Precise purity-evaluation of high-purity copper by residual resistivity ratio

To evaluate the purity of high-purity copper with high accuracy by the residual resistivity ratio (RRR), the optimum annealing condition before the RRR measurement and the diameter dependence (size effect) of RRR for high-purity copper (5N and 6N grade copper) wires, 0.2-2 mm in diameter, annealed under the optimum condition have been investigated. The most suitable annealing temperature and period were determined to be 923 K and more than 14.4 ks (4 h), respectively. For the size effect on RRR, the relationship between RRR_W (RRR measured for a copper wire with diameter d), RRR_B (RRR of the bulk copper) and the specimen diameter d (mm) was found as follows: RRR_W^-1=RRR^B-1 + (3.8 × 10^-5)·d^-1 The product of ρ·λ=6.5 × 10^-16 Ωm² for copper was also obtained from the slope of the above equation. Furthermore, it became clear that the difference in RRR_W is dependent on the specimen diameter and increases with purity of the specimen. Therefore, the influence of the size dependence on RRR must be considered carefully whenever to evaluate and to compare the purity of high-purity metals by RRR.