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 (9), mellow melting point, and sturdiness.
Silicon-nitride does not melt and decomposes at 1900degC when under normal pressure. It has a specific temperature of 711.8J/kg*degC. The microhardnesses of the a-phase and b-phase are respectively 1016GPa & 24.532.65GPa. As it is a strong compound with covalent bonds, no liquid phase will form below its decomposition temperatures (about 1900degC). Silicon nitride is sintered by using oxide additives. Oxide materials are Y2O3, AL2O3, etc., which promote sintering. When the addition is higher than 20%, the reaction is based on using the SiO2 oxide layer formed on the silicon-nitride particles’ surface and the added oxide in order to form a liquid and penetrate into 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 resistance. Because of the strong covalent bond at high temperatures and low diffusion coefficient, Si3N4 must be manufactured under high temperature and pressure using sintering agent. 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 was mainly used as a binding phase for refractory materials. 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. The silicon carbide is combined with silicon nitride using metal Si Nitrided as a binder. It can then be used to make a silicon nitride-bonded silicon carbide, which is widely used in the blast furnace body. 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 refractory phase, Baosteel used the corrosion resistance Si3N4 against molten iron and steel.
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