| dc.description.abstract | Every year, millions of tons of high purity oxygen are produced mainly by cryogenic distillation. Since this process is energetically demanding, there is a need for more energy efficient technologies, and the use of oxygen permeable membranes has been highlighted as a promising alternative. However, to yield sufficient oxygen flux the current membrane materials require high operating temperatures (700-900°C) and/or reducing atmosphere. It has been reported that these operating conditions lead to chemical and mechanical instability, and eventually to demixing and breakdown of the membranes. Thus, there is a need to engineer materials that maintain a high oxygen transport rate at lower temperatures. Hexagonal manganites have been suggested as candidate materials for this purpose. RMnO3 phases in air have been believed to be oxygen stoichiometric. However, certain compositions of RMnO3 (R = Dy, Y) hexagonal manganites have recently been reported to have the capability of accommodating oxygen at lower temperatures (200-300°C), making them an excellent candidate for oxygen permeable membranes. Unfortunately, this material has an anisotropic thermal expansion, which leads to the formation of microcracks in the materials during the production process.
The aim of this thesis is twofold: first, to evaluate the choice of chemical composition for the fabrication of an asymmetric membrane for oxygen separation of hexagonal YMnO3, by investigating the effect of 15% Ti-doping and 15% Y- deficiency. Second, to assess possible fabrication routes to a functioning membrane.
In this work, hexagonal YMnO3, Y0.85MnO3 and YMn0.85Ti0.15O3 have been synthesized by conventional solid state synthesis and a modified citric acid synthesis. The effects of the chemical composition on the crystal structure, the sintering properties, and the oxygen exchange at different temperatures have been investigated. A superior ability to accommodate and exchange oxygen with the surroundings is observed for the Ti-doped sample, with oxygen exchange all the way down to 50°C. Furthermore, the Ti-doped sample is found to stabilize the P63/mmc phase at room temperature, thus reducing the microcracking in the ceramic. The sintering capability of the Y-deficient sample is exceeding the sin- tering capabilities of the two other compositions. Several parameters of the fabri- cation process have been explored, and the current findings strongly supports the hypothesis that hexagonal manganites have great potential to be used in oxygen permeable membranes. A combination of Ti-doping and Y-deficiency is suggested as a very promising composition for future fabrication. | en |
