Reaction mechanism in combustion synthesis of α-Si ₃ N4 powder using NaN ₃

A combustion synthesis method for the synthesis of α-Si ₃ N ₄ from a reactant compact composed of Si, NaN ₃ , and NH ₄ X 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 Si ₃ N ₄ . In addition to NH ₄ Cl, which was considered previously to function as a catalytic agent, other ammonium halides (i.e., NH ₄ F, NH ₄ Br, and NH ₄ I) were found to be capable of catalyzing the synthesis reaction with NH ₄ Cl being the most effective. Si ₃ N ₄ could not be produced when NaN ₃ was replaced by C ₃ H ₆ N6NaN ₃ 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 NaN ₃ reduces SiX _x (formed by reaction of Si and NH ₄ X), promoting the nitridation reaction to form Si ₃ N ₄ . NaN ₃ thus plays a role as a reducing agent in the synthesis reaction.