The molecular formula of silicon nitride is Si3N4, which belongs to covalent bonding compound. Silicon nitride ceramics belong to polycrystalline material, and the crystal structure belongs to hexagonal system, which is generally divided into α、β Both crystal orientations are composed of [SiN4] 4-tetrahedron, in which β- Si3N4 has high symmetry and small molar volume. It is a thermodynamic stable phase at temperature, but α- Si3N4 is easy to form dynamically. At high temperature (1400 ℃ ~ 1800 ℃), α Phase transition occurs and becomes β This phase transition is irreversible, so α The phase is conducive to sintering.

 

Appearance of silicon nitride products

 

The appearance of silicon nitride obtained from different crystalline phases is different, α- Si3N4 is white or grayish white, loose wool or needle, β- Si3N4 is dark in color and appears as dense granular polyhedron or short prism. Silicon nitride whiskers are transparent or translucent. The appearance of silicon nitride ceramics is gray, blue gray to gray black, which varies with different density and phase ratio, as well as other colors due to additives. The silicon nitride ceramic surface has metallic luster after polishing.

 

Electrical insulation

 

Silicon nitride ceramics can be used as high-temperature insulating materials, and their performance indexes mainly depend on the synthesis method and purity. The free silicon not nitrided in the material and the magazines such as alkali metal, alkaline earth metal, iron, titanium and nickel brought in during the preparation can deteriorate the electrical properties of silicon nitride ceramics. Generally, the specific resistance of silicon nitride ceramics at room temperature and in dry medium is 1015 ~ 1016 ohms, and the dielectric constant is 9.4 ~ 9.5. At high temperature, silicon nitride ceramics still maintain a high specific resistance. With the improvement of process conditions, silicon nitride can enter the ranks of common dielectrics.