Joining mixed conducting oxides using an air-fired electrically conductive braze

The lanthanum strontium cobalt ferrites (LSCF) are a well-known family of mixed oxygen-ionic/electronic conductors that are of interest for use in high-temperature electrochemical devices, such as solid oxide fuel cells and oxygen separation membranes. One of the challenges however in developing these types of devices is joining the constituent metallic and ceramic components, particularly with a technique that is compatible with the electrochemically active membrane materials. A new joining process has been developed which employs a silver-copper oxide braze material. In the present study, we found that braze compositions containing between 1.4 and 16 mol % CuO offer the best combination of wettability, joint strength, and electrical conductivity. Our results indicate that the wettability of the Ag-CuO brazes on LSCF substrates increases with CuO content and that electrical conductivity through the joint remains high as long as the silver content is greater than 66 mol %. Results from long-term, high-temperature resistance experiments confirmed the latter finding, as minimal degradation in conductivity was observed in the various test joints over 500 h of testing at 750 degreesC. Data obtained from three-point bend testing showed that joints formed with a braze composition of 1.4 and 16 mol % CuO displayed maximal average flexural strength. (C) 2004 The Electrochemical Society.