Superconducting critical temperatures, critical magnetic fields, lattice parameters, and chemical compositions of ‘‘bulk’’ pure and alloyed Nb3Sn produced by the bronze process

Abstract

Superconducting critical temperatures Tc and magnetic fields Hc2, lattice parameters a0, and chemical compositions were measured for ‘‘bulk’’ layers (∼6 μm or greater) of ‘‘pure’’ and alloyed Nb3Sn which were made by the bronze process. The values of Tc, a0, and the composition of pure Nb3Sn layers were ∼18 K, 0.52900±0.00005 nm, and 25±0.5 at. % Sn, respectively, independent of heat-treatment temperature (between 650–780 °C) and of the bronze composition, as long as the thickness of the layers was greater than ∼6 μm. Small additions of Ti (∼1 at. %) or Ta (∼3 at. %) slightly increased the value of Tc (by ∼0.2–0.4 K) above that for pure Nb3Sn. However, additions of larger amounts of these elements or addition of other transition elements (V, Zr, and Mo) significantly decreased Tc. Also, small additions of these elements significantly increased Hc2. Specifically, the largest values of Hc2 (∼27 T at 4.2 K) were obtained for Nb3Sn layers containing ∼1.5 and ∼4 at. % of Ti and Ta, respectively, compared with a value for the pure Nb3Sn of 23.5 T at 4.2 K. The value of a0 decreased with all of the alloying additions; these variations can be explained qualitatively by several models for the lattice parameter of A15 compounds, but none of them can quantitatively predict the variations. In one system, (Nb,Ti)3Sn, values of the normal-state resistivity just above the transition temperature were measured: adding 3 at. % Ti raises the value to ∼55 μΩ cm from the value of 10–15 μΩ cm for pure Nb3Sn. This increase in the resistivity is thought to be a primary reason for the increased Hc2 for this system as well as the other types of alloyed Nb3Sn which were studied here.

The upper critical field of filamentary Nb3Sn conductors

“…29 The general consensus on ternary additions as Ta and Ti is that they introduce scattering sites, thereby raising the resistivity and thus 0 H c2 ͑0͒. 18,33 These ternary additions may also prevent the formation of tetragonal phases which reduce 0 H c2 ͑0͒. 20,34 A third characteristic of wires is the presence of residual strain.…”

“…1,2 Single-and polycrystal data, measured over the complete field range, indicate either 0 H c2 ͑0͒Ϸ24.5 T for the tetragonal phase or 29 T for the cubic phase. 3,4 The limited amount of wire 0 H c2 ͑0͒ data, mostly extrapolated from 4.2 K, ranges from about 20 T up to Ͼ30 T. 17,18,[21][22][23] This large spread causes confusion as to what value is to be regarded as realistic in multifilamentary conductors and was a prime motivation for us to make a more systematic study of the influence of the inhomogeneities that are present in wires. We believe that the broad range of performance boundaries for wires in the literature is mostly a side effect of property distributions caused by these inhomogeneities, combined with the use of either arbitrary or variable criteria for 0 H c2 .…”

“…17,18,[21][22][23] Extrapolation of data measured up to 22 T indicates a zero-temperature upper critical field ͓ 0 H c2 ͑0͔͒ in thin films ranging from 26 to 29 T, depending on resistivity. 1,2 Single-and polycrystal data, measured over the complete field range, indicate either 0 H c2 ͑0͒Ϸ24.5 T for the tetragonal phase or 29 T for the cubic phase.…”

“…For wires, 0 H c2 ͑T͒ data are available in limited fields [15][16][17][18][19][20] while few publications report single high-field points at liquid-helium temperature. 17,18,[21][22][23] Extrapolation of data measured up to 22 T indicates a zero-temperature upper critical field ͓ 0 H c2 ͑0͔͒ in thin films ranging from 26 to 29 T, depending on resistivity.…”

Evidence that the upper critical field of Nb3Sn is independent of whether it is cubic or tetragonal

“…To remedy this lack, we have fabricated homogeneous binary Nb 3 Sn bulk samples to provide the compositional variation of µ 0 H c2 with one sample set made in one, consistent fashion. Here we show whether the sample transforms to the tetragonal state or not is irrelevant to µ 0 H c2 (0), which can equal 29.1 ± 0.2 T in both cases, values as high as any optimally Tior Ta-doped wire [4] [6].…”