Nanostructured Materials

Nanostructured MaterialsProcessing, Properties, and ApplicationsSecond Edition

Edited by Carl C. Koch North Carolina State University, Raleigh, North Carolina

Copyright 2007 by William Andrew, Inc. 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. Front cover depicts a simulation of nanocrystalline grains. Images provided by Professor Donald Brenner, North Carolina State University. Cover by Hannus Design Nanostructured is a registered trademark of Hybrid Plastics, Inc., and is used in this book with the consent of Hybrid Plastics. Library of Congress Cataloging-in-Publication Data Nanostructured materials : processing, properties, and applications / edited by Carl C. Koch. p. cm. Includes bibliographical references and index. ISBN-13: 978-0-8155-1534-0 (978-0-8155) ISBN-10: 0-8155-1534-0 (0-8155) 1. Nanostructured materials. I. Koch, C. C. TA418.9.N35N3535 2006 620.5dc22 2006034353 Printed in the United States of America This book is printed on acid-free paper. 10 9 8 7 6 5 4 3 2 1 Published by: William Andrew Publishing 13 Eaton Avenue Norwich, NY 13815 1-800-932-7045 www.williamandrew.com Sina Ebnesajjad, Editor in Chief (External Scientic Advisor) NOTICE To the best of our knowledge the information 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 information. This book is intended for informational purposes only. Mention of trade names or commercial products does not constitute endorsement or recommendation for their use by the Publisher. Final determination of the suitability of any information or product for any use, and the manner of that use, is the sole responsibility of the user. Anyone intending to rely upon any recommendation of materials or procedures mentioned in this publication should be independently satised as to such suitability, and must meet all applicable safety and health standards.

Materials Science and Process Technology Series Series Editor: Gary E. McGuire International Technology Center, Research Triangle Park, North CarolinaVacuum Deposition onto Webs, Films, and Foils, Charles A. Bishop, 978-0-8155-1535-7, 494 pp., 2007 Ultrananocrystalline Diamond: Synthesis, Properties, and Applications, Olga A. Shenderova and Dieter M. Gruen, eds., 978-0-8155-1524-1, 620 pp., 2006 Diffusion Processes in Advanced Technological Materials, Devendra Gupta, ed., 0-8155-1501-4, 550 pp., 2005 Handbook of Ellipsometry, Harland G. Tompkins and Eugene A. Irene, eds., 0-8155-1499-9, 886 pp., 2005 Thin Film Materials Technology, Kiyotaka Wasa et al., 0-8155-1483-2, 534 pp., 2004 Tribology of Abrasive Machining Processes, Ioan D. Marinescu, et al., 0-8155-1490-5, 751 pp., 2004 Handbook of Thin Film Deposition Processes and Techniques, 2nd Ed., Krishna Seshan, ed., 0-8155-1442-5, 657 pp., 2002 Nanostructured Materials, Carl C. Koch, ed., 0-8155-1451-4, 636 pp., 2002 Handbook of Hard Coatings, Rointan F. Bunshah, ed., 0-8155-1438-7, 568 pp., 2001 Handbook of VLSI Microlithography, 2nd Ed., John N. Helbert, ed., 0-8155-1444-1, 1022 pp., 2001 Wide Bandgap Semiconductors, Stephen J. Pearton, ed., 0-8155-1439-5, 591 pp., 2000 Handbook of Chemical Vapor Deposition, 2nd Ed., Hugh O. Pierson, 0-8155-1432-8, 506 pp., 1999 Hybrid Microcircuit Technology Handbook, 2nd Ed., James J. Licari, and Leonard R. Enlow, 0-8155-1423-9, 601 pp., 1998 Handbook of Magneto-Optical Data Recording, Terry McDaniel, and Randall H. Victora, eds., 0-8155-1391-7, 967 pp., 1997 Ultra-Fine Particles, Chikara Hayashi, R. Ueda and A. Tasaki, eds., 0-8155-1404-2, 467 pp., 1997 Handbook of Refractory Carbides and Nitrides, Hugh O. Pierson, 0-8155-1392-5, 362 pp., 1996 Diamond Chemical Vapor Deposition, Huimin Liu, and David S. Dandy, 0-8155-1380-1, 207 pp., 1995 Handbook of Compound Semiconductors, Paul H. Holloway and Gary E. McGuire, eds., 0-8155-1374-7, 936 pp., 1995 Handbook of Vacuum Arc Science and Technology, Raymond L. Boxman, Philip J. Martin, and David M. Sanders, eds., 0-8155-1375-5, 771 pp., 1995 High Density Plasma Sources, Oleg A. Popov, ed., 0-8155-1377-1, 465 pp., 1995 Molecular Beam Epitaxy, Robin F. C. Farrow, ed., 0-8155-1371-2, 790 pp., 1995 Handbook of Deposition Technologies for Films and Coatings, 2nd Ed., Rointan F. Bunshah, ed., 0-8155-1337-2, 887 pp., 1994 Contacts to Semiconductors, Leonard J. Brillson, ed., 0-8155-1336-4, 702 pp., 1993 Diamond Films and Coatings, Robert F. Davis, ed., 0-8155-1323-2, 437 pp., 1993 Electrodeposition, Jack W. Dini, 0-8155-1320-8, 381 pp., 1993 Handbook of Carbon, Graphite, Diamonds and Fullerenes, Hugh O. Pierson, 0-8155-1339-9, 419 pp., 1993 Handbook of Multilevel Metallization for Integrated Circuits, Syd R. Wilson, Clarence J. Tracy, and John L. Freeman, Jr., eds., 0-8155-1340-2, 901 pp., 1993 Handbook of Semiconductor Wafer Cleaning Technology, Werner Kern, ed., 0-8155-1331-3, 645 pp., 1993 Chemical Vapor Deposition of Tungsten and Tungsten Silicides, John E. J. Schmitz, 0-8155-1288-0, 249 pp., 1992 Chemistry of Superconductor Materials, Terrell A. Vanderah, ed., 0-8155-1279-1, 838 pp., 1992 Electrochemistry of Semiconductors and Electronics, John McHardy and Frank Ludwig, eds., 0-8155-1301-1, 375 pp., 1992

Handbook of Sputter Deposition Technology, Kiyotaka Wasa and Shigeru Hayakawa, 0-8155-1280-5, 316 pp., 1992 Handbook of Plasma Processing Technology, Stephen M. Rossnagel, Jerome J. Cuomo, and William D. Westwood, eds., 0-8155-1220-1, 547 pp., 1990 Handbook of Polymer Coatings for Electronics, 2nd Ed., James Licari, and Laura A. Hughes, 0-8155-1235-X, 408 pp., 1990 Handbook of Semiconductor Silicon Technology, William C. OMara, Robert B. Herring, and Lee P. Hunt, eds., 0-8155-1237-6, 815 pp., 1990 Characterization of Semiconductor Materials, vol. 1, Gary E. McGuire, ed., 0-8155-1200-7, 342 pp., 1989 Handbook of Ion Beam Processing Technology, Jerome J. Cuomo, Stephen M. Rossnagel, and Harold R. Kaufman, eds., 0-8155-1199-X, 456 pp., 1989 Diffusion Phenomena in Thin Films and Microelectronic Materials, Devendra Gupta, and Paul S. Ho, eds., 0-8155-1167-1, 604 pp., 1988 Handbook of Contamination Control in Microelectronics, Donald L. Tolliver, ed., 0-8155-1151-5, 509 pp., 1988 Ionized-Cluster Beam Deposition and Epitaxy, Toshinori Takagi, 0-8155-1168-X, 239 pp., 1988 Semiconductor Materials and Process Technology Handbook, Gary E. McGuire, ed., 0-8155-1150-7, 689 pp., 1988 Chemical Vapor Deposition for Microelectronics, Arthur Sherman, 0-8155-1136-1, 227 pp., 1987

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Contents

Contributors....................................................................................... Preface...............................................................................................

xv xvii 1

PART I PROCESSING .................................................................... 1 Chemical Synthesis of Nanostructured Particles and Films ........................................................................................ Shi Yu, Cheng-Jun Sun, and Gan-Moog Chow 1.1 Introduction .............................................................................. 1.2 Particles .................................................................................... 1.2.1 Nucleation and Growth .................................................. 1.2.2 Dispersion and Agglomeration....................................... 1.2.3 Metals ............................................................................. 1.2.4 Ceramics ......................................................................... 1.2.5 Host-Derived Hybrid Materials...................................... 1.2.6 Surfactant Membrane-Mediated Synthesis .................... 1.2.7 Cytotoxicity of Nanoparticles ........................................ 1.3 Films and Coatings................................................................... 1.3.1 Metals ............................................................................. 1.3.2 Ceramics ......................................................................... 1.4 Summary................................................................................... Acknowledgements ......................................................................... References .......................................................................................

