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Materials Science and Engineering for the 1990s: Maintaining Competitiveness in the Age of Materials APPENDIXES: ISSUES IN MATERIALS RESEARCH
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Materials Science and Engineering for the 1990s: Maintaining Competitiveness in the Age of Materials This page in the original is blank.
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Materials Science and Engineering for the 1990s: Maintaining Competitiveness in the Age of Materials APPENDIX A Synthesis OVERVIEW The term synthesis, as used in this appendix, refers to that part of synthesis and processing in which attention is focused primarily on the chemical and physical means by which atoms and molecules are combined to produce materials. In this view of synthesis, both the scientific problems of finding new routes to synthesis and the technological problems of finding ways to synthesize materials in useful quantities and forms are included. Thus the term synthesis as used here has a strong component of basic science. The principal findings of the committee concerning the nature of this field and its current status in the United States are the following: Both intellectual opportunities and technological needs provide motivation for innovative research in the synthesis of materials. Synthesis underlies much of the progress in materials science and technology. Advances in synthesis yield new materials having new properties. Advances in synthetic techniques also may be used to enhance the properties of known materials by improving control of structure and composition. The physical phenomena that emerge in new or enhanced materials often lead to new technologies. Thus the health of U.S. technology requires active, aggressive, exploratory programs in materials synthesis. Close interactions between scientists and engineers engaged in materials synthesis, characterization, processing, and, ultimately, manufacturing are highly desirable in order to achieve rapid economic exploitation of research in synthesis. While U.S. excellence in molecular synthesis in areas such as medicine
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Materials Science and Engineering for the 1990s: Maintaining Competitiveness in the Age of Materials and agricultural chemicals is clear, a comparable excellence in the synthesis of materials is lacking. In some areas involving materials synthesis, such as biotechnology, the United States still enjoys a leading position that needs to be preserved and strengthened. In other areas, such as polymer synthesis and solid-state chemistry, however, the United States clearly has fallen behind. Because of weaknesses in materials synthesis, there is growing concern that the United States is losing its dominance in basic materials research. In the past, U.S. scientists have been able to make up for weakness in synthesis by maintaining greater strength in characterization and analysis of physical phenomena. This system is no longer working, however, because foreign scientists with superior capabilities in synthesis are increasingly able to bring their own modern methods of characterization and analysis to bear on the study of new materials. Although many U.S. universities have strong academic programs in chemistry, materials science, and physics, very few have programs in solid-state synthesis, and fewer still have programs in which synthesis is strongly coupled to characterization and analysis. Moreover, academic materials research groups too often specialize in activities that require only narrow educational backgrounds. The decline of basic research in U.S. industry and the growing tendency to focus primarily on short-term product development in industrial laboratories have had an especially adverse effect on national capabilities in materials synthesis. Additional findings of the committee are as follows: New educational experiences for many scientists and engineers will be required if the materials research community is to be able to carry out tightly coupled programs in synthesis, characterization, analysis, and processing. Both undergraduate and graduate academic curricula need to be reexamined with the goal of helping students achieve a level of basic understanding of materials synthesis and materials properties that will allow them to participate in fruitful exchanges of research ideas. Postdoctoral fellowships to encourage interdisciplinary research and, particularly, to encourage involvement in materials synthesis should be provided for talented students currently in doctoral programs in chemistry, physics, and materials science. All of the major providers of support for basic research should participate in funding such fellowships. Basic research in synthesis of materials should be increased in universities. Programs in polymer synthesis and inorganic solid-state chemistry, in particular, seem too few in number, especially in view of the wealth of U.S. talent in physical and organic chemistry. Scientists and engineers should be supported and encouraged to carry out interactive research in materials synthesis coupled with characterization
