Silicon nitride-application prospect and properties of it

Silicon nitride is a compound synthesized under artificial conditions.  Although silicon nitride was synthesized directly more than 140 years ago, it was only remembered as a stable "refractory" nitride.  After the Second World War, the rapid development of science and technology, the urgent need for high temperature resistance, high hardness, high strength, corrosion resistant materials.  After a long period of efforts, silicon nitride was not taken seriously until 1955. In the mid-1970s, silicon nitride ceramic products with high quality, low cost and wide application were really produced.  

Silicon nitride's structure and characteristics  

Silicon nitride is very strong, especially hot-pressed silicon nitride, and is one of the hardest substances in the world.  It is very high temperature resistance, strength has been able to maintain the high temperature of 1200℃ without falling, after heating will not melt into a melt body, until 1900℃ will decompose, and has amazing chemical corrosion resistance, almost all inorganic acid and 30% of the following caustic soda solution, also can withstand a lot of organic acid corrosion;  It is also a kind of high performance electrical insulation material.  Chemical formula Si3N4.  White powdery crystal; Melting point 1900℃, density 3.44 g/cm (20℃);  There are two variants: α type is hexagonal dense packing structure;  The β type is a crystalline structure.  Silicon nitride is gray with impurities or excess silicon.  

The production of silicon nitride has the following methods: at 1300 ~ 1400℃ powdery silicon and nitrogen reaction;  The mixture of silicon dioxide and carbon is burned in nitrogen containing a small amount of hydrogen by reacting pure silicon with ammonia at 1500℃.  The ammonolysis product Si(NH2)4 of SiCl4 was completely thermally decomposed.  Silicon nitride can be used as catalyst carrier, high temperature resistant material, coating and abrasive, etc.  

Silicon nitride ceramics have the characteristics of high strength and high temperature resistance. Among the ceramic materials, its comprehensive mechanical properties are the best, and its thermal shock resistance, oxidation resistance, wear resistance and corrosion resistance are good. It is the first candidate material for ceramic used in thermal engine parts.  In the mechanical industry, silicon nitride ceramics are used as bearing ball bearings, rollers, ball seat rings, molds, new ceramic cutting tools, pump plungers, spindle sealing materials, etc.  

In the chemical industry, silicon nitride ceramics are used as wear - and corrosion - resistant components.  Such as ball valves, pump bodies, combustion carburetors, filters, etc.  

In the treatment of gold industry, silicon nitride ceramics because of high temperature resistance, small friction coefficient, with self lubrication.  For most of the metal, alloy solution stability, therefore, can make metal material processing die, such as dial core bar, extrusion, wire drawing die, roll, transfer roll, heating fixture, hot couple set camp, metal heat treatment support, crucible, liquid aluminum guide camp, aluminum lining and so on.  

Silicon nitride's application field  

Silicon nitride ceramic material has the advantages of high thermal stability, high oxidation resistance and high dimensional accuracy.  Because of the silicon nitride is the covalent compound high bond strength, and oxide film formed in the air, and so also has good chemical stability, oxidation under 1200 ℃, 1200 ~ 1600 ℃ generated protective film to prevent further oxidation, and is not aluminum, lead, tin, silver, brass, nickel and so on many kinds of molten metal or alloy infiltrating or corrosion,  But can be magnesium, nickel chromium alloy, stainless steel and other molten corrosion.  

Silicon nitride ceramic materials can be used in high temperature engineering components, metallurgical industry and other aspects of the advanced refractory, corrosion resistant components and sealing components in the chemical industry, machining industry tools and cutting tools.  

Because silicon nitride can form a strong bond with silicon carbide, alumina, thorium dioxide, boron nitride, etc., it can be used as a bond material to modify with different proportions.  

In addition, silicon nitride can be used in solar cells.  After the silicon nitride film is plated by PECVD method, not only can it be used as an antireflection film to reduce the reflection of incident light, but also, in the deposition process of silicon nitride film, hydrogen atom of reaction product enters the silicon nitride film and silicon wafer, playing a role of passivation defect.  Here, the ratio of silicon nitride to silicon nitrogen atoms is not a strict 4:3, but fluctuates within a certain range according to different technological conditions. Different atomic ratios correspond to different physical properties of the film.  

Used in ultra-high temperature gas turbine, aircraft engine, electric furnace, etc.  

Silicon nitride is used as advanced refractory, such as si3N4-sic refractory used in blast furnace body and other parts combined with sic;  If combined with BN as Si3N4-BN material, used for horizontal continuous casting separation ring.  Si3n4-bn horizontal continuous casting separation ring is a kind of fine structure ceramic material with uniform structure and high mechanical strength.  Good heat resistance to impact and will not be wet by liquid steel.