Joining of metal-ceramic using reactive air brazing for oxygen transport membrane applications

Abstract

This study examined the joining of dense Ce0.9Gd0.1O2−δ–La0.6Sr0.4Co0.2Fe0.8O3−δ (GDC–LSCF) ceramics to high temperature metal alloys for the fabrication of multilayered oxygen transport membrane (OTM) stacks. Reactive air brazing using a silver-based paste was performed at 1050°C for 30min in air to join GDC–LSCF/high temperature alloys, such as AISI 310S, Inconel 600 and Crofer 22 APU. The effects of the various filler materials, including CuO, GDC, LSCF, and GDC–LSCF mixture, in the Ag paste were also examined. The Ag–10wt% CuO braze filler ensured in a reliable and compact joining without the formation of cracks and voids at the joining interface, while the addition of other ceramic fillers resulted in incomplete joining. Although none of the GDC–LSCF/metal alloy joints showed gas leakage at room temperature, the GDC–LSCF/Crofer joint only maintained the gas-tightness up to 800°C under pressurized air up to 7bars, which was explained by the microstructural rigidness of the oxide layer formed on the filler/alloy interface at high temperatures. This was supported by the minimal decrease in shear strength of the GDC–LSCF/Crofer joint, which was 91.1 and 88.3MPa for the as-brazed and isothermally aged joint at 800°C for 24h in air, respectively.