The increasing demand for improving the high-field (16-22 T) performance of Nb$_3$Sn conductors requires a better understanding of the properties of modern wires much closer to irreversibility field, H$_{Irr}$. In this study we investigated the impact of Ta, Ti and Hf doping on the high-field pinning properties, the upper critical field, H$_{c2}$, and H$_{Irr}$. We found that the pinning force curves of commercial Ti and Ta doped wires at different temperatures do not scale and that the Kramer extrapolation, typically used by magnet designers to estimate high-field critical current density and magnet operational margins from lower field data, is not reliable and significantly overestimates the actual H$_{Irr}$. In contrast, new laboratory scale conductors made with Nb-Ta-Hf alloy have improved high-field J$_c$ performance and, despite contributions by both grain boundary and point defect pinning mechanisms, have more predictable high-field behavior. Using Extended X-ray Absorption Fine Structure spectroscopy, EXAFS, we found that for the commercial Ta and Ti doped conductors, the Ta site occupancy in the A15 structure gradually changes with the heat treatment temperature whereas Ti is always located on the Nb site with clear consequences for H$_{c2}$. This work reveals the still limited understanding of what determines H$_{c2}$, H$_{Irr}$ and the high-field J$_c$ performance of Nb$_3$Sn and the complexity of optimizing these conductors so that they can reach their full potential for high-field applications.

Ta, Ti and Hf effects on Nb$_3$Sn high-field performance: temperature-dependent dopant occupancy and failure of Kramer extrapolation

To meet critical current density, J c , targets for the Future Circular Collider (FCC), the planned replacement for the Large Hadron Collider (LHC), the high field performance of Nb3Sn must be improved, but champion J c values have remained static for the last 10 years. Making the A15 phase stoichiometric and enhancing the upper critical field H c2 by Ti or Ta dopants are the standard strategies for enhancing high field performance but detailed recent studies show that even the best modern wires have broad composition ranges. To assess whether further improvement might be possible, we employed Extended X-ray Absorption Fine Structure (EXAFS) to determine the lattice site location of dopants in modern high-performance Nb3Sn strands with J c values amongst the best so far achieved. Although Ti and Ta primarily occupy the Nb sites in the A15 structure, we also find significant Ta occupancy on the Sn site. These findings indicate that the best performing Ti-doped stand is strongly sub-stoichiometric in Sn and that antisite disorder likely explains its high average H c2 behavior. These new results suggest an important role for dopant and antisite disorder in minimizing superconducting property distributions and maximizing high field J c properties.

/pdf/evidence-from-exafs-for-different-ta-ti-site-occupancy-in-3ih03yrztx.pdf

Evidence from EXAFS for Different Ta/Ti Site Occupancy in High Critical Current Density Nb 3 Sn Superconductor Wires

To meet critical current density, J$_c$, targets for the Future Circular Collider (FCC), the planned replacement for the Large Hadron Collider (LHC), the high field performance of Nb$_3$Sn must be improved, but champion J$_c$ values have remained static for the last 10 years. Making the A15 phase stoichiometric and enhancing the upper critical field H$_{c2}$ by Ti or Ta dopants are the standard strategies for enhancing high field performance but detailed recent studies show that even the best modern wires have broad composition ranges. To assess whether further improvement might be possible, we employed EXAFS to determine the lattice site location of dopants in modern high-performance Nb$_3$Sn strands with J$_c$ values amongst the best so far achieved. Although Ti and Ta primarily occupy the Nb sites in the A15 structure, we also find significant Ta occupancy on the Sn site. These findings indicate that the best performing Ti-doped stand is strongly sub-stoichiometric in Sn and that antisite disorder likely explains its high average H$_{c2}$ behavior. These new results suggest an important role for dopant and antisite disorder in minimizing superconducting property distributions and maximizing high field J$_c$ properties.

/pdf/evidence-from-exafs-for-different-ta-ti-site-occupancy-in-w8css61kka.pdf

Evidence from EXAFS for Different Ta/Ti Site Occupancy in High Critical Current Density Nb$_3$Sn Superconductor Wires

https://iopscience.iop.org/article/10.1088/1361-6668/ab4d9e/pdf

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

X-ray absorption techniques have in recent years been developed into powerful probes of the electronic and structural properties of materials difficult to study by other techniques In particular, the extended x-ray absorption fine structure (EXAFS) technique can be applied to a variety of cryogenic materials Three examples will be used to demonstrate the power of the technique The first is the determination of the lattice location of dilute alloying additions such as Ta and Zr in Nb3Sn The Ta additions are shown to reside predominately in Nb lattice sites, while Zr is not uniquely located at either Nb or Sn sites In addition to structural information, temperature dependent EXAFS studies can be used to determine the rms deviations of atomic bond lengths, providing information about the temperature dependence of interatomic force constants For Nb3Sn deviations are found from simple harmonic behavior at low temperatures which indicate a softening of the Nb-Sn bond strength The final example is the study of interfacial properties in thin film systems This is accomplished by making x-ray absorption measurements under conditions of total external reflection of the incident x-rays As some examples will show this technique has great potential for studying interfacial reactions, a process used in the fabrication of many superconducting materials

S. M. Heald

Papers

The Characterization of Cryogenic Materials by X-Ray Absorption Methods