Reversible poisoning by sulphur, oxygen and other gases of oxide-coated cathodes at high temperatures

A simple mass spectrometer has been used to select and study the behaviour of the negative ions of sulphur and oxygen emitted by oxide-coated cathodes on recovery from the poisoning of their electrom emission by these agents over the temperature range 1075-1300° K. Only the atomic ions were observed, and the behaviour of the 32 S - ion emitted on recovery from sulphur poisoning was similar to that of the 16 O - ion emitted on recovery from oxygen poisoning. The ion 13 CH - also had a slight poisoning effect. Measurements of the energy distributions of these ions showed that, during recovery, many more of them were sputtered off the top surface of the cathode than were evaporated from its pores. These results support the hypothesis that high-temperature reversible poisoning is caused by the adsorption of negative ions on the internal and external surfaces of the porous oxide-coating.