Thermo-mechanical Investigations and Predictions for Oxygen Transport Membrane Materials

One of the most efficient ways to realize an Oxy-fuel process is the utilization of ceramic oxygen transport membranes (OTMs) for air separation, since this process provides a significantly lower efficiency loss compared to conventional cryogenic separation technologies. Driven by the difference in oxygen partial pressure, the oxygen transport takes place via oxygen vacancies in the crystal lattice of the membrane. Thin membrane layers supported by a porous substrate are considered as the most efficient design solution for such air separation units. The porous substrate should provide mechanical stability of the entire membrane structure. The operational temperatures are rather high, since the release of oxygen atoms from the lattice at elevated temperatures aids the transport processes. Due to their favorable permeation properties, which are an essential functional prerequisite, several materials were suggested as promising membrane and substrate materials, namely: Ba$_{0.5}$Sr$_{0.5}$Co$_{0.8}$Fe$_{0.2}$O$_{3-\delta}$, La$_{0.58}$Sr$_{0.4}$Co$_{0.2}$Fe$_{0.8}$O$_{3-\delta}$, Ce$_{0.9}$Gd$_{0.1}$O$_{1.95-\delta}$ and as alternative substrate material, the novel Fe21Cr7Al1Mo0.5Y alloy. The current study aims at the thermo-mechanical characterization and comparison of those materials. Fundamental mechanical characteristics such as elastic behavior and fracture properties were evaluated to warrant the long-term functionality of these materials. However, the long-term reliability of the component does not only depend on its initial strength, but also on strength degradation effects. In particular, the sensitivity to environmentally enhanced crack propagation at subcritical stress levels was assessed and also used as a basis for a strength–probability–time lifetime prediction. Creep behavior and time to rupture were characterized, since at operation relevant (elevated) temperatures long-term failure may occur due to creep damage. The mechanical limit of the thin membrane layer and its effect on the stability of the substrate material was also addressed. Complementary numerical simulations were carried out to permit an assessment of the experimentally obtained mechanical characteristics since standard analytical relationships (ASTM C 1499) are limited to flat mono-layer specimens. The mainly experimentally based work was additionally supported by numerical simulations to assess the effects of the final membrane´s geometrical arrangement (i.e. tubular and planar) and thickness ratios of particular layers, in order to optimize the membrane design