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Reactive air brazing of porous SiO₂ ceramic (p-SiO₂) was achieved using Ag-CuO filler metal. When brazing p-SiO₂, two main problems existed. Firstly, the wettability of the Ag filler metal on the surface of p-SiO₂ was poor. Secondly, the residual stress caused by the mismatch of the coefficient of thermal expansion was high in the joint. In order to solve these problems, the effects of CuO contents on the p-SiO₂ brazed joint were analyzed. In a wetting experiment, the addition of CuO significantly improved the wettability of the Ag-CuO/p-SiO₂ system. With the content of CuO increasing, the contact angle decreased from 90° to 0°. In addition, when the content of CuO increased to 0.5 mol%, the contact angle decreased from 90° to 52°. Then, during brazing p-SiO₂ with the Ag-xCuO filler metal, the typical interfacial microstructure of the joints brazed at 1000 °C for 30 min was p-SiO₂ ceramic/Ag (s,s) + SiO₂ + CuO/Ag (s,s)/Ag (s,s) + SiO₂ + CuO/p-SiO₂ ceramic. Meanwhile, Ag-CuO infiltrated into the p-SiO₂ ceramic and an infiltration layer formed. The infiltration layer was composed of Ag (s,s) + SiO₂ + CuO and the infiltration layer was conductive to form a good gradient transition of the coefficient of thermal expansion (CTE). Then, the residual stress in the joint was released and the shear strength improved. In addition, with the content of CuO increasing, the depth of the infiltration layer increased. Furthermore, when the content of CuO was 0.5 mol%, the maximum shear strength of the joint was 55 MPa.