Palladium Membrane with High Density of Large-Angle Grain Boundaries to Promote Hydrogen Diffusivity

A higher density of large-angle grain boundaries in palladium membranes promotes hydrogen diffusion whereas small-angle grain boundaries suppress it. In this paper, the microstructure formation in 10 mu m thick palladium membranes is tuned to achieve a submicronic grain size above 100 nm with a high density of large-angle grain boundaries. Moreover, changes in the grain boundaries' structure is investigated after exposure to hydrogen at 300 and 500 degrees C. To attain large-angle grain boundaries in Pd, the coating was performed on yttria-stabilized zirconia/porous Crofer 22 APU substrates (intended for use later in an ultracompact membrane reactor). Two techniques of plasma sprayings were used: suspension plasma spraying using liquid nano-sized powder suspension and vacuum plasma spraying using microsized powder as feedstock. By controlling the process parameters in these two techniques, membranes with a comparable density of large-angle grain boundaries could be developed despite the differences in the fabrication methods and feedstocks. Analyses showed that a randomly oriented submicronic structure could be attained with a very similar grain sizes between 100 and 500 nm which could enhance hydrogen permeation. Exposure to hydrogen for 72 h at high temperatures revealed that the samples maintained their large-angle grain boundaries despite the increase in average grain size to around 536 and 720 nm for vacuum plasma spraying and suspension plasma spraying, respectively.

Automated summary

This study achieved submicron grain size with a high density of large-angle grain boundaries in 10 μm thick palladium membranes by tuning the microstructure formation. Two different plasma spraying techniques were used to develop membranes with comparable density of large-angle grain boundaries. The samples maintained their large-angle grain boundaries even after exposure to hydrogen at high temperatures.