CARBOTHERMAL PREPARATION OF SILICON-NITRIDE - INFLUENCE OF STARTING MATERIAL AND SYNTHESIS PARAMETERS

C and SiO2 of different types were mixed and heat-treated (1410-degrees to 1550-degrees-C), according to a standardized temperature program, in flowing N2 at different pressures. It was found that a starting material combination of C (115 m2/g) and SiO2 (50 m2/g) yielded pure Si3N4 after about 2 h at 1550-degrees-C and 1.3 MPa. By adjustment of the pressure in the range 2 to 6 MPa, irreversible evaporation of SiO(g) as well as formation of nonequilibrium phases was suppressed, and the amounts of residual C and O in the sample were controlled. Even a small amount of CO in the N2 was observed to retard the nitridation. Possible explanations are discussed, based on thermodynamic calculations on the Si-O-N-C system.