Absorption of cesium by polycrystalline graphite-sticking coefficient studies

Abstract

The interaction of metal vapors with graphite is a subject of considerable scientific and technological importance. Graphite is known to getter cesium (Cs) atomic vapor and this has found important space applications. Results of our experimental investigations of the absorption of Cs by polycrystalline graphite are reported. We measure the sticking coefficient γ of Cs on graphite using an atomic beam apparatus. For polycrystalline graphite γ starts with the initial value of 1 (every impinging Cs atom sticks to the surface) and decreases slowly with time (~days), for continuous exposure to the Cs atomic beam. This is in sharp contrast to single crystal graphite where γ is essentially 0. For polycrystalline graphite, the rate of decline of γ with time is found to be essentially independent of the incident Cs beam flux over the range of fluxes investigated experimentally. A theoretical model for the rate of decline of γ based on the diffusion of Cs into polycrystalline graphite is developed. Polycrystalline graphite consists of crystallites graphitized in a rather porous structure. In general, for such an inhomogeneous medium several types of diffusion with different activation energies come into play. We assumed that under our experimental conditions grain boundary diffusion (surface diffusion along the grains) is the single dominant mechanism of migration of Cs through polycrystalline graphite. The experimental results show a good fit with the predictions of this model. This represents the first quantitative study of the Cs gettering property of graphite at room temperature.