A new approach to the determination of oxygen nonstoichiometry (δ) of MIEC oxides as a continuous function of pO2 at high temperatures was developed. The description of the model allowing one to distinguish the contribution of oxygen released from the samples to the partial pressure of oxygen at the outlet of the continuous-flow fixed-bed reactor after the stepwise change of the oxygen partial pressure of inlet gas from 0.2 to 10(-5) atm and to calculate the dependence of δ on pO2 is presented. The criterion for assessing the achievement of quasi equilibrium release of oxygen from the MIEC oxides is proposed. The adequacy of the method was confirmed by comparing the obtained and published data for well-studied SrCo0.8Fe0.2O3-δ and SrFeO3-δ MIEC oxides.

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The kinetics of oxygen release from grossly nonstoichiometric perovskite SrCo0.8Fe0.2O3−δ (SCF) with mixed conductivity chosen as a model object was studied for the first time using the oxygen partial pressure relaxation technique. The use of isostoichiometric conditions during the investigation of the oxygen exchange in SCF characterized by a wide homogeneity region made it possible to get information that cannot be obtained using traditional measurements under isobaric conditions. This approach allowed us to discover a kinetic compensation effect: successive oxygen release from SCF was found to result simultaneously in the increase of the apparent activation energy and pre-exponential factor of the reaction rate.

We have studied oxygen release from a membrane material with the composition SrCo 0.8 Fe 0.2 O 3 -δ of various sizes in the temperature range 600-900°C in a flow reactor at oxygen partial pressures from 0.2 to 10 -5 atm. The results demonstrate that the oxygen release from samples 50 µm to 2 mm in size at temperatures above 800°C can be described in terms of a quasi equilibrium model. For fine powders, ≤63 µm in size, the tem perature range of quasi equilibrium oxygen release begins at t ≥ 600°C.

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