| The thermionic emission of positive cesium ions from zeolite has been studied. The extracted positive cesium ion current densities are described by the Richardson equation, J(,0) = A SQRT.(T) exp (-(PHI)(,0)/kT).;After 10('-4) coul/cm('2) of positive ions are extracted, the emitted current density, J(,0), begins to decay exponentially with time, until the steady state is reached. The steady state emitted current is up to two orders of magnitude lower than the initially extracted current. In steady state, the Richardson work function and pre-exponential factor are large, having values of 2.49 eV and 2.12 x 10('3) A/cm('2) K(' 1/2), respectively. The changes are attributed to the filling of ion surface states which increases the ionic work function as ion emission occurs.;Accompanying the decrease in the emitted current, J(,0), is a decrease in the electric field inside the zeolite bulk. The current of positive ions inside the zeolite, J(,z), is described by the Nerst-Planck equation, J(,z) = (sigma)E(,z) - qD (PAR-DIFF)p/(PAR-DIFF)z, where (sigma) is the ionic conductivity, D is the ionic diffusion coefficient, and p is the positive ion density. The two currents, J(,z) and J(,0) are, in general, unequal. This inequality results in an accumulation of positive ions beneath the emitting surface. As the surface ion states become filled, J(,0) and therefore J(,z), decay with time. The build-up of positive ions beneath the emitting surface increases until steady state is reached at which time E(,z) is approximately three orders of magnitude less than its initial value.;By adsorbing electronegative gas atoms at the emitting surface, the thermionic emission of positive cesium ions is significantly increased. Operation is limited to very short pulses where the filling of surface ion states is minimal. Adsorption of fluorine and chlorine rich gases lowers the Richardson work function and preexponential factor. Specifically, fluorine reduces (PHI)(,0) by over an order of magnitude and the pre-exponenetial factor by eight orders of magnitude. This results in over a 1000 fold increase in J(,0) at 800(DEGREES)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.;For very short times, on the order of fractions of a second relatively large current densities of several mA/cm('2) of positive ions can be extracted. The Richardson work function, (PHI)(,0), and preexponential factor, A, are small having values of 1.36 eV and 2.5 A/cm('2) K(' 1/2), respectively. |
