy ofthe composite thin films. For cobalt-polymer films, Ms shows no temperature dependence, but Hc.L is temperature dependent. Thus, the anisotropy observed originates not only from shape anisotropy but from the sum of the shape anisotropy caused by the structure of the film and the magneto crystalline anisotropy of the hcp-Co. Qualitatively, it was estimated that the shape and crystalline anisotropies contribute equally.
346
Microstructure of Cobalt-Polymer Composite Thin Films An SEM image of the cross-section (parallel to the polymer incident plane) of a cobalt-polymer composite thin film is shown in Figure 6. For this sample, WI for the polymer beam was equal to 80°, V2 for the cobalt beam was equal to 0° ,and the cobalt content was 16 vol%. This film grew with a columnar structure in which the columnar grains grew in a direction of 27° from the direction normal to the film plane and in a direction facing toward the incident direction of the polymer beam. According to the TEM and SEM observations of the cross~sections of the films, bundles of columns are formed in a direction normal to the polymer incident plane. The selected area electron diffraction patterns of the cross-section of the film exhibited weak but clear patterns for a-Co and CoO. This indicates that many a-Co UFPs exist within these columns. The morphological effect of the columnar structure of the composite thin films is the main factor that gives rise to the appearance of the large perpendicular magnetic anisotropy. The bundle structure of the columns produces differences in the magnetization curves in the two directions within the film plane.
Figure 6. SEM image of the cross-sectional structure of a cobaltpolymer composite thin film (XOZ plane).
347
Ultra-Fine Particles Figure 7 shows a TEM image ofthe microstructure of a single column inside a composite thin film. In the figure, the directions of the normal to the film plane, the column growth, the a-Co UFP array, and the oriented <001> axis of the a-Co are shown. The <001> axis is distributed over an angle of ± 10 o. The columnar axis is inclined about 5 0 from the normal to the film plane and toward the direction of the incident polymer beam. The direction of the symbol a-Co UFP distribution is inclined 50 0 from the normal to the film plane and toward the direction ofthe incident cobalt beam, while the <001> axis of the a-Co tilts by about 40 0 • The dark field image of the same field shows the presence of numerous a-Co UFPs with diameters of 5 - 20 nm throughout the column in Figure 8. No segregation within the column was observed. Some of the a-Co UFPs are also arranged in arbitrary directions. A diagram showing the columnar microstructure and the distribution of the particles is shown in Figure 9. The column is composed of a mixture ofpolymer and cobalt UFPs, where the cobalt UFPs are in regular arrays and pointing in different directions. It appears that the magneto crystalline anisotropy originates from the arrangement of the <001 > axis of the a-Co particles.
Summary Two-component, composite magnetic thin films of cobalt and polymer were produced via simultaneous vapor deposition of cobalt and poly(ethylene terephthalate) polymer and their magnetic properties and microstructures were evaluated. The incident beams of the cobalt and polymer were coplanar and arranged in opposing directions toward the substrate. The polymer and cobalt beams entered at angles of symboll\J 1 and l\J2relative to the direction normal to the film plane. The effects of varying l\J 1 and l\J2 on the magnetic properties and microstructures were examined. For the analysis of the films, a vibrating sample magnetometer (VSM) and a torque magnetometer were used to measure the magnetic properties, transmission and scanning electron microscopes were used to
348
1000 A
Figure 7. TEM image ofthe microstructure ofa single column inside a cobalt-polymer composite thin film (XOZ plane).
349
Figure 8. Dark field image of the microstructure of a single column inside a cobalt-polymer composite thin film (XOZ plane).
observe the cross-sectional microstructure, and an x·ray microprobe analyzer was used to measure the cobalt composition. The results showed that with low temperature substrates (lOOK) the use of high angles of "', (e.g., '" ~ 80°) produces composite thin films having large perpendicular magnetic anisotropies in the direction nonnal to the ftlm plane. The maximum value of the perpendicular magnetic anisotropy constant, K.L, was 5 350
Applications Columnar axis
t
Polymer vapor beam
CrOlllHleCtion
Substrate
1 Figure 9. Diagram of the columnar microstructure.
105 erg/cc and the coercivity in the direction normal to the film plane, RCi-, was 1000 Oe. The internal structures of the thin films were composed of columns of polymer containing regularly distributed a-Co UFPs. Furthermore, bundles of the columns were formed in a direction perpendicular to the polymer incident beam. The perpendicular magnetic anisotropy of the thin films arises from the shape magnetic anisotropy that originates from the columnar structures. On the basis of the oriented <001> axis of the a-Co particles and the temperature dependence of RCi-, however, the magneto crystalline anisotropy of hcp-Co also contributes to the perpendicular magnetic anisotropy observed. X
351
Ultra-Fine Particles
Prospects The objective of this study was to create a magnetic recording medium having superior magnetic properties, high mechanical strength, and good chemical corrosion resistance that could be produced via a new manufacturing method that combines a strongly magnetic metal with a polymer to produce two component composite magnetic films. The results suggest the possibility of obtaining a new recording medium that can be used for perpendicular magnetic recording and which can achieve high recording densities. In recent years, Co-Cr metallic thin film medium have been studied extensively [10-21]. The Co-Cr medium has advanced from the stage of basic research on the magnetic properties and microstructures of the thin films to the evaluation of their recording/reading characteristics and magnetic recording head-medium interfaces. Furthermore, the tribology of the wear and fracture of these thin film media is being examined. When these issues are resolved, this medium will become a practical medium in high density recording applications. In contrast, cobalt-polymer composite thin film research has just begun. The perpendicular magnetic property of this medium is comparable to that of the Co-Cr medium, but the film manufacturing processes needs additional work. The first problem in reaching commercial applications of this medium is to increase the film deposition rate for mass production. It is necessary to keep the substrate temperature of the plastic film close to room temperature while maintaining the magnetic properties that have been identified in the present study. The second problem is to increase the degree of polymerization of the polymer within the composite thin films, thereby improving the strength of the films and the adhesive strength to the substrate. Through these improvements, the important yet difficult issue of the tribology of the Co-Cr metal thin films are expected to be overcome by the cobalt-polymer composite films. This expectation arises because the magnetic head 352
Applications interface problems associated with conventional particulate recording media are essentially the same ones that the composite thin films face. When the film deposition problems are resolved, one must reevaluate the composite thin films as practical materials for perpendicular recording medium in regard to their recording/recording, endurance, travel characteristics, corrosion, and storage characteristics.
References 1. Fujiwara, T., Isshiki, M., Koike, Y. and Oguchi, T., IEEE Trans. Magn. MAG-18, 6: 1200 (1982). 2. Fujiwara, T., IEEE Trans. Magn. MAG-21, 5: 1480 (1985). 3. Isshiki, M., Suzuki, T., Ito, T., Ido, T. and Fujiwara, T., IEEE Trans. Magn. MAG-21 , 5: 1486 (1985). 4. Suzuki, T., Ito, T., Isshiki, M. and Saito, N., IEEE Trans. Magn. 25,5: 4060 (1989). 5. Kunieda, T., Shinohara, K. and Tomago, A., Proc. ofIERE, 59: 37 (1984). 6. Shinohara, K., Plastics Fab. Tech 12,4:25 (1985). 7. Tomago, A., Shinohara, K., Nochi, N., Kunieda, T., Murai, M., and Yoshida, H., National Technical Report 31, 6: 899 (1985). 8. Suzuki, T., Electrochem. 54, 10: 836 (1986). 9. Hokkyo, J., J Magnetics Society ofJapan, 17,5: 777 (1993). 10. Iwasaki, S., IEEE Trans Magn MAG-16, 1: 71 (1980). 11. Nakamura, Y. and Iwasaki, S., IEEE Trans. Magn. MAG-18, 6: 1167 (1982). 12. Ouchi, K. and Iwasaki, S., IEEE Trans. Magn. MAG-18, 6: 1110 (1982). 13. Sugita, R., Nanbu, T., Echigo, N. and Sakamoto. Y., IEEE Trans. Magn. MAG-22, 5: 1182 (1986). 14. Nakamura. Y. Tagawa, 1. and Iwasaki, S., IEEE Trans. Magn. MAG-23, 5: 2856 (1987). 15. Ouchi, K. and Iwasaki, S., IEEE Trans. Magn. MAG-23, 5: 2443 (1987). 353
Ultra-Fine Particles 16. Yamamoto, S., Nakamura, Y and Iwasaki, S., IEEE Trans. Magn. MAG-23, 5: 2070 (1987). 17. Sugita, R., Nanbu, T. and Sakamoto, Y, IEEE Trans. Magn. MAG-23, 5: 2449 (1987). 18. Ouchi, K.,J Magnetics SocietyojJapan, 13, SI: 611 (1989). 19. Awano, H. et aI., 46th Meeting of the Magnetics Society of Japan, p. 46 (1986). 20. Nakatsuka, Y et aI., IEEE Trans. Magn. MAG-22, 5: 1002 (1986). 21. Karimoto, H., Sumita, I. and Nakayama, Y, J Magnetics Society ojJapan, 11,2: 129 (1987). 22. Kurokawa, H., Mitani, T. and Yonezawa, T., IEEE Trans. Magn. MAG-23, 5: 2410 (1987). 23. Iwasaki, K. and Makino, Y, 40th Annual Meeting Phys. Soc. Jpn Abst., p. 115 (1985). 24. Iwasaki, K. and Makino, Y., 11th International Colloquium on Magnetic Films & Surfaces ICMF-ll, AB-18 (1985). 25. Iwasaki, K, Hayashi UFP Project Research Seminar, ERATO, p. 9 (1986). 26. Iwasaki, K, Hayashi UFP Project Research Seminar, ERATO, p. 45 (1986). 27. Iwasaki, K., U.S. Patent, 4671971 (1987). 28. Iwasaki, K. and Makino, Y, J Japan Society oj Powder & Powder Metallurgy, 41, 5: 595 (1994).
354
Applications 4.4 Catalytic Applications of Gas Evaporated Ultra-Fine Particles (by Toyoharu Hayashi)
Research on UFPs has concentrated on properties that arise from microscopic or quantum size effects, but little is known about their capability as catalysts. In this section, a description of the application of UFPs in catalysis is presented. Metal UFPs can be produced by gas evaporation in which metals are evaporated in an inert gas atmosphere (e.g., 0.01 - 100 torr of helium or argon). By selecting the gas pressure and evaporation rate, UFPs ranging in size from several nanometers to several tens of nanometers in diameter can be produced [1] (see Figure 1). By changing the gas to ammonia, methane, or oxygen, UFPs comprised of nitrides, carbides, or oxides, respectively, can be produced although such materials were not used in this study. There are several conventional methods that can be used to prepare catalysts. These include the impregnation method in which metallic salts and clusters are impregnated into inorganic oxide carriers and then heated and reduced. Another method is the Raney method in which Raney alloys are dissolved in an alkaline solution to produce catalysts. These methods produce highly dispersed metal catalysts with diameters ranging from several nanometers to several tens of nanometers [2] Thus, in terms of particle diameter alone, the UFPs with diameters of 30 nm that are produced by the gas evaporation method are not unique. These UFPs are expected to have the following advantages. 1. When the UFPs are used as a suspended catalyst in a liquid, the parameter of particle-carrier interactions can be eliminated. 2. Gas evaporated UFPs have large specific surface areas of several tens of square meters per gram, which is adequate as practical catalysts.
355
s
Gas Evaporation Technique
~ I
~
:I ~
~
a..
Collection plate ,
l")
~
r..I
""
"
VJ VI
0'1
Vacuum pumping system f-
"\ \ j"" Crucible
Gas inlet Ar,He,N 2 f - 02' NH3,
C~,~S
Electric power supply Figure 1. Apparatus for UFP preparation by the gas evaporation method.
Applications 3. Gas evaporated UFPs have no porous structures, which allows for elimination of the catalytic reactions that occur in pores. This is desirable in terms of the activity (rate of reaction) and selectivity of the catalyst. 4. Gas evaporated UFPs are made by a physical process, so contamination due to impurities can be minimized relative to conventional chemical processes. 5. This method is well suited to the preparation of bimetallic or compound metallic catalysts An appropriate model reaction system was selected with the above mentioned features in mind. The system would provide clarification of the characteristics of gas evaporated UFPs used as catalysts. Experimental
To clarify the catalytic capabilities of gas evaporated UFPs, a reaction was selected that used a catalyst suspended in a liquid so that the UFPs would not sinter during the reaction. A hydrogenation reaction of unsaturated carbon-carbon or carbon-oxygen bonds that react at low temperatures was selected. Hydrogenation Reaction Activation of nickel ultra-fine particles [3]
Nickel UFPs with average diameters of 30 nm were required as catalysts for hydrogenation. Because the surface of commercially available nickel UFPs is covered with an oxide layer, these must be activated by reducing the oxide. Without the reduction treatment, the UFPs showed no activity in hydrogenation reactions. Nickel UFPs were also prepared using the gas evaporation method and their activity was tested without exposure to the atmosphere. The activity was low, perhaps because of inadequate handling. 357
Ultra-Fine Particles
Both the temperature required to reduce the surface oxide and the oxygen content of the nickel UFPs was determined by thermogravimetric analyses with the samples in a hydrogen atmosphere. The results are shown in Figure 2. Reduction started at 155°C and the oxygen content was found to be 5.5% of the total nickel UFP weight before reduction. The samples were reduced in borosilicate glass tubes with an inside diameter of 10 mm and which contained a filter. A O.I-g sample of the nickel UFPs was placed inside the tube and a hydrogen gas flow was used to both fluidize and reduce the sample. Heating was done over the range of 160 - 180 °C within a I-min period (see Figure 3). This reduction method activated the nickel UFPs but it did not cause much sintering. The specific surface areas before and after the reduction treatment of 30nm nickel UFPs via the BET method were 27.3 m2/g and 27.0 m2/g, respectively. These values are essentially identical considering the accuracy of the BET method used. This concurs with the determinations made based on electron microscopy (Figure 4 ).
300
-8 r-..
IN!
LJ
'-/
II l!l
-
....
-4
W
..
3:
LL
0 W U
z
0 '--'
200
"".."",,~
w
0::
::> I-
«
0:: W ll..
100
0
-««
::E
w I-
0 ::
>-
10
20
30
40
TIME(MIN)
Figure 2. Thermogravimetric measurement of nickel UFPs in flowing hydrogen gas. A: weight change; B: time (min); and C: temperature. 358
Applications
Glass tube
Frit
Figure 3. Reduction of the surface oxide layers of nickel UFPs by fluidization. A: porous plate; B: furnace; and C: glass tubing.
