Electrical Contact Resistance in REBCO Stacks and Cables With Modified Surfaces
Shengchen Xue, M.D. Sumption, Dean Panik, C J Thong, Xiaolei Guo, M. Majoroš, E. W. Collings
Abstract
Rare-earth barium copper oxide (REBCO) coated conductors are of interest for fabricating high performance cables and magnets for magnetic field <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">B</i> > 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 <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">, η</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">int</sub> of a ten-layer REBCO tape stack was measured to be 106 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\mu$</tex-math></inline-formula> Ω*cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> at 4.2 K and 145 µΩ*cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> 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 <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">η</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">int</sub> of 10 µΩ*cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> at 4.2 K and 19 µΩ*cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> 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 <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">η</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">int</sub> of 2.7 µΩ*cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> at 4.2 K and 4.6 µΩ*cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> at 77 K. Both techniques led to <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">η</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">int</sub> that were relatively insensitive to changes in temperature. We were able to achieve <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">η</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">int</sub> < 10 µΩ*cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> with sintering under pressure, and <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">η</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">int</sub> < 3 µΩ*cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> with Ni electroplating alone.