Anomalous Thermionic Emission of Some Borides and Carbides of Rare Earth and Transition Elements

Thermionic emission constants, $A^{*}$, up to several thousand times larger than the $A_0$ (120 amp/${\mathrm{cm}}^2$-°${\mathrm{K}}^2$) predicted theoretically for metals have been reported for several compounds involving transition metals and rare earths with boron and carbon. It is suggested that such anomalously large emission constants, as well as some anomalously small ones, are due to the relatively large distances between metal atoms as a result of which the energy bands originating from the incomplete atomic $f$ and/or $d$ sublevels are narrow enough for nondegeneracy to occur in the experimental temperature range.