Development of hexagonal manganites for use in oxygen permeable membranes
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Mixed ionic-electronic conducting membranes can provide a cheaper and more environmental friendly alternative to cryogenic distillation for production of 100 % pure oxygen. However, most of today's state-of-the-art materials intended for this application suffer from chemical degradation and a reduced lifetime due to the high working temperatures (>800). Hexagonal manganites have recently shown to accommodate an excess oxygen content in the temperature range of 200-400 oC. This is accompanied by reversible oxygen absorption upon cooling between 400-250 oC. In contrast to the perovskite materials used today, which have conduction through the formation of oxygen vacancies, the hexagonal manganites conduct through the formation interstitial oxygen ions. Hence, the hexagonal manganites can reduce problems associated with high operating temperatures and can be regarded as materials for next generation of membranes. In this thesis hexagonal YMnO3 is explored for the possible application as an oxygen permeable membrane. A challenge with YMnO3 is that the material experiences anisotropic thermal and chemical expansion upon cooling after sintering, which results in microcracking of the material. It was shown that this problem can be solved by substitution of titanium in Mn-site. The electrical conductivity of YMnO3 was investigated in the temperature range of 200-600 oC with in situ changes between oxidizing and reducing atmospheres. Based on the obtained results it is proposed that YMnO3is a p-type conductor and that the mechanism for conduction is by polaron hopping between Mn3+ and Mn4+.