Surface studies of the Medicus nickel matrix cathode

It is shown that activation of the Medicus nickel matrix cathode results in the development of a surface microstructure consisting of alkaline earth oxide-rich regions, 20–40 μm in diameter, distributed over the nickel-rich cathode coating. An in situ correlation between thermionic emission current and surface composition indicates a primary dependence upon the relative Ba to Ni concentration. A time dependent increase in the average calcium concentration at the surface has also been observed; compositional analyses of material evaporating from the cathode revealed that this calcium may accumulate at the surface due to the preferential release of barium and strontium. The accumulation of calcium at the surface, as well as the loss of barium, are believed to contribute to emission degradation during cathode life. Ion sputtering studies suggest the dispensing of barium and the dependence of the surface microstructure upon the initial thermal/ electrolytic activation. Although barium concentrations in excess of the oxide stoichiometry are measured, the presence of elemental barium monolayers has not been verified. It is concluded that owing to these chemical, compositional and morphological effects, the nickel matrix cathode should not be viewed simply as an oxide cathode physically supported by a metal matrix.