Oxygen transport in Ce0.8Gd0.2O2 - δ-based composite membranes

Gadolinia-doped ceria electrolyte Ce0.8Gd0.2O2 - δ (CGO) and perovskite-type mixed conductor La0.8Sr0.2Fe0.8Co0.2O3 - δ (LSFC), having compatible thermal expansion coefficients (TECs), were combined in dual-phase ceramic membranes for oxygen separation. Oxygen permeability of both LSFC and composite LSFC/CGO membranes at 970-1220 K was found to be limited by the bulk ambipolar conductivity. LSFC exhibits a relatively low ionic conductivity and high activation energy for ionic transport (∼ 200 kJ/mol) in comparison with doped ceria. As a result, oxygen permeation through LSFC/CGO composite membranes, containing similar volume fractions of the phases, is determined by the ionic transport in CGO. The permeation fluxes through LSFC/CGO and La0.7Sr0.3MnO3 - δ/Ce0.8Gd0.2O2 - δ (LSM/CGO) composites have comparable values. An increase in the p-type electronic conductivity of ceria in oxidizing conditions, which can be achieved by co-doping with variable-valence metal cations, such as Pr, leads to a greater permeability. The oxygen ionic conductivity of the composites consisting of CGO and perovskite oxides depends strongly of processing conditions, decreasing with interdiffusion of the phase components, particularly lanthanum and strontium cations from the perovskite into the CGO phase

Oxygen ionic conduction in brownmillerite CaAl0.5Fe0.5O2.5+δ

The oxygen permeability of CaAl0.5Fe0.5O2.5+δ brownmillerite membranes at 1123-1273 K was found to be limited by the bulk ionic conduction, with an activation energy of 170 kJ/mol. The ion transference numbers in air are in the range 2 × 10-3 to 5 × 10-3. The analysis of structural parameters showed that the ionic transport in the CaAl0.5Fe0.5O2.5+δ lattice is essentially along the c axis. The largest ion-migration channels are found in the perovskite-type layers formed by iron-oxygen octahedra, though diffusion in tetrahedral layers of the brownmillerite structure is also possible. Heating up to 700-800 K in air leads to losses of hyperstoichiometric oxygen, accompanied with a drastic expansion and, probably, partial disordering of the CaAl0.5Fe0.5O2.5+δ lattice. The average thermal expansion coefficients of CaAl0.5Fe0.5O2.5+δ ceramics in air are 16.7 × 10-6 and 12.6 × 10-6 K-1 at 370-850 and 930-1300 K, respectively

Oxygen nonstoichiometry and defect chemistry of perovskite-structured SrFe1−xMoxO3−δ solid solutions

The oxygen nonstoichiometry ofmixed-conducting perovskite-structured SrFe1−xMoxO3−δ (x=0, 0.07, 0.15 and 0.25) was measured by oxygen coulometric titration in the range of oxygen partial pressure 10−20–0.5 atm and temperature 800–950 °C. Based upon a defect chemical analysis of the non-stoichiometry data it is shown that significant reduction of Mo6+ occurs under reducing conditions. The trend observed in the oxidation enthalpy suggests a weakening of the Fe\\O bond strength along with the degree of substitution of Fe by Mo, which behavior is consistent with the concomitant increase in lattice constant. Atomistic simulations were carried out to support the defect chemical analysis, showing that the Mo cations in SrFe1−xMoxO3−δ retain a full, 6-fold coordination shell, whilst Fe cations can have 0, 1 or 2 oxygen vacancies in the first coordination shell

Oxygen permeability of LaGa0.65Ni0.20Mg0.15O3-δ ceramics: Effect of synthesis method

