Mixed ionic-electronic conducting (MIEC) ceramic-based membranes for oxygen separation

This article focuses on perovskite materials for application as cathode material in solid oxide fuel cells. In order to develop new promising materials it is helpful to classify already known perovskite materials according to their properties and to identify certain tendencies. Thereby, composition-dependent structural data and materials properties are considered. Structural data under consideration are the Goldschmidt tolerance factor, which describes the stability of perovskites with respect to other structures, and the critical radius and lattice free volume, which are used as geometrical measures of ionic conductivity. These calculations are based on the ionic radii of the constituent ions and their applicability is discussed. A potential map of perovskites as a tool to classify simple ABO3 perovskite materials according to their electrical conduction behavior is critically reviewed as a structured approach to the search for new cathode materials based on more complex perovskites with A and/or B-site substitutions. This article also covers the approaches used to influence electronic and the ionic conductivity. The advantage of mixed ionic electronic conductors in terms of the oxygen exchange reaction is addressed and their important properties, namely the oxygen-exchange coefficient and the oxygen diffusion coefficient, and their effect on the oxygen reduction reaction are presented.

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Materials design for perovskite SOFC cathodes

Oxygen nonstoichiometry and transport properties of strontium substituted lanthanum ferrite

The equilibrium thermal expansivity (βT) and oxygen-vacancy chemical expansivity (βC) of polycrystalline La1-xSrxCoO3-δ (LSC; x = 0.2, 0.4, and 0.7) have been measured at 600 °C < T < 900 °C and 10-4 atm < PO2 < 0.21 atm using controlled-atmosphere dilatometry. These measurements show only a moderate dependence of βT on temperature, suggesting that increases often observed in the coefficient of thermal expansion when measured at constant PO2 are primarily thermally induced chemical expansion associated with changes in oxygen stoichiometry. The dependences of βT and βC on the temperature, oxygen-vacancy concentration, and Sr content (x) were characterized and found to follow a consistent nonlinear trend, which may result from the relaxation of lattice strain with increasing defect concentration. A slowly relaxing secondary expansion effect (and/or expansion hysteresis) was also discovered. Possible causes of this behavior, including phase transition/segregation at high vacancy concentration, are discussed.

Thermal and Chemical Expansion of Sr-Doped Lanthanum Cobalt Oxide (La1-xSrxCoO3-δ)

Ion–electron transport in strontium ferrites: relationships with structural features and stability

Crystal structure and thermal properties of La 1-x Sr x FeO 3-δ (x = 0, 0.1, 0.3, 0.4, 0.5, and 0.75) have been studied by high-temperature X-ray diffraction and thermal analysis in air and nitrogen (p(O 2 ) 10 -3 atm) atmosphere. The first-order phase transition from orthorhombic-to-rhombohedral La 1-x -Sr x FeO 3-δ (x = 0, 0.1) was strongly shifted to lower temperatures with increasing Sr content. The phase-transition temperature was observed significantly lower in polycrystalline ceramics compared with fine powders. The temperature depression of the phase transition in the ceramics was qualitatively explained by stresses induced both by the anisotropic thermal expansion of LaFeO3 and the observed volume contraction of the phase transition. Rhombohedral La 1-x Sr x -FeO 3-δ (x = 0.3, 0.4, 0.5) were observed to transform to the cubic perovskite structure during heating. The second-order phase-transition temperature decreased with increasing Sr content and decreasing partial pressure of oxygen. On the basis of the present findings, a pseudobinary phase diagram of the LaFeO 3 -SrFeO 3-δ system is presented. Finally, a severely nonlinear thermal expansion was observed for the Sr-rich materials at high temperature. The high thermal expansion in this region is due to a chemical expansion resulting from a reduction of the valence state of Fe.

Crystal structure and thermal expansion of La1-xSrxFeO3-δ materials

Non-linear thermal evolution of the crystal structure and phase transitions of LaFeO3 investigated by high temperature X-ray diffraction

Phase equilibria in the pseudo-binary system SrO–Fe2O3

The fracture strength, fracture toughness and apparent Young’s modulus of LaFeO3 ceramics in the temperature region 25–800 °C are reported. The fracture strength of the material was observed to increase from 202 ± 18 MPa at room temperature to 235 ± 38 MPa at 800 °C. The room temperature fracture toughness was 2.5 ± 0.1 MPa m1/2. The fracture toughness decreased to 2.1 ± 0.1 MPa m1/2 at 600 °C, followed by an increase to 3.1 ± 0.3 MPa m1/2 at 800 °C. The temperature dependence of the fracture toughness correlates well with the crystallographic strain, |(a−c)|/(a+c), and ferroelastic toughening of LaFeO3 materials is inferred. Non-elastic stress–strain behaviour of the LaFeO3 materials due to ferroelasticity was confirmed by cyclic compression experiments, and residual strain was observed in the material after unloading.

Anita Fossdal

Papers

Crystal structure and thermal expansion of La1-xSrxFeO3-δ materials

Non-linear thermal evolution of the crystal structure and phase transitions of LaFeO3 investigated by high temperature X-ray diffraction

Phase equilibria in the pseudo-binary system SrO–Fe2O3

Mechanical properties of LaFeO3 ceramics

Phase equilibria and microstructure in Sr4Fe6−xCoxO13 0≤x≤4 mixed conductors