New results on the oxygen permeability of perovskite-type oxides SrCoo.sB~.203_ a (with B' =Cr, Fe, Co and Cu) and Lao.6Sro.4CoO3_6 are presented. The occurrence of order-disorder transitions at elevated temperatures (790-940°C) in these phases has been confirmed by DSC measurements and, in some cases, by X-ray powder diffraction of samples either slowly cooled or quenched from high temperature after annealing in different atmospheres. The oxygen permeability found upon exposing opposite sides of sealed disc specimens to a stream of air and of helium, respectively, increases sharply (between 5-6 orders of magnitude up to 0.3-3 × 10-7 mol cm -2 s-1 ) at the onset of the transition from a low-temperature vacancy-ordered state to defect perovskite, except for SrCoo.sFeo.203_a. In the latter case only a slight anomaly is found in the Arrhenius plot of the oxygen permeability at ~ 790 ° C. The comparatively high oxygen flux through SrCo0.aFeo.203_a observed at intermediate temperatures is interpreted in terms of a two-phase mixture of a vacancy-ordered state and disordered perovskite, while above ~ 790°C the sample is single-phase of defect perovskite structure.

The surface oxygen exchange of Lao.3Sro,, Co0,_6 has been studied by 'SO/'hO exchange followed by dynamic SIMS analysis in the temperature region 700-900°C at a po, of 2.1 X lo4 Pa. The activation energy for the surface exchange rate (k) is 283 kJ mol -' and for the oxygen tracer diffusion coefficient (D *) 56t4 kJ mol-' The absolute values for both k and D * are among the highest known in the literature. The characteristic thickness at a po, of 2.1 X lo4 Pa is calculated to lie in the range 0.70 to 1.3 mm and is a function of temperature. The latter reflects the different activation energies for k and D*. The surface oxygen exchange coefficient is proportional to p& with n =0.41 to.02 at WYC, which has been explained by a rate determining step involving an adsorbed oxygen species and an oxygen vacancy. No oxygen pressure dependence for the tracer diffusion coefficient was observed at this temperature.

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The crystal structure of La1−xSrxCoO3−δ (0≤x≤0.6) has been studied, using powder X-Ray diffraction. The crystal structure shows a transition from rhombohedral distorted perovskite for LaCoO3−δ into cubic perovskite for La0.4Sr0.6CoO3−δ. The cubic unit cell parameter is ac=3.8342(1) Å for La0.4Sr0.6CoO3−δ, the space group probably being Pm3m. Using a hexagonal setting, the cell parameters for La0.5Sr0.5CoO3−δ, are a=5.4300(3) Å, c=13.2516(10) Å; a=5.4375(1) Å, c=13.2313(4) Å for La0.6Sr0.4CoO3−δ; a=5.4437(1) Å, c=13.2085(5) Å for La0.7Sr0.3CoO3−δ; a=5.4497(2) Å, c=13.1781(6) Å for La0.8Sr0.2CoO3−δ and a=5.4445(2) Å, c=13.0936(6) Å for LaCoO3−δ with the space group probably being R3c.

Perovskite powders of La 0.3 Sr 0.7 CoO 3−d were prepared by the thermal decomposition of precursor complexes derived from nitrate solutions using ethylenediaminetetraacetic acid (EDTA) as a complexing agent. The calcination temperature is 920°C. Powders thus obtained have a low carbon contamination. Dense ceramics with a relative density of about 96% have been prepared after sintering at 1150°C.

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Oxygen permeation rates are reported for 2 mm thick La1-xSrxCoO3-δ (0 ≤x≤0.8) in the temperature range 700 ≤T 1100 °C and oxygen partial pressure range 7.0 x 103 - 1.0 x 105 Pa. With the exception of x = 0.6 permeation rates increase with an increase in Sr doping level. The oxygen permeation can be described by Wagner theory. An anomaly is observed in the oxygen permeation between 750 - 775 °C, which is attributed to an order-disorder transition. It is further shown that the partial pressure at the low partial pressure side of the membrane has an important influence on the permeation rate. The (calculated) ionic conductivities and oxygen vacancy diffusion coefficients are among the highest reported in literature.