Heat Treatment Optimization on Nb Sn Strands Based on Electrical and Physical Properties

• Published in: IEEE transactions on applied superconductivity Vol. 32; no. 6; pp. 1 - 4
• Main Authors: Dematte, F., Bruzzone, P., Sarasola, X., Pfeiffer, S., Castro, E. Rodriguez, De Marzi, G., Muzzi, L.
• Format: Journal Article
• Language: English
• Published: New York IEEE 01.09.2022; The Institute of Electrical and Electronics Engineers, Inc. (IEEE)
• Subjects: Cables; Conductors; Core loss; Critical current density; Critical current measurement; Current measurement; Diameters; Field coils; Filaments; Grain size; Heat treating; Heat treatment; Magnetic field measurement; Magnetic hysteresis; Nb<inline-formula xmlns:ali="http://www.niso.org/schemas/ali/1.0/" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"> <tex-math notation="LaTeX"> _{3}</tex-math> </inline-formula>Sn wire; Niobium; Optimization; Physical properties; Prototypes; Scaling laws; Strands; Superconducting magnets; Temperature measurement; Tin
• ISSN: 1051-8223; 1558-2515
• DOI: 10.1109/TASC.2022.3159312

The electrical and physical properties of Nb<inline-formula><tex-math notation="LaTeX">_{3}</tex-math></inline-formula>Sn strands are strongly dependent on the heat treatment during which tin diffuses into niobium by solid-state diffusion. During diffusion, Nb<inline-formula><tex-math notation="LaTeX">_{3}</tex-math></inline-formula>Sn grains grow at the Nb/bronze interface. The shape and size of the grain depend on the temperature of the last step of the heat treatment, its duration and the size of the Nb filaments. The volume of reacted Nb<inline-formula><tex-math notation="LaTeX">_{3}</tex-math></inline-formula>Sn together with the grains' structure influence the non-copper critical current density J<inline-formula><tex-math notation="LaTeX">_{c}</tex-math></inline-formula> and the magnetization. Therefore, an optimization of the heat treatment with respect to J<inline-formula><tex-math notation="LaTeX">_{c}</tex-math></inline-formula> and hysteresis loss is important when working on the design of superconducting cables. This contribution presents the results of a heat treatment optimization performed on a 1<inline-formula><tex-math notation="LaTeX">\,</tex-math></inline-formula>mm diameter, internal Sn Nb<inline-formula><tex-math notation="LaTeX">_{3}</tex-math></inline-formula>Sn strands produced by Kiswire Advanced Technology (KAT) for two React&Wind conductor prototypes for the Toroidal Field Coil of the EUROfusion DEMO: a <inline-formula><tex-math notation="LaTeX">\mathbf {66}\,</tex-math></inline-formula>kA/<inline-formula><tex-math notation="LaTeX">\mathbf {12}\,</tex-math></inline-formula>T and a <inline-formula><tex-math notation="LaTeX">\mathbf {105}\,</tex-math></inline-formula>kA/<inline-formula><tex-math notation="LaTeX">\mathbf {12}\,</tex-math></inline-formula>T prototype. For the optimization, four heat treatment schemes were considered and their evaluation based on I<inline-formula><tex-math notation="LaTeX">_{c}</tex-math></inline-formula> measurements at 4.2<inline-formula><tex-math notation="LaTeX">\,</tex-math></inline-formula>K, in the range of 9<inline-formula><tex-math notation="LaTeX">\,</tex-math></inline-formula>T to 15<inline-formula><tex-math notation="LaTeX">\,</tex-math></inline-formula>T, on SEM micrographic studies on grain size and shape and on hysteresis loss measurements on a vibrating sample magnetometer (VSM). Based on these results, a heat treatment schedule is proposed for the prototype DEMO conductor and the scaling law for J<inline-formula><tex-math notation="LaTeX">_{c}</tex-math></inline-formula> is updated.