3 3 5 5 6 9 16 23 25 28 30 30 32 34 35 35

2 Synthesis of Nanostructured Materials by Inert-Gas Condensation Methods ................................................................. 47 C. Suryanarayana and Balaji Prabhu 2.1 2.2 2.3 2.4 2.5 2.6 Introduction ............................................................................ Classication .......................................................................... Synthesis of Nanostructured Materials .................................. Early Studies on Inert-Gas Condensation.............................. The Principle of Inert-Gas Condensation .............................. Evaporation Techniques ......................................................... 2.6.1 Thermal Evaporation.................................................... 2.6.2 Laser Vaporization........................................................ vii 47 47 48 50 51 56 56 57

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Contents 2.6.3 Sputtering ..................................................................... 2.6.4 Electrical Arc Discharge .............................................. 2.6.5 Plasma Heating............................................................. 2.7 Particle Transport ................................................................... 2.8 Particle Collection .................................................................. 2.9 Nucleation and Growth .......................................................... 2.10 Limitations of the Classical Nucleation Theory .................... 2.11 Crystal Structure and Morphology ........................................ 2.12 Inuence of Process Variables on Particle Size..................... 2.12.1 Inert-gas pressure ....................................................... 2.12.2 Inert-Gas Temperature................................................ 2.12.3 Inert-Gas Type............................................................ 2.12.4 Inert-Gas Flow Rate ................................................... 2.12.5 Evaporation Rate ........................................................ 2.12.6 Nozzle Diameter......................................................... 2.12.7 Chamber Size and Growth Distance.......................... 2.12.8 Effect of the Position of the Evaporation Source...... 2.12.9 Effect of Reactive Gases............................................ 2.13 Advantages of IGC................................................................. 2.14 Drawbacks of IGC ................................................................. 2.15 Recent Developments in IGC ................................................ 2.16 Conclusions ............................................................................ Acknowledgements ......................................................................... References ....................................................................................... 58 59 60 60 61 62 64 65 67 68 71 72 73 74 75 75 76 76 78 79 80 83 84 84

3 Thermal Sprayed Nanostructured Coatings: Applications and Developments ......................................................................... 91 George E. Kim 3.1 Introduction .............................................................................. 91 3.2 Thermal Spray Technology ...................................................... 92 3.2.1 Types of Processes ......................................................... 93 3.2.2 Applications.................................................................... 95 3.2.3 Thermal Spray Processes ............................................... 96 3.3 Thermal-Sprayed Nanostructured AluminaTitania Coating and United States Navy Applications ...................................... 98 3.3.1 Nanostructured AluminaTitania Coating ..................... 98 3.3.2 United States Navy Applications ................................... 99 3.4 Development and Application of Nanostructured Titania-Based Coating for Industrial Application.................. 102 3.4.1 Background................................................................... 102 3.4.2 Failure Analyses ........................................................... 103

Contents 3.4.3 Case StudyNanostructured Titania-base Coating of Ball Valve Components for High-pressure Acid Leach Autoclaves ......................................................... 3.5 Conclusions ............................................................................ Acknowledgements ....................................................................... References .....................................................................................

ix

106 116 116 117

4 Nanostructured Materials and Composites Prepared by Solid State Processing ................................................................. 119 H.J. Fecht and Yu. Ivanisenko 4.1 Introduction and Background................................................. 4.2 Phenomenology of Nanostructure Formation........................ 4.3 High-Energy Ball Milling and Mechanical Attrition............. 4.3.1 Examples ...................................................................... 4.3.2 Mechanism of Grain Size Reduction ........................... 4.3.3 PropertyMicrostructure Relationships........................ 4.4 Phase Stability at Elevated Temperatures .............................. 4.5 Severe Plastic Deformation (SPD)......................................... 4.5.1 General ......................................................................... 4.5.2 High-Pressure Torsion .................................................. 4.5.3 Cold Rolling of Thin Sheets ........................................ 4.5.4 Friction Induced Surface Modications....................... 4.6 Summary and Outlook ........................................................... Acknowledgements ....................................................................... References ..................................................................................... 119 120 122 122 130 135 140 143 143 144 159 161 164 165 165

5 Nanocrystalline Powder Consolidation Methods ..................... 173 Joanna R. Groza 5.1 Introduction ............................................................................ 5.2 Thermodynamics, Mechanisms and Kinetics of Nanocrystalline Powder Densication................................... 5.2.1 Thermodynamic and Kinetic Effects ........................... 5.2.2 Sintering Mechanisms .................................................. 5.2.3 Role of Impurities ........................................................ 5.2.4 Green Density of Nanopowders................................... 5.2.5 Pore Size and Its Effects on the Densication Behavior ....................................................................... 5.2.6 Grain Growth................................................................ 5.3 Methods for Full Densication of Nanopowders .................. 5.3.1 Characterization of Nanomaterials Densication: Density and Grain Size Measurements........................ 5.3.2 Conventional Sintering................................................. 173 175 175 178 185 186 193 196 200 200 202

x

Contents 5.3.3 Pressure Effects in Nanopowder Consolidation .......... 5.3.4 Pressure-Assisted Consolidation Methods................... 5.3.5 Nonconventional Sintering Methods............................ 5.4 Summary................................................................................. Acknowledgements ....................................................................... References ..................................................................................... 206 210 213 216 217 217

6 Electrodeposited Nanocrystalline Metals, Alloys, and Composites............................................................................ 235 Uwe Erb, Karl T. Aust, and Gino Palumbo 6.1 Introduction ............................................................................ 6.2 Synthesis of Nanostructured Materials by Electrodeposition .................................................................... 6.3 Structure of Nanocrystalline Metal Electrodeposits .............. 6.4 Properties ................................................................................ 6.4.1 Properties with Weak Grain Size Dependence ............ 6.4.2 Properties with Strong Grain Size Dependence .......... 6.5 Applications............................................................................ 6.5.1 Nanocrystalline Copper for Printed Wiring Boards .... 6.5.2 Nanocrystalline Metals in Microsystem Components .................................................................. 6.6 Summary................................................................................. References ..................................................................................... 235 236 239 246 247 253 276 278 280 283 283

7 Computer Modeling of Nanostructured Materials.................. 293 Donald W. Brenner 7.1 Introduction ............................................................................ 7.2 Modeling Methods ................................................................. 7.2.1 Molecular Dynamics and Monte Carlo Modeling....... 7.2.2 Atomic Potential Energies and Forces......................... 7.2.3 Multiscale Modeling .................................................... 7.3 Nanostructured Materials ....................................................... 7.3.1 Nanoparticle Properties ................................................ 7.3.2 Microstructure Modeling.............................................. 7.3.3 Sintering and Grain Growth Dynamics ....................... 7.3.4 Mechanical Deformation and Fracture ........................ 7.3.5 Shock Loading.............................................................. 7.3.6 Vibrational Properties................................................... 7.3.7 Nanoalloys .................................................................... 7.4 Prospects for Future Modeling............................................... References ..................................................................................... 293 294 294 296 297 299 299 302 307 311 316 318 319 322 324

Contents

xi

PART II PROPERTIES ................................................................. 329 8 Diffusion in Nanocrystalline Materials ..................................... 331 Wolfgang Sprengel Introduction ............................................................................ Modeling of Interface Diffusion ............................................ Diffusion in Grain Boundaries of Metals .............................. Diffusion in Nanocrystalline Metals ...................................... 8.4.1 Specic Aspects............................................................ 8.4.2 Nanocrystalline Pure Metals ........................................ 8.4.3 Correlation between Diffusion and Crystallite Growth ........................................................ 8.4.4 The Nanocrystalline Soft-magnetic Alloys Fe73.5Si13.5B9Nb3Cu1 and Fe90Zr7B3 ............................... 8.4.5 Nanocrystalline Hard-magnetic Nd2Fe14B Compounds................................................................... 8.4.6 Diffusion of Hydrogen in Nanocrystalline Metals ...... 8.5 Diffusion in Nanocrystalline Ceramics.................................. Acknowledgements ....................................................................... References ..................................................................................... 8.1 8.2 8.3 8.4 331 332 334 335 335 340 344 345 349 350 352 356 356

9 Nanostructured Materials for Gas Reactive Applications ...... 365 Michel L. Trudeau 9.1 Introduction ............................................................................ 9.2 Catalysis and Electrocatalysis ................................................ 9.2.1 Impact of Structure on Catalysis and Electrocatalysis Processes ............................................ 9.2.2 Nanostructure Design ................................................... 9.3 Gas Sensors ............................................................................ 9.3.1 Physical Principles of Semiconductor Sensors............ 9.3.2 Nanostructured Design of Sensing Materials .............. 9.3.3 Novel Nanostructured Systems .................................... 9.4 Hydrogen Storage................................................................... 9.4.1 Properties of Hydrogen-Storage Compounds .............. 9.4.2 Metallic Hydrides ......................................................... 9.4.3 Complex Hydrides........................................................ 9.4.4 Porous or Surface-Related Nanostructures .................. 9.5 Conclusion .............................................................................. Acknowledgements ....................................................................... References ..................................................................................... 365 366 367 370 383 385 392 399 401 403 405 412 414 417 418 418

xii

Contents 439 439 441 441 442 443 444 449 449 451 453 455 455 458 461 462 467 473 474 474 487 487 488

