Superconducting properties and critical current density of Nb-Ti/Ti multilayers

Abstract

AbshacMultilayers of Nb47Ti superconductor (S) andtitanium pins (N) have been made with a critical temperature T, and an upper critical field Hc2 approaching bulk values. There is no proximity effect suppression of T, and Hc2 for layer thickness ds = dN = 10 nm, in contrast to the strong suppression for comparable multilayers made with Cu, Cu-alloy, or Nb pins. This may be because the proximity length of the Ti layers, -10 nm, is less than or equal to their thickness. The critical current density J, exhibits multiple peaks in J,(H), which suggests that matching effects contribute to the overall pinning. J, for 20 nm bilayers is comparable to that of Nb47Ti tapes, which have a higher number density of pins but a lower pin volume fraction. The weak proximity coupling suggests that much higher J, can be obtained if smaller bilayer periods can be made with good adhesion. The results are compared to other Nb-Tfli multilayer experiments, and stability limitations are also discussed.

Advances in high-field superconducting composites by addition of artificial pinning centres to niobium-titanium

“…Multilayer experiments show that Cu [9] and Cu-alloy [91] pins exhibit strong suppression of T c , and this has a more profound impact on J c than ξ N does. On the other hand, Ti pin layers do not suppress T c [88], and changing ξ N by alloying Ti may benefit the pinning force. An advantage of Ti pins is high resistivity, and alloys of Ti with a group V or group VI elements are known to exhibit extremely high resistivity [1].…”

“…Because the ribbonlike pin shape appears to be inevitable, multilayers of Nb-Ti and artificial pins can be used as models for round wire composites. This has been exploited by several research groups, using Nb47Ti superconductor and Nb [8,86,87], Ti [8,87,88] or Cu [9,[89][90][91] pin layers, and pure Nb and Ti layers [92]. Ideal multilayers allow the ultimate limits of J c in APC composites to be probed.…”

“…Multilayers also provide useful analyses of the proximity effect, flux pinning and dimensionality. Much of this fundamental work is still in progress [88,90,91]; nonetheless, the preliminary results point out new ways to approach the problems below.…”