Nanostructured palladium membrane synthesis by magnetron sputtering

Palladium membranes are of technological interest since they show a very high selectivity for hydrogen. Diffusion through the palladium is often the rate-limiting step in hydrogen transport through the membrane. Hydrogen flux can be improved by reducing the membrane thickness and increasing the hydrogen diffusivity. Nanostructured palladium has a higher hydrogen diffusivity than conventional palladium due to its large Volume fraction of grain boundaries. They were generated as ultrathin membranes by d.c. magnetron sputtering onto porous Vycor(R) glass substrates in an argon atmosphere. The nanostructured films exhibited no cracks on the submicron scale when examined with environmental scanning electron microscopy. The membranes delaminated from the substrates when exposed to hydrogen at room temperature, owing to the alpha-->beta phase transition. Heating the ultrathin nanostructured palladium membranes to 200 degrees C led to some grain growth. Stabilization against the phase transition and grain coarsening is critical to applications of nanostructured membranes in hydrogen separations, and can be achieved by the alloying of palladium with another metal.