Thermo-mechanical Properties of Mixed Ionic-Electronic Conducting Membranes for Gas Separation

In this work membrane materials with mixed ionic / electronic and protonic / electronic conductivity for oxygen (Oxygen Transport Membranes - OTM) and hydrogen separation (Hydrogen Transport Membranes - HTM) were investigated regarding the thermo-mechanical properties. In case of OTM, perovskite-type materials Ba$_{0.5}$Sr$_{0.5}$(Co$_{0.8}$Fe$_{0.2}$)$_{1-x}$Zr$_{x}$O$_{3-\delta}$ (BSCF·Z100x), where x = 0.01, 0.03, 0.05 and 0.1, as well as alternative SrTi$_{1-x}$Fe$_{x}$O$_{3-\delta}$ (ST·F100x) with x= 0.03, 0.05 and 0.07, while the fluorite structured La$_{5.4}$WO$_{12-\delta}$ (LWO54) and Nd$_{5.5}$WO$_{12-\delta}$ (NWO55) were investigated as HTM membrane materials. Compressive creep tests were carried out for all compounds in different temperature (900 – 1450 °C)and stress regimes (20 – 100 MPa) in air, vacuum and Ar / 4 % H$_{2}$ 2.5 % H$_{2}$O-atmosphere. The observed activation energies and stress exponents point to diffusional creep as the predominant creep mechanism. In case of BSCF-Z100·x ceramics, this was further supported by the fact that the grain-size-normalized steady-state creep ratevaries little for the different BSCF-Z100·x compositions. It was confirmed that Zr substitution does not significantly affect the thermal hysteresis of the creep behavior as observed for pure BSCF. Regarding ST∙F100x and LWO54 materials all materials maintained their main structure after the tests. Coming to the HTM materials, the creep mechanism for LWO54 was suggested to be cation aided diffusion with a common migration of La$^{3+}$ / W$^{6+}$ as rate controlling species along grain boundaries / through lattice. ST∙F100x, LWO54 and NWO55 materials are promising membrane materials regarding creep resistance. Elastic and fracture properties were determined for dense and porous tape casted LWO54. Young’s moduli via Vickers indentation, ring-on-ring and impulse excitation technique at room and elevated temperatures show a decrease by ~ 20 % when the material is heated up from room temperature to 1000 °C in air and Ar / 4 % H$_{2}$ atmosphere. Strength decreases by ~30 % when it is heated up to 1000 °C in air for dense materials while at room temperature it can be increased by a factor ~ 2 for homogeneous microstructure. Subsequent fractographic analysis reveals agglomerates of large irregular pores as fracture origins. For porous LWO54 the strength is decreasing with porosity and the presence of the secondary phase La$_{6}$W$_{2}$O$_{15}$. Micromechanical properties at room temperature by Vickers indentation test are also determined