The upper critical field of filamentary Nb3Sn conductors

Abstract

We have examined the upper critical field of a large and representative set of present multifilamentary Nb 3 Sn wires and one bulk sample over a temperature range from 1.4 K up to the zero-field critical temperature. Since all present wires use a solid-state diffusion reaction to form the A15 layers, inhomogeneities with respect to Sn content are inevitable, in contrast to some previously studied homogeneous samples. Our study emphasizes the effects that these inevitable inhomogeneities have on the field-temperature phase boundary. The property inhomogeneities are extracted from field-dependent resistive transitions which we find broaden with increasing inhomogeneity. The upper 90%-99% of the transitions clearly separates alloyed and binary wires but a pure, Cu-free binary bulk sample also exhibits a zero-temperature critical field that is comparable to the ternary wires. The highest 0 H c2 detected in the ternary wires are remarkably constant: The highest zero-temperature upper critical fields and zero-field critical temperatures fall within 29.5± 0.3 and 17.8± 0.3 K, respectively, independent of the wire layout. The complete field-temperature phase boundary can be described very well with the relatively simple Maki-DeGennes model using a two-parameter fit, independent of composition, strain state, sample layout, or applied critical state criterion.

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

“…There are strong indications that Sn richer A15, which is closer to stoichiometry and thus has a higher degree of or dering in the A15 lattice, has an increased strain sensitivity, as was suggested elsewhere [22]. Increasing the Sn content in the A15, however, also increases the effective upper critical magnetic field [19] which would reduce the strain sensitivity at constant magnetic field as was shown above. It is, for now, unclear how these opposing effects will average out.…”

“…The potential benefits of matching the spacing of the pinning sites to the vortex spacing can be estimated to be a 50% increase in the maximum pinning force [22], combined with an approximate 40% increase in pinning efficiency at 50% of H* 2 (i.e. 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].…”

“…Longer reaction does not result in a further increase of the A15 volume. The [19,28]. Courtesy of C. M. Fischer and P. J.…”

“…This ternary wire carried about 1750 Amm -2 , and 217 Amm"" 2 at 20 T and 4.2 K. This was combined with an increased effective critical field of 25.5 T, compared to an extrapolated 21 T for the binary wires. In the end of the 1990's also new optimized binary and ternary 192 filament wires were manufactured by SMI with reported non-Cu current densities of 1955 Amm -2 and 2250 Amm -2 , respectively [19]. The latter value translates, using 2380 jum 2 and 935 ;Um 2 for the nonCu area and the fine grain A15 area respectively [20], to over 5700 Amm -2 in the fine grain A15.…”

“…This high average Sn content is amongst others a major deter minant for the achieved 3000 Amm 2 [45]. For now, modern PIT wires achieve 2500 Amm"' 2 (see section 4) but composi tional and A15 fraction optimization could lead to a significant further increase [19,28,40] (see section 6).…”