Oxygen permeability and thermal expansion of ferrite-based mixed conducting ceramics

In order to evaluate promising directions in the development of mixed-conducting membrane materials for oxygen separation and partial oxidation of natural gas, a series of ferritebased ceramics were studied, including La1-xSrxFe1-yGayO3-8 (x = 0.5 - 0.8; y = 0 - 0.4), La1-xSrxFe1-yAlyO3-delta (x = 0.7 - 1.0; y = 0 - 0.5), La0.3Sr0.7Fe0.7-XAl0.3CrXO3-delta (x = 0.1 - 0.2), (Sr2Fe3)(1-X)(SrCo)(X)O-Z (x = 0 - 0.8), CaFe0.5Al0.5O2.5+delta and Ln(3-x)Ca(X)Fe(5)O(12-delta) (Ln = Gd, Y; x = 0 - 0.5). The maximum oxygen permeation is observed for perovskite-type solid solutions with high oxygen deficiency, which exhibit, however, excessive thermal and chemically induced expansion. As for cobaltite- and gallate-based mixed conductors, the increase in ionic transport is accompanied with increasing limiting role of the surface exchange processes. The stability of perovskite-related ferrites in reducing atmospheres, which is comparable to that of LaFeO3-delta and iron oxide, may be moderately increased or decreased by donor- or acceptor-type doping, respectively. In addition, the substitution of iron with cations having a more stable oxidation state, such as Ga3+, Al3+ or Cr3+/4+, partly prevents the lattice expansion induced by oxygen nonstoichiometry variations, although the solubility of these dopants in the ferrite lattice is limited.