Phase and microstructural characterizations for Ce0.8Gd0.2O2--FeCo2O4 dual phase oxygen transport membranes

Abstract

Dual phase oxygen transport membranes were prepared via solid state reaction at 1200 ℃. The sintered membranes were characterized via X-ray diffraction, back scattered electron microscopy and electron backscatter diffraction, and associated with image analysis and calculations to quantify phase compositions and microstructural features including volume fractions, grain sizes, and contiguity. The characterizations reveal a multi-phase system containing Ce 1-x Gd x O 2-δ' (x ≈ 0.1) (CGO10), and Fe y Co 3-y O 4 (0.2 < y < 1.2) (FCO), CoO and Gd 0.85 Ce 0.15 Fe 0.75 Co 0.25 O 3 (GCFCO) in the sintered membranes. In addition, a novel model is utilized to assess the evolution of the ambipolar conductivity with respect to microstructural features. Both experimental and calculated results indicate that if the grain sizes of all phases in the composites are similar, the optimal ambipolar conductivity is reached with a volume ratio of ionic conducting phase to electronic conducting phase close to 4:1. Meanwhile, the GCFCO phase dominates the effective electronic conductivity.

Mechanical reliability of Ce0.8Gd0.2O2−δ‐FeCo2O4 dual phase membranes synthesized by one‐step solid‐state reaction

“…The content of formed GCFCO remains stable upon cooling 71 . Since the average Gd content in GCFCO was reported as almost constant, 48,53 a similar amount of GCFCO in the surface and the bulk (Figure 4) implies that the total consumed Gd amount from Ce 0.8 Gd 0.2 O 1.9 is the same. In other words, the Gd content left in CGO is the same at the surface and the bulk, and, hence, the lattice constants of CGO should not be different.…”

“…As illustrated in Figure 1, four phases are revealed: CGO fluorite, FCO spinel, CoO rock salt, and GdFeO 3 ‐type perovskite. The composition of GdFeO 3 ‐type perovskite was reported as Gd 0.85 Ce 0.15 Fe 0.75 Co 0.25 O 3 (GCFCO) 48,72 . However, the CoO rock salt phase, being a result of uncomplete oxidization, 72 is only observed in the 50CGO:50FCO composite.…”

“…The lattice constants of CGO and GCFCO were derived from the XRD data, while it was not possible to obtain a value for the lattice constant of FCO due to the variation of the cobalt‐to‐iron ratios 53 . The formation of GCFCO consumes Gd from the Ce 0.8 Gd 0.2 O 1.9 in the initial powder mixtures, and reverses the reaction equilibrium in Equation (), which results in a reduction of oxygen vacancies and leads to a shrinkage of the CGO lattice.…”

“…The residual stress () was calculated by 61 :where and are elastic modulus and Poisson's ratio, and is the lattice spacing in a stress‐free sample; is the lattice spacing at an angle . The values for CGO, FCO, and CoO were 229 GPa, 209 GPa, and 193 MPa, respectively 53 . The was assumed to be 0.3, which is a typical value for ceramics 62 …”

“…Aiming toward future scaling‐up of dual phase membranes for industrial applications, mechanical behavior and robustness, in addition to chemical performance and stability, are of critical importance to warrant long‐term safe operation 7,20 . Only a limited number of reports focused on the mechanical properties of dual phase membranes, 49‐53 reporting values for, for example, elastic modulus, hardness, fracture strength, and fracture toughness. The subcritical crack growth behavior of dual phase membranes, though, has not yet been studied.…”