Investigation of deterioration mechanism of inorganic insulation under high temperature

This paper describes the characteristics of inorganic insulators under high temperature. The test materials are mica, alumina-cloth and alumina-plate that are components of mica-alumina combined insulation that we have suggested to be able to use over 500°C. Initial breakdown voltage tests and metal diffusion tests under high temperature are carried out in nitrogen gas atmosphere. It is clear that because there are many openings in alumina-cloth, the breakdown voltage does not depend on temperature. And also it is clear that there is two metal behavior mechanisms. One is that the openings of the insulator are filled up with copper that is oxidized, the other is the metal diffuses in alumina-cloth not through the bulk but through the surface. Finally in combined insulation the breakdown voltage depends on the thickness of mica and the mica works as a barrier to metal diffusion