Microfabricated Palladium-based Membranes for Hydrogen Separation

An innovative process for the microfabrication of Pd-based membranes (Pd and Pd-Ag) on a <110> silicon wafer is presented. Pd-Ag alloy films containing 23wt% Ag are prepared by a dual-sputtering from pure Pd and Ag targets. In the first step, deep grooves are KOH etched in one side of a <110> oriented silicon wafer, leaving membranes with a thickness of ca. 50 μm. After Pd alloy deposition on the unetched side, the silicon membranes are removed by etching, leaving Pd-Ag membranes. Membranes with a porosity, i.e. the ratio of effective separation area over the total area, of up to 20% may be fabricated. Anodic bonding of thick glass plates (containing powder blasted flow channels) to both sides of the silicon substrate is used to package the membranes and create a robust module. The mechanical strength of the membrane is found to be adequate, pressures of up to 3 bars at room temperature do not break the 1000 nm thick membrane. With 700 nm Pd-Ag membrane a high separation flux of ca. 3.6 mol H2 /m.s is obtained with a minimal selectivity of 1500 for H2 with respect to helium (He) at 723 K and 0.83 bars H2 retentate pressure.