Oxygen ionic transport in dense LaGa0.65Ni0.20Mg0.15O3-δ membranes, prepared by the standard ceramic synthesis technique and via glycine-nitrate process (GNP), was studied using measurements of the total conductivity, oxygen permeation and faradaic efficiency (FE). At 1223 K oxygen transfer through LaGa0.65Ni0.20Mg0.15O3-δ ceramics is mainly determined by the bulk ambipolar conductivity, while decreasing temperature leads to a greater role of the surface exchange rate. In spite of moderate difference in the ceramic microstructures, the surface exchange limitations are considerably higher for the membranes prepared by the standard ceramic route compared to GNP-synthesized material. Thermal expansion and partial ionic and electronic conductivities were found essentially independent of the synthesis method. The level of oxygen ionic conduction in LaGa0.65Ni0.20Mg0.15O3-δ, characterized by the activation energy of about 150 kJ/mol and ion transference numbers in the range 1 × 10-3-5 × 10-2 at 973-1223 K, is higher than that in La(Ga,Ni)O3-δ perovskites and comparable to La2NiO4-based phases

Structural characterization of mixed conducting perovskites La(Ga,M)O3-δ ( M = Mn, Fe, Co, Ni)

Comparative analysis of the structure refinement results of perovskite-like LaGa0.5M0.5O3-δ (M = Mn, Fe, Co, Ni) and data on other LaGaO3-based phases, heavily doped with transition metal cations, shows that on doping the structural changes in these oxides follow common trends for the perovskite-type systems. The maximum ionic conductivity, observed in various perovskites when the tolerance factor values are approximately 0.96-0.97, was found to correlate with the transition from orthorhombic to rhombohedral structure and maximum lattice distortion. The perovskite unit cell distortion near the orthorhombic-rhombohedral phase boundary may hence play a positive role in the ionic transport processes. © 2002 Elsevier Science Ltd. All rights reserved

Oxygen nonstoichiometry and defect chemistry of perovskite-structured SrFe1−xMoxO3−δ solid solutions

The oxygen nonstoichiometry ofmixed-conducting perovskite-structured SrFe1−xMoxO3−δ (x=0, 0.07, 0.15 and 0.25) was measured by oxygen coulometric titration in the range of oxygen partial pressure 10−20–0.5 atm and temperature 800–950 °C. Based upon a defect chemical analysis of the non-stoichiometry data it is shown that significant reduction of Mo6+ occurs under reducing conditions. The trend observed in the oxidation enthalpy suggests a weakening of the Fe\\O bond strength along with the degree of substitution of Fe by Mo, which behavior is consistent with the concomitant increase in lattice constant. Atomistic simulations were carried out to support the defect chemical analysis, showing that the Mo cations in SrFe1−xMoxO3−δ retain a full, 6-fold coordination shell, whilst Fe cations can have 0, 1 or 2 oxygen vacancies in the first coordination shell

Redox stability and high-temperature electrical conductivity of magnesium-and aluminium-substituted magnetite

Spinel-type magnetite-based oxides, possessing relatively high electrical conductivity, are considered as promising consumable anode materials for high temperature pyroelectrolysis, a breakthrough low-CO2 steel technology to overcome the environmental impact of classical extractive metallurgy. The present work was focused on the analysis of phase stability, thermal expansion and high-temperature electrical conductivity in (Fe,Mg,Al)(3)O-4 system under oxidizing and mildly reducing conditions. Metastable, nearly single-phase at room temperature (Fe,Mg,Al)(3)O-4 ceramics was obtained by sintering at 1753-1773 K for 10 h in argon atmosphere. Thermal expansion and redox induced dimensional changes were studied on heating, using TG, XRD and dilatometry. The results revealed that magnesium improves the tolerance against oxidative decomposition and minimizes unfavorable dimensional changes in ceramic samples upon thermal cycling. Co-substitution of iron with aluminium and magnesium was proved to be a promising strategy for improvement of refractoriness and phase stability of Fe3O4-based spinels at elevated temperatures, without significant reduction in the electrical conductivity. (C) 2013 Elsevier Ltd. All rights reserved