Because of kinetic limitations, self-sustaining combustion synthesis reactions cannot be initiated in dense powder compacts. In compacts of Mo + 2Si, self-propagating waves can be initiated in samples with less than 78% relative density. At this and higher densities, no waves could be initiated without field-activation. In the presence of an electric field (at values of 7 and 13 V⅐cm −1 ), reactant compacts with densities up to 95% could sustain a combustion wave to produce MoSi 2 . In the absence of a field (for lowerdensity samples) the wave propagated in a non-steady-state (pulsating) mode, while under the influence of a field the wave propagated in a steady-state mode. The dependences of wave velocity and combustion temperature on the relative density of the reactants were qualitatively similar, showing maxima at a relative density of about 65%. These observations are explained in terms of the contribution of a liquid phase in the MoSi 2 -Mo 5 Si 3 binary to the synthesis kinetics. Although not detected by X-ray diffraction analysis, small amounts of Mo 5 Si 3 were discerned at the grain boundaries of the MoSi 2 product. The particle size of the silicide synthesized from 95% dense reactants was significantly smaller than those synthesized from reactants with lower densities, but the reason for this observation is not well understood at this time.
The effect of AlN on the structure formation of SiC was investigated. SiC was synthesized in the presence of AlN under vacuum at 1500°C, and the result was cubic SiC. The synthesis of AlN-SiC composites through the reaction Si 3 N 4 ؉ 4Al ؉ 3C ؍ 3SiC ؉ 4AlN was also investigated and compared with synthesis via field-activated self-propagating combustion (FASHS). Reactants were heated in a vacuum furnace at temperatures ranging from 1130°to 1650°C. Below 1650°C, the reaction is not complete and at this temperature the product phases are AlN and cubic SiC. At 1650°C, the product contained an outer layer which contained -SiC only and an inner region which contained AlN and cubic SiC. 2H-SiC and AlN composites synthesized via field-activated selfpropagating combustion were annealed at 1700°C under vacuum. The AlN dissociated and evaporated and the 2H-SiC transformed to the cubic  phase. Reasons for the differences in products of furnace heating and FASHS are discussed.
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