Hydrogenation using nickel ultra-fine particles [3] The hydrogenation of 1,3-cyclo-octadiene CD proceeds in two steps, and produces only cyclo-octene @ and cyclo-octane Q). These features make this reaction ideal for the evaluation of catalytic activity.
o o o 1 The catalytic activity of nickel UFPs was compared to those of two Raney nickel catalysts under the same reaction conditions. One ofthe Raney nickel catalysts was made by the W-4 method using
359
NiUFP
~
100nm
Figure 4. Electron micrograph of nickel UFPs after H2 reduction. 360
Applications a Ni-AI alloy (50 ± 1 wt% Ni) and the other used a high temperature preparation similar to the W-4 method, except the temperature of the aluminum extraction by alkaline solution was done at 93°e. The second method used removes more aluminum, producing a catalyst with higher activity. As for the catalyst properties, the ratio of the rates of the first hydrogenation reaction (VI) and the second hydrogenation reaction (v2), v/ V 2, defines the reaction selectivity for cyclo-octene. As a practical reaction of interest is the hydrogenation of the remaining butadiene in a C'4 distillate, while minimizing as much as possible the hydrogenation of the butenes that are present. Figure 5 shows the results for the hydrogenation of 1,3-cyclooctadiene using the three catalysts described above. With nickel UFPs, the first stage shows a sharp increase in the hydrogen absorption, which corresponds to the first hydrogenation reaction in which cyclo-octadiene is converted to cyclo-octene. The lower slope for the later stage corresponds to the second hydrogenation reaction, in which cyclo-octene is converted to cyclo-octane. The cyclo-octene selectivity defined by the v/v 2 ratio is shown in Table 1. The modified W-4 Raney nickel had a higher activity because there was less aluminum in the catalyst compared to the W-4 Raney nickel. However, both catalysts had poor selectivity. The nickel UFPs have comparable or better activity in the first stage than the modified W-4 Raney nickel. This is most likely due to the absence of aluminum impurities and due to the lack of diffusion through fine pores, so the reaction proceeds directly on the particle surfaces. The selectivity of nickel UFPs is 5 - 10 times higher than that for Raney nickel. The morphology of Raney nickel is sponge-like, in contrast to the simpler structure of the nickel UFPs. This difference may be responsible for the selectivity. Although the Raney nickel catalysts used were of a limited variety, it is clear that the gas evaporated UFPs have interesting catalytic properties.
361
Ultra-Fine Particles
80 .........
E
~
5}... 70 ::::s::::
RNi-2
(Y)
o ~60 a
.
--
E c 50 o
Ni-UFP (30 nm)
70-
~---
......
0.
~40 (/)
c o u
c 30
(];
O'l
o -020 >.
..c
10
10
20
30
40
50
time (min) Figure 5. 1,3 cyco-octadiene hydgrogenation. Reaction conditions: hydrogen pressure = 1 atm; T = 30°C; 1,3 cyclo-octadiene = 0.2 ml; catalyst = O.1g; and ethanol solvent =150 ml.
0:
Nickel UFPs; <1:>: RNi-l Raney nickel by the W-4 method .: RNi-2 nickel by high temperature preparation 362
Applications Table 1. Results From the Hydrogenation Reaction of 1,3 Cycooctadiene. Catalyst
1st Stage Hydrogen Consumption Rate (mmol/sec Ni g atom)
Selectivity A
Ni UFPs
7.7
210
Ni UFPS
5.3
110
RaneyNi B
4.5
24
RaneyNi
1.1
16
C
A: The 1st stage hydrogen consumption rate/the 2nd stage hydrogen consumption rate. B: Modified W-4 method using a leaching temperature of93 C: Raney nickel by the W-4 method.
0c.
Enantio-face selective hydrogenation of methyl acetoacetate using nickel ultra-fine particles To synthesize optically active compounds, numerous homogeneous noble metal catalysts have been developed. To separate the product and catalyst, attempts have been made to replace the catalyst with heterogeneous catalysts. It is also worth using base metals rather than of noble metals. The hydrogenation of 13-keto acid esters, such as methyl acetoacetate, has been attempted using L(+)-tartarate treated Raney nickel, nickel reduced from nickel oxide, and other nickel materials [5]. It is known that these hydrogenation reactions are strongly affected by impurities in the catalysts. For example, the remaining aluminum in Raney nickel substantially decreases the optical activity yield, which is a measure of the selectivity of this reaction. Nickel metal powders produced by the
363
Ultra-Fine Particles reduction of nickel oxide are of high purity and have a high optical yield, but the reaction rate is low due to the small specific surface areas. When high purity catalysts are needed, as they are for these reactions, nickel UFPs having a large surface area and high purity should be appropriate. This has been confirmed experimentally. Nickel UFPs have a high activity and produce a high optical yield.
rnethylacetoacetate
rnethy13-hydroxybutyrate
Nickel UFPs were reduced according to the previously described method. The surface of these were modified via Izumi's method [5] using an aqueous solution of optically active tartarate. Figure 6 shows the reaction processes along with the experimental conditions. The optical yield was nearly constant for reactions done at temperatures between 80 and 140°C, and it was higher under higher hydrogen pressures (100 kg/cm2) than at lower pressures (10 30 kg/cm2). The modification by tartarate under alkaline conditions for a long period at high temperatures produced better results. An apparent activation energy of 10.2 kcallmol was obtained from the Arrhenius plot. This value is identical to the previously reported value. The conditions were varied and a maximum optical yield of 85 %, was obtained which is comparable to the maximum yield previously attained.
Synthesis of Methanol [6] When one uses the gas evaporation method to prepare bimetallic or compound catalysts, there are several features that are available that were not available with conventional catalysts. With the impregnation method, bimetallic catalysts are produced by impregnating two different metallic salts into the carrier, followed by drying, heating, and reduction. Bimetallic catalysts are used in an attempt to obtain catalytic activities that are unavailable by using a single element catalysts by taking advantage of a binary alloy or a
364
Applications
150..-------------------,
REDUCE])
~~-"lf----;;-)( ~N~,i.~0 ~ 120°c)
1
2
3
4
time ( hour) Figure 6. Enantio-face selective hydrogenation of methyl aceoacetate. Reaction conditions: catalyst: 0.8 g and; acetoacetate, 9ml; and solvent: methylpropionic acid (reaction temperatures are indicated in the figure). Catalyst preparation conditions: activation of nickel catalysts by reduction followed by dipping for 1 hour at 85°C in an aqueous solution ofL-(+)-tartarate (pH 4.1). 365
Ultra-Fine Particles compound state. For such catalysts it is important to avoid separation or precipitation of one of the components during the drying stage and to avoid the segregation of components between the surface and the interior ofthe bimetallic particles during heating and reduction. Such segregation can arise from differences in the affinity the metals have for the gas atmosphere. For gas evaporation, the synthesis ofUFPs can be done in a variety of atmospheres. Some of the difficulties associated with the impregnation method should be able to be overcome by using UFPs produced by the gas evaporation method. To confirm this prospect, we prepared Cu-Zn UFPs and used them as catalysts for the synthesis of methanol from carbon monoxide and hydrogen. The Cu-Zn UFPs were oxidized slowly prior to use, so the UFPs consist of copper and ZnO, and are not strictly bimetallic. In the past, methanol catalysts were produced by the following methods. 1. Co-precipitation method, in which a catalyst precursor is prepared by adding materials to modify the hydrogen ion density, such as sodium carbonate and ammonia, to aqueous solutions of copper and zinc salts. 2. Mixing method, in which a catalyst precursor is prepared by mixing compounds containing copper and zinc. 3. Impregnation method, in which an aqueous solution containing catalytically active elements are impregnated on an inorganic carrier to make a catalyst precursor. 4. Thermal decomposition method, in which metal carbonyl clusters are adsorbed onto the carrier and heated 5. Raney alloy leaching method, in which an alloy containing Cu and Zn is leached by an alkaline aqueous solution. These preparation methods introduce many factors that can reduce the catalytic activity or selectivity. These preparation methods 366
Applications include the need to carefully control the hydrogen ion concentration, to use pure reagents for inducing precipitation of the catalytic metals, and to control contamination caused by impurities. In the gas evaporation method, however, copper and zinc are fused at the atomic level. Thus, the catalysts obtained are expected to show new features in terms of their activity and selectivity. The production and characterization of Cu-Zn UFPs was discussed in Section 2.2. From electron microscopy analysis of the Cu-Zn UFPs, the UFPs were found to consist of copper metal cores (diameters of 15 - 45 nm) with protrusions ofZnO on the surface (diameters of2 - 3 nm). The EELS and electron diffraction confirmed that the individual particles consist of copper and ZnO. To evaluate the activity and selectivity for methanol synthesis, the catalysts were tested in liquid suspensions. The gas evaporated UFPs were compared with a co-precipitated CuZnO catalyst. The results indicate that both catalysts have comparable activity and selectivity. Whether these are affected by the by-products remains to be explored (see Table 2). Table 2. Methanol Synthesis reaction (CO + H2 .... CH30H, CU/ZnO compound UFPs). Catalyst
CO Conversion Rate A (%)
Methanol Selectivity B (%)
UFPs
Cu-ZnO (64:18)
98
CU-ZnO (57:24)
99
Cu-ZnO-AI2O J
93
Cu-ZnO
98
Coprecipitation
Liquid suspension reaction: catalyst. 0.5g; xylene. 20 ml; HICD mixture, 60 kg/cm 2; reaction temperature, 250°C; and reaction time, 2 hours. A: The CO conversion rate was derived from the following relation: [ 1- (unreacted CO (mol))/(unreacted CO(mol) + C in all products except CO 2 (g atom)] x 100% B: The methanol synthesis selectivity was derived from the following relation: [I - (synthesized methanol (mol))/C in all products except CO 2 (g atom))] x
100%.
367
Ultra-Fine Particles Prospects The characteristics of gas evaporated UFPs as new catalysts were clarified in this study [7-9]. The activity and selectivity that were found for these materials, however, must be correlated with their structural features. To develop these materials into practical catalysts will require more studies that explore such features as their stability, costs, and the design of specific UFPs for particular reactions.
References 1. S. Yatsuya, S. Kasukabe, and R. Uyeda, Jpn J Appl. Phys., 12:
1675 (1987). 2. M. Haruta, S. Tsubota, T. Kobayashi, H. Kageyama, M. J. Genet, and B. Delmon, J. Catal., 144: 175 (1993). 3. T. Hayashi and T. Nagayama, J Jpn Chern. Soc., 1050 (1984). 4. T. Hayashi and T. Nagayama, Jpn Chern. Soc., 49th Spring Meeting, 2Q15 (1984). 5. T. Harada, S. Onaka, A. Tai, and Y. Izumi, Chern. Lett. 1131 (1977). 6. M. ada, E. Fuchida, and T. Hayashi, Oyo Buturi 56:395 (1987). 7. M. Noda, S. Shinoda, and Y. Saito, Jpn. Chern. Soc. 1017 (1984). 8. E. Lee, S. Sou, T. Kim, 919th Meeting, Jpn Mech. Engr. Soc., 82018 (1982). 9. J. Iwamura, S. Nishiguchi, and K. Yoshida, Local Meeting of J. Chern. Soc. 2B08 (1984).
368
Applications
4.5 Chemical Heat Pump (by Yasukazu Saito)
About a half of the energy consumed in Japan goes to waste [1]. In particular, heat <100°C has little use, but it would become a useful energy source if the temperature was raised to 150-200°C. If the temperature rise was larger, it would be even more desirable. Chemical heat pumps use the absorption and emission of heat that accompanies chemical reactions to increase the temperature [2]. When a newly developed 2-propanol/acetone/hydrogen system is used as an operating medium, the heat pump can provide 200°C heat using an 80°C heat source and a 30°C coolant. Here, a catalyst plays a key role. A New Chemical Heat Pump System Acetone can be hydrogenized quickly and selectively when a suitable catalyst is chosen. Based on the restriction imposed by the chemical equilibrium involved, this reaction becomes unfavorable above 202 ° C at 1 atm, [3] but acetone and hydrogen can be converted to 2-propanol until the equilibrium is reached, which results in the emission of the heat of reaction. On the other hand, 2propanol absorbs heat when it is converted into acetone and hydrogen by the use of a suitable catalyst. When the reaction occurs in the liquid phase, the hydrogen has very low solubility and separates into the gas phase [4]. The temperature of the system is around 80°C at which the mixture of 2-propanol/acetone boils. By cooling the system at 30°C, 2-propanol (b.p. 82.4 ° C) easily separates, condenses from the mixed gas containing acetone (56.3 °C), and returns to the liquid-phase reactor. The exchange with the outside is limited to heat and the work of separation becomes the driving force of this heat pump. Table 1 summarizes the chemical reactions involved.
369
Ultra-Fine Particles Table 1. Enthalpy Changes in the 2-Propanol/Acetone/Hydrogen Chemical Heat Pump. (CH3)2CHOH -. (CH3)2CO + H2 vapor vapor liquid
~H
= 100.4 kJ mol- l
(CH3)2CHOH + H2 -. (CH3)2CHOH vapor vapor liquid
~H
= -55.0 kJ mol- 1
(CH3)2CHOH -. (CH3)2CHOH liquid vapor
~H
= -45.4 kJ mol- l
Generally, the heat efficiency (11 ) of a heat pump is given by the following equation.
Here, low quality heat QL is supplied at temperature TL' then a part of the heat, Qc, is discarded at temperature Tc, and high quality heat QH is recovered at temperature TH' From the First and Second Laws of thermodynamics we have the following relationships.
QL / TL = QH / TH+ Qc / Tc Using these relations, the maximum efficiency, 11 as follows.
max'
can be derived
By setting T L equal to 80° C, T c equal to 30°C, and T H equal to 200°C, a value of 0.39 is obtained for 11 max' That is, about 60% of the low quality 80°C heat is discarded via cooling, and the remainder 370
Applications can be raised to 200°C. This is significant in terms of the thermodynamics of the energy chemistry because one can effectively use the low quality heat and obtain a useful energy source by merely putting the system in contact with a low cost heat source. The heat pump consists of a vapor-phase exothermic reactor, RH ; a liquid-phase endothermic reactor, R L ; a distillation tower, D; and a heat exchanger, E, as shown in Figure 1. The liquid-phase reactor acts as a reboiler for the distillation tower as well. The reaction vapor is mainly acetone and hydrogen. This is sent from the top of the distillation tower to the vapor-phase exothermic reactor via the heat exchanger and is converted by the catalyst to an equilibrium mixture that contains 2-propanol. The heat of hydrogenation is recovered at the reaction temperature. The vapor exiting the vapor-phase exothermic reactor moves to the distillation tower after pre-heating unreacted vapor in the heat exchanger. In the tower, 2-propanol condenses and the liquid component enters the liquid-phase endothermic reactor, in which 2-propanol is converted to acetone and hydrogen by the catalyst. This is also a reboiler for the distillation tower in which the 2-propanol/acetone mixture boils and the acetone preferentially vaporizes. The hydrogen gas dissolves only little in the liquid phase, with most of it separating into the vapor phase. The liquid phase endothermic reactor receives the heat of reaction and that of vaporization and then regenerates acetone and hydrogen from the 2-propanol that enters. The heat efficiency of the system shown in Figure 1 was calculated assuming that the components in the distillation tower and the reaction inside the exothermic reactor are in equilibrium, and that the value for TL is 80°C, the temperature of the distillation tower (bottom) temperature is 78°C, the value for T c is 30°C, there is a total pressure of 1 atm, and that there is a hydrogen/acetone ratio of five in the exothermic reactor. The heat efficiency is highest for high quality heat at 200°C as shown in Figure 2 and reaches a maximum of 0.36 [5]. This is about 90% of the maximum heat efficiency. Figure 2 also shows the heat efficiency, 11H' and the excergy efficiency, 110 which are defined as follows.
