Internal oxidation for the fabrication of multifilamentary Nb3Sn wire with high-Jc

To enhance the critical current density (J_c) to meet the requirements of the Future Circular Collider (FCC-hh), we implemented internal oxidation in multifilamentary Rod-In-Tube wires. Internal oxidation involves alloying Nb with highly electropositive elements (Hf or Zr) and introducing an oxygen source (OS), typically SnO₂ powder. This process promotes the precipitation of oxide nanoparticles (HfO₂ or ZrO₂), which act as artificial pinning centers (APCs) and refine the Nb₃Sn grain structure, thereby increasing grain boundary density.

For wires reacted at 650 °C for 200 h, the combined presence of Hf and OS reduced the Nb₃Sn grain size to below 50 nm and increased the layer J_c to 3700 A/mm² at 15 T and 4.2 K. In contrast, wires fabricated from the same precursor alloys but without OS exhibited grain sizes exceeding 100 nm and a layer J_c below 2100 A/mm² under the same conditions.

The APCs enhance flux pinning through a point-defect mechanism, as evidenced by a shift of the pinning force peak to 0.3 of the reduced field (b, defined as the applied field divided by the upper critical field). Transmission electron microscopy (TEM) shows that HfO₂ forms smaller APCs than ZrO₂ (approximately 4 nm and 15 nm, respectively, after reaction at 650 °C for 200 h). In addition to average particle size, the size distribution of APCs is critical to ensure effective interaction with the flux-line lattice.

Increasing the heat-treatment temperature modifies the APCs size distribution, reducing their pinning efficiency, leading to a decrease in layer J_c. This occurs even though the Nb₃Sn grain size remains below 70 nm, highlighting the dominant role of APCs in J_c enhancement.

Overall, these results demonstrate that incorporating an internal oxidation in industrial multifilamentary Nb₃Sn wires is essential to achieve significant J_c improvement through a modified pinning mechanism.