Reactive air brazed ceramic-metal seals for SOFC: Mechanical properties and long-term behaviour

Solid Oxide Fuel Cells (SOFC) are highly effective electrochemical devices for the power conversion of fossil and biomass fuels. For SOFC ceramic and metallic components have to be joined and sealed for operation at high temperatures up to 850°C in dual atmospheres. The mechanical integrity of the sealants and their long-term stability under operating conditions are essential requirements for a reliable operation of SOFC stacks. In this respect metallic brazes are considered to have advantages in comparison to the widely used brittle glasses or glass ceramics. An approach for the integration of ceramic components such as the cell into SOFC stacks is the reactive air brazing technology. The present study focuses on mechanical properties and long-term stability of reactive air brazed ceramic -metal joints at high temperatures in air. Fracture experiments before and after ageing have been carried out in order to investigate 4-point bending strength and failure mechanisms of different silver-based reactive air brazes. Characterization of fracture surfaces revealed details of fracture origin and crack propagation. The formation of cracks in the interfacial layers between braze/ceramic and braze/steel were identified as origins for failures of the joints. Inductive brazed 4-point bending specimens showed higher strength than samples brazed in a conventional furnace due to thinner interfacial layers. After thermal ageing of the brazed bending bars at 850°C in air the bending strengths decreases while the thickness of the interfacial layers increases. Failure mechanisms and the pote ntial for improvements of the reactive air brazing technology are discussed in this context.