Scandium wetting of tungsten surfaces in “scandate” thermionic cathodes

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.