Interaction of oxide cathodes with gas molecules and ions

Oxidation and reduction experiments have been done on oxide coated cathodes, using a number of gases commonly occurring in vacuum devices, notably CH4, H2, H2O, CO, CO2, and O2. The results are interpreted in terms of a chemical model of the cathode, which results in reaction probabilities; for the oxidizing gases, these are normalized to the reaction probability of Mg from the core. It is shown that this probability is near unity when the coating is subjected to external oxidizing influences. The reaction probabilities found in this way are O2:1 , CO2:O216 H2O: 0,009. For the reducing gases the reaction probabilities are, in principle, absolute; H2O 5.10^-6, CO:2.10^-6, CH4=3, 6.10^-6With CH4two stages can be discerned: first, cracking at the cathode surface with the probability mentioned; second, reaction of the resulting carbon with the coating with a maximum rate of about 1.10¹³atoms cm^-2. s^-1. This interpretation is backed by an experiment in which the cathode was bombarded with argon and methane ions.