Electrical Contact Resistance in REBCO Stacks and Cables With Modified Surfaces

• Published in: IEEE transactions on applied superconductivity Vol. 32; no. 6; pp. 1 - 6
• Main Authors: Xue, Shengchen, Sumption, Mike D., Panik, Dean, Thong, Chee J., Guo, Xiaolei, Majoros, Milan, Collings, Edward W.
• Format: Journal Article
• Language: English
• Published: New York IEEE 01-09-2022; The Institute of Electrical and Electronics Engineers, Inc. (IEEE)
• Subjects: Acidic oxides; Barium; Conductors; Contact pressure; Contact resistance; Copper; Copper oxides; Current sharing; Electric cables; Electric contacts; Electroplating; Engineering; Fasteners; Heat treatment; HTS cables; HTS magnets; Magnet coils; Magnets; Nickel; no-insulation coils; Physics; Pressure measurement; Rare earth elements; Sintering; Superconducting cables; Superconducting magnets; superconducting materials; Temperature measurement
• ISSN: 1051-8223; 1558-2515
• DOI: 10.1109/TASC.2022.3165736

Rare-earth barium copper oxide (REBCO) coated conductors are of interest for fabricating high performance cables and magnets for magnetic field B > 22.5 T. One critical challenge is the control of the current sharing between conductor tapes to enable self-protection to take place when a localized disturbance (hot spot) appears in the cable or magnet coil. Current sharing can be enhanced by reducing the inter-strand contact resistance (ICR). In this work, we explore ICR reduction with several tape surface modification techniques. Under a pressure of 13.3 MPa the contact efficiency , η int of a ten-layer REBCO tape stack was measured to be 106 <inline-formula><tex-math notation="LaTeX">\mu</tex-math></inline-formula>Ω*cm 2 at 4.2 K and 145 µΩ*cm 2 at 77 K. Further increases in pressure had only a small effect on ICR. To further reduce ICR, we proposed two other techniques: (1) Heat treatment of the ten-layer YBCO coated conductor tape stack at 300 °C for 30 min and under various pressures. We concluded that 20 MPa was the minimum pressure required to initiate substantial conductor-to-conductor sintering and an optimum result was achieved by a processing pressure higher than 23.4 MPa which produced η int of 10 µΩ*cm 2 at 4.2 K and 19 µΩ*cm 2 at 77 K when measured under 13.3 MPa. (2) The second approach was to Ni-plate the REBCO tapes. The acidic plating solution removes the native oxide on the Cu surface and the process replaces it with a thin layer of Ni. The result was an η int of 2.7 µΩ*cm 2 at 4.2 K and 4.6 µΩ*cm 2 at 77 K. Both techniques led to η int that were relatively insensitive to changes in temperature. We were able to achieve η int < 10 µΩ*cm 2 with sintering under pressure, and η int < 3 µΩ*cm 2 with Ni electroplating alone.