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.
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