Vacuum surface flashover characteristics of an alumina insulator with a metalized layer

Alumina ceramics are often used as vacuum electrical device insulators owing to their excellent properties, high hardness quality, high heat resistivity, and high insulation and low gas desorption properties. As alumina is chemically stable, directly brazing it with a metal is difficult. Thus, an alumina surface is metalized using the Mo-Mn method and subsequently brazed to a metal. When a high negative voltage is applied to the alumina surface, the edge of the metalized region becomes a cathode triple junction. In this case, surface flashover can occur, which limits the equipment's surface-flashover withstand voltage. Understanding the vacuum surface-flashover characteristics that depend on the edge condition of a metalized layer is important. Herein, surface-flashover tests in vacuum were conducted using column alumina samples under different metalized-edge conditions. The results showed that the withstand voltage and conditioning speed obviously changed with the edge condition of the metalized layer.