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

TL;DR

This study uses Density Functional Theory to analyze the work function and surface stability of tungsten-based thermionic cathodes, revealing how Ba-O species influence surface properties under operational conditions.

Contribution

It provides detailed theoretical insights into the surface energetics and adsorbate compositions of W-Ba-O systems, which were previously not well characterized.

Findings

Ba-O species lower the work function by creating surface dipoles.

Surface facet proportions are significantly affected by oxygen and barium presence.

Wulff construction aligns with sintering kinetics data for fine tungsten particles.

Abstract

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.