Influence of adsorbed hydrogen molecules on the hydrogen permeation and recycling

Hydrogen permeation through a thin iron membrane has been measured using a molecular hydrogen gas and/or a weakly ionized rf-discharge as particle sources. Transient and steady state values of the permeation flux $\varnothing_{p}$ were obtained in the regime where the surface penetration of H and the recombinative release of H$_{2}$ are rate-determining. $\varnothing_{p}$ is then /Waelbroeck 84a/ proportional to the flux density of the particles which impinge on the membrane upstream. The proportionality factors depend on the nature (atomic or molecular) of the incident particies, an temperature (in particular for a molecular driving gas), and on the ratio of the surface recombinative release rate constants (2$\sigma$k$_{r}$ )$_{u}$ and (2$\sigma$k$_{r}$ )$_{d}$ up- and downstream, respectively. The pressure P of the molecular gas, the power coupled to the rf-discharge, the temperature of the membrane, and its electrical potential with respect to the plasma luve been varied during the rf-plasma-driven permeation measurements. The upstream pressure was l0$^{-3}$ mbar for most measurements whereas the downstream side was evacuated. The asymmetry factor A $\equiv$ (2$\sigma$k$_{r}$)$_{u}$ / (2$\sigma$k$_{r}$)$_{d}$ of the membrane is then about one and independent of P at high temperatures. lt increases strongly when T is lowered from 300 to 100°C where it reaches a value $\backsimeq$ 10$^{3}$. lt depends more weakly on T below 100°C . The value A depends on P in the intermediate T domain where, moreover, $\varnothing_{p}$ varies non monotonously with time after changes of P : the permeation flux density increases rapidly with a characteristic permeation time of $\backsimeq$ 1 sec. when the pressure is increased, then it decreases slowly (characteristic time : $\backsimeq$ 1 hour) because this pressure increase leads also to the increase of A. The exposure of the membrane to a current I$_{e}$ of electrons with an energy E $\gtrsim$ 10eV upstream leads, below 300°C, to a slow decrease of A towards a new steady state value. Increases of I$_{e}$ at a given E or of E at a given I$_{e}$ lower this value of A. The time to reach steady state is shorter than after simple pressure variations ; it decreases when I$_{e}$ increases