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Materials Science and Engineering for the 1990s: Maintaining Competitiveness in the Age of Materials and analysis of the properties of novel materials, and also coupled with the processing of materials. Because materials synthesis is intrinsically “small science,” this is an area in which it should be possible to find funding modes that will encourage interactive research while preserving the flexibility and creativity that are associated with projects led by individual principal investigators. The U.S. system of federal and state-supported laboratories provides a framework within which it is possible to develop programs in synthesis that are closely related to technologically significant materials problems. This capability should be exploited more effectively than it has been in the past. In particular, federal laboratories might serve in part as resource facilities for the preparation of special materials to be used in R&D projects. Industrial corporations that depend on materials in order to compete successfully for world markets should recognize the benefit of maintaining in-house capabilities in materials synthesis. The long-term economic advantage of having ready access to superior materials usually justifies investment in research in the synthesis of new materials, the modification of existing materials, and the development of efficient synthetic methods. Present U.S. corporate strategy, however, appears in most cases to favor withdrawal from long-range research, including research in the synthesis of materials. This general decline of industrial research has posed a dilemma for this committee throughout its various attempts to formulate meaningful recommendations, and the difficulty seems particularly severe with regard to research in synthesis. On the one hand, the committee strongly believes that research directly related to the synthesis of commercially useful materials can be performed successfully only in an industrial environment. On the other hand, most of the economic and political causes for the decline of industrial research are well beyond the scope of this report. Accordingly, the committee’s position is simply that focused but flexible efforts, such as those suggested in the summary chapter of this report, should be made to help industrial organizations carry out research in materials synthesis, and that universities and national laboratories should participate in these efforts. THE MEANING OF SYNTHESIS As used in this appendix, the term synthesis refers to the chemical and physical means by which atoms and molecules are combined to form materials. Synthesis is the heart of the science of chemistry. It is the activity in which new materials are produced and in which new paths for the manufacture of materials are invented. It is the major source for the discovery of new chemical and physical phenomena in solids. Synthesis is a crucial component in the development of new technologies and in the improvement of existing technologies.
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Materials Science and Engineering for the 1990s: Maintaining Competitiveness in the Age of Materials Synthesis, processing, and manufacture of solid materials form a continuum of intellectual and technological activities. The boundaries between these three elements of materials science and engineering are becoming increasingly blurred as the need for strong coupling between basic research and applied technology becomes more and more clear. A practical reason for this blurring is the frequent need to choose synthetic reactions in such a way that products are suitable for further processing. For example, in the preparation of ceramics, it is generally desirable that the synthetic reactions produce precursor particles that are small enough to be compacted easily and sintered at relatively low temperatures. Another reason for the blurred distinction is that modern techniques for preparing materials often are direct combinations of what we conventionally have considered to be separate operations of synthesis and processing. A pertinent example here is the reactive injection molding of polymers in which the synthetic chemical reactions occur simultaneously with the “process” of molding. In this appendix, the focus is on activities that seem to the committee to fall primarily in the category of “synthesis.” Discussion of topics that are primarily “processing” is saved for the appendix with that title. The reader should keep in mind that this distinction may sometimes appear to be artificial. HISTORICAL BACKGROUND Most of the major innovations in materials technology of the past two centuries have depended directly on the development of novel synthetic routes to existing or new materials. For example, in the early part of the nineteenth century, metallic aluminum was a curiosity available only in very small quantities. With the new synthetic route from cryolite developed by Hall, aluminum became a major commercial material. The field of synthetic polymers had its beginnings at the turn of the twentieth century with the development by Baekeland of phenol formaldehyde resins. However, the real revolution in this field began in the 1930s with synthesis of thermoplastics such as nylon, polyethylene, and polyester, which could be processed directly into film, fibers, or molded plastics. The plastics revolution, which has influenced every aspect of our lives, was made possible by novel synthetic approaches to both the monomers and the polymers. Thus synthesis not only has led to new and novel materials but also has provided low-cost alternatives that have made products affordable. Rapid growth of the chemical industry in the 1950s and 1960s was so dependent on synthesis of new materials that most chemical companies maintained R&D centers consisting primarily of synthetic chemists. However, starting in the mid-1960s, some profound changes have occurred in U.S. industrial research that have altered sharply the way materials-related industries can respond to competitive challenges or generate new opportunities.
