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.

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.

Tensiometers are commonly used for measuring soil water matric pressures. Unfortunately, the water-filled reservoir of conventional tensiometers limits their applicability to soil water matric pressures above approximately 20.085 MPa. Tensiometers filled with a polymer solution instead of water are able to measure a larger range of soil water matric pressures. We designed and constructed six prototype polymer tensiometers (previously called osmotic tensiometers) consisting of a wide-range pressure transducer with a temperature sensor, a stainless steel casing, and a ceramic plate with a membrane preventing polymer leakage. A polymer chamber (0.1-2.2 cm 3 ) was located between the pressure transducer and the plate. We tested the polymer tensiometers for long-term operation, the effects of temperature, response times, and performance in a repacked sandy loam under laboratory conditions. Several months of continuous operation caused a gradual drop in the osmotic pressure, for which we developed a suitable correction. The osmotic potential of polymer solutions is temperature dependent, and requires calibration before installation. The response times to sudden and gradual changes in ambient temperature were found to be affected by polymer chamber height and polymer type. Practically useful response times (,0.2 d) are feasible, particularly for chambers shorter than 0.20 cm. We demonstrated the ability of the instrument to measure the range of soil water pressures in which plant roots are able to take up water (from 0 to 21.6 MPa), to regain pressure without user interference and to function properly for time periods of up to 1 yr.

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Yttria-stabilized cubic zirconia (YSZ)-palladium dual phase composites have been investigated. The percolative composite containing 40 ~01% Pd (ZYPd40) showed a much larger oxygen permeability than that of the non-percolative composite containing 30 ~01% Pd (ZYPd30). For a 2.0 mm thick percolative composite, an oxygen flux of 4.3 x lOWa mol/cm'/s was measured at 1100°C with oxygen partial pressures at the feed and permeate sides being 0.209 and 0.0 14 atm, respectively. This value is two orders of magnitude larger than that observed for a 2.0 mm thick non-percolative composite at the same temperature with the oxygen partial pressures at the feed and permeate sides being 0.209 and I .5 x 1 OV' atm, respectively. From the dependence of the oxygen permeation on the temperature and on the oxygen partial pressures, it was concluded that the transport of the oxygen ions through the YSZ phase in the percolative system was the rate limiting step.