Mapping conditions for the formation of high-performance scandate cathodes: New insights into the role of sc

TEM and SEM studies have shown that tungsten grains in low work function scandate cathodes are well-faceted and exhibit a characteristic shape consisting of (001), (110), and (112) surfaces. These surfaces appear with characteristic relative surface areas. The relative surface areas of facets expressed on a crystallite are controlled by the relative energies of each facet present. In turn, relative surface energies are controlled by the structure, composition, and chemical potential (equivalently, partial pressure) of species present in a cathode system— generally, in the case of scandate cathodes, Ba, Sc, O and W. Combining quantum mechanical calculations of the atomic structure and energy of Ba/Sc/O-adsorbed W facets with thermodynamic models of chemical potential dependent surface energies, we have screened a large number of possible surface configurations and chemical conditions to identify a small number of candidate structures and ranges of stability that are consistent with experimental observations. These results represent a map of chemical conditions necessary to yield experimentally observed cathode grain shapes, and suggest a role for Sc that explains both the superior thermionic emission properties of low work function scandate cathodes and manufacturing/reproducibility problems that have limited scandate cathode adoption by industry and the military.