K. Tachikawa

T. Mito

National Institute for Fusion Science (NIFS-Repository)

Development of New High Field and High Current Density Superconductors for Fusion Devises Tachikawa, K. (Faculty of Engr., Tokai Univ.), Mito, T.      Nb3Sn superconductors with improved high-field performance have been prepared by the Jelly Roll (JR) process using Sn-Ta based sheet.1) In this study, new JR-processed Nb3Sn wires with Sn-B based sheet have been successfully fabricated. Then the structure and superconducting properties of Sn-Ta and Sn-B based sheet wires have been compared.      Sn-B based alloys with Sn/B atomic ratio of 4/1-8/1 were prepared by a reaction among constituent metal powders at 750?. A small amount of Ti and Cu were added to the alloys. A tightly consolidated button was obtained, which was pressed to a plate, and then rolled into a sheet 80?m in thickness. The sheet was laminated with a Nb sheet 100?m in thickness, and wound into a JR composite. The composite was encased in a Nb-3.3at%Ta sheath, and drawn into a wire 1.4? in diameter. Finally the wires were heat treated at 725?~770? in vacuum.      The Ti in the alloy has a strong affinity against both B and Sn, and accumulates around B particles, which effectively improves the bonding between B particles and the Sn matrix. The Cu in the alloy decreases the reaction temperature to form Nb3Sn layer from 900? to 750?.Thick Nb3Sn layers with nearly stoichiometric Sn concentration are synthesized in the wire through the diffusion between the Sn-B based sheet and the Nb sheet. The Nb3Sn layer is composed of nearly uniform and equiaxed grains. The Nb3Sn layers formed in the JR wires using Sn-Ta and Sn-B based sheet show almost the same microstructure.      Fig. 1 summarizes the Tc transitions of JR wires and a commercial bronze (16wt%Sn+0.3wt%Ti) processed wire. The Tc transition of the bronze processed wire is onset 17.2K and offset 16.8K. The JR wires show offset Tc of ~18.1K with a transition width of less than 0.1K. Thus the JR wires indicate much higher and sharper Tc transition than the bronze processed wire. This may be attributed to the stoichiometric and uniform Sn concentration in the Nb3Snlayers formed in the JR wires. The Sn-B based sheet wires show slightly higher Tc compared to the Sn-Ta based sheet wires.     Fig. 2 illustrates Bc2 transitions of Sn-Ta and Sn-B based sheet JR wires at 4.2K. Both wires show nearly the same Bc2 transition of onset 27.2T, midpoint 26.8T and offset 26.4T. Tc and Bc2 (4.2K) values of JR processed wires are at the higher end of those in Nb3Sn wires. The workability of the Sn-B based alloy is even better than that of the Sn-Ta based alloy. Present study has revealed that Sn-B based alloy is a promising new material for Nb3Sn wire fabrication through the JR process. Fig.1  Tc transitions of JR wires with quoted sheet composition. The transition of a bronze processed wire is also indicated. Fig.2  Bc2 (4.2K) transitions of Sn-B and Sn-Ta based sheet JR wires. 1) Tachikawa, K. Tsuyuki, T. Hayashi, Y. Nakata, K. and Takeuchi, T.: Adv. Cryogenic Engineering (Material), Vol 54(2008)pp244-251 Sheet Composition: JR-1  8/1(Sn/B)-6at%Ti+3wt%Cu : JR-2  4/1(Sn/Ta)-3at%Ti+3wt%Cu Voltage for JR Wires (?V)Magnetic Field (T)4.2K Current : 0.3AJR-1JR-2Voltage for JR Wires (?V)? JR4/1(Sn/Ta)-4Ti+3Cu sheet? JR4/1(Sn/B)-4Ti+3Cu sheet? JR4/1(Sn/B)-4Ti+3Cu sheet? JR8/1(Sn/B)-4Ti+3Cu sheet? JR8/1(Sn/B)-4Ti+3Cu sheet? Bronze (16Sn-0.3Ti) Cu ratio 0.3114§2. Dev lop t f  igh Field and High Current Densit  erconductors for Fusion iseschikawa, K. (Faculty of Engr., Toka  Univ.),  ito, T.

Development of New High Field and High Current Density Superconductors for Fusion Devises

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