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Silicon Carbide Ascendancy

Hangzhou JWG Technology Co.Ltd | Updated: Sep 20, 2017

Nowadays with the development of science and technology, information, energy, materials, biological engineering has become the four pillars of social productivity development, and silicon carbide due to the chemical performance is stable, high thermal conductivity, thermal expansion coefficient, low density and good wear resistance, high hardness, high mechanical strength, resistance to chemical corrosion, is developing rapidly in the field of materials, widely used in ceramic ball bearings, valves, semiconductor material, gyro, measuring instrument, aerospace and other fields

The structure and chemical properties of silicon carbide. Silicon carbide is dominated by covalent bonds, and covalent bonds account for about 88%. The basic structure of the lattice is interpenetrating SiC4 and CSi4 tetrahedron, which constitute many different crystal types. In theory, silicon carbide is composed of SiC tetrahedron, and the difference is the combination of parallel or antiparallel. There are 75 varieties of SiC, such as alpha-sic, beta-sic, 3c-sic, 4h-sic, 15r-sic, and so on. All these structures can be divided into cubic crystal, six-square crystal and rhomboid, and alpha-sic and beta-sic are the most common ones. Alpha SiC is the high temperature stability, beta SiC is stable at low temperature in 2100 ~ 2400 ℃ can be transformed into alpha SiC. Beta SiC can at about 1450 ℃ temperature mixture was prepared by a simple silicon and carbon.

The chemical stability of silicon carbide is closely related to its oxidation property. Silicon carbide itself is easy to oxidize, but it oxidizes and forms a layer of silicon dioxide, which gradually blocks the oxidation process. In the air oxidation silicon carbide to start at 800 ℃, but it is slow; As the temperature rises, the oxidation rate accelerates rapidly. The oxidation rate of silicon carbide is 1.6 times faster in oxygen than in air, and the oxidation rate slows down over time. The typical parabolic shape can be obtained if the number of oxidation Numbers is graphically in time, which reflects the blocking effect of silicon dioxide protective layer on the oxidation rate of silicon carbide.

When oxidized, the silicon carbide is easily oxidized if there are substances that can remove the silicon dioxide film or cause it to rupture. For example, metals such as iron and manganese have several valence, which can oxidize silicon carbide and produce low-melting compounds with silicon dioxide, so they can erode silicon carbide. FeO, for example, in 1300 ℃ 1300 ℃, and MnO in the erosion of silicon carbide, CaO, MgO style at 1000 ℃ can erosion of silicon carbide.

Water vapor reaction with silicon carbide at high temperatures is quite strong, above 1100 ℃, as the case is different, can generate silicon, carbon and silicon dioxide. Silicon carbide at about 1000 ℃, and sulfur compounds such as hydrogen sulfide can generate red brown vulcanization silicon (SiS2 or SiS). This reaction is also one of the reasons that the color of silicon carbide becomes red when burnt.

Advantages of silicon carbide

Silicon carbide has the following advantages compared with today's widely used metal materials:

1) high mechanical strength and rigidity

High mechanical strength can effectively prevent deformation of materials, which is crucial. The mechanical strength of silicon carbide is higher than that of corundum. For example, the compressive strength of silicon carbide is 224MPa, and the corundum is only 75.7 MPa. The bending strength of silicon carbide was 15.5 MPa, while the corundum was 8.72 MPa.

2) high hardness and wear resistance

The hardness of silicon carbide is very high, and the hardness of mohs is 9.2~ 9.6, which is second only to diamond and boron carbide, which is higher than that of the corundum. Compared with metal steel materials, it is not only high in hardness, but also has a small friction coefficient in the non-lubricated state. The friction force is relatively small, the surface roughness is small, and it has good wear resistance. In addition, it has a strong resistance to external materials and improves the surface tolerance.

3) the density is small

Silicon carbide materials are less dense than metals, making them lighter.

4) high temperature and low thermal expansion coefficient

Silicon carbide is made at high temperature. In some high temperature environments, the materials need to have some degree of processing strength, but also to meet the machining accuracy, the silicon carbide ceramics can achieve these two points. Silicon carbide, the highest temperature about 800 ℃, and steel under temperature is 250 ℃ only. A rough calculation, the average thermal expansion coefficient of silicon carbide within 25 ~ 1400 ℃ for 4.4 x 10-6 / ℃. The thermal expansion coefficient of silicon carbide determination results showed that the quantity compared with other abrasive and high temperature materials are much smaller, such as the thermal expansion coefficient of corundum can be as high as (7 ~ 8) x 10-6 / ℃.

5) high thermal conductivity

The thermal conductivity of silicon carbide is very high, which is another important characteristic of the physical properties of silicon carbide. The thermal conductivity of silicon carbide is much larger than other refractory materials and abrasive materials, about four times the thermal conductivity of corundum. Silicon carbide with low thermal expansion coefficient and high thermal conductivity, the parts are in the process of heating and cooling of thermal stress is small, which is why silicon carbide parts especially the cause of the heat shock.

6) corrosion

Because silicon carbide with high melting point (decomposition temperature), chemical inertness and reting resistance, so the silicon carbide can be used for abrasive, ceramic firing kiln shed board, sagger and used in the zinc smelting industry vertical distillation furnace with silica brick, aluminum carbide cylinder electrolytic cell line, crucible, small furnace material of silicon carbide ceramics and other products.

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