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Suggested Citation:"Executive Summary." National Research Council. 1989. Research Opportunities for Materials with Ultrafine Microstructures. Washington, DC: The National Academies Press. doi: 10.17226/1488.
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Suggested Citation:"Executive Summary." National Research Council. 1989. Research Opportunities for Materials with Ultrafine Microstructures. Washington, DC: The National Academies Press. doi: 10.17226/1488.
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Suggested Citation:"Executive Summary." National Research Council. 1989. Research Opportunities for Materials with Ultrafine Microstructures. Washington, DC: The National Academies Press. doi: 10.17226/1488.
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Suggested Citation:"Executive Summary." National Research Council. 1989. Research Opportunities for Materials with Ultrafine Microstructures. Washington, DC: The National Academies Press. doi: 10.17226/1488.
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Suggested Citation:"Executive Summary." National Research Council. 1989. Research Opportunities for Materials with Ultrafine Microstructures. Washington, DC: The National Academies Press. doi: 10.17226/1488.
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Suggested Citation:"Executive Summary." National Research Council. 1989. Research Opportunities for Materials with Ultrafine Microstructures. Washington, DC: The National Academies Press. doi: 10.17226/1488.
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EXECUTIVE SUMMARY 1 Executive Summary Materials synthesis--the preparation of materials from atomic or molecular precursors--and materials processing--the manipulation of microstructures to effect desired properties--are both critical to the development of advanced materials with engineered properties. Ceramics, polymers, and semiconductors are just some of the materials for which significant new achievements in synthesis and processing are taking place. Current research is focused on the design, synthesis, and processing of ultrafine material microstructures that extend into the nanoscale (less than 100 nanometer) regime. This research has been inspired by the realization that significant beneficial changes in the properties of materials can be achieved by progressively reducing the scale of their microstructure, while maintaining chemical and microstructural uniformity. Another incentive has been the discovery of novel and improved materials properties when their microstructure approaches nanoscale dimensions. These findings have generated much interest in materials synthesis and processing research in the United States and abroad. This report presents a state-of-the-art assessment of the activity in this rapidly growing field of research and identifies new areas of research opportunity and some potential application areas for the future. The focus is on materials with submicron-sized microstructure (i.e., less than 100 nanometers), regardless of the specific type of material being considered. The scope includes what is known about metal, ceramic, and polymeric materials and their composites, but the report deliberately avoids significant discussion of semiconductor and superconductor materials, electronic, magnetic, and optical properties of multilayers, and rapidly solidified materials. These have been adequately treated in other publications. The structure-related properties of materials with phase structures on a nanometer scale (nanophase materials) are expected to be different from normally available single-crystal, polycrystal, or amorphous materials because their aggregate atomic structure may be unique. Initial investigations

EXECUTIVE SUMMARY 2 indicate some support for this view. Properties measured on nanocrystalline metals have been tabulated and compared with values for their coarse-grained counterparts and similar glassy materials. The change in a number of properties appears to be significantly greater in going from conventional crystalline material to the nanocrystalline form than in going from crystalline to glassy solid. The nature of such changes and their relationship with the underlying material's structure will be clarified only by further research in this interesting and potentially useful new area of materials synthesis and processing. Nanometer-scale materials have certain distinguishing features that must be characterized in order to understand the relationships between their unique composition and structures and their properties. Since many materials properties (e.g., magnetic, optical, and electrical) depend strongly on the atomic arrangements in the material, whereas others such as mechanical properties can also depend on morphological structure, knowledge of the structure of nanophase materials is important on both atomic and nanometer scales. Among the features that need to be explained are grain and pore size distributions and morphologies, the nature of their grain boundaries and interphase interfaces, composition profiles across grains and interfaces, perfection and the nature of intragrain defects, and identification of residual trapped species from processing. Because of the ultrafine-scale of these nanophase materials, characterization can be a challenge in itself, and some traditional characterization tools are no longer easily applied. For the characterization of the structure and morphologies of nanoscale materials, traditional tools such as electron microscopy and x-ray and neutron scattering are both necessary and useful. However, for microchemical analysis of the materials on the requisite fine scale, further advancements in the state of the art of instrumental capabilities will be necessary to obtain the desired lateral spatial resolution. A number of recent developments in microstructural and microchemical analysis methods will almost certainly have a significant impact on characterization of nanoscale materials. Among the most promising new methods currently available are the field ion microscope with atom probe capabilities, the scanning tunneling microscope with atomic force probe capabilities, and such analytical methods as electron energy loss spectroscopy utilizing ultrafine probe sizes. In addition, the various analytical probes based on new high-luminosity synchrotron radiation sources will undoubtedly make significant contributions. Clearly, the ability to synthesize and examine nanophase materials in situ under carefully controlled conditions, such as ultrahigh vacuum, will be necessary to fully explain their unique characteristics. In exploring research opportunities for materials with ultrafine microstructures, all elements of the materials science and engineering

