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The excellent performance of SiC ceramics is closely related to its unique structure.
SiC is a compound with a strong covalent bond, and the ionicity of the Si-C bond in SiC is only about 12%. Therefore, SiC has high strength, large elastic modulus, and excellent wear resistance. Pure SiC will not be attacked by acid solutions such as HCl, HNO3, H2SO4 and HF, and alkali solutions such as NaOH. Oxidation is easy to occur when heated in air, but the SiO2 formed on the surface during oxidation will inhibit the further diffusion of oxygen, so the oxidation rate is not high. In terms of electrical properties, SIC ceramic heat sinks are semiconducting, and the introduction of a small amount of impurities will show good electrical conductivity. In addition, the silicon carbide ceramic substrate has excellent thermal conductivity.
SiC has two crystal forms, α and β.
The crystal structure of β-SiC is a cubic system, and Si and C respectively form a face-centered cubic lattice; there are more than 100 types of polymorphs such as 4H, 15R, and 6H, among which 6H polytypes are used in industrial applications. The most common one. There is a certain thermal stability relationship between various types of SiC bodies. At temperatures below 1600 ° C, SiC exists as β-SiC. Above 1600 ° C, β-SiC slowly transforms into various polymorphs of α-SiC. 4H-SiC is easily formed at about 2000 ° C; 15R and 6H polytypes need to be easily formed at a high temperature above 2100 ° C; for 6H, SiC is very stable even at temperatures exceeding 2200 ° C. The difference in free energy between various polytypes in SiC is small, so the solid solution of trace impurities will also cause the thermal stability relationship between polytypes to change.
The production process of SiC ceramics is briefly described as follows:
The preparation technology of silicon carbide powder is divided into solid phase synthesis method and liquid phase synthesis method according to the state of its raw materials.
The solid-phase method mainly includes a carbothermal reduction method and a silicon carbon direct reaction method. The carbothermal reduction method includes the Archison method, the vertical furnace method, and the high-temperature converter method. The Acheson method was first invented by Acheson.In the Acheson electric furnace, silicon dioxide in quartz sand is reduced by carbon to produce SiC.It is essentially an electrochemical reaction under the action of a high temperature and strong electric field. Historically, the SiC particles obtained by this process are relatively coarse. In addition, the process consumes a large amount of electricity, which is used for production and is a heat loss
Liquid phase synthesis
The liquid phase method mainly includes a sol-gel method and a polymer decomposition method. Ewell proposed the sol-gel method for the first time, but the real use of ceramics began around 1952. In this method, an alkoxide precursor prepared by a liquid chemical reagent is dissolved in a solvent at a low temperature to form a uniform solution. An appropriate coagulant is added to hydrolyze and polymerize the alkoxide to form a uniform and stable sol system. After being left for a long time or dried, it is concentrated to a mixture or polymer of Si and C at the molecular level, and it is continuously heated to form a two-phase mixture of Si and C which is uniformly mixed and has a small particle size. A carbon reduction reaction occurs at about 1460 to 1600 ° C Finally, SiC fine powder was prepared. The main parameters for controlling sol-gelation are the pH value of the solution, the concentration of the solution, the reaction temperature and time. This method is easy to achieve the addition of various trace ingredients during the process operation, and the mixing uniformity is good; but the residual hydroxyl groups and organic solvents in the process products are harmful to the human body, the cost of raw materials is high and the shrinkage during processing is insufficient.
High-temperature decomposition of organic polymers is an effective technique for the preparation of silicon carbide:
One type is the heating gel polysiloxane which undergoes a decomposition reaction to release small monomers, eventually forming SiO2 and C, and then the SiC powder is obtained by carbon reduction reaction. The other type is to heat the polysilane or polycarbosilane to release small monomers to generate a skeleton, and finally form a SiC powder.
At present, sol-gel technology is used to make SiO2 into a sol / gel material based on SiO2 hydroxide derivatives, which ensures that sintering additives and toughening additives are evenly distributed in the gel, in order to form high-performance silicon carbide Ceramic powder provides the conditions.
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