Evaluation of tantalum-based materials for hydrogen separation at elevated temperatures and pressures

The hydrogen permeability of bulk tantalum and tantalum coated with thin films of palladium was measured at temperatures from 623 to 1173 K and hydrogen partial pressures from 0.1 to 2.6 MPa in a flowing gas system. Palladium coatings were deposited by both electroless plating (1-2 mum thick I'd layer) and cold plasma-discharge sputtering with two different thicknesses. 0.04 or 1.2 mum. All samples studied showed declining permeability values with surface fouling over time. The highest absolute values of permeability were observed at the lowest temperatures of study (623-773 K). However, permeability data taken at 1173 K tended to be more consistent with less scatter. The overall trend exhibited declining permeability values as a function of temperature. As confirmed by surface analysis, the palladium coating was lost from the coated tantalum samples. In one test, a 1.2 mum sputter-coated palladium film was observed peeling off the tantalum surface after 49 h at 773 K. The tantalum surface of all tested samples exhibited oxidation, distortion and cracking. Tantalum hydride formation was observed at 623 K on a palladium sputter-coated sample. Surface fouling limited the hydrogen permeability of all samples studied. (C) 2003 Elsevier Science B.V. All rights reserved.