The Thermionic Constants of Tungsten as a Function of Crystallographic Direction

Abstract

A method is described for recrystallizing tungsten wire into long single crystals occupying the whole cross section of the wire and so oriented that one of the face diagonals is parallel to the axis of the wire. The temperature dependence of the emission associated with the different crystal directions has been investigated for one such specimen and fitted to the empirical equation ${i}_{0}={A}^{*}{T}^{2} \mathrm{exp} (\frac{e{\ensuremath{\phi}}_{0}}{\mathrm{kT}})$. The values of ${\ensuremath{\phi}}_{0}$ range from 4.35 volts for the (111) crystal direction to 4.65 volts for the (110) crystal direction. The values of ${A}^{*}$ range from 125 for the (112) and (001) directions to 15 for the (110) direction. A possible explanation for the low values of ${A}^{*}$ is suggested. These results show that the values of ${\ensuremath{\phi}}_{0}$ and ${A}^{*}$ obtained from the usual measurements on polycrystalline wire are characteristic of a weighted average of the properties of the various crystal surfaces occurring and have, therefore, only empirical value, indicating that all attempts to relate observed to theoretical values of emission constants have so far no great significance.