Main issue of this project is the question whether nanoscaled electrolyte thin films lead to improvements of performance of  SOFCs (solid oxide fuel cells). 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 8 mol% Y₂O₃-doped ZrO₂ (8YDZ). But controverse experimental findings are published up to now.

To answer this question of grain-size dependency of the ionic conductivity a series of 8YDZ thin films with mean grain sizes in the range of about 5 nm (Fig. a)) up to the microscale is investigated. A sol-gel based method in combination with spin coating of sapphire substrates was utilized to deposit the thin films. After a rapid thermal annealing process, the thin films were subsequently annealed at a temperature in the range of 650-1350 °C to adjust mean grain size and porosity.

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. 2b). Since the thin films are prepared by a route, that strongly differs from the procedure that is used to prepare microcrystalline ceramic electrolyte substrates (pressing, sintering at high temperature), the samples in addition yield new insights into the phase diagram of YDZ.

Poster presentation: