SOFC electrolytes: Yttria-doped zirconia (ZrO2)
The main aim of this project was to achieve a better understanding of the degradation of the ionic conductivity of polycrystalline 8.5 mol% Y2O3-doped ZrO2 (8YDZ). This frequently applied electrolyte material for fuel-cell applications shows a significant reduction (40 %) of its ionic conductivity at temperatures of about 1000 °C after 2500 h. The degradation is correlated with the coarsening of nanoscaled regions occurring in the metastable tetragonal (t´´) phase in 8YDZ as shown in the following series of dark-field TEM micrographs (a) in the as-sintered state, b) after 2500 h, and c) after 5800 h annealing. The tetragonal phase appears bright in the dark-field TEM micrographs. Recent chemical analyses on the nanoscale have shown that this microstructural evolution is accompanied by the local spinodal decomposition on the cationic sublattice.
Another issue in this field of research is the question whether electrolyte thin films lead to improvements of SOFC performance. The reduction of the mean grain size down to the nanoscale and, thus, the strongly increasing grain-boundary content is discussed in literature as possibility to reduce the ohmic resistivity of 8YDZ. To answer this question of grain-size dependency of the ionic conductivity a series of 8YDZ thin films with mean grain sizes from 5 nm (Fig. a)) to the microscale was investigated. The investigations were focused on the characterization of the microstructure (crystallite size, phases, porosity) and purity of the prepared thin films (Fig. c)) as a function of annealing temperature. The matrix phase of the grains in the thin films is cubic. In addition, nanoscaled precipitates of a metastable tetragonal YDZ phase (t´´-phase) are present as in microcrystalline 8YDZ. The presence of this additional phase is indicated by weak additional reflections as indicated in the electron diffraction pattern in Fig. b.
B. Butz, P. Kruse, H. Störmer, D. Gerthsen, A. Müller, A. Weber, E. Ivers-Tiffée, Correlation between microstructure and degradation in conductivity for cubic Y2O3-doped ZrO2, Solid State Ionics 177, 3275 (2006), DOI: 10.1016/j.ssi.2006.09.003
B. Butz, H. Störmer, D. Gerthsen, M. Bockmeyer, R. Krüger, E. Ivers-Tiffée, M. Luysberg, Microstructure of nanocrystalline Y-doped zirconia thin films obtained by sol-gel processing, Journal of the American Ceramics Society 91, 2281 (2008), DOI: 10.1111/j.1551-2916.2008.02400.x
B. Butz, R. Schneider, D. Gerthsen, M. Schowalter, A. Rosenauer, Decomposition of 8.5 mol% Y2O3-doped Zirconia and its contribution to the degradation of ionic conductivity, Acta Materialia 57, 5480 (2009) dx.doi.org/10.1016/j.actamat.2009.07.045
C. Peters, A. Weber, B. Butz, D. Gerthsen, E. Ivers-Tiffée, Grain-size effects in YSZ thin film electrolytes, J. Am. Ceram. Soc. 92, 2017 (2009), DOI: 10.1111/j.1551-2916.2009.03157
B. Butz, Yttria-Doped Zirconia as Solid Electrolyte for Fuel-Cell Applications, Dissertation KIT (2009) http://digbib.ubka.uni-karlsruhe.de/volltexte/1000015724