High-temperature tolerance of the silver-copper oxide braze in reducing and oxidizing atmospheres

Abstract

The silver-copper oxide–based reactive air brazing technique was developed as a method of joining complex-shaped ceramic parts. To investigate the viability of this approach for high-temperature application, a series of air-brazed alumina joints were independently exposed to either oxidizing or reducing atmosphere at 800 °C for 100 h. Those samples that were thermally aged in air maintained good joint strength, similar to that of the original as-brazed samples. Microstructural analysis revealed no significant change in joint microstructure after long-term oxidation at elevated temperature, indicating excellent stability of the Ag–CuO-based filler metal in this environment. On the other hand, exposure of the air-brazed alumina joints to hydrogen under the same aging conditions resulted in a measurable decrease in joint strength. Scanning electron microscope analysis conducted on the fracture surfaces of the broken hydrogen-exposed specimens indicated that the source of joint failure was debonding along the interface between the filler metal and alumina substrate. This was due in large part to internal reduction of CuO precipitates within the filler metal to copper and accompanied by the simultaneous formation of porosity at these sites, both within the bulk of the joint as well as along the filler metal/substrate interfaces. Pore formation was noticeably present in filler metals prepared with a high concentration of copper oxide.

Ceramic Integration Technologies for Solid Oxide Fuel Cells

“…It is generally accepted that the application of silver‐based brazes at high temperatures is limited by the reactivity of silver in presence of air, the vapor pressure of silver and also the decreased mechanical strength of silver‐brazed joints . Furthermore, the interaction mechanisms of these brazes in contact with ceramic (YSZ) and metallic (SOFC steels) components have to be clarified.…”

Microstructure evolution of Ag-Cu/BaCo0.7Fe0.2Nb0.1O3-δ joints in air and argon atmospheres

“…J.Y. Kim et al [14] exposed the Ag-CuO-based/Al 2 O 3 joints to air and hydrogen, respectively, at 800°C for 100 h. The joint microstructure had no significant change in air, while the joint strength had a measurable decrease in hydrogen and was accompanied by the formation of holes. A. Kaletsch et al [15] treated the BSCF/Ag-Cu/AISI 314 joints in air at 850°C for 500 h. The porosity at the interface between the braze and ceramic increased and the strength of the joint decreased.…”

A novel Ag based sealant for solid oxide cells with a fully tunable thermal expansion

“…In planar intermediate temperature (650-850 °C) SOFC/SOEC stacks, this is typically achieved by sealing the solid oxide cell to a ferritic stainless steel interconnect material [8][9][10]. Several joining technologies are currently under consideration, of which glass/glass-ceramic bonding [11][12][13][14][15][16][17] and reactive air brazing (RAB) [18][19][20][21][22][23][24][25][26] have attracted the greatest attention during the last few decades.…”