10 Magnetic Nanoparticles and Their Applications ..................... Sara A. Majetich 10.1 Introduction .......................................................................... 10.2 Fundamental Physics of Magnetic Nanoparticles ............... 10.2.1 Bulk Ferromagnetism............................................... 10.2.2 Magnetic Clusters..................................................... 10.2.3 Molecular Magnetism .............................................. 10.2.4 Ideal Monodomain Particles Made from Ferromagnetic Materials .......................................... 10.2.5 Macroscopic Quantum Tunneling............................ 10.2.6 Surface and Interface Effects in Nanoparticles ....... 10.2.7 Magnetostatic Interactions between Nanoparticles............................................................ 10.2.8 Exchange Interactions Between Nanoparticles........ 10.3 Applications of Monodomain Magnets ............................... 10.3.1 Ferrouids ................................................................ 10.3.2 Biomedical Applications .......................................... 10.3.3 Imaging with Magnetic Nanoparticles..................... 10.3.4 Data Storage Media.................................................. 10.3.5 Magnetoresistive Devices ........................................ 10.4 Conclusions .......................................................................... Acknowledgements ....................................................................... References ..................................................................................... 11 Magnetic Properties of Nanocrystalline Materials .................. Akihisa Inoue, Akihiro Makino, and Teruo Bitoh 11.1 Introduction .......................................................................... 11.2 FeMB (M = Zr, Hf or Nb) Amorphous Alloys and their Crystallization-Induced Nanostructure........................ 11.3 Soft Magnetic Properties and Structural Analyses of FeMB (M = Zr, Hf or Nb) Nanocrystalline Ternary Alloys ...................................................................... 11.4 Improvement of Soft Magnetic Properties by the Addition of Small Amounts of Solute Elements ................. 11.5 Soft Magnetic Properties and Structure of Cu-free Quaternary FeZrNbB Alloys ..................................... 11.6 Soft Magnetic Properties and Structure of FeNbBPCu Alloys Produced in Air ...................... 11.6.1 Structure and Soft Magnetic Properties................... 11.6.2 Effect of Grain-Size Distribution and Curie Temperature of Intergranular Amorphous Phase on Soft Magnetic Properties ....................................

492 499 503 507 507

513

Contents 11.7 Improvement of High-frequency Permeability by the Dissolution of Oxygen in the Surrounding Amorphous Phase..................................................................................... 11.7.1 As-sputtered Structure.............................................. 11.7.2 Magnetic Properties.................................................. 11.8 Applications.......................................................................... 11.9 Conclusions .......................................................................... References .....................................................................................

xiii

520 520 524 530 533 534

12 Mechanical Behavior of Nanocrystalline Metals ..................... 537 Julia R. Weertman 12.1 Introduction .......................................................................... 12.2 Models and Computer Simulations of Mechanical Behavior of Nanocrystalline Materials ................................ 12.2.1 Models of Deformation............................................ 12.2.2 Molecular Dynamics Computer Simulations........... 12.3 Characterization of Nanocrystalline Metals ........................ 12.3.1 Density, Pores and Microcracks............................... 12.4 Mechanical Behavior............................................................ 12.4.1 Elastic Properties of Nanocrystalline Metals........... 12.4.2 Hardness, Yield and Ultimate Strengths .................. 12.4.3 Ductility of Nanocrystalline Metals......................... 12.4.4 Experiments that Shed Light on Deformation Mechanisms.............................................................. 12.5 Conclusions .......................................................................... References ..................................................................................... 537 538 538 543 547 548 553 553 555 557 558 560 561

13 Structure Formation and Mechanical Behavior of Two-phase Nanostructured Materials ....................................... 565 Jrgen Eckert 13.1 Introduction .......................................................................... 13.2 Methods of Preparation........................................................ 13.2.1 Rapid Solidication Techniques .............................. 13.2.2 Mechanical Attrition................................................. 13.2.3 Devitrication of Metallic Glasses .......................... 13.3 Phenomenology of Nanostructure Formation and Typical Microstructures........................................................ 13.3.1 Rapidly Solidied Materials .................................... 13.3.2 Conventional Solidication and Devitrication of Bulk Samples ....................................................... 13.3.3 Mechanically Attrited Powders................................ 565 567 567 569 574 580 581 598 617

xiv

Contents 13.4 Mechanical Properties at Room and Elevated Temperatures ........................................................................ 13.4.1 Al-Based Two-Phase Nanostructured Alloys........... 13.4.2 Mg-Based Amorphous and Nanostructured Alloys ....................................................................... 13.4.3 Zr-Based Alloys........................................................ 13.4.4 Ti-Based Alloys........................................................ 13.4.5 Mechanically Attrited Composites........................... 13.5 Summary and Outlook ......................................................... Acknowledgements ....................................................................... References .....................................................................................

628 629 635 641 651 654 662 664 665

14 Nanostructured Electronics and Optoelectronic Materials .... 677 Raphael Tsu and Qi Zhang 14.1 Introduction .......................................................................... 14.2 Physics of Nanostructured Materials ................................... 14.2.1 Quantum Connement: Superlattices and Quantum Wells ......................................................................... 14.2.2 Dielectric Constant of Nanoscale Silicon ................ 14.2.3 Doping of a Nanoparticle......................................... 14.2.4 Excitonic Binding and Recombination Energies..... 14.2.5 Capacitance in a Nanoparticle ................................. 14.2.6 Structure, Bonds and Coordinations of Si Nanostructure: Porous Si and Si Clusters................ 14.3 Applications.......................................................................... 14.3.1 Porous Silicon .......................................................... 14.3.2 Photoluminescence in nc-Si/SiO2 Superlattices....... 14.3.3 Luminescence from Clusters.................................... 14.3.4 Semiconductor/Atomic/Superlattice: Si/O Superlattice ...................................................... 14.3.5 Amorphous Silicon/Oxide Superlattice ................... 14.3.6 nc-Si in an Oxide Matrix ......................................... 14.3.7 Electronic Applications of Si/O Superlattices ......... 14.3.8 Single Electron Transistor........................................ 14.3.9 Quantum Dot Laser.................................................. 14.4 Challenges in Quantum Dot Devices................................... 14.5 Epilogue................................................................................ Acknowledgements ....................................................................... References ..................................................................................... 677 677 677 678 680 682 685 688 691 691 692 694 695 699 700 702 704 708 711 712 713 713

Index .................................................................................................... 719

Contributors

Karl T. Aust University of Toronto Toronto, Ontario, Canada Teruo Bitoh Akita Prefectural University Yurihonjo, Japan Donald Brenner North Carolina State University Raleigh, NC, USA Gan-Moog Chow National University of Singapore Kent Ridge, Singapore Jrgen Eckert IFW Dresden, Institute for Complex Materials Dresden, Germany Uwe Erb University of Toronto Toronto, Ontario, Canada Hans Fecht University of Ulm Ulm, Germany Joanna Groza University of California, Davis Davis, CA, USA Akihisa Inoue Tohoku University Sendai, Japan xv

Yu. Ivanisenko University of Ulm Ulm, Germany George E. Kim Perpetual Technologies, Inc. Quebec, Canada Carl Koch North Carolina State University Raleigh, NC, USA Sara Majetich Carnegie Mellon University Pittsburgh, PA, USA Akihiro Makino Tohoku University Sendai, Japan Gino Palumbo Integran Co. Toronto, Ontario, Canada Balaji Prabhu University of Central Florida Orlando, Florida Wolfgang Sprengel University of Stuttgart Stuttgart, Germany Cheng-jun Sun National University of Singapore Kent Ridge, Singapore

xvi C. Suryanarayana University of Central Florida Orlando, FL, USA Michel Trudeau Hydro-Quebec Research Institute Varennes, Quebec, Canada Raphael Tsu University of North CarolinaCharlotte Charlotte, NC, USA

Contributors Julia Weertman Northwestern University Evanston, IL, USA Shi Yu Singapore-Massachusetts Institute of Technology Alliance Singapore Qi Zhang Advanced Photonix, Inc. Dodgeville, WI, USA

Preface to Second Edition

IntroductionNanostructure science and technology has become an identiable, if very broad and multidisciplinary, eld of research and emerging applications in recent years. It is one of the most visible and growing research areas in materials science in its broadest sense. Nanostructured materials include atomic clusters, layered (lamellar) lms, lamentary structures, and bulk nanostructured materials. The common thread to these various material forms is the nanoscale dimensionality, i.e., at least one dimension less than 100 nm, more typically less than 50 nm. In many cases, the physics of such nanoscale materials can be very different from the macroscale properties of the same substance. The different, often superior, properties that can then occur are the driving force behind the explosion in research interest in these materials. While the use of nanoscale dimensions to optimize properties is not new, as will be outlined below, the present high visibility and denition of the eld is mainly attributable to the pioneering work of Gleiter and coworkers in the early 1980s [1]. They synthesized nanoscale grain size materials by the in situ consolidation of atomic clusters. The study of clusters preceded this work by researchers such as Uyeda [2]. The International Technology Research Institute, World Technology Division (WTEC), supported a panel study of research and development status and trends in nanoparticles, nanostructured materials, and nanodevices between 1996 and 1998. The main results of this study have been published [3] and formed one of the drivers for the U.S. National Nanotechnology Initiative. This report attempted to cover the very broad eld of nanostructure science and technology and included assessments of the areas of synthesis and assembly, dispersions and coatings, high surface area materials, functional nanoscale devices, bulk nanostructured materials and biologically related aspects of nanoparticles, nanostructured materials, and nanodevices. A conclusion of the report is that while many aspects of the eld existed well before it was identied as a eld in the last decade, three related scientic/technological advances have made it a coherent area of research. These are: xvii

xviii

Preface 1. New and improved synthesis methods that allow control of the size and manipulation of the nanoscale building blocks, 2. New and improved characterization tools for study at the nanoscale (e.g., spatial resolution, chemical sensitivity), and 3. Better understanding of the relationships between nanostructure and properties, and how these can be engineered.

