Synthesis of silicon nitride by carbothermal reduction and nitriding of silica: Control of kinetics and morphology

Abstract

Abstract Reaction of finely dispersed high-purity silica and carbon in nitrogen flow begins at approximately 1250°C, and effective synthesis of α-silicon nitride was carried out at 1510–1550°C. Kinetics of the process is accelerated by seeding with a fine α-silicon nitride powder and the intensity of the seeding effect increases with increasing surface area of the seeding powder. The appropriate seeding and control of silicon monoxide evolution results in highly uniform and isometric α-silicon nitride crystals. If silicon monoxide evolved in the early stages of the reaction is not effectively consumed for silicon nitride growth, its reoxidation and redistribution of silica in the reaction mixture take place. Such heterogeneities, formed during the reaction, significantly slow down the overall process. These observations are consistent with a mechanism involving crystallisation of silicon nitride from gaseous silicon monoxide, carbon monoxide and nitrogen as the rate-limiting step. Silicon nitride is most probably formed in the sites where solid carbon is not present (i.e. in sites where carbon activity is lower than unity). The equilibrium temperature of the reaction Si 3 N 4 + 3C ⇌ 3SiC + 2N 2 falls within the range 1510–1550°C (at 101 kPa N 2 ). An addition of free silicon to the reaction system promotes β-silicon carbide formation.