Grain boundary conductivities are determined by complex impedance measurements (1-106 Hz) on high-purity ceramics prepared by the alkoxide synthesis and on less pure ceramics obtained from a commercial powder. The grain size was varied systematically in the region 0.36-55 tam. The grain boundary conductivity is strongly influenced by the gsain size, impurities and cooling procedure. The grain boundary conductivity increases linearly with the grain size for small grain sizes (0.3 to 2-4 tam) and is constant for larger grain sizes The calculated specific conductivity of the grain boundary for pure materials is about 100 times smaller than that of the bulk. The grain boundary thickness was estimated to be 5.4 rim. The activation energy of the grain boundary conductivity is 7 kJ mole -1 higher than that of the bulk. 0 378-5963]82/0000-0000/$ 02.75

Attention is drawn to the possible involvement of the surface exchange kinetics in limiting the rate of oxygen permeation through mixed-conducting oxide ceramics. A theoretical approach is provided with which it is possible to distinguish between surface exchange-and bulk diffusion controlled kinetics of oxygen permeation. New results on the oxygen permeability of perovskites Lao.aSro.2CoO3_~ and SrCoo.sFeo.203_6 are presented. The importance of the exchange reaction relative to diffusion in limiting overall oxygen transport through (La,Sr) (Co,Fe)O3_6 perovskite-type oxides is emphasized.

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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.

Complex admittance measurements are performed on high‐purity ceramics prepared by means of the alkoxide synthesis and on less pure ceramics obtained from the citrate synthesis. The results on ceramic materials with grain sizes ranging from 0.4 to 20 μm are compared with those from a single crystal. The activation enthalpy for grain boundary conductivity ΔHgb = (118 ± 2) kJ/mol for the samples studied, is independent of composition, grain size, and preparation method. Grain boundary conductivity values and consequently the relevant pre‐exponential factors are an order of magnitude smaller for the alkoxide materials than for the citrate materials. The ratio of grain bulk and grain boundary conductivity (σb/σgb) for alkoxide materials with grain‐sizes 0.4 to 0.8 μm varies from 8.5 to 1.0 in the temperature range 500 to 700 °C.