Drift and Diffusion of Activation in Oxide-Coated Cathodes

Two experiments pertaining to oxide-coated cathodes are described. One of these involves the drift of activators in oxide-coated cathodes under the influence of an electric field and the other involves the diffusion of these activators in the absence of an electric field. The drift experiment revealed a temporal nonuniform increase in the activation of the conductivity. However, it failed to show any significant spatial nonuniformity. Pulse measurements of the current-voltage characteristics showed the origin of the temporal nonuniformity to be associated with nonohmic properties of the coating which were related to the Nergaard effect. The results of the diffusion experiment indicated a diffusion coefficient at 1000°K for the crystalline defects undergoing diffusion of (1.5±0.8)×10−7 cm2/sec and an activation energy of the diffusion of 0.67±0.4 eV. Comparison of this diffusion coefficient with data obtained from the literature makes possible the identification of the mobile defects involved in activation processes in oxide-coated cathodes as being barium vacancies. On the bases of these results, a ``mobile acceptor model'' for oxide-coated cathodes is presented which suggests that activation is achieved by the removal of the highly mobile acceptors (barium vacancies) which compensate the relatively immobile donors (oxygen vacancies) and not by an increase in the density of the donors as has usually been assumed. Nergaard's theory explaining the millisecond decay of thermionic emission from oxide-coated cathodes is modified in terms of the mobile acceptor model, and the kinetic processes involved in the activation of oxide-coated cathodes are discussed.