Abstract
Surface topography and hydrogen permeation properties of Porous Stainless Steel (PSS) substrates for thin–films deposition of Pd–based hydrogen separation membrane were investigated. Hydrogen permeance through the as–received PSS substrates demonstrated a wide range, despite a similar average surface pore size of 15 micron determined by SEM and confocal laser microscopy analyses. The surface pores of the PSS substrates were modified by impregnation of varying amounts of tungsten (W) powder. Maximum hydrogen flux reduction of only 28% suggested that W has a limited effect on the hydrogen permeation through the PSS substrate. Therefore, it is suggested that hydrogen transport through PSS substrates is mainly controlled by the substrate geometrical factors, particularly the ratio of the porosity to tortuosity (e τ ). The variation in the permeance between the nominally similar PSS substrates indicates the importance to independently assess the hydrogen transport characteristics of each of the components in a composite membrane.
Highlights
Dense palladium (Pd) metal offers excellent permeability for hydrogen, based on the solution–diffusion mechanism [1]
Whilst Scanning Electron Microscopy (SEM) images suggest an average diameter of approximately 15 μm for the surface pores, further analyses of the 3 Dimensional images obtained by confocal laser microscope (Fig. 1b) showed the both surface pore diameter and the pore depth ranged between 10 to 25 μm in a good agreement with the previously reported values by Li et al [45]
In order to deposit a continuous thin film of less than 5 μm in thickness, the present work indicates that precise control of the amount of tungsten powder is required to minimise the formation of powder clusters on the surface whilst maximising the extent of the pore filling
Summary
Dense palladium (Pd) metal offers excellent permeability for hydrogen, based on the solution–diffusion mechanism [1]. Failure of Pd or Pd–alloy films during thermal cycling and hydrogen loading was attributed to the rising shear stresses as a result of the different elongations of metallic layer and ceramic support at the interface [36]. The scale of such a shear stress was shown to be directly related to the thickness of the metallic film and thermal stability with a lower thickness of metallic film can be achieved only at the expense of reduced hydrogen selectivity
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