Residual resistance ratio of Cu stabilizer in commercial REBCO tapes

TL;DR

This study measures the residual resistance ratio (RRR) of copper stabilizers in commercial REBCO tapes, identifying key factors like grain size, impurities, and annealing that influence RRR, crucial for magnet design.

Contribution

It provides the first comprehensive analysis of RRR in commercial REBCO tapes, linking microstructural factors and impurities to RRR variations.

Findings

RRR averages above 50 in tested samples

RRR is strongly correlated with grain size and boundary resistivity

Thermal annealing significantly increases RRR with an activation energy of 0.4 eV.

Abstract

Residual resistance ratio (RRR) of Cu stabilizer in REBCO coated conductor is an important design parameter for REBCO magnets. In this work, we measured RRR of electroplated Cu stabilizer in commercial REBCO tapes. Over 130 samples were measured for the quality assurance programs of REBCO magnet projects at the National High Magnetic Field Laboratory, USA (NHMFL). The average RRR value was above 50. In order to investigate the factors that influence RRR, several samples were analyzed by using scanning electron microscopy, secondary ion mass spectroscopy, and inductively coupled plasma mass spectroscopy. We found that, in our samples, RRR was strongly correlated with the grain size. We demonstrated that RRR was primarily determined by grain boundary resistivity. Lower RRR was also strongly correlated with higher concentration of chlorine impurity. This is explained by that higher chlorine impurity hindered the grain growth in the room temperature self annealing process resulting smaller grain. Smaller grain resulted in lower RRR. In addition, thermal annealing significantly enhanced RRR. An activation energy of 0.4 eV was obtained from the annealing experiment which corresponds to the activation of Cu grain growth.