371
R~ 1 , 'I.r r
'r
I
-
D
W
......:l
tv
TL Figure RH : D: TH :
Tc:
r
t~ ., .. ~ ::
~RL
,F:
s~ ~
~
.
I~ J Ijj}j.\
I .I..
~
1~
1::1.! gJ
!11:~:1l<:1 f::~
il. l":l ~
"'l
T
H
~H
1. Flow chart for the 2-propanol/acetone/hydrogen chemical heat pump. Vapor-phase exothermic reactor RL : Liquid-phase endothermic reactor Distillation to\lJer E: Heat exchanger High recovery temperature TL : Low quality heat temperature Cooling temperature
Applications
I1 H=Q .. /Ql
L0
150
170
TH/'C 190
210
230
.-----,...------.---r---.-----"T------.
11. =QH(I- Tc/THl/Q l (I-1(/T l )
08
0.6
I
<::-
04
00 410
430
450
470
490
510
TH/K
Figure 2. Chemical heat for the 2-propanol/acetone/hydrogen system. Heat efficiency and energy efficiency of the pump. Setting temperatures: heat supply, T) = 80°C, cooling, Tc = 30°C distillation tower bottom = 78°C. Vapor phase composition (exothermic reactor entrance): hydrogen/acetone ratio = 5.0 Parameter Xb: acetone mole fraction in quasi-condensed component. 373
Ultra-Fine Particles
The energy efficiency is a function of the heat recovery temperature, TH, relative to a value of30 a C for Te. The maximum for T]€ is about 0.9. Such high heat efficiency results because the chemical heat pump depends on thermodynamic changes, such as chemical reactions and phase transformations. This heat pump will only become technically and economically feasible, however, when the catalyst is inexpensive and has a long life time, can be handled easily, has a high selective, has high reaction rates on the endothermic side, and has high transfer rates on the high temperature side [6]. Table 2 summarizes the characteristics of the present chemical heat pump in comparison with the Type 2 absorption heat pumps and metal hydride chemical heat pumps that are based on hydrogen storage alloys [7]. Japan is a country with volcanic activity and geothermal sources are an important supply of domestic energy. The use of high temperature water that is produced along with steam is a challenge for the future [8]. At a geothermal power generating plant that uses 150 a C steam, a process design and feasibility study were done for a process using a heat pump to obtain additionalI50 a C steam from hot water at I50 a C [9]. Based on the system shown in Figure 1, a variety of modifications were made and the heat efficiency was calculated to be 32%. The modified system that was calculated to produce such efficiency is shown in Figure 3. Because no corrosive materials are involved, relatively low cost carbon steels can be used for the construction. Based on the following assumptions: a catalyst cost that is 5% that ofthe plant cost (4 yen per kcal for high quality heat), a yearly operational period of 8000 h, a finance interest of 6%, and other operating costs such as labor, cooling water, and electricity, it was estimated that the system could be depreciated over a 5.1 year period by selling I50 a C pressurized steam produced at 10 t/h for 3000 yen/to The cost of the cooling water for the system is the most costly item in the operational expenses.
374
Table 2. Comparison of Heat Pumps Using Absorption, Metallic Hydrides and Organic Compounds.
w
-....l
Absorption
Metallic Hydride
Organic Compound
Driving Force for Heat Pump
Compression work
Compression work
Separation work
Working Medium
Stearn LiBr aq. solution
Hydrogen Metal hydride
Hydrogen Organic compound
State Change
Liquid/vapor
Solid/vapor
Liquid/vapor
Chemical Change (reaction pair)
None
Hydrogen absorption emission (2 pairs)
Hydrogenation Dehydrogenation
Heating (middle T)
Stearn generation (2 places)
Hydrogen emission (2 places)
Dehydrogenation (1 place)
Cooling (low T)
Condensation
Hydrogen absorption
Separation of organic compound
Heat Generation (High T)
Stearn absorption by concentrated solution
Hydrogen absorption
Hydrogenation
System Features
Supply hot water vapor to concentrated solution
Hydrogen compression by heat
Hydrogen emission from liquid to vapor phase Use of catalyst
Operation Features
Continuous
Cyclic
Continuous
Heat Storage
None
Yes
Yes
Heat Transfer
VI
::to..
~ :::
£
g-.
:: lOll
s a-
s·~ E-IOI
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eI
t-
o
w
-l 0\
8 6
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!. t
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t==-=-='*
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:
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:
:
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I.
; I R-IOI
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E-103A,B E-I05 P-IOIA,B P-I02A,B
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Figure 3. Design of a 2-propanoVacetone/hydrogen chemical heat pump system using high temperature waste water from a geothermal power generating plant producing high temperature steam. Heat supply, 150° Cwaste water; heat output, 150°C pressurized steam; 10 t/hr; exothermic reaction, 170°C (1.5 atm); and endothermic reaction, 78° C (1.75 atm).
~ \oil
Applications Catalytic Activities of Metal Nickel Ultra-Fine Particles and Their Applications Metal UFPs prepared by the gas evaporation method have the following interesting properties in terms of catalysts [10]: (1) highly crystalline spherical particles, (2) relatively uniform particle size distribution, (3) large specific surface area without small pores, and (4) high chemical purity (except for the surface oxides). The reason for the high crystallinity and non-porous spherical structure is the high temperature they are subjected to during preparation. The advantage of not having small pores is that diffusion within small pores can lead to subsequent byproducts and lower selectivity. When UFPs suspend well, highly concentrated solutions can be prepared, which should provide for not only increased reaction rates per unit catalyst weight, but also increased reaction rates per unit liquid volume. On the other hand, thermally stable systems can be expected when they are used under mild reaction conditions. In particular, long life times can be expected for endothermic reactions. Nickel UFPs (average particles diameter of 20 om, specific surface area of43.7 m2/g) have been used after hydrogen reduction as suspended liquid phase catalysts for the low temperature endothermic reaction of the present chemical heat pump. The need for the hydrogen reduction process arises from the stabilization treatment that is used to cover the surface of the UFPs with oxide layers [11]. Because 2-propanol is a polar solvent, we can obtain a suspended solutions by using ultrasonic dispersion. When such a solution is boiled, hydrogen gas is generated vigorously, yielding acetone in the same molar quantity. A solution of a 1: 1 mixture of2propanol and acetone (b.p. 64.4°C) also generates hydrogen. The liquid-phase dehydrogenation of2-propanol with a suspension offine nickel particles as the catalyst can be described by the following kinetic relation, which includes the inhibition term for acetone, as reported for a nickel boride catalyst [12].
v = 1C / (
1 + K [acetone] )
Here 1C is the reaction rate constant and K is the equilibrium adsorption constant. 377
Ultra-Fine Particles When Pt(II) bisacetylacetonate was dissolved into 2propanol in the above reaction, and precipitated platinum on the surface ofthe suspended nickel particles via reduction treatment, the catalytic activity of the suspended nickel UFPs for the dehydrogenation reaction showed a marked improvement [13]. The catalytic activities of solid suspended catalysts, homogeneous catalysts, and immobilized catalysts are compared for the same weight basis in Table 3. The metal nickel UFPs, especially those modified with platinum, have high reactivities. Such a surface treatment is a promising means for improving the activity of UFP catalysts [14]. Table 3. Comparison of Catalyst Activities for Liquid Phase Dehydrogenation of 2-Propanol. Activity (mmol h-1g- 1)
Catalyst Metal Ni UFPs
136
Metal Ni UFPs modified with Pt
286
Boride Ni powders
60
RaneyNi
55
17.3
RhiOAc)2(PPh3)6 Ru(OAcF)iCO)(PPh3)6
14
Silica modified Ru( 0 Ac)2(PPh2(CH2)2)6
7.74
Rh2CliSnCI3)6
4.0
Resin immobilized Ru(OAcF)iCO)(PPh3)6
0.84
378
Applications To increase the thermal efficiency ofthe present heat pump, it is desirable to keep the concentration of acetone in the liquid phase 2propanol dehydrogenation stage at as high level as possible. This reduces the thermal load in the distillation separation and lowers the boiling point ofthe solution. This is the most critical requirement for the catalyst used in the low temperature endothermic reaction. For the high temperature exothermic reaction of acetone hydrogenation, a nickel UFP catalyst supported on porous graphite was evaluated [15]. Because the reaction temperature is high, the selectivity, rather than the activity, is important. It has been pointed out [16] that efficient heat transfer at high temperatures is critical. Metallic nickel UFPs prepared by gas evaporation were found to be useful as catalysts for liquid phase 2-propanol dehydrogenation and for acetone hydrogenation. The elements of the chemical heat pump technology described here are basically similar to those of traditional chemical engineering processes. A superior catalyst was required, where an improved catalyst upgraded the system as a whole. The use oflow quality heat is important in countries such as Japan that have poor energy resources. It is hoped that the technology is developed further. References
1. Okamoto, H., Handbook of Waste Heat Recovery, p. 29, Fuji Techno Systems (1981). 2. Raldow, W. M. and Wentworth, W. E., Solar Energy 23: 75 (1979); Yoshida. K., Saito, Y eds., Chemical Heat Pump Design Handbook, Science Forum (1985); Kashiwagi, K., Kameyama, H., and Sakata, A. eds., Applications of HighPerformance Chemical Heat Pump, Science Forum (1991); Saito, Y, Yamashita, M., Ito, E., and Meng, N., Inti. J. Hydrogen Energy 19: 223 (1994). 3. Stull, D. R, Westrum, Jr., E. F., and Sinke, G. C., The Chemical Thermodynamics of Organic Compounds, p. 649, John Wiley and Sons, New York (1969).
379
Ultra-Fine Particles 4. Seidell, A., Solubilities ofInorganic and Metalorganic Compounds, 3rd Edn., vol. 1, p. 564, D. Van Norstrand, New York (1958). 5. Saito, Y., Kameyama, H., and Yoshida, K., Int!. J. Energy Research 11: 549 (1987) 6. Yoshida. K., Saito,Y. eds., Chemical Heat Pump Design Handbook, p. 55, Science Forum (1985). 7. Saito, Y., Seisan Kenkyu 38: 459 (1986). 8. Resources and Energy Agency ed., 1985 Annual Book on Resources and Energy, p. 755, MITI Resource Study Group (1985). 9. Sato, S. and Toyoyama, M., Chemical Heat Pump Design Handbook, p. 131, Science Forum (1985). 10. Hayashi, T. and Saito, Y., Chemistry 39: 667 (1984); Saito, Y. Chemistry Review No. 48, UFPs Science and Applications, p. 193, Gakkai Shuppan Center (1985); Saito, Y., High Polymers 35: 356 (1986); 41; Hayashi, T. and Saito, Y., Chemistry 41: 680(1986); Saito, Y. andNoda,M.,Mat. Sci. 23: 186 (1987). 11. Noda, M., Shinoda, S. and Saito, Y., Nihon Kagaku Gakkaishi: 1017 (1984). 12. Mears, D. E. and Boudart, M.,A.I.Ch.E. Journal 12: 313 (1966). 13. Noda. M., Shinoda, S. and Saito. Y., Catalyst 27: 359 (1985). 14. Noda, M., Shinoda, S. and Saito, Y., Bull. Chem. Soc. Jpn. 61: 2541 (1988) . 15. Nakagawa, N., Kato, K., Kameyama, H., Noda, M. and Saito, Y., Chikunetsu, Zonetsu Gijutu, Chern. Engr. Soc. Symp. Series Vol. 8, p. 117, (1985) . 16. Kameyama, H., Yamashita, M., and Saito, Y., Catalyst 31: 285 (1989) .
380
Applications 4.6 Film Formation by the Gas Deposition Method (by Seiichiro Kashu and Eiji Fuchita)
The Concept for Gas Deposition The following idea originated from Director Hayashi at the beginning of the Ulta-Fine Particle Project regarding the handling of UFPs with diameters below 0.1 /lm. Because of the size of UFPs, they 1) will not fall due to the effects of gravity once they are dispersed in a gas, 2) will assume the velocity of a moving gas that they are mixed with, and 3) will probably show distinct behavior that is different than those of gases and UFPs when the mixtures are collided with objects at high speeds (due to the much greater mass of the UFPs compared to that of the gas molecules) [1]. These phenomena were known, but it was a unique concept to attempt to use these features in a process. The idea for using mixtures of UFPs in gases is shown schematically in Figure 1. The UFPs are mixed with a gas and ejected from an orifice at high speeds onto a hard surface, thereby depositing the UFPs as a film or small protrusions. The method of gas transport and spraying of UFPs, called gas deposition, was initiated in this fashion.
Confirmation of UFP Film Formation The gas deposition premise was confirmed by experimentation using slowly oxidized and freshly produced UFPs. For both materials, UFP films were formed by the method shown in Figure 1.
Film formation by slowly oxidized UFPs Metal and alloy UFPs are produced by the gas evaporation method in an inert atmosphere of argon or helium, but they are treated by the slow-oxidation process to prevent burning in air. This is
381
Ultra-Fine Particles
Transfer pipe Transfer pipe
<== Gas and UFP
<:== Gas and UFP
Nozzle "'-.. Nozzle --..........
UFP
Fil::;i:t:::===--=X==:::::l~ <==
Substrate Metal Glass Plastics Ceramics etc.
~UFF Cr"t.1
Figure 1. Schematic of the method for gas transport and spraying of metal and compound metal UFPs. a) Deposition ofUFPs in a film; and b) forming small protrusions
typically the way UFPs are supplied to users. Here, nickel UFPs (manufactured by Vacuum Metallurgical Co., Ltd.) having a specific surface area of 37.8 m 2/g and an average diameter of 20 nm were used. A diagram of the experimental apparatus used is shown in Figure 2. Nickel UFPs in the mixing chamber floated when a gas entered the chamber and were then carried out with the gas. The mixture was sprayed from a small nozzle onto a substrate kept at a small distance from the tip ofthe nozzle (mixing chamber pressurized to 10 kPa). When the substrate was moved as shown by the arrow, 382
Applications
......--- +--
CARRIER GAS
[~!]
COOLING OR HEATING
MIXING APPARATUS
Figure 2. Gas deposition method using slowly oxidized UFPs. a film having a width the same as that of the nozzle diameter was formed. Various substrate materials were used including glass, metals, plastics, and ceramics, but the effects of the substrate hardness or roughness were small or non-existent. The films that were formed were relatively firm. The films were formed at room temperature and required no heating of the substrate (as is needed in vacuum deposition). The above method is referred to as gas deposition [1]. In the method shown in Figure 2, the UFPs can be heated just before deposition by resistance heating of the nozzle. This heats the carrier gas, which in turn heats the UFPs. This process allows for modification of the properties of slowly oxidized UFPs, as will be discussed. 383
Ultra-Fine Particles Film formation using freshly formed UFPs The method shown in Figure 3 was used for making a film from freshly formed UFPs without exposure to the atmosphere (under the least contaminating conditions). This is a new handling method that allows for films to be formed in a simple way. In the synthesis chamber, the helium, argon, or hydrogen gas pressure is kept at 10 kPa, while the spraying chamber is evacuated (to 100 - 400 Pa) by a vacuum pump, as shown in Figure 3. Freshly generated UFPs are transported to the spraying chamber by the gas flow and deposited to form a film. UFPs produced by the gas evaporation method are the best suited for this gas deposition method because the gas used during the formation process can be used for transporting the UFPs.
