Electron Emission from Adsorbed Films on Tungsten

Abstract

Electron emission from tungsten filaments covered more or less completely with adsorbed films.—(1) Complete film of oxygen. The emission corresponds to the equation $i=A{T}^2{e}^{\frac{-b}{T}}$ where $A=5\mathrm{}\times {10}^{11}$ amp/${\mathrm{cm}}^2$ ${\mathrm{deg}.\mathrm{}}^2$; $b={107,000}^{\circ }$. It was necessary to supply oxygen continuously to keep the film complete. (2) Thorium film covering a fraction $\theta$ of a thoriated tungsten filament. The data obtained with three tubes agree with the empirical expression $i_{\theta }=[{a_1}^{\theta }+{a_2}^{(1-\theta )}-1]A_0{T}^2{e}^{\frac{-[b_1\theta +b_2(1-\theta \left)\right]}{T}}$ where $a_1$ and $a_2$ are the numerical values of $A$ for a complete film of Th on W and for pure W respectively, expressed in terms of the dimensional unit $A_0=1\mathrm{}$ amp/${\mathrm{cm}}^2$ ${\mathrm{deg}.\mathrm{}}^2$, and $b_1$ and $b_2$ are the respective values of $b$. The present experiments give $a_1=7\mathrm{}$, $b_1={31,200}^{\circ }$; from the results of other workers, $a_2=60\mathrm{}$, $b_2={52200}^{\circ }$. (3) Caesium films on tungsten covered with oxygen. Complete films of Cs were not obtained, but the results indicate $A_{C_s}<0.003\mathrm{}$ amp/${\mathrm{cm}}^2$ ${\mathrm{deg}.\mathrm{}}^2$ and $b_{C_s}$ slightly less than 8300°. Such low values, together with the values for an oxygen film, substituted in Eq. (1) give a curve which shows a maximum emission for $\theta =0.993\mathrm{}$ instead of the expected 1.00. Various experiments on the effect of varying the pressure of Cs vapor and on the effect of evaporating Cs from the surface confirm this conclusion, and also the variation of $A$ and $b$ with $\theta$. (4) Caesium films on tungsten. The maximum emission is computed to be for $\theta =0.987\mathrm{}$. (5) Effect of adding nitrogen to fully activated thoriated tungsten is to increase the emission about five fold at 1400°K. A pressure of 0.1μ is sufficient. The effect seems to be due to a change in the $A$ value.