282 research outputs found
Work function and surface stability of tungsten-based thermionic electron emission cathodes
Materials that exhibit a low work function and therefore easily emit
electrons into vacuum form the basis of electronic devices used in applications
ranging from satellite communications to thermionic energy conversion. W-Ba-O
is the canonical materials system that functions as the thermionic electron
emitter used commercially in a range of high power electron devices. However,
the work functions, surface stability, and kinetic characteristics of a
polycrystalline W emitter surface are still not well understood or
characterized. In this study, we examined the work function and surface
stability of the eight lowest index surfaces of the W-Ba-O system using Density
Functional Theory methods. We found that under the typical thermionic cathode
operating conditions of high temperature and low oxygen partial pressure, the
most stable surface adsorbates are Ba-O species with compositions in the range
of Ba0.125O to Ba0.25O per surface W atom, with O passivating all dangling W
bonds and Ba creating work function-lowering surface dipoles. Wulff
construction analysis reveals that the presence of O and Ba significantly
alters the surface energetics and changes the proportions of surface facets
present under equilibrium conditions. Analysis of previously published data on
W sintering kinetics suggests that fine W particles in the size range of
100-500 nm may be at or near equilibrium during cathode synthesis, and thus may
exhibit surface orientation fractions well-described by the calculated Wulff
construction
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