Contact resistance between two REBCO tapes: the effects of cyclic loading and surface coating

Abstract

No-insulation (NI) superconducting REBCO magnets have advantages of self-quenchprotection, a very high engineering current density and high mechanical strength, and the potential to reach very high magnetic fields. However, NI REBCO magnets have drawbacks of a long magnet charging time and high field ramp losses. These can be mitigated by controlling the turn-to-turn contact resistivity (R c ). In an effort to control R c , we consider two approaches. One is coating a REBCO conductor with various resistive thin films, and the other is to use a stainless steel (SS) tape as an interlayer which is also coated with different metallic films. We present experimental results of R c of an as-received sample under cyclic contact pressure of 2.5-25 MPa up to 30 000 cycles. After an initial increase in R c for the first 10-20 cycles, R c decreases to about one tenth of its initial value after a few hundred cycles. A warm-up and cool-down thermal cycle does not significantly change the low R c resulting from a previously high number of load cycles. We also studied R c of REBCO tapes that are coated with different resistive layers and interlayers. In order to increase R c , we experimented with electro-and electroless plating of Ni, Cr, and Ni-P. We also measured R c with a thin metallic interlayer as a coil co-winding material which included Cu, SS, and SS plated with Ni and Cu. A SS interlayer increases R c by about three orders of magnitude; while the Cu plated SS interlayer only increases R c by one order of magnitude. Finally, we treated the as-received REBCO surface by oxidation using an Ebonol® C solution. This controlled oxidation allowed the R c to be controlled over a wide range.

Contact resistivity due to oxide layers between two REBCO tapes

“…The contact resistivity measurements were performed at both 77 K and 4.2 K under cyclic contact pressure in a previously described facility [7]. For stainless steel tape  c measurements, the sample was sandwiched between two as-received REBCO tapes.…”

“…The stainless steel tapes received from different manufacturers can have very different surface conditions depending on the details of the manufacturing process. This may partially explain the considerable difference in  c reported by different groups [7,27,28]. Evidently surface oxides and their effects on  c is critically important, but they have not been characterized in the context of  c control.…”

“…Previously we measured  c of short samples at 77 K and 4.2 K under different contact pressures [6]. The control of  c by coating the surface of REBCO tapes with various materials has been attempted; and the effect of cyclic contact pressure was studied [7]. But a technique that reliably controls a wide range of  c values which is consistent over a large number of pressure cycles is yet to be developed.…”

CSD-Grown Y1−xGdxBa2Cu3O7−δ-BaHfO3 Nanocomposite Films on Ni5W and IBAD Technical Substrates

“…Here, chemical solution deposition (CSD), a scalable and low-cost method for the fabrication of CCs [37][38][39][40], and, specifically, the TFA-MOD route [41], has been used to grow YGBCO-BHO nanocomposite films by the "in situ" approach on two different technical substrates: Ni5W and IBAD. Y 3+ and Gd 3+ ions were chosen for the mixed phase because this combination has already been prepared in the past by pulsed laser deposition (PLD) or metal-organic chemical vapor deposition (MOCVD) with excellent properties both on single crystals [24] and buffered metallic tapes [42][43][44][45][46]. CSD has already shown promising properties for YGBCO films [19][20][21][22][23][24], but still the effort is not comparable to YBCO or GdBCO.…”

Transverse pressure effect on superconducting Nb3Sn Rutherford and ReBCO Roebel cables for accelerator magnets

“…Earlier literature on the electrical tape-to-tape resistance in coated conductors mainly focused on soldered or welded lap joints that are needed for current leads or to extend the length of magnet windings [187][188][189][190][191][192][193][194][195]. However, recent interest in no-insulation pancake-or racetracktype coils for stationary applications [196][197][198][199][200] as well as the steady development of various ReBCO cable types [100,102,[201][202][203][204] has raised the question of how the direct contact resistance between two tapes, i.e. the electrical resistance without intermediate soldering or welding layer, is influenced by the internal layout of the coated conductor and by external factors such as temperature, transverse pressure or surface condition.…”