The current go-to braze for SOFC applications is the Ag-CuO reactive air braze (RAB). As with other RAB brazes, the ability to use this braze with ceramic materials (such as yttria stabilized zirconia (YSZ) and oxide-protected stainless steel (SS)) is enabled by a braze component (in this case Cu) reacting with the brazing atmosphere to form a surface oxide that facilitates wetting. [1-3]. Although Ag-CuO provides an easy way to join YSZ and SS for SOFC applications, the insufficient ~45° wetting angle of the conventional Ag-4 wt.% Cu braze [1, 2] often leads to braze joint manufacturing defects (Type I pores). Further, the reduction of CuO during dual atmosphere (H₂/O₂) SOFC operation results in porosity (Type II pores) that: i) mechanically weaken the braze/SS and braze/YSZ interfaces and ii) facilitates H₂invasion into the center of the braze, which in turn, hastens the development of gaseous water pockets within the braze (Type III pores) caused by dissolved oxygen and hydrogen meeting.

This poster will present a new method for silver brazing YSZ and SS without the need for reactive air elements. As shown in Figure 1, this procedure (which is performed under SOFC electrode compatible oxygen partial pressures ≥ 1x10^-10atm) produces a braze that wets both the YSZ and SS. In addition, this procedure also produces Ag-YSZ bonding which has been shown by Kuhn et. al [4] to have a fracture energy two times higher than conventional Ag-CuO braze joints. It is expected that the elimination of Type I and Type II pores enabled by this new brazing method will increase SOFC braze joint lifetimes.

References

1. Kim, J.Y., J.S. Hardy, and K.S. Weil, Effects of CuO Content on the Wetting Behavior and Mechanical Properties of a Ag-CuO Braze for Ceramic Joining. Journal of the American Ceramic Society, 2005. 88(9): p. 2521-2527.

2. Kim, J.Y., J.S. Hardy, and K.S. Weil, Silver-Copper Oxide Based Reactive Air Braze for Joining Yttria-Stabilized Zirconia. Journal of Materials Research, 2005. 20(3): p. 636-643.

3. Chatzimichail, R., et al., Interfacial properties of (Ag plus CuO) brazes used as sealing materials in SOFC stacks. Journal of Materials Science, 2014. 49(1): p. 300-313.

4. Kuhn, B., et al., Mechanical Performance of Reactive-Air-Brazed (RAB) Ceramic/Metal Joints for Solid Oxide Fuel Cells at Ambient Temperature. Journal of Power Sources, 2009. 193(1): p. 199-202.

Figure 1. Back scattered electron cross-sectional images of a) the entire joint sandwich, b) the SS/braze interface and c) the braze/YSZ interface.