Combustion synthesis of α-Si3N4 assisted by molten salt additives

Abstract

Combustion synthesis (CS) of silicon nitride (Si3N4), assisted by molten salt additives under high N2 pressure, is reported. The effect of salt additives (NaCl, MgCl2, and MgCl2∙6H2O) on the reaction temperature and the final α-Si3N4 content is studied. The maximum reaction temperature (Tmax) decreased with an increase in the amount of the salt additives. NaCl is found to be the most suitable as it results in 57.8% α-Si3N4 at 30 mass% concentration. MgCl2 is strongly hygroscopic, and MgCl2∙6H2O decomposes at very low temperature. Therefore, they absorb heat at low temperatures, which makes it difficult to reach the ignition temperature, thereby hindering the reaction propagation. Si3N4 is necessary as a diluent for creating pores in the raw materials to allow effective penetration and contact of N2 gas with the Si particles.