Ag–Cu–Ti Braze Materials for Sealing SOFCs

Two Ag–Cu–Ti brazing alloy compositions were studied as gas seals for solid oxide fuel cells (SOFCs). Yttria-stabilized zirconia pellets were bonded using Ag–35.25Cu–1.75Ti (Cusil-ABA) or Ag–20.1Cu–1.0Ti (average composition; Ag-Cusil). Both as-brazed filler metals contained metallic Ag and Cu phases, along with continuous titanium oxide layers at the filler metal-zirconia interfaces. The brazed joints were then aged at $700°C$ for up to $100h$ in air or while separating $H2$ and air atmospheres. After annealing in air for $100h$ at $700°C$, degradation in microstructure and hermeticity was more pronounced in Cusil-ABA. However, in $H2$-air atmosphere, Cusil-ABA performed better than Ag-Cusil. Ag-Cusil aged in $H2$ air delaminated from zirconia and its microstructure showed large interconnected pores in Ag-rich regions. The results were confirmed by single-cell tests using each filler metal as a gas seal. Observations of the post-test Cusil-ABA microstructure revealed little degradation. These results indicate that Cusil-ABA is better suited than Ag-Cusil as a SOFC gas seal.