Chemistry of Advanced Materials - ACS Symposium Series (ACS

Chapter 1

Chemistry of Advanced Materials Dieter M. Gruen

Downloaded by GEORGE MASON UNIV on July 3, 2014 | http://pubs.acs.org Publication Date: December 15, 1997 | doi: 10.1021/bk-1998-0681.ch001

Materials Science and Chemistry Divisions, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, IL 60439

With the end of the Cold War and the increasing globalization of the U.S. economy, R & D expenditures must be increasingly justified based on future economic benefits. In the case of materials science, this justification is not difficult to make, since the results of these efforts are often closely linked on a fairly short time scale to important new industrial processes and products. In large measure, this is due to the fact that the hallmark of the discipline is its dependence on a mixture of basic and applied research and on interdisciplinary borrowing. In the organization of this volume, subject areas of current technological interest were chosen to provide an understanding of the important roles of material scientists, in general, and materials chemists, in particular, in the synthesis and characterization of advanced materials. The U.S. economy is experiencing fundamental structural change in the wake of the Cold War and the increasing globalization of economic activity. These changes are having far-reaching effects on the nation's science and technology enterprise. The complex issues associated with the role of technological innovation in economic growth and change need to be better understood and addressed. One of the factors complicating this task is that we can no longer justify large expenditures in scientific research in Cold War terms. From the point of view of the business community, these expenditures rarely — i f ever — yield returns that reflect favorably in a corporation's near term earnings report. In the longer term, substantial profits may be made but, by that time, the scientific discovery that gave rise to the new business opportunity -- be it the transistor, the diode laser, carbon fibers, or nylon - is lost sight of. The connection between the discovery and the profit derived therefrom has been lost and cannot be forged anew in a convincing way. That is the dilemma in which the scientific community finds itself today. Materials scientists and materials chemists, in particular, understand very well the role they play in inventing new technologies from which future economic benefits will be 2

© 1998 American Chemical Society

In Synthesis and Characterization of Advanced Materials; Serio, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1997.


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derived. They also know very well that, without sufficient investments in skills, research and development, infrastructure, plants, and equipment, the whole edifice of our complex society will be undermined. The scientific community can only hope that policy makers and society as a whole will come to recognize that support of the scientific enterprise is absolutely crucial to continued economic prosperity. Of course, scientists can help shape the course of events by communicating to others their deeply held convictions. The materials chemistry community can argue strongly for this point of view because the results of that scientific effort are often closely and demonstrably linked on a fairly short time scale to important new industrial processes and products. In large measure, this is due to the fact that the hallmark of the discipline is its dependence on a mixture of basic and applied research and on interdisciplinary borrowing. These characteristics -- coupled with intense personal motivation - lead to high levels of scientific innovation. Materials chemistry applies the insights of chemical thermodynamics, kinetics, and quantum mechanics to problems in material science. In the organization of this volume, it was felt that subject areas of great current technological interest should be chosen which illustrate the important contributions of materials chemists. The subjects covered include the synthesis and characterization of thin films, Group III nitrides, fullerenes and other carbon materials, ceramics, catalysts, and polymeric materials, covering a broad spectrum of interdisciplinary activities. Rather than in bulk form, more and more materials are finding wide-ranging uses as films varying in thickness from a few nanometers to many microns. Quantum well, multi-layer and thin film composites with diamond-like hardness are just a few of the exciting "atomically engineered" materials that are transforming the approaches to modern materials science. Materials chemistry is fulfilling an important function by moving many of these areas closer to practicality through improvements in synthesis processes and in the performance characteristics of advanced materials. For example, although the story of carbon is as old as mankind itself, it is not fully told, and new chapters -- sometimes new volumes -- are written from time to time. We appear to be in a period of a rapidly evolving carbon science right now. Not only is diamond film growth by chemical vapor deposition (CVD) an active area of research and development, but the discovery of fullerenes -- the third allotrope of carbon -- has opened up a whole new field of science with vast potential for both basic knowledge and applications. The paper that follows, authored by Professor Sir Harold W. Kroto, discusses the profound implications of the discovery of C on the way we think about the structure of graphite and other layered materials. Because an important branch of chemistry -- organic chemistry — deals with carbon and its compounds, carbon as a material quite naturally arouses the interests of chemists, particularly of materials chemists. There can be no doubt that the discovery of the fullerenes by Kroto, Smalley and Curl, and their collaborators at Rice University in 1985, was one of the great moments in carbon chemistry. The recipe given us by Kratschmer and Hoffman in 1990 for the synthesis of macro amounts of C has given rise not only to the new 6 0