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Materials Science and Engineering for the 1990s: Maintaining Competitiveness in the Age of Materials A number of industrial organizations—the metals industry is a prime example—have cut back on R&D to the point where little or no basic work is pursued. In these cases, the potential for improving antiquated commercial processes through new synthetic routes has become nonexistent. In the area of ceramics, a number of important new directions have emerged in the past 20 years that suggest real opportunities, for example, stabilized zirconia, ceramics derived from sol-gel techniques, new types of refractory fibers, ceramic composites, and, most recently, the high-temperature superconductors. However, for the most part, the traditional ceramics industry does not possess the research capabilities necessary to pursue development of these novel concepts. Some materials-dependent companies in the United States do recognize the long-range value of research in the synthesis of new materials and devote significant resources to such programs. Unfortunately, there are too few examples of this kind to be able to report a positive trend. The history of programs in materials synthesis at U.S. universities, with one or two striking exceptions, parallels the weak effort in industry. Indeed, the lack of industrial job opportunities in this area has been a disincentive for academic programs. Conversely, the lack of academic programs and the resulting national shortage of materials scientists trained in synthesis have made it especially difficult for industrial laboratories to start new research projects in advanced materials. The level of activity in materials synthesis has remained low in academic institutions despite the fact that many of the leading research-oriented universities traditionally have maintained strong efforts in organic molecular synthesis. U.S. academic institutions even have been weak performers in the synthesis of polymeric materials. In the committee’s opinion, there are fewer than 10 innovative polymer synthesis programs in U.S. universities. Synthesis of inorganic solids is in little better shape. In the area of thin-film microelectronic materials formed by chemical vapor deposition, much of the chemistry appears to be done by electrical engineers who are interested primarily in the device characteristics of the materials being produced. While this situation is not necessarily bad, the lack of special expertise in synthetic chemistry leads to dependence on standard chemicals that can be acquired from commercial sources and standard methods of materials preparation. It seems likely that really new approaches to the design of materials systems will require novel synthetic techniques. There are some signs of change in the academic scene, but there is also much inertia in the system that will hinder rapid strengthening of programs in materials synthesis. Part of the difficulty is that people who are creative in synthesis often are not aware of the needs and opportunities for new materials. Further, the culture of chemistry has been such that a synthetic chemist could make new materials, albeit molecular materials, and fully
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Materials Science and Engineering for the 1990s: Maintaining Competitiveness in the Age of Materials characterize them without ever going beyond the boundaries of a conventional chemistry department. The characterization of new solid materials, on the other hand, generally requires a much broader range of instrumental capabilities and scientific expertise. In short, modern materials research is necessarily an interactive enterprise involving many scientific specialties. The fact that synthesis is an absolutely essential part of this picture is the principal message of the next paragraphs. THE ROLES OF SYNTHESIS IN MATERIALS RESEARCH It is useful to distinguish three separate roles that are played by synthesis in materials research: The synthesis of new materials. The synthesis of known materials by new techniques or in novel forms. The synthesis of known materials for special purposes, for example, for use in research or in technological development. The historic weaknesses of U.S. institutions in materials synthesis have caused problems to emerge in connection with each of these roles. These problems will be discussed in the order listed above. Research in the synthesis and characterization of new materials has been demonstrated repeatedly to be the basis for the discovery of new phenomena and for the development of new technologies. Since about 1970, the United States has relied largely on foreign laboratories for the synthesis of new chemical compounds. The common experience was that foreign scientists would publish compositions and structures but would not be very quick to study the properties of novel materials. Subsequent discoveries of new phenomena usually occurred in the United States. Examples of discoveries or technological developments that have taken place in this way include charge-density waves, conducting polymers, high-field superconductors, intercalation compounds, and high-dielectric-constant microwave resonators. This informal arrangement—“they” discover the materials and address questions of the existence and stability of particular structures, and “we” then discover new phenomena and properties—has never really been satisfactory. In the first place, it requires that U.S. scientists be able to prepare the new compounds themselves by following the published recipes. At past levels of funding, even that step was only marginally possible, with much of the best work being done only in industrial laboratories such as AT&T Bell Labs, Du Pont, or IBM. Indeed, many scientists involved in materials research could not obtain samples of desired substances for study and were not able to prepare them themselves. More importantly, foreign scientists are increasingly able to search for
