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| Hauptseite > Publikationsdatenbank > Selbstheilende plasmagespritzte Mn$_{1,0}$Co$_{1,9}$Fe$_{0,1}$O$_{4}$-Schutzschichten in Festoxidbrennstoffzellen |
| Hauptseite > Publikationsdatenbank > Selbstheilende plasmagespritzte Mn$_{1,0}$Co$_{1,9}$Fe$_{0,1}$O$_{4}$-Schutzschichten in Festoxidbrennstoffzellen |
| Book/Dissertation / PhD Thesis | FZJ-2018-03606 |
2018
Forschungszentrum Jülich GmbH Zentralbibliothek, Verlag
Jülich
ISBN: 978-3-95806-327-3
Please use a persistent id in citations: http://hdl.handle.net/2128/19076
Abstract: Applying dense chromium protective layers between interconnector and cathode can efficientlydiminish chromium related degradation of solid oxide fuel cells (SOFCs). Especiallyatmospherically plasma sprayed (APS) Mn$_{1.0}$Co$_{1.9}$Fe$_{0.1}$O$_{4}$ (MCF) coatings demonstrated their effectiveness concerning Cr retention within stacks tested in Jülich. Nevertheless, strong microstructural and phase changes of these coatings during operation were reported in literature, but not fully understood yet. This thesis was part of a collaborative project called “SOFC-Degradation” (grant no.03SF0494A), which was focused on different degradation phenomena emerging during SOFC operation. The goal of the present work was to investigate the basic mechanisms leading to the observed changes of APS-MCF coatings during operation. The results should enable long-term prediction and facilitate accelerated test-procedures. Additionally, wet powder spraying (WPS) was investigated as a cost efficient alternate coating technique. APS-MCF coatings were heat treated in air in combination with common steel substrates and cathode contact layers to simulate SOFC operation conditions. During the APS-process MCF is reduced and deposited in a rock salt configuration ((Mn,Co,Fe)$_{1}$O$_{1}$), which is metastable at room temperature. By annealing these coatings in air, the material transforms to the low temperature (T < 1100 °C) stable spinel phase ((Mn,Co,Fe)$_{3}$O$_{4}$). This phase transformation is connected to an oxygen uptake and is accompanied by a volume expansion of the material. Inside the crack-network that can be found in as-sprayed coatings, this volume expansion leads to a densification and crack-healing. Subsequently, the phase transformation, which is also anoxidation process, is dominated by solid state diffusion of cations as soon as the cracks areclosed. Thereby cobalt is enriched at the layer’s surface as its diffusion coefficient is higher than that of manganese or iron within the layer formed. The outward diffusion of cations generates a counterflow of vacancies into the bulk, where they accumulate to form small pores. A two-phase-system composed of a cobalt-rich rock salt phase and a manganese- and iron-rich spinel phase can be observed in the coating’s bulk. Extending the annealing time results in a decrease of rock salt phase and an increase of spinel phase. When the transformation to the spinel phase is completed, solid state diffusion strives for a homogeneous distribution of elements on the long term. An increase of the annealing temperature accelerates the observed phenomena. [...]
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