1-3 x1Q3torr.cc/s ~
0-1torr " II.".----I-.nozzle size
--
~O-6x130
evaporation chamber
Figure 3. Gas deposition method using as-formed UFPs.
384
Applications
Floatation and Transport of Ultra-Fine Particles in a Gas It was confirmed that UFPs float and mix in a gas and that they can be transferred by the gas. Hayashi examined this behavior and speculated as follows: 1) When sub-micron UFPs float in a gas, the gravitational falling velocity is quite small. In a stationary gas, the UFP behavior can be described by fluid dynamics, in which it is assumed that the UFPs are giant molecules; and 2) The UFPs inside the moving gas rapidly reach a steady-state velocity (i.e., the velocity of the gas). Naruse [3] did calculations on these systems using a density for the UFPs in the gas of 8 g/cm3 and a particle diameter of 10 urn, see Table 1. The falling velocity is proportional to the density of the UFPs, thus the velocity is hardly affected by the type of UFPs and is about 1 cm/hr in the atmosphere for 10-urn diameter UFPs. Table 2 shows the time required for UFPs in a gas to attain a steady-state velocity. The results for the same UFPs indicate very short (practically negligible) times to reach a steady-state velocity. The velocity of the gas by which UFPs are usually transported is much faster than the falling velocity given in Table 1, meaning that UFPs can be mixed with a gas and transported freely. Table 1. The Steady-State Falling Velocities ofUFPs.
Gas Atmosphere Used
Atmospheric Pressure 1 atm (lOOkPa)
1 torr (133Pa)
He
2.6 x 10-4
2.0
Ne
1.2 x
Ar
0.8 x 10-4
to-4
X
10-1
0.9 x
to-I
0.6 X 10- 1
Calculated using a density of 8 g/cm 3 and a particle diameter of10 nm.
385
Ultra-Fine Particles Table 2. Time (sec) Required for UFPs in a Gas to Attain 95% of the Steady-State Velocity.
Gas Atmosphere Used
Atmospheric Pressure 1 atm (lOOkPa) 1 torr (133Pa)
He
2.5 x 10-6
1.8 X 10-3
Ne
1.1 x 10-6
0.8
X
10-3
Ar
0.8
10-6
0.5
X
10-3
X
Calculated using a density of 8 g/cm 3 and a particle diameter of 10 nm. Hayashi devised the following applications based on the above UFP behavior. 1. When the UFP transporting gas exhibits lamellar flow and the pressure is appropriate, the collisions among individual UFPs can be minimized (when the UFPs are relatively well dispersed). 2. Contamination of the UFPs can be avoided by using an inert gas (argon, helium, etc.) or a reducing gas (hydrogen). 3. The floatation of individual UFPs allows for the mixing of more than two types of UFPs in a gas. This leads to the deposition of uniformly mixed UFPs without effects due to differences in the densities. The following experiments were done on the basis of the above concepts.
Formation of UFP Films by Gas Deposition Films made of slowly oxidized UFPs can be produced by the method shown in Figure 2 and films of fresh UFPs by the method 386
Applications shown in Figure 3. Nozzles with circular cross sections were chiefly used (round nozzles were used because it is easy to get small tubes, such as commercial 0.1 - l.O-mm ID stainless steel tubes). The film width was about equal to the inner diameter of the tubing. The film thickness was determined by the amount of UFPs ejected from the nozzle (the mass ofUFPs transported by gas) and the sweeping rate across the substrate. Film thicknesses of several hundred nanometers (about ten layers of UFPs) to several hundred microns were produced. The cross-sectional geometry of the deposited films was measured using the arrangement shown in Figure 4. This is a conventional apparatus for surface roughness-morphology measurements. The contact needle was made of diamond with a 5~m radius of curvature and a load of 0.4 g. The cross-section of a deposited silver UFP film is shown in Figure 5a. The center is thicker because a circular nozzle was used. This film had a smooth surface with sub-micron order roughness. In comparison, Figure 5b shows a cross-section of a film made from a commercial silver paste that was cured at ambient temperature. Because the solvent was evaporated from the film during drying, the film had an irregular surface and, as a result of using micron sized silver powder particles, the roughness of the surface was on the order of microns.
Diamond
needle
~ lo"d
'----T-i----.....--
I~order
0·4g
scanning
position
Figure 4. Measurement of the cross-sectional morphology of the deposited films. 387
Ultra-Fine Particles
20"'01
A.
Ag
UFP
film
0.40101 ~
------------- -----B.
Ag
paste film
Figure 5. The cross sections of a) gas deposited silver UFP film; and b) film made from a commercial silver paste.
Using the apparatus to measure the surface morphology, the change in the cross-sectional morphology of the deposited film was examined by repeating the measurements with a O.4-g load. If it is assumed that a 311m diameter area is in contact with the O.4-g load, then the stress is 42 kg/mm2• Figure 6 shows the cross-sections for the 5th and 20th measurements, which show the wear after 15 measurements. This result shows that 1) a layer about 0.2 11m thick was removed from the surface (corresponding to two layers ofUFPs), and 2) the edge of the film is weaker than the center. Similar to a previous attempt to improve the adhesion of vacuum deposited films, the UFP film strength and adhesion to the substrate needs to be improved. Figure 7 shows a scanning electron micrograph of the crosssection and surface of a gas deposited silver UFP film. It is clear that the UFPs, which have an average diameter of about 0.1 11m, are densely packed. It can also be seen that the surface is very smooth. 388
Applications
5th measurements
measurements
Figure 6. Cross-sections obtained on the 5th and 20th measurements, which indicate the changes caused by repeated measurements.
Figure 8 shows the cross section and surface of a copper UFP film. Figure 9a shows an enlarged micrograph of the lower part of the film and Figure 9b shows a section from the middle of the film. No individual particles can be seen in the lower part of the film, indicating that the UFPs are fused together. In the middle section of the film, individual UFPs can be seen. These micrographs indicate that the structures are essentially similar to those ofvacuum deposited films. The gas deposition method is suitable for compacting UFPs, which cannot be consolidated by the conventional powder metallurgy techniques of filling a mold and pressurizing. In the conventional powder metallurgy techniques, UFPs tend to fill the gap in the mold and cause sticking. Compression is not enough to squeeze the air 389
AQ -UFP deposit
surface
i
Figure 7. Scanning electron micrograph of the cross-section and
surface of a gas deposited silver UFP film. 390
Cu - UFP deposit
surface
cross sectio
E
::J.
N N
1 um
Scanning electron micrograph of Cu UFP film by using a gas deposition.
Figure 8. Cross-section and surface of a copper UFP film.
391
/ ' surface
upper section
middle section
bottom section o
1000A
'\\ baseplate
Figure 9. a) Enlarged micrograph of the lower part of the sample shown in Figure 8; and b) micrograph of the middle section of the same sample. 392
Applications
from the gaps between the UFPs. The green density is also difficult to increase by the usual handling methods. Two reasons contribute to the success of compacting UFPs to a fairly high density by gas deposition: 1) the particle size is less than 0.1 IJ.m and 2) the ejection velocity (the velocity of the gas carrying the UFPs through the nozzle) exceeds several tens of meters per second (normally about 100 m/sec). The degree of contamination on the surface of the UFPs appears to also affect the compaction. Characteristics of Gas Deposited UFP Films The electrical resistance and shape of gas deposited nickel UFP films was measured as shown in Figures 2 and 3 and the specific resistivity was determined as well (see Figure 10) [4]. The measured values depend on the prior treatment and method of deposition. The film produced by using slowly oxidized nickel UFPs did not exhibit conductivity irrespective of the type of gas used (inert or reducing), apparently due to the surface oxide layer. When hydrogen gas was used as a carrier, the film began to show conductivity when the gas temperature exceeded about 80 o e. As the gas temperature was increased, the conductivity of the film increased. These nickel UFP films were characterized by the dependence of the resistivity on the preparation method and subsequent treatment. The temperature coefficient of the resistivity of UFP films is lower than that of bulk or vacuum deposited films (see Figure 11). Figure 12 shows the specific resistivity of conductive silver and copper UFP films. For silver UFP films formed using hydrogen gas as a carrier (evaporated in hydrogen gas and also transported by hydrogen gas), the resistivity of the film is on the same order as the bulk material (about 1 x 10-6 Q-cm). The resistivity data for a silver paste film cured at room temperature is also given. To obtain a resistivity comparable to the silver UFP film (about 1 x 10-5 Q-cm), the silver paste film must be baked at high temperatures. Because the gas deposition method can be used for the room temperature production of conductive films, this allows for the use of a wider 393
Ultra-Fine Particles
character of U·F·P·
carrier gas
surface conditioned (partial ) oxidation
Ar
No.
1 I--
2
(he~fing)
atmosphere at spray air Ar
3 r--- as grown
Ar air
4
0-11-
I
I
t
•• •
CX)
-5
-
-2-
-
0 '-3,o
-
-4-
-
-bulk - 5 1-------""""":' ........~ I I I (20°C)
-
.
q
-
I
I
I
No.1
I
No.2 No.3 NO.4 condition ( refer to the attached Table)
Figure 1O. Specific resistance of nickel UFP films. 394
Applications
carrier gas
character of UFP surface conditioned
0
(partial oxidation)
• A
atmosphere at spray
H2 (heating)
air
Ar
Ar
Ar
Ar
as grown (Ar.H2)
as grown (Ar.N2)
CII
U
c: ~l1lU
'iii
I
0
CII'-M
6-
c:
4-
--b 0 .... <11
:Q <11
ou
CII '-
I
I
I
I
I
-& bulk
-e film
8eJ8 8
2-
8
%
01-
-
0
:::J
~CII
a.
-
E -4f-
CII
l-
I
I
I
I
I
-5 -4 -3 -2 -1 0 1 2 Specific resistance of Ni-UFP deposit.
3
loge n·cm) ( temperature range: 20 - 50·C )
Figure 11. Temperature coefficients of the resistance of nickel UFP films. 395
Ultra-Fine Particles
-10- 2 E
I I
·
U
I
I
~
I
-
I I
· 8 I
I
~-1O-3 ell
----------
a.
ell
"0
0-
~10-4
-
wejght ratio JAg/binder = 5/3
ell
I I I I I I I I I I
§
I
8
I I
..... I I
8
0
ttl
.L.
Ag paste
0
U C
I
I I I
8
-5
iii 10
0 0
I
'iii
I I I I I
8
...: I
I I
I I
ell
I · -8- Ag bulk
~
-
I I
.~
I
I
'~10-6
I
I
-3- Cu bulk
I I I
-r I I
If)
Ar
He
H2
(Ag-UFP)
Ar
I
I
He
H2
(Cu-UFP)
Figure 12. Effect of gases on the specific resistance of silver and copper UFP films. range of substrates, such as glass and plastics, and eases the processing requirements. Copper UFP films have an order of magnitude higher resistivity than silver UFP films. The bulk resistivities of silver and copper are comparable, so this represents the higher activity of copper UFPs and the effects of the surface oxide layers on the UFPs. Gold UFP films have also been evaluated because gold is used for connections in integrated circuits. The gold films also have an order of magnitude higher resistivity than the bulk as was found for silver (Figure 13). This is within the limit for practical uses. 396
Applications
-
E ~ 10- 3.----~----.----r-----.
8 o
o 8 o
o o
-e bulk Ag
-&
Au
Cu
Figure 13. Specific resistance ofUFP films.
Figure 14 shows a pattern formed by using gold UFP films, where the nozzle (0.3-mm ID) was fixed and the substrate was moved in two dimensions. Figure 15 shows a similar pattern, but this was formed by using a rectangular nozzle (0.25 mm x 0.75 mm). Here one can obtain a line width of 0.20 mm with a pitch of 0.58 mm, which produces a cross section that has a mesa-like shape rather than an arc due to the circular nozzle as was shown in Figure Sa. Uniformly Mixed Binary UFP Films
The gas deposition method was used to deposit uniformly mixed binary films consisting of two types ofUFPs. Two gases that carry two different UFPs were mixed and the combination was ejected through a nozzle. Mixtures of two metals and a metal and
397
~
circular nozzle
Figure 14. Scanned pattern of gold UFP films. I
.... '"
0
s quare
nozzle
0·25 Figure 15. SEM micrograph of scanned pattern of gold UFP films.
398
Applications a ceramic (a composite material) were produced. For both materials, uniform mixing of submicron particles was achieved.
Mixed film of silver and iron UFPs The choice of silver and iron is based on the immiscibility of these two elements. Slowly oxidized UFPs of silver and iron were mixed in a predetermined proportion (selected amounts ofUFPs were ground in a mixing bowl for 1 hr) and formed into a disk, after which it was subjected to scanning x-ray microanalysis (XMA). The results shown in Figure 16 indicate good mixing if the analysis covered an area that was 100 ~m in diameter, but when a focused beam with a diameter of 3 ~m was used, the mixing was found to be far from complete. This was probably due to the coagulation of UFPs before the mixing process, which the mechanical mixing could not redisperse to a homogeneous mixture.
100,..------------------------,100 (spot size. ~100~m) ~ ssize. p~3 o m)t
90~@
90
80 80
70
70
~ 60 ~50
. _/\ v~
,r/'- ® V •
~
rv-J\,Jt ,t.•.I\J",.•
v·
40
M 1\.
30
I
.....
~
V
,J\f'r,
50 :: CII
I.L
40
_ A,tv,. 1jlfv '\J ~
1"'\
~VvV - ~
30 20
20
10
10
o
I
1 mm
I
I
Fe + Ag UFP mixing in mortar (1 hour) _
~q ~m
I
0
pressing at 100kg/cm2
Figure 16. X-ray microanalysis of a mixture of silver and iron UFPs in a mixing bowl (analyses using beam diameters of 3 and 100 ~m). 399
Ultra-Fine Particles The apparatus shown in Figure 17 was used to produce a mixed UFP film via gas deposition. The left side of Figure 18 shows the XMA results for a mixed film, which indicates that uniform mixing was attained at the submicron scale. The right side of Figure 18 shows the results from analysis of a binary vacuum deposited film with two concurrent sources (Figure 19). This method was ideal for making uniformly mixed films, but the results showed that the gas deposition method could provide comparable mixing in the films that were produced.
t ran s f e r
pipe
r.:=====:::::;::::;--;::===~;:::;l
(gas
.
(Ag+Fe)UFP
i
:::llFP
Ag-llFP
~ :::llFP I Fe-llFP
\t1 Fe
generator
generator
Figure 17. Experimental apparatus for the formation of a mixed film of silver and iron UFPs. 400
Applications
X-ray Micro Analysis
(20 KV ) 0·03jJA
r lOO 90 80
100 ~
~~
70
;;!
