Microstructure evolution of Ag-Cu/BaCo_0.7Fe_0.2Nb_0.1O_3-δ joints in air and argon atmospheres

Microstructure evolution of the joints formed by Ag-6.6 mol% Cu brazing BaCo0.7Fe0.2Nb0.1O3-δ (BCFN) ceramic after long-term heat treatment in different atmospheres was revealed. After heat treatment in air, the layered structure in the original interfacial reaction layer disappears, accompanied by the aggregation and growth of the interfacial products. The thickness of the reaction layer increased first and then stabilized. Under the action of the concentration gradient, the residual copper oxide diffused along the pores of BCFN matrix and continued to react with the matrix until it exhausted. The types of interfacial products remained unchanged, which were Ba2Cu2O5 and Co-Cu-O (solid solution). However, serious decomposition of interfacial products (Ba2Cu2O5) occurred due to the low oxygen partial pressure after heat treatment in argon, resulting in the formation of holes and cracks. Part of cobalt precipitated from the BCFN matrix and formed a cobalt-rich phase at the junction of Ag-Cu alloy and interfacial reaction zone. Results showed that the joint microstructure is unstable at high temperature. Controlling the amount of the interfacial products formed during brazing is key to improving the stability of the joint, which provides a theoretical support for further optimizing the sealing quality of membrane components.