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INTRODUCTION. 7 1 Introduction. 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. From a historical viewpoint, this concept received its initial impetus in the fields of ceramics and polymeric materials, because traditional methods of materials processing were not applicable. Examples of recent innovations in ceramics (including glass) are laser-pyrolysis of monodispersed particles and hydrothermal synthesis of composites. Of recent interest in polymeric materials has been the development of rigid-rod polymers, self-assembled polymer architectures, polymer blends and alloys, and block copolymers. Developments of comparable significance have occurred in the semiconductor and structural materials fields. Chemical vapor deposition, reactive sputtering, ion-beam processing, and other vapor-phase methods have become the enabling technologies for surface processing in the thin-film device and integrated- circuit industries. These same technologies have recently been adapted to structural applications, such as wear- resistant coatings for bearings, cutting tools, and mirrors. A long history also exists in the area of synthesis of multilayered structures by vapor-deposition methods. There materials have been found to exhibit striking properties, such as a supermodulus effect. Current research is focused on the design, synthesis, and processing of ultrafine material microstructures, extending 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 materials properties when the scale of the microstructure approaches nanoscale dimensions. Birringer and coworkers (1986) tabulated a number of properties measured on nanocrystalline metals and
INTRODUCTION. 8 compared them with values for their coarse-grained counterparts and similar glassy materials. Some of these comparisons are shown in Table 1. The changes in this variety of materials properties are significantly greater in going from conventional crystalline material to the nanocrystalline form than are observed in going from crystalline to glassy solid. Prepared by conventional techniques, these latter changes are generally less than 10 percent. A series of symposia have addressed the synthesis and processing of nanoscale ceramics and polymers (Hench and Ulrich, 1984; Karasz, 1985; Hench and Ulrich, 1986; Mackenzie and Ulrich, 1988). The materials research community has responded by initiating a new series of symposia at Materials Research Society meetings that extended submicron-scale microstructure concepts beyond nonmetallics to the full range of materials. Table 1 Properties of Nanocrystalline Materials Compared With Their Conventional Coarse-Grained Counterparts* Property Material Nanocrystal Conventional Polycrystal Thermal expansion [10-6 K-1] Cu 31 (+80%) 17 Density [g/cm3] Fe 6 (-25%) 7.9 Saturation magnetization @ 4 K [emu/g] Fe 130 (-40%) 222 Susceptibility [10-6 emu/Oe g] Sb 20 (+2000%) -1 Fracture stress [kp/mm2] Fe (1.8% C) 600 (+1000%) 50 Superconducting Tc [K] Al 3.2 (+160%) 1.2 * The percentages in parentheses represent changes from the reference crystal value (Birringer et al., 1986). The present report gives a state-of-the-art assessment of activity in this exploding field of research and attempts to identify new areas of research opportunity and some potential application areas for the future. The
