Silicon nitride is an inorganic substance with the chemical formula of Si3N4. It is an important structural ceramic material with high hardness, lubricity, and wear resistance. It is an atomic crystal; Oxidation resistance at high temperatures.
Moreover, it can resist cold and hot shock, and it will not break when heated to 1000 ℃ or more in the air, and then cooled and heated sharply. It is precise because silicon nitride ceramics have such excellent characteristics that people often use it to manufacture mechanical components such as bearings, gas turbine blades, mechanical seal rings, permanent molds, etc. If the heating surface of engine parts is made of silicon nitride ceramic which is resistant to high temperatures and not easy to heat transfer, it can not only improve the quality of the diesel engine, save fuel, but also improve the thermal efficiency. China, the United States, Japan, and other countries have developed such diesel engines.
Silicon nitride history:
Henry Edin St. Clair de Ville and Friedrich Weiler first reported the synthesis method of silicon nitride in 1857. In the synthesis method they reported, another crucible containing silicon was buried in a crucible filled with carbon to reduce the penetration of oxygen. They reported the product of a nitride they called silicon, but they failed to understand its chemical composition. In 1879, Paul Schuetzenberger mixed silicon with the lining material (a paste that can be used as the lining of the crucible, which is obtained by mixing charcoal, coal, or coke with clay) and heated it in the blast furnace, and reported it as a compound with the composition of Si3N4. In 1910, Ludwig Weiss and Theodor Engelhart heated silicon under pure nitrogen to obtain Si3N4. In 1925, Friedrich and Sittig used carbothermal reduction to heat silicon dioxide and carbon to 1250-1300 ℃ in a nitrogen atmosphere to synthesize silicon nitride.
Silicon nitride crystal structure and characteristics:
Silicon nitride (Si3N4) has three crystal structures, which are α、β and γ three-phase. α and β Two phases are the most common type of Si3N4 and can be prepared under normal pressure. γ The phase can only be synthesized under high pressure and high temperature, and its hardness can reach 35GPa.
Application of silicon nitride
Silicon nitride is used as advanced refractory, such as SI3N4-SIC refractory combined with sic for blast furnace shaft and other parts; For example, SI3N4-BN material is used for horizontal continuous casting separation ring in cooperation with BN. SI3N4-BN series horizontal continuous casting separation ring is a fine structure ceramic material with a uniform structure and high mechanical strength. It has a good heat resistance and impact resistance, and will not be wetted by molten steel, meeting the technological requirements of continuous casting.
The production method of silicon nitride:
There are two production methods of silicon nitride ceramic products, namely, the reaction sintering method and the hot pressing sintering method. The reaction sintering method is to form a mixture of silicon powder or 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 certain strength, mechanical processing can be carried out on the machine tool, and then further nitriding at 1350~1450 ℃ for 18~36h until all of them become silicon nitride. The size of the product is accurate and the volume is stable. The principle of hot pressing sintering is to press the silicon nitride powder and a small number of additives (such as MgO, Al2O3, MgF2, AlF3, or Fe2O3) under the pressure of 19.6MPa and 1600~1700 ℃. Generally, the products produced by the hot-pressing sintering method have higher density and better performance than those produced by reaction sintering.
Other applications of silicon nitride
Silicon nitride ceramic materials have excellent properties such as high thermal stability, strong oxidation resistance, and high dimensional accuracy of products. Because 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 ℃, and the formation of a protective film at 1200~1600 ℃ can prevent further oxidation. It will not be soaked or corroded by aluminum, lead, tin, silver, brass, nickel, and many other molten metals or alloys, but can be corroded by magnesium, nickel-chromium alloy, stainless steel, and other molten solutions.
Silicon nitride ceramic materials can be used for parts of high-temperature engineering, advanced refractory materials in the metallurgical industry, anti-corrosion parts and sealing parts in the chemical industry, tools and cutting tools in the mechanical processing industry, etc.
Because silicon nitride can form a strong combination with silicon carbide, aluminum oxide, thorium dioxide, boron nitride, etc., it can be used as a binding material and modified with different proportions.
In addition, silicon nitride can also be used in solar cells. After the silicon nitride film is deposited by the PECVD method, it can not only be used as an antireflection film to reduce the reflection of the incident light but also, during the deposition process of a silicon nitride film, the reaction product hydrogen atoms enter into the silicon nitride film and silicon wafer, which plays a role in passivating defects. The ratio of silicon nitride to silicon nitride atoms here is not strictly 4/3 but fluctuates within a certain range according to the different technological conditions. The physical properties of the films corresponding to different atomic ratios are different.
For ultra-high temperature gas turbines, aircraft engines, electric furnaces, etc.
Now, do you know the magic silicon nitride?