An intermetallic powder-in-tube approach to increased flux-pinning in Nb₃Sn by internal oxidation of Zr

Abstract

We report on the development of multifilamentary Nb 3 Sn superconductors by a versatile powder-in-tube technique (PIT) that demonstrates a simple pathway to a strand with a higher density of flux-pinning sites that has the potential to increase critical current density beyond present levels. The approach uses internal oxidation of Zr-alloyed Nb tubes to produce Zr oxide particles within the Nb 3 Sn layer that act as a dispersion of artificial pinning centres (APCs). In this design, SnO 2 powder is mixed with Cu 5 Sn 4 powder within the PIT core that supplies the Sn for the A15 reaction with Nb1Zr filament tubes. Initial results show an average grain size of ∼38 nm in the A15 layer, compared to the 90-130 nm of typical APC-free high-J c strands made by conventional PIT or Internal Sn processing. There is a shift in the peak of the pinning force curve from H/H irr of ∼0.2 to ∼0.3 and the pinning force curves can be deconvoluted into grain boundary and point-pinning components, the point-pinning contribution dominating for the APC Nb-1wt%Zr strands.

Ta, Ti and Hf effects on Nb3Sn high-field performance: temperature-dependent dopant occupancy and failure of Kramer extrapolation

“…This implies that the Nb-Ta-Hf alloyed Nb3Sn does not have a suppressed actual HIrr as was observed in Nb-Zr alloyed Nb3Sn wire. 26,27 In fact we measured µ0HIrr (4.2 K) ∼23.4 T, about 1 T larger than the best Ta-doped RRP sample studied in this work, making Nb-Ta-Hf alloy a promising material for high-field Nb3Sn optimization.…”

Ternary Nb3Sn superconductors with artificial pinning centers and high upper critical fields

“…Further improvements are being pursued. respectively, which are about half of those RRP and PIT wires reacted at 650-675 °C [4,30], but much larger than the average grain sizes of the early APC samples with 1% Zr and sufficient O reacted below 650 °C, which were 35-45 nm [15,19,20]. For APC-A, this is because it was reacted at 675 °C and it had only 0.6%Zr in the Nb alloy: lower Zr content leads to fewer ZrO2 particles for grain refinement.…”

“…Brookhaven National Lab (BNL) [19], as well as Ohio State University (OSU) [20]: both effects of refined grain size (<40 nm) and shift in Fp-B curve peak were clearly verified [19,20]. Hyper Tech Research Inc. (HTR) fabricated some 61-filament binary PIT-APC wires, which were drawn to Ds of 24 µm [18].…”

Origin of the enhanced Nb3Sn performance by combined Hf and Ta doping

“…Although a 36% J c increase ( J c (4.2 K,16 T) ~ 1300 A/mm 2 ) was achieved in small filament diameter RRP (Restacked Rod Process) wires 8 , other approaches have also been pursued. The most promising is the introduction of artificial pinning centres by internal oxidation of the Nb alloy to form oxide nanoparticles, a technique which was first developed at GE (General Electrics) in 1994 using Nb-Zr to form ZrO 2 10 , 11 , and which was recently re-examined using SnO 2 as oxygen source 12 , 13 . Despite a clear shift to higher field of the pinning force maximum caused by the introduced pinning centres, adding only Zr causes a suppression of the irreversibility field H Irr (and upper critical field H c2 ), clearly an undesirable feature for a high-field superconductor.…”