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The physical properties, production methods and application fields of silicon nitride

wallpapers News 2020-10-15
Silicon nitride is an inorganic substance with the chemical formula Si3N4. It is an essential structural ceramic material with high hardness, inherent lubricity, wear resistance, and atomic crystals; it is resistant to oxidation at high temperatures. Moreover, it can resist the impact of cold and heat. It will not be broken if it is heated to more than 1000℃ in the air, cooled rapidly and then heated rapidly. It is precise because of silicon nitride ceramics' excellent characteristics that people often use to manufacture bearings, turbine blades, mechanical seal rings, permanent molds, and other mechanical components. If silicon nitride ceramics, which are resistant to high temperatures and difficult to transfer heat, are used to make the heating surfaces of engine components, not only can the quality of the diesel engine be improved, fuel savings and thermal efficiency can be improved.
Physical properties

The relative molecular mass is 140.28. Gray, white or off-white. It is a high-temperature insoluble compound, no melting point, strong resistance to high-temperature creep, and the reaction sintered silicon nitride without a binder has a load softening point above 1800℃; hexagonal system. The crystal is a hexahedron. The density of Si3N4 prepared by the reaction sintering method is 1.8~2.7g/cm3, and the thickness of Si3N4 prepared by the hot pressing method is 3.12~3.22g/cm3. The Mohs hardness is 9~9.5, the Vickers hardness is about 2200, and the microhardness is 32630MPa. The melting point is 1900°C (under pressure). It usually decomposes at about 1900°C under normal pressure. The specific heat capacity is 0.71J/(g·K). The heat of formation is -751.57kJ/mol. The thermal conductivity is (2-155) W/(m·K). The coefficient of linear expansion is 2.8~3.2×10-6/℃ (20~1000℃). Not soluble in water. Soluble in hydrofluoric acid. The temperature at which oxidation starts in the air is 1300~1400°C. The specific volume resistance is 1.4×105 ·m at 20°C and 4×108 ·m at 500°C. The modulus of elasticity is 28420~46060MPa. The compressive strength is 490MPa (reaction sintered). It reacts with dicalcium dinitride at 1285°C to form calcium dinitrogen silicide. At 600°C, it reduces transition metals and releases nitrogen oxides. The bending strength is 147MPa. It can be prepared by heating silicon powder in nitrogen or reacting silicon halide with ammonia. The resistivity is 10^15-10^16Ω.cm. It can be used as a high-temperature ceramic raw material.

production method

There are two production methods for silicon nitride ceramic products, namely, reaction sintering and hot pressing sintering. The reaction sintering method shapes silicon powder or a mixture of silicon powder and silicon nitride powder according to the general production method of ceramic products. Then in the nitriding furnace, pre-nitriding at 1150~1200℃, after obtaining a particular strength, it can be machined on the machine tool and then further nitriding at 1350~1450℃ for 18~36h, until all becomes silicon nitride. The product thus obtained has accurate dimensions and sound volume. The hot-pressing sintering method is to press silicon nitride powder and a small number of additives (such as MgO, Al2O3, MgF2, AlF3 or Fe2O3, etc.) at a pressure of 19.6MPa or more and under the conditions of 1600~1700℃. Generally, products made by hot-pressing sintering have higher density and better performance than products made by reaction sintering.

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Silicon nitride ceramic materials have excellent properties such as high thermal stability, strong oxidation resistance and high product size accuracy. Since silicon nitride is a covalent compound with a high bond strength and can form an oxide protective film in the air, it also has good chemical stability. It will not be oxidized below 1200°C. The formation of a protective film at 1200~1600°C can prevent further. It is rusted and is not infiltrated or corroded by many molten metals or alloys such as aluminum, lead, tin, silver, brass, nickel, etc., but can be corrupted by molten magnesium, nickel-chromium alloy, stainless steel, etc.

Silicon nitride ceramic materials can be used for high-temperature engineering components, advanced refractories in the metallurgical industry, corrosion-resistant parts and sealing components in the chemical industry, tools cutting tools in the machining industry, etc.

Since silicon nitride can form a strong bond with silicon carbide, aluminum oxide, thorium dioxide, boron nitride, etc., it can be used as a bonding material to be modified in different proportions.

Silicon nitride can also be applied to solar cells. After the silicon nitride film is plated by the PECVD method, not only can it be used as an anti-reflection film to reduce the reflection of the incident light, but also, during the deposition of the silicon nitride film, the hydrogen atoms of the reaction product enter the silicon nitride film, and silicon wafer Played a role in passivating defects. The ratio of the silicon nitride ratio to silicon atoms is not strictly 4:3 but fluctuates within a specific range according to different process conditions. The physical properties of the film corresponding to different atomic ratios are other.

They are used in ultra-high temperature gas turbines, aircraft engines, electric furnaces, etc.
 
Trunnano is one of the world's largest producers of silicon nitride powder. If you have any questions or needs, please contact Dr. Leo by email: brad@ihpa.net.

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