A New Model for the Mechanism of Operation of Scandate and Refractory Oxide Cathodes

Refractory oxide thermionic cathodes are modeled as a top layer of high-band-gap semiconducting cubic nanocrystals with surface donor sites activated by a monolayer of barium dispensed from a substrate. An interpretation of the common experimental observation of an apparent deviation from Child's law for the space-charge-limited region for scandate cathodes in simple diodes as a temperature-dependent resistance of the top layer of the cathode supports this model. Using a previously developed phenomenological theory based on the uncertainty principle, it is shown that the work function depends on the number of donor sites per unit area of the nanocrystals that comprise the top layer and the diameter of the metal atomic component of the oxide. The model is applied to provide quantitative values for the basic semiconductor properties of a top-layer scandate cathode from published experimental data.