Thermionic Emission and Electron Diffraction from Thin Films of Barium Oxide

Abstract

A high-vacuum, sealed-off glass, electron diffraction tube was developed for observing the physical structure of evaporated BaO films on a nickel substrate. Simultaneously, measurements of thermionic emission could be made. The thermionic activity of the deposited film increased until approximately 20 monolayers (measured using a radioactive tracer technique) had been laid down, and no further increase was found up to 50 monolayers. This emission density (0.4A/cm2 at 1000°K) is comparable with that obtainable from sprayed oxide coatings. With the receiver at less than 450°K the oxide film showed an amorphous structure. With the receiver at 800°K the film was crystalline and was believed to exist in clumps. This crystalline structure became amorphous if the films were heated to 1070°K and this form persisted as the oxide film was removed by evaporation. The relation of the thermionic emission from thin BaO films to that from sprayed oxide cathodes is discussed.