EXECUTIVE SUMMARY 3 continuum need to be addressed, including, the synthesis and processing of nanophase materials, their characterization and properties, and potential application areas. Some of these techniques and materials are summarized here. • Organic molecular composites are polymeric materials consisting of two or more components dispersed at the molecular level. Examples include compatible polymer blends that are noncrystalline, thermodynamically stable single-phase materials as well as blends of components that are processed into a homogeneous state but are not at thermodynamic equilibrium. The latter blends are polymer analogs of nonequilibrium multiphase alloys. Blends of two amorphous polymers can exhibit significant physical attributes through synergistic effects. The field of polymer blends is extremely active, with much current interest centered on synthetic routes to produce “miscibility windows” wherein the blend can be produced and processed. With further research into synthetic methods, efforts directed at producing various copolymers, and exploration of novel processing of both neat materials and various blends, truly outstanding high-performance materials will certainly be realized. • Metallic composites are produced by mechanical reduction of two-phase starting materials, which are either mixtures of powders or castings of phases that are mutually insoluble in the solid state. The resulting microstructure has a very dense and uniform dispersion of ultrafine phases. The mechanical reduction technique has the advantage over conventional techniques for forming in situ composites in that it is less dependent on limitations of the phase diagram. Cryomilling, a recent innovation in this field, dispenses with the need to add the dispersed phase and relies on in situ liquid-solid reactions to produce the desired dispersion. • Metal-ceramic composites (the so-called cermets) are usually synthesized by traditional powder metallurgy methods. Because of the limitations in such methods, it has not yet been possible to extend the range below 1 µm particle size, much less below 100 nm. Thus the challenge is to devise new processing routes for significantly reducing the scale of bicontinuous metal-ceramic composite structures. A promising approach is the controlled decomposition of molecular precursors that encompass within their molecular architecture the correct atomic fractions of the elemental species. This new approach is widely applicable and is limited only by the constraints imposed by molecular design of the precursor solutions. Another approach is the gas-condensation method for synthesizing nanophase materials from mixed ultrafine powders of metals and ceramics. Such ultrafine structures present the opportunity to synthesize a new class of cutting-tool materials that will have the ability to form and maintain a very fine cutting edge that is resistant to chipping. For this reason, it is believed that nanophase composite cermets will find great utility for such high-value-added applications as microtome blades and surgeon's scalpels.

