Aging behavior of reactive air brazed seals for SOFC

The mechanical integrity of solid oxide fuel cells (SOFC) and the long-term stability underoperating conditions are basic requirements for a reliable operation of SOFC stacks. In this respect the use of metallic brazes as sealing material is considered to have advantages incomparison to the widely used brittle glasses or glass ceramics. In this study the mechanical properties of reactive air brazed YSZ-steel joints and their long-term stability at high temperatures in air and dual atmospheres are investigated. Silver-based braze compositions like Ag-4CuO are used for reactive air brazing of gas tightness samples. During aging in air at 850°C for 800 h the thickness of the interfacial oxide layers increases with time, while the bending strength decreases. Microstructural analysis reveals the formation of voids within the brazing zone and the development of multilayered reactionlayers. In contrast, the aging in dual atmospheres at 850°C causes strong degradation andthe total loss of gas tightness. However, the addition of small amounts of TiH2 to the brazecompositions enhances the dual atmosphere tolerance of the reactive air brazed seals.