C
50
..
en
«
40 30
90 ..,,-~~
lO)Jm
60 ~
sampll' currl'nt
.~ 'iii
e "-
"0 III
-1rspot sizl' ~3)Jm
'01>-"
10 0
70
:6
i.. I
60 50
--
40
u..
e
u
~
30
~
.0
20
80
-=----~
Figure 18. X-ray microanalysis of a Ag-Fe UFP film.
subst.r<-J.tt.'~
shut,l.c"r
Figure 19. Co-evaporation of silver and iron. 401
20 10 0
;l!41
Ultra-Fine Particles Mixed films of nickel and TiN UFPs Nickel and TiN were selected as a representative combination of metal and ceramic UFPs because I) this system is used as a cement composite and 2) this system cannot be produced by the binary vacuum deposition method. Nickel and titanium UFPs were produced using the apparatus shown in Figure 17, but ammonia gas was fed into the titanium evaporation chamber. The titanium UFPs reacted with the ammonia gas to form TiN UFPs. The degree of mixing is indicated on the left side of Figure 20, which shows uniform mixing of nickel and TiN at the submicron level, similar to that found for the silver and iron system.
x- ray 100
Micro Analysis ( 20KV • 0·03J.lA) spot size: ¢3J.lm
90
90
.-,~
80
)?
~
Ti
70 60 .-..
~
z
40 30 20 10
80 70 60 .-.. -e
50~
-e
:::- 50
100
(TiN-Ni UFP d~~oSit) by gas depOSition
(TiN thin film) by P.V.D.
I-
40 .-..
~~~ ~~
0
Figure 20. X-ray microanalysis of a TiN-Ni UFP film.
402
-
30
30 ;;!
20
20
10
10
0
0
z
Applications In the physical vapor deposition method (PVD), TiN is formed by evaporating titanium atoms through the discharging space of the nitrogen gas. Thus, single-chamber, simultaneous evaporation cannot be done with this system, unlike the Ag-Fe system. The right side of Figure 20 shows the analysis data for a TiN film formed by activated ion plating. The gas deposition method can produce composite films that cannot be formed by vacuum deposition techniques. This is one of the advantages of the gas deposition of UFPs.
Mixed films of lead and zinc UFPs A feature of gas deposition is the absence of gravitational effects on UFPs floating in a gas. Submicron scale mixing of lead (p =11.3 g/cm3) and zinc (p = 7.1 g/cm3) was investigated (successfully accomplished in the zero-gravity environment of space) to confirm if mixing could be done using the gas deposition method. An x-ray image of the mixed film is shown in Figure 21, which was obtained using the zinc Ka and lead La lines. The white areas show the high concentration regions of each element. The results show a uniform mixture of lead and zinc. The compositional analysis is shown in Figure 22. This shows that a simple terrestrial apparatus can be used to reproduce the results obtained in zero-gravity processes. As demonstrated above, the gas deposition ofUFPs facilitates the macroscopic laminar fusion of metals and ceramics while the entire process is done at low temperatures. Hayashi [5] proposed that this process referred to as "Cryometallurgy."
Formation of UFP Films and Applications Several examples of the use of UFPs as constituents in films, other than the gas deposition method, are known. Abe and others at Matsushita used the apparatus shown in Figure 23. A UFP film was formed with as-generated Sn02 UFPs without air contact. These films were used as sensors [6]. The film was produced by condensing the evaporated metal vapor in a reactive gas and depositing the 403
Ultra-Fine Particles
404
Applications 100.----------.,.....,.....-------.100 90 90 PbL.c 80 80
70 sample current ----------:------160
70 ..... -.!!
50 ..... -.!!
~60 .0 0..
50 40
0
'-"
40 c: N
20kV - 0.03J.1A ) ( spot size: (J 3J.1m
30 20
30 20 10
Zn K.c
10
OL----------------l 0 I
o
I I
I
I
I
I
I
I
I
,
50 100 distance (J.lm)
Figure 22. X-ray microanalysis of a Pb-Zn UFP film.
hol.der
l=j~I~:::DI:::n~~~.I.:nd~uct.l.on dO
~
~
a:::
co~1
U1.I\.ur.1u~.
pUU1P
J.nductJ.on • • n_ra'tor
powvr K,onur.tor
Figure 23. Synthesis of UFPs and subsequent film formation via evaporation in a reactive gase [6].
405
Ultra-Fine Particles
particles on a substrate between electrodes while controlling the particle diameter, size distribution, and composition. Using oxygen as a reactive gas (70 Pa), SnOz UFP films with a high sensitivity to isobutane gas were produced. At an oxygen pressure of 7 Pa, the SnOz films were sensitive to water vapor and had different characteristics from sensors made with conventional thin and thick films or sintered bodies. Oxidized tin UFPs were used by Kosaka's group at the University of Osaka Prefecture to produce gas sensitive films [7]. A gas reaction method was employed to make the UFPs. Tin was evaporated from an electrically heated boat in a furnace and transported by a nitrogen carrier gas and combined with oxygen gas downstream to produce an oxidized tin vapor. Immediately afterward, the vapor was quenched, causing supersaturation, which resulted in the generation of tin oxide UFPs that were introduced into a film forming chamber through the top (see Figure 24). The UFPs were accelerated to a velocity of about 200 m/sec by the time they passed through the nozzle and collided with a substrate placed normal to the gas flow. This resulted in the formation of a UFP film. By measuring the voltage-current characteristics of the films, the effects of the atmospheric gas and the temperature on the film properties were studied. The films responded selectively to gases and they have the potential to be used as gas sensors. Similarly, Iwama of Daido Institute of Technology formed UFP films and evaluated the films by using electrical resistivity measurements [8]. This method is shown in Figure 25. The raw materials were evaporated in a basket shaped tungsten filament in a vacuum evaporator. Films formed on a glass substrate were studied using the initial values of the electrical resistance and their subsequent aging characteristics. These UFP films formed without exposure to air can be expected to see wider applications in such areas as gas sensors and other devices. Gas deposition as well as other techniques will find use in the preparation of these films.
406
Applications ultra-fine
particles
~
glass
nozzle (;O.8mm)
=>
pump
Figure 24. Film formation chamber for using tin oxide UFPs [7].
cool.ing
IJb
er
I
\
substrate
~ pump
gus
l.nleL
power
genct:+a L o r
Figure 25. Experimental apparatus for formation ofUFP films [8].
407
Ultra-Fine Particles
Prospects for the Gas Deposition Method
The gas deposition method has been established as an effective method for handling UFPs. It can be used in 1) replacing, at least partially, vacuum evaporation and screen printing and 2) for developing new materials. The former can take advantage of the features of UFP films such as 1) high rates of film formation in a limited region, 2) room temperature operation (no substrate heating), 3) freedom to select the composition, and 4) wide range of film thicknesses (several tens of microns to several hundred microns). In terms of the development of new materials, the gas deposition method is little affected by gravity and differences in the densities of the elements mixed for process pressures from atmospheric pressure down to about 130 Pa. Consequently, one can easily simulate the environment of space with a simple apparatus (e.g., those shown in Figures 2,3, and 17). There have been attempts to make new alloys in space, but the gas deposition experiments can provide preliminary results for such research. According to Hayashi, the films that are produced by gas deposition can also be used as a means of quality control in the production of UFPs on a large scale. By confirming that the properties of the films are within acceptable limits, one can assure that the characteristics of the UFPs being produced meet specifications. It has been recognized that the handling ofUFPs is critical in the development of applications using UFPs. Different handling techniques are being developed, but the gas deposition method takes good advantage of the characteristic features of UFPs. It is hoped that this method will find a wide range of uses in basic studies and in industrial applications [9-14]. References 1. Hayashi, C., Oyo Butsuri 54: 687 (1985).
2. Kashu, S., ERATO Hayashi Ultra-Fine Particle Project 1984 Research Report, p. 11 (1984). 408
Applications 3. Hayashi, C., Aerosol Research 1: 23 (1986). 4. Kashu, S., Fuchita, E., Manabe, T., and Hayashi, C., Jpn J Appl. Phys. 23: L900 (1984). 5. Hayashi, C., Kagaku Sosetsu No. 48, p. 1 (1985). 6. Abe, A. and Hayakawa, S., Elect. Materials 19: 79 (1980). 7. Adachi, M., Okuyama, K., Kosaka, Y., and Tanaka, K., Aerosol Research 1: 123 (1986). 8. Iwama, S. and Hayakawa, K., Jpn J Appl. Phys. 20: 335 (1981). 9. Hayashi, C., Kashu, S., Oda, M., and Naruse, F., Materials Science and Engineering, A163: 157 (1993). 10. Oda, M., Katsu, 1., Tsuneizumi, M., Fuchita, E., Kashu, S., and Hayashi, c., Mat. Res. Soc. Symp. Proc., vol. 286: 121 (1993). 11. Fuchita, E., Setoguchi, K., Katsu, 1., Mizutani, R., and Oda, M., Proceedings of the 8th International Microelectronics Conference (lMC 94), Omiya, Japan, April 20-22, 46 (1994). 12. Fuchita, E., Oda, M., and Hayashi, c., Materia Japan, 34(4): 455 (1995). 13. Kashu, S., Matsuzaki, Y., Kaito, M., Toyokawa, M., Hatanaka, K., and Hayashi, C., Proc. 2nd Internat. Symp. on Superconductivity (lSS-89) 413 (1989). 14. Kashu, S., Kaito, M., Hatanaka, K., and Hayashi, C., Proc. 3rd Internat. Symp. on Superconductivity (lSS-90) 643 (1990).
409
Ultra-Fine Particles 4.7 Surface Processing Using Solidified CO 2 Ultra-Fine Particles (by Takeshi Manabe and Seiichiro Kashu)
Inorganic UFPs of metals and ceramics have been studied for some time in regard to their synthesis and applications. The Hayashi Ultra-Fine Particle Project has also examined the formation and properties of organic UFPs. Another area that has also been explored by the Hayashi project is the synthesis and applications of solidified gas UFPs [1]. This work was pursued by Manabe of Teisan [2]. The formation of CO 2 UFPs and their subsequent use as noncontaminating and selective abrasives for the removal of photoresist films on silicon wafers was explored. A key advantage to the use of CO 2 UFPs as a photoresist stripper is that when the UFPs are removed by evaporation, they leave no residue on the substrate. This is a new processing technique and the first application of such solidified gas particles. Synthesis of Microscopic CO2 Particles The use of solid CO 2 pellets that were obtained by crushing dry ice has been explored. No prior examples have been found, however, for the use of microscopic CO2 particles. Synthesis of CO 2 UFPs by gas evaporation When evaporated metal vapor is quenched, the coalescence of vaporized atoms occurs and UFPs are formed. Using the same principle, CO 2 UFPs may be formed by cooling CO 2 gas at room temperature. Figure 1 shows a diagram of the method. Here, CO 2 gas and a nitrogen carrier gas at ambient temperature were mixed and fed into a double-wall cooling chamber with the inside wall cooled by liquid nitrogen. The CO2 gas was then quenched and CO 2 UFPs were produced. Representative synthesis conditions are as follows.
410
Applications
f---~~ vacuum
.;;;c:::==r.....-..l----.=> ,,, ,, ,,
'' ''
'''
\! ,' "" "" " " ""
:: ::
i
L--_4E-.:....
C02
Figure 1. Synthesis of CO2 UFPs by gas evaporation. Flow rate ofN2 carrier gas: Flow rate of CO2 gas: Pressure in the cooling chamber:
600-800 ml/min about 10 mllmin about 1.3 kPa (about 10 torr)
Because a white smoke is continuously formed, one can directly see the formation of CO2 UFPs. Using this method, however, no means could be found to increase the speed of the UFPs after they were produced. The next effort was to find a method to accelerate the CO2 UFPs so that they could be used for abrasive processing of surfaces.
CO2 UFP synthesis by free expansion According to the phase diagram for CO2, the triple point is at 0.52 MPa (52 kg/cm2) and -56.6°C. Liquid does not exist below this temperature and pressure. Under atmospheric pressure, CO2 does not melt, but vaporizes directly from the solid, hence the name "dry ice." When a vessel is filled with CO2 at a high pressure of 50 - 60 kg/cm2 , the CO2 separates into a gas phase and a liquid phase. Therefore, unlike a vessel filled with nitrogen or hydrogen, as long as the temperature is held constant, the pressure will be constant if liquid phase is present. 411
Ultra-Fine Particles When liquefied CO2 in a container is allowed to expand freely through a needle valve to atmospheric pressure, a steady state flow of solid and gas results. This change of state can be understood from the Moriel diagram shown in Figure 2. When CO2 liquid that is under a pressure of 6 MPa at room temperature expands while passing through a needle valve, CO2 UFPs are formed along with vaporized CO2 gas that becomes a carrier jet for the particles as they pass through the nozzle.
Selective Stripping of Resist Films A patterning method for integrated circuit production uses photoresist films on silicon wafers. Here the photoresists are selectively exposed to light and developed. The undeveloped part of the photoresists is then removed by exposing the films to an oxygen plasma or by immersion in a chemical solution. The undeveloped part of resist films was selectively stripped by projecting a jet of CO 2 UFPs onto the films. As shown in Figure 3, a sample was formed by sputter coating a 100-nm thick layer of chromium on a glass substrate and then spin-coating a 500-nm thick photoresist-film (AZ-1350) on the chromium film. The wafer was spun at a high speed and a drop of the resist was dropped on the center. The centrifugal forces spread the resist uniformly over the surface of the wafer. This sample was set on a sample table that can be moved in one direction. The sample was 4 mm away from the nozzle, which was a tube (I-mm ID and 40-mm length) connected to a needle valve, see Figure 4. For nozzle-sample spacings of less than 4 mm, accumulations of CO2 UFPs were formed on the surface, while larger spacings reduced the stripping ability because the UFPs evaporated, at least in part, before they impacted on the surface. The sample processing chamber was at ambient temperature and pressure, but it was back-filled with nitrogen, which prevents the condensation of water vapor from the air. To strip AZ-1350 resists, CO2 was sprayed at 20 mllmin and then the sample was moved under the nozzle at a rate of 15 mm/min. The results for the removal of the 412
120
N
5
-.... 10 Ol ~
~ J
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)
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)
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w
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V
)
""
/
SOUD ~GAS
/ V
/
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LIQUID
+GAS
,
r\ \
GAS
.~ oJ
20
40 60 80 100 120 140 160 180 .L keal/kg
~ ~
-.... ~
l::l
Figure 2. Moriel diagram of CO2•
::to c::l
a
Ultra-Fine Particles
r---------,~ RESIST 1-----------1
<
CHROMIUM
I......----------'~
GLASS
Figure 3. Structure of a hard mask.
FILTER
flow meter nee le rooter vaL e -r'~--+------l
ta I
I
materi~
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U
Figure 4. Solidified CO 2 jet apparatus. 414
resist are shown in Figure 5. The widths of the stripped regions and the nozzle diameter were about equal and the edges are relatively sharp. No damage to the chromium layer was found even after the resist had been removed. Removal of Plastics A jet of CO2 UFPs was used to remove a plastic similar to the
resist films. Material removal was successful for films of acrylics, polycarbonates, and hard polycWorinated vinyls. Figure 6 shows the
removal for an acrylic board, which proceeded as shown in Figure 7. The removal of plastic materials demonstrates the capability of the CO, UFP jets.
...
500nm
..
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Figure 5. Morphology ofa photo-resist film treated with ajet of CO, UFPs.
415
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o
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....0
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416
Applications
I
O.4mm , ,
Figure 7. Morphology of an acrylic sample treated with a jet of CO2 UFPs.
