Very high upper critical fields and enhanced critical current densities in Nb3Sn superconductors based on Nb–Ta–Zr alloys and internal oxidation

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%.

在二氧化锆（ZrO₂）纳米颗粒存在下抑制铌三锡（Nb₃Sn）晶粒生长，是将Nb₃Sn超导导线的临界电流密度（critical current density）提升至满足未来环形对撞机（Future Circular Collider）设定要求的最具前景的途径之一。本研究针对由Nb-1 wt%Zr、Nb-7.5 wt%Ta、Nb-7.5 wt%Ta-1 wt%Zr及Nb-7.5 wt%Ta-2 wt%Zr合金制备的Nb₃Sn，探究了通过锆（Zr）内部氧化生成的ZrO₂纳米颗粒对其超导性能与微观组织的影响。本研究采用单股导线构型（monofilamentary wire configuration）：外径0.22 mm的Nb合金管内部填充粉末状金属氧化物（SnO₂、CuO或MoO₃）作为氧源，管外表面依次沉积铜（Cu）、锡（Sn）与铜层。经电感测量（inductive measurements）验证，采用内部氧化Zr的Nb合金制备的样品，其Nb₃Sn层的临界电流密度优于以Nb-7.5 wt%Ta为原料的对照样品。采用Nb-7.5 wt%Ta-1 wt%Zr与Nb-7.5 wt%Ta-2 wt%Zr制备的样品，在10~15 T以上的高磁场（high magnetic fields）下临界电流密度更高，且在4.2 K温度下的上临界场（upper critical field）超过28.5 T（以99%正常态电阻率为判定准则）。采用Nb-7.5 wt%Ta-2 wt%Zr制备的样品，在4.2 K下获得了29.2 T的上临界场纪录值。本研究提出并探讨了若干假说，以解释上临界场的这一异常提升：分别考量未氧化Zr对Nb₃Sn超导性能的潜在影响，以及氧化Zr对Nb₃Sn形成过程与微观化学组成的作用。无论样品类型如何，本研究观测到的Nb₃Sn晶粒长径比（aspect ratio）均为1.5~1.7。沿短轴方向对比时，采用内部氧化Zr的样品，其晶界截距（grain boundary intercepts）的平均距离（采用线截距法（lineal intercept method）测定）较以Nb-7.5 wt%Ta为原料的对照样品小约40%；沿长轴方向，该平均距离的降幅为20%~30%。