Electrical Contact Resistance in REBCO Stacks and Cables With Modified Surfaces

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 he 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 133 MPa the contact efficiency, eta(int) of a ten-layer REBCO tape stack was measured to he 106 mu Omega(*)cm(2) at 4.2 K and 145 mu Omega(*)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 degrees 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 eta(int) of 10 mu Omega(*)cm(2) at 4.2 K and 19 mu Omega(*)cm(2) at 77 K when measured under 133 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 eta(int) of 2.7 mu Omega(*)cm(2) at 4.2 K and 4.6 mu Omega(*)cm(2) at 77 K. Both techniques led to eta(int) that were relatively insensitive to changes in temperature. We were able to achieve eta(int) < 10 mu Omega(*)cm(2) with sintering under pressure, and eta(int) < 3 mu Omega(*)cm(2) with Ni electroplating alone.