Ta, Ti and Hf effects on Nb<sub>3</sub>Sn high-field performance: temperature-dependent dopant occupancy and failure of Kramer extrapolation

Tarantini, C.; Balachandran, Shreyas; Heald, Steve M.; Lee, Peter J.; Paudel, Nawaraj; Choi, Eun Sang; Starch, William; Larbalestier, D. C.

doi:10.1088/1361-6668/ab4d9e

Cited by 22 publications

(11 citation statements)

References 36 publications

(69 reference statements)

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“…In fact, fitting these data shows that only a minor amount of Ta, ~ 12%, sits on the Sn site. Interestingly, this level of split site occupancy for Ta is compatible with that found for the Ta-doped RRP wire after similar heat-treatment 23 , suggesting that the presence of Hf does not affect the Ta occupancy behaviour. A similar analysis of the A15 layer for the Hf edge shows no clear indication of Hf being in the A15 structure (Fig.…”

Section: Resultssupporting

confidence: 83%

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

Tarantini

Kametani

Balachandran

et al. 2021

Sci Rep

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In recent years there has been an increasing effort in improving the performance of Nb3Sn for high-field applications, in particular for the fabrication of conductors suitable for the realization of the Future Circular Collider (FCC) at CERN. This challenging task has led to the investigation of new routes to advance the high-field pinning properties, the irreversibility and the upper critical fields (HIrr and Hc2, respectively). The effect of hafnium addition to the standard Nb-4Ta alloy has been recently demonstrated to be particularly promising and, in this paper, we investigate the origins of the observed improvements of the superconducting properties. Electron microscopy, Extended X-ray Absorption Fine Structure Spectroscopy (EXAFS) and Atom Probe Tomography (APT) characterization clearly show that, in presence of oxygen, both fine Nb3Sn grains and HfO2 nanoparticles form. Although EXAFS is unable to detect significant amounts of Hf in the A15 structure, APT does indeed reveal some residual intragrain metallic Hf. To investigate the layer properties in more detail, we created a microbridge from a thin lamella extracted by Focused Ion Beam (FIB) and measured the transport properties of Ta-Hf-doped Nb3Sn. Hc2(0) is enhanced to 30.8 T by the introduction of Hf, ~ 1 T higher than those of only Ta-doped Nb3Sn, and, even more importantly the position of the pinning force maximum exceeds 6 T, against the typical ~ 4.5–4.7 T of the only Ta-doped material. These results show that the improvements generated by Hf addition can significantly enhance the high-field performance, bringing Nb3Sn closer to the requirements necessary for FCC realization.

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Section: Resultssupporting

confidence: 83%

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

Tarantini

Kametani

Balachandran

et al. 2021

Sci Rep

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“…It is seen that the magnetic and transport J c, non-Cu values are more or less on the same level. However, the B irr (4.2 K) extracted from the magnetic J c -B curve is about 21.8 T, while that from the transport result is about 24 T. A previous highfield M-µ 0 H loop measurement at 4.2 K by Tarantini et al [28] reported a B irr (4.2 K) of ∼22 T for a similar RRP ® strand. That study also pointed out the discrepancy between magnetic and transport B irr values.…”

Section: Resultsmentioning

confidence: 78%

Significant reduction in the low-field magnetization of Nb₃Sn superconducting strands using the internal oxidation APC approach

Sumption

Wan

et al. 2023

Supercond. Sci. Technol.

