Thermionic and Adsorption Characteristics of Thorium on Tungsten

Variation of thermionic emission of tungsten with surface density of adsorbed thorium.—Thorium was deposited on a tungsten ribbon by evaporation from a thorium wire. A study was made of the dependence of the thermionic emission on the two parameters: $T$, the temperature, and $f$, a quantity which is proportional to the amount of thorium on the tungsten surface. At a fixed temperature 1274°K it was found that as the amount of thorium on the tungsten surface was increased, the thermionic emission increased to a maximum, then decreased, and asymptotically approached a constant value. For the maximum, $f$ is defined to be 1.0. The maximum value and the final constant value of the emission current were respectively 5.7×${10}^5$ and 5.7×${10}^4$ times the value of emission current characteristic of clean tungsten. Moreover the final constant value of the emission agreed to within a factor of 2 with the value characteristic of clean thorium. From $f=0.0\mathrm{}$ to $f=0.8\mathrm{}$ the relation between the emission current and $f$ satisfied the following empirical equation ${log}_{10}i=-3.14-6.54\mathrm{}{\epsilon }^{-2.38f},$ where $i$ is the emission current in amperes per ${\mathrm{cm}}^2$. For $0.8<f<\mathrm{}2.0\mathrm{}$, the values of emission currents are tabulated. For any fixed $f$, the emission obeys Richardson's equation. All the Richardson lines for $0<f<\mathrm{}1\mathrm{}$ intersect in a common point at an extrapolated temperature of 12,500°K, and for $f\ge 1$ the lines intersect in a common point for which the temperature is 3250°K. These results obtained by depositing thorium on a tungsten ribbon have been compared with results obtained from thoriated tungsten wire. Thoriated tungsten wire can be activated by diffusion of thorium from the interior to the surface. For a while every atom that diffuses to the surface sticks to it so that $f$ increases linearly with the time; later when evaporation is no longer negligible the rate of accumulation, $\frac{\mathrm{df}}{\mathrm{dt}}$, gets less and less; a steady state is reached when the diffusion rate equals the evaporation rate. It is unnecessary to assume "induced evaporation" to explain these results.