Mechanical properties and microstructure of dense ceramic membranes for oxygen separation in zero-emission power plants

Abstract

In the present work, the mechanical properties as well as microstructure of perovskitestructured $Ba_{0,5}Sr_{0,5}Co_{0,8}Fe_{0,2}O_{3-\delta}$ (BSCF) dense ceramic membranes for oxygen separation were investigated. The main supplier of the material was the Fraunhofer- Institut fur Keramische Technologien und Systeme (IKTS), Hermsdorf, Germany. High temperature uniaxial compressive tests on tubular specimens allow the precise determination of material's creep resistance under varied load in the wide range of application relevant temperatures and various oxygen partial pressures. Additionally, it allows to establish the influence of grain size on creep. The results of the creep test were described by the steady state creep equation, which allows to predict the creep resistance of the material under varied conditions (like: stress, oxygen pressure, grain size, temperature). Creep tests performed in vacuum, revealed the existence of two creep regimes, characterised by different activation energy. Change of creep mechanism occurs at around $800^{\circ}C$. In air, creep behavior is much more complicated: during heating sequence, also two regimes are visible, however transition temperature is shifted to $850^{\circ}C$. This difference is related to materials instability and hexagonal phase formation. Moreover, hysteresis was found- creep rates obtained at $850^{\circ}C$ during cooling sequence are significantly higher than one obtained during heating. In order to verify creep results obtained for air, additional specimens with cylindrical geometry were tested. Furthermore, additional creep tests were conducted on a newly developed $Ba_{0,5}Sr_{0,5}Co_{0,8}Fe_{0,2}O_{3-\delta}$ (BCFZ) material. Complementary to creep characterization, also bending test on O-ring specimens, machined from tubes, were carried out. Temperature dependence of Young's modulus and strength were established. Subsequent fractographic analyses revealed elongated pores and agglomerates as fracture origins. Additionally, micromechanical measurements provided information concerning the micro-hardness and Young's modulus at the room temperature. In order to gain information about material stability, a long-term (2 weeks) annealing of BSCF in the air in the range of application-relevant temperatures ($750 – 950^{\circ}C$) was performed. The study was complemented by qualitative microstructural investigation performed by means of light microscopy and revealed significant material instability. Precipitates of hexagonal phase are present after annealing in air below $850^{\circ}C$, and their volume fraction is temperature-dependent (maximum occurs at $750^{\circ}C$). Post operation analysis of tubes operated in a demonstration unit under conditions expected in a real power-plant operation allows to investigate microstructural changes in the material and find the axial temperature profile in the membrane tube. Finally, all collected results were summarized and conclusions were drawn.