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.
Preparation of barium cerate-based thin films using a modified Pechini process
Thin films of BaCe0.8Gd0.2O3 have been successfully prepared on both dense and porous substrates using a modified Pechini process. To increase the extent of chelation, ethylene diamine tetraacetic acid (EDTA) has been used to replace citric acid in the Pechini process. Results indicate that the most important processing parameter is the molar ratio of complexation/polymerization agent (EDTA) to metal ions (C ratio). Films derived from sols with C ratio smaller than 1 are usually cracked, while those derived from sols with C ratio greater than 2.5 are porous. Crack-free and uniform films are obtained from the sols with C ratios between 1.5 and 2.5. Thin-film membranes of barium cerate electrolytes have also been deposited on porous substrates by modifying the surface of the porous substrate with an intermediate polymer film derived from EDTA and ethylene glycol.
The effect of deposition conditions on the structure and electrical properties of YBaCuO superconducting thin films
Abstract Thin superconducting films of yttrium barium copper oxide (YBCO) grown onto MgO(100) and YSZ(100) substrates have been prepared by rf magnetron sputtering and by chemical sol gel methods. The structure, morphology and compositional properties were studied by X-ray diffraction, scanning electron microscopy and Auger electron spectroscopy, respectively. The electrical resistance was measured by a four-probe dc method. The results indicate that the method of film preparation, the substrate material and the nature of the post-deposition annealing have a significant influence on the microstructure and the superconducting properties of YBCO films.