Grain Boundary Diffusion of Hydrogen in Nano-Structured Pd/Ag Alloy Membranes

Palladium and its alloys have long been used as hydrogen separation membranes due to their extremely high permeability and selectivity to hydrogen over all other gases [1]. The hydrogen permeation process begins with selective chemisorption of the gas onto the metal surface. As the adsorption process is the point in the permeation sequence where the majority of gases become excluded, it follows that a cleverly designed device could be created to take advantage of the so-called ‘fast’ diffusion paths of surface and grain-boundary diffusion to further enhance permeability without sacrificing selectivity. The contribution of grain-boundary diffusion to the overall permeation rate is dependent on the relative volume in the membrane occupied by grain-boundaries versus bulk material. Typically, grain boundaries only make up a miniscule fraction of the overall volume and therefore only contribute an appreciable amount to the overall diffusion process at temperatures low enough to make the bulk diffusion process nearly stagnant. However, in the case of a nanostructured membrane this paradigm is no longer valid. The fabrication methods associated with extremely thin membrane deposition typically lead to highly non-equilibrium microstructure with an average grain size on the order of tens of nanometers [2]. In order to exploit the potential advantages of grain boundary diffusion the nano-scale grains must persist throughout operation. To avoid the tendency for the grain structure to relax to a more equiaxed, coarse-grained morphology the self-diffusion of metal atoms in the film must be minimized by operating the membranes at a temperature much lower than the membrane melting temperature. Figure 1 shows the microstructural changes in a thin, sputtered, Pd/Ag alloy film before and after annealing. The initial fine-grained structure on the bottom surface of the membrane is due to a combination of low substrate temperature during deposition and the Ti adhesion layer onto which the Pd/Ag layer was deposited. After annealing at 400 C the grains have coarsened and the top and bottom structure are identical.