Surface Switching Characteristics of Variable Permittivity Dielectrics
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Flashover voltage, lifetimes, and switch performance of insulators utilizing square thin and thick film electrodes were examined to determine the viability of using thin electrodes for reliable surface discharge switching. Gold, silver, and platinum were sputtered (0.25 µm) and screen printed (15 µm) onto Al2O3, TiO2, and modified BaTiO3 (MBT), then tested in air at 10^5 Pa, under vacuum (10^-3/ torr), and while immersed in an insulating fluid, SF-2 (manufactured by 3M). For the measured range of 0.5 to 3 mm in air, the flashover voltage for all three insulators was found to have a linear dependence on the electrode separation distance with 15 µm thick screen printed electrodes and a square root dependence with 0.25 µm thick sputtered electrodes. Delay times of approximately 20 ns with a corresponding jitter of 6 ns were observed across all three insulators under triggered flashover. Insulators in air with sputtered electrodes had lifetimes of approximately 5 flashovers for dc flashover and 40 for triggered flashover. Screen printed TiO2 and MBT had dc lifetimes of approximately 10 flashovers in air, and 3 flashovers in vacuum and SF-2. Screen printed TiO2 and MBT had triggered lifetimes of greater than 200 flashovers in air, and <3 flashovers in vacuum and SF-2. Screen printed Al2O3 had dc and triggered lifetimes of greater than 200 flashovers in>air, vacuum and SF-2. Insulator failure during dc flashover was determined to be due to the formation of a conductive channel between the anode and cathode. Formation of the channel was attributed to insulator thermal and dielectric properties and the presence of vaporized electrode species in the gap region during flashover.