Preparation of thin Pd-based composite membrane on planar metallic substrate

Pd-based membranes are of increasing interest for hydrogen separation and purification due to their good thermal stability, high permeability and perfect selectivity. The membranes are generally prepared in a composite structure consisting of a thin Pd-based layer on a porous substrate to reduce Pd cost and increase hydrogen permeability. The integrity of a composite Pd-based membrane, and accordingly its selectivity, are highly dependent on the quality of the substrate. Porous stainless steel is an attractive substrate due to its similar thermal expansion coefficient to the Pd-based films, ease of fabrication, good weldability, ductility and low cost. However, the surface roughness and largest pore size of even the best commercially available porous stainless steel substrates are too large, typically ∼10-20 μm. Therefore, a proper pre-treatment is essential to deposit a thin pinhole-free Pd-based layer on the substrate. This paper presents a method to pre-treat the porous stainless steel substrate, including polishing, etching and coating with alumina particles. Polishing decreased the surface roughness of the substrate. Alumina coating served as several roles: further smoothening the surface, decreasing the pore size of the substrate and preventing interdiffusion between the Pd-based layer and substrate at elevated temperatures. Permeation properties and surface morphologies of the substrate are investigated before and after pre-treatment.