Intrinsic defects and conduction characteristics of $Sc_2O_3$ in thermionic cathode systems

TL;DR

This study uses ab initio calculations to investigate the intrinsic defect-driven conduction in $Sc_2O_3$ for thermionic cathodes, revealing that impurities are likely essential for high conductivity and emission.

Contribution

It provides a detailed defect energetics analysis of $Sc_2O_3$ and demonstrates that intrinsic defects alone cannot account for observed conduction, highlighting the role of impurities.

Findings

Intrinsic defects are insufficient for high conductivity in $Sc_2O_3$.

Impurities at very low concentrations significantly enhance conduction.

Experimental high emission currents likely depend on impurity-induced conductivity.

Abstract

Recent experimental observations indicate that bulk $Sc_2O_3$ (~200 nm thick), an insulator at room temperature and pressure, must act as a good electronic conductor during thermionic cathode operation, leading to the observed high emitted current densities and overall superior emission properties over conventional thermionic emitters which do not contain $Sc_2O_3$. Here, we employ ab initio methods using both semilocal and hybrid functionals to calculate the intrinsic defect energetics of Sc and O vacancies and interstitials and their effects on the electronic properties of $Sc_2O_3$ in an effort to explain the good conduction of $Sc_2O_3$ observed in experiment. The defect energetics were used in an equilibrium defect model to calculate the concentrations of defects and their compensating electron and hole concentrations at equilibrium. Overall, our results indicate that the conductivity of $Sc_2O_3$ solely due to the presence of intrinsic defects in the cathode operating environment is unlikely to be high enough to maintain the magnitude of emitted current densities obtained from experiment, and that presence of impurities are necessary to raise the conductivity of $Sc_2O_3$ to a high enough value to explain the current densities observed in experiment. The necessary minimum impurity concentration to impart sufficient electronic conduction is very small (approximately $7.5x10^{-3}$ ppm) and is probably present in all experiments.