Palladium membrane thermal expansion coefficient

The selection of porous substrate. Ceramic supports are chemically stable, have small pore sizes and a more uniform pore size distribution, essential for the formation of thinner and uniform membrane layer. But they are brittle, and the large difference in thermal expansion coefficients between palladium layer and ceramic supports leads to membrane cracking and loss of adherence. On the other hand, metallic porous supports made of stainless steel (PSS) have a more similar thermal expansion coefficient, reducing thermal stresses, but the large pore size and wide pore size distribution make the formation of a uniform selective layer problematic. 

The challenge, for these membranes, is related to stabihty under thermal cycling, due to a mismatch in thermal expansion coefficients between the ceramic and the palladium, particularly when hydrogenated (Ayturk et al., 2006). At high temperature, total stress can be considered as the sum of thermal stress and H2 stress due to H2 dissolution within the Pd lattice. As shown in Table 3.5, the thermal expansion coefficient of stainless steel is about three times higher than that of alumina. It is also interesting to note that Hastelloy C and zirconia oxide exhibit very close values. Pall... 

Moreover, since also the mismatch of palladium and alumina thermal expansion coefficients is reduced, high temperature delamination and defects formation in the membrane are also reduced. It is also possible to control the palladium thickness (to about 5 pm) by increasing the hydrogen flux and so minimizing the cost of the composite membrane. 

Already by 1963, for a patent granted in 1966, Straschil and Lopez realized that the match of coefficient of thermal expansion between palladium membranes and (porous) substrates was critical, and stated that it would be virtually impossible to compensate for differences in dilation due to absorption of hydrogen [38]. They patented the use of dimpled or corrugated foils to accommodate differential thermal and chemical expansion [38]. Buxbaum and Hsu, in a 1992 patent, maintained that a rough substrate surface produced by abrasion with steel wool was critical for adherence of palladium on surfaces of Nb, Ta, V and Zr [39]. Other patents recommend corrugated or undulating configurations to allow for both thermal and chemical expansion [24, 26, 27, 29]. 

---