The Poisoning of Impregnated Cathodes

Abstract

The poisoning of barium— calcium aluminate impregnated tungsten cathodes by various gases has been investigated experimentally. It is shown that oxygen, water vapour, carbon dioxide and air poison these cathodes, while carbon monoxide, nitrogen and hydrogen do not. Poisoning takes place with the former gases if a critical pressure, depending on the gas is exceeded, this pressure increasing with cathode temperature. For a normal operating temperature of 1100° c the approximate critical pressures for the gases are as follows: O₂ 10₋₁mm, H₂0 3x I0⁻⁶ mm, C0₂10⁻⁶mm and air 5 x 10mm. Cathodes with two different porosities of tungsten show somewhat different poisoning characteristics. The amount of poisoning increases rapidly with the pressure when once the critical pressure p_ois exceeded, an equilibrium value of the emission poisoning ratio dependent on plp0 being reached. For very severe poisoning, tho reactivation time increases with the time the cathode has been held in the poisoned condition, but for small degrees of poisoning the recovery rate depends only on cathode temperature. A theory correlating the various observations is proposed, based on the probabilities of gas adsorption, and rates of barium production, evaporation and migration.