Synthesis and Characterization of Advanced Materials (1984)

The synthesis and characterization of advanced materials (SACAM) are carried out by an interdisciplinary community whose contributions have been at the heart of progress in materials science and technology. This community includes researchers in solid-state physics; metallurgy; ceramics; materials science; geoscience; electrical engineering; and inorganic, physical, organic, and solid-state chemistry. Only recently has awareness of group identity begun to develop within this community.

We will begin with some definitions: first, the word “synthesis” as used in the SACAM Workshop and this report. Although synthesis is generally used, “preparation” might be preferable, for it would include both the careful control of microstructure and extrinsic properties of materials, which are in the province of the ceramist and metallurgist, and the ab initio preparation or synthesis of materials, which is in the domain of the chemist. In this sense, both synthesis and preparation and, concomitantly, their synergistic interaction, have been essential to many achievements in solid-state science and technology. We believe that they will continue to be the driving forces of progress in materials science and technology.

Second, “characterization” refers to the measurement of the chemical and physical properties of materials. The purpose of characterization is generally twofold: to understand closely related materials on the basis of their chemical bonding, atomic structure, and microscopic and macroscopic perfection and to improve and specify materials for particular applications.

Third, “advanced materials,” which can best be defined briefly by examples, such as the following:

• Semiconductors (excluding “routine” Si and GaAs)

• Magnetic materials

• Ferroelectric materials

• Piezoelectric materials

• High-strength and electronic ceramics

• Metallic organic materials, especially those with established connections between bonding, structure, and properties, where synthesis of desired structures is critical, for example, linear chain conductors and layered compounds

• Inorganic polymers, for example, SN_x

• Active organic polymers, such as resists and piezoelectric materials

• New superconducting materials, particularly binary and ternary compounds, alloys, and solid solutions

• High-performance composites

• Amorphous materials for which the preparation requirements are unusual, such as amorphous metals and fibers

• Alloys and composites for which specialized care in preparation and processing control is essential

• Some single-crystal materials

• Various other inorganic materials such as oxides, layered materials, tunnel junction materials, intercalation compounds, chalcogenides, and pnictides

Advanced materials are those at the forefront of science and technology at any given time, the materials critical to improvements in the technologies at the heart of our economic system.

Because of the overlap between charges to Panels 1 and 2, there is a degree of duplication between the reports of the two panels. There were somewhat different emphases in the two groups, however, so it has seemed useful to include both accounts.

To assess the importance of SACAM to future technology and to explore the strengths and needs of the present SACAM community, it is helpful to consider past achievements. Such an exercise provides models for future efforts and shows how obstacles have been overcome.

Semiconductor science and technology have been central to modern electronics since the invention of the transistor and the development of the basic preparative techniques for making semiconductors, especially single-crystal Si and III–V compounds. This work took place over more than 20 years and included the development of such methods as zone refining, float-zone crystal growth, crystal pulling, and liquid-phase epitaxy. Si and GaAs were once exotic advanced materials. Indeed, at the degree of perfection and crystal size needed for very-large-scale integrated-circuit devices, advanced LEDs, solid-state lasers, and magnetic bubbles, they are still advanced materials, and substantial synthesis and characterization research is still needed.

Several activities allied with preparation have played key roles in the development of semiconductor materials. Characterization of the chemical purity and electrical properties and careful studies of point and extended defects have been essential for progress at every stage.

The theoretical description of impurity and dopant partition and of the interaction of defects has also been essential. Thus, in semiconductor materials research and development, advances have depended on the interdisciplinary activities of metallugists, chemists, and solid-state physicists. The interactive style of research developed in the work on semiconductor electronic materials has pervaded and been a model for much of materials science.

Another area of SACAM contributions to electronics is the preparation of magnetic materials. The descriptive and structural chemistry of oxide magnetic materials, alloys, and intermetallic compounds was developed through decades of continuous work. As a result, an understanding of spinel and garnet structure materials has been achieved. The magnetic interactions have become so well understood that these materials might be regarded as the “fruit flies” of magnetochemistry. These successes depended heavily on the development and exploitation of single-crystal techniques, such as flux growth, for the preparation of refractory oxides. Studies of properties of single crystals have resulted in a basic understanding of intrinsic properties. Controlling microstructure and understanding the connection between magnetic properties and extrinsic structure have been essential to progress.

