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The properties of Si3N4:
The molecular form of silicon nitride Si3N4 is where N makes up 39.94% and Si 60.06%. Si and N are bound by a strong, covalent bond (30% ionic). This gives Si3N4 a high Mohs hardness (Mohs Hardness 9), a high melting point, and stability.
Silicon-nitride does not melt and decomposes at normal pressure of 1900degC by sublimation, with specific heat of 718.8J/kg*degC. The microhardnesses of the a-phase and b-phase are respectively 1016GPa & 24.532.65GPa. As it is a covalently-bonded compound, liquid phase will not form under its decomposition temperatures (about 1900degC). Silicon nitride is sintered by using oxide additives. Oxide materials are mainly Y2O3, AL2O3, etc., which promote sintering. When the addition is higher than 20%, the reaction is to use a SiO2 oxide layer formed on top of the silicon-nitride particles to create a liquid phase that can penetrate into the crystal. The material must have a high degree of diffusion.
Material Si3N4 has a high thermodynamic stability. In an oxidizing environment, silicon nitride is suitable up to 1400degC. It can also be used at up to 1850degC when used in neutral or reducing environments.
Applications of silicon-nitride materials to refractory materials
The excellent properties of silicon nitride are well known for their high temperature structural materials. These include high-temperature resistance, wear resistance and corrosion. Because of the strong covalent bond at high temperatures and low diffusion coefficient, Si3N4 must be manufactured under high pressure and temperature. Otherwise, it would be difficult to produce silicon nitride material of high quality. This production cost and these equipment limitations are hard for the metallurgical industries to accept. The research on refractories began late, and was not comprehensive. There are many theories derived from ceramics but little innovation. In the past silicon nitride mainly existed as a phase of bonding in refractory products. The nitriding of metal Si combined with fine powder and corundum, or silicon carbide to produce a mixture of hard-to sinter products.
The ceramic shed consists of fine powder and silicon carbide. It uses metal Si Nitrided to Form Silicon Nitride to Combine Silicon Carbide, and is used to make a silicon nitride-bonded silicon carbide material that can be used to produce blast furnace bodies, among other parts. Material has better high-temperature properties than the clay-bonded silicone carbide shed. This solves the problem caused by the oxidation and swelling of the shed when using the clay-bonded shed. As the binding material for the second phase of Baosteel’s project, Si3N4’s corrosion resistance against molten iron and steel is used.
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