Composite Pd/Ta metal membranes for hydrogen separation

The fabrication of a composite metal membrane using Vapor deposition techniques is reported. Commercially obtained tantalum foils are cleaned by ion milling and subsequently coated with palladium metal. The test foil was 13 mu m tantalum sheet coated with 1 mu m of palladium on each side. For comparison, several other foils of differing tantalum and palladium thicknesses were investigated, These membranes having an area of about 2 cm(2) provide a hydrogen flux ranging from 18.7 standard cubic centimeters per minute (seem) at 300 degrees C to 27.4 seem at 365 degrees C with a hydrogen differential pressure of about 355 torr. The deposited palladium films were oriented along the (111) crystal axis. Gas flux versus pressure measurements revealed that these membranes do not show the p(0.5) dependence upon applied differential pressure expected for foils in which bulk diffusion is rate limiting. This indicates that surface effects are becoming significant in determining hydrogen flux.