Speaker
Description
We introduced internal oxidation in Rod-in-Tube (RIT) Nb3Sn subelements, used in simplified restacked wire, by using SnO2 powders as an oxygen source (OS). During Nb3Sn synthesis, the OS decomposed, resulting in a grain size below 60 nm. This grain refinement is consistent with an increase of the critical current density (Jc) towards the performance target of 1500 A/mm² at 16 T and 4.2 K for the dipole magnets of the Future Circular Collider (FCC).
Internal oxidation relies on the combined effects of the oxygen source (OS) and a high oxygen-affinity element, such as Hf or Zr, alloyed with Nb. This process inhibits Nb3Sn grain growth and enhances Jc through the formation of nano-oxide precipitates. While internal oxidation is typically implemented in Powder-In-Tube (PIT) wires, where incorporating an OS is less challenging, RIT wires—for example those produced via the Rod-Restacked Process (RRP)—are preferred for accelerator-magnet applications due to their superior mechanical properties and reduced Ic degradation during Rutherford cabling.
A key challenge is that the RIT process does not use powders, and it involves high-temperature treatments (e.g., hot extrusion), which risks premature OS decomposition that could harden the Nb alloy. In our approach, each Nb-alloy filament was assembled with the OS, then deformed and restacked into subelements. The filament assembly procedure was specifically conceived to prevent OS decomposition during hot extrusion.
This achievement represents a promising step toward the commercial-scale introduction of internal oxidation in RIT wires. While further optimization of wire design is needed (e.g., reducing Cu excess and increasing filament and subelement count), this work marks a progress in meeting the stringent performance requirements for next-generation high-field magnets.
