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| Dissertation / PhD Thesis/Book | PreJuSER-20618 |
2012
Forschungszentrum Jülich GmbH Zentralbibliothek, Verlag
Jülich
ISBN: 978-3-89336-781-8
Please use a persistent id in citations: http://hdl.handle.net/2128/4557
Abstract: Membrane-based gas separation systems are noteworthy among technological options for carbon capture and storage (CCS), which is an important strategy to reduce CO$_{2}$ emitted from point sources, e.g. mainly fossil power plants. In Oxyfuel-Combustion and Pre-Combustion of CCS power plant concepts oxygen separation from air is required. To meet this requirement oxygen transport membranes (OTM) consisting of gastight mixed ionic electronic conductors (MIEC) are proposed, which are associated with significantly lower efficiency losses compared with conventional air separation technologies. For cost effective application a maximum oxygen flux has to be achieved to reduce the membrane area. This can be met by reduction of membrane thickness. Therefore, the reduction of the membrane thickness to the micrometer range or even below is aimed in the present thesis. Ce$_{0.8}$Gd$_{0.2}$O$_{2-\delta}$ (CGO) with fluorite crystal structure and La$_{0.58}$Sr$_{0.4}$Co$_{0.2}$Fe$_{0.8}$O$_{3-\delta}$-(LSCF) with perovskite crystal structure were developed as thin film membrane. CGO is expected to be more stable than other potential MIEC membranes in reducing atmospheres and to achieve sufficient oxygen permeation, e.g. in syngas production or petrol chemistry. LSCF is expected to be highly permeable with an acceptable chemical stability in Oxyfuel-combustion. Various porous ceramic substrates were prepared by vacuum-slip-casting and warm-pressing, and then characterized for porosity, gas-permeability and surface roughness. Subsequently, two approaches to fabrication of thin film membranes were investigated, which are wetchemical deposition (WCD) and physical vapor deposition (PVD). For WCD, nano-dispersions and colloidal sols were prepared for membrane top-layer and/or interlayer. When CGO nano-dispersion (NDCGO) was spin-coated as thin film membrane, the gastightness of sintered membranes was increased with decrease in spinning time and increase in concentration of NDCGO. With decrease in cooling rate for sintering process and high molecular weight binder for higher concentration of NDCGO, crack-free layers were achieved. He leak rates of sintered and reduced membranes reached the range of 10$^{-4}$ and 10$^{-3}$ mbar$\cdot$1$\cdot$sec$^{-1} \cdot$cm$^{-2}$, respectively. For PVD, CGO membranes were deposited by reactive magnetron sputtering. According to the substrate properties and applied bias power, different deposition behavior was observed. Particularly for 8YSZ (8 mol% Y$_{2}$O$_{3}$ stabilized ZrO$_{2}$) substrate, four-zone-model of membrane was derived related to substrate strength and bias power. Without bias assist only porous films were deposited. Applying bias power enabled compact membrane but caused delamination at the same time. Adopting higher presintering temperature of substrate improved substrate strength and thus realized delamination-free compact membranes. LSCF membranes were deposited by magnetron sputtering without bias assist. LSCF membranes were porous on 8YSZ substrates, but gastight on CGO interlayers. Concentration of CGO nano-dispersion and presintering temperature of CGO interlayers rarely influenced the gastightness of deposited LSCF membrane. He leak rates of CGO and LSCF membranes reached the range of 10$^{-4}$ and 10$^{-3}$ mbar$\cdot$1$\cdot$sec$^{-1} \cdot$cm$^{-2}$, respectively.
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