Chen Peng, Trociewitz Ulf P, Davis Daniel S, Bosque Ernesto, Hilton David, Kim Youngjae, Abraimov Dmytro, Starch William, Jiang Jianyi, Hellstrom Eric E, Larbalestier David C
Applied Superconductivity Center, National High Magnetic Field Laboratory, Florida State University, 2031 East Paul Dirac Drