BRAZING OF MIEC CERAMICS TO HIGH TEMPERATURE METALS

Mixed ionic/electronic conductors (MIEC) are in the center of interest as oxygen transport membranes (OTM). For OTM-reactors the perovskite ceramic Ba0.5Sr0.5Co0.8Fe0.2O3-x (BSCF) is considered as the best solution due to its high oxygen permeation flux. For an industrial application the gastight joining of BSCF components to metal partners is a special challenge in particular because of its high thermal expansion and low strength. The so far best joining solution for BSCF-tubes, which was used in an oxygen separation module at RWTH Aachen University, is the butt-to-butt joint with Reactiv Air Brazing (RAB) brazing, but the joining stresses are very high. Aim of this study is to develop a brazing technique for BSCF tubes in order to minimize joining stresses. These are mainly caused by the mismatch of the thermal expansions of the partners but can be reduced by optimization the joining design. Also the RAB braze was modified to adapt the thermal expansion to the brazing partners. The design of the joint was changed from butt-to-butt-joint to sleeve joint. Different variations of sleeve joints were simulated, tested and compared to the butt-to-butt-joint.