Residual Resistance of Ultra High Purity Copper by Sulfate-Electrorefining and Zone-Refining

Abstract

Ultra-high-purity (99.9999% or higher) copper rod having a RRR value of 5000 was prepared by a two-stage refining process, i.e., a combination of two processes, sulfate-electrorefining and zone-refining. In the first process, the potential of the cathode electrode was maintained at a level of 0.2V (vs NHE) at 25°C to prevent the deposition of more active metals than copper. In the second process, the width of the molten zone was made smaller than the diameter (16 mm) of the copper rod to obtain a higher refining efficiency. A wire of 0. 5 mm in diameter was drawn from the refined rod. The RRR value of the ultra-high-purity copper wire annealed in a N₂ gas atmosphere at 500°C for two hours is as high as 4100, while that of conventional oxygen-free copper wire is usually 200. Ultra-high-purity copper will be suitable for applications in the superconducting field, electronic devices and so on.