Insulation coating on electro-heating porous silicon carbide ceramic

Alumina (A1₂O₃) sol was prepared by the sol-gel method using aluminium isopropoxide {Al[OCH(CH₃)₂]₃} as primal raw material. Then porous SiC ceramic was used as matrix and coated by the dipping and drawing method. After the coated ceramic was heated, a compact A1₂O₃ coating was formed on the surface and the inner wall of the pores of the ceramic. Then the coated ceramic was insulated with fluid when it was used as a heating element. The scanning electron microscope photograph showed that the surface and the wall of the pores of the ceramic were coated by A1₂O₃ sol. The A1₂O₃ has good properties such as high dielectric constant and coefficient of resistance, inoxidizability and corrosion resistance, so the resistivity of coated ceramic was evidently increased. The higher resistivity can prevent the possibility that the fluid will be conducted. The A1₂O₃ coating also improved the inoxidizability of the SiC ceramic. The experimental results indicated that the percentage composition of A1₂O₃ coating is 43.1% in mass (the same below), and SiO₂ and Si are 38.8% and 18.1%, respectively. The A1₂O₃ is the main composition of the coating, and SiO₂ results from the oxidation of residual silicon. When the ceramic was sintered in the ihrigizing process, it was inevitable that some silicon would remain in the final ceramic. The residual silicon was oxidized when the ceramic was leaving the furnace and cooled in air, so the oxidization product SiO₂ was left in the ceramic. The other part was produced during the heat treatment of the coating. Therefore, the content of SiO₂ was higher. It is obvious from the above analysis that the Si came from the ceramic matrix.