With the recent intense interest in the broad eld of nanostructure science and technology, a number of books, articles, and conference proceedings have been published on this broad topic. A partial listing of these publications is given in the bibliography below, starting with the review of Gleiter in 1989. A two-fold justication was given for another book in this rapidly advancing eld in the preface of the rst edition. These were, rst that since many areas of the eld are moving rapidly with increased understanding from both experiment and simulation studies, it would appear useful to record another snapshot of the eld. This justication is certainly true for the second edition since in the over four years since the rst edition was published, many new advances have occurred and the updated chapters reect them. The second justication for the rst edition was that because this eld is so broad, the book has been designed to focus mainly on those areas of synthesis, characterization, and properties relevant to application that require bulk, and mainly inorganic materials. An exception was the article by Tsu and Zhang on electronic and optoelectronic materials. The exceptions in this second edition are the updated chapter by Tsu and Zhang on the above area, and a new chapter on magnetic nanoparticles and their applications by Majetich. Before a brief description of the updated chapters, the new chapters, changes in some authorship, and the organization of the book is presented, a historical perspective will be given to suggest how the eld has developed and what new information has been provided by reaching the limit of the nanoscale.

Historical PerspectiveNanoscale microstructural features are not new, either in the natural world or in materials engineering. There are examples of nanoscale ferromagnetic particles found in microorganisms, e.g., 50 nm Fe3O4 in the organism A. magnetotactum [4]. A number of examples exist of improvement in mechanical properties of structural materials when a ne microstructure was developed. Early in the last century, when

Preface

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microstructures were revealed primarily with the optical microscope, it was recognized that rened microstructures, for example, small grain sizes, often provide attractive properties such as increased strength and toughness in structural materials. A classic example of property enhancement due to a rened microstructurewith features too small to resolve with the optical microscopewas age-hardening of aluminum alloys. The phenomenon, discovered by Alfred Wilm in 1906, was essentially explained by Merica, Waltenberg, and Scott in 1919 [5], and the microstrutural features responsible were rst inferred by the x-ray studies of Guinier and Preston in 1938 [6]. With the advent of transmission electron microscopy (TEM) and sophisticated x-ray diffraction methods, it is now known that the ne precipitates responsible for age-hardening, in Al-4% Cu alloys, for example, are clusters of Cu atomsGuinier-Preston (GP) Zonesand the metastable partially coherent precipitate [7,8]. Maximum hardness is observed with a mixture of GPII (or , coarsened GP zones) and , with the dimensions of the plates typically about 10 nm in thickness by 100 nm in diameter. Therefore, the important microstructural feature of age-hardened aluminum alloys is nanoscale. Critical length scales often determine optimum properties which are structure sensitive. Mechanical properties such as strength and hardness are typical, and as above, microstructural features such as precipitates or dispersoids are most effective when their dimensions are nanoscale. In ferromagnetic materials, the coercive force has been found to be a maximum if spherical particles (e.g., Fe3C in Fe) which act as domain wall pinners have a diameter about equal to the domain wall thickness, i.e., about 50 nm [9]. Similarly, in type II superconductors, it has been found that uxoid pinning, which determines the magnitude of the critical current density, is most effective when the pinning centers typically have dimensions of the order of the superconducting coherence length for a given material. For the high eld superconductors, the coherence length is usually about 1020 nm, and indeed the commercial superconductors have pinning centers that approximate these dimensions. In Nb3Sn, the grain boundaries are the major pinning sites and optimum critical current densities are obtained when the grain sizes are about 50 nm [10]. Many other examples could be given of the long term use of nanoscale materials in elds such as catalysis.

OrganizationAs was done in the rst edition of this book, Part I covers the important synthesis/processing methods for the production of nanocrystalline

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materials. Part II focuses upon selected properties of nanostructured materials. Potential, or existing, applications of nanocrystalline materials are described as appropriate throughout the book. Chapter 1, Chemical Synthesis of Nanostructured Particles and Films, by Yu, Sun, and Chow, is an updated version of Ch. 1 of the rst edition by Chow and Kurihara. The chemical methods for nanoparticle synthesis described include aqueous, polyol, sonochemical, precursor, organometallic, hydrolysis, solvothermal, and sol-gel methods. The cytotoxicity of nanoparticles is discussed in this updated chapter. Other methods discussed are host-derived hybrid materials, surfactant membrane mediated synthesis, and a variety of lms and coatings. Chapter 2, Synthesis of Nanostructured Materials by Inert-Gas Condensation Methods, by Suryanarayana and Prabhu, is new in this edition. It covers the important technique that was used by Gleiter and co-workers that stimulated the present eld of nanocrystalline materials. This chapter reviews the principles of the inert-gas condensation method, explains the synthesis of nanophase materials via this technique, and discusses the process parameters that inuence the constitution and particle size of the product phase. Chapter 3 by Kim is an updated version of the chapter by Lau and Lavernia now entitled Thermal Sprayed Nanostructured Coatings: Applications and Development. Thermal sprayed nanostructured coatings are a prime example of a method that has already matured to the point of application. Thermal sprayed nanostructured oxide coatings in particular have been shown to be practically advantageous for both military and industrial applications. After reviewing the technology of thermal spray a number of applications are described. Chapter 4 by Fecht and Ivanisenko, Nanostructured Materials and Composites Prepared by Solid State Processing, is the updated version of the chapter by Fecht in the rst edition. The methods described, such as mechanical attrition and other severe plastic deformation methods have become popular methods to produce nanocrystalline materials from the top-down. The promise to scale up from laboratory to industrial quantities is one of the advantages of these methods. The mechanisms believed responsible for this nanocrystalline synthesis as well as the stability of the nanocrystalline microstructures at elevated temperatures are reviewed. A major problem with nanocrystalline materials made in particulate form is the requirement for consolidation into bulk for most applications. Chapter 5 by Groza is an updated version of her chapter in the rst edition. Nanocrystalline Powder Consolidation Methods are reviewed and include conventional sintering methods as well as a variety of full-density

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consolidation techniques. The challenge in processing nanocrystalline powders is to fully densify them without losing the initial metastable features (nanoscale grain size and, sometimes, metastable phases). The stateof-the-art in consolidation of nanocrystalline powders is presented and the remaining challenges are discussed. While chapters 1, 2, and 4 describe processing methods for nanocrystalline materials that result in particulates that require subsequent compaction, i.e. two-step processing, there are one-step processing methods available that eliminate the need for compaction with its attendant problems. A notable and commercially attractive one-step method is electrodeposition. Pioneers in this eld, Erb, Aust, and Palumbo, update their former chapter into chapter 6, Electrodeposited Nanocrystalline Metals, Alloys, and Composites. This chapter describes the processing methods as well as the structure and properties of the electrodeposited nanostructured materials. Recent breakthroughs in the mechanical and magnetic properties of electrodeposited nanocrystalline materials are presented. A variety of applications for electrodeposited nanocrystalline coatings are reviewed. Computer simulation of nanomaterials comprises virtual processing and so was included in Part I of the rst edition. The chapter in the rst edition was written by Professor Phil Clapp who has subsequently retired. Chapter 7, now entitled Computer Modeling of Nanostructured Materials, has been written by Professor Donald Brenner. This chapter describes the various modeling techniques including molecular dynamics and Monte Carlo modeling, atomic potential energies and forces, and multiscale modeling. The modeling of nanoparticle properties, microstructure, sintering and grain growth dynamics, mechanical deformation, and nanoalloys are reviewed. Part II of the book deals with selected properties of nanocrystalline materials. Chapter 8, Diffusion in Nanocrystalline Materials, is an updated version of the chapter by Wurschum, Brossmann, and Schaefer in the rst edition. This chapter is written by Sprengel, a colleague of the former authors. This chapter reviews the data for diffusion in nanocrystalline materials. It describes modeling of interface diffusion, diffusion in grain boundaries of metals, and then gives examples of diffusion behavior for a variety of nanocrystalline materials including pure metals, soft magnetic materials, hard magnetic materials, ceramics, and diffusion of hydrogen in nanocrystalline metals. Chapter 9, Nanostructured Materials for Gas Reactive Applications, is an updated version by Trudeau of his chapter in the rst edition which brings in important results since the rst chapter was written. This large

