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- Bagni, Tommaso, et al.
(author)
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Investigating the effect of rolling deformation on the electro-mechanical limits of Nb3Sn wires produced by RRP® and PIT technologies
- 2024
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In: Superconductors Science and Technology. - : Institute of Physics (IOP). - 0953-2048 .- 1361-6668. ; 37:9
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Journal article (peer-reviewed)abstract
- Future high-field magnets for particle accelerators hinge on the crucial development of advanced Nb3Sn wires engineered to withstand the large stresses generated during magnet assembly and operation. The superconducting properties of Nb3Sn enable the design of compact accelerator-quality magnets above 10 T, but at the same time the brittleness and strain sensitivity of the material impose careful consideration of the mechanical limits. In addition, accelerator magnets are wound using Rutherford cables and the cabling process generates deformations in the wire that can affect its electro-mechanical performance. This paper reports on the impact of the rolling deformation on the transverse stress tolerance of high-performance restacked-rod-process (RRP (R)) and powder-in-tube (PIT) Nb3Sn wires. Rolling deformation was used to mimic the effect of cabling on the wire shape. Deformed samples were compared to reference round wires in term of stress dependence and irreversible limit (sigma(irr)) of the critical current (I-c) under transverse compressive loads up to 240 MPa. Experiments were performed at 4.2 K, 19 T, on resin-impregnated single wires that imitate the operating conditions in a Rutherford cable of an accelerator magnet. The results show that rolling deformation has a detrimental effect on the initial I-c of PIT wires, but it does not influence the behavior of the wire under stresses above 70 MPa. On the other hand, the deformation of RRP (R) wires leads to an improved sigma(irr) without affecting the initial I-c. Additionally, a 2D-mechanical finite element method model of the RRP (R) wire was developed to investigate the impact of the wire geometry on the plastic deformation of the copper matrix, which induces residual stresses on Nb3Sn and is the main cause for the permanent reduction of I-c. Based on the model results, an alternative layout of the wire was proposed that improves its stress tolerance without affecting its electrical transport properties.
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| 3. |
- Lonardo, F., et al.
(author)
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Influence of the Heat Treatment on the Layer JC of Internal-Sn Nb3Sn Wires With Internally Oxidized Nanoparticles
- 2024
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In: IEEE transactions on applied superconductivity (Print). - : Institute of Electrical and Electronics Engineers (IEEE). - 1051-8223 .- 1558-2515. ; 34:5
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Journal article (peer-reviewed)abstract
- 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.
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