The Effect of Doping on the Voltage Holdoff Performance of Alumina Insulators in Vacuum

Abstract
Alumina ceramics doped with various combinations of Cr, Mn, and Ti were fabricated into hollow cylindrical insulators, then assembled into vacuum diodes and tested with 25 μs voltage pulses. Four doped alumina ceramics with total dopant levels of 2.6 to 4.7% wt and partial Cr plus Ti dopant levels of 1.0 to 2.4% performed very well. They had voltage holdoffs significantly better (25 to 40%) than plain alumina ceramics. Ceramics with total dopant levels of 5.3 to 5.6% performed only slightly better than plain alumina. Ceramics with partial Cr plus Ti dopant levels of 3.8 to 4.0% had significantly improved voltage holdoff capabilities, but undesirably high permanent failure rates (19 to 33%). Ceramics with total dopant levels greater than 7% performed very poorly, nearly all suffered permanent damage from breakdowns. These results suggest that desirable upper limits for doping are 5% for total dopants and 3% for Cr and Ti combined. Thus, doping a 94% Al203 alumina ceramic with appropriate amounts of Cr, Mn, and Ti can significantly improve the voltage holdoff capability, of insulators fabricated from such ceramics. These doped ceramic insulators are suitable for use in ultrahigh vacuum applications.