A combustion synthesis (SHS) process has been developed for the synthesis of Si 3 N 4 powder under low nitrogen pressures. Si and NaN 3 powders were used as the reactants, and NH 4 Cl powder was added as a catalytic agent. These powders were mixed and pressed into a cylindrical compact. The compact was wrapped up with an igniting agent (i.e., Ti 1 C), and the synthesis reaction was triggered by the combustion of the igniting agent. Addition of NH 4 Cl was found necessary for the combustion synthesis reaction under low nitrogen pressures (,1.2 MPa). The product as synthesized is mostly in the form of agglomerated fine particles (0.1-1 mm in diameter) and is composed mainly of a-phase and a minor amount of b-phase. Effects of various experimental parameters (N 2 pressure, NaN 3 , NH 4 Cl, and Si 3 N 4 contents) on the product conversion and the combustion temperature were investigated. A possible reaction mechanism was proposed that explains the effects of the experimental parameters on the synthesis reaction.

nitrides, 4 intermetallics, 5 and composites, 6 have been produced The effect of the addition of aluminum on the ignition and by this method. self-propagating high-temperature synthesis (SHS) reaction Considerable effort has been exerted toward studying the between titanium and carbon was experimentally investicombustion synthesis reactions theoretically and experimengated. Although TiC was the only product compound in the tally. 7-10 However, consistent agreement between experimental final product, the reaction between titanium and aluminum results and theoretical prediction is currently insufficient. This was believed to occur before the ignition of the reaction insufficiency is primarily the consequence of the numerous between titanium and carbon, as evidenced by a hump that simplifying assumptions incorporated into the theoretical modappeared in the temperature profile during heating. The els and the lack of a firm understanding of the reaction mechaignition temperature of the reaction between titanium and nisms in the SHS process. carbon was significantly decreased by the addition of alumi- Holt et al. 11 and Dunmead et al. 12 studied the kinetics of num. This was explained by the fact that the aluminum titanium-boron and titanium-carbon combustion reactions, provides an easier route for reactant mass transfer, thus respectively, by measurement of the combustion temperature significantly increasing the reaction rate of the reaction and wave velocity. The formation of TiC-metal composites by between titanium and carbon. As the temperature was the SHS reaction of titanium, carbon, and metals also has been increased to beyond the melting point of aluminum during studied by Dunmead et al., 12 Fu et al.,13 and Choi and Rhee. 14 heating, aluminum melted and titanium dissolved into it.Dunmead et al. 12 studied the reaction kinetics in the titaniumAs the temperature was increased further (.1050؇C), the carbon-nickel system and concluded that the SHS reaction of titanium-containing aluminum spread over the carbon parthe titanium-carbon-nickel system is controlled by dissolution ticles. Ignition is believed to have occurred by the reaction of carbon into a titanium-nickel melt and that the product is of titanium and carbon at the interface by the diffusion of formed by the precipitation of TiC particles from the melt. Fu titanium through the aluminum melt to the interface. The et al. 13 performed a fundamental study on the SHS preparation rate of heating and the density of the reactant compact also of TiB 2 -Al composites by measuring the combustion temperaaffected the ignition temperature. These were explained by ture, propagation rate, and ignition delay time. They found that their influence on the extent of the capillary spreading of a maximum propagation rate was obtained when the aluminum the titanium-containing aluminum melt on the carbon parcontent is ϳ10 vol% and proposed that a molten metallic phase ticles or by the formation of a TiAl x layer at the interface formed during reaction provides better conditi...

A SHS process has been established for synthesis of AlN powder under low nitrogen pressures. Al and NaN3 powders were used as the metal and nitrogen sources, respectively. The compact of the mixture of the reactants was wrapped up with aluminum foil and then wrapped up with an igniting agent (i.e., Ti + C). The synthesis reaction was triggered by the combustion of the igniting agent. The wrappings were found necessary in achieving high product conversions under low nitrogen pressures (<1 MPa). The product conversion was also affected by the reactant composition and the nitrogen pressure. High conversions were obtained when the mass ratio of Al to NaN3 was 1/2 or lower and the nitrogen pressure was 0.5 MPa or higher. The AlN powders as synthesized were observed to have two major types of morphology, i.e., granular particles with 0.5–3 μm in diameter and fibers with aspect ratios of 10–800.

No abstract