The newest addition to the family of magnetic materials, the transition metal/rare earth intermetallic compounds, benefited from the measurement of the anisotropic properties of single-crystal materials. The exploitation of this crystalline anisotropy required the development of powder-metallurgy techniques for the consolidation of these highly reactive materials and the exploitation of new chemistry to produce fine powders at reasonable cost from inherently inexpensive rare earth oxides. Although these materials are intrinsically more expensive than oxide materials, their high coercive force makes them extremely resistant to demagnetization and thus cost effective for many device applications.

Techniques have been developed for the routine preparation of optical fibers for communications. These methods, which are based on rapid, controlled, vapor-phase chemical reactions, permit technologists to produce materials with only a few parts per billion of critical, optically absorbing impurities. With such high-purity materials, the optical losses at the wavelengths of interest for optical communication are sufficiently low that repeater spacings can be many kilometers. These techniques also allow the careful control of deliberately added impurities over dimensions of micrometers so that the index of refraction can be tailored to produce the desired waveguide characteristics. Glasses for fiber transmission provide a continuing need and stimulus for additional SACAM contributions.

Cost considerations have dictated that, wherever possible, ceramic systems be used in electronics. Accordingly, the ability to control the microstructure of ceramic materials is among the most remarkable accomplishments in controlling the properties of inorganic substances of technological importance. The emphasis in much of electronic materials synthesis has been on the preparation of materials in a state of sufficient purity and perfection that they exhibit their intrinsic properties. These properties are often unique and result in entire families of devices based on single crystals, where the quantities of materials required per device function are quite small. The unique challenges of ceramic SACAM research have been to control the grain structure and the state of grain boundaries. The roles of solid-state chemists in

providing background understanding of defect chemistry and of physicists and engineers in characterization and end-use guidance have been significant. Luculox and its relatives—optically transparent ceramics for lamp envelopes and windows—are typical of the technological achievements. The key to these advances was the discovery that MgO inhibited grain growth in Al₂O₃ and the subsequent careful exploitation of this discovery so that it could be used under actual processing conditions. Other examples of SACAM achievements based on detailed understanding of ceramic processing include the efficient fabrication of UO₂ fuel rods for fission reactors, ZrO₂ ceramics for oxygen detectors, which find use in emission control in internal combustion engines, ballistic armor for military and civil police applications, tungsten carbide tool tips for high-speed machining, and β-Al₂O₃ and related ceramic materials for solid-state battery electrodes. The key has been the careful control of grain structure to make a useful polycrystalline material at a cost low enough to permit exploitation.

Electronic ceramics, such as piezoelectric lead zirconite titanate and its cogeners, doped ZnO and related nonlinear resistance (varistor) materials, and complex ceramic conductors and their relatives, which are essential in thick-film hybrid integrated circuits, are all examples of additional SACAM contributions. The set of innovative processing techniques that permit the rapid and cheap preparation of Al₂O₃ ceramics for electronic and autocatalyst bed applications is an additional example.

High-pressure synthesis is an important area of scientific research and has produced a number of high-technology materials. A key element in these successes was the timely exploitation of modern engineering materials to produce the experimental conditions necessary for new syntheses; for example, high-pressure autoclaves, pumps, and gauges. Two of the most important products of this work are synthetic quartz and diamond. As a result, the U.S. electronics industry is now independent of foreign sources of natural quartz, and the U.S. machine tool industry is independent of foreign sources of industrial-grade abrasive diamond.

