Characteristics of Silicon Carbide in Silicon Carbide Refractory Materials

Silicon carbide, also known as emery or refractory sand, is a ceramic-like compound formed by bonding silicon and carbon. Emery exists in nature in the form of a rare mineral such as moissanite. There are very few natural ones, and the ones used in industry are all synthetic raw materials. Silicon carbide is the main raw material for silicon carbide bricks and silicon carbide castables. Today I will give you a brief introduction to some aspects of silicon carbide, hoping it will be helpful to you.

There are at least 70 crystal forms of silicon carbide. α-SiC is the most common allomorph. It is formed at high temperatures above 2000℃. It has a hexagonal crystal structure (like wurtzite). The unit cell is a face-centered cubic structure. Each unit cell contains 4 C atoms and 4 Si atoms. Similar to diamond structure.

β-SiC, with a cubic crystal structure, is similar to diamond. It is generated below 2000℃. β-SiC will transform into α-SiC above 2100℃. As for another type of silicon carbide, μ-SiC is the most stable and there is a more pleasant sound when colliding. These two types have not yet been commercially applied.

The silicon carbide produced industrially in my country is divided into two types: black silicon carbide and green silicon carbide, both of which are hexagonal crystals. It is a hard material with a microhardness of 2840~3320kg/mm² and a Mohs hardness of 9.5.

The decomposition (sublimation) temperature of silicon carbide is 2600℃. The thermal expansion coefficient of SiC is not large. In the range of 25℃~1400℃, the average thermal expansion coefficient of SiC is 4.4×10-6/℃.

At present, high-density silicon carbide is produced through industrial synthesis through hot-pressing sintering. To put it simply, silicon carbide is made of quartz sand, petroleum coke (or coal coke), wood chips (salt is required when producing green silicon carbide), etc. The raw materials are smelted at high temperatures in a resistance furnace. Pure silicon carbide is colorless, while the brown-to-black color produced in industrial production is due to iron-containing impurities, and the green color is due to chromium. The rainbow-like luster on the crystal is due to the protective layer of silica produced on its surface.