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important eld is reviewed with examples from catalysis and electrocatalysis, semiconductor gas sensors, and hydrogen storage materials. Of special interest is the sensitivity to nanocrystalline structure. It is speculated that reducing the nanocrystallite size below 10 nm may have more dramatic effects on such properties as catalysis. Chapter 10 is a new chapter in this edition. Majetich reviews Magnetic Nanoparticles and Their Applications. After a brief introduction to the phenomenon of ferromagnetism, an in-depth description of the physics of monodomain ferromagnetic particles is given. Applications based upon magnetic nanoparticles are discussed and include such topics as magnetic recording media, spin valve devices, and tunnel junction structures as possible magnetic random access memory. Both current and future applications based on magnetic nanoparticles are described in terms of their basic properties, and the material challenges are identied. Chapter 11 by Inoue, Makino, and Bitoh is an updated and expanded version of the chapter by Inoue and Makino in the rst edition. Magnetic Properties of Nanocrystalline Materials focuses on the soft magnetic properties of bulk ferromagnetic nanocrystalline alloys prepared by the crystallization of amorphous precursors. The formation of nanogranular bcc and amorphous structures in the FeZrNbBCu, FeZr NbB, FeNbBPCu, and FeHfO systems are described along with their superior soft magnetic properties and their engineering applications. Chapter 12, Mechanical Behavior of Nanocrystalline Metals by Weertman, is an updated review of this dynamic eld of research. She brings in the new results from both experimental studies and the simulation of mechanical behavior by molecular dynamics calculations. An experimental breakthrough is the observation in some nanocrystalline materials of both high strength and good ductility. Computer simulation has allowed access to the smallest nanocrystalline grain sizes that are difcult to attain experimentally without the introduction of processing artifacts. Chapter 13, Structure, Formation, and Mechanical Behavior of TwoPhase Nanostructured Materials by Eckert, is updated from his chapter in the rst edition. The methods used to produce bulk two-phase nanostructured materials are described. The mechanical behavior of such materials is then discussed. Of special interest in this updated chapter is the report of research from the authors laboratory of enhanced plasticity in a Ti-base alloy with a nanocrystalline matrix and micron-scale ductile dendrites. This material exhibited both high strength and good ductility.

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The subject of functional nanostructured materials for electronic and optoelectronic applications is a large and important area. While this eld is not stressed in this book, it was felt that a chapter outlining some of the important features of this area should be included. Tsu and Zhang have updated their chapter from the rst edition, entitled Nanostructured Electronic and Optoelectronic Materials. Functional nanocrystalline materials, typically thin lms or quantum dots, are covered. An in-depth treatment of several topics related to Si semiconductors is given. This includes the physics of nanostructured materials which covers the dielectric constant, the capacitance, doping and exiton binding energies of a nanoparticle. Possible devices requiring nanoscale features are described. Such devices are light emitting diodes (LEDs) and quantum eld effect transistors (QD-FETs).

References1. Gleiter, H., Progress in Materials Science, 33:223315 (1989). 2. Uyeda, R., Progress in Materials Science, 35:196 (1991). 3. Siegel, R.W., Hu, E., and Roco, M.C., (eds), Nanostructure Science and Technology, Kluwer Academic Publishers, Dordrecht, Netherlands (1999). 4. Kirschvink, J.L., Koyayashi-Kirschvink, A., and Woodford, B.J., Proc. Natl Acad. Sci., USA, 89:76837687 (1992). 5. Merica, P.D., Waltenburg, R.G., and Scott, H., Bulletin AIME, June: 913 (1919). 6. Guinier, A., Nature, 142:569 (1938); Preston, G.D., ibid, 570. 7. Silcock, J.M., Heal, T.J., and Hardy, H.K., J. Institute of Metals, 82:239 (195354). 8. Cohen, J.B., Metall. Trans. A., 23A:2685 (1992). 9. Swisher, J.H., English, A.T., and Stoffers, R.C., Trans. ASM, 62:257 (1969). 10. Scanlan, R.M., Fietz, W.A., and Koch, E.F., J. Appl. Phys., 46:2244 (1975).

BibliographyGleiter, H., Nanocrystalline Materials, Progress in Materials Science, 33:223315 (1989). Siegel, R.W., Nanostructured Materials-Mind Over Matter, NanoStructured Materials, 3:1 (1993). Hadjipanayis, G.C., and Siegel, R.W., Nanophase Materials: SynthesisProperties-Applications, Kluwer Press, Dordrecht, Netherlands (1994). Gleiter, H., Nanostructured Materials: State of the Art and Perspectives, NanoStructured Materials, 6:3 (1995). Edelstein, A.S., and Cammarata, R.C., (eds.), Nanomaterials: Synthesis, Properties, and Applications, Institute of Physics, Bristol (1996).

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Suryanarayana, C., and Koch, C.C., Nanostructured Materials, in NonEquilibrium Processing of Materials, edited C. Suryanarayana, Pergamon, Elsevier Science Ltd., Oxford, UK (1999) p. 313. Dekker Encyclopedia of Nanoscience and Nanotechnology, Marcel Dekker Inc., New York, NY (2004).

Carl C. Koch Raleigh, North Carolina

October, 2006

PART 1 PROCESSING

1 Chemical Synthesis of Nanostructured Particles and FilmsShi Yu,1 Cheng-Jun Sun,2 and Gan-Moog Chow1,21

Molecular Engineering of Biological and Chemical Systems, SingaporeMassachusetts Institute of Technology Alliance (SMA), Singapore

2

Department of Materials Science and Engineering, National University of Singapore, Singapore

1.1 IntroductionThe performance and properties of materials depend on atomic structure, composition, microstructure, defects, and interfaces which are controlled by thermodynamics and kinetics of the synthesis and processing. Nanostructured materials, often characterized by a physical dimension (such as particle size or grain size) of less than 100 nm, attract much interest due to their unique properties compared to conventional materials. Current advances in synthesizing and processing of functional materials for high technology emphasize the bottom-up approach to assemble atoms, molecules, and particles, from the atomic or molecular scale to the macroscopic scale. The tailor-designed arrangement of atoms from the nanoscale to the macroscale for optimized properties may be realized by materials chemistry. Increasing recent interests have been found in chemical synthesis and processing of nanostructured materials.[12,3,4,5,6,7,8,9,10,11,12,13,14] Chemical synthesis of materials may be conducted in solid, liquid, or gaseous state. The traditional solid-state approach involves grinding and mixing of solid precursors, followed by heat treatment at high temperatures to facilitate diffusion-controlled chemical reactions to obtain the nal products. Mixing and grinding steps are usually repeated throughout the heating cycle, with great efforts to mix materials at the nanoscale and provide fresh surfaces for further chemical reactions. Grain growth, if not prevented, occurs at elevated temperatures resulting in undesirable large grain size.Carl C. Koch (ed.), Nanostructured Materials: Processing, Properties, and Applications, 2nd Ed., 346 2007 William Andrew, Inc.

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Processing

Material diffusion in liquid or gas is, advantageously, many orders of magnitude larger than in the solid phase, allowing for synthesis of nanostructures at lower temperatures. Reduced reaction temperatures discourage detrimental grain growth. Many functional materials can be synthesized in aqueous or nonaqueous solutions. Water, for example, is one of most common solvents. There are three general classes of aqueous reactions: acid/base reaction, precipitation, and reduction/oxidation (redox). The reactants may be solids, liquids, or gases in any combination, in the form of single elements or multi-component compounds. A multi-element compound often acts as precursor where the components of the nal product are in a mixture with atomicscale mixing. Many precursors may be prepared by precipitation. In precipitation, the mixing of two or more reactant solutions leads to formation of insoluble precipitate or a gelatinous precipitate. Caution and care must always be taken in handling reactants and precursors, reaction by-products and post-reaction wastes, particularly when complex and hazardous chemicals are involved. Special procedures may be required to remove any impurities from the products and to avoid postsynthesis contamination. Although many laboratory-scale reactions may be scaled up economically to produce large quantities of materials, the laboratory-scale reaction parameters are not necessarily linearly related to those of large-scale reactions. Parameters such as temperature, pH, reactant concentration, and time ideally should be correlated with factors such as supersaturation, nucleation and growth rates, surface energy, and diffusion coefcients, in order to ensure the reproducibility of reactions. Chemistry is based on the manipulation of atoms and molecules, and indeed has a very long history in the synthesis of materials comprising nanostructures. The elds of colloids and catalysts are such examples. The recent popularity of nanoscience not only revitalized the use of many old chemical methods, but also motivated many new and modied ones to be developed for the synthesis of nanostructured materials. The wide scope of chemical synthesis and processing of nanostructured materials spans structural, optical, electronic, magnetic, biological, catalytic, and biomedical materials. A comprehensive review of every aspect of this eld is not possible in this chapter. The previous overview chapter[15] has been revised here and updated, with addition of new topics. Some of the topics covered in the earlier review are not addressed here. The current overview highlights current advances in chemical synthesis of nanostructured particles and lms. Selected examples mainly are metals, ceramics, and hybrid materials with novel magnetic and optical properties for highdensity magnetic data storage and biomedical applications. The synthesis of monodisperse nanoparticles is emphasized due to the importance of

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size-dependent properties and feasibility of particle organization to form two-dimensional and three-dimensional superlattices. For other materials, interested readers are encouraged to consult the chapter in the previous edition.[15] This chapter is organized according to the class of material and type of synthetic approach. However, due to the fact that many advanced materials are hybrid and are prepared using multidisciplinary techniques, a clear distinction is not always possible. Cited references,[114] archival journals,[16] and latest conference proceedings may be consulted for further details.

