Some experiments on the breakdown of heater-cathode insulation in oxide-cathode receiving valves

Sintered alumina used for the insulation of valve cathode heaters is preferentially liable to sudden failure in service when the heaters are at a positive potential with respect to the cathode. The phenomenon has been studied in conventional valves run in varying conditions and also, in more detail, on simple two-electrode systems using recrystallized alumina as the insulant. The sudden failure is shown to be the last of three stages, (a) a time-consuming degradation of the insulant with rate exponentially related to operating temperature, (b) a period of intermittent sparking and arcing, and (c) a local weld between heater and cathode which appears as a more or less permanent circuit fault. The initial stage is accompanied by a transfer of tungsten from the heater across the alumina, and it is this process which is strongly enhanced when the heater is the positive electrode. Production of positive tungsten ions under electron bombardment from the cathode is postulated, and it is shown that tungsten transfer and also insulation failure are greatly retarded when the alumina contains a pore-free layer. This may be achieved in the laboratory by surface fusion or by the use of monocrystalline sapphire. Reasons for the apparent absence of widespread heater-cathode insulation failures are suggested.