Ba x Sc y O z on W (0 0 1), (1 1 0), and (1 1 2) in scandate cathodes: Connecting to experiment via μ O and equilibrium crystal shape

Qunfei Zhou, Xiaotao Liu, Tyler Maxwell, Bernard Vancil, T. John Balk, Matthew J. Beck

Research output: Contribution to journalArticlepeer-review

30 Scopus citations

Abstract

Quantum mechanical calculations of the structure and properties of a range of Ba x Sc y O z adsorbate layers on W (0 0 1), (1 1 0), and (1 1 2) surfaces reveal the importance of O chemical potential in controlling the surface structure, surface energy, equilibrium crystal shape, and work function of W particles in Sc-containing (scandate) cathodes. Using the experimentally observed shape of W particles present in a high-emission scandate cathodes as a constraint, screening of ∼2000 possible combinations of surface configurations reveals that the observed W grains are terminated with Ba 0.50 O/W (0 0 1) , Ba 0.25 O/W (1 1 0) and Ba 0.50 O/W (1 1 2) surfaces, and were equilibrated in an O-poor environment with μ O between ∼−8.5 and −8.0 eV/at. Examination of the surface structures studied reveals that competing Ba-O and O-W interactions control the net surface dipole and that this dipole directly correlates with computed work functions, implying that the surface dipole alone is sufficient to explain low work functions observed for scandate cathodes. Analysis of the present results suggests that the role of Sc in scandate cathodes is to tune μ O and that difficulties in manufacturing scandate cathodes likely arises from variability in the availability of O at the cathode surface during activation and operation.

Original languageEnglish
Pages (from-to)827-838
Number of pages12
JournalApplied Surface Science
Volume458
DOIs
StatePublished - Nov 15 2018

Bibliographical note

Publisher Copyright:
© 2018

Keywords

  • Density functional theory
  • Equilibrium crystal shape
  • Scandate cathodes
  • Thermionic cathodes
  • Tungsten surfaces
  • Work function

ASJC Scopus subject areas

  • Condensed Matter Physics
  • Surfaces, Coatings and Films
  • Surfaces and Interfaces

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