Characterization of residual-resistance-ratio of Cu stabilizer in commercial REBCO tapes

Abstract

Residual-resistance-ratio (RRR) of Cu stabilizer in REBCO coated conductor is an important design parameter for REBCO magnets. Cu stabilizer with high RRR is especially beneficial for quench protections of REBCO magnets. In this work, we study RRR of electroplated Cu stabilizer in commercial REBCO tapes. We present RRR of over 180 samples measured for the quality assurance programs of REBCO magnet projects at the National High Magnetic Field Laboratory, USA (NHMFL). To investigate the factors that influence RRR, several samples were analyzed extensively by using scanning electron microscopy, secondary ion mass spectroscopy, and inductively coupled plasma mass spectroscopy. We found that RRR is strongly correlated with the grain size of Cu, which suggests that resistivity at low temperatures is dominated by grain boundary resistivity. In addition, low RRR corresponds to high concentration of chlorine impurity. This is explained by that higher chlorine impurity hindered the grain growth in the self-annealing process at room temperature which resulted in smaller grain size and low RRR. Annealing at 300C significantly enlarged the grain size and enhanced RRR. Due to the concern of critical current degradation, however, annealing is not recommended as a practical method to improve RRR of Cu in REBCO tapes.