In this study the gas-tightness 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 all tested samples remained hermetic. However, microstructural analysis reveals the formation of voids within the brazing zones and severe changes of the interfacial oxide layers. It is shown, that due to the diffusion of chromium and manganese from the steel substrate the thickness of the reaction layers increases and the phase composition changes. Furthermore, in previous work a decrease of the bending strength of reactive air brazed YSZ-steel joints due to the microstructural changes is observed. In contrast, the aging in dual atmospheres at 850°C causes strong degradation and the total loss of gas-tightness. Reducing atmospheric conditions lead to different reactions profiles of the silver based brazes in contact with the YSZ electrolyte. Complex reaction products are identified and reported.
Off-grid power generation is considered as an attractive market for fuel cells. For this purpose, a compact SOFC battery hybrid system was developed that operates on commercially available fuels. Due to its progressive thermal packaging, the mobile prototype achieves a net efficiency of 22 % with commercial BoP components. It was possible to reduce the cold start time of the system by direct cathode preheating with burner off-gas. Thus, the system is designed for frequent start-up and cycling capability. In consequence the planar SOFC stack was tested with rapid heating rates up to 20 K/min in furnace and hotbox environment. The degradation caused by system cycling is less than 0.1 % per cycle. Finally, the optimization of stack temperature conditions has improved the lifetime of the SOFC stack significantly.
Abstract. Besides the well-known application as circuit boards and housings, multilayer low-temperature cofired ceramics (LTCC) offer a flexible and temperature-stable platform for the development of complex sensor elements. Commercial LTCC qualities are usually available with a matching set of metallization pastes which allow the integration of various electrical functions. However, for the integration of ceramic sensor elements based on LTCC into standardized steel housings it is necessary to compensate the mismatching thermal expansion behaviour. Therefore balancing elements made of Kovar ® (Fe-29 wt% Ni-17 wt% Co) and alumina ceramic (Al 2 O 3 ) can be used. These components have to be joined hermetically to each other and to the LTCC sensors. In this study, brazing experiments were performed for combinations of Kovar-Al 2 O 3 and Kovar-LTCC with Ag-Cu-Ti-and Ag-Cu-In-Ti-based commercial braze filler metals, Cusil-ABA ® and Incusil ® -ABA, respectively. For both active braze filler metals, optimized processing parameters were investigated to realize hermetic Kovar-Al 2 O 3 and Kovar-LTCC joints.
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