Solid oxide fuel cells are the most promising power conversion units for fossil and biomass fuels combining ceramic and metallic components. Because of long-term high temperature operation and thermal cycling the standard materials cannot be applied for integration of active ceramic components. Different approaches are used nowadays for the integration of ceramic cells into stacks and of system components in the power plant. In this paper an overview on the application of glass and glass ceramic sealants, active and reactive air brazes as well as compressive mica sealants for solid oxide fuel cells is made. The crystallizing glasses are more favorable for cell integration in the stack for operation in a broad temperature range. Metallic brazes are mainly limited to integration of anode or metal supported cells in ferritic alloy gaskets for operation at T < 750°C and mica is a most suitable solution for system component integration in hot regions of power plants as well as for gas tight electrical isolation. In addition, this study regards aspects of degradation processes of glass ceramic and metallic sealants which allow estimations for the long-term behavior of joints.
Reactive Air Brazing (RAB) has been developed as a method for joining ceramics and steel using CuO as a reactive agent that interacts with the surface of the ceramic, enabling a wetting by the molten filler metal. A major benefit of this method is the fact that the joining process can be carried out in an ambient atmosphere, in contrast to active brazing, which needs to be performed in a vacuum furnace. In the past, several investigations were conducted to improve the mechanical bonding. But a sealed gas tightness is also important for applications like solid oxide fuel cells. In this regard, the reduction of porosity, which is neccessary in order to achieve reproducible joints with a long lifetime, presents a challenge. Therefore, it is necessary to conduct fundamental analyses of the reasons and mechanisms of the formation of voids in the interfacial area to guarantee a reliable joint quality.In this study, the authors used an ultrasonic testing method to evaluate the porosity in brazements produced with varying process parameters, using a light furnace. A major goal was to assess the influence of the cooling stage on the pore formation. The advantage of this non-destructive method is the possibility to scan the entire joint area using just one scan. These investigations were flanked by SEM analyses on different cross sections.
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