EXECUTIVE SUMMARY 4 • The ultimate goal of synthetic chemists is to approach the specificity exhibited by nature in controlling the macromolecules of biology, which have extremely well-defined, yet complex, composition and stereoregularity and accomplish a wide variety of functions in a most elegant manner. Researchers in synthetic polymer chemistry are only just beginning to utilize biotechnological approaches to produce synthetic polymers of heretofore never-achieved specificity. • Progress in nanoscale materials science can also produce advances in ultrafine-scale semipermeable membranes. Membrane microstructures need to be combined with membrane properties, such as flexibility and environmental tolerance. Either man-made assemblies or self-assembled composite materials, in which one phase possesses selective high transport or for which methods are available to selectively remove one of the phases, can provide suitable membranes. The exploitation of polymer gels is also an avenue toward new membrane materials. In particular, gels formed from rod-like macromolecules show promise as interesting materials. Indeed, the general area of phase separation of multicomponent polymer materials to provide ultrafine-scale composites is promising, since the length scale of phase separation can be readily controlled by synthesis of well-defined starting materials and time and temperature processing history. • Detailed understanding of the relationships between catalyst structure and properties has in the past been hampered by limited structural information on catalysts. New and improved characterization techniques combined with the potential for synthesizing catalysts with controlled microstructures hold potential for making connections between structure and properties in model catalyst systems that closely mimic real catalysts. Such studies also hold potential for synthesizing materials with improved catalytic properties. The application of new catalysts that replace current catalysts will be based primarily on performance criteria. New applications will be based on the potential for new product schemes and the economics for the entire process, of which the catalyst is just one part. Broad classes of catalytic reactions that make use of ultrafine catalytic particles include emission control catalysis, catalytic reforming, synthesis gas catalysis, Ziegler process for polyethylene, and oxidation catalysis. Possible future catalytic processes include catalytic activation of fuel for energy conversion. Since the performance of a catalyst is determined by measuring product yield, testing the activity for a particular reaction is the best way to choose a catalyst. Moreover, important discoveries are made doing kinetic measurements, where serendipity can play a role. Catalyst characterization that is not related to performance is ancillary if the concern is economics. • Composite materials made up of single domain and superparaelectric particles have yet to be investigated in a systematic way with proper control of the connectivity and surrounding environment. The controlled synthesis of ultrafine ferroelectric grains will do much to stimulate research in this area. Surface treatment of the ferroelectric phase allows control of the

EXECUTIVE SUMMARY 5 mechanical boundary conditions. Since polymers are about a hundred times more compliant than ceramics, if a ceramic grain is surrounded by polymer, the mechanical constraints are relatively small. Electrical boundary conditions can also be controlled by adjusting the dielectric constant and conductivity of the surrounding phase. • Periodicity and scale are important factors when composites are to be used at high frequencies where resonance and interference effects occur. When the wavelengths are on the same scale as the component dimensions, the composite no longer behaves like a uniform solid. Multidomain crystals and ceramics have been used as acoustic phase plates and high-frequency transducers. The extension of this thinking to phenomena associated with optical excitations automatically focuses attention on equivalent nanoscale structures. A wide range of potential property modifications, including shape-induced optical birefringence, shape-controlled optical nonlinearity, and potential modes for inducing optical bistability, remain to be explored. It is clear that there will be corresponding magnetic nanocomposites and that for these materials additional versatility can be expected because of nanoscale interaction with the transport phenomena and the associated optical absorption. • Many applications for nanophase materials have been identified, but progress has been hampered by lack of sufficient quantities of material for performance evaluation studies and field testing. Thus, in addition to significantly augmenting the current level of support for basic research in the field, there is a need also to support work dedicated to the scale-up and manufacture of nanophase materials. Although the study of nanophase materials is still in its infancy, it is clear that an exciting new area of research has been opened up and that new materials with novel and useful properties will emerge. Just which avenues of endeavor will be most profitable, however, is not clear. The demonstrated success in synthesizing nanophase metals, single-phase and multiphase alloys, and ceramics with different and sometimes improved properties over those previously available indicates that the possibilities are very promising.

EXECUTIVE SUMMARY 6

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Materials with nanoscale structure (i.e. a structure of less than 100 nanometers in size) represent a new and exciting field of research. These materials can be produced in many ways, possess a number of unique properties compared with coarser-scaled structures, and have several possible applications with significant technological importance. Based on a state-of-the-art survey of research findings and commercial prospects, this new book concludes that much work remains to be done in characterizing these structures and their exceptional properties, and presents recommendations for the specific research and development activities needed to fill these gaps in our understanding.

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