New braze material for planar solid oxide fuel cell applications

"Solid oxide fuel cells (SOFC) are power generation devices that can convert the chemical energy from a wide range of fuels and energy-carriers directly into electricity with high efficiency. One of the major challenges for the viability of commercial SOFC devices is the development of suitable sealing technologies for the separation of the air and fuel in the system at high temperature (∼750°C). Silver-copper oxide reactive air brazes (RAB) are the most widely used SOFC brazes. However, the conventional Ag-4 wt.% Cu composition has a high wetting angle of 450303° in air on yttria-stabilized-zirconia (YSZ) that leads to manufacturing defects (denoted as Type I pores). Also, many reactive elements that oxidize during brazing to promote braze wetting (such as CuO) are easily reduced by SOFC fuels. This results in Type II pores that decrease the braze interfacial strength and provide a quick path for H2 invasion into the braze (where the H2 reacts with diffused O2 to form gaseous water pockets, denoted as Type III pores). In the present work, two approaches were proposed to replace the current RAB brazing system by either designing a new, silver-free braze or to develop a CuO-free, silver-based braze. For the first approach, an integrated computational-experimental method was used. Thermo-CalcRTM was utilized to fast screen through hundreds of alloy systems to identify candidate braze compositions. Material compositions, melting ranges, mechanical properties, oxidation resistance, as well as wetting characteristics of these candidate alloys were analyzed. Substrate surface pre-treatments, active element additions, and novel brazing schemes were also investigated. Unfortunately, the current study failed to find a suitable silver-free braze for SOFC applications. In the second approach to find a CuO-free, silver-based SOFC braze, a novel silver-nickel brazing method was developed. It was demonstrated that transient porous nickel interlayers, instead of reactive element additions, can be used to promote Ag wetting on YSZ and produce high-quality YSZ-stainless steel braze joints. Mechanical tests on these reactive-element-free, silver-based SOFC braze joints, both before and after 500 hours of 750°C oxidation in air, show that the braze and braze interface strength are higher than the underlying YSZ