Characterization of nanocrystalline copper powders prepared by melting in a cryogenic liquid

Nanocrystalline copper powders were produced by the cryogenic melting technique. The molten metal is overheated while maintained in levitation by r.f. induction in a flux of liquid nitrogen. A calefaction layer is produced at the interface between the two phases, and it is believed that the nanoparticles are formed by rapid condensation of supersaturated metal vapour. Nanocrystalline copper powders were obtained with a production rate of 60 g h−1 and a yield of 75%. The structure and the composition of the powder specimens were investigated by transmission electron microscopy. The particles with an average size of 35 nm are nearly spherical and partially aggregated. It was found that the particles had the structure and lattice parameter of bulk copper, and that the Cu2O oxide phase was also present. This oxide appears as a strained 2.5 nm layer surrounding a metallic copper kernel. Additionally, many twin boundaries are seen crossing the particles.