The effect of CuSn intermetallics on the interstrand contact resistance in superconducting cables for the large hadron collider

Abstract

The LHC superconducting cables are submitted to a 200°C heat-treatment in air in order to increase the resistance between the crossing strands (R C ) within the cable. During this treatment the as-applied Sn-Ag alloy strand coating is transformed into a CuSn intermetallic compound layer. The microstructure, the surface topography and the surface chemistry of the non-reacted and reacted coatings have been characterised by different techniques, notably focused ion beam (FIB), transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS). Based on the results obtained by these techniques the different influences that the intermetallics have on R C are discussed. The desired R C is obtained only when a continuous Cu 3 Sn layer is formed, i.e. a sufficient wetting of the Cu substrate by the tinning alloy is crucial. Among other effects the formation of the comparatively hard intermetallics roughens the surface and, thus, reduces the true contact area and it strongly affects the oxide growth on the strand surface. The oxide formed on the fully reacted coatings, which may contain essentially Cu-oxides, appears to be more stable, both mechanically and thermally, as compared to the oxide formed on the tinning alloy.

On the formation of voids in internal tin Nb3Sn superconductors

“…As compared to other samples, e.g. Sn solder joints and Sn coatings on Cu substrates [8], the void growth is observed at relatively high temperature, possibly because of stresses in the strand that prevent void growth at lower temperatures.…”

Electrical Interconnection of Superconducting Strands by Electrolytic Cu Deposition

“…In case of soft soldering the resistance of the typically 50-100 -thick solder layer has to be taken into account as well, while the resistance of the roughly 1 -thick intermetallics is comparatively small [6].…”

Electrical resistance of Nb3Sn/Cu splices produced by electromagnetic pulse technology and soft soldering

“…For the simulations only the bulk resistivities of the Cu parts and the solder have been considered. The additional resistance possibly caused by thin intermetallic layers, constriction resistances due to porosity and contact resistances are not taken into account [10,11]. The resistance of the thin unreacted diffusion Nb-Ta barriers in the RRP strand is neglected as well.…”