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

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 Ba 0.125 O to Ba 0.25 O 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.

A mathematical model of solid-state dewetting of barium thin films on W(112)

“…Dewetting is a self-regulating phenomenon that ensures a thin layer of barium on the surface. Recent theoretical studies [5] find that Ba is unstable on the clean tungsten low index surfaces, consistent with dewetting. In the work shown by Zhou et al [20], similar calculations show that barium adsorption on clean tungsten is unstable relative to the Ba-oxides on W (100), W (111) and W (112).…”

A review on recent progress of thermionic cathode

“…The generally accepted mechanisms for the improved performance are "dipole model" [40] and "excess barium adsorption model" [41] with the emphasized function of active Ba. According to these works, Ba first diffuses from the inside of the matrix to W surface through the pores to form the Ba-O dipole layer at high temperatures, which leads to a sharp decrease of the work function and promotes the electron emission.…”

“…In recent years, DFT methods were applied to investigate the work function and surface stability of cathodes [40]. The model that combined the work function of different surface grain orientations was further developed, which is calculated according to the area fractions of different surface grain orientations in real polycrystalline W specimens provided via electron backscatter diffraction (EBSD) microscopy [44].…”

“…Surface structures depicting the most stable adsorbed species in the ratio of Ba/O < 1 for the a (001), b (011), c (111), d (210), e (211), f (221), g (310), and h (311) surface orientations under ther-mionic cathode operating conditions using the Visualization for electronic and structural analysis (VESTA) code[40] …”

A review on scandia doped tungsten matrix scandate cathode

“…The results indicate that the Ba 0.25 Sc 0.25 O structure is the most stable structure and holds the lowest work function compared to other Ba x Sc y O z monolayers. They found that in the W-Ba-W system the most stable surface adsorbates were Ba-O species with compositions in the range of Ba 0.125 O-Ba 0.25 O per surface of a W atom, and Ba could form work function-lowering surface dipoles [53].…”