Materials that exhibit a low work function and therefore easily emit electrons into vacuumform the basis of electronic devices used in applications ranging from satellitecommunications to thermionic energy conversion. W–Ba–O is the canonical materialssystem that functions as the thermionic electron emitter commercially used ina range of high-power electron devices. However, the work functions, surface stability,and kinetic characteristics of a polycrystalline W emitter surface are still notwell understood or characterized. In this study, we examined the work function andsurface stability of the eight lowest index surfaces of the W–Ba–O system usingdensity functional 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 range ofBa0.125O–Ba0.25O per surface W atom, with O passivating all dangling W bondsand Ba creating work function-lowering surface dipoles. Wulff construction analysisreveals that the presence of O and Ba significantly alters the surface energeticsand changes the proportions of surface facets present under equilibrium conditions.Analysis of previously published data on W sintering kinetics suggests that fine Wparticles in the size range of 100-500 nm may be at or near equilibrium during cathodesynthesis and thus may exhibit surface orientation fractions well described bythe calculatedWulff construction. ? 2017 Author(s). All article content, except whereotherwise noted, is licensed under a Creative Commons Attribution (CC BY) license.
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