Joint Strength of a Braze Sealant with Metallic Interconnect in Solid Oxide Fuel Cell

Abstract

To meet the quick start-up characteristics of metal-supported solid oxide fuel cell (MS-SOFC), brazing seals have been used in many MS-SOFC systems. During the cyclic operation of SOFC, thermal stresses are unavoidably generated due to temperature gradient and mismatch of coefficient of thermal expansion (CTE) between components. Excessive thermal stresses may damage the structural integrity of an SOFC stack, which in turn reduces the efficiency and lifetime of the SOFC. Therefore, it is essential to study the mechanical properties of the joint between braze sealants and metallic interconnects at room temperature (RT) and operating temperatures for developing a reliable MS-SOFC stack. The aim of this study is to investigate the mechanical strength of a braze sealant/metallic interconnect joint in SOFC. The braze sealant used is a silver-based alloy which is developed at the Institute of Nuclear Energy Research (INER) for MS-SOFC. The metallic interconnect used is a commercial ferrite stainless steel (Crofer 22 H). Two types of sandwich-like joint specimens are made to determine the shear and tensile joint strength at RT and 750 °C. Experimental results indicate that a bonding time of 30 min produces better joint strength at RT than 20 and 40 min, and it is thus selected as the bonding time for the specimens used in the subsequent tests. For tensile loading mode, the joint strength at RT and 750 °C is of 30.63 ± 3.2 MPa and 13.43 ± 0.99 MPa, respectively. For shear loading mode, the joint strength at RT and 750 °C is of 40.02 ± 2.88 MPa and 10.06 ± 0.36 MPa, respectively. Fractography analysis reveals that regardless of testing temperature, fracture of the tensile specimens occurs at the interface between the braze sealant and the Cr2O3 oxide layer and occasionally within the Cr2O3 oxide layer. When the testing temperature of the shear specimens is increased from RT to 750 °C, the location of rupture changes from the interface between the braze sealant and Crofer 22 H to the interface between the braze sealant and the Cr2O3 oxide layer.