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science of fullerene chemistry, but also to an intensive search for other new forms of carbon. Carbon nanotubes and carbon onions are examples of forms of carbon that owe their existence to the curiosity caused by C . The vast, and hitherto unknown, ways in which carbon atoms can arrange and rearrange themselves in space continues as a subject of intense interest in the scientific community, along with the possibilities for using these materials in novel applications. One of the outstanding problems confronting the large-scale utilization of fullerenes is the cost associated with present-day methods of production based largely on carbon arc techniques. New — and hopefully more cost-effective — approaches to the synthesis of the fullerenes will have to be found. Those currently being studied range from the use of solar rather than electric energy to the use of premixed hydrocarbon/oxygen flames to the pyrolysis of hydrocarbon precursors and plasmaenhanced C V D methods. The wonder is that, once the fullerenes were shown to exist, they now show up everywhere, even in nature. The synthesis of the fascinating boronand nitrogen-doped fullerenes, as well as of endohedral metal-doped fullerenes has been accomplished. C can also serve as a "combinatorial pincushion" for the efficient synthesis of new drugs. Research on the Group III nitrides has seen a spectacular increase in interest in the last few years as a result of new uses that have been discovered for these materials. Interest has centered on cubic boron nitride for surface hardness, on A IN for its thermal conductivity, and on GaN as a wide bandgap semiconductor. Another forefront area in materials science, to which materials chemistry is making important contributions, is research on oxides, nitrides, and carbides. Although known to inorganic and physical chemists for many decades, these materials are coming into their own in high-technology applications. Such applications depend critically on one's ability to tailor structure and properties in such a way as to optimize the interplay between form and function. A very important use of these materials is as catalysts. Special structural features of metal vanadomolybdates, for example, enhance their function as selective oxidative dehydrogenation catalysts. Transition metal carbides and nitride catalysts are of interest because of their resistance to poisoning. It is important to develop new methodologies for preparing catalysts with better activity, selectivity, and long term stability. Catalysis was one of the first technological areas that required close cooperation between materials chemists and other materials scientists. A particularly challenging emerging area is that of nanoparticles and particularly the synthesis of nanoscale oxide powders. Many properties — mechanical, electrical, and optical — of nanoscale materials are very different from polycrystalline materials composed of micron-sized crystallites. The reason is that the surface free energy becomes important, even in determining phase stability. Several percent of the atoms making up the material reside at the grain boundaries with profound consequences for material properties. Again, as in the case of the fullerenes, in order to find applications, cost-effective methods of production must be found. The insights of materials chemists are therefore vitally important in devising new, more efficient methods of synthesis. These methods, in addition to being more efficient, should also


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enable one to have better control over the nanaocrystallinity, and hence over the properties of the material. Nucleation and formation mechanisms, of course, form the basis for understanding the synthesis of any nanocrystallite, whether produced at relatively low temperatures from solutions or at higher temperatures, as in flames. Efficient solvents and process conditions have been developed for the synthesis of ceramic powders, such as alpha aluminum oxide, at sizes ranging from tens of microns to tens of nanometers. The synthesis of nano powders such as β''-alumina, S i 0 and T i 0 by flame spray, bulk pyrolysis, and combustion spray techniques has been accomplished by invoking new methodologies which draw heavily on the insights most familiar and available to materials chemists. A l l of these high temperature methodologies will profit from improved spectrometry diagnostic capabilities, which are now available. A discussion of the chemistry of advanced materials would be incomplete without the inclusion of polymeric materials. This is a classic illustration of an area where materials chemists can make important contributions to materials science because of the preeminent role of chemistry in the polymerization step. In some cases, polymers are used as precursors for other types of advanced materials. The synthesis of aluminosilicates from alkoxide precurors is a case in point. To accomplish this task requires, first of all, the synthesis and characterization of the polymer alkoxide precursors. In most cases, a polymeric material is the end result, as with the synthesis of high temperature polymers (e.g., thermosets) The interdisciplinary nature of materials science and the central role of chemistry has been clearly demonstrated in this volume by bringing together in a cohesive way a wide diversity of topical areas which are of current interest in materials science. It is hoped that volumes of this type will promote an understanding of the important roles of material scientists, in general, and materials chemists, in particular, in the synthesis and characterization of advanced materials.


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Acknowledgments The author gratefully acknowledges the support of this work by the U.S. Department of Energy, BES Materials Sciences, under contract number W-31-109-ENG-38.


Chemistry of Advanced Materials - ACS Symposium Series (ACS

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