A. P. Nemudry

Model for anomalous transport of oxygen in nonstoichiometric perovskites: 1. General formulation of the problem

An attempt to explain the phenomenon of anomalous fast oxygen anion transport previously observed in the course of low temperatures topotactic oxidation of perovskite-related nonstoichiometric compounds has been made. According to the model developed the fast oxygen uptake relates to the high concentration of extended defects which exist in these compounds or can arise during oxidation. Experimentally observed specific kinetics has been described as being a result of fast oxygen transport taking place along the extended defects and followed by slow diffusion into undisturbed areas of the lattice.

Investigation of the microstructural features of SrCo0.8Fe0.2O3−δ perovskite

Nonstoichiometric oxides with perovskite structure ABO3−δ are the subject of intense experimental and theoretical investigations. Rapt attention to this class of materials is connected with wide possibilities of their practical application, including their use as oxygen-permeable membranes and electrodes for solid oxide fuel elements [1, 2]. It is known that grossly nonstoichiometric and doped oxides are characterized by the high concentration of structural defects (oxygen vacancies, dopant ions) and by excess Gibbs energy. Excess of energy can be reduced either due to defect ordering and their localization as structural elements (the formation of superstructures) or due to the elimination of defects from the structure with the formation of extended defects (for example, the crystallographic shear planes) [3]. An intermediate case is nanostructuring, i.e., ordering of oxygen vacancies inside nanosized domains where the defects which are excess with respect to domain structure (oxygen interstitials, vacancies) are ejected to the vicinity of the domain boundaries, where they have higher mobility [4, 5]. This phenomenon has been recognized previously as microdomain texture or microtwinning [6–9]. For materials in which high oxygen transport characteristics are required (oxygen-permeable membranes, electrodes for SOFCs), nanostructuring may play the key role because it is accompanied by the formation of the high concentration of interfaces (domain, antiphase, and twin boundaries) that can be the channels of enhanced diffusion with lower activation barriers [5, 10]. One of the materials exhibiting record-breaking oxygen permeability is perovskite having the composition SrCo0.8Fe0.2O3−δ [11]. Investigation of the factors determining anomalously high mobility of oxygen ions in this nonstoichiometric perovskite still remains an actual problem. The aim of the present work is detailed examination of the structure and microstructure of SrCo0.8Fe0.2O3−δ perovskite.

Model for anomalous transport of oxygen in nonstoichiometric perovskites: Analytical and numerical solutions

The present work is a development of the model for anomalous transport of oxygen in nonstoichiometric perovskites which includes precise analytical solutions of the mathematical model formulated earlier, their comparison with approximate solutions and numerical modeling of the process.

Study of high-temperature oxygen permeability in Sr1 − xLaxCo0.8 − yNbyFe0.2O3 − z perovskites

The oxygen permeability of ceramic SrCo0.8 − yFe0.2NbyO3 − z (0 ≤ y ≤ 0.2) and La0.3Sr0.7Co0.6Fe0.2Nb0.2O3 − z disc membranes as a function of temperature and oxygen partial pressure was studied. Kinetic analysis was performed based on the experimental data on oxygen permeability as a function of oxygen partial pressure.

Oxygen release from SrCo0.8Fe0.2O3 − δ

We have studied oxygen release from a membrane material with the composition SrCo0.8Fe0.2O3 − 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 temperature range of quasi-equilibrium oxygen release begins at t ≥ 600°C.

Oxygen permeability of hollow fiber membranes of composition Ba0.5Sr0.5Co0.78W0.02Fe0.2O3–δ

Hollow fiber membranes of composition Ba0.5Sr0.5Co0.78W0.02Fe0.2O3–δ (BSCFW2) have been synthesized by the phase inversion method. The structure of hollow fiber membranes has been studied by scanning electron microscopy, and the phase composition of the material before and after the tests has been determined.

Study of the structural features of nonstoichiometric SrCo0.8–xFe0.2WxO3–δ (0 < x < 0.1) perovskites

The work studies a partial tungsten substitution for cobalt in the structure of nonstoichiometric SrCo0.8Fe0.2O3–δ (SCF) perovskite with mixed oxygen electron conductivity. It is shown that samples of the composition SrCo0.8–xFe0.2WxO3–δ (SCFW) at x ≥ 0.03 undergo the endotaxial phase separation with the formation of nanosized domains with a structure of ordered Sr2CoWO6 double perovskite, which are distributed in the matrix of nonstoichiometric SrCo0.8–xFe0.2WxO3–δ perovskite. For the materials with 0.02 < x < 0.1, a decrease in the oxygen stoichiometry is accompanied by nanostructuring of the matrix: the formation of nanosized domains in which oxygen vacancies are ordered with the formation of the brownmillerite-like structure.

Modification of mixed conducting Ba0.5Sr0.5Co0.8Fe0.2O3–δ by partial substitution of cobalt with tungsten

The Ba0.5Sr0.5Co0.8–xWxFe0.2O3–δ (х = 0–0.1) materials prepaMIECred by partial substitution of cobalt in BSCF with tungsten were studied. The tungsten solubility limit in the structure of cubic perovskite BSCF was shown to be ~2%. The doping with the highly charged W6+ (2%) cation improved the functional properties of BSCF: it increased the oxygen permeability and membrane stability in the CO2-containing atmosphere and suppressed the cubic–hexagonal perovskite polymorphic transition. This stabilizes high oxygen fluxes during long-term stability tests.

Study of the oxygen permeability of ceramic membranes based on nonstoichiometric perovskites SrCo0.8–xFe0.2WxO3–δ

The effect of the substitution of highly charged cations W6+ for Co3+/Co4+ cations in the SrCo0.8Fe0.2O3–δ structure on the transport properties of new membrane materials has been studied.

High-temperature oxygen nonstoichiometry determination in mixed ionic-electronic conducting oxides

We have developed a new method for determining oxygen nonstoichiometry (3 − δ) as a continuous function of oxygen partial pressure