Lay summary
Background:Sub-µm thin films of oxygen-ion conductive materials are of interest as solid electrolyte in miniaturized high temperature electrochemical devices such as micro solid oxide fuel cells (SOFCs), solid oxide electrolysis cells (SOECs) and gas sensors. Due to a significant reduction of the ohmic resistance, the use of thin film technologies promises lower operational temperatures, an enhanced performance as well as an increased lifetime and wider eligible material range compared to traditional processing techniques. The fabrication as well as the analysis of the oxygen ion conduction in thin films of the ion conductors is challenging as cross-plane measurements are strongly influenced by holes, cracks, or voids in the films. Cross plane measurements are most important for the characterization, as they closely resemble the geometry of an operational SOFC. This would allow in principle to apply polycrystalline films, which are relatively easy to prepare with many methods, for the cross-plane measurements. This is unfortunately also not really possible, as grain boundaries, which serve as segregation centers for impurities, and short circuits created by metal migration along the grain boundaries, strongly influence the measurements. In addition, the influence of microstrain and varying degree of crystallinity from amorphous to nanocrystalline or mixed phases on the ionic conduction in these fluorite related structures is unclear.Goal:The thin film electrolyte in a micro-SOFC is a key component for the operation of the fuel cell. It is therefore necessary to measure the true ion conductivity of material without any influences of the film preparation and crystallinity.Approach:The best approach to measure the true ion conductivity of a material is the application of the single crystalline material, and one possibility is the preparation in the form of thin films by pulsed laser deposition. In literature different approaches are described for the deposition of single crystalline ion conducting materials, and we will adopt these approaches for the deposition of relevant materials. The second important aspect in addition to the single crystalline film preparation is the analysis of the cross plane ion conductivity.We will apply an approach, named ion exchange depth profiling, where the diffusion of isotopic oxygen (18O) in the material is analyzed by secondary ion mass spectrometry and by standard cross plane conductivity measurements with and without blocking electrodes.