Electrical conduction and discharge processes associated with aluminium oxide

Abstract

At elevated temperatures alumina insulators are a source of small electrical pulses of approximately 10(^-14) coulombs when subjected to an electrical stress. This phenomenon, termed pulse breakdown, has been studied on single crystal and polycrystalline alumina over the temperature range 0ºC to 900ºC. The study has been made over a range of ambient gas pressures and at voltages up to 3 kV. The d.c. conductivity of alumina has also been investigated over this range of conditions. The study has shown that pulse breakdown is dependent on the ambient gas, the electrodes and the alumina surface. The results have been interpreted by means of a surface discharge model, in which discharges occur at both electrodes. The surface discharge at the cathode has been studied in detail and is shown to be produced by a corona discharge which is triggered by the desorption of impurities from the alumina surface. The discharge did not occur once single crystal alumina had been heated to 900ºC. The possible explanations for this effect are considered. The d.c. conductivity of single crystal alumina has been shown to be affected by chemical treatment of the alumina surface. This is due to the surface conduction of the alumina being affected by chemisorption. The activation energy for the surface conduction of alumina has been shown to be 1.7 eV. The very much higher activation energy of 3.9 eV attributed to the bulk conduction shows that alumina is not an ionic conductor below 900ºC.