Correlation between oxygen surface exchange rate and surface structure in the La1.5Sr0.5Ni1-yCoyO4+δ ceramic membranes.

The La 1.5 Sr 0.5 Ni 1- y Co y O 4+δ (y = 0, 0.2, 0.4) complex oxides were synthesized by the citrate-nitrate method. The disk-shaped dense ceramic membranes with thickness of 0.8 and 1.2 mm for each composition were prepared by uniaxial pressing of the powders and sintering at 1350 °C in air. The phase purity of the samples was confirmed by the X-ray diffraction (XRD). The oxygen excess in the samples gradually increased with cobalt substitution and was almost independent of temperature. Oxygen permeation studies through the membranes with different thickness revealed that the cobalt doping eliminated the surface exchange limitations leading to a significant increase in the oxygen surface exchange coefficients (k ex). On the contrary, the bulk oxygen diffusion was deteriorated by the cobalt doping, although the oxygen-ion conductivity remained almost unchanged. The electron backscatter diffraction (EBSD) analysis of the surface for the studied ceramic membranes indicated that the observed increase in the k ex values could be explained by the effect of cobalt on the crystallographic orientation of crystallites. It is suggested that the cobalt doping increases the surface area of the ( 1 ‾ 10) , (1 ‾ 11) and (011) faces leading to a higher number of 3 d -metals in the surface layer of the membranes. [ABSTRACT FROM AUTHOR]