silicon carbide ceramic

Structural ceramics, also known as engineering ceramics, are a class of advanced ceramics that mainly exert the mechanical, thermal, chemical and other effects of materials. Structural ceramics have the properties of high temperature resistance, wear resistance, corrosion resistance, oxidation resistance, and low creep under high temperature. They can withstand the harsh working environment that metal materials and polymer materials are not competent for. They are widely used in aviation, machinery, automobile, metallurgy, chemical industry, electronics and other fields, and have become a class of ceramic materials that are developing very rapidly.Structural ceramics mainly include oxide ceramics, nitride ceramics and carbide ceramics. Silicon carbide ceramic are mainly introduced below. Silicon carbide, commonly known as carborundum, also known as carborundum, is a typical covalent bond compound that almost does not exist in nature. In 1890, when Eword and G. Acheson wanted to synthesize diamond by adding silicon into carbon as a catalyst, they prepared silicon carbide. Today, it is still being studied and developed.

Silicon carbide was initially used because of its super hard performance, which can be prepared into various grinding wheels, abrasive cloth, abrasive paper and various abrasives, and is widely used in the mechanical processing industry. In the Second World War, silicon carbide ceramic was found that it could also be used as a reducing agent and a heating element in steelmaking, thus promoting its rapid development. With the further research, it is found that it has many excellent properties, such as high temperature stability, high thermal conductivity, acid and alkali corrosion resistance, low expansion coefficient, and good thermal shock resistance.

There are mainly two crystal forms of silicon carbide, namely: cubic β- SIC4 and hexagonal α- SIC。 The basic structural units of silicon carbide lattice are SIC4 and CSI4 tetrahedrons interpenetrating each other. The tetrahedrons share the same edge to form a plane layer, and the vertices are connected with the next layer of tetrahedrons to form a three-dimensional mechanism. Since different tetrahedron stacking sequences can form different structures, hundreds of variants have been found so far. Generally, concise and intuitive symbols, namely the letters C, H, R, are used to represent the lattice type, and the number of layers contained in the unit cell is used to show the difference. Although the lattice constants of these polymorphs are different, there is no obvious change in the substances in them. silicon carbide ceramic is a typical valence compound, but there are also some ionic types. According to theoretical calculation, 78% of the total energy of SI-C bond belongs to covalent state and 22% belongs to ionic state. Due to the small size of S and C atoms, the bond length and strong covalence, silicon carbide ceramic has a series of characteristics, such as high hardness, certain mechanical strength and difficult sintering.

Silicon carbide is a typical covalent bond bound stable compound. In addition, its diffusion coefficient is low, which makes it difficult to densify by conventional sintering methods. It is necessary to add some sintering aids to increase the surface energy or surface area and adopt special processes to obtain dense silicon carbide ceramics. According to the sintering process, silicon carbide can be divided into recrystallized silicon carbide ceramics, reaction sintered silicon carbide ceramics, pressureless sintered silicon carbide ceramics, hot pressed sintered silicon carbide ceramics, high-temperature hot isostatic pressing sintered silicon carbide ceramics and chemical vapor deposition silicon carbide. The properties of silicon carbide prepared by various processes are quite different, that is, SIC prepared by the same process has poor performance due to different raw materials and additives.