Improving the voltage holdoff performance of alumina insulators in vacuum through quasimetallizing

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. A 7 Mn:1 Ti mix performed very well, being found to be as effective on a 94% Al2O3 alumina as on the previously investigated 95% Al2O3, 1% Cr2O3 alumina. Mixes of 6 Mn:1 Ti:1 Ca and 6 Mn:3 Cr performed about as well as the 7 Mn:1 Ti mix, but no better. Quasimetallizing with pure Mn improved the voltage holdoff capability of alumina by about half as much as when using the 7 Mn:1 Ti mix. Mixes with relatively high titanium content (4 Mn:3 Ti and 3 Mn:3 Ti:2 Cr) significantly increased the voltage holdoff capability of the alumina, but unfortunately were much more prone than the 7 Mn:1 Ti mix (or plain alumina) to suffer severe and permanent damage when a breakdown did occur. Quasimetallizing with appropriate formulations has been shown to change the surface characteristics of alumina in two ways: it decreases the surface resistivity of the alumina, and it decreases the secondary electron emission yield of the alumina. Each change improves the voltage holdoff characteristics of the alumina