Enhancement of the upper critical field of Nb3Sn utilizing disorder introduced by ball milling the elements

Abstract

Nb 3 Sn was prepared by milling Nb and Sn powder mixtures followed by limited reactions to restrict disorder recovery. Although disorder reduced the superconducting critical temperature T c , the concomitant electron scattering increased the upper critical field µ 0 H c2 to as high as 35 T at 0 K, as determined by the Werthamer-Helfand-Hohenberg equation. H c2 was higher for longer milling times and lower annealing temperatures. Substitution of 2% Ti for Nb did not appreciably enhance H c2 , suggesting that alloying mitigates the benefits of disorder. Since alloyed Nb 3 Sn wires  have µ 0 H c2 (0) ≈ 29 T, wires based on heavily milled powders could extend the field range for applications if they can be made with high current density.Practically all high-field superconducting magnets operating above ~8 T at 4.2 K use Nb 3 Sn wires. This includes laboratory solenoids, large solenoids for fusion, dipoles and quadrupoles for particle accelerators, and very high field magnets for nuclear magnetic resonance. The operating envelope for Nb 3 Sn, presently ~8 < µ 0 H < ~21 T at 4.2 K (up to ~23 T at 2.0 K), is determined by the temperature dependence of the upper critical field H c2 (T) and the field H and temperature T dependence of the critical current density J c . While J c depends strongly on the development of a homogeneous fine-grained microstructure through control of the Nb 3 Sn formation reaction, H c2 is determined by the physics of the superconducting state itself.Very recently, Godeke et al. examined the factors that affect the H c2 (T) curve in a range of commercial and experimental Nb 3 Sn composite wires. 1 Generally, their results fall into two classes: 1) pure Nb 3 Sn wires which exhibit T c ≈ 18.0 K, µ 0 H c2 ′ ≡ µ 0 dH c2 /dT| Tc of about -2.0 T K -1 , and an extrapolated µ 0 H c2 (0) ≈ 26 T; and 2) alloyed wires with 1 to 2% Ti or Ta substituted for Nb which exhibit T c ≈17.5 K, µ 0 H c2 ′ ≈ -2.3 T K -1 , and µ 0 H c2 (0) ≈ 29 T. These wires' behaviors were described very well by theoretical descriptions developed during the 1960s, in particular the Werthamer, Helfand, and Hohenberg (WHH) 2 equationSince H c2 ′ is directly proportional to the normal-state electrical resistivity ρ N , alloying the Nb 3 Sn phase with Ti and/or Ta adds electron scattering and improves H c2 at low temperatures. Alloying also changes the electronic density of states and the electron-phonon coupling, since adding more than about 2% of Ti or Ta results in a drop of T c that more than counteracts the increase in H c2 ′. Hence, the maximum observed enhancement is 2-3 T at 4.2 K. 3 In this Letter, we report that greater improvements in µ 0 H c2 (0) can be realized by introducing disorder instead of by alloying. Niu and Hampshire showed recently that highenergy ball milling of PbMo 6 S 8 superconductors, followed by controlled annealing in a hot isostatic press, resulted in a tripling of H c2 ′ at the expense of a modest reduction of T c from 15 to 12 K. 4 The µ 0 H c2 (0) values estimated by the WHH formula increased from 45 ...

A review of the properties of Nb3Sn and their variation with A15 composition, morphology and strain state

“…The solid curve in the right graph in figure 7 is calculated using (11). It is interesting to note that a linear extrapolation of the cubic H c2 (β) results yields around 35 T for the stoichiometric composition, in agreement with the reported record value by Cooley et al [40]. Unfortunately, however, compositional information is not available for this bulk sample, a high degree of uncertainty is stated for their reported resistivity value of 300 µΩcm, and the corresponding T c value of 15.1 K would place the Sn content around 23%.…”

State of the art powder-in-tube niobium–tin superconductors

“…around 15 T) [19,31,32]. An additional benefit of grain refinement is that it will increase scattering and thus the normal state resistivity, which in turn has the potential to further enhance H C 2 if the mean free path is not too severely reduced [68,69].…”

Bulk nanocrystalline superconducting YBa2Cu3O7-x

“…loss filamentary conductors made with YBCO. A number of improvements are possible in high density nanocrystalline YBCO materials: Flux pinning is expected to be very strong in high density, nanocrystalline samples prepared by ball milling [11][12][13]; The grain boundaries may be very clean due to the improved diffusion under high pressure; The densification of the material can improve the connectivity between grains thereby increasing J C [15,16], and the upper critical field (B C2 ) in the nano-structured superconductors may be increased by shortening the electron mean-freepath as has been found in the conventional superconductors such as Chevrel phase [11], elemental [17] and A15 compounds [18]. Improving B C2 of YBCO is particularly important for high magnetic field applications operating at liquid nitrogen temperatures.…”