The efficient conversion of petroleum into the variety of fuels needed by modern industry and the conversion of petroleum feedstocks to petrochemicals and polymeric materials depend on the availability of highly specific solid catalysts. The synthesis of these catalysts, their characterization, and the understanding of their role in breaking and forming chemical bonds in petroleum-based materials have been major successes of modern science and technology. The role of solid-state chemistry and synthesis is often overlooked in these achievements. For example, controlled zeolite catalysts, Ziegler-Natta catalysts, and metal-alloy catalysts are vital in modern technology; all of these have resulted from extensive SACAM efforts.

The efficient and pollution-free production of elemental chlorine by electrochemical processes is a key part of present industrial chemistry and provides an essential industrial chemical, Cl₂, for scores of industrial syntheses. The discovery, through preparative techniques and characterization, that ruthenium-titanium oxides can replace mercury anodes had great impact on our chemical industry. These new anodes not only increase efficiency, thus conserving energy, but also eliminate mercury pollution.

Control of glass “structure” by selective leaching before final consolidation has given us comparatively inexpensive, thermally shock-resistant families of glasses, such as vycor. Careful control of recrystallization has produced even more remarkably controlled thermal properties in, for example, the pyrocerams. Rapid quenching has opened the glassy state to previously non-glass-forming materials, such as metals. Indeed, glassy metals have remarkable mechanical strength and unusual magnetic properties. All of these achievements resulted from controlled synthesis and careful characterization.

With the notable exception of semiconductors, polycrystalline solids are the basis of much of our technology. In these materials it is extrinsic structure (grain boundaries, impurities, inclusions, and dislocations) coupled with the intrinsic molecular or crystal structure that determine the technological importance of the material. Materials that are appropriate for a variety of complex functions generally have higher intrinsic value.

Technically sophisticated systems (gas turbines, nuclear power plants, space capsules) require the simultaneous optimization of several properties, such as strength, density, and chemical stability. Accordingly, the materials employed usually have coupled chemistry and microstructure.

The key to developing new alloys and composite materials is to understand and manipulate the microstructure and thereby the properties of the materials. Four classes of tools are required for progress: (1) theoretical models of the structure and binding of localized states of matter, (2) computational procedures and equipment to evaluate the models and suggest experiments to validate them, (3) observational techniques and equipment to provide structural and compositional information on a local scale, and (4) an extensive and precise quantitative data base for phase equilibria.

All of these tools have been and are being applied to the development of technologically important materials. Examples of past accomplishments and areas of particular promise for the future are phase state and phase equilibria; transport phenomena; and interface, surface, and grain boundary phenomena.

Historically, the major body of phase-state information has been obtained by measurement and computation of bulk properties, augmented, when appropriate, with simple rules for predicting phase type from electron concentration and pre-existing knowledge of crystal structure. Crystal structure cannot be predicted a priori except in the simplest of cases. One example of a simple rule is the PHACOMP calculation of phase stability from knowledge of a specific alloy composition and multiple regression analysis of previous data relating composition to volume fraction of constituent phases. This method has been applied successfully to superalloy design and specification. These superalloys are critical for the efficient power generation and propulsion devices necessary for a modern industrial society.

The recently developed theory of atomic clusters, coupled with powerful computational techniques, shows promise for improving the prediction of phase stability. Clearly this is an area of opportunity.

In service, materials are often subject to changes that are diffusion-controlled, and product life is limited by time and temperature under stress. Nevertheless, detailed analysis of diffusion mechanisms and their relation to composition are rarely a part of an alloy-development program.

A recent report demonstrates that in 25 years of high-temperature alloy development, the prime factor accounting for the 50 percent increase in allowable operating temperature has been the decrease of the diffusion coefficient. This analysis includes the recent spectacular improvement resulting from fiber and lamellar eutectics represented by TaC- and Mi₃Nb-reinforced, nickel-base superalloys.

Another example of the importance of transport phenomena in technologically significant materials is identification and development of structural materials for fast breeder reactors. The extensive swelling of austenitic alloys caused by a fast neutron flux can be drastically reduced by control of both the major alloying elements, Fe, Cr, and Ni, and a host of minor impurities. The effects can be understood from a knowledge of the changes in diffusion rates that accompany these changes in composition.

