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 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
$\mu$
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.