High voltage breakdown of titania in vacuo

The electrical breakdown of titania in a vacuum using static electric fields is investigated experimentally, by examination of current and temperature variation at different applied voltages. Thermal instability is found to occur at voltages above a critical value. This leads to sufficiently high temperatures to cause the reduction of the titania into a lower-order semiconducting oxide, which carries the breakdown current. The breakdown of titania and a vacuum gap in series is also investigated. Evidence gathered from temperature and current readings suggests that, at high voltages, part of the voltage originally across the vacuum gap appears across the titania. Evidence to support this theory is obtained from measurements of the mechanical force appearing across the vacuum gap which enable the voltage redistribution to be calculated. The voltage redistribution is due to an excess charge appearing on the surface of the ceramic, due to pre-breakdown currents in the vacuum gap. When the voltage across the titania reaches a large enough value, thermal instability occurs leading to breakdown of the titania.