Reactions between oxide cathodes and gases at very low pressures

Abstract

A method is described by which the behaviour of oxide cathodes under the impact of gases at pressures between 10-7 and 10-5mm Hg can be studied. From the cathode under investigation saturated pulsed emission and space-charge-limited emission currents are drawn simultaneously. Whilst the pulsed emission is used as a measure of thermionic emissivity, the space-charge-limited emission is employed for operating an ionization gauge which indicates the gas pressure directly in front of the cathode. The experimental tube is connected to the pump via a capillary, and the rate of gas flowing into the cathode can be determined from measurements of the two pressures at either end of capillary. The resistance of a cathode to attack by oxygen is found to be independent of the material used for the core but dependent to a considerable extent on the presence of small traces of other gases and of barium originating from the getter. The influence of emission current, cathode temperature and gas pressure on rate of poisoning by oxygen is examined. By comparing the rate of oxygen flow into the cathode with the number of ions produced in the experimental tube, it is shown that the poisoning effect observed is not due to ions. From the rate of flow of oxygen the quantity of barium consumed during poisoning is calculated and found to be of the order of 0.01 mol%. When exposure to oxygen ceases, complete recovery of the cathode is observed at temperatures between 1000 and 1100°K but not above 1200°K. The effect of carbon dioxide on oxide cathodes is similar to that of oxygen but, at pressures below 5 × 10-6mm, its poisoning effect is more pronounced for cathodes with passive cores. There are indications that carbon dioxide as opposed to oxygen reduces the emitting area of the cathode. Carbon monoxide poisons cathodes with active nickel cores but activates those with platinum cores. Nitrogen does not affect the emission appreciably whilst water vapour, hydrogen and methane activate at pressures below 10-5mm. The activating effect of methane is due to the reducing action of carbon which is deposited at the cathode surface owing to dissociation of the methane. The action of the carbon reverses into a poisoning one if the methane pressure exceeds the equilibrium pressure of the carbon monoxide originating from the reaction between carbon and barium oxide.