1. Silicon nitride belongs to the hexagonal crystal system and has α-type (low temperature) and β-type (high temperature) transformations. When the temperature rises to 1600°C, it changes from α type to β type, and it is difficult to change back. The physical properties of α and β types are shown in the table.
It has high thermal conductivity, small thermal expansion coefficient, good thermal shock resistance, high temperature strength and wear resistance. However, in a high-temperature oxidizing atmosphere, oxidation begins at 1200 to 1400°C and becomes violently oxidized above 1400°C. The synthesis method of silicon nitride involves grinding silicon powder and heating it to 1200~1450°C in N2 gas to form an exothermic reaction of 3Si+4N2=2Si3N4+737J.
2. Aluminum silicon oxynitride, Si-Al-O-N solid solution. It has a β-Si3N4 crystal structure and has the advantages of strong oxidation resistance, corrosion resistance and high strength.
3. Silicon oxynitride is produced by mixing high-purity silicon powder and SiO2 powder at a molecular ratio of 3:1, grinding them, and heating them to 1450°C by adding a mixed gas of 80% N2 and 20% H2.
4. Aluminum nitride is a hexagonal crystal with lead-zincite (hexagonal) type crystal. It is white and the single crystal is colorless and transparent. The density is 3.26g/cm3 and the sublimation temperature is 2450°C. The product has high compressive strength, low thermal conductivity and slightly larger linear expansion coefficient.
5. The crystals of boron nitride and graphite are similar, in the shape of hexagonal plates, with a fat feel and white color, so it is called white graphite. The density is 2.2g/cm3, the chemical properties are neutral, and it is stable to above 2000°C in a reducing atmosphere.
