Structural and defect chemistry guidelines for Sr(V,Nb)O3-based SOFC anode materials.
Structural and defect chemistry guidelines were used for Nb-substituted SrVO3-δ materials, designed to meet SOFC anode requirements, with emphasis on redox tolerance, thermochemical compatibility with other SOFC materials, electrical conductivity and adjustable changes in oxygen stoichiometry for their prospective impact on electrocatalytic performance. SrV1-xNbxO3-δ (x = 0-0.30) ceramics were prepared by solid-state synthesis and sintered at 1773 K in a reducing atmosphere. XRD and SEM/EDS showed that under these conditions a single-phase cubic perovskite structure appears up to x ≈ 0.25. Electrical conductivity is metallic-like and nearly p(O2)-independent. Although substitution by niobium decreases the conductivity, which still exceeds 100 S cm(-1) for x ≤ 0.20 at temperatures below 1273 K, it also expands the stability domain of the cubic perovskite phase and suppresses partly high thermochemical expansion characteristic of parent SrVO3-δ. The upper p(O2) limit of phase stability was found to shift from ∼2 × 10(-15) atm for the undoped material to ∼2 × 10(-12) atm for x = 0.30, whereas the average thermal expansion coefficient at 773-1223 K decreased from 22.7 × 10(-6) to 13.3 × 10(-6) K(-1). SrV1-xNbxO3-δ perovskites undergo oxidative decomposition in air, which causes dimensional and microstructural changes. However, sluggish kinetics of oxidation under inert gas conditions results in nearly reversible behavior in relatively short-term redox cycles between reducing and inert atmospheres. Subtle structural changes and a close correlation with point defect chemistry clarify these sluggish changes and provide guidelines to retain the metastability.
Electrical properties and chemical stability of BaCe0.9Y0.1O3-BaWO4 composites synthesized by co-sintering and impregnation method
Abstract BaCeO 3 -based materials exhibit high electrical conductivity, however undergo degradation in CO 2 and H 2 O containing atmospheres. In this work the BaCe 0.9 Y 0.1 O 3 -BaWO 4 composites were prepared by co-sintering and via impregnation methods. The main motivation was to locate the barium tungstate on the grain boundary of Y-doped barium cerate and enhance the functional properties of final material comparing to single phase BaCe 0.9 Y 0.1 O 3 − δ . It was found that used synthesis procedures lead to two phase materials with doped barium cerate as the main phase and BaWO 4 as the second one. The Electrochemical Impedance Spectroscopy (EIS) measurements performed at dry air, 100% HR air and 5% H 2 in Ar atmospheres for temperature range 200–700 °C allowed to determine the electrical conductivity of synthesized materials. Introduction of BaWO 4 second phase into host BaCe 0.9 Y 0.1 O 3 − δ modified the electrical properties of bulk and grain boundary for materials synthesized by both co-sintering and for impregnation method. However, the effect of synthesis procedure was observed mainly for grain boundary contribution. Chemical resistance of composites toward CO 2 and water vapour was estimated based on the CO 2 /H 2 O exposure test in which samples were exposed to CO 2 and H 2 O rich atmosphere at 25 °C for 700 h. It was observed that chemical stability of composites in corrosive atmosphere was significantly higher comparing to single phase BaCe 0.9 Y 0.1 O 3 − δ regardless to the synthesis procedure.
Solid-state reactive sintering mechanism for large-grained yttrium-doped barium zirconate proton conducting ceramics
A cost-effective solid-state reactive sintering (SSRS) method has recently been developed to synthesize high quality, fully dense, and large-grained yttrium-doped barium zirconate (BZY) ceramic pellets from the raw materials of BaCO3, ZrO2, Y2O3, and NiO, resulting in total proton conductivities as high as 3.3 × 10−2 S·cm−1at 600 °C under a wet argon atmosphere [J. Tong et al., Solid State Ionics, 2010, 181, 496]. In the present work, the mechanisms for the rapid formation of the cubic perovskite phase of BZY, pellet densification, and grain growth during SSRS synthesis are investigated in detail using a suite of experimental techniques. The pre-reaction addition of NiO to the precursor powders is confirmed to accelerate the formation of the cubic perovksite BZYphase. The rapid and full densification of NiO-modified pellets at relatively low temperature (1350 °C) is ascribed to the formation of the impure phase BaY2NiO5 and its subsequent role as a sintering aid. The dramatic further grain growth after densification is facilitated by the partial decomposition of the BaY2NiO5 (which is located primarily at grain boundaries) and its incorporation into the cubic perovskite structure of BZY.