DIFFUSION POROSITY IN FILMS OF IMPREGNATED M-TYPE WBa HOT CATHODES AND ITS INFLUENCE ON THEIR EMISSION CHARACTERISTICS

Abstract

The paper examines how the emission characteristics of impregnated M-type cathodes depend on variation in the film composition and structure in the cathode operation process. Diffusion porosity has been established to appear in the 49.5% Os–49.5% Ir–1% Аl and 50% Os–50% W films, and its influence on the emission characteristics of the M-type cathodes has been studied. It is assumed that the diffusion porosity leads to decrease in the M-type cathode work function because of increase in the total specific surface area of the film and to faster delivery of barium and its compounds to the emitting layer through mass transfer facilitated by both Knudsen flow of atoms through pores and surface diffusion along pore walls. Degradation in the emission characteristic of the M-type cathode is due to the film structural destruction. The performed experiments confirm the existence of barium-containing three-dimensional crystallites on the emitting surface of the M-type WBa cathode film.