Improving the voltage holdoff performance of alumina insulators in vacuum by quasimetallizing or doping

Abstract

Treatment of the surface of an alumina insulator with coatings incorporating varying amounts of Cr, Mn, and Ti can significantly increase the vacuum voltage holdoff capability of the insulator (up to 25%). During processing (quasimetallizing) the coating penetrates into the alumina, so it is insensitive to mechanical damage. This quasimetallizing treatment is also compatible with subsequent metallizing and brazing of the alumina insulator. Quasimetallizing with appropriate formulations decreases both the surface resistivity of the alumina and the secondary electron emission yield of the alumina. Each change improves the voltage holdoff characteristics of the alumina. Similar improvements in voltage holdoff capability can be obtained by doping the plain alumina to the 5% (by weight) level with additives of 4 parts Mn/1 part Ti or 2 Mn/1 Ti/1 Cr. Such doped ceramics have the advantage of requiring fewer processing steps than do the quasimetallized ceramics. Both doped and quasimetallized ceramics are suitable for use in ultrahigh vacuum apparatus.