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Materials Science and Engineering for the 1990s: Maintaining Competitiveness in the Age of Materials new phenomena and new properties of materials in their own laboratories—often the same laboratories in which the materials being studied originally were synthesized. In this way, they are often able to capture both the scientific excitement and the technological advantages of their research. The recent discovery of high-temperature superconductors is a case in point. These materials were first synthesized in France, and their superconducting properties were discovered in Switzerland (at an IBM laboratory). Much of the best work in this field is now being carried out in Japan, where scientific leaders were early to recognize the need for strong coupling between synthesis and other areas of materials research, and have acted to meet this need by supporting appropriate programs at universities and at industrial and state-supported laboratories. In each country’s laboratories, synthesis capability and the study of materials properties are linked. The lack of similarly strong programs in the United States will lead to further decline in our scientific leadership and technological capabilities. Consequently, for the future health of all of materials science and engineering, a much stronger focus on the synthesis of new materials seems urgent. New techniques are needed for the synthesis of known materials that are useful either for scientific or for technological purposes but that cannot currently be prepared in the proper form, in adequate quantities, or at reasonable expense. Examples of needed techniques in this category include efficient methods for preparing advanced ceramics and special polymeric materials. In addition to the development of methods to address already known needs, basic investigations of novel techniques need to be supported in order to provide a foundation for future developments. Such investigations might range from fundamental studies of metastable solids to exploration of the effects of various methods of preparation on the stability and lifetime in service of materials used in electronic devices. The motivation for developing new methods for synthesizing known materials is primarily technological, and therefore such research is best performed by industrial organizations that understand their own goals and constraints. As mentioned above, however, U.S. industry has largely abandoned research of this kind. Current trends in federal support for materials science and engineering—note, for example, the National Science Foundation’s rationale for science and technology centers—imply an expectation that some of the necessary effort in technologically important areas such as synthesis might be taken up by research groups at universities. The synthesis of known materials for special scientific or technological purposes is essentially a service function. For example, solid-state physicists and electronic engineers depend strongly on the availability of carefully prepared samples of the materials that they are using. Sample preparation, however, is not a particularly challenging or rewarding activity for research chemists. Nevertheless, such synthesis often requires considerable effort and
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Materials Science and Engineering for the 1990s: Maintaining Competitiveness in the Age of Materials expertise as well as expensive and well-maintained equipment. Research-grade samples generally cannot be purchased commercially because the cost of their preparation seldom can be recovered on a direct-charge basis. Yet the phenomena to be discovered might be scientifically and technologically very exciting. Because there is an obvious need for special materials to be used in R&D, it seems very much in the national interest that some mode be found for the support of laboratories that will perform this kind of synthesis. Perhaps some part of this function could be served by government-related laboratories if funding were available for that purpose. But a satisfactory solution to the problem of availability of research-grade materials can come only as a part of a broader effort to upgrade the status of materials synthesis in all of the nation’s research institutions. NEEDS FOR MATERIALS SYNTHESIS: SOME EXAMPLES In the remaining parts of this appendix, the various roles played by materials synthesis are illustrated by selected examples of needs and opportunities for research in this area. In this section, some needs that arise in commercial and defense-related applications are discussed. In the next section, some opportunities of a more fundamental nature are described. Information Industry The large-scale need for synthesis of new polymeric materials is a relatively recent development in the information industry. Until a few years ago, the only places where polymers played key roles in the manufacture of large mainframe computers were in lithographic processes for patterning metal lines and in the bindings of the particulate disks used for magnetic storage. In the 1980s, this picture has changed to the point where one can predict that future progress will depend on the availability of polymers that do not exist today. These needs extend throughout the entire hierarchy of computer hardware, including semiconductor chips and packages, optical and magnetic recording systems, and printers and displays such as electrophotography, impact printing, ink jet printing, and liquid crystal displays. For illustrative purposes, attention is focused here on the needs for new polymeric synthesis in the packaging of electronic components. An important driving force in packaging technology is the rapid growth in the capacities of RAM chips from 50 kilobits just a few years ago to 1 megabit today and to a projected 64 megabits by the mid-1990s. With higher device densities, problems of wireability and reliability become more critical. A whole new set of materials needs emerges for these advanced structures.
