Selection criteria for insulator materials in high-current arc environments

Insulator materials that are used in surface discharge switches or rail accelerators are subjected to the intense heat and chemistry of the arc plasma and must maintain their mechanical and electrical properties in order for the device to be dependable. The selection criteria for insulators used in these applications are presented. These criteria reflect an expected level of insulator performance and their usefulness in the selection of candidate insulator materials is determined by comparing the expected and the experimental performances of a given group of ceramic insulators. They are shown to be inadequate means of predicting the performance of the ceramic insulators considered here. The ceramics that were chosen include A12O3—SiC (alumina with 25% silicon carbide whisker reinforcement), CZA 500 (alumina and zirconia composite), MTF (aluminum titanate), Si3N4 (silicon nitride), TTZ (zirconia stabilized with magnesium oxide), and ZDY (zirconia stabilized with yttrium oxide). These materials were tested in a surface discharge switch (-300 kA, 20 fis pulse) using atmospheric air and nitrogen as discharge gases. EDX (Energy Dispersive X-ray) surface analysis is performed on the A12O3O-3—SiC, CZA 500, and MTF to determine the surface contaminants present and the possible failure modes that are caused by such contaminants. The temperature increase on the insulator's surface is assumed proportional to the I R heating in the arc discharge, and the temperature scaling, as a function of the arc current and arc resistance, is discussed. Methods to improve insulator performance in high current arc environments are also discussed.