Temperature Effects on Desorption Behavior and Characteristic Wulff Shapes of Scandate Cathodes

Abstract

Scandate cathodes have been shown to exhibit superior emission properties to other classes of thermionic cathodes. However, a deeper understanding of their fundamental operating behavior is crucial before widespread technological integration. It is a widely held hypothesis that the electron emission of scandate cathodes is related to Ba availability on the surfaces of the porous W matrix. In addition, extensive characterization of scandate cathodes motivates an additional hypothesis that high-performance cathodes all contain W nanoparticles of a particular shape. These attributes, Ba availability and nanoparticle shape, are significantly affected by temperature-the former due to desorption and the latter to the temperature-dependence of surface excess free energies. Here, we report computed Ba desorption rates for surfaces at $850^{\circ}C$ (operating temperature). We show that total Ba evaporation from the Ba/O-decorated W(110) surface $is\sim 13$ orders of magnitude higher than the Ba/O-decorated W(112). We also report that temperatures on the order of $850^{\circ}C$ imply that chemical environments suffcient to reduce BaO are required to form the W particle shapes observed in high-performing Sc cathodes.