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Materials Science and Engineering for the 1990s: Maintaining Competitiveness in the Age of Materials The present trend is to build more of the circuitry directly into the chips and to package many interconnected chips within single modules, cards, or boards. One can readily translate this trend into specific needs for new materials. Increasing the density of devices on chips requires development of submicron metal lines and an increase in the depth of circuitry from two or three levels to five or six. These developments will require, respectively, new photoresists with strong sensitivities at short wavelengths, and insulating polymers that planarize and have unusually low dielectric constants. Increasing the complexity of the package causes problems of reliability associated with mechanical stresses at the interconnections between the various devices. These stresses arise from the mismatch in thermal expansion coefficients between the silicon of the chip, the alumina of the module, and the epoxy fiberglass in the card or board. Reducing these stresses requires development of polymeric substrates for modules, cards, and boards that better match the thermal expansion coefficient of the silicon chip. Transportation Industry Several needs for synthesis of advanced materials are common to both the aircraft and the automobile industries. For example, lightweight structural materials are needed for fuel efficiency. Significant progress has been made over the past 15 years in the development of lightweight graphite-fiber-reinforced epoxy composites. The synthesis of tougher matrix resins might allow such composites to be used more widely in structural elements such as aircraft wings. Graphite fiber composites are relatively expensive for use in automobiles, and thus the primary emphasis has been on fiberglass-reinforced resins. In the future, we might look for the synthesis of inexpensive, self-reinforcing, injection-moldable liquid crystalline polymers to bring about major advances in structural materials for automobiles. In fact, certain types of liquid crystalline polymers retain mechanical properties up to about 350°C and might even be used in parts of engines. Another need in the transportation industry is for improved flame-resistant materials for use in the interiors of aircraft and automobiles. Currently, too little effort is directed at designing new kinds of textiles that are intrinsically flame resistant and that do not produce smoke or toxic gases in a fire. The challenge is not only to achieve the key feature of flame resistance, but also to provide comfort and wearability, and to make such textiles available at costs competitive with those of currently used materials. The search continues for inexpensive, high-temperature ceramics that can be used to increase the efficiency of combustion engines. The intrinsic brittleness of ceramics remains a major stumbling block, notwithstanding recent progress in the preparation of composites reinforced with stabilized zirconia and fiber-reinforced composites. Success in this area will require synthesis
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Materials Science and Engineering for the 1990s: Maintaining Competitiveness in the Age of Materials of ceramic materials that can be processed into finished shapes quickly and cheaply. Many needs for new materials arise in the design of advanced supersonic aircraft. The development of lightweight structural skins that retain their strength at 250°C will be necessary for aircraft designed to fly at Mach 3. Even more formidable are the materials requirements for the national aerospace plane, which is expected to have surface temperatures in the engines and leading edges that are in excess of 1500°C. Energy Industry A number of our present energy technologies could be improved in efficiency or safety through the synthesis of new materials. The success of technologies like solar energy conversion or automobiles powered by fuel cells or batteries will depend on whether new materials and economically viable ways of making them can be discovered. Unfortunately, much of the momentum of research in these areas has been lost in recent years. One example of the role of new materials in what may be a future energy technology is associated with fuel cells. Fuel cells are devices for converting chemical energy directly to electricity with high theoretical efficiency, with no moving parts, and without the pollution that accompanies combustion. The advantages of fuel cells seem evident, but the development of a technology based on them is limited by the materials available for use as electrodes. For example, there is no fuel cell capable of directly consuming liquid fuel because there is no known electrode at which such fuels can be oxidized at useful rates. Development of new electrode catalysts for fuel cells might lead to dramatic improvements in the efficient use of energy