Temperature Dependence of the Work Function of Single-Crystal Faces of Copper

Abstract

Measurements of the temperature dependence of the work functions of the (100), (110), (111), (211), and (221) crystal faces of copper have been made in the temperature range 20 to 700\ifmmode^\circ\else\textdegree\fi{}C. The measurements were obtained by a diode retarding-field method in vacuo below ${10}^{\ensuremath{-}9}$ torr. Constant current operating conditions permitted a direct continuous monitoring of changes in the anode work function. While absolute work function measurements could not be obtained by this method, a precision measurement of the temperature dependence was possible. The single-crystal anodes were given rigorous heat treatment in high vacuum resulting in drift-free measurements which were reproducible. The results from the (100) and (111) faces, when combined with previously reported values of the absolute work function, demonstrate a relationship in which the work function smoothly approaches a value independent of crystallographic orientation at the melting point. A simple thermodynamic argument is presented to explain the results. Published measurements of the temperature dependence of the work function of single-crystal faces of tungsten are shown to be consistent with this interpretation.