strain ; thin films; interfaces; micro solid oxide fuel cells; ion conductors; pulsed laser deposition
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Fluri Aline, Gilardi Elisa, Karlsson Maths, Roddatis Vladimir, Bettinelli Marco, Castelli Ivano E., Lippert Thomas, Pergolesi Daniele (2017), Anisotropic Proton and Oxygen Ion Conductivity in Epitaxial Ba 2 In 2 O 5 Thin Films, in The Journal of Physical Chemistry C
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Shi Yanuo, Bork Alexander Hansen, Schweiger Sebastian, Rupp Jennifer Lilia Marguerite (2015), The effect of mechanical twisting on oxygen ionic transport in solid-state energy conversion membranes, in Nature Materials
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A solid oxide fuel cell (SOFC) is an electrochemical device that converts chemical energy (normally hydrogen containing) into electric energy and consists of two electrodes and an oxygen ion conducting oxide as electrolyte. State-of-the-art oxygen-ion conducting oxides in SOFCs are doped CeO2 or Y2O3 stabilized ZrO2 (YSZ). Existing SOFC systems operate at rather high temperatures (above 900°C) in order to achieve suitable ionic conductivity across the electrolyte material, which imply severe restrictions (material specifications) and are therefore the most important drawback of this technology. The application of thin films of the electrolyte, allows a significant reduction of the ohmic losses across the ionic conductor promising lower operating temperatures, increased lifetimes, and a wider range of eligible materials. Thin film deposition technologies are therefore the key for these type of developments and play also the key role in the fabrication of micro solid oxide fuel cells (µSOFC) prototypes. Such micro-fuel cells exhibit a free-standing fuel cell membrane integrated on a substrate (e.g. Si wafer) with an active total fuel cell thickness of less than 1 micron (for anode-electrolyte-cathode thin films) producing power in the hundreds mW to several W range. One important part of the µSOFC is the ion conducting oxide layer, but a review of the available data shows that the key numbers, i.e. the conductivity and activation energy of conduction, for a given material, such as YSZ scatter over several orders of magnitude (for conductivity). There are many possible origins for the large scatter of the data, but it is very important to understand the reason for variations in the properties, which would allow to create improved ion conducting structures for µSOFCs. Within this project two likely origins of increased conductivity in thin films will be tested, i.e. the role of hetero-interfaces which induces strain and homo-interfaces which may be described as grain boundaries, including interfaces with a high number of defects and misfit dislocations. These two effects will be studied by growing heterolayer superlattices (with a varying number of layers, but also single layers) with a high control of the interface near order and chemistry (using RHEED) and quantitative overall strain (using the optical strain monitor at PSI, which can by used during PLD together with RHEED). For the second approach, i.e. the homointerfaces, special thin film microstructures, i.e. zigzag or even helicoidal structures, will be prepared by the variable-angle pulsed laser deposition process. The properties, especially electric conductivity, of these thin films will be characterized in detail by various electrochemical methods (e.g. impedance combined with optical monitoring) and secondary ion mass spectrometry using 18O2. These characterizations (in plane and cross plane) will be performed for the thin film structures on various substrates, as well as on free standing membranes, which will be prepared by selectively etching the substrate underneath the electrolyte film. The characterization of the properties (electrochemical, but also mechanical) is of key importance for the performance and design of µSOFCs and whether it is possible to engineer ion conductors with optimized properties for micro devices.