resources and could help solve pollution problems. The energy industry also has a need for synthesis of passive materials. For example, the nuclear-based technologies need new, highly nonreactive materials for waste storage, and they also need to improve techniques for suppressing corrosion in cooling systems. Achieving superior materials properties in these cases will require the sort of materials synthesis in which the United States has not been strong. Since the nuclear industry is currently in a state of flux, it is unlikely that materials synthesis will receive a great deal of attention. Solar energy conversion has been proven theoretically to be feasible in the sense that direct conversion of light to electricity with efficiency greater than 15 percent has been achieved in the laboratory. It is evident, however, that large-scale generation of electricity directly from sunlight will not be possible until inexpensive and efficient photovoltaic technology becomes available. At present, thin-film materials look the most promising. There is already a profitable photovoltaic industry in Japan, but, even there, large-
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Materials Science and Engineering for the 1990s: Maintaining Competitiveness in the Age of Materials scale energy production has yet to be realized. Solar energy technology seems a good bet for the future, but advances in the synthesis of photovoltaic materials will have to precede further development. Like solar energy, nuclear fusion looks promising in the long run. In this future technology for central power generation, the materials problems are severe and somewhat less well defined than in the photovoltaic area. Some of the problems of conventional nuclear energy will be encountered here, but the most challenging research is likely to be focused on the development of structural materials that will survive the extremely harsh conditions expected in the fusion reactor. For the most part, such materials do not exist today, and future success in this area will be determined by the ability of scientists to come up with innovative solutions. Defense and Aerospace Industries The advanced materials needed for defense and for aerospace have very special purposes. They often do not have the commercial applications that would justify support for their development in the private sector. Thus government agencies must take the lead in supporting synthesis of novel materials for military applications. Outstanding needs include systems having unique optical and electronic properties as well as systems having unusual strength and toughness. The materials needed for defense and aerospace span such a wide range that only a few possibilities for materials synthesis in this area can be described here. One such possibility is the development of multitechnology chips that combine infrared detectors with electronic and optical processors. The technical issues here include problems of synthesizing each of the component materials in the presence of the others without destroying the function of any of them. The Department of Defense has unique needs for new materials in developing its “low observables” or “stealth” technology. The synthesis of such materials requires improved understanding of the physics of detection and of the properties of materials that will be needed to defy detection. The interplay between synthesis, processing, and physics in this area must be strong, especially if the resulting materials are to maintain structural properties with minimal increase in mass and volume. The Department of Defense also needs unusual materials for sensing and surveillance. Applications range from tracking submarines to detecting toxic chemicals. For example, it would be very useful to find a material superior to polyvinylidenefluoride as an element in acoustic detectors. Electronic ceramics, redox polymers, biomaterials, and membranes are all likely to play roles in advanced chemical sensing systems. These few examples illustrate the diversity of defense- and aerospace-
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Materials Science and Engineering for the 1990s: Maintaining Competitiveness in the Age of Materials related materials problems that might be solved, at least in part, by synthesis. The needs for defense are as wide ranging as those in the private sector. The difference is that the military needs will be the sole responsibility of government because, for the most part, commercial markets for advanced military materials are limited. However, the innovative research in synthesis that will be necessary for meeting these needs is not currently under way in academic, government, or industrial laboratories. OPPORTUNITIES IN SYNTHESIS Synthesis provides scientists and engineers with the raw materials for their work, whether that work be fundamental or applied. The wide variety of technological needs described briefly in the preceding paragraphs tend