Emission mechanism of high current density scandia-doped dispenser cathodes

Abstract

The emission mechanism of scandia-doped dispenser (SDD) cathodes is characterized based on systematic investigation of microstructure, surface behavior, evaporation, and emission. High current density SDD cathodes have been developed in recent years to meet the requirements of advanced vacuum electron devices. Space charge limited current densities of over 100 A/cm2 have been reliably reached at a temperature of 950 °Cb. SDD cathodes are composed of a tungsten matrix with submicron structure and uniformly dispersed materials containing Sc, Ba, Ca, Al, and O as nanosized particulates in the matrix. The authors conclude that the copious electron emission from this cathode type results from a layer of Ba–Sc–O on W formed after activation on top of the matrix. This layer is about 100 nm thick and maintains a stable composition over its lifespan. Its emission can be explained in terms of a semiconductor layer with characteristics that differ from a metal-like emission model. The Ba–Sc–O layer is formed by codiffusion of Sc, Ba, and O (generated inside the W-matrix) to the surface of the W-matrix during activation. Furthermore, this layer guarantees a low evaporation rate of emission-relevant materials and thus enables a longer lifespan.