Abstract
The inhibition of Nb3Sn grain growth in the presence of ZrO2 nanoparticles appears to be one of the most promising method for pushing the critical current densities of Nb3Sn superconducting wires to levels that meet the requirements set for the Future Circular Collider. We have investigated the effect of ZrO2 nanoparticles formed by the internal oxidation of Zr on the superconducting properties and microstructure of Nb3Sn formed from Nb-1 wt%Zr, Nb-7.5 wt%Ta, Nb-7.5 wt%Ta-1 wt%Zr and Nb-7.5 wt%Ta-2 wt%Zr alloys. A monofilamentary wire configuration was used, with a 0.22 mm outer diameter Nb-alloy tube containing a core of powdered metal oxide (SnO2, CuO or MoO3) as oxygen source and successive deposits of Cu, Sn and Cu on the outer surface. As determined from inductive measurements, the layer critical current densities of the samples based on Nb alloys with internally oxidized Zr were superior to those based on Nb-7.5 wt%Ta. The samples based on Nb-7.5 wt%Ta-1 wt%Zr and Nb-7.5 wt%Ta-2 wt%Zr showed higher critical current densities at high magnetic fields (above 10–15 T), and upper critical fields exceeding 28.5 T at 4.2 K (99% normal state resistivity criterion). A record value of 29.2 T of the upper critical field at 4.2 K was obtained on samples based on Nb-7.5 wt%Ta-2 wt%Zr. Hypotheses are proposed and discussed for explaining this unexpected increase of the upper critical field, by considering the possible effects of non-oxidized Zr on the superconducting properties of Nb3Sn and of the oxidized Zr on the formation and microchemistry of Nb3Sn. Regardless of sample type the Nb3Sn grains observed in our samples have an aspect ratio of 1.5–1.7. When compared in the short axis direction, the mean distance between grain boundary intercepts (lineal intercept method) is ∼40% smaller in the samples with internally oxidized Zr than in the reference samples based on Nb-7.5 wt%Ta. In the long axis direction the reduction is of 20%–30%.
Highlights
The generation of high magnetic fields is one of the most important applications of superconductivity, from the point of view of the mass of material that is used and because of the huge impact on energy consumption and how this enables applications that would otherwise be unfeasible or prohibitive in terms of cost
Based on these critical current density determinations, our samples did not show a positive effect of MoO3 as an oxygen source in Nb-1Zr, the values obtained being comparable with those obtained on the reference wires without Zr
When used as an oxygen source for the internal oxidation of Zr in samples based on Nb-1Zr, CuO was found to lead to critical current densities slightly lower than those of similar samples having SnO2 as oxygen source, as seen in figure 2
Summary
The generation of high magnetic fields is one of the most important applications of superconductivity, from the point of view of the mass of material that is used and because of the huge impact on energy consumption and how this enables applications that would otherwise be unfeasible or prohibitive in terms of cost. The dipole and quadrupole electromagnets used to guide and focus the beams of charged particles in high-energy physics accelerators like the Large Hadron Collider (LHC) of CERN [1] constitute one such example. These applications are the driving force for research and development in the field of applied superconductivity. The work reported here is motivated by this need to further optimize the properties of Nb3Sn wires in view of achieving and exceeding the requirements of the FCC study
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