Properties and applications of silicon carbide

Silicon nitride is an important structural ceramic material. It is a superhard material, which is lubricated and wear-resistant. It is an atomic crystal. Antioxidant at high temperature. Moreover, it can resist the cold and heat shock, heating in the air to more than 1000℃, cooling sharply and then heating sharply, it will not break. It is because of its excellent properties that silicon nitride ceramics are often used to manufacture mechanical components such as bearings, turbine blades, mechanical sealing rings and permanent moulds.

Silicon nitride is very strong, especially hot pressed silicon nitride, which is one of the hardest materials in the world. It is extremely resistant to high temperature, the strength has been able to maintain to 1200℃ high temperature without falling, heat will not melt into a melting body, until 1900℃ will decompose, and has amazing chemical corrosion resistance, can withstand almost all inorganic acid and caustic soda solution below 30%, also can withstand a lot of organic acid corrosion; It is also a kind of high performance electrical insulation material.

Silicon nitride – chemical formula Si3N4. White powdery crystals; The melting point was 1900℃, and the density was 3.44 g/cm (20℃). There are two variants: the leume type is a hexagonal dense accumulation structure; The type is like SPAR structure. Silicon nitride is gray with impurities or excess silicon.

Silicon nitride hardly interacts with water; Ammonium salt and silica were formed by slow hydrolysis in the solution of strong acid. Soluble in hydrofluoric acid, and dilute acid does not work. The solution of strong strong base can slowly corrode silicon nitride, and the molten strong base can quickly convert silicon nitride to silicate and ammonia. Silicon nitride can reduce the transition metal (see transition element) oxides, lead oxide, zinc oxide and tin dioxide at more than 600℃, and release nitrogen oxides and nitrogen dioxide. The following reactions occurred between silicon nitride and Ca3N2 nitride at 1285℃ :

Casin2 Ca3N2 + Si3N4 ─ – > 3

The relative molecular mass is 140.28. Gray, white, or off-white. It is a high-temperature insoluble compound with no melting point and strong resistance to high-temperature creep. The softening point of reaction sintering of silicon nitride without binder is above 1800℃. Hexagonal system. The crystal is hexahedral. The density of Si3N4 obtained by reaction sintering was 1.8~2.7g/cm3, and the density of Si3N4 obtained by hot pressing was 3.12~3.22g/cm3. Mohs hardness 9~9.5, vickers hardness is about 2200, micro hardness is 32630MPa. Melting point 1900℃ (under pressure). It is usually decomposed at about 1900℃ under normal pressure. The specific heat capacity is 0.71J/(g·K). The heat of formation is -751.57 kilojoules per mole. The thermal conductivity is (2-155) W/(m·K). The linear expansion coefficient is 2.8~3.2×10-6/℃(20~1000℃). Insoluble in water. Soluble in hydrofluoric acid. The temperature at which oxidation begins in air is 1300~1400℃. Specific volume resistance is 1.4×105 ·m at 20 ° c and 4×108 ·m at 500 ° c. The elastic modulus is 28420~46060MPa. The compressive strength is 490MPa (reaction-sintered). At 1285℃, it reacts with dicalcium disulfide to produce calcium disilicate. At 600℃, the transition metal is reduced and nitrogen oxides are released. The bending strength is 147MPa. It can be made by heating silicon powder in nitrogen or reacting silicon halide with ammonia. Resistivity in 10 ^ 15-10 ^ 16 Ω. Cm. It can be used as high temperature ceramic material.

Silicon nitride ceramic has the characteristics of high strength and high temperature resistance. Among the ceramic materials, its comprehensive mechanical properties are the best. It has the properties of heat shock resistance, oxidation resistance, wear resistance and corrosion resistance. In the mechanical industry, silicon nitride ceramics are used for ball bearings, rollers, ball seat rings, tools, new ceramic tools, pump plunger, spindle sealing materials, etc.

In the chemical industry, silicon nitride ceramics are used as wear – and corrosion-resistant components. Such as ball valve, pump body, combustion carburetor, filter, etc.

In the gold treatment industry, due to the high temperature resistance of silicon nitride ceramics, the friction coefficient is small, has the self-lubricity. For most metals, alloy solution stability, therefore, can be made of metal material processing tools, such as shifting rods, extrusion, wire casting mold, roller, transfer roller, heating fixture, thermocouple nesting, metal heat treatment supporting parts, crucible, aluminum liquid guide, aluminum lining.

Silicon nitride ceramic materials are also widely used in the electronics, military and nuclear industries.

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