Applications of Solid Gas Ultra-Fine Particles Selectively removing resist films on a hard mask has been successful using a solid CO2 UFP jet made by free expansion of high pressure CO2 through a nozzle. This process did not cause damage to the substrate. The removal of plastic materials using CO 2 UFPs was also demonstrated. Because the CO2 stripper vaporizes after use, no residue is left, so clean samples are obtained. As a stripper, other solidified gas UFPs can also be used, if they are of submicron sizes and have a suitable hardness. For example, H20 UFPs can also be used. Further development of solidified gas UFPs is expected.
417
Ultra-Fine Particles
References 1. Hayashi, C., Aerosol Research 1: 23-29 (1986). 2. ERATO, Hayashi Ultra-Fine Particle Project Research Report, p. 11-15 (1984).
418
5 PROSPECTS FOR THE FUTURE OF ULTRA-FINE PARTICLES
(by Chikara Hayashi)
Development of Applications
1) The Ultra-Fine Particle Project consisted of four groups, each of which had three to six researchers. The Basic Materials Property Group headed by R. Uyeda and S. Iijima employed a specially designed electron microscope. Using this microscope, the atomic arrangements of several types of metal and ceramic UFP samples were observed and recorded at a rate of one frame every 1/60 sec on a VTR. This allowed for the direct examination of UFPs, sintering processes, and catalytic and other reactions with gaseous atoms, which involve interfacial phenomena. These phenomena were observed as electronic images along a direction allowing clear visualization of the interfaces. The development of this technique is believed to be a significant contribution to the science of interfacial phenomenon. With further improvements in the spatial and temporal resolution and image analysis capability, along with other analysis methods, many scientifically important results should be obtained. Iijima demonstrated the basic technology for the study of interfacial phenomenon. This approach should develop further, leading to new scientific theories and industrial technologies. 2) The gas deposition method demonstrated by Kashu and others is one application ofUFPs that meets the basic requirement for becoming widely used, which is simplicity. Independent of whether the UFPs are metallic, ceramic, organic, frozen gas, or combinations thereof, films can be obtained with good yields in various shape, such as lines, points, or films. This method is similar to conventional 419
Ultra-Fine Particles spraying, but the results are significantly different. At the UFP level, a qualitative difference appears that is unlike what is expected from simple extension of conventional spraying techniques. With the gas deposition method, UFPs of high temperature sintering oxides can be compacted at room temperature. When rapid heating with focused heat sources (e.g., laser, focused xenon discharge radiation, etc.) is used simultaneously, continuous multi-layer film formation can be accomplished. In contrast, one can etch a surface by using ice UFPs. This is a promising process for the creation of new composite materials. Such materials may require the development of special UFPs with well defined compositions, sizes, and shapes. Thus, various gas deposition techniques must be further developed. Kashu also confirmed that the properties of UFPs are reflected in the deposited films, which can serve as a quality control tool for UFP production. 3) The gas evaporation method can also be used to make UFPs that consist of large organic molecules that are of pharmaceutical importance, as demonstrated by Toyotama: These UFPs are soluble in water, implying a huge potential for their use in making medicines. Here, we also need to understand the science of the water solubility of such UFPs. Reactions with microbes may be another important theme to study in this area. 4) The formation of coatings of organic high polymers around magnetic UFPs and the control of these particles using externally applied heterogeneous magnetic fields will undoubtedly impact both biological research and biotechnology development. The basic study in this area was done by Kakuta. 5) The development of a technique to uniformly deposit UFPs on a substrate may be important in the field of electronic materials technology. Umemura showed a possible method to produce uniform arrays. Other methods can be developed for integrated circuit If stable manufacturing, as well as other specialty needs. arrangements can be obtained, these may be used to make transmitting and receiving components, recording media, etc. Parallel developments in suitable UFP synthesis techniques, however, are still needed. 420
Prospects for the Future In the above, we have reviewed developments from the UltraFine Particle Project. In addition, many comments have been made by others regarding the actual use of UFPs for magnetic recording media, uniform or dispersed ceramic or glassy sintered bodies, refrigeration heat exchangers, and other areas. It has been pointed out that it is important to accomplish the following. 1. Make inexpensive, but usable quality ultra-fine powders (aggregated bodies ofUFPs). 2. Clarify the properties of ultra fine powders. 3. Develop handling methods for ultra fine powders. Of the three points mentioned above, it is probably not necessary to comment further on 1 and 3, but it is necessary to consider 2 in more detail. Materials science since the 19th century has pursued the properties of materials independent of the size and shape. Material properties were presumed to be size or shape independent, but is this true of the properties of ultra-fine powders? For example, if an array of UFPs has a special property different from the bulk material, are the properties associated with that of the material absolute? Are they a result of the environment? Or are they a matter of how the properties are determined? We often refer to the features ofUFPs or ultra-fine powders, but not to the properties ofthe materials because we cannot necessarily define these properties. This uncertainty, however, does not diminish the importance ofUFPs as new materials nor the need to clarify their behavior. It is obvious that inexpensive production of UFPs is important. However, the experience with UFPs for magnetic recording media indicates that once the utility of UFPs is demonstrated using an available technique (e.g., with evaporated UFPs), many other techniques will be developed using chemical engineering processes. This ultimately results in economical production methods. Once the goal is set, it becomes a competitive challenge. Thus, it is just as important to expand the horizon using 421
Ultra-Fine Particles
well characterized synthesis methods, such as gas evaporation, and to determine the heretofore unknown properties of UFPs. Problems Unresolved by the Ultra-Fine Particle Project
I) Twenty years prior to the start of the Ultra-Fine Particle Project, the Kubo theory was introduced. This was followed by many other studies in the fields of physics and chemistry. In the Ultra-Fine Particle Project, the electronic and optical properties of UFPs were not pursued, because these require UFPs with more uniformly controlled sizes, shapes, surfaces, structures, and compositions. Furthermore, such aspects cannot adequately be pursued within the five-year period of such a project. It was anticipated that twodimensional superlattices must be fabricated and analyzed. The recent discovery of high temperature superconducting oxides has made a strong impact on our society. The high temperature ceramic UFPs have superior weatherability and heat resistance compared to metallic or organic UFPs. Because practical superconductors require thin films, multi-layers, or fine multi-conductor structures, it is likely that such applications will require UFP starting materials, unlike the metallic superconductors. 2) The dispersion of organic high polymer UFPs into water, as discussed earlier, is an interesting aspect that requires the development of a theoretical basis of understanding. The biological UFP applications group started studying the interaction between macrophages and metal or oxide UFPs and discovered many new phenomena. The group began this work in uncharted waters and the five-year period ended before many more discoveries could be made. Scientifically, the combination of molecular level and cellular level experiments should be pursued, while, industrially, biological and electronic technologies should benefit from new developments in UFP applications. 3) The size ofUFPs is between that of atoms and microscopic matter. UFPs exhibit properties that can be defined as metals, ceramics, plastics, or living matter. As mentioned earlier, the concept that the properties of a material are independent of its size has been 422
Prospects for the Future a central theme in materials science. How the special behavior of UFPs should be treated has been a problem that was faced from the start of this project, and one that remains unresolved. The mean free path for gases based on molecular theory or for electrons in a solid dictates a macroscopic property that is in relation to the size of the matter. It is yet unclear what aspects best characterize UFP properties. Can this be related to the average spacing between UFPs? This area leaves many questions unanswered. 4) In the Ultra-Fine Particle Project, we only lightly dealt with the topic of clusters. An increasing number of researchers are examining this domain for clusters under 1 nm in size. 5) Through the work done by Uyeda and Iijima, the Ultra-Fine Particle project established the basic techniques for the direct observation ofthe atomic arrangements ofUFPs. However, another key physio-chemical analysis method, namely, analysis of energy transfer, was not developed. UFPs separated in a vacuum and in flight were supplied with radiative energy from the surroundings in an experiment by Okada (Hitachi) after a suggestion by Uyeda. The results were inconclusive. Basic techniques for observing energy transfer need to be developed for the next breakthrough.
Problems with the Environment and the System The Exploratory Research for Advanced Technology program (ERATO) pursues new and creative technologies in areas associated with matter and life, with the aim of spawning new industries. These efforts rely on project teams that involve people in industry, academe, and government, and which cross all national boundaries. These projects require the formation of bridges between science and technology and are not aimed at mission-oriented research (development). These are the explanations given for the ERATO program by the sponsoring organization, the Research Development Corporation of Japan. After the first five years of operation of the ERATO program, four projects were completed with much international interest regarding the system itself. New knowledge that can develop into new industries is expected to result, and the 423
Ultra-Fine Particles expectations regarding the system itself are quite high. The next challenge, indeed, is to actually develop the new industries. It can be said that the goal of industry is to pursue missionoriented research, while that of academic fields is to create and cultivate symposia and specialty sessions within symposia. These roles can be ultimately extended to the international scene and on to new industries. Such a process would be an ideal administration of science and technology, but whether that goal is reached is outside the scope of ERATO. If the results of ERATO were to be evaluated by monetary gains, mission-oriented research must be done, but this would diminish the general worth of the creative science and technology that is being done. Such problems are common in governmental endeavors on research and development. In non-governmental organizations, there is usually a final decision maker, but government efforts lack such central control. In the ERATO system, one advantage is that a researcher returning to a corporate laboratory can initiate new developments based on ideas that arose during the project. However, the continuing pursuit of pure scientific discovery has no organizational support beyond ERATO. This aspect requires further consideration. Finally, it should be mentioned that the UFP Forum benefited from support from the ERATO project. The Ultra-Fine Particle Project had as one of its goals the creation of new technologies and even new industries. That is, UFPs can be used in a variety of ways, and the project had an obligation to promote new uses ofUFPs and their applications. Consequently, public dissemination of information and many seminars have been done. These efforts have resulted in an increase in the number of researchers in the UFP field in Japan by ten-fold, from 50 to 500. The corresponding population of researchers in the UFP field at the international level seems stagnant and further efforts are needed. In Japan, several UFP symposia were held by various scientific and technical societies. Technical journals have also had special issues on UFPs. These efforts have been effective in
424
Prospects for the Future spreading the information on UFPs to a broad range of researchers. Such efforts need to be continued for at least the next five years. General References 1. Hayashi, C., Physics Today, December (1987). 2. Hayashi, c., J. Vac. Sci. Techno!., A5(4) JuI/Aug, 1375-84 (1987). 3. Hayashi, C., Proc. Tenth Internatl. Conf. on Vac. Metallurgy, 1-23, Metallurgical Industry Press, Beijing, China. 4. Hayashi, C., Materials Science and Engineering, A163, 157-161 (1993). 5. Morinaga, H., Futatsuki, T., Ohmi, T., et aI., J. Electrochem. Soc., 142(3), March (1995).
425
Appendix
BACKGROUND ON THE ERATO PROGRAM (by Genya Chiba)
The Exploratory Research for Advanced Technology program (ERATO) was initiated in 1981 by the Science and Technology Agency of the Government of Japan. It is administered by the Research Development Corporation ofJapan (JRDC), a public research corporation owned and operated by the Japanese government. The first four projects that were initiated in 1981 came to successful conclusions in 1986. Eleven projects were started in the second phase. The entire program was recognized for its numerous accomplishments. Using the first five years ofexperience as a foundation, efforts were made to develop this program further. To judge the projects at this stage may not be appropriate because most are basic research programs. Before establishment of the program, there were some questions concerning the basic concept of setting up, under various limitations, research projects that were focused on human resources. The projects had no permanent laboratories and space was leased from various institutions. This required the cooperation of industry and academia. This somewhat revolutionary concept is working well, though certain societal changes may be necessary to recognize thevalue of individual creativity and to develop the mechanisms for discovering hidden talents.
427
Ultra-Fine Particles This program, which began with four projects that included the tntra-Fine Particle Project, was born in recognition ofthe fact that Japan needed to pioneer innovative, basic research. Basic research has no model and, as with artistic endeavors, talent, effort and luck are important factors. Each project director sought out young scientists from both Japan and abroad. As a group, they engaged in research that would possibly define a new direction in science and technology.
ERATO PROGRAM FRAMEWORK The purpose of the ERATO program was to foster the creation of advanced technologies and to advance future interdisciplinary scientific activities, while searching for a better system for doing basic research. Recognizing the need for individual talents and flexible research administration to foster creative research, a new organization was created and placed into practice. There has been some question regarding the inventiveness of Japanese. However, this program presumed that Japanese ingenuity exists and focused on eliminating the obstacles to free, creative activities. The program was organized as follows. People-Based Organization Japanese research has traditionally been operated with an organizational-based focus. The program was designed to provide a system that centered around the project director and a group ofyoung researchers so that individuals in the group could reach their full potentials. Team work among the individuals was also essential. Avoiding Conformative Thinking and Establishing an Open System Research activities tend to emerge from mutual interactions between various thoughts and values. The ERATO program was thus organized to attract researchers from universities, government laboratories, and industry, both from Japan and abroad. Each group 428
Appendix is heterogeneous, and is required to open their doors to overseas scientists.
Flexibility The objective of the ERATO program was to create new concepts, so the program set no concrete goals or methods other than the basic guidelines for the projects. New knowledge and concepts are often born by chance in the process of working in a set direction. The projects needed the flexibility to actively pursue knowledge and concepts. At the same time, the groups needed the flexibility to change plans, decrease their scale, or terminate weak projects.
Mobility Advances in science and technology are accelerating and becoming more dynamic. Research organizations in Japan are based on permanent employment and are not suited to deal with rapid changes, especially in innovative technological research. Thus, a new system should avoid setting up fixed facilities and should limit the duration of research. These approaches are not new and they adopt the rotating research staffsystem proposed some years ago. It is worth noting that the ERATO program was initiated in the 1980s as Japan's increasing contributions to the international community have been called for. The program had initial start-up problems, but functioned well during the first phase.
RESEARCH PROJECTS Each research project was under the complete control of its project director. The first four projects, started in 1981, dealt with materials comprised offinite numbers of atoms and molecules. Three additional projects were started between 1982 and 1984 and dealt with biological fields in terms ofinformation science and attempted to
429
Ultra-Fine Particles define a new direction in medical and pharmacological areas. Two projects started in 1985 explored the physical and chemical aspects of the nanometer world through measurement and processing techniques. Three projects started in 1986 dealt with the information and life sciences based on the basic units of photons and molecules. Three projects started in 1987 explored circuit elements and materials based on molecular level techniques. These projects are listed in Table 1 (Note: The ERATO Program still operates today). Table 1. ERATO Projects.
PROJECT TITLE
DIRECTOR
RESEARCH FOCUS
Hayashi Ultra-Fine Particle Project 1981-1986
Dr. Chikara Hayashi, Chairman, ULVAC Corp.
Metal, oxide and organic particles 100A in diameter
Masumoto Amorphous & Intercalation Compounds Project 1981-1986
Dr. Tsuyoshi Masumoto, Director, The Research Institute for Iron, Steel and Other Materials, Tohoku University
Amorphous non-metals and intercalation compounds
Ogata Fine Polymer Project 1981-1986
Dr. Naoya Ogata, Professor, Faculty of Science & Technology, Sophia University
Condensation type function polymers
Nishizawa Perfect Crystal Project 1981-1986
Dr. Jun-ichi Nishizawa, President, Tohoku University
Static induction transistors and thyristors
Mizuno Bioholonics Project 1982-1987
Dr. Den'ichi Mizuno, Professor, Faculty of Pharmaceutical Science, Teikyo University
Self organization of living bodies
Hayaishi Bioinformation Transfer Project 1983-1988
Dr. Osamu Hayaishi, Director, Osaka Bioscience Institute
Information transfer of prostaglandins
430
Appendix Horikoshi Superbugs Project 1984-1989
Dr. Koki Horikoshi, Professor, Dept. of Bioscience and Biotechnology, Tokyo Institute of Technology and Chief Scientists, RIKEN (Institute of Physical & Chemical Research)
Microorganisms that live under conditions of extreme temperature and pressure
Yoshida NanoMechanism Project 1985-1990
Mr. Shoichiro Yoshida, Managing Director, NIKON Corp.
