Silicon carbide Properties
Silicon carbide (SiC), also known as carborundum sand, is rarely found in nature and is commonly used as a synthetic raw material in industry. It gains excellent properties of thermal resistance and wear resistance. It has two crystal forms: low-temperature morphology β- SiC, belonging to cubic structure, and high-temperature form α- SiC belonging to the hexagonal structure. Silicon carbide has two types according to color: green silicon carbide and black silicon carbide.
Its true density is 3.21g/cm3, and the decomposition (sublimation) temperature is 2600 ℃.
It is a hard material with a Mohs hardness of 9.2.
The thermal expansion coefficient of SiC is not significant, and the average thermal expansion coefficient of SiC is 4.4 within the range of 25 ℃~1400 ℃ × 10-6/℃.
Performance of silicon carbide
Silicon carbide has a high thermal conductivity (58.6W/m · K). Usually, the higher the SiC content, the lower the temperature, and the greater the thermal conductivity. Low thermal expansion coefficient and high thermal conductivity can make SiC refractory materials have good thermal shock stability.
At low temperatures, the chemical properties of Silicon carbide are relatively stable, with excellent wear and corrosion resistance. It is also resistant to corrosion in boiling hydrochloric acid, sulfuric acid, and hydrofluoric acid. However, at high temperatures, it can react with certain metals, salts, and gases. Silicon carbide remains stable in a reducing atmosphere until 2600 ℃, but oxidation occurs in a high-temperature oxidizing atmosphere:
SiC+2O2 → SiO2+CO2
In addition, Silicon carbide material is a non-oxide with strong covalent bonding and poor sintering ability with oxides.
SiC is widely used as an additive to improve material properties, especially slag resistance and thermal shock stability, due to its advantages such as low thermal expansion coefficient, high thermal conductivity, high high-temperature strength, good slag resistance, and the ability to form protective oxidation.
Silicon carbide material Usages:
Silicon Carbide (SiC) in Shaped Refractory Materials
In shaped refractory materials, Silicon carbide can be used as the main component to make SiC products or as an additive to make semi-SiC products.
SiC refractory material refers to a type of advanced refractory material with SiC as the main component, which is fired from industrial SiC as the raw material, also known as SiC products. SiC products can be classified based on SiC content, type of binder, and addition amount. The performance of materials largely depends on the bonding condition between SiC particles in the material. So SiC products are usually classified based on the type of bonding phase. According to the different binding phases, there are silicon carbide ceramics such as oxide-bonded SiC, nitride-bonded SiC, self-bonded SiC, silicon infiltration reaction sintered SiC, etc.
Semi-SiC refractory products are those containing silicon carbide as a secondary or auxiliary component. According to its different materials, there are clay clinker SiC products, high aluminum oxide carbide products, and corundum SiC products. Due to the presence of silicon carbide in these products, their thermal shock stability, thermal conductivity, and strength are significantly improved.
Adding a small amount of silicon carbide to clay clinker SiC products has a significant effect on improving the thermal shock stability of the products. As the content of SiC fine powder in the ingredients increases, the thermal shock stability of the products gradually improves. Adding an appropriate amount of SiC (the most suitable amount is 30%) to high aluminum SiC products, and adding an appropriate amount of phosphoric acid, the products have high thermal shock stability, good thermal conductivity, and high strength. Adding a small amount of SiC fine powder to corundum SiC products can significantly improve their thermal shock stability. As the amount of SiC fine powder increases, the thermal shock stability regularly increases. For example, using brown corundum as the aggregate, adding 10% SiC fine powder, using phosphoric acid as the binder, high-pressure forming, and heat treatment at 1450 ℃ to produce slide rail bricks for steel rolling heating furnaces, the application effect is good.
Silicon Carbide (SiC) in Amorphous Refractory Materials
In amorphous refractory materials, silicon carbide can be used as the main component to make SiC-based castables. It works as an additive to improve the performance of other castables, especially in terms of slag resistance and thermal shock stability. The research on the improvement of castable properties by SiC mainly focuses on aspects such as corundum castables and high alumina castables.
The most common application of SiC in amorphous refractory materials is for the working lining of the blast furnace tapping channel, which has a history of more than 20 years and has good performance. At present, Al2O3-SiC-C castable is widely used in larger blast furnaces both domestically and internationally, greatly extending the service life of the iron channel. In addition, amorphous refractory materials containing SiC are widely used in the iron and steel industry as liners for hot metal pretreatment, cupola, and induction furnaces; The combustion chamber side wall lining and boiler tube protective lining of the garbage incinerator; Cement kiln preheater lining in the cement industry; The cyclone separator lining of thermal power plants, the combustion chamber, lining, and high-temperature separator of circulating fluidized bed boilers; The firing kiln shed boards, as well as silicon and aluminum outlets in the ceramic industry.
In summary, the addition of SiC can improve the high-temperature strength and thermal shock stability of Al2O3-SiO2 based castables. However, research on the resistance of SiC to lead slag corrosion has not been reported yet.
But SiC is thermodynamically easy to react with oxygen in the air. In practical applications, especially under high temperatures, low oxygen pressure, and long-term effects, the oxidation rate of SiC is very fast.
Through the study of the microstructure of the high-temperature oxidation layer on the surface of SiC, it was found that the oxidation layer generated by SiC materials in the range of 1040~1560 ℃ has the following characteristics on their high-temperature oxidation resistance:
1) Below 1360 ℃, the oxidation layer formed on the surface of SiC particles is very thin. There is no significant changes in microstructure. The oxidation resistance is good and it is in a stable stage of oxidation resistance.
2) When the temperature exceeds 1360 ℃, the thickness of the oxide layer on the surface of SiC increases significantly with the increase of temperature. The formed oxide layer has many pores. However, due to the gradual increase of the oxide layer, SiC still exhibits sufficiently high antioxidant performance. This process is a transitional stage.
3) Above 1520 ℃, the thickness of the oxide layer is larger and the outer surface is relatively flat. However, SiO2 in the molten state has strong flowability, making the oxidation layer at the edges and corners of SiC particles thinner. The gas from SiC oxidation reaction is prone to escape and form pores. That provides a channel for oxygen to enter, accelerating the oxidation rate of SiC. This stage is a rapid oxidation stage.
4) There is no obvious transition zone between the SiO2 layer formed on the surface and the SiC matrix.