to “pull” research in synthesis. At the same time, materials synthesis is “pushed” by advances in science and engineering. This appendix concludes with a few examples that seem to fall in the latter category of opportunities for research. Ultrapure Materials New synthetic methods and new procedures for handling materials during synthesis are now yielding substances of unprecedented purity and performance. The synthesis of very pure substances is becoming increasingly important both in microelectronics and in the development of new structural materials. The need for extremely pure silicon in microelectronic devices is well known. Currently, very pure and atomically perfect III–V semiconductor crystals are being used in advanced electrooptical devices. The production of volatile precursors (SiH4 for silicon, AsH3 and gallium alkyls for GaAs) in 99.9999 percent purity presents a challenge to the engineers who are developing the technology for producing these ultrapure semiconductors. Similarly, the production of actinide-free ceramics for packaging radiation-sensitive very large scale integrated circuitry requires reengineering the synthetic technology. In the area of structural materials, it is becoming clear that oxygen and carbon impurities can limit the strength of fibers used in composites. Crack initiation and growth in solids can be attributed to impurities and defects. Thus, as in the case of microelectronic materials, the ability to synthesize extremely pure ceramics—and to do this in economically significant quantities—will determine the success of this technology. The success of molecular precursors in solid-state synthesis often depends upon the use of ultrapure molecular materials. For example, a promising new method for producing ceramic fibers starts with the synthesis of a preceramic polymeric material that can be processed into a fiber and then py-
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Materials Science and Engineering for the 1990s: Maintaining Competitiveness in the Age of Materials rolyzed to form the ceramic. The purity of the preceramic polymer turns out to determine the strength of the ceramic fiber. In another example, very pure organometallic precursors can be used in synthesizing complicated ternary and quaternary III–V compounds for use in the preparation of device-quality materials. Lasers with unusually low threshold currents have been produced in this way. Finally, molecular precursors to metal lines and thin films may be useful in device fabrication. In all of these examples, synthetic chemistry must be integrated with the design of devices and the engineering of production processes. Organic nonlinear optical materials provide another illustration of the opportunities for research in the synthesis of ultrapure substances. It is becoming widely appreciated that the nonlinear optical properties of organic and organometallic molecules can be superior to those of inorganic solids. Preparation of new organic and organometallic molecular solids for use in nonlinear optics represents a special opportunity for academic chemists because the synthetic techniques are relatively commonplace in major research-oriented chemistry departments in the United States. What makes this a new opportunity is the need to prepare organic and organometallic solids of a purity and optical quality seldom demanded in typical chemical applications. New strategies for designing and preparing organic and organometallic solids with good optical properties are needed. As in the preceding examples, practical applications of new optical materials will require technologies for producing the solids in useful forms—monolithic crystals, crystal core fibers, oriented films, and the like. Synthesis will have to be integrated with fabrication, as discussed in the paragraphs on shape-limited synthesis below. One of the most critical and pervasive needs for ultrapure materials is in fundamental physical studies of structure-property relationships. For example, very pure samples of polymers with narrow distributions of molecular weights are needed in order to test modern theories of the behavior of polymers in dense fluids. Thus, for both fundamental and applied purposes, laboratories dedicated to the preparation of ultrapure materials must have high priority in programs aimed at upgrading U.S. capabilities in materials science and engineering. Shape-Limited Synthesis The term shape-limited synthesis refers to a relatively new approach to the preparation of materials—a combination of both synthesis and process-ing—in which one of the chemical reactants is formed from the beginning in the shape of the final product. In more conventional technologies, new materials first are synthesized in whatever form emerges from the reactor—often a powder—and then processed by various methods—e.g., molding and pressing—to achieve the desired shape. This approach requires that the
