Effects of bulk doping on surface insulating performance of alumina ceramic in vacuum

The voltage hold-off capability of 95% alumina insulator in vacuum is usually less than that of a similar-sized vacuum gap and also is far below its dielectric breakdown strength, due to its low surface flashover strength. In this work, bulk doping was taken into consideration to modify the surface of 95% alumina in hope to improve its insulating property in vacuum and satisfying results were achieved. The as-blended 95% alumina powder was selected as initial powder to be doped with oxides combination such as MnCO₃, Cr₂O₃, MgO and TiO₂ separately or combinatorially. Conventional manufacturing procedures were applied for preparing all ceramic samples. Samples doped with proper Cr₂O₃ and MgO and those doped with proper MnCO₃ and Cr₂O₃ were found to possess higher surface hold-off voltage than the original 95% alumina. It was pointed out through properties measurements that proper bulk doping was effective to reduce both the secondary electron emission (SEE) coefficient and surface resistivity of 95% alumina and so to enhance its surface insulating ability.