HYDROGEN PERMEATION THROUGH COPPER-COATED PALLADIUM

The rate of hydrogen uptake and release by metals can be strongly affected by surface barriers for adsorption and desorption. The rate of hydrogen permeation through a Pd membrane was measured for both incident molecules (10(-3)-10(-4) Pa) and neutral atoms (10(16)-10(19) H-0/m2.s) for membrane temperatures of 300-570 K. The pressure dependence of H-2-driven permeation was used to identify regimes where the permeation was controlled by bulk processes (diffusion-limited) and surface processes (surface-limited). The dependence of the H-0-driven permeation rate on the direction of permeation was used to separate the contribution of each surface to the overall surface-limited permeation rate. One of the membrane surfaces was coated in situ with copper evaporated from a hot source. This same surface could be monitored in situ by Auger electron spectroscopy. At temperatures below 450 K, stable copper coatings were made with thicknesses ranging from approximately 3 to 25 nm. The thin Cu coatings led to a decrease in the H-2-driven permeation rate. The permeation rate was found to increase, however, for H-0 atoms incident on the Cu-coated surface. This is consistent with a barrier for H-2 dissociation and H recombination at the Cu/vacuum interface. Membranes with such a barrier, in conjunction with a source of H-0 atoms, have applications as hydrogen pumps.