The surface exchange coefficient and chemical diffusion coefficient of oxygen for the perovskites La 0.6 Sr 0.4 Co 1)y Fe y O 3)d (y=0.2, 0.5 and 0.8) were measured using the conductivity relaxation technique. Measurements were performed between 600 and 800 °C in an oxygen partial pressure range between 10 )4 and 1 bar. Both transport coefficients decrease markedly with decreasing oxygen partial pressure below about 10 )2 bar at all temperatures. This is attributed to ordering of oxygen vacancies. Implications for using La 0.6 Sr 0.4 Co 1)y Fe y O 3)d as an oxygen separation membrane are discussed.

The present paper deals with the analysis of experimental data from conductivity relaxation experiments. It is shown that evaluation of the chemical diffusion and surface transfer coefficients for oxygen by use of this technique is possible only if accurate data for the conductivity transient can be measured at short times, i.e., immediately after the step change in the surrounding oxygen partial pressure. The flushing behavior of the reactor volume may, however, significantly influence the early stage of the relaxation process. Large errors in the transport parameters are obtained from fitting the relaxation data to the theoretical equations if this phenomenon is not properly recognized. Equations are presented which describe the transient conductivity taking into account the finite flush time of the reactor. The regimes of surface-and diffusion-controlled kinetics are discussed quantitatively.

The chemical diffusion coefficient and oxygen-transfer coefficients of selected compositions in the series La 1Ϫx Sr x CoO 3Ϫ␦ were studied using the conductivity relaxation technique. Measurements were performed in the temperature range 600-850°C and oxygen partial pressure 10 Ϫ4 to 1 bar. Chemical diffusivity and oxygen surface transfer in the La 1Ϫx Sr x CoO 3Ϫ␦ perovskites appear to be highly correlated. The general trend displayed is that both parameters decrease with decreasing p O 2 below about 10 Ϫ2 bar at all temperatures. This is attributed to ordering of induced vacancies at low oxygen partial pressures. The observation that the correlation between both parameters extends even to the lowest p O 2 values in this study suggests a key role of the concentration of mobile oxygen vacancies, rather than of the extent of oxygen nonstoichiometry, in determining the rates of both processes. The characteristic thickness L c , which equals the ratio of the chemical diffusion coefficient to the surface transfer coefficient, shows only a weak dependence on oxygen partial pressure and temperature. For different compositions La 1Ϫx Sr x CoO 3Ϫ␦ , L c is found to vary between 50 and 150 m.

The conductivity relaxation (CR) method is often used for measuring the surface transfer rate,K tr , and the bulk diffusion coefficient, D;for oxygen transport in mixed conducting oxides (MIECs). The time domain analysis of the obtained CR response is rather complex and is based on 'ideal' behaviour for the diffusion process. It is quite favourable to perform the data analysis in the frequency domain, where 'non-ideal' responses are easily recognised. Besides, frequency domain analysis (impedance spectroscopy) can yield reliable parameter estimates. Using a discrete Fourier-transform procedure, the time domain responses can be transformed to a frequency domain impedance-type expression. This approach can be applied to any system for which a driving force and a resulting flux can be defined.

Oxygen permeation experiments were performed on dense mixed-conducting ceramic-metal composite membranes (thickness 0.2 to 2 mm) Bil,5Ero50,-Ag with 10.0, 27.8, and 40.0 volume percent (v/o) silver, respectively, in the temperature range 873 to 993 K and oxygen partial pressure range to 1 bar 0,. The oxygen fluxes increased with increasing silver content. In the cerinets with a nonpercolative silver phase (10.0 and 27.8 v/o), the increased oxygen flux relative to that of pure Bi,,ErO50, was attributed to faster kinetics of surface oxygen exchange in the presence of silver. Percolativity of the silver phase in the 40 v/o Ag composition enhances the ambipolar diffusion of oxygen ions and electrons. High oxygen fluxes (-0.25 mmol m-2 s-' at 873 K) were observed with the latter composition, which were shown to be fully limited by the surface exchange kinetics. The activation energy for oxygen permeation in the temperature range 848 to 1003 K is about 85 to 95 kJ/mol for the compositions without percolativity of silver and 115 kJ/mol for the composite with 40 V/O Ag, which reflects a change of the rate-limiting step upon passing the percolation threshold. Results from both permeation and isotopic exchange measurements on the composition with Ag percolativity indicated the kinetic order of the surface process in oxygen to be 1/4, indicating a process fundamentally different from that on pure Bi,,Er,,O,.