Significant enhancement of compositional and superconducting homogeneity in Ti rather than Ta-doped Nb3Sn

Abstract

Nb 3 Sn wires are now very close to their final optimization but despite its classical nature, detailed understanding of the role of Ta and Ti doping in the A15 is not fully understood. Long thought to be essentially equivalent in their influence on H c2 , they were interchangeably applied. Here we show that Ti produces significantly more homogeneous chemical and superconducting properties. Despite Ta-doped samples having a slightly higher T c onset in zero-field, they always have a wider T c -distribution. In particular, whereas the Ta-doped A15 has a T c -distribution extending from 18 down to 5-6 K (the lowest expected T c for the binary A15 phase), the Ti-doped samples have no A15 phase with T c below ∼12 K. The much narrower T c distribution in the Ti-doped samples has a positive effect on their in-field T c -distribution too, leading to an extrapolated µ 0 H c2 (0) 2 Tesla larger than the Ta-doped one. Ti-doping also appears to be very homogeneous even when the Sn content is reduced in order to inhibit breakdown of the diffusion barriers in very high J c conductors. The enhanced homogeneity of the Ti-doped samples appears to result from its assistance of rapid diffusion of Sn into the filaments and by its incorporation into the A15 phase interchangeably with Sn on the Sn sites of the A15 phase. a) Electronic mail: tarantini@asc.magnet.fsu.edu 2 Application of large quantities of Nb 3 Sn conductors first for ITER 1 and now for the High Luminosity upgrade of the Large Hadron Collider (LHC) 2 has motivated major recent R&D and raised multiple questions about how best to optimize the superconducting strand, 3 especially with respect to balancing the conflicting requirements of small hysteretic loss, high critical current density (J c ) and a high residual resistance ratio (RRR) in the Cu stabilizer. 4 However, few recent studies have investigated the fundamental properties of the A15 phase in these newer wires, which has a composition range with T c varying from 18 K (stoichiometric) to 6 K on the Sn-poor (18at.%Sn) side of the binary. Controlling this composition range is crucial to optimizing Nb 3 Sn properties. The classical nature of Nb 3 Sn implies broadly well-known properties. 5,6 For example the very similar effect of Ti and Ta in enhancing H c2 was clearly shown many years ago by Suenaga et al., 7 who also noticed a slight increase of T c upon doping that was greater for Ta. Ta, Ti or Ti+Ta doping are now standard for enhancing the in-field performance. 8 However, for a long time Ti and Ta were both thought to substitute on the Nb site, but more recent studies suggested that Ti can substitute on the Sn site. 9,10 A comparison of the in-field physical and compositional properties of Ti and Ta-doped A15 phase in the most modern, highest-J c wires taking into account these factors has not been performed yet. Maintaining a high RRR in the Cu stabilizer is crucial but frequently in conflict with the need to obtain the largest possible quantity of high quality (i.e. homogeneous A15 close to the stoichiometri...

Evidence from EXAFS for Different Ta/Ti Site Occupancy in High Critical Current Density Nb3Sn Superconductor Wires

“…They were slightly underreacted to avoid any Sn diffusion into the stabilizer Cu. In addition, a second similarly made RRP ® “108/127” wire series was included: a Ti-doped (Ti#2) and a Ta-doped (Ta#2) wire from our earlier T c -distribution comparison study 9 (OST #14895 and #12879). These samples were heat treated by Arup Ghosh at Brookhaven National Laboratory at 662–665 °C for 48 h and reacted close to their maximum J c as is reflected in the ≈3000 A/mm 2 (12 T, 4.2 K) performance.…”

“…Noticeable is also the larger H k of the Ta + Ti strand, despite its inferior J c (12 T) performance when compared with both Ti#1 and Ta#1. For completeness the “54/61” series was analyzed by specific heat to determine the 0–15 T T c -distributions, in the same way that we previously characterized the “108/127” wires 9 . This characterization is shown in Fig.…”

“…In fact these dopants can raise the H c2 (0) by ~3 T 7 (conventionally attributed to additional electron scattering increasing the normal state resistivity and thus H c2 (0)) 8 . For the strand design closest to meeting the FCC critical current target (which uses the “internal-Sn” process), we have previously shown that doping with Ti rather than Ta produces a more homogeneous A15 composition and a tighter distribution of T c. Furthermore, Ti appears to accelerate the diffusion reaction that creates the A15 phase from its Sn and Nb constituents 9 . An early EXAFS (Extended X-ray Absorption Fine Structure) study of Ta doped samples 10 and an ALCHEMI (Atom Location by Channeling Enhanced Microanalysis) study of both Ti-doped and Ta-doped samples 11 using older generation strands indicated that both dopants substitute on the Nb sites.…”

Unified Scaling Law for flux pinning in practical superconductors: II. Parameter testing, scaling constants, and the Extrapolative Scaling Expression

“…This correlates with the use of Ta doping and distributed diffusion barriers for the first two conductors, versus Ti doping and a single diffusion barrier for the last three. With recent micrographic information regarding increased compositional inhomogeneities for Ta versus Ti doping (Tarantini et al 2016), this increase in the temperature curvature parameter could also correlate with compositional inhomogeneities (see figure 6 and the discussion on compositional inhomogeneities below). However, further compositional studies and scaling parameter measurements are needed.…”

“…This is a significant consideration in relating magnetization to transport I c measurements. Energy-dispersive x-ray spectroscopy (Tarantini et al 2016) show greater radial compositional variations across the A15 layer for RRP ® conductors with Ta doping, compared with Ti doping. Figures 16(a) and (b) show graphs of the raw scaling data for the USL prefactor K(T,ε) for the Luvata and OST-RRP ® conductors, analyzed with the magnetic self-field correction (red diamond symbols), and without correction (green circles).…”