Mechanical factors at the nanometer level
Kuroda Solid Surface Project 1985- I990
Dr. Haruo Kuroda, Professor, Facully of Sciencc, University of Tokyo
Chemistry of solid surfaccs
Goto Quantum Magneto Flux Logic Project 1986-1991
Dr. Eiichi Goto, Professor, Dept. Of Science, Kanagawa University
Logic clements using quantum magneto flux
Hotani Molecular Dynamic Assembly Project 1986-1991
Dr. Hirokazu Hotani, Professor, Faculty of Science & Tech., Teikyo University
Sclf control of proteins and lipids
Inaba Biophoton Project 1986- I991
Dr. Humio Inaba, Director, Research Institute of Electrical Communication, Tohoku University
Information derived from the light emitted from cells
Nishizawa Terahertz Project 1987- I992
Dr. lun-ichi Nishizawa, President, Tohoku University
Electronic elements using thc intermediate region between light waves and electrical waves
Furusawa MorphoGenes Project 1987-1992
Dr. Mitsuru Furusawa, Board Director, Manager of Molecular Biology Research Lab., Daiichi Pharmaceutical Co., Ltd.
Revelation of the role of genetic information involved in gastrulation processes
Kunitake Molecular Architecture Project 1987-1992
Dr. Toyoki Kunitake, Professor, Faculty of Engineering, Kyushu University
Organic synthesis via self organization
PRESENT STATUS The ERATO program has been functioning well. Japanese society apparently has sufficient flexibility to absorb the program, due
431
Ultra-Fine Particles in part to the tireless efforts ofthe project directors and research staff This program has been evaluated widely by groups in advanced countries and has received visits from over 200 groups that have found the program to be unique and dynamic. From the date of its inception until 1987, there were 15 research projects with 15 directors (11 from universities, one from a government corporation and three from industry), 53 research groups, and over 300 research and other staff members. The average age ofthe research staffwas 32, with 60% coming from industry and 30% as postdoctoral fellows. This includes many foreign staff and Japanese returnees from overseas. The remaining 10% came from universities, national laboratories, and research institutes (See Table 2). Organizations that have sent their staff to these projects exceeded 100 by 1987 and included foreign corporations. Table 2. Organizations and Countries of Origin of ERATO Researchers. COMPANIES Advantest Dai Nippin Printing Fuji Film Furujawa Electric Hitachi Japan Aviation El. Japan Synthetic Rubber Kaneka KyowaHakko Matsushita Res. lnst. Mitsubishi Gas Chern. Mitsubishi Petroleum Mitsui Petroleum Nihon Shokuhin Kako Nippon Shoji Oji Paper Osaka Cement Rigaku Electric Seiko Electronics Shin-Nippon Musen
Akashi Eisai Fujisawa Pharmaceutical Godo Shusei Hitachi Chemical Japan Metals & Chern. Jasco Kao Maruzen Petrochem. Meiji Seika Mitsubishi Kasei Mitsui Eng. & Shipbld. Mitsui Toatsu Nikon Nippon Shinyaku Oki Electric Otsuka Chemical Riken Shimadzu Vacuum Metallurgical
432
RE. Marubishi Fuji Chemical Fujitsu Hamamatsu Photonics ldemitsu Kosan Japan Mining JGC Kumiai Chemical Matsushita Electric Mitsubishi Electric Mitsubishi Metals Mitsui Kinzoku Morinaga Milk Nippondenso Nippon Steel Olympus Q.P. Sanyo Electric Shin-Etsu Handotai Shionogi
Appendix Shiseido Sumitomo Chemical Teisan Titan Kogyo Toppan Printing ToyoInk Ueno Fine Chern. Ind. Zeria Pharmaceutical
Sony Sumitomo Pharrn. Terumo TodoKogyo Toray Research Toyo Jozo ULVAC
Stanley Electric Suntory The Nippon Syn. Chemical Tohoku Metal Toshiba Electric Toyota Central Lab. Yaskawa Electric
TOTAL: 83 companies, 144 researchers
UNIVERSITIES Gakushuin University Keio University Nagoya University Saitama University Tohoku University Tokyo Inst. of Tech.
Hiroshima University Kyoto University Nihon University Takasaki City University Of Economics University of Tokyo Tokyo Medical & Dental University
Hokkaido University Meijo University Osaka University Teikyo University Toho University Toyama Medical & Pharm. University
Waseda University ** FOREIGN UNIVERSITIES** Brandeis University University of Arizona University of Manitoba
Cornell University University of Hawaii Yale University
Swiss Inst. Tech. University of Illinois
TOTAL: 27 universities, 63 researchers ** UNIVERSITIES HOLDING DUAL POSTS** Gakushuin University Teikyo University
Osaka Medical Univ.
TOTAL: 4 universities, 5 researchers
433
Sophia University
Ultra-Fine Particles COUNTRIES Australia East Germany India South Korea The Netherlands West Germany
Canada France Italy Sweden United Kingdom
China Hungary New Zealand Taiwan United States
TOTAL: 16 countries, 26 researchers
NATIONAL LABORATORIES National Chemical Laboratory of Industry National Research Institute for Metals TOTAL: 2 national laboratories, 4 researchers OTHER INSTITUTES Semiconductor Research Institute The Research Institute for Electric and Magnetic Materials TOTAL: 2 institutes, 4 researchers
Research laboratories are leased from various organizations distributed throughout Japan (e.g., companies, universities, research parks, etc.). While it is true that such a system does create some inconvenience for project directors and staff, it does allow for the build-up of better facilities, reduction in operating costs, and minimization of other problems, such as family relocation. The program was operated with fixed project periods of five years, with a fixed budget of 1.5 to 2 billion yen, and within the confines of a government corporation. Nonetheless, these constraints did not prove to be an obstacle to the operation of the program. It can be viewed that the conscience of a society in rapid fluctuation has made the management of this program possible. 434
Appendix ACCOMPLISHMENTS
Research accomplishments ofbasic research include scientific knowledge and the creation of seeds for new technologies. The discovery of the amoebae-like movement of 3-nm diameter gold particles in the Ultra-Fine Particle Project is science in itself and it may directly contribute to semiconductor and catalysis technologies. Some results may lead to new areas in science and technology and a few have already been developed into technological endeavors. Thus far, over 50 major findings have been reported. A symposium is held each year to aid in the dissemination of results. Over 850 presentations were given at national and international meetings during the first five years. During that time, over 400 patent applications were submitted. A partial list of accomplishments can be found in Table 3. FUTURE DEVELOPMENT
The results of basic research will eventually be evaluated by society. Some scientific results will be pursued further in academia, while some will be developed further through applied research and industrial development. From the corporate viewpoint, the research results must contain the potential for further development. In turn, the real potential must first be recognized. The evaluation of results from ERATO projects is still an ongoing affair. Some of the researchers will continue their work upon returning to their home organizations. An unexpected aspect of this program has also resulted. This is the aspect of providing a training ground for young researchers in an interdisciplinary environment. Because a variety of researchers are gathered to make the groups heterogeneous, young researchers are able to encounter different ways of thinking at early stages of their careers. The program is proceeding well, and will continue to strive to provide a new environment for innovative research.
435
Ultra-Fine Particles Table 3. Examples of Research Results.
Hayashi Ultra-Fine Particle Project Highly selective UFP catalysts, behavior of UFPs in biocells, unique superconductivity of superlattice films, atomic level observations of UFP behavior, high performance alloys via UFPs, lattice arrays of UFPs, individually isolated UFPs, etc. Masumoto Amorphous & Intercalation Compounds Project Amorphous metal UFPs, new compound ceramics based on boron nitride, hygroscopic amorphous complex oxides, ferromagnetic amorphous ferrous thin films, ferromagnetic ferrite glass synthesis, painted sensors, bi-directional photo-controlled memory materials, etc. Ogata Fine Polymer Project Ambient temperature/pressure synthesis of polyester, organic conductive polymers, highselectivity separation materials, condensation monomer films, man-made enzymes, high polymer UFPs with single molecular chains, high-crystallinity high-polymer synthesis, etc. Nishizawa Perfect Crystal Project Pilot-scale manufacture of ultra high-density SIT integrated circuits, production of GaAs single crystals, photo-excited molecular layer epitaxial growth, pilot-scale manufacture of SIT photocenters, synthesis of GaAs by a low pressure eptaxial method, double-gate type thyristor, etc. Mizuno Bioholonics Project Proposal of a new concept for a pattern image analysis method, preparation of antibodies against human atherosclerosis, reconstitution of actin outside living bodies, discovery of treatment methods using macrophages (cancer remission, atherosclerosis reversal, etc.), design of a holistic computer for pattern recognition, etc. Hayaishi Bioinformation Transfer Project Discovery of the sleep effect of prostaglandin D2, discovery of a clue to the treatment of dementia in elderly, discovery of the effect of prostaglandin D2 on the promotion of nerve differentiation, clarification of the distribution of prostaglandin receptors in the brain, discovery of the increase in AIDS infection among homosexual partners, etc. Horikoshi Superbugs Project Discovery of an enzyme that can decompose cane sugar and of an enzyme that can decompose trehalose, which takes part in the energy cycle of living bodies, discovery of selenium resistant microorganisms, discovery of a triangular microorganism, etc.
436
INDEX
A a-phase 60 Accelerating effect 24 Acceleration voltage 156 Accicular magnetic metal particles 339 Aceoacetate 365 Acetone 369, 371 Acetone hydrogenation 379 Adduct particles 210 Aerosols 213, 217, 262, 317 Aerothermodynamics 160, 165 Agglomeration 199 Albumin 308 Aldehyde 295 Alkali halide 333, 334, 335, 336 Alkoxide method 255, 257, 260 Alkoxides 201, 214 Alkyl compounds 201 Alloy catalysts 253, 255, 260 Alloy composition control 318 Alloy particles 256 Alloy UFPs 253, 315 Alumina 31-36, 52, 64, 89 conditions 51 Alumina carriers 65, 66 Alumina particles 60 Alumina purity 50
Aluminum particle diameter 43 Aluminum hydroxide 52, 58 Alveolar macrophages 262, 271, 272, 276 Amino groups 297 Amplitude distribution 25, 26 Analysis methods 238 Anatase 198, 202 Annihilation 164 Anthracene 291 Antibodies 263, 297, 308 Antibody immobilized particles 308 Antigen 158 Anti-metabolites 279 Anvil 93, 94 Applications 3, 197, 313, 316, 319, 324, 337, 352, 410, 419, 422 Arc method 47 Arrays 337, 420 Astigmatic aberration 22 Atomic transport phenomena 11 ATP 280
B Bacterial magnetic particles 302, 303, 306, 308, 309 Bacterium 263
437
Index Ballistic experiments 233 Ballistic flights 219 Batch processes 170 Beam UFP. See UFB beam Beam current 109 Beam velocity 107, 109, 110 Behavior ofUFPs 423 BET method 358 Bethe's method of wave mechanics 27 Bethe's stopping coefficient 131 BHK-21 264, 266 Bimetallic catalysts 357, 364 Binary systems 135 Binary UFPs 144 Biolistic delivery 299 Bioreactors 293 BiosYnthesized materials 287 Biotechnology 293 Bisacetylacetonate 378 Bismuth 128 Bleeding device 18 Boundary conditions 218 Bovine serum albumin 298 Brookite 198 Brownian motion 279 Brownian particles 164 Bullet treatments 263 Bundle structure 347 Butler-Banerjee diagram analysis 303
c ~H2 188 C3b complement 263 Calcium 114, 115 Carbide stripes 333 Carbides 208, 209, 331 Carbon black 201 Carbon whiskers 88 Carbonizing treatment 89
Carbonyl clusters 65, 66, 68, 69, 71 Carcino embryonic antigens (CEA) 308 Carrier particles 293 Catalysis 355 heterogeneous 64 Catalyst carrier 58 Catalyst properties 361 Catalysts 119, 253, 260, 314, 357, 363, 364, 368, 377 Catalytic activity 255, 359, 378 ofCu-Zn 141 CEA antibodies 308 Cellular magnetic field 283 Cellular model 280 Cellular motion 284 Cementite 89 Ceramics 419, 422 CH4 decomposition 185, 187 Charged particle 108 Chemical flame process 205, 207 Chemical heat pump 314, 369, 372, 374, 376, 379 Chemical methods 214 Chemical oscillation phenomena 243 Chemical vapor deposition (CVO) 201 Chlorides 201 Chromatic aberration 22, 25 Chromium 41 Clapeyron-Clausius equation 167 Cloud formation 42 Cluster physics 217 Cluster size distribution 217 Clusters 33, 215, 423 Co-Cr medium 352 Co-Cr metal thin films 352 Co-evaporation 401 Co-precipitation method 366 Co-Sm 318 CO2 410, 411, 414, 415, 417
438
Index Coalescence 42, 216 Cobalt 5, 343 Cobalt-polymer 314, 340, 341, 342, 344, 346, 347, 349, 350, 352 Coercivity 153, 155, 316, 343, 345, 346, 351 Collection methods 176 Collection of samples 147 Collision and coalescence 216 Collisions 42 Colloidal particles 262, 293 Colloids 217 Column growth 348 Columnar microstructure 348 Columnar structure 351 Columns 347 Compacting UFPs 389, 393 Complement body 263 Composite magnetic thin films 348 Composite thin films 353 Composition control 137 Compound catalysts 364 Compound metallic catalysts 357 Compound particle synthesis 135 Compound UFPs 144 Compressed samples 94 Condensation reactions 214 Conservation of mass equation 162 Conservation of momentum equation 162 Contamination 50 of the sample 29 Continuous processes 170 Convection 218 Coolant 48 Copper 319, 367, 389, 391 Coulomb interaction 23 Cowley-Moodie method of physical optics 27 Cryometallurgy 403 Crystal thick 27
