  Although thermionic emission has been studied for more than 100 years, recent
interest in novel electron devices for military and civilian use has led to a
surge in demand for cathodes with enhanced emission properties (e.g. higher
current density, more uniform emission, lower operating temperatures, or
extended in-service longevity). Sc-containing "scandate" cathodes have been
widely reported to exhibit superior emission properties compared to
previous-generation thermionic cathodes, including oxide, B-, and M-type
cathodes. Despite extensive study spanning several decades, the mechanism by
which the addition of Sc enhances cathode emission remains ambiguous, and
certain limitations -- non-uniform emission, low reproducibility, inconsistent
longevity -- continue to prevent widespread commercial integration of scandate
cathodes into electron devices. This review attempts to survey the literature
to-date addressing the fabrication, structure, and properties of scandate
cathodes, with particular attention to studies addressing the role of Sc in
enhancing emission.