Grain boundaries and internal interfaces frequently limit the structural performance of metals, alloys, and composites. The strength and ductility of the boundaries and interfaces are, in turn, controlled by the structures produced at these interfaces where segregation occurs. Modern analytical tools, such as the electron microscope, microprobe, and Auger spectrometer, which permit chemical and structural analysis on a localized scale, are now being effectively used in studying structure and composition of grain boundaries and other internal interfaces in solids.

Primary and secondary recrystallization of metals and alloys, such as copper, nickel, and transformer steels, are controlled in part by segregation of minor additives, both intentional and inadvertent. The amelioration of, temper embrittlement in steels will require understanding and control of the segregation of impurities at grain boundaries, which controls the embrittlement kinetics. Corrosion phenomena, particularly those associated with stress corrosion, are influenced by local chemical

conditions in the solid and solutions. Both, in turn, are modified by segregation of alloying elements and impurities on a local scale.

Multiphase composite materials, which permit new levels of performance and allow materials to be tailored to specific needs, depend for their mechanical properties on the transfer of load between the phases. Bond strengths at interfaces are markedly influenced by phase stability, local structure, and composition. Opportunities exist to develop phase-compatible fibers, coatings, and matrix materials for new composities. The boron-aluminum composites and carbon-reinforced polymers are two examples of synthetic composites that have unique properties for aircraft skin and engine use. Natural composites, such as eutectic superalloys produced by directional solidification, are currently under development for use as blading in aircraft turbine engines. Composites offer a unique opportunity for improved efficiency and reduced weight in aircraft applications and are finding use in consumer products in the less sophisticated form of glass-fiber-reinforced polymers.

The achievements that we have discussed are merely a prelude to the achievements that SACAM can yield in the future. Trends in technology point to increased importance of advanced materials and the likelihood that SACAM will pace progress in many critical areas. Some major opportunities are the following.

Electronic materials will continue to play a key role in many areas of technology, especially communications and data processing. Several driving forces important to the future of the electronic materials industries have as their central feature the need for improved advanced materials. Probably foremost is the continuing increase in the scale of integration of silicon integrated circuits (SICs). Automata of all sorts, such as inexpensive small personal computers (the hand-held calculator) and distributed computation and control devices (the microprocessor), will have increasing impact on society. Super-large-scale computers, which will make possible

real-time calculations for a variety of problems, notably meteorological equilibria and kinetics necessary for the accurate prediction of weather, will also become a reality. These developments will come about only as a result of increases in the memory and calculating power obtainable per unit investment. Less costly data processing results directly from putting more circuit functions on a single Si chip. At present, large-scale integration (LSI) has brought 262-kbit memories close to everyday use. The packing density in SICs has been doubling approximately every 2 years as feature sizes become smaller. It is currently possible to fabricate LSI circuits with feature sizes of the order of a micrometer. Submicrometer featured sizes or less are expected within a few years. However, advanced materials will be required. The conventional conductors and dielectrics used in SICs will require improvement or replacement. Thus, SiO₂ may not be adequate as the insulator in semiconductor circuits as the feature size becomes smaller. We need materials of higher dielectric constant, materials less subject to pinhole formation, materials that can be made in thinner layers more reproducibly, and materials that have a smaller likelihood of imperfections over a small area. Perhaps detailed study of silicon-nitrogen chemistry could lead to the production of such materials. The techniques for preparation of new Si dielectrics should be compatible, insofar as possible, with semiconductor processing as currently carried out with respect to times, temperatures, and pressure, since there is an immense investment in processing equipment and procedures. The same is true of improved conductors. Thus, silicide chemistry and preparative efforts in general are ripe for SACAM contributions, which could be the key to the next generation of very-large-scale integration (VLSI).

