Entwicklung geträgerter Ba0,5_{0,5}Sr0,5_{0,5}Co0,8_{0,8}Fe0,2_{0,2}O3−δ_{3-\delta} Sauerstoff-Permeationsmembranen

Oxygen Transport Membranes (OTMs) are a promising way of obtaining high-purity oxygen. Compared to conventional methods, membranes require less energy than cryogenic air separation. OTMs consist of gastight, ceramic, mixed ionic-electronic conductors (MIEC) and allow oxygen transport via oxygen vacancies in the crystal lattice. Therefore, the theoretically achievable purity of these OTMs is 100%. The most promising class of materials are the perovskites, which has a high ionic and very high electronic conductivity. The perovskite with the highest oxygen permeability is the Ba$_{0,5}$Sr$_{0,5}$Co$_{0,8}$Fe$_{0,2}$O$_{3-\delta}$ (BSCF), which has also been used in this work. Further potential for improvement of the oxygen permeation can be provided by a thin, supported membrane,an optimization of the microstructure of the porous support as well as by the use of porous activation layers on top of the membrane. An aim of the first part of the work is the development of thin membranes on top of a porous support. For this purpose, supports of different porosity and pore size were prepared by tape casting using different pore formers. The thin membrane layers were manufactured by screen printing and tape casting. The preparation of screen-printed membrane layers as well as porous activation layers was carried out on pre-sintered supports respectively sintered membranes. Composite membranes (thin membrane layer and porous support) were prepared by sequential tape casting and subsequent co-firing. Regarding deflection and leakage, the tape cast and co-fired membranes achieved the best results. The influence of membrane microstructure on oxygen permeation has been studied on composite membranes with 26%, 34% and 41% support porosity and 20$\mu$m and 70$\mu$m membrane layer thickness. This increase of support porosity as well as the reduction of membrane thickness led to an increase in the oxygen permeation. The increase of the oxygen permeation by decreasing the membrane layer thickness is lower than the Wagner equation would have suggested and this issue will be discussed in this chapter. Ways of reducing the limiting factors are to be sought in the use of porous surface layers, tailoring the support microstructure and in the use of vacuum conditions instead of a sweep gas on the support side. Limiting factors for oxygen transport through the composite membrane were identified and separated by systematic choice of the boundary conditions during permeation measurements. Limiting factors are surface transport processes, concentration polarization in the porous support and the transport through the membrane. From the acquired data, a transport model has been developed to describe the oxygen transport through the composite membrane

Creep behavior of porous La0.6_{0.6}Sr0.4_{0.4}Co0.2_{0.2}Fe0.8O3−δ_{O3−δ} oxygen transport membrane supports

Advanced oxygen transport membrane designs rely upon a thin functional layer supported by a porous substrate material that carries the mechanical loads. The creep deformation behavior needs to be assessed in order to warrant a long-term reliable operation at elevated temperatures. The current study reports the creep behavior of porous La0.6Sr0.4Co0.2Fe0.8O3−δ in air for a temperature range of 750–950 °C. Stress exponents and activation energies are derived from the deformation data. A comparison with the creep rates of dense material revealed a progressively increasing creep rate with increasing temperature that might be related to surface diffusional effects. Additional tests at room temperature revealed non-linear stress–strain curves and an apparent ferroelastic creep due to domain switching in the rhombohedral phase that is stable at lower temperature

Stability aspects of porous Ba0.5Sr0.5Co0.8Fe0.2O3−δ

Porous substrates are a prerequisite for advanced oxygen transport membranes. In particular, phase stability and mechanical robustness are of concern for long term performance and reliability. These aspects were investigated for porous Ba0.5Sr0.5Co0.8Fe0.2O3−δ material using annealing experiments along with microstructural investigations, depth-sensitive micro-indentation and ring-on-ring bi-axial bending tests. Annealing studies revealed phase instabilities at elevated temperatures that were also characterized by X-ray diffraction analysis. Elastic modulus and fracture stress were strongly affected by the porosity, whereas their temperature dependency agreed with the behavior of dense material

Strength and Elastic Modulus of Lanthanum Strontium Cobalt Ferrite Membrane Materials

Mixed ionic electronic ceramic transport membranes have a large potential for industrial oxygen supply and carbon emission reduced fossil power plant concepts. Although permeation and phase stability are main development aspects, mechanical robustness is of concern especially for the long term performance and reliability under application relevant thermo-mechanical loads. Lanthanum strontium cobalt ferrite materials appear to be advantageous, especially with respect to CO2 stability. However, the effect of the A-site stoichiometry on the mechanical properties needs to be assessed. Furthermore, advanced design concepts rely on thin layers supported by a porous substrate and therefore also the porosity is an important factor. Hence, these aspects were investigated for dense and porous La0.6Sr0.4Co0.2Fe0.8O3−δ and dense La0.38Sr0.62Co0.2Fe0.8O3−δ. The specimens were investigated using a ring-on-ring bending set-up. The work summarizes the effect of the stoichiometry and porosity on the mechanical properties and compares the temperature dependencies of elastic moduli and fracture stresses

Mechanical properties of pure and doped cerium oxide

Advanced concepts for oxygen transport membranes focus on thin layers supported by a porous substrate. To warrant long term reliable operation of such systems, an assessment of the mechanical stability of each membrane component is required. With respect to chemical stability, cerium oxide and doped cerium oxides are promising materials for this application. The present work addresses the mechanical properties of cerium oxide variants, aiming to elucidate properties of relevance to the manufacture of reliable cerium oxide components. In particular Ce0.8Gd0.2−yPryO2−δ membrane materials and the influence of an addition 2 mol% of CoO is assessed. Materials preparation and compositional variations are compared with respect to elastic modulus, hardness and fracture toughness, as characterized using depth sensitive indentation. Furthermore, the creep behavior of doped and undoped cerium oxides, that is important for the long term structural stability at temperatures relevant to the oxygen-membranes operation, is assessed and critically discussed