Thermionic Emission in the Presence of Adsorbed Films

Abstract

The thermionic emission of electrons from heterogeneous platinum surfaces in the presence of sulphur dioxide shows a strongly time dependent effect. The effect is shown to be dependent upon the pressure of sulphur dioxide used, and the temperature and nature of the previous treatment of the platinum. This is interpreted as a result of the occurence of a surface-sensitive reaction leading to the gradual formation of a monolayer of platinum sulphide. The experimental observations are explained in terms of a nucleation and growth theory, founded on the theories of Poisson and Avrami for the case of a two-dimensional phase change. Poisson has worked out the necessary mathematics to explain a transformation due to the constant generation of new nuclei whilst Avrami has explained mathematically transformations involving the growth of a fixed number of nuclei present at the start of the transformation. These theories are combined in order to interpret the sigmoidal and half-sigmoidal transformation curves obtained experimentally for the platinum/sulphur dioxide system. Several minor studies of the action of sulphurous gases on the thermionic emission of electrons from platinum are presented for comparative purposes with the main study of the platinum/sulphur dioxide system. Other minor studies show the results obtained by the action of some sulphurous gases on the thermionic emission of electrons from tungsten and iridium. The Farragher theory concerning the modification of the Richardson equation to account for the increase in work function brought about by adsorption, is extended to the system iridium/ sulphur dioxide which involves an observed variation in work function with surface coverage.