Pd/Ag membranes on porous alumina substrates by unbalanced magnetron sputtering

The demand for hydrogen is increasing, particularly for high-purity product. The majority of this demand occurs within the chemical industry, but the second greatest requirement is for the electronics industry, which has increased its use of hydrogen greatly in the last 10 years. A further demand on hydrogen production is expected to materialise with the advent of the various types of fuel cells now being actively developed. Currently, the most effective method to produce hydrogen of the quality required for electronics processing is by the use of hydrogen-selective membranes. Dense, thin-walled Pd/Ag tubular membranes currently in operation are expensive, due to the cost of the raw materials used. A number of tubes are required to work in parallel to obtain the hydrogen flows required for industrial use, increasing both the overall cost and the space required for the permeator units. The aim here is to produce composite components consisting of thin film permeators deposited onto porous supports. As the hydrogen permeation rate relates inversely to the permeator thickness, thin film/porous substrate composite permeators offer greatly increased rates of permeation. The present investigation looks at the deposition of Pd/Ag films on porous ceramic supports by unbalanced magnetron sputtering. The RF substrate bias and level of external heating have been systematically varied in a full experimental array. Subsequently, coatings were examined to determine the structure, pinhole density, composition, N2 gas-tightness and, where appropriate, H2 permeability. All coatings were found to contain pinholes. The N2 gas flow ranged from 8×10−9 to 1×10−5 mol m−2 s−1 Pa−1, depending on the substrate type and condition, and H2/N2 permeation separation ranged from 4 to >80. The most important factor influencing pinhole formation was the initial condition of the substrate surfaces. In addition to this, some variation in composition was noted.