电致发热多孔碳化硅陶瓷的绝缘涂层

以异丙醇铝{AlOCH(CH3)2]3}为原料,用溶胶-凝胶法制备氧化铝(Al2O3)溶胶,然后再以碳化硅(SiC)多孔陶瓷为基体,用浸渍提拉法对陶瓷进行涂层,涂覆完成后进行热处理即可在陶瓷表面和孔隙内部形成致密的Al2O3涂层.当涂层后的陶瓷用作电加热元件时,就可以达到陶瓷和流体绝缘的目的.从扫描电镜照片可以看出:在陶瓷表面及其孔隙内部确实涂覆了Al2O3涂层.Al2O3具有高电阻系数、高介电常数,抗氧化、耐腐蚀性等优异性能,所以,涂层后陶瓷的电阻率明显增加,可以弥补电致发热过程可能引起危险性的缺陷.Al2O3的涂覆也很好地改善了陶瓷成分SiC的氧化问题.结果表明:涂层中Al2O3的质量分数(下同)为43.1%,二氧化硅(SiO2)为38.8%,硅(Si)为18.1%.Al2O3是涂层物质的主要成分,SiO2有两种来源,一是作为基体的多孔陶瓷在渗硅过程中的剩余硅在陶瓷冷却过程中氧化生成存留于陶瓷中;二是陶瓷中的剩余硅在涂层的热处理过程中再次氧化形成.所以SiO2的含量相对较高,其中的硅显然就是陶瓷中的残留硅.

INSULATION COATING ON ELECTRO-HEATING POROUS SILICON CARBIDE CERAMIC

Alumina(Al2O3)sol was prepared by the sol-gel method using aluminium isopropoxide{AlOCH(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 Al2O3 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 Al2O3 sol.The Al2O3 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 Al2O3 coating also improved the inoxidizability of the SiC ceramic.The experimental results indicated that the percentage composition of Al2O3 coating is 43.1%in mass(the same below),and SiO2 and Si are 38.8%and 18.1%,respectively.The Al2O3 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 oxi-dization 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.