On the Leakage Current Flowing through Alundum Coating of Heaters in Vacuum Tubes

Abstract

Alundum is usually used as coating on heaters in vacuum tubes for insulation. But alundum, when heated, becomes semi-conductive causing a current to leak through the space between the heater and the cathode often originating hum of the tube. This paper deals with several causes of current leak involved in the processing of tube production.
1). By heating, BaSr(CO₃)₂ on cathode gives out CO₂ or CO by dissociation, and if the gas pressure is higher than 0.1mmHg, CO₂ or CO is further dissociated to O₂ and C. The carbon produced is deposited on the surface of alundum and diffuse into it thus incresing the leakage current.
2). By applying high voltage to the alundum layer, the conductivity becomes higher by the increase of the number of elecrrons excited thermally to the conduction band resulting in eventual increase of the leak.
3). With D. C. voltage on the alundum layer at a certain temperature above some 1250°K, thermal breakdown takes place. The breakdown voltage is lowered by the rise of temperature but is raised by prolonged aging- The pulsive breakdown voltage is always higher than D. C. breakdown voltage and depends upon the duty cycle.