Abstract
The surface structure and electron emission mechanism in high-performing “scandate” thermionic cathodes has long been a point of debate, in part due to the challenges inherent in observing emitting surfaces under operational conditions. Critically, the distribution and role of Sc in enabling high emitted current densities at low operating temperatures remains unclear. Here, using density functional theory calculations of the structure and energy of bare and Sc-covered W slabs, the wetting behavior of Sc on W has been explored. Computed surface excess energies reveal that, despite the bulk immiscibility of W and Sc, a single monolayer of Sc wets (001), (110), and (112) W surfaces at very low oxygen chemical potentials. The addition of further layers of Sc was found to be thermodynamically unfavorable. In contrast to previous studies, the present findings suggest the existence of a Sc interlayer directly atop W in scandate cathodes, representing a novel example of surface-energy-driven metal-on-metal wetting leading to self-assembly of atomically-thin layers.
Original language | English |
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Article number | 102476 |
Journal | Surfaces and Interfaces |
Volume | 35 |
DOIs | |
State | Published - Dec 2022 |
Bibliographical note
Funding Information:This research was funded by the Defense Advanced Research Projects Agency (DARPA) Innovative Vacuum Electronics Science and Technology (INVEST) program ( 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.
Funding Information:
This research was funded by the Defense Advanced Research Projects Agency (DARPA) Innovative Vacuum Electronics Science and Technology (INVEST) program (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.
Publisher Copyright:
© 2022 Elsevier B.V.
Keywords
- Scandate cathodes
- Scandium
- Surface energy
- Thermionic cathode
- Tungsten
- Wetting layer
ASJC Scopus subject areas
- Chemistry (all)
- Condensed Matter Physics
- Physics and Astronomy (all)
- Surfaces and Interfaces
- Surfaces, Coatings and Films