To pattern smaller feature sizes, families of improved resists are needed. Resists are materials for which solubility can be changed by exposure to radiation and that can thus be patterned to produce the circuit features desired in SICs. Optical resists are the mainstay of the semiconductor industry for direct processing, although master masks for patterning many devices are regularly made using electron-beam resists. As feature sizes become smaller, the diffraction limit of light will become increasingly restrictive. Accordingly, much SACAM research is concerned with organic polymeric materials for which solubility is altered by exposure to

light, electron beams, and x rays, with the hope that from these studies resist materials for the micrometer and submicrometer regime can be developed. All of these efforts require careful synthesis and characterization and are essential to future progress in integrated-circuit technology.

The role of SACAM in the development of low-loss and high-bandwidth optical fibers is well known. However, new families of materials might have losses that are orders of magnitude lower than the silicate-based glasses now used for optical fibers. There are classes of non-oxide glasses in which, with careful synthesis and control, such low losses might be achieved. The impact would be substantial if repeater spacings could be much greater than is now possible.

There are many other opportunities for SACAM in electronics. Examples include high-speed circuit elements, such as Josephson logic junctions, and new display materials. The latter could result in cheaper interfaces, which, together with the broad bandwidths of optical fibers and the cheap computational and control power of VLSI, could lead to a much larger market. Electronics, in spite of its remarkable impact on communications and computation and the key role of advanced materials in its development, is in many ways just beginning to show what it can do for technology and what SACAM can do for it.

An ever larger fraction of our resources is going into the production, conversion, storage, and conservation of energy. Survival of our technological society and improvement of less-developed societies will depend on the more efficient use of energy resources and on environmentally sound new resources. Here we discuss some SACAM opportunities in energy research and development.

• Heterogeneous Catalysis. Petroleum fuels require expensive pollution-control devices. Efforts will be directed toward the development of catalytic processes to eliminate the pollutants at the refinery. These efforts will require advances in understanding catalytic activity. Efficient catalysts for shale, coal, and other abundant sources of hydrocarbons are needed for both breaking

chemical bonds to produce useful fuel and feedstocks and for forming bonds for property modification.

• Electrochemical Materials. Electrochemical processes such as those used in chlorine and aluminum manufacturing consume large amounts of energy. The opportunity exists for the development of new electrode materials using the Yi-Nb-Cl₂ electrode as a model.

• High-Temperature Hostile-Environment Materials. New energy sources, including fission, fusion, geothermal energy, and ocean thermal energy, and new approaches to higher-efficiency energy generation, such as magnetohydrodynamics, all involve demanding temperature, corrosion, or radiation environments. The ability of the SACAM community to provide nitrides, carbides, silicides, superalloys, and composites for these critical areas could well determine the extent and reliability of advances in these technologies.

• Solar Energy and Hydrogen Utilization and Storage. Progress in photovoltaics, liquid-junction solar cells, solar batteries, solar collectors, photodissociation techniques for producing hydrogen, and hydrogen storage is tightly tied to SACAM. Key contributions to energy production can be made in all of these areas. Both new advanced materials and older materials prepared by innovative lower-cost techniques are needed.

• Lighting Materials. Further opportunities in energy conservation through improved design of lamp envelopes, phosphors, and the like are closely coupled to the availability of new advanced materials.

• Materials Substitution. Increased vehicle efficiency through weight reduction depends on the development of new lighter materials meeting the requisite engineering criteria.

Advanced materials can play a vital role in improving the quality of the environment and making it possible to pursue desirable technologies without adverse environmental effects. In regard to efficient and environmentally satisfactory energy production and utilization, solar energy, fission, and fusion will depend for their ultimate success on yet-to-be-developed advanced materials. Photovoltaic solar energy requires low-cost solar-cell materials; fission requires inexpensive encapsulants for spent fission products that are

environmentally stable for hundreds of years; and fusion requires materials that maintain their structural integrity under intense neutron irradiation.

Improved heterogeneous catalysts for vehicles and boiler stacks could result in significantly improved emission control. Heterogeneous catalysts could also make possible the reduction of high sulfur content of some fuels, notably petroleum and coal, before their use. Clearly, catalysts that facilitate the more-efficient and less-expensive removal of sulfides and other pollutants from stack gases would be a boon.

So far we have considered the opportunities for SACAM arising from societal and technological needs; the research itself generates additional opportunities.

The significantly lower cost of polycrystalline materials usually dictates their choice over single-crystal materials when they meet requirements. Consequently, ceramic materials have found many applications. However, improvement in mechanical properties, such as strength, ductility, and durability, and improved control of electronic properties over wider ranges would greatly increase the economic attractiveness and utility of ceramic materials. Research techniques, many of them originally used in physical metallurgy, are unraveling many of the mysteries of grain growth and control during densification and sintering in ceramics. Much basic progress is possible in the area of grain-boundary, interface morphology and microstructure characterization and control. Characterization of impurities in grain boundaries by the use of the analytical scanning transmission electron microscope (STEM), studies of the basic mechanisms of sintering, and studies of the relationship of the properties of ceramics to those of single crystals are examples in which progress in fundamental science driven by SACAM can lead to great benefits.

In the study of solids with interactions in less than three dimensions, theory and SACAM interact vigorously. Metal-organic chemists, solid-state chemists, solid-state physicists, and theorists have made this a fruitful field in U.S. university research. The theoretical results often suggest classes of materials and particular materials that would be worth synthesizing. Theories with

predictive capability stimulate synthesis. Technological results have already come from such materials, for example, intercalation compounds for battery electrodes. We believe that this area will continue to be productive and should be pursued for the intellectual challenge it presents, to advance the connection between solid-state theory and synthesis, and for the likely technological spin-offs.

New techniques of synthesis are in many respects the life blood of SACAM. Emphasis on developing new techniques, exploiting techniques developed in other branches of science, and extending older techniques to new classes of materials should accelerate progress. Molecular beam epitaxy (MBE) and ion implantation have been applied to comparatively limited classes of materials. Their extension to the synthesis of more diverse materials should be productive, especially in view of the sensitive control of properties over small dimensions that MBE makes possible. Low-temperature techniques, including quenching from both the vapor and the liquid, have not been exploited for many classes of materials, and their further use by the SACAM community should be encouraged.

We believe that with respect to catalysis the characterization and understanding of real surfaces could be the next area where scientific opportunities will become a driving force for technological progress. The materials on which these studies should be made will have to be provided by SACAM techniques. SACAM researchers would like to begin to look at catalytic material with a complexity approaching that of catalysts used in technology; however, it is essential that these materials be prepared reproducibly with stoichiometry and defect chemistry well characterized and understood.

The need for theoretical work of many sorts is obvious. One aspect of theory that tends to be overlooked is the development of qualitative models to help the synthesist choose the most promising classes of materials and compounds to work with. Clearly, this kind of qualitative modeling, which permits classes of materials to be investigated for interesting properties without always being synthesized, can increase the effectiveness of synthesis.

U.S. industry is conducting a great deal of excellent research and development in SACAM. Tutorials based on

this activity could be valuable to university programs. Universities are conducting some of the best fundamental research in SACAM; therefore, more interaction would benefit both industry and academia. To increase academic-industry coupling, we recommend that the exchange of personnel be encouraged as much as possible. Such exchanges could range from seminars to permanent movement of people from industry into university teaching appointments. Sabbaticals of from several months to 1 or 2 years could be particularly useful. The NSF Industry/University Cooperative Research Projects program could provide an effective means of encouraging cooperative ventures between universities and industries.

A large fraction of the sabbaticals taken by SACAM researchers in U.S. industry involve substantial interaction with the European SACAM community. There is a vigorous SACAM community in Europe with which U.S. researchers should interact; however, there is a possible problem in that the U.S. SACAM academic community is smaller and less active than the European, which could hamper reciprocal interaction.

The panel agreed that more solid-state chemists are needed in SACAM research. Solid-state chemistry is crucial to many SACAM activities.

The Panel also believes that the unique contributions of the national laboratories to SACAM should be studied. Further, the Panel found that data are lacking on the number of individuals in SACAM, the number being trained in SACAM, and similar demographic information. Such data are essential for future planning in the field.