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Title: 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 commercially used 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 0.125O–Ba 0.25O 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. In conclusion, analysis of previously published data on W sintering kinetics suggests that fine W particles in the sizemore » 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.« less

Authors:
 [1]; ORCiD logo [1];  [2]
  1. Univ. of Wisconsin-Madison, Madison, WI (United States). Dept. of Materials Science and Engineering
  2. Univ. of Wisconsin-Madison, Madison, WI (United States). Dept. of Electrical and Computer Engineering
Publication Date:
Research Org.:
Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States). National Energy Research Scientific Computing Center
Sponsoring Org.:
USDOE Office of Science (SC)
OSTI Identifier:
1498060
Resource Type:
Accepted Manuscript
Journal Name:
APL Materials
Additional Journal Information:
Journal Volume: 5; Journal Issue: 11; Journal ID: ISSN 2166-532X
Publisher:
American Institute of Physics (AIP)
Country of Publication:
United States
Language:
English
Subject:
36 MATERIALS SCIENCE

Citation Formats

Jacobs, Ryan, Morgan, Dane, and Booske, John. Work function and surface stability of tungsten-based thermionic electron emission cathodes. United States: N. p., 2017. Web. doi:10.1063/1.5006029.
Jacobs, Ryan, Morgan, Dane, & Booske, John. Work function and surface stability of tungsten-based thermionic electron emission cathodes. United States. doi:10.1063/1.5006029.
Jacobs, Ryan, Morgan, Dane, and Booske, John. Mon . "Work function and surface stability of tungsten-based thermionic electron emission cathodes". United States. doi:10.1063/1.5006029. https://www.osti.gov/servlets/purl/1498060.
@article{osti_1498060,
title = {Work function and surface stability of tungsten-based thermionic electron emission cathodes},
author = {Jacobs, Ryan and Morgan, Dane and Booske, John},
abstractNote = {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 commercially used 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–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. In conclusion, 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.},
doi = {10.1063/1.5006029},
journal = {APL Materials},
number = 11,
volume = 5,
place = {United States},
year = {2017},
month = {11}
}

Journal Article:
Free Publicly Available Full Text
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Cited by: 12 works
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Figures / Tables:

Figure 1 Figure 1: Work function versus surface energy (calculated under thermionic cathode operating conditions) for the (001), (011), (111), (210), (211), (221), (310), and (311) surfaces (symbol types) that are bare (blue points) or contain adsorbed O (green points), Ba (red points), Ba–O where Ba/O = 1 (purple points), and BaxOymore » where x(Ba)/y(O) < 1 (black points). Data of all work functions and surface energies shown here can be found in the spreadsheet which is part of the supplementary material.« less

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    Figures/Tables have been extracted from DOE-funded journal article accepted manuscripts.