1.2 Particles 1.2.1 Nucleation and GrowthIn a solution or mixture, chemical reagents or precursors react to form stable nuclei followed by the growth of particles. Reactants can be solids or liquids and sometimes gases. Aqueous or nonaqueous solvents are used. Precipitation of solids in solution has been well studied.[17,18] For coprecipitation of multicomponent particles, attention is required to control the conditions to achieve chemical homogeneity of the nal product. Different ions may precipitate under different conditions of pH and temperatures with different solubility product constants. After a reagent such as a reducing or oxidizing agent is added to the reactant solution or mixture, chemical reactions occur and the solution becomes supersaturated with the product. The supersaturation drives the chemical system to deviate from the minimum free energy conguration. The state of thermodynamic equilibrium is restored by condensation of nuclei of the reaction product. Homogeneous nucleation does not involve foreign species as nucleating aids. Heterogeneous nucleation however allows formation of nuclei on foreign species. Kinetic factors compete with the thermodynamics of the system in a growth process.[19] Factors such as reaction rates, transport rates of reactants, accommodation, removal, and redistribution of matter compete with inuences of thermodynamics in particle growth. The reaction and transport rates are affected by concentration of reactants, temperature, pH, and order of introduction of and degree of mixing of reagents. The structure and crystallinity of particles may be inuenced by reaction rates and impurities. Factors such as supersaturation, nucleation and growth rates, colloidal stability, and recrystallization and aging processes have effects on the particle size and microstructure. Supersaturation generally shows predominant inuence on the morphology of precipitates. At low

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supersaturation, the particles are small, compact, and well-formed, and the shape depends on crystal structure and surface energies. At high supersaturation, large and dendritic particles form. At even higher supersaturation, smaller but compacted, agglomerated particles form.[18] The interface-controlled growth of a small particle in solution becomes diffusion-controlled after the particle exceeds a critical size.[20]

1.2.2 Dispersion and AgglomerationIf the formation of all nuclei occurs at nearly the same time in a supersaturated solution, subsequent growth of the nuclei results in formation of particles with a very narrow size distribution, provided that subsequent secondary nucleation does not occur.[21] Homogeneous nucleation as a single event requires the use of proper concentrations of reagents. Foreign nuclei should be removed before reaction to prevent heterogeneous nucleation that may otherwise result in a wide size distribution of particles. A narrow size distribution may be maintained as long as agglomeration and Ostwald ripening of particles do not occur in solution. The synthesis of stable colloids and dispersion of agglomerated particles have been extensively investigated.[22] Colloids and sols refer to the dispersion of particles (with particle sizes less than 100 nm) within a continuous uid matrix. The small particles approach and then separate from each other by Brownian motion, and as a result, settling out of particles from solution does not occur. Note that random agglomeration between particles may still occur by Brownian motion. Agglomerates of small particles or particles with size larger than 100 nm tend to settle out of solution. In aqueous solvents, particles with a surface oxide layer or a hydrated surface may become charged under appropriate conditions. Electrostatic repulsion, with a force proportional to the inverse of second power of separation distance, occurs between two particles carrying the same charge. The attractive van der Waals force is proportional to the inverse of the distance with an exponent of 36. The net attractive or repulsive force between the particles in such a suspension is the sum of the electrostatic repulsion and the attractive van der Waals forces. The DLVO theory (Derjaguin, Landau, Verwey, and Overbeek) describes the effects of attraction and repulsion of particles as a function of separation distance.[23] On the DLVO plot of potential energy versus the separation distance of particles, there exists a positive potential energy peak, which separates the negative potential energy of the primary minimum and secondary minimum. The height of the potential energy peak must be 25 millivolts

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(corresponding to the thermal energy of Brownian motion at 20C) at ambient conditions, in order for a dispersion of particles to remain stable. In an appropriate solvent, an electric double layer is formed surrounding the particle. The stable distance of particle separation depends not only on the charges of particles, but also the concentration of other ions in the diffuse region of the double layer. When there is a sufcient number of such ions or ions with multiple charges in the diffuse layer, the charge repulsion will be neutralized. The collapse of the double layer leads to particle contacts and agglomeration.[23] Nanostructured particles possess large surface areas and often form agglomerates as a result of attractive van der Waals forces and the tendency of the system to minimize the total surface or interfacial energy. Coagulation refers to the formation of strong, compact aggregates (corresponding to the primary minimum on the DLVO plot of potential energy versus particle separation), and occulation refers to the formation of a loose network of particles (corresponding to the secondary minimum on the DLVO plot). Agglomeration of particles may occur during any of the following stages: synthesis, drying, handling, or subsequent processing. In many applications and processing where dispersed particles or stabilized dispersions are required, undesirable agglomeration in each synthesis and processing step must be prevented. To produce unagglomerated particles, surfactants may be used to control the dispersion during chemical synthesis, or disperse as-synthesized agglomerated ne particles. A surfactant, usually an organic compound, lowers the surface or interfacial tension of the medium in which it is dissolved. A surfactant is a surface-active agent that need not be completely soluble and may decrease surface or interfacial tension by spreading over the surface. It has an amphipathic structure in that solvent, i.e., a lyophobic (solvent repulsive) and lyophilic group (solvent attractive). Surfactants are classied as anionic, cationic, zwitterionic (bearing both positive and negative charges), or non-ionic (bearing no charges). The effectiveness of a surfactant is measured by the maximum reduction in surface or interfacial tension by the surfactant, whereas, surfactant efciency refers to the surfactant concentration that is needed to reduce the surface or interfacial tension by a certain amount from that of the pure solvents. For example, water and oil may be dispersed in each other provided a suitable surfactant is used to stabilize the microemulsion. The surfactant establishes itself at and denes the boundary between the two liquids. The relative quantity of a surfactant determines the amount of surface that can be covered and, therefore, the extent to which the size and number of droplets of one liquid is dispersed in the other. When the major component is apolar (oil),

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the dispersion is one in which the water (polar) phase forms the droplets or reverse micelles. The polar head group of the surfactant is pointing inward toward the water phase while the hydrocarbon tail is pointing outward into the oil phase. The radius of the water droplet is related to the amount of water and surfactant. Repulsive interparticle forces are needed to prevent particle agglomeration during synthesis. A common method is to disperse the particles by electrostatic repulsion resulting from interactions between the electric double layers surrounding the particles. This may be achieved by adjusting the pH of the solution or adsorbing charged surfactant molecules on the particle surfaces. Such stabilization is generally effective in dilute systems of aqueous or polar organic media, and is very sensitive to the pH and effects of other electrolytes in the solution. At the isoelectric point, the pH where the particles have no net surface charges, agglomeration may occur. The isoelectric point varies for different materials. In most nonaqueous solvents without signicant ionization, electrostatic repulsion has a lesser contribution to stabilization of particles. Another approach to dispersion involves the steric forces produced by adsorbed surfactant on particle surfaces. The lyophilic, nonpolar chains of surfactant molecules extend into the solvent and interact with each other. The interactions between nonpolar chains are subject to much less van der Waals attraction and provide a steric hindrance to interparticle approach. To optimize steric stabilization, the size of surfactant molecules must be large enough to be a barrier without entangling each other. When the particles approach one another, the stretched-out lyophilic chains of the adsorbed surfactant are forced into a smaller spatial connement. This interaction leads to a thermodynamically unfavorable decrease of the entropy of the system, thus, the particles will be prevented from approaching each other by this entropic repulsion. Entropic stabilization becomes even more signicant when the temperature of the dispersion is increased. Steric stabilization may occur in the absence of electric barriers and is effective in both aqueous and nonaqueous media. It is also less sensitive to impurities or trace additives than electrostatic stabilization and is particularly effective in dispersing high concentrations of particles. Dry, high-surface-area powders agglomerate by van der Waals forces and hydrogen bonds. When these agglomerates need to be used in a dispersed form during subsequent processing, deagglomeration can be achieved by breaking the agglomerates using methods such as milling or ultrasonication in an appropriate solvent containing a suitable surfactant for dispersion.[22] The deagglomerated powders may then be carried in a

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liquid for further processing such as injection molding and polymer-based casting. Though surfactants may be used to stabilize particles against agglomeration, the presence in the nal product may adversely affect the properties of the materials. Subsequent removal of surfactants from the particle surface could result in undesirable agglomeration and particle growth under certain conditions. Careful consideration is warranted in optimizing the properties and control of particle dispersion.

