Metal–metal bonding process using cuprous oxide nanoparticles

This work performs metal–metal bonding by using cuprous oxide (Cu2O) nanoparticles prepared by reduction in aqueous solution. A colloid solution of Cu2O nanoparticles was prepared by mixing Cu(NO3)2 aqueous solution and NaBH4 aqueous solution. Cu2O nanoparticles with a size of 111 ± 34 nm, cubic crystal phase, and crystal size of 21.2 nm were produced at initial concentrations of 0.010 M Cu(NO3)2 and 0.010 M NaBH4 at a temperature of 40 °C. The Cu2O particles contained not only CuO bonds but also Cu0Cu0 bonds, which indicated formation of fine cluster-like domains composed of Cu0Cu0 bonds. The shear strength required for separating the metallic Cu discs bonded by using the particles as a filler at 400 °C in H2 gas was as high as 27.9 MPa, which was comparable to the shear strengths of metallic Cu particles and CuO particles reported in our previous works. Metallic Cu single crystallites were produced during the bonding process. The presence of the fine cluster-like domains promoted epitaxial particle growth of the metallic Cu and formation of the micron-sized domains composed of nano-sized and submicron-sized single crystals, which provided the strong bonding.