Development of a mixed-conducting layered oxide for asymmetric oxygen permeable membranes
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Over 100 million tons of pure oxygen gas are produced each year, making it one of the most used chemicals worldwide. As the conventional production process, cryogenic distillation, is very energy demanding, a new, cheaper and more energy efficient production process is needed. Oxygen separation from air using mixed ionic-electronic ceramic (MIEC) membranes is ranked as one of the most promising alternatives to cryogenic distillation, but state-of-the-art materials show material degradation due to high operating temperatures. Hexagonal manganites have been proposed as a new candidate material for MIEC membranes, as they show high oxygen storage at 200-300C due to transport of oxygen interstitials. In this study, [Ho0.98In0.02]0.97Mn0.85Ti0.15O3 (HIMTO) was chosen as the thin film material for an asymmetric membrane. Indium was added to increase densification during sintering, and to stabilize the hexagonal P63cm crystal structure. Titanium was used as a dopant to decrease the anisotropy of the thermal expansion, increase the oxygen storage capacity, and to increase the thermal stability of interstitial oxygen. The porous supports were made of Y- and Ti-doped HoMnO3 or Ti-doped YMnO3, with 5 wt% carbon black added as a pore former. The final porous supports showed some gas permeability. Nanocrystalline HIMTO powders were produced using a modified citric acid synthesis method. Partial reduction in H2 was added as a pre-annealing step, which resulted in phase pure HIMTO. The oxygen storage capacity of the material was tested using thermogravimetric analysis, and compared to stoichiometric and Ti-doped HoMnO3. The nanocrystalline powder was suspended in ethanol using a surfactant, and deposited on top of the porous supports using drop casting, dip coating, and a combination of the two. The films were sintered at temperatures ranging from 1350 to 1450 C. Dip coating gave the best results, giving homogeneous film coverage and a thickness of 10-13 µm. The highest sintering temperatures gave the highest film density, but none of the thin films were completely dense, mostly due to microcracking and low green body density. Further work should therefore focus on improving the density of deposited thin film.