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

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.