Scandate cathode surface characterization: Emission testing, elemental analysis and morphological evaluation

Xiaotao Liu; Qunfei Zhou; Tyler L. Maxwell; Bernard K. Vancil; Matthew J. Beck; T. John Balk

doi:10.1016/j.matchar.2018.12.013

Scandate cathode surface characterization: Emission testing, elemental analysis and morphological evaluation

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

Research output: Contribution to journal › Article › peer-review

33 Scopus citations

Abstract

This paper describes an investigation of the surfaces of scandate cathodes that exhibited good emission characteristics during close-spaced diode testing. Morphological evaluation with electron microscopy showed that micron-sized tungsten particles constituting the cathode surface are faceted and decorated with nanoscale Ba/BaO dots, along with larger Sc₂O₃ and BaAl₂O₄ particles. The facets of tungsten grains were determined to be {100}, {110} and {112}, based on Wulff analysis of the tungsten crystal shape. Moreover, {112} facets are prevalent in the Wulff shape, which agrees with the experimentally observed facet surface areas. A hypothesis is proposed to account for the amount of metallic Ba that covers the cathode surface at operating temperatures, and which transforms during cooling of the cathode after emission testing, to form nanoscale hemispherical particles that oxidize upon air exposure. According to computation, the coverage of Ba atoms during cathode operation corresponds to 0.5 (+0.05/−0.12) monolayer, which represents full coverage of the available tungsten surface. This investigation using a range of materials characterization techniques has provided a picture of the structure and composition of scandate cathode surfaces. It is noted that these results do not support the existence of a Ba-Sc-O layer occupying the top 10–100 nm of tungsten grain surfaces.

Original language	English
Pages (from-to)	188-200
Number of pages	13
Journal	Materials Characterization
Volume	148
DOIs	https://doi.org/10.1016/j.matchar.2018.12.013
State	Published - Feb 2019

Bibliographical note

Publisher Copyright:
© 2018

Funding

This work was financially supported by the Defense Advanced Research Projects Agency (DARPA) Innovative Vacuum Electronics Science and Technology (INVEST) program, under grant number N66001-16-1-4041 . The views, opinions, and/or findings expressed are those of the authors and should not be interpreted as representing the official views or policies of the Department of Defense or the U.S. Government. The authors also thank Dr. Nicolas Briot and Dr. Dali Qian of the University of Kentucky Electron Microscopy Center, for helpful suggestions and stimulating discussion regarding the FIB-related work and TEM investigation, respectively.

Funders	Funder number
Innovative Vacuum Electronics Science and Technology Program
Defense Advanced Research Projects Agency	N66001-16-1-4041

Keywords

Elemental analysis
Emission testing
Facet
Scandate cathode
Surface characterization
Wulff construction

ASJC Scopus subject areas

General Materials Science
Condensed Matter Physics
Mechanics of Materials
Mechanical Engineering

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10.1016/j.matchar.2018.12.013

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title = "Scandate cathode surface characterization: Emission testing, elemental analysis and morphological evaluation",

abstract = "This paper describes an investigation of the surfaces of scandate cathodes that exhibited good emission characteristics during close-spaced diode testing. Morphological evaluation with electron microscopy showed that micron-sized tungsten particles constituting the cathode surface are faceted and decorated with nanoscale Ba/BaO dots, along with larger Sc2O3 and BaAl2O4 particles. The facets of tungsten grains were determined to be {100}, {110} and {112}, based on Wulff analysis of the tungsten crystal shape. Moreover, {112} facets are prevalent in the Wulff shape, which agrees with the experimentally observed facet surface areas. A hypothesis is proposed to account for the amount of metallic Ba that covers the cathode surface at operating temperatures, and which transforms during cooling of the cathode after emission testing, to form nanoscale hemispherical particles that oxidize upon air exposure. According to computation, the coverage of Ba atoms during cathode operation corresponds to 0.5 (+0.05/−0.12) monolayer, which represents full coverage of the available tungsten surface. This investigation using a range of materials characterization techniques has provided a picture of the structure and composition of scandate cathode surfaces. It is noted that these results do not support the existence of a Ba-Sc-O layer occupying the top 10–100 nm of tungsten grain surfaces.",

keywords = "Elemental analysis, Emission testing, Facet, Scandate cathode, Surface characterization, Wulff construction",

author = "Xiaotao Liu and Qunfei Zhou and Maxwell, {Tyler L.} and Vancil, {Bernard K.} and Beck, {Matthew J.} and Balk, {T. John}",

note = "Publisher Copyright: {\textcopyright} 2018",

year = "2019",

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doi = "10.1016/j.matchar.2018.12.013",

language = "English",

volume = "148",

pages = "188--200",

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AU - Liu, Xiaotao

AU - Zhou, Qunfei

AU - Maxwell, Tyler L.

AU - Vancil, Bernard K.

AU - Beck, Matthew J.

AU - Balk, T. John

PY - 2019/2

Y1 - 2019/2

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AB - This paper describes an investigation of the surfaces of scandate cathodes that exhibited good emission characteristics during close-spaced diode testing. Morphological evaluation with electron microscopy showed that micron-sized tungsten particles constituting the cathode surface are faceted and decorated with nanoscale Ba/BaO dots, along with larger Sc2O3 and BaAl2O4 particles. The facets of tungsten grains were determined to be {100}, {110} and {112}, based on Wulff analysis of the tungsten crystal shape. Moreover, {112} facets are prevalent in the Wulff shape, which agrees with the experimentally observed facet surface areas. A hypothesis is proposed to account for the amount of metallic Ba that covers the cathode surface at operating temperatures, and which transforms during cooling of the cathode after emission testing, to form nanoscale hemispherical particles that oxidize upon air exposure. According to computation, the coverage of Ba atoms during cathode operation corresponds to 0.5 (+0.05/−0.12) monolayer, which represents full coverage of the available tungsten surface. This investigation using a range of materials characterization techniques has provided a picture of the structure and composition of scandate cathode surfaces. It is noted that these results do not support the existence of a Ba-Sc-O layer occupying the top 10–100 nm of tungsten grain surfaces.

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KW - Surface characterization

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Scandate cathode surface characterization: Emission testing, elemental analysis and morphological evaluation

Abstract

Bibliographical note

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Keywords

ASJC Scopus subject areas

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