Crystal growth process 74 Crystal habits 67, 131 Crystal structure analysis 28 Crystal structure data 54 Crystal structures metastable 51 Crystalline equilibrium 74 Crystalline modifications 198 Crystallization process 80 Crystallography 4 Cu-Ni alloys 254 Cu-Zn 137, 139, 140, 144, 366, 367 Culture medium 264 Curie-Wulff theory 74, 131 CVD method 178, 179, 201, 211 1,3 Cyclo-octadiene 361, 362, 363 Cytoskeleton 279
D DC plasma jet 173 Debye temperatures 246 Defects planar 75 Dehydration reactions 260 Dehydrogenation 377 Dense aerosols 213 Deposition method 48 Diamond structure 6 Diffusion 218, 230 Dimers 213 Double-layer structure Cu-Zn 141 Dry ice 411 Dulbecco modified MEM 264 Dynamic behavior of metallic UFPs 119 Dynamic observation 29
E E. coli 308 EELS 17, 21
439
Index Einstein relation 164 Electrical resistance 94 Electrical resistivity 153 Electro·static potential 23 Electron beam wavelength 21 Electron beam for reading! writing 324 Electron beam heating 48 Electron diffraction 9, 27 Electron dose 333 Electron irradiation 71, 326, 328, 330, 335 Electron microscope 8, 113 Electron microscopy 4 Electron scattering 23 Electrostatic potential distribution 26 Enantio-face selective hydrogenation 365 Encapsulated UFPs 294, 297 Endoplasmic reticula 286 Endothermic processes 241 Energy source 371 Ensemble effect 255 Enzyme activity 306 Enzyme treatments 304 Enzymes 293, 297 ERATO I, 314, 426-435 Evaporated UFPs 315 Evaporation 48, 213, 218 explosive 222 IFP 177, 178 Evaporation conditions 46 Evaporation equipment 44 Evaporation methods 40 Evaporation rate 41 EXAFS 257 Expansion adiabatic 102 Explosive behavior 229, 231 Explosive evaporation 222 Extinction characteristics 52
F Falling velocity 385 Fc 263 Fe-Co 315, 318 Fe-Co alloys 257 Fe-Ni alloys 257 Fe-Ni/SiOs catalysts 258 Fe304 271 Fe304 particles 158 Ferromagnetic metallic UFPs 82 Ferromagnetism 5, 315 Fibroblast 270 Film characteristics 393 Film formation 153, 381, 383, 384, 386, 405 Film morphology 388 Films 314 Fine powders 7 Flagellum motor 270 Floatation 385, 386 Fluid mechanics 160 Formalin 272, 279 Formation of coatings 420 Fonnation ofUFPs 332, 333 Fonnic acid 260 Forming regular arrays 337 Free-fall capsules 219 Free-fall experiment 220 Frequency domain optical recording 324 Ff-IRIPAS 242 Fusion growth 60, 61 Fusion process 33 Future 419
G y-alumina 58, 59 y-phase 60 Gas bleeding device 18 Gas condensation 40 Gas deposited films 314 Gas deposited nickel 393
440
Index Gas deposition 381, 383, 384, 389, 393, 397, 403, 408, 420 Gas evaporation 40, 218, 420 physical 213 zero gravity 222 Gas evaporation experiment 221 Gas evaporation method 50, 133, 144, 286, 290, 356, 367 Gas evaporation synthesis apparatus 136 Gas evaporation system 213 Gas phase reaction 205, 210 Gas sensors 406 Gaseous reaction method 200, 201, 207, 208 Gases kinetic theory 40 Gels 256, 259 Geothermal power 376 Geothermal sources 374 Glucose oxidase 298, 305, 306, 307 Gold 37, 38, 128, 313, 328 Gold clusters 120 Gold films 396 Gold single crystal particles 36 Grain boundaries 31, 76 Gram staining 301 Granulocytes 310 Graphite films 89 Gravity 218, 219, 233 Green bodies 200 Green density 393 Green structure 199 Green UFPs 322 Gus-gene 299
H Halides
205, 206, 208, 333, 334, 335 Hardness 96, 97
Heat efficiency 371, 374 Heat exchanger 371 Heat of hydrogenation 371 Heat pump 314, 369, 370, 371, 372, 374, 376, 379 Heat sources 133 Heating UFPbeam 116 Heating device 16 Heating methods 47, 50, 94 Heterogeneous catalysis 64 High density recording 324, 333, 337, 339, 352 High-resolution electron microscopy (HREM) 11 Hole burning 337 Hot isostatic pressing (HIP) 92 HREM 11 Hydrogen-reduction 215 Hydrogenation 362, 363 Hydrogenation reaction 254, 357, 361, 363 Hydrolysis 207 Hydrophilicity 289 Hydrophobicity 289 Hydroxides 240, 243 Hydroxyl 295 Hypennagnetite 249 Hysteresis 343
I IFP evaporation method 177, 178 IFP process 170, 171, 173, 193, 194 Image formation 27 Immobilized antibodies 308 Immobilized particles 306, 308 Impregnation method 257, 355, 364, 366 Impurities 44, 50, 357 In-flight oxidation process 246
441
Index In-flight plasma processes (IFP processes) 170 Induction heating 47, 133 Injection into plasma 174 Inorganic UFPs 276 Ion irradiation 19 Ionized iron 157 Ionizing UFPs 151 Iron 98, 99, 100, 101, 147, 149, 154, 158, 245, 247, 257 Iron chloride 213 Iron oxide 150 Iron particles 5 Iron sulfide 11 Iron-polymer 345 Irradiation 325, 326, 327, 330 Irradiation dose 330 Island structures 4, 325 Isolated iron 149, 158 Isolated nickel 148 Isolated UFPs 146, 150, 151, 158 Izumi's method 364
Lattice images 129 Lattice parameters 303 Lattice structure ofUFPs 314 Lead and zinc 403 Lectin 263 Ligand effect 255 Lipase treatment 304 Liquid-phase endothermic reactor 371 Liquid-phase reactor 369 Longitudinal magnetic recording 340 Lorentz equation 22 Lorentz force 23 Lothe-Pound theory of nucleation 181 Low gravity experiments 233 Luminescence 243 Lung function 271 Lymphocytes 310 Lysozyme treatment 304
J
Macrophages 262, 271 Magnesia 50 Magnesimn 116 Magnetic anisotropy 344, 345 Magnetic hysteresis 343 Magnetic metal particles 339 Magnetic particles as carriers 294 Magnetic properties 322 Magnetic recording 316, 317, 339, 352, 421 Magnetic relaxation phenomenon 272 Magnetic UFPs 271, 272, 276, 277, 279, 283, 297, 309, 310, 313 characteristics of 300 Magnetite 149, 271, 303 Magnetosomes 301
1.774 276 J774.2 264, 265 Jet torch 171 JRDC 426
K 13-Keto acid esters 363 Killer T cells 311 Kubo theory 5, 290 Kupffer's stellate cells 262
L Langevin function 283 Langmuir equation 167 Laser process 205 Lattice arrays 128 Lattice defects 331
M
442
Index Magnetotactic bacteria 300, 301, 302 Material properties defined 4 Metal carbonyl 65, 66 Metal catalysts 64 Metal clusters 71 Metal halides 205 Metal smoke 41 Metallic UFPs 315 dynamic behavior 119 Metastable crystal structures 51 Methanol synthesis 142, 367 Methyl acetoacetate 363, 365 MgO 203 sintering of 204 Microcapsules 263 Microclusters 48 Microcrystals 239 Microstructures 20 uniform 199 Miller indices 64 Mitochondria 286 Mixed film 400, 403 Mixing method 366 Mixing temperature 210 Monocytes 262, 310 Morphology 138, 153 Mossbauer spectroscopy 245, 249 Motion ofUFPs 121 MS-l 300 Multiple twinned particles 54
N NaCI 336 Navier-Stokes equation 162 NBT reduction capability 309, 310 Neck 31, 32 Needles 42 Ni-Al alloy 361 Nickel 89, 138, 148, 237, 239, 243, 257, 319, 357, 361, 364, 377, 378, 382, 393
Nickel and TiN 402 Nickel catalysts 359 Nickel hydroxides 240 NiO 237, 239, 242 Niobium oxide 10, 27 Nitrates 256 Nitrides 208, 209 Non-specific phagocytosis 263 Nozzle chamber 113 Nucleation 42, 181, 215 Nucleation rate 205 Nucleus formation 216
o Optical disc systems 324 Optically active compounds 363 Optimum focus 26 Organic membrane 304 Organic solvent 147, 158 Organic UFPs 286, 287, 290 properties of 289 Oscillation phenomena 243 Oxidation 50, 242, 315 rate equation 85 Oxidation treatment 241, 249 Oxidative luminescence 243 Oxide 9, 13, 50, 240, 245, 420 intermediate products 241 Oxide carriers 64 Oxide layer 237 Oxide support 119 Oxide UFPs 207, 317 Oll:ychlorides 201 Oxygen 240 Oxygen content 358 Oxygen plasma generation 171
p Partial vapor pressure 167 Particle characteristics of 209 defined 215
443
Index Particle beam instrument 15 Particle collection 158 Particle collisions 42 Particle diameter 355 Particle diameter effect 12, 54 Particle growth 120 Particle size 120, 121, 161, 204, 213, 214 Particle size control 193, 257 Particle size distribution 199, 217 Particle synthesis 205 Particle velocity 112 Particle-carrier interactions 355 Particles charged 108 mesoscopic 107 Pb-Zn 404, 405 Percolation phenomenon 229 Perpendicular magnetic anisotropy 344, 350 Perpendicular magnetic recording 352 Phagocytic ability 310 Phagocytic activity 309 Phagocytosic cells 263 Phagocytosis 262, 263, 266, 267, 271 Phase contrast transfer function 25, 26 Phase object 23, 26, 27 Photochemical hole burning 337 Photoresist 412 Photoresist stripper 410 Physical vapor deposition (PVD) 201 Physio-chemical analysis method 423 Pigment 198 Planar defects 75 Planar fault 76 Plasma flame method 47 Plasma process 205, 207 Plasma reactor 188 Plasma tail flame 189
Plasma torch 171, 172, 173 Platelets 42 Platinum 48, 119, 378 Platinum clusters 33, 34, 71 Plug-flow type bioreactors 293 Poly(ethylene terephthalate) 341, 348 Polymeric film 158 Polymerization 294 Polymers 287, 341, 420 Population balance 217 Pores 357 Powder manufacturing methods 200 Powder metallurgy 389 Powder production methods 290 Powder technology 7 Powders characteristics of 197, 199 sinterability of 200 PPG process 179, 180 Pressure 94 Pressureless sintering 199 Producing UFPs methods 134, 135 Production techniques 290 2-Propanol 377 2-Propanol dehydrogenation 379 2-PropanoVacetone 371 2-PropanoVacetonel hydrogen 369, 372, 373, 376 Properties ofUFPs 313, 421 Pseudo-pods 265, 279 Pulmonary magnetic fields 271, 276 Pumping system 44 Purity 50 alumina 65 PVD 201 Pyrene 287, 288
Q Quantum size effects 355 Quenching 184, 185, 194
444
Index R
s
Radiation damage 331 Raft structure 68 Raney alloy leaching method 366 Raney method 355 Raney nickel 361, 363 Raney nickel catalysts 359 Reaction processes 205 Reactive quenching 184, 185, 188, 194 Reading/writing 324 Receptor mediated phagocytosis 263 Recognition mechanisms 264, 276 Recording 324, 337, 339 Recording medium 316, 334, 339, 340 Recovery 176 Reduction method 358 Reduction process 377 Refractory carbides and nitrides 202 Relaxation 272, 276, 280, 281 Relaxation curve 275 Relaxation rate 279, 283 Resistance electrical 94 Resistance of nickel 394 Resistivity 153, 155, 156 Resistivity ofUFP films 393 RF plasma reactions 210 RF plasma torch 171, 173 RF-DC coupled plasma torch 171 Rhodium 119 Rhodium clusters 29, 68, 69, 71 Rotating particle 268 Rotation ofUFPs 271, 272, 279, 283 Rotational Brownian motion 279 Rotational motion 270 Rotational movement 268 Ruffling motion 265 Rutile 198
Sample chamber 113 Sample heating 16 Sample holder 44 Samples collection of 147 Saturation vapor pressure 167 SC 181, 183 Scanning-transmission electron microscopy 10 Schiff salt fonnation reaction 297 Segregation 366 Selective fonnation 332 Shear defonnation 127 Si 3N 4 184, 188, 194, 200, 202 SiC 184, 188, 191, 193, 194, 200, 202, 210 Silicon 31, 32, 74, 78, 114 Silicon carbide 92 Silicon grain boundary structures 33 Silicon particles spherical 78 Silicon wafer 326, 327 Silver 104, 116, 226, 228, 266, 267, 387, 390 Silver and iron 399, 400 Sinterability of powders 200 Sintering 92, 197, 198, 199, 319 Sintering characteristics of MgO 204 Sintering mechanisms 12, 31 Sintering oxides 420 Sintering phenomenon ofUFPs 16 Size ofUFPs 329, 422 Smoke 40, 41, 42, 47, 62, 213, 222, 227, 411 expansion of 232 explosive behavior 227, 229 iron 98, 99 silicon carbide 92 SN 181, 182 5n02 406
445
Index Snow fonnation 42 Solid CO2 410 Solid gas spraying apparatus 314 Solidified gas UFPs 410 Soot 160 Specific phagocytosis 263 Specific surface area 88, 355 Spherical aberration 25 Spherical particles 60, 74, 77, 207 Spherical silicon 82, 85 Spherical structure 377 Spinel structure 52, 68 Spraying apparatus 314 SQUID 271 Stacking faults 75, 76, 124 Steady-state velocity 385 Stellate cells 262 Stigmater device 22 Stripping ability 412 Structural analyses 257 Styrene 254 Sugar 263 Superconductors 422 Supersaturation 215, 216 Supersaturation ratio 205 Surface analysis techniques 82, 254 Surface area 58, 88 Surface energy 83 Surface features 88 Surface oxidation 82, 246 Surface oxide layer 237, 240, 242 Synthesis apparatus 145 Synthesis chamber 98 Synthesis conditions 137, 160 Synthesis from gas phase reactions 205 Synthesis method 144 Synthesis ofUFPs 135 Synthesis rate 161, 164
T Tail flame temperature Tartarate 364
180
Theory of nucleus fonnation 215 Thennal decomposition 215, 366 Thennal efficiency 379 Thennal oxidation 85 Thennal stability 58 Thennodynamics 162, 180 Thick crystal 27 Thin films 340 TiB 2 production 179 TiN 206, 210, 403 Tin 41, 406 Tin oxide 407 Ti02 198, 202 Torches 171 Transmission electron microscope 20 Transport in a gas 385 Transporting gas 386 Trimers 213 Trypsin 304 Tungsten atom 27 Twin planes 124, 125, 126 Twinned particles 54, 123 Twins 75, 76, 101
u UFP beam 104, 106, 107, 116 UFPs defined 2, 195 Ultra fine powders 7 Ultra-Fine Particle Project 1, 428
v Vacuum chamber 44 Vacuum deposition 322 Vacuum evaporation 40 Vapor deposition 325 Vapor growth 42 Vapor pressure 219, 232 Vapor region 42 Vapor-phase exothennic reactor 371 Vapor-Solid (V-S) mechanism 184
446
Index Velocity 385 beam 107 particle 112 VEROS 48 Vibrating sample magnetometer 341, 348 VinylpolY1fierization 294 Vinyltrimethoxysilane, see vrs vrs 294, 296
W W-4 method 361 W-4 Raney nickel 361 Wave mechanics 27 Wavelength of an electron beam 21 Whiskers 88 White cells 262
x X-ray diffraction 20, 60 X-ray powder diffraction 28
z Zero gravity 219, 222, 224, 229 Zeta potential 276, 290 Zinc 367 Zinc black 5 Zinc ferrite particles 306 Zircon sand 170 Zirconia 63 ZnO 139, 143 Zymosan particles 265
447
