An Ag based brazing system with a tunable thermal expansion for the use as sealant for solid oxide cells

An Ag-Al₂TiO₅ composite braze was developed and successfully tested as seal for solid oxide cells. The thermo-mechanical properties of the Ag-Al₂TiO₅ system and the chemical compatibility between this composite braze and relevant materials used in stacks were characterized and the leak rates as a function of the operation temperature were measured. The thermal expansion coefficient in the Ag-Al₂TiO₅ system can be tailored by varying the amount of the ceramic filler. The brazing process can be carried out in air, the joining partners showed a good chemical stability and sufficient low leak rates were demonstrated. Furthermore, the long-term stability of the Ag-Al₂TiO₅ composite braze was studied under relevant SOFC and SOEC conditions. The stability of brazed Crofer/Ag-Al₂TiO₅/NiO-YSZ assemblies in reducing atmosphere and in pure oxygen was investigated over 500 h at 850 °C. Additionally, a cell component test was performed to investigate the durability of the Ag-Al₂TiO₅ seal when exposed to dual atmosphere. The seals performed well over 900 h under electrolysis operation conditions (-0.5 A cm², 850 °C), and no cell degradation related to the Ag-Al₂TiO₅ sealing was found, indicating that the developed braze system is applicable for the use in SOFC/SOEC stacks.