1.2.3 MetalsInterest in nanostructured metals and semiconductors arises from their unique electronic and physical properties and diverse high-tech applications including high-density magnetic recording, catalysis, pharmaceuticals, and medical diagnosis. Many references are available in the literature.[24,25,26] Here, emphasis is given to recent advances in this area. Aqueous Methods. Water has a high permittivity which makes it a good solvent for polar or ionic compounds. Therefore, many chemical reactions occur in aqueous media. Precious, elemental metal nanoparticles for catalytic and biomedical applications, such as Au, Ag, Pt, and Pd nanoparticles may be prepared by adding liquid reducing agents to aqueous solutions of respective salts in the presence of a stabilizer. The choice of reducing agents may drastically affect the nucleation rate and particle growth, which in turn inuences the particle size and size distribution. Commonly used reducing agents include sodium borohydride, hydrazine, sodium citrate, and alcohols.[25,27] For example, crystalline Ag nanowires were prepared by reducing AgNO3 with sodium citrate in the presence of NaOH at 100C. The quantity of NaOH was an important factor in determining the morphology of the nal product.[28] Sizecontrolled synthesis of chemically clean Ag nanoparticles was also reported using reduction of silver oxide by hydrogen gas in water. Particles with a diameter between 15 and 200 nm were synthesized by varying the reaction time. The advantages of this method are the easy scale-up for production of naked particles with long-term stability.[29] However, the reaction must be handled with great caution because hydrogen gas is explosive when mixed with air in concentrations larger than 4%. The particle size and morphology of metal nanoparticle may be controlled by choosing suitable capping agents and varying the ratio

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Processing

of concentrations of capping agent to metal salts. For example, monodisperse silver nanocubes with a mean edge length of 55 5 nm were synthesized in water by a modied silver mirror reaction at 120C. nHexadecyltrimethylammonium bromide (HTAB), as an ionic surfactant, played a key role in the formation of the nanocubes. Its micelles directed the silver metal to nucleate and grow into nanoparticles other than the usual silver mirror. An increase in the molar ratio of HTAB/[Ag(NH3)2]+ led to an obvious shape evolution of Ag nanoparticles from spheres to cubes, due to the anisotropic adsorption (molar ratio dependent) of the surfactant on the silver crystal faces.[30] Although the aqueous approach to making metal powders is not new, its use in the synthesis of metal nanoparticles requires special attention to avoid undesirable contaminated products. Impurities such as salts and other reaction by-products may not be completely removed, even by repeated washing procedures, if they are entrapped inside the particles or agglomerates during a fast and ill-controlled reaction. Because of the high reactivity of metal nanoparticles due to large surface area, special care must be taken during washing and ltering of the nanoparticles to avoid undesirable hydrolysis or oxidation. Subsequent drying often requires vacuum-assisted procedures to avoid oxidation. Polyol Method. The polyol method has been used to make nely dispersed single elemental metal particles such as Cu, Ni, Co, and others in the micron and submicron size range.[3132,33,34] In this method, precursor compounds such as oxides, nitrates, and acetates are either dissolved or suspended in ethylene glycol, diethylene glycol, or 1,2-propanediol. The polyol acts as both solvent and reducing agent. The mixture is heated to reux between 160C and 194C. During the reaction, the precursors are reduced and metal particles precipitate out of solution. Submicron size particles may be synthesized by increasing the reaction temperature or inducing heterogeneous nucleation via the addition of foreign nuclei or forming foreign nuclei in-situ. A higher temperature favors the nucleation step and this, in turn, favors the monodispersity of particles when more nuclei are formed. Nanocrystalline particles such as Ni, Co, Pt, Ag, Au, CoNi, and FeNi have been recently synthesized using this method.[3536,37,38,39,40,41,42] The morphology of nanostructured metal synthesized by the polyol method may be controlled by choosing appropriate capping agents. Facetselective capping agents promote the abundance of a particular shape by selectively interacting with a specic crystallographic facet via chemical adsorption. A typical example is the shape-controlled polyol synthesis of Ag nanoparticles. The primary reaction involves the reduction of AgNO3

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with ethylene glycol in the presence of a capping agent poly(vinyl pyrrolidone) (PVP) at 160C. The morphology and dimensions of Ag are found to strongly depend on reaction conditions such as temperature, the concentration of AgNO3, and the molar ratio between the repeating unit of PVP and AgNO3. When the temperature is reduced to 120C or increased to 190C, the product is dominated by nanoparticles with irregular shapes. To obtain Ag nanocubes, the initial concentration of AgNO3 has to be higher than 0.1 M, otherwise Ag nanowires are the major product. Silver nanocubes of various dimensions can be obtained by controlling the growth time.[36,43] Moreover, the addition of a small amount of hydrochloric acid improves the monodispersity and shape perfection of synthesized Ag nanocubes. It is suggested that hydrochloric acid plays an important role in selectively etching and dissolving twinned silver nanoparticles that form at the earlier stages of reaction. The presence of protons further slows down the reduction reaction and thereby facilitates the formation of single-crystal seeds.[44] Nanostructured metal with different morphologies may also be synthesized by controlling the reduction kinetics in the polyol process. For example, morphological control over Pt nanoparticles is realized by varying the amount of NaNO3 added to a polyol process, where H2PtCl6 is reduced by ethylene glycol to form PtCl42 and Pt at 160C. As the molar ratio between NaNO3 and H2PtCl6 is increased from 0 to 11, the morphology of Pt nanoparticles evolves from irregular spheroids with rounded proles to tetrahedra and octahedra with well-dened facets. It is proposed that nitrate is reduced to nitrite by PtCl42 in the early stage of the synthesis, and the nitrite can then form stable complexes with both Pt(II) and Pt(IV) species. As a result, the rate of reduction of Pt precursors by ethylene glycol is signicantly reduced. The change in reaction kinetics alters the growth rates associated with different crystallographic directions of the Pt nanocrystals and ultimately leads to formation of different morphologies.[39] The presence of a trace amount of Fe species (FeCl2 or FeCl3) in a polyol synthesis can also alter the growth kinetics of Pt nanostructures and hence morphology. Depending on the way the Fe species and oxygen (from air) are supplied to the reaction system, Pt nanostructures in the form of spheres, star-shaped particles, branched multipods, and nanowires are prepared as the major product for each run of synthesis.[40] Metal nanoparticles have also been synthesized using a modied polyol process. The modication includes addition of other solvents and sodium hydroxide. In the synthesis of monodisperse Co nanoparticles, cobalt acetate tetrahydrate, oleic acid, and diphenylether (DPE) are mixed and

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heated to 200C under N2. Trioctylphosphine (TOP) is added at 200C, and the mixture is heated to 250C. When a solution of 1,2-dodecanediol in DPE is injected into the mixture at 250C, the color of the reaction solution changes from blue to black, indicating the formation of Co nanoparticles. After size-selective precipitation, monodisperse Co nanoparticles are obtained. The particle size of Co nanoparticles can be controlled by tailoring the concentration or composition of stabilizers. For example, increasing the concentration of oleic acid and TOP by a factor of 2 yields nanoparticles having average diameters between 3 and 6 nm, while substituting tributylphosphine (TBP) for TOP increases the average size to 1013 nm. Substituting cobalt acetate tetrahydrate with nickel acetate tetrahydrate, using oleic acid, TBP and tributylamine (TBA) as stabilizers, Ni nanoparticles with average diameters ranging from 8 to 13 nm are synthesized. When a mixture of cobalt acetate tetrahydrate and nickel acetate tetrahydrate is used, Co/Ni alloy nanoparticles form.[45] Monodisperse FePt nanoparticles with an average particle size of 3 nm are synthesized by heating a solution of platinum(II) acetylacetonate, iron(II) acetylacetonate, 1,2-hexanedecanediol, oleic acid, and oleylamine in octylether solution at 286C for 30 min.[46] Compared with commonly used ethylene glycol or glycerol, using long-chain 1,2-diols may prevent the formation of insoluble particles because the precursors and generated nanoparticles are well dispersed during reaction. In the preparation of NiFe nanoparticles in ethylene glycol (EG) (Fig. 1.1), strong alkaline solution is essential for disproportionation of Fe. The addition of NaOH results in a competition between the formation of metal hydroxide and that of metal EG complex. In the Figure 1.1 A TEM micrograph of presence of NaOH, the metal FeNi nanoparticles synthesized by the polyol method. Reprinted with hydroxide formation is predominpermission from [47]. Yin, H., and ant with only a small amount of Chow, G. M., Electroless Polyol complex precipitated from solution. Deposition of FeNi-based Powders The formation of Ni(OH)2 inhibited and Films, J. Mater. Res., the reduction of Ni(II) to Ni, 18:180187 (2003). 2003, Materials Research Society. because Ni(OH)2 has a more nega-

