Application of Si-NaN3-Na2SiF6 - C Powder Mixture Combustion for Synthesis of Highly Dispersed Si3N4-SiC Ceramic Composition

The process of burning mixtures of powder reagents: sodium azide (NaN3), halide salt Na2SiF6, silicon and carbon at different ratios of reagents, in an atmosphere of nitrogen gas at a pressure of 4 MPa, for self-propagating high-temperature synthesis (SHS) of a highly dispersed powder ceramic composition Si3N4-SiC has been investigated. The thermodynamic calculations confirmed the possibility of synthesis of the Si3N4-SiC of different compositions in the combustion mode. The combustion process has been experimentally studied with thermocouple measurements of temperatures and combustion velocities. The study of combustion product using scanning electron microscopy and X-ray phase analysis showed that the use of azide SHS allows the synthesis of a powder product of Si3N4-SiC of various compositions with a small content of free silicon admixture. With a silicon carbide content of no more than 10%, the composition of the synthesized product practically coincides with the theoretical one and consists of ultrafine particles mainly of the α-modification of silicon nitride of equiaxed and fibrous forms with a particle size of 100–300 nm and a fiber diameter of 50–250 nm with a length of up to 5 μm, When the content of silicon carbide is more than 10%, the composition of the synthesized product differs from the theoretical one by a lower content of silicon carbide, an approach of the content of the β-modification of silicon nitride to the content of the α-modification, and an enlargement of the particle size to the micron level.