Abstract Body

REBa₂Cu₃O_y (RE=rare earth, Y, Gd, etc., REBCO) coated conductors are narrowed and twisted to reduce the AC losses [1]. Since it is practically important to investigate the mechanical properties, such as bending and twisting, of the critical current (I_c) for such wires, we have been studying the effect of torsion on I_c. In this study, we have investigated the in-plane critical current density (J_c) distribution of REBCO coated conductors (CCs) with low I_c due to torsion and observed the microstructure by Scanning Electron Microscopy (SEM) to confirm the relation between the defect structure and I_c. For the I_c measurement for 2 mm-width REBCO CCs, we cut it 100 mm in length and soldered it with copper wire for voltage leads. The REBCO CCs were then clamped using a pair of Cu blocks, which were used as current leads. The torsion angle was applied by rotating the upper Cu block manually. We measured current-voltage curves by the four-probe method using pulse current, ensuring accurate results and I_c estimation. The measurement was terminated when I_c reached approximately 40% of the initial I_c. Then, to identify the location of I_c reduction, trapped magnetic field distribution and in-plane J_c distribution were measured by Scanning Hall-probe Microscopy. After that, the surface microstructures were observed by SEM to confirm whether defects equivalent to the percentage of I_c reduction were generated. As a result, longitudinal cracks up to 180 µm in the width direction at the tape edge were observed, and the cracks are connected to each other. Exfoliation of crystal grains also confirmed. Due to these defects, the effective width in the tape-width direction was 800 µm at the narrowest part, which was about 40% of the 2 mm-width of the REBCO coated conductors. This is comparable to the I_c reduction rate, suggesting that the continuous cracks and exfoliation cause I_c reduction. The details will be discussed.

References

[1] N. Amemiya, “AC Losses in Tc Superconductors-AC Loss Characteristics of Tapes and Power Transmission Cables-” J. Cryo. Soc. Jpn. Vol. 45 （2010） 376

Acknowledgment

The results of this research were obtained as a result of NEDO's commissioned work and JSPS Grant-in-Aid for Scientific Research JP22H02021, JP22H02019.

Keywords: REBa₂Cu₃O_y, Critical current, Torsion dependence, In-plane J_c distribution