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tive reduction potential than Ni(II). On the other hand, the formation of Fe(OH)2 leads to disproportionation of Fe(II), which is an alternative way to synthesize Fe. This disproportionation can signicantly increase the yield of Fe. The composition of NiFe particles is independent of reaction time. However, the particles with reaction time of 60 min show small size with narrow size distribution compared to particles obtained in shorter or longer time. It was suggested that the formation of a metalEG complex acts as a reservoir of solute and prevents further metal particle nucleation during the growth process.[35,47] Compared to aqueous methods, the polyol approach results in the synthesis of metal nanoparticles protected by surface-adsorbed glycol, thus minimizing the problem of oxidation. The use of a nonaqueous solvent such as the polyol also further reduces the problem of hydrolysis of ne metal particles that often occurrs in the aqueous case. Sonochemical Methods. Ultrasound has been used in chemical synthesis of nanostructured materials. High energy sonochemical reactions, without any molecular coupling of the ultrasound with the chemical species, are driven by the formation, growth, and collapse of bubbles in a liquid. This acoustic cavitation involves a localized hot spot of temperature of about 5000 K, a pressure of 1800 atm and a subsequent cooling rate of about 109 K/s, due to the implosive collapse of a bubble in the liquid.[48] Volatile precursors in low-vapor-pressure solvents are used to optimize the yield. Ultrasonic irradiation is carried out with an ultrasound probe, such as a titanium horn operating at 20 kHz. When water is sonicated, the very high temperatures and pressures of collapsing gas bubbles lead to thermal dissociation of water vapor into *OH and H* radicals.[49] In the presence of a primary alcohol (RCH2 OH; R==H or alkyl group), the formation of RCHOH* radicals during sonication was proposed. Metal nanoparticles may be prepared by reducing prospective salts with the reducing species H* and RCHOH* radicals. For example, Au nanoparticles in the size range of 925 nm are synthesized by reduction of tetrachloroauric(III) acid in the presence of aliphatic alcohols and sodium dodecyl sulfate in aqueous solutions using 20 kHz ultrasound. The temperature of the reaction solution was maintained at 20 5C. The extent of reduction of AuCl4 is found to be dependent on the concentration of the alcohol at the bubblewater interface. The particle size can be tailored by varying the alcohol concentration in solution and the hydrophobicity of the alcohol. The particle size decreases with increasing alcohol concentration and alkyl chain length. It is suggested that as alcohol is adsorbed onto Au in aqueous solution, the adsorbed molecules stabilize the particles at a smaller size and prevent further growth of the

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colloids. The amount of alcohol adsorbed depends on the alcohol used and its concentration.[50] Other noble-metal nanoparticles with a narrow size distribution, such as Pt and Pd, are prepared by sonochemical reduction of aqueous solution containing H2PtCl6 or K2PdCl4 in the presence of PVP as a capping agent.[51] Precursor Methods. The conventional approach to preparing alloys and composites is to rst grind and mix the solid precursors using some mechanical means, and then carry out appropriate chemical reactions to obtain nal products. The communition and mixing in the solid state are generally limited to the submicron level. Consequently, material diffusion in chemical synthesis is limited to this spatial scale, which has a direct inuence on the time and temperature of reactions and the nal chemical homogeneity of the product. With great efforts such as high energy milling, solid state mixing of constituents at the atomic scale is possible. If the precursors are mixed at the atomic or molecular level, the synthetic reactions may then be carried out at shorter times and reduced temperatures due to the shorter distance for material diffusion. Intimate contact of constituents at the atomic scale also provides a better means to control the stoichiometry and homogeneity of the nal product. These advantages motivate the synthesis of precursor materials which have the constituents as atomic neighbors (for example, as in a compound). These precursors are subsequently subjected to thermochemical reactions to synthesize alloys and composites with improved properties compared to the same materials obtained by traditional solid-state reactions. Organometallic Methods. An organometallic compound is an organic compound containing a metal, in which a metal atom is bonded directly to a carbon atom. Organometallic compounds are advantageous chemical precursors since the constituents, in molecular proximity to each other, may be decomposed at relatively low temperatures to form the nal product desired. The biggest disadvantage of this approach is that most of the reactions involve air-sensitive reactants, therefore, a glove box or schlenck line technique must be used. Because of the air-sensitive nature of some of the reactants, greater care must also be taken in preparation of solvents and the choice of atmospheres. Generally, organometallic routes produce only small amounts of material. There has been a renewed interest in the synthesis of monodisperse magnetic nanoparticles with unique properties such as high magnetization. Monodisperse Co nanoparticles are prepared by pyrolysis of Co2(CO)8 in diphenylether (DPE). An as-prepared solution of Co2(CO)8 in DPE is injected into a hot solution containing DPE and surfactants

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(oleic acid and tributylphosphine) at 200C under N2. Thermal decomposition of Co2(CO)8 results in nucleation of Co and release of CO gas. The reaction solution was held at 200C for 1520 min, allowing the nanoparticles to grow. The solution was cooled, and the nanoparticles were isolated by size-selective precipitation. Generally, a higher temperature and larger ratio of metal-precursor to surfactants produces larger nanoparticles. Fe nanoparticles are synthesized by replacing Co2(CO)8 with Fe(CO)5 in such a procedure at 250C.[45] Face centered tetragonal (fct) FePt alloys have potential application in high-density magnetic information storage due to the large uniaxial magnetocrystalline anisotropy and good chemical stability. The chemical synthesis strategy combines reduction of platinum acetylacetonate by 1,2-hexadecanediol and thermal decomposition of iron pentacarbonyl. Both chemical reactions are initiated by reuxing a mixture containing metal precursors, dioctylether, oleic acid and oleylamine at 300C for 30 min under airtight conditions. The composition of the resulting alloys can be adjusted by controlling the molar ratio of Fe(CO)5 to platinum salt. For example, a 3 : 2 molar ratio of iron pentacarbonyl to platinum acetylacetonate gives Fe48Fe52, and a 2 : 1 molar ratio produces Fe52Pt48. The particle size of FePt alloys can be tailored to be from 3 to 10 nm in diameter with a standard deviation of less than 5%. This involves rst growing 3 nm seed particles in situ followed by adding more reagents to allow the existing seeds to grow to the desired size.[52] However, the assynthesized FePt nanoparticles have a face centered cubic (fcc) structure and are superparamagnetic. To obtain the ordered fct phase (the so-called Ll0 structure), the as-synthesized nanoparticles typically have to be heated to 550C. Heat treatment at these temperatures leads to undesirable agglomeration of particles and a dramatic increase in particle size. Direct synthesis of fct FePt nanoparticles is reported using Collmans reagent, Na2Fe(CO)4, as a reducing agent for Pt(II). In this method, a 1 : 1 molar ratio of platinum acetylacetonate to Na2Fe(CO)4, and a surfactant oleylamine are sonicated and then reuxed in tetracosane at 389C under an inert atmosphere. Magnetic measurements of samples produced directly in solution show a coercivity of 1300 Oe at 290 K and 3100 Oe at 10 K.[53] Monodisperse CoPt3 nanocrystals are synthesized via simultaneous reduction of platinum acetylacetonate and thermodecomposition of cobalt carbonyl in different coordinating mixtures in the presence of 1-adamantanecarboxylic acid. The average particle size can be varied from 1.5 to 7.2 nm by controlling reaction conditions and types of coordinating mixture. As-synthesized CoPt3 particles are single crystal, however, with chemically disordered fcc structure.[54]

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1.2.4 CeramicsChemical methods such as precipitation[12,55] and solgel processing can be used to synthesize ceramic nanoparticles. As-synthesized powders, depending on the synthesis technique used, may require subsequent heat treatment for dehydration, removal of organics, and controlled crystallization to form oxides with desirable structure and crystallite size. Hydrolysis. Precipitation from solution generally involves formation of an insoluble hydroxide which can then be converted to its oxide by heat-assisted dehydration. Metal (hydrous) oxide particles are synthesized by forced hydrolysis involving controlled deprotonation of hydrated cations. For example, by heating suitable metal salts with a dened amount of water in diethylene glycol (DEG), various metal oxides such as CoO, SiO2, TiO2, Fe2O3, ZnO, and Nb2O5 nanoparticles with a mean particle diameter between 30 and 200 nm are obtained. The concentrations of the metal precursor and water are important in controlling the particle size.[5960,61] Water in the metal hydrates is used as an alternative water source for the polyol-mediated preparation. Transition metal ferrite nanoparticles are synthesized by heating transition metal hydrates in DEG in the presence of alkaline hydroxide under an argon atmosphere. Complexation of the transition metal cations with DEG in the presence of alkaline hydroxide enables control over the rate of their hydrolysis. The growth of nanoparticles can be terminated by adding long-chain carboxylic acid, which binds to their surface as a capping ligand.[62,63] Magnetite nanoparticles can also be synthesized by hydrolysis of hydrated ferric salt in 2-pyrollidone at the boiling temperature of 2-pyrollidone under a nitrogen atmosphere. It is proposed that thermal decomposition of 2-pyrrolidone results in carbon monoxide and azetidine. Azetidine catalyzes the hydrolysis of FeCl36H2O to form ferric oxide hydroxide (FeOOH). The FeOOH is then partially reduced by CO and dehydrates to form Fe3O4.[64] Organometallic Methods. A complete separation of nucleation from growth is crucial for synthesis of monodisperse nanoparticles. A general scheme for pr