silicon nitride synthesis and application
Silicon nitride is an important structural material. It is a kind of superhard material with lubricity and wear resistance. It is a covalent bond compound with the ratio of nitrogen atom to silicon atom of 4:3. It is an atomic crystal and a hexagonal crystal structure. Except hydrofluoric acid, it does not react with other inorganic acids, and has strong corrosion resistance and oxidation resistance at high temperature. Moreover, it can resist cold and hot shock. It can be heated to over 1000 ℃ in the air, and then rapidly cooled and then heated, it will not be broken. In terms of physical properties, silicon nitride materials have the characteristics of high hardness, wear resistance, large elastic modulus, high strength, high temperature resistance, small thermal expansion coefficient, large thermal conductivity, good thermal shock resistance, low density, small surface friction coefficient, and good electrical insulation performance, while the chemical properties, it also has the advantages of corrosion resistance and oxidation resistance. It is generally believed that there are two isomers of silicon nitride under normal conditions, namely α phase and β phase. One cell of α phase is si12n16, while that of β phase is si6n8.
1. silicon nitride synthesis
At present, the most researched preparation methods of silicon nitride in the world are carbothermal reduction, direct nitridation of silicon powder, ammonolysis of halogenated silicon, combustion synthesis under low ammonia pressure, gas phase reaction, etc. Next, we will introduce carbothermal reduction method, silicon powder direct nitriding method and ammoniacal halogenation method.
2. silicon nitride application
Silicon nitride is widely used because of its superior mechanical, chemical, electrical, optical and thermal properties. Silicon nitride has the characteristics of high strength, thermal stability and high concentration of doping, which makes it possible to prepare electrical / optical devices working in high temperature and high radiation environment; its high strength, thermal shock resistance and high chemical stability make it an excellent refractory; and its high strength, wear resistance and other characteristics make it widely used as pressure and wear-resistant devices.
Traditional silicon nitride ceramics are made by sintering large particles and multiphase powders, so their brittleness, uniformity, reliability, toughness and strength are poor, so their application is limited. Nano silicon nitride has the following characteristics: small particle size, large specific surface area and high chemical properties, which can significantly improve the densification degree of silicon nitride ceramics in the sintering process, reduce the sintering temperature, and save energy; it can also make the composition and structure of silicon nitride ceramics uniform, improve the performance of materials and improve its reliability; it can also control the formation of materials from the nano level The structure of silicon nitride ceramics is beneficial to give full play to the potential properties of silicon nitride ceramics.
Therefore, the strength, toughness and Superplasticity of nano silicon nitride ceramic materials developed by nanotechnology have been greatly improved, which overcomes many shortcomings of nano silicon nitride as engineering ceramics and opens up a new application field of engineering ceramics.