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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 (9), mellow melting point, and stable structure.
Silicon-nitride does not melt and decomposes at a temperature of 1900degC when under normal pressure. The specific heat is 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 190degC). 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 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 1800degC when used in a neutral atmosphere or reducing one.
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 strength, corrosion resistance and good wear 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 had mainly been used 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 materials.
The ceramic shed consists of fine powder and silicon carbide. The silicon carbide is combined with silicon nitride using metal Si Nitrided as the bonding phase. This material can be used for blast furnace parts and 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 silicon carbide 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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