Investigations in the tape-to-tape contact resistance and contact composition in superconducting CORC(R) wires

Conductor on Round Core (CORC(R)) wires and cables, constructed from multiple layers of helically wound REBa2Cu3O7-delta tapes, are a promising cable technology for high field magnet applications. An important feature of high-temperature superconductor cables is the ability to share current between conductors, allowing current to bypass drops in I (c) and minimizing the risk of hot spot formation, which could lead to potential burnout in the superconductor. In contrast to stacked-tape cables, which have continuous contact between tapes, in CORC(R) the transfer points occur at discrete tape crossovers. The tape-to-tape contact resistance, R (c), plays a critical role in the current sharing capabilities and current distribution in cables. For the work reported here, special CORC(R) wires were manufactured using different winding parameters to investigate variations in R (c). Variations comprised inclusion of a lubricant, different lubricant conductivity, inclusion of pre-tinning, and heating briefly to melt the solder. Cables were first tested as straight lengths, followed by bending to a 10 cm diameter. In straight cables R (c) values ranged from 1 to over 1000 mu Omega cm(2), depending on cabling parameters, with the highest values being found for cables made by the present 'standard' process. Bending the cables to a 10 cm diameter decreased R (c) by a factor 2-5. Tinning with PbSn decreased R (c) by three orders of magnitude compared to standard CORC(R) wires, and heat treating wires with tinned conductor resulted in only a small further decrease in R (c). Based on the measured R (c) at an electric field of 1 mu V cm(-1) the resulting current transfer length between layers can range from a few millimeters to a tens of centimeters. Examination of contacts with a laser confocal microscope showed plastic deformation of the copper at the edges of the contact overlap area, apparently caused by thicker plating at tape edges digging into the copper of neighboring layers. These images reveal that only a fraction of the total contact surface may actually be touching when there is nothing to compensate for height differential. Images of the PbSn coated tapes indicated that application of solder produces a much more uniform contact surface and higher contact area. Furthermore, imaging of CORC(R) cross-sections confirmed that in the non-tinned cables there are many regions where tapes are not in contact, while in contrast the PbSn cable shows significantly more contact between the tapes. These different imaging techniques reveal that tape surface morphology is a significant parameter in determining R (c).