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Materials that exhibit a low work function and therefore easily emitelectrons into vacuum form the basis of electronic devices used in applicationsranging from satellite communications to thermionic energy conversion. W-Ba-Ois the canonical materials system that functions as the thermionic electronemitter used commercially in a range of high power electron devices. However,the work functions, surface stability, and kinetic characteristics of apolycrystalline W emitter surface are still not well understood orcharacterized. In this study, we examined the work function and surfacestability of the eight lowest index surfaces of the W-Ba-O system using DensityFunctional Theory methods. We found that under the typical thermionic cathodeoperating conditions of high temperature and low oxygen partial pressure, themost stable surface adsorbates are Ba-O species with compositions in the rangeof Ba0.125O to Ba0.25O per surface W atom, with O passivating all dangling Wbonds and Ba creating work function-lowering surface dipoles. Wulffconstruction analysis reveals that the presence of O and Ba significantlyalters the surface energetics and changes the proportions of surface facetspresent under equilibrium conditions. Analysis of previously published data onW sintering kinetics suggests that fine W particles in the size range of100-500 nm may be at or near equilibrium during cathode synthesis, and thus mayexhibit surface orientation fractions well-described by the calculated Wulffconstruction.

Work function and surface stability of tungsten-based thermionic electron emission cathodes