Design of Nb-W-Mo alloy membrane for hydrogen separation and purification

The alloying effects of tungsten and molybdenum on the hydrogen solubility of niobium are investigated quantitatively. It is found that the equilibrium hydrogen pressure at the hydrogen concentration of 0.2 (H/M), PDBTC, increases linearly with increasing the amount of tungsten or molybdenum in niobium. On the basis of these results, the PDBTC - composition diagram for Nb-W-Mo system has been proposed. Using this diagram, Nb-8 mol%W-8 mol%Mo alloy has been designed which is expected to resist against hydrogen embrittlement under at least 0.2 MPa of hydrogen pressure. Then, the hydrogen solubility and the permeability of the designed alloy have been investigated in a fundamental manner. It is shown that the dissolved hydrogen concentration in Nb-8 mol%W-8 mol%Mo alloy does not exceed the critical value of the ductile-to-brittle transition hydrogen concentration (DBTC, which is about 0.2 (H/M)) even in a 0.25 MPa of hydrogen pressure at 773 K. This alloy membrane exhibits excellent hydrogen permeability without showing any hydrogen embrittlement during hydrogen permeation under this pressure and temperature conditions.