How the trapping of charges can explain the dielectric breakdown performance of alumina ceramics

The aim of this work is to bring new elements to the understanding of breakdown phenomena. Different alumina materials are studied in breakdown tests and by absorbed current measurement. The association of these two characterisation methods displays correlations between breakdown strength, the ability to trap or diffuse electrical charges and microstructural parameters. Different materials have been studied with a growing quantity of defects: structural defects in the single crystal, grain boundaries in very pure polycrystalline alumina (99.99%), impurities in less pure polycrystalline alumina (99.8%), interfaces in the case of alumina with a dispersion of zirconia and finally the adding of sintering aids. It appears that materials capable of diffusing injected charges have higher breakdown values than those trapping charges locally. Two other favourable behaviours are characterized: trapping charges at the injection point in order to limit the injection process, and trapping followed by reemission which relaxes the insulator and delays breakdown.