Niobium–tin

History

Nb3Sn was discovered to be a superconductor in 1954, one year after the discovery of V3Si, the first example of an A3B superconductor. In 1961 it was discovered that niobium–tin still exhibits superconductivity at large currents and strong magnetic fields, thus becoming the first known material to support the high currents and fields necessary for making useful high-power magnets and electric power machinery.

Notable uses

The central solenoid and toroidal field superconducting magnets for the planned experimental ITER fusion reactor use niobium–tin as a superconductor.{{cite web

At the Large Hadron Collider at CERN, extra-strong quadrupole magnets (for focussing beams) made with niobium–tin are being installed in key points of the accelerator between late 2018 and early 2020.{{cite journal | author-link = Lucio Rossi

Composite wire

Mechanically, Nb3Sn is extremely brittle and thus cannot be easily drawn into a wire, which is necessary for winding superconducting magnets. To overcome this, wire manufacturers typically draw down composite wires containing ductile precursors. The "internal tin" process includes separate alloys of Nb, Cu and Sn. The "bronze" process contains Nb in a copper–tin bronze matrix. With both processes the strand is typically drawn to final size and coiled into a solenoid or cable before heat treatment. It is only during heat treatment that the Sn reacts with the Nb to form the brittle, superconducting niobium–tin compound. The powder-in-tube process is also used.{{cite journal | url-access = subscription

The high field section of modern NMR magnets are composed of niobium–tin wire.

Strain effects

Inside a magnet the wires are subjected to high Lorentz forces as well as thermal stresses during cooling. Any strain in the niobium tin causes a decrease in the superconducting performance of the material, and can cause the brittle material to fracture. Because of this, the wires need to be as stiff as possible. The Young's modulus of niobium tin is around 140 GPa (gigapascals) at room temperature. However, the stiffness drops to as low as 50 GPa when the material is cooled below 50 K.{{cite book |editor1-first= A. F |editor1-last= Clark |editor2-first= R. P |editor2-last= Reed |editor1-first= A. F |editor1-last= Clark |editor2-first= R. P |editor2-last= Reed

:\varepsilon {m}=\frac{V{c}E_{c}{ \frac{\Delta L}{L_c}-\frac{\Delta L}{L_f} }-\sigma_{cu,y}V_{cu}-\sigma_{bz,y}V_{bz}}{V_fE_f+V_cE_c}. where εm is the pre-strain, ΔL/Lc and ΔL/Lf are changes in length due to thermal expansion of the niobium tin conduit and strengthening fiber respectively; Vc, Vf, Vcu, and Vbz are the volume fractions of conduit, fiber, copper, and bronze; σcu,y, and σbz,y are the yield stresses of copper and bronze; and Ec, and Ef are the Young's modulus of the conduit and the fiber.{{cite book |editor1-first= A. F |editor1-last= Clark |editor2-first= R. P |editor2-last= Reed :\varepsilon c=\varepsilon{co}{ 1-\frac{B}{B_{c2m}} }. where εc is the critical strain, εco is a material dependent parameter equal to 1.5% in tension (−1.8% in compression) for niobium tin, B is the applied magnetic field, and Bc2m is the maximum upper critical field of the material.{{cite book |editor1-first= A. F |editor1-last= Clark |editor2-first= R. P |editor2-last= Reed

Developments and future uses

Hafnium or zirconium added to niobium–tin increases the maximum current density in a magnetic field. This may allow it to be used at 16 tesla for CERN's planned Future Circular Collider.

References

References

1. (1954). "Superconductivity of Nb₃Sn". [[Physical Review]].
2. Geballe, Theodore H.. (1993). "Superconductivity: From Physics to Technology". [[Physics Today]].
3. Godeke, A.. (2006). "A review of the properties of Nb3Sn and their variation with A15 composition, morphology and strain state". [[Supercond. Sci. Technol.]].
4. (2005). "A success story: LHC cable production at ALSTOM-MSA". Fusion Engineering and Design (Proceedings of the 23rd Symposium of Fusion Technology).
5. (October 2022). "Survey Of High Field Superconducting Material For Accelerator Magnets".
6. [https://news.fsu.edu/news/science-technology/2020/07/17/maglab-awarded-1-5m-by-u-s-department-of-energy-to-develop-better-superconductors/ ''MagLab awarded $1.5M by U.S. Department of Energy to develop better superconductors'' July 2020]