Development of Pure Copper with Superior Electrical Conductivity at Cryogenic Temperatures

Abstract

Pure copper, specifically oxygen–free copper (OFC), is widely used in superconductive and low temperature refrigeration technologies for its high electrical (and thermal) conductive properties at cryogenic temperatures. This property, commonly expressed in Residual Resistivity Ratio (RRR), is associated with purity, or impurity concentration. A high purity copper with low impurity concentration will exhibit high RRR. In addition to impurity concentration, the form in which the impurities exist within the matrix affects RRR, which is influenced by heat treatment. Specifically, for OFC, heat treatment at high temperatures will cause the impurities to dissolve into the matrix, resulting in decrease in RRR. Thus, to obtain high RRR, it is necessary to decrease impurity concentration, which often requires complex purification processes. In this research, we aimed to develop a pure copper material that offers high RRR over a wide heat treatment temperature range, using industrially feasible methods and with OFC as the base material. Impurities having negative effect on RRR were investigated, and an additive element was selected to target and minimize the negative effect of the impurities without itself causing a negative effect on RRR. As a result, we developed a pure copper material using an extremely small amount of calcium as an additive element, that not only exhibits RRR comparable to higher purity copper, but also maintains high RRR over a wide heat treatment temperature range due to the formation of CaS.