Influence of the Heat Treatment on the Layer JC of Internal-Sn Nb3Sn Wires With Internally Oxidized Nanoparticles

• Published in: IEEE transactions on applied superconductivity 2024-08, Vol.34 (5), p.1-5
• Main Authors: Lonardo, F., Bovone, G., Buta, F., Bonura, M., Bagni, T., Medina-Clavijo, B., Ballarino, A., Hopkins, S. C., Boutboul, T., Senatore, C.
• Format: Article
• Language: English
• Subjects: Critical current density; Crystal defects; Flux pinning; Force; Grain size; Hafnium; Heat treating; Heat treatment; internal oxidation; Magnetic measurement; Metals; Nanoparticles; Nb<named-content xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:ali="http://www.niso.org/schemas/ali/1.0/" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" content-type="math" xlink:type="simple"> <inline-formula> <tex-math notation="LaTeX"> _3</tex-math> </inline-formula> </named-content>Sn; Niobium base alloys; Oxidation; Physics; pinning force; Point defects; Precipitates; superconducting wire; Thickness; Thin films; Tin dioxide; Wires; Zirconium
• ISSN: 1051-8223; 1558-2515
• DOI: 10.1109/TASC.2024.3355353

We evaluated various heat treatments (HT) for maximizing the Nb 3 Sn layer thickness while retaining a refined grain microstructure in low filament count internal-Sn Nb 3 Sn Rod-In-Tube wires with internally oxidized nanoparticles. These wires were manufactured in our laboratory using SnO 2 as oxygen source and Nb alloys containing Ta and Zr or Hf. By reacting the wires at 650 °C for 200 hours we obtained relatively thin reaction layers but high layer critical current densities (layer J C ) of ∼3000 A/mm 2 for Hf-containing wires and ∼2700 A/mm 2 for Zr-containing wires, both at 4.2 K and 16 T. Notably, both of these values are over the layer J C threshold of 2500 A/mm 2 , which is estimated to be necessary for attaining the corresponding Future Circular Collider (FCC) target non-Cu J C of 1500 A/mm 2 . Following this heat treatment, the fine-grained Nb 3 Sn area occupies only ∼35% of the filament area for Hf-containing wires and ∼20% for Zr-containing wires. After heat treatments with a reaction step at 700 °C these values increase to 70-80% and ∼60%, respectively, with only a minor increase of the grain size. However, we observed a noticeable decrease in the layer J C for these HT. Magnetic measurements show that the high J C wires exhibit a point defect contribution from precipitates to the pinning force, which is missing in wires with depressed J C values. The higher heat treatment temperatures may have caused excessive coarsening of the oxide precipitates, to sizes unsuitable for flux pinning. Reaction heat treatment temperatures in the range of 650 °C to 700 °C and durations between 50 and 200 hours may provide a better compromise between the Nb 3 Sn layer thickness, its grain size and nanoparticle size.