Fundamental Studies of Controlled Microstructure Developmentby the Self-Propagating High-Temperature Synthesis Process

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A comprehensive investigation of the preparation of advanced materials by the Self-Propagating High-Temperature Synthesis (SHS) Process will be conducted. (The SHS process is a powder- based process, where at least one of the reactant materials is a finely-divided powder. Two primary classes of SHS reactions are those involving two solids (S/S) and those involving a solid and a gas (G/S); the reactions in both cases are characteristically exothermic. When the reactant mixture is "ignited" at one end, a self-sustaining or self-propagating "combustion wave" proceeds through the packed bed of material, leaving a condensed product or product mixture in its wake.) Enhanced and precise control of the product microstructure is a major goal of this program. The study is expected to identify and quantify key micromechanistic processes which determine the type, size, and distribution of the product phases (as well as the porosity characteristics) in the synthesized material. Activity in this program will focus on the intrinsic reaction kinetics in samples with a well-defined micro-structure. The samples will consist of a metallic or bimetallic foil embedded in an SHS powder mixture. The powder will be ignited in the usual fashion so that both the powder and the foil participate in the SHS reaction. The diffusion, melting, and/or reaction at the foil interface will thus be driven on the same temperature and time scales as a conventional SHS reaction. Thus, a "microstructure" which can be readily characterized will be created, and an idealized large, flat interface will be subjected to the same temperature history as material in a totally powder-based SHS reaction. Diffusion coefficients, reaction kinetic parameters, and resulting phase distributions will all be obtained. Both a solid-solid and a gas-solid system will be studied. A relatively simple model of this idealized configuration will be developed to aid in the extraction of the fundamental kinetic parameters. This study is expected to establish a strong processing-microstructure-material properties relationship; the enhanced fundamental understanding of the interrelationships involved in SHS processing will lead to development of standardized approaches for producing materials with a controlled/tailored microarchitecture. Emphasis in this particular program will be placed on experimental and theoretical studies of the intrinsic kinetics involved; additional tasks regarding other aspects of SHS processing, particularly as applied to production of "functionally gradient" materials will be carried out on a parallel program being funded concurrently by the Army Research Office. Fundamental studies of the detailed mechanisms involved in SHS processing are needed for attainment of the full potential of this novel method for production of advanced materials with unique properties; without such understanding of these mechanisms, production of products with properties optimized for various end uses will remain very difficult and time consuming, with a long learning process having to be repeated for each new application. Fine-tuning the SHS process to produce materials with well-defined (and controlled) composition gradients and/or pore structures will make major contributions to such widely varied applications as catalyst support materials, heat-engine components, porous preforms for composite materials fabrication, disk brakes, high-temperature filter media, high-surface area adsorption materials, piezoelectric devices, and shape-selective separation media.