Development of Pure Copper with Superior Electrical Conductivity at Cryogenic Temperatures

Pure copper, specifically oxygen-free copper (OFC), is widely used in superconductive and low-temperature refrigeration technologies owing to its superior electrical and thermal conductive properties at cryogenic temperatures. These properties, which can be expressed in terms of the residual resistivity ratio (RRR), are associated with the purity of copper or with its impurity concentration. High-purity copper with a low impurity concentration exhibits a high RRR. In addition to the impurity concentration, the existing form of impurities within the matrix affects the RRR, which is influenced by heat treatment. Specifically, for OFC, high-temperature heat treatment causes the impurities to dissolve into the matrix, resulting in a decrease in the RRR. Thus, to obtain a high RRR, the impurity concentration must be reduced, which often requires complex purification processes. In this study, we aimed to develop a pure copper material with a high RRR over a wide range of heat treatment temperatures using industrially feasible methods with OFC as the base material. For this, impurities with a negative effect on the RRR were investigated, and an additive element was selected to target the impurities and minimize the negative effect of these impurities without adversely affecting the RRR. We successfully developed a pure copper material using an extremely small amount of Ca as an additive element. This material not only exhibited an RRR comparable with that of high-purity copper but also maintained the high RRR value over a wide range of heat treatment temperatures, owing to the formation of CaS. The high RRR pure copper developed in this study is a promising material for use in components employed in superconductor and refrigeration applications such as in magnetic resonance imagining (MRI), in nuclear magnetic resonance (NMR), fusion reactors, maglevs, and particle accelerators.