Hydrogen sorption by barium films

Abstract

The capillary method, under Knudsen flow conditions and at constant pressure on the sorbent, is in this work applied to the study of hydrogen sorption on evaporated barium films. Particular precautions have been taken to reduce to a minimum the influence of the filament and electron current of the ionization gauge (Alpert type) on the measurement of the sorption velocity. To this end, thoriated tungsten filaments have been employed and very low ionizing electron currents (0.01 mA) have been used. The experimental results have been obtained in the temperature range between 195 and 473°K, on barium films of approximately 2 mg, evaporated from a stabilized barium-aluminium alloy and having an apparent surface area of 100 cm 2. A preliminary investigation of the curves giving velocity of sorption as a function of time, shows that the shape of such curves is dependent on the pressure range in which they are obtained. At the higher pressures the kinetics of adsorption follows very closely the diffusion laws, as is shown by the nearly linear relation existing between quantity sorbed and square root of time. On the other hand at lower pressures, the graphs show an almost horizontal portion, corresponding to a constant velocity of sorption lasting a considerable period of time. These differences in behaviour show that for hydrogen, as for other gases, the sorption by barium films is made up of a number of phenomena superimposed on each other. The different appearance of the various phenomena for different gases can, with all probabilities, be attributed to the different coefficients of diffusion of the gases and the relative activation energies.