Performance of as-reacted and multiple bent (‘pre-bent’) practical Nb₃Sn bronze route wires with different architectures

Abstract

Six practical composite wires of Nb3Sn (Furukawa Electric Co, Ltd) with different architectures (design and reinforcement) were compared from the normal state resistance R, critical temperatures Tconset, Tcoffset and ΔTc, upper critical field Bc2 (at 4.2 K) and critical current density Jc points of view. Wires were as follows: three of near-the-edge reinforcement design with Nb reinforcement of 0, 21, 50 vol% in the CuNb region, and two of central reinforcement design with Nb of 21 and 50 vol%. One wire with near the edge 50% vol Nb reinforcement had a different reinforcement/superconductor ratio. As-reacted wires show very different patterns of R, Tconset, Tcoffset, ΔTc, Bc2 and Jc. For the superconducting parameters this is probably due to different 3D thermal residual strains. Data suggest that the architecture of the as-reacted wire can control residual strain values and distribution. During multiple bending of the wires at room temperature (named pre-bending), introduction of the reinforcement improves relaxation of the 3D residual strain and especially of the lateral components. As a consequence, Bc2 and Jc versus pre-bending strain, εpb, are enhanced to values closer to those of the Nb3Sn in the stress-free state. Relative enhancement of these critical parameters for the reinforced wires is higher than for the reinforcement-free wire. Evolution during pre-bending and maximum attained absolute values of the superconducting parameters can be grouped roughly as a function of near-the-edge or central reinforcement design. Variation of the superconducting parameters suggests that the pre-bent state may depend on the as-reacted one (which is a function of wire architecture and processing history); when Jc was low as in the case of as-reacted wires with central reinforcement, during pre-bending, despite a relatively high Jc enhancement, this parameter attained lower absolute maximum values than for the other wires. Depending on the wire, when pre-bending strain εpb = 0.8–1%, the enhancement of non-Cu Jc at 4.2 K in the pre-bent wire is by up to 43.5% at 15 T compared to the as-reacted case. The maximum absolute value of Jc is obtained for the near-the-edge reinforcement pre-bent wires with 21 and 50% vol Nb. Bc2 of these wires was lower than for the other reinforced wires and higher than for the reinforcement-free wire. Pre-bent wires with near-the-edge reinforcement are identified as new and promising candidates for fabrication of react-and-wind coils with improved performance.

Microstructural and Magnetic Characterization of ${\hbox{CuNb}}/{\hbox{Nb}}_{3}{\hbox{Sn}}$ Wires With Different Architectures

“…Reinforcement material (RM) of the JR wires was realized by the jelly-roll technique (Cu-50vol%Nb), while that for IS wires was fabricated by the in-situ process (Cu-21vol%Nb). The micrographs depicting the cross section of these wires are found elsewhere [4]. The microstructural characterization of the wires was performed by scanning electron microscopy (SEM).…”

“…This last contribution is related to the number of barriers: one Nb barrier for NR, IS2, JR2, and JR3 wires; two barriers for IS1 and JR1 wires [4]. Regarding pure Nb, there are three possible sources in the conductor: Nb at the core of the filaments , Nb at the reinforcement material and Nb at the barrier .…”

“…On the other hand, the Nb Sn filaments experience additional compressive strain caused by the differences in thermal expansion coefficients between the reinforcement and the Nb Sn compound. Therefore, the appropriate choice of the reinforcement material, as well as its position in the high strength Nb Sn wire is of fundamental importance for the improvement of the superconducting performance of such a conductor [4].…”

“…In this condition, Cu-Nb composites are exposed to temperatures as high as 600-700 C, which is the usual temperature range necessary to form the A15 superconducting phase [4]. As Manuscript a consequence, significant changes occur in the microstructure of such composites during annealing, mainly those concerning the morphology of Cu-Nb interfaces [5], [6].…”

Rutherford flat cable composed of CuNb-reinforced Nb3Sn strands

“…Relationship between the critical current and the transverse compressive stress for the CuNb/Nb 3 Sn strands and an ordinary Cu/Nb 3 Sn strand [9].…”