Deterioration of mica-alumina composite insulations at high temperatures

Sodium-immersed electromagnetic pumps will be critical for commercialization of fast breeder reactors in the near future. In order to operate the pumps in molten sodium satisfactorily, it is indispensable to have electrical insulation that can withstand temperatures of above 500°C for a prolonged period of time. Several mica-alumina composite insulation types were aged under a high electric field in nitrogen at high temperatures. All composite insulations aged at 750°C experienced dielectric breakdown after around 1000 hours. However, it was possible to detect the onset of dielectric breakdown by monitoring the ac current. Mica splittings insulation was found to be superior in dielectric performance to mica paper insulation. Thermal degradation contributed to the dielectric deterioration of these insulations.