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Abstract
| We evaluated various heat treatments (HT) for maximizing the Nb3Sn layer thickness while retaining a refined grain microstructure in low filament count internal-Sn Nb3Sn Rod-In-Tube wires with internally oxidized nanoparticles. These wires were manufactured in our laboratory using SnO2as 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 JC) of ∼3000 A/mm2for Hf-containing wires and ∼2700 A/mm2for Zr-containing wires, both at 4.2 K and 16 T. Notably, both of these values are over the layer JCthreshold of 2500 A/mm2, which is estimated to be necessary for attaining the corresponding Future Circular Collider (FCC) target non-Cu JCof 1500 A/mm2. Following this heat treatment, the fine-grained Nb3Sn 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 JCfor these HT. Magnetic measurements show that the high JCwires exhibit a point defect contribution from precipitates to the pinning force, which is missing in wires with depressed JCvalues. 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 Nb3Sn layer thickness, its grain size and nanoparticle size. |