The mobility of sodium on tungsten

Abstract

Although the existence as well as some of the properties of adsorbed films of alkali and alkali earth metals on metals of higher work function have long been known, it is only recently that these films have been shown to be capable of migrating over the surface of the adsorbent, and that attempts have been made to measure diffusion coefficients and the activation energy associated with the diffusion. Langmuir and Taylor, investigating the properties of cæsium films, found it necessary to postulate that the cæsium was mobile in order to account for the high value (α ≅ 1.0) for the condensation coefficient on quite concentrated films. Later they were able to make a direct measurement of the diffusion coefficient. They obtained a value of D at 812° K of 3.4 x 10 -5 cm 2 secs -1 , and an activation energy of 0.61 volt. Langmuir has also shown that cæsium films exist in two phases and has measured the diffusion coefficient by following the movement of the boundary between these two phases.