Sc-Containing (Scandate) Thermionic Cathodes: Mechanisms for Sc Enhancement of Emission

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 nonuniform emission, low reproducibility, and inconsistent longevity continue to prevent widespread commercial integration of scandate cathodes into electron devices. This second part of a two-part review explores the various mechanisms proposed for how Sc enhances electron emission and summarizes ongoing scientific challenges relating to the development and deployment of scandate cathodes.