An investigation of fundamental phenomena affecting the performance of sputtered Pd alloy thin film membranes for hydrogen separation

Abstract

Pure hydrogen flux has been obtained for sputtered Pd77%Ag23% membranes with thickness of 2.2-11.2 μm. A general thickness dependent behaviour is observed upon permeation under hydrogen. However, permeability is not constant indicating that phenomena other than bulk diffusion are limiting the transport through the thinner membranes. Atomic force microscopy (AFM) reveals a thickness dependent increase in surface roughness upon hydrogen stabilization.

Sputtered Pd77%Ag23% membranes with thickness ranging from 2.2 μm to 8.5 μm have been subjected to a three-step heat treatment in air (HTA). The results show that the hydrogen flux increases stepwise upon HTA of each side and mainly affects the thinner membranes. After HTA, permeability appears to become a material constant, indicating that bulk diffusion is now the rate-limiting step. AFM results indicate that after HTA an increase in membrane surface area is found. However, the surface roughness increased already after hydrogen stabilization, hence the further increase in surface area do not fully explain the gain in permeability.

Hydrogen solubility has been obtained by volumetric sorption analysis for all samples in Pd77%Ag23% membranes. The solubility of hydrogen decreases as the membrane thickness increases. Topography studies by AFM revealed that as thickness decreases, higher average grain boundary density apparently enhances hydrogen solubility.

The hydrogen solubility has also been obtained for Pd-Cu and Pd-Cu-Ag membranes with different composition. The obtained isotherms display poorer linearity and reproducibility especially at high pressure (> 33.7 kPa). This can be a result of possible composition-depending phase change.

A 4.7 μm thick Pd77%Ag23% membrane has been exposed to CO over a short period of time prior and after HTA. As the temperature increases, a less reduction in hydrogen flux is registered in both heat-treated and not-heat treated membranes. In general, the experiments verify the reported reduction of the inhibiting effect of CO upon HTA.