A ceramic breaks maximum voltage rating is determined with an external pressure of 1 atmosphere (air) and an internal pressure of high vacuum. For reasons beyond these Notes scope, different gases have different breakdown voltages (BVs). For example, a paper from Rensselaer Polytechnic shows heliums BV under some conditions is roughly half that of air. In general, the simpler the gas physical structure, the lower its BV. Atomic inert gases, particularly He, have a low BV at most backfill pressures. Complex gas molecules, such as SF6, have high BVs since the many possible bond bending and stretching modes help dissipate energy when the molcule is hit by an electron. That tends to quench ionization and electron multiplication (which causes discharges). SF6 is used as a backfill gas in power transmission lines switches and breakers. To assess the effects of pressure on a gas BV, look for references to Paschens Law. On Wikipedias reference to the law, there is a link that shows the BV for air declines from 50,000V at 760 Torr to 350V at 0.2 Torr. Clearly, this can seriously affect experiments requiring low gas pressures and high voltages.
Sub-Category: Electrical Feedthroughs