The relative molecular weight is 140.28. Gray, white, or non-white. It is a high-temperature insoluble compound without a melting point with strong resistance to high-temperature creep. Softening point under load of reacted-sintered silicon nitride without binder above 1800°C; hexagonal system. The crystal has a hexagonal shape. The density of Si3N4 obtained by reaction sintering is 1.8~2.7 g/cm3, and the density of Si3N4 obtained by hot pressing is 3.12~3.22 g/cm3. The Mohs hardness is 9~9.5, the Vickers hardness is about 2200, and the microhardness is 32630 MPa. The melting point is very high and can exceed 2000°C. Specific heat capacity is 0.71 J/(g-K).
The heat of formation is 751.57 kJ/mol. Thermal conductivity is (2-155)W/(m-K). The linear expansion coefficient is 2.8~3.2×10-6/°C (20~1000°C). Insoluble in water. Soluble in hydrofluoric acid.
The temperature at which oxidation begins in the air is 1300~1400°C. The volume resistivity is 1.4×105 m at 20°C and 4×108 m at 500°C. The modulus of elasticity is 28420~46060MPa. The compressive strength is 490 MPa (reaction sintering). It reacts with calcium nitride to form calcium silicide at 1285°C, and reduces the transition metal at 600°C to release nitrogen oxides. The flexural strength is 147 MPa. It can be produced by heating silicon powder in nitrogen or by reacting silicon halide with ammonia. The resistivity is 10^15-10^16Ω.cm. Can be used as a raw material for high-temperature ceramics.