Transient porous nickel interlayers for improved silver-based Solid Oxide Fuel Cell brazes

Abstract Silver-copper oxide reactive air brazes are the most widely used Solid Oxide Fuel Cell (SOFC) brazes. However, the conventional Ag-4 wt.% Cu composition has a high wetting angle of ∼40° in air on yttria-stabilized-zirconia (YSZ) that leads to manufacturing defects (denoted as Type I pores). Also, many elements that oxidize during brazing to promote braze wetting (such as Cu) are easily reduced by SOFC fuels. This results in Type II pores that decrease the braze interfacial strength and provide a quick path for hydrogen permeation into the braze (where the hydrogen reacts with diffused oxygen to form gaseous water pockets, denoted as Type III pores). The present work demonstrates 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 h of 750 °C oxidation in air, show that the braze and braze interface strengths are higher than the underlying YSZ|NiO-YSZ substrate. The elimination of Type I and Type II porosity enabled by this new technique should result in improved braze lifetimes for SOFC and other ceramic-to-metal sealing applications.