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Materials Science and Engineering for the 1990s: Maintaining Competitiveness in the Age of Materials synthesized material not only possess a set of desired properties, but also be suitable for later-stage processing. This latter requirement has constrained scientists to focus much of their attention on those materials that can be processed using commercially available equipment. This constraint has not encouraged innovative materials synthesis. The idea of chemically converting a precursor body into a new composition while retaining the original shape first was demonstrated by the preparation of fibers with compositions that had been inaccessible by traditional fiber-forming methods. Thus fibers of boron nitride were prepared by nitriding fibers of boric oxide, and cross-linked phenolic fibers were formed by reacting Novolac resin filaments with formaldehyde. One variation of this approach is to incorporate into the precursor fiber all of the necessary reactants. For example, silicon carbide fibers can be prepared directly by the pyrolysis of a carbosilane fiber. For many ceramic processing steps, it is desirable to have monodisperse, submicron, spherical particles. One of the most practical applications of sol-gel technology is the preparation of spherical ceramic particles with precisely controlled sizes. Similarly, the engineering of new gas-phase processes for synthesizing polyolefins can make possible the production of dry, flowable microspheres of polypropylene and high-density polyethylene. Chemically modifying surfaces to achieve surface passivation of sensitive materials is another version of shape-limited synthesis. For example, in the processing of silicon into semiconductor chips, the surface of silicon can be very thinly oxidized to protect the active devices during subsequent processing. A similar procedure would be highly desirable for passivating gallium arsenide surfaces. For the future, the ability to control surface chemistry will be essential in the design of devices with nanometer dimensions. The versatility of the shape-limited approach for preparing new fibers or thin coatings is considerable. On the other hand, to prepare larger shapes, it is essential to minimize changes in density between the starting material and the product. Such changes may produce stresses that cannot easily be relieved in this kind of process. Innovative techniques for overcoming this limitation would be most desirable. New Synthetic Methods At present, too little research is being carried out in the United States on truly novel synthetic methods. Most current research on synthesis emphasizes the use of conventional techniques for tailoring structures of molecules to achieve specific properties. Few completely new synthetic approaches are being developed, and, as a result, U.S. scientists are becoming comparatively limited in the techniques they can bring to bear on problems of synthesis.
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Materials Science and Engineering for the 1990s: Maintaining Competitiveness in the Age of Materials Perhaps even more important, opportunities for unexpected discoveries are being lost. The committee believes that there is no lack of good ideas to be explored. For example, laser-assisted chemical processing is a new approach for synthesis of metals as films or deposited wires. It has recently been demonstrated that metals such as copper, gold, chromium, and cadmium can be thermally deposited with laser assist onto surfaces by using the appropriate organometallic precursor. Similarly, relatively cool plasmas are being used to pyrolyze hydrocarbons into carbon radicals that deposit as diamond films. The engineering of industrial-scale plasma processes presents major challenges. With hot plasmas, it may be necessary to cool the gases at rates of millions of degrees per second in order to trap the thermodynamically unstable species that are necessary for synthesis. There is also an opportunity for stronger efforts aimed at improving catalytic processes. Major progress is being made these days in understanding the structures of surfaces and how they function as catalysts. The ability to modify such surfaces chemically at the angstrom level—for example, via molecular beam epitaxy or atomic layer epitaxy—offers opportunities for the design of completely new types of catalytic systems. A final example of novel synthesis concerns solid-state preparation techniques. The most commonly used methods involve reacting mixtures of powdered substances at high temperatures. Low-temperature methods, such as the increasingly popular sol-gel techniques, not only are more efficient but also allow the preparation of many new phases that are unstable at high temperatures. Again, there is no lack of opportunities for innovation. But innovation cannot occur unless there is financial support and institutional encouragement for research in synthesis.
Representative terms from entire chapter: