Synthesis and Characterization of Advanced Materials (1984)

We now briefly discuss the accomplishments of SACAM and show that SACAM activities are beginning to change from the past emphasis on electronic materials to a broader effort encompassing materials for advanced communications and automation and development of new materials for energy conversion and conservation.

Semiconductor science has been at the heart of modern electronics since the invention of the transistor. The development of the basic preparative techniques for making semiconductors, especially single-crystal Si and III–V compounds, was a landmark achievement of SACAM. 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. Advanced synthesis and characterization research is still needed to achieve the degree of perfection and crystal size required for very-large-scale integrated-circuit devices, advanced light-emitting diodes (LEDs), solid-state lasers, and magnetic bubbles.

Several activities allied with preparation have played key roles in semiconductor materials advances. Characterization of the chemical purity and electrical properties and careful studies of point and extended defects have been essential for progress at every stage, as have the theoretical description of impurity and dopant partition and the interaction of defects. Thus in semiconductor materials research and development, advances have depended on the interdisciplinary activities of metallurgists, chemists, and solid-state physicists.

Another important SACAM contribution to electronics is the preparation of magnetic materials. The descriptive and structural chemistry of these materials was developed through decades of continuous work. As a result, a reasonably complete picture of the crystal chemistry of single-crystal spinel and garnet structure materials is now available. Indeed, the magnetic interactions have become so well understood that it might be argued that these materials have been the “fruit flies” of magnetochemistry. These successes depended heavily on the invention and exploitation of single-crystal techniques, such as flux growth, for the preparation of refractory oxides. The capability for producing single crystals made it possible to gain a thorough understanding of intrinsic properties. Controlling microstructure and understanding the connection between magnetic properties and extrinsic structure have also been essential to progress. The present families of important materials include polycrystalline ferrites, which are used in cores and memories, and single-crystal magnetic garnets, which are used in bubble-domain memories.

Luminescent materials play an essential role in modern instrument and appliance displays, TV picture tubes, and fluorescent lighting. (Of course, these materials could be classified as either electronic or optical materials, depending on whether the basic mechanism or the application is of primary concern.) The careful preparation and control of the chemistry of II–VI and ZnSiO₄ phosphors and the discovery of the luminescent properties of Eu, which is now used extensively in color TV picture tubes, are examples of the contributions of SACAM to these technologies.

SACAM has made substantial contributions to the development of the optical materials that are widely used in modern technology. Such materials include yttrium aluminum garnet, which is used in solid-state lasers, and a variety of nonlinear optical materials, such as LiNbO₃. These are examples of the contributions of synthesis chemists, ceramists, and metallurgists to the preparation of carefully controlled single crystals of high complexity, high reactivity, and high melting points. Another notable contribution is the discovery and perfection of the techniques now used for the routine preparation of

optical fibers. These techniques, which are based on rapid, controlled, vapor-phase chemical reactions, permit technologists to make 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 communications 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. It is clear that glasses for fiber transmission constitute a crucial and challenging opportunity 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 significant accomplishments of SACAM. Electronic ceramics, such as piezoelectric lead zirconate titanate, doped ZnO and related nonlinear resistance (varistor) materials, and the complex ceramic conductors used in thick-film hybrid integrated circuits are examples of contributions of ceramic science to modern technology where SACAM has been instrumental. We also mention, as an additional example, the processing techniques that have made possible the preparation of ceramics for electronic substrates and autocatalyst bed applications.

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. For example, zeolite catalysts, Ziegler-Natta catalysts, and metal-alloy catalysts are vital in modern technology. All of these catalysts and many more have resulted from extensive synthesis programs. However, only now are fundamental investigations beginning on the effects of composition and structure of catalysts, their characterization, and the understanding of their role in breaking and forming chemical bonds in petroleum-

based and other materials. The effort directed toward the cleaning and conversion of coal for energy and chemical applications is barely under way. This program will require the application of the most advanced tools and techniques available to the SACAM community.

The efficient and pollution-free production of elemental chlorine by electrochemical processes is a key part of current industrial chemistry. Thus, the discovery that ruthenium-titanium oxides can replace mercury anodes was of great importance. These new dimensionally stabilized anodes permit higher current efficiencies, thus conserving energy and, at the same time, eliminating mercury pollution.

The development of advanced batteries has progressed considerably during the last decade or so through preparation and characterization research in the solid-state sciences. First, a solid electrolyte, β-alumina, was found to exhibit ionic conductivities at 300°C as high as those of aqueous solutions. Then excellent reversibility was discovered for the layered materials, such as TiS₂, in lithium cells at ambient temperatures. There are now extensive research and development programs based on these findings, and, in addition, lithium-alloy anodes are playing a critical role in the development of molten-salt batteries.

The ready availability of high pressures has made it feasible to produce a number of new materials with application to advanced technologies. Two of the most important of these are synthetic quartz and diamond. As a consequence, the United States has become independent of overseas sources of these two materialist

Selective leaching before final consolidation has yielded comparatively inexpensive, thermally shock-resistant families of glasses. Careful control of recrystallization has produced even more remarkable thermal properties in the Pyrocerams. Rapid quenching has opened the glassy state to metals, resulting in new materials with remarkable mechanical strength and unusual

magnetic properties. All of these achievements have resulted from controlled synthesis and careful characterization.

Amorphous materials play a role in a number of technologies. One of the best-known examples is selenium for xerography; a second is the use of amorphous silicon for solar cells. The development and exploitation of this material, production of which relies on arc discharge synthesis techniques, is just beginning. The SACAM community can be expected to play a central role in the continuing development of amorphous silicon, as well as other materials, for solar cells.

In areas as diverse as ferromagnetism, the band structure of semiconductors, superconductivity, defect solids, and layer and chain structures, the observation of unusual properties in a novel material has been the initial stimulus for new theoretical and experimental research, which has opened broad areas of science. Thus advances in materials preparation frequently precede and stimulate theoretical activities and are often the precursors of the effort to establish the scientific bases of new fields. Advances in understanding the connections between useful properties and chemical bonding and crystal structure have been closely coupled to the availability of well-characterized classes of materials that are difficult to synthesize. For example, our understanding of the magnetochemistry and spectroscopy of the magnetic and nonmagnetic garnets required the availability of well-characterized single crystals of scores of garnet compositions. The technological fruits of these scientific acievements included bubble garnet memories and low-threshold optically pumped lasers.

One of the strengths of materials science is the close coupling between research and technology. This relationship is particularly true in SACAM. Thus, many of the specific achievements that we have listed were closely associated with and heavily dependent on basic research and could be discussed again here, with a slight change in viewpoint, as research accomplishments. For example, the accurate characterization of carefully prepared single-crystal semiconductors led to the understanding that holes and electrons obey the law of mass action, which was an important scientific finding and a key step

in the advance of semiconductor technology. Another example of an important basic scientific advance, which resulted from semiconductor studies and stimulated technological progress, is the conceptualization of the Frank-Read source, the dislocation mechanism whereby basic crystal growth kinetics can be understood.