Among the various substrate materials, porous ceramics
are the most popular because of their superior chemical
stability and mature commercial availability. To achieve an
ultra-thin and defect-free palladium membrane, a porous
ceramic substrate material must possess a low surface
roughness and a small pore size with a narrow pore size
distribution. This kind of material often has an asymmetric
structure. Namely, it is composed of a micro or nanoporous
layer and a macroporous base, between which one or more
intermediate layers are also necessary. Consequently, fabri-
cation of asymmetric porous ceramics is accompanied by
a multi-time coating and sintering process, which causes high
cost and greatly offsets the advantage of the composite
membrane concept [9]. It may be argued that the cost of