Defect chemistry and transport properties of BaxCe 0 . 85 M 0 . 15 O 3 -
The site-incorporation mechanism of M dopants into ABO3 perovskites controls the overall defect chemistry and thus their transport properties. For charge-balance reasons, incorporation onto the A-site would require the creation of negatively charged point defects (such as cation vacancies), whereas incorporation onto the B-site is accompanied by the generation of positively charged defects, typically oxygen vacancies. Oxygen-vacancy content, in turn, is relevant to proton-conducting oxides in which protons are introduced via the dissolution of hydroxyl ions at vacant oxygen sites. We propose here, on the basis of x-ray powder diffraction studies, electron microscopy, chemical analysis, thermal gravimetric analysis, and alternating current impedance spectroscopy, that nominally B-site doped barium cerate can exhibit dopant partitioning as a consequence of barium evaporation at elevated temperatures. Such partitioning and the presence of significant dopant concentrations on the A-site negatively impact proton conductivity. Specific materials examined are BaxCe0.85M0.15O3(x 0.85 − 1.20; M Nd, Gd, Yb). The compositional limits for the maximum A-site incorporation are experimentally determined to be: (Ba0.919Nd0.081)(Ce0.919Nd0.081)O3, (Ba0.974Gd0.026)(Ce0.872Gd0.128)O2.875, and Ba(Ce0.85Yb0.15)O2.925. As a consequence of the greater ability of larger cations to exist on the Ba site, the H2O adsorption and proton conductivities of large-cation doped barium cerates are lower than those of small-cation doped analogs.
Characterization and Rietveld refinements of new dense ceramics Ba3−xSrxTb3−xCexO9 (x = 1 and 1.5) perovskites
Ba3−xSrxTb3−xCexO9 (x = 1 and 1.5) ceramics (BSTC) with a relative density of 93% and a grain size distribution of 0.2–3 µm were prepared by the mixed-oxides reaction route. The crystalline structures, microstructures, valence states, and electrical properties of two ceramics were analyzed using X-ray powder diffraction (XRPD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), electron paramagnetic resonance (EPR), and electrical measurements. Rietveld analyses of XRPD patterns show that BSTC1 is indexed as a trigonal structure with the space group R-3c, and BSTC3/2 is indexed as an orthorhombic perovskite structure with the space group Pmcn. The EPR, XPS, and electrical conductivity results confirm that Ce and Tb ions in BSTC exist as Ce4+ and mixed-valence states of Tb4+/Tb3+, respectively. At room temperature, the two BSTC ceramics exhibit a similar semiconducting behavior. The relationships between electrical conductivity and temperature/frequency are provided. The defect chemistry is discussed.
Rational selection of advanced solid electrolytes for intermediate temperature fuel cells
Abstract Discussed here is a strategy for selecting perovskite-based solid electrolytes with the potential for achieving high ionic conductivities at intermediate temperatures (≈600 °C). Activation energies for anionictransport (either O2- or proton) have been shown to be influenced by: (1) the average metal-oxygen bond energy in the perovskite, (2) the lattice free volumes, obtained by subtracting the ionic volumes of cations and O2- in the unit cell volume, (3) the parameter rcritical (rc) which corresponds to the radius of the opening between the two A-site cation and one B-site cation through which the mobile anion must pass, and (4) the lattice polarizability. Low activation energeis for anion migration appear to favor: (1) that the overall lattice possess a moderate metal-oxygen binding energy, (2) perovskite solid electrolytes possess free volumes of 30–35A 3, (3) that the lattice minimally polarizes the mobile anion, and (4) preferred ( r critical r 2- 0 ) 2 ratios for A-A-B saddle points ≅0.5 High ionic conductivities have also been achieved for the perovskite-related brownmillerites A2B2O5 which possess a high intrinsic population of anion vacancies in their lattice. Solid electrolytes evolving from this complimentary rationale, which has included BaTh0.9Gd0.1O3, Sr2Gd2O5 and Sr2Dy2O5, have been incorporated into fuel cells operating at intermediate temperatur es. La0.9Sr0.1CoO3, BaCo0.8Fe0.2O3, Ag and Au have been found candidate cathodes for the intermediate-temperature fuel cell applications.