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Nb3Sn superconductors are promising for building accelerator magnets for future energy-frontier circular colliders. A critical factor for this application is the low-field persistent-current magnetization because it leads to several critical issues: e.g., low-field instability (including flux jumps), hysteresis loss, and field errors in magnet bores. Suppression of low-field magnetization requires reduction of low-field critical current density (Jc) or effective subelement size (deff). However, reduction of deff of state-of-the-art Nb3Sn conductors – the restacked-rod-process (RRP®) type – below 40-50 μm without a pronounced decrease in high-field Jc is difficult. On the other hand, the internal oxidation method which forms artificial pinning centers (APC) in Nb3Sn offers an alternative approach to reducing the low-field magnetization. Compared with a conventional Nb3Sn conductor whose flux pinning force versus field (Fp-B) curve peaks at ~20% of its irreversibility field (Birr), the Fp-B curve peaks of APC conductors shift to higher fields due to the point pinning effect, leading to flattening of the Jc-B curves. The goal of this paper is to quantitatively study how much the APC approach can reduce the low-field magnetization. We measured the Jc-B curves of an RRP® conductor and two APC conductors (reacted at 700°C) from zero field to Birr using a high-field vibrating sample magnetometer (VSM). The results showed that the APC conductors have higher non-Cu Jc at high fields (e.g., 32-41% higher at 16 T) and simultaneously lower non-Cu Jc at low fields (e.g., 28-34% lower at 1 T) compared with the RRP®. This effect is due to a competition between their Nb3Sn layer fraction ratios and layer Fp ratios. Suppose they reach the same 16 T non-Cu Jc, then the 1 T non-Cu Jc and magnetization of the APC conductors are only half or even less compared with the RRP® conductor.

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“…A study of the doping mechanism has indicated that the site displacement behavior in the Nb 3 Sn lattice differs between these elements. Extended x-ray absorption fine structure measurements revealed that Ti atoms are preferentially localized in Nb sites, whereas Ta atoms can sit in both Nb and Sn sites [19,20]. This phenomenon may account for the difference in the enhancement of B c2 when doping with these elements.…”

Section: Introductionmentioning

confidence: 94%

In-depth S/TEM observation of Ti–Hf and Ta–Hf-doped Nb₃Sn layers

Banno,

Moronaga,

Hara

et al. 2024

Supercond. Sci. Technol.

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In superconducting Nb3Sn layers with coherence lengths of approximately 3 nm, grain boundaries act as effective pinning sites. Thus, grain refinement is an essential issue that directly affects the superconducting critical characteristics of the Nb3Sn layer. In recent years, Hf addition to Nb3Sn wires co-doped with Ta has attracted notable interest as a method that enables grain refinement down to several tens of nm. In-depth characterization of the Nb3Sn grain morphology in Hf-doping is crucially important to correlate the microstructure with the flux pinning characteristics. In this article, the grain morphologies of Ti–Hf and Ta–Hf-doped Nb3Sn layers were clarified by scanning transmission electron microscopy (STEM) and TEM-based automated crystal orientation mapping (ACOM-TEM). STEM/energy dispersive X-ray spectroscopy (EDS) revealed no significant oxide precipitates in our samples. The grain size distribution was attained by ACOM-TEM. Although Hf-doping attained a grain refinement effect in the Nb3Sn layer in both doping cases, the degree of this effect was relatively small for Ti–Hf. Kernel average misorientation (KAM) analysis by scanning electron microscopy-electron backscatter diffraction (SEM-EBSD) unveiled no appreciable difference between the internal strain states of the Nb-alloy parent phases in Ti–Hf and Ta–Hf. One remarkable new finding through STEM/EDS was the presence of a Cu–Hf compound phase in the Nb3Sn layer. The Cu–Hf compound sounds analogous to the Cu–Ti compounds that form when Nb–47Ti with Cu matrix is heat treated. The STEM/EDS maps revealed a larger amount of Cu flow from the Cu-Sn side along the grain boundaries. The large Cu deposition on the grain boundaries might facilitate grain growth in Nb3Sn. Those findings make a novel contribution to the literature as they provide a deep insight into Nb3Sn phase formation via Hf doping.

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

Cited by 22 publications

References 36 publications

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

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

Significant reduction in the low-field magnetization of Nb₃Sn superconducting strands using the internal oxidation APC approach

In-depth S/TEM observation of Ti–Hf and Ta–Hf-doped Nb₃Sn layers

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

Cited by 22 publications

References 36 publications

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

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

Significant reduction in the low-field magnetization of Nb3Sn superconducting strands using the internal oxidation APC approach

In-depth S/TEM observation of Ti–Hf and Ta–Hf-doped Nb3Sn layers

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

Significant reduction in the low-field magnetization of Nb₃Sn superconducting strands using the internal oxidation APC approach

In-depth S/TEM observation of Ti–Hf and Ta–Hf-doped Nb₃Sn layers