Enhanced emission of positive cesium ions from zeolite

Abstract

The thermionic emission of positive cesium ions from zeolite has been significantly increased in the pulsed regime due to the adsorption of electronegative gas atoms at the zeolite emitting surface. Extracted current densities are described by the Richardson equation, J0 = A(T)1/2 exp(−Φ0/KT). Adsorption of fluorine and chlorine rich gases lowers the Richardson work function Φ0 and the pre-exponential factor A. Specifically, fluorine adsorption reduces Φ0 to less than 0.1 eV and the pre-exponential factor by eight orders of magnitude. This results in over a 1000 fold increase of J0 at 800 °C. The changes are caused by a negative outward dipole layer at the emitting surface due to chemisorbed fluorine atoms. The coverage of these fluorine atoms obeys the Langmuir-Freundlich isotherm equation. Exposure of zeolite to oxygen results in an increase of up to an order of magnitude in A only. This is attributed to the production of additional active ion sites at the emitting surface.