Reaction mechanism in combustion synthesis of α-Si₃N4 powder using NaN₃

A combustion synthesis method for the synthesis of α-Si₃N₄ from a reactant compact composed of Si, NaN₃, and NH₄X and wrapped up with an igniting agent was investigated. Wrapping the reactant compact with the igniting agent (i.e., a mixture of Ti and C powders) was found necessary for the synthesis of Si₃N₄. In addition to NH₄Cl, which was considered previously to function as a catalytic agent, other ammonium halides (i.e., NH₄F, NH₄Br, and NH₄I) were found to be capable of catalyzing the synthesis reaction with NH₄Cl being the most effective. Si₃N₄ could not be produced when NaN₃ was replaced by C₃H₆N6NaN₃ was thus considered to exert an essential effect on the combustion synthesis reaction other than functioning as a solid-state nitrogen source as considered previously. It was proposed that Na vapor produced by decomposition of NaN₃ reduces SiX_x (formed by reaction of Si and NH₄X), promoting the nitridation reaction to form Si₃N₄. NaN₃ thus plays a role as a reducing agent in the synthesis reaction.