Joining Oxygen Transport Membranes by Reactive Air Brazing

In recent years Oxygen Transport Membranes (OTM) became more important for a number of technological applications. OTM can be used for oxygen supply in emission free oxyfuel power plants but also in steelmaking and chemicals industry. Suitable OTM materials are perovskite type ceramics with mixed ionic and electronic conductivity. Working conditions of those membranes are oxidizing atmospheres at elevated temperatures. By joining the membranes to metallic components heat resistance and gas tightness is required. Because of a decomposition of perovskite at low oxygen partial pressures, Reactive Air Brazing (RAB) seems to be the only option for brazing those materials. The material which is actually used for OTM applications is Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3-d (BSCF) and the most common brazing alloy for this purpose is AgCu(O). BSCF is a very reactive material and by brazing to austenitic steel (X15CrNiSi25-20) brittle reaction layers occur in the brazing interface, because of reactions with copper-oxide and a few alloying elements of the steel. The thickness of the reaction zone depends on brazing temperature, brazing time and copper concentration in the braze filler. Moreover, it can be expected, that those reaction zones increase at working conditions, because the joints are exposed to high temperatures over a long period. These long term changes in the microstructure and the influence of reaction layers in the interface on mechanical strength and creep behavior of such joints are currently investigated at the Institute for Materials Applications in Mechanical Engineering at Aachen University. Besides microstructural investigations with respect to the brazing conditions, mechanical properties of the entire joint are simulated by finite element analysis. The results are validated by experiments.