Kinetics and Mechanisms of Rapid Heterogeneous Reactions During Combustion Synthesis

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Abstract - VarmaThis work aims at a fundamental understanding of combustion synthesis especially in the volume combustion synthesis (VCS) mode. A knowledge of reaction mechanisms and chemical kinetics of VCS that incorporates phase changes and microstructure evolution is necessary to control product structure and properties. To achieve this, a set of new experimental methods is used. The kinetics of high-temperature (up to 3000o K) heterogeneous reactions at heating rates up to 106 oK/sec is studied by computer-assisted electrothermo- graphy (CAE). To overcome difficulties in determining laws from the time dependence of the reaction rate, time-resolved X-ray diffraction (TRXRD) is used; this provides continuous measurement of the kinetics of phase formation under reaction conditions. Temperature distribution across the sample during all stages of VCS is monitored by a set of thermocouples and by a high-speed infrared imaging system. Direct observation of microstructural transformation is done with a modified digital high-speed microscopic video recorder with rates up to 12,000 frames/sec. The first stage of the research will look at formation of nitrides and binary intermetallics of nickel, aluminum, titanium, and niobium. In the second stage, more complex systems will be examined including direct formation of ceramic-metal composites.