Thermionic and Adsorption Characteristics of Thorium on Tungsten

Abstract

Variation of thermionic emission of tungsten with surface density of adsorbed thorium.---Thorium was deposited on a tungsten ribbon by 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\ifmmode^\circ\else\textdegree\fi{}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\ifmmode\times\else\texttimes\fi{}${10}^{5}$ and 5.7\ifmmode\times\else\texttimes\fi{}${10}^{4}$ 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 $f=0.0$ to $f=0.8$ the relation between the emission current and $f$ satisfied the following empirical equation ${log}_{10}i=\ensuremath{-}3.14\ensuremath{-}6.54{\ensuremath{\epsilon}}^{\ensuremath{-}2.38f},$ where $i$ is the emission current in amperes per ${\mathrm{cm}}^{2}$. For $0.8lfl2.0$, the values of emission currents are tabulated. For any fixed $f$, the emission obeys Richardson's equation. All the Richardson lines for $0lfl1$ intersect in a common point at an extrapolated temperature of 12,500\ifmmode^\circ\else\textdegree\fi{}K, and for $f\ensuremath{\ge}1$ the lines intersect in a common point for which the temperature is 3250\ifmmode^\circ\else\textdegree\fi{}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 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 rate. It is unnecessary to assume induced evaporation to explain these results.Variation of emission from thoriated tungsten with applied field.---It was found that for both the ribbon and the thoriated tungsten wire the dependence of emission on applied field changed as $f$ was varied. For the thoriated tungsten wire the dependence of the thermionic constants $A$ and $b$ on applied field was most pronounced for $0.3lfl0.6$.Evaporation and migration of thorium on tungsten surface.---Evaporation and migration of thorium on the tungsten surface were studied. The rate depends on the temperature and the fraction of the surface covered ($f$). For $0.2lfl1.0$ the rate of is approximately an exponential function of $f$. At 2200\ifmmode^\circ\else\textdegree\fi{}K and $f=0.2$ the rate of was ${10}^{\ensuremath{-}4}$ layers/sec. and at $f=0.8$ was 31\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}4}$ layers/sec. It was found that thorium could be deposited on one side of the tungsten ribbon and then made to migrate to the other side of the ribbon. This migration occurred at an appreciable rate above 1500\ifmmode^\circ\else\textdegree\fi{}K and was not complicated by up to 1655\ifmmode^\circ\else\textdegree\fi{}K. It was found that the migration coefficient depended on $f$ as well as on $T$. For a given set of conditions an approximate value of the heat of migration was calculated to be 110,000 calories per mol.