Thickness dependent effects of solubility and surface phenomena on the hydrogen transport properties of sputtered Pd77%Ag23% thin film membranes

The hydrogen solubility and permeation in Pd77%Ag23% membranes have been determined as a function of temperature and membrane thickness. Unexpectedly, the solubility of hydrogen is found to systematically increase as the membrane thickness decreases from 11.2 to 2.2 µm. Topography studies by atomic force microscopy in conjunction with previously reported characterization suggest linkage of the hydrogen solubility to the density of grain boundaries. A higher average grain boundary density for thinner membranes results from the nucleation and growth proceeding during membrane fabrication by sputtering. For the membranes and conditions (no membrane pretreatment; 300-400 °C; ΔpH2≤200 kPa) applied here, surface phenomena affect the hydrogen transport at thicknesses below ~5 µm. Determination of the solubility constants hence allowed the extraction of the bulk diffusivity parameters from the permeability measurements over the thicker membranes (6.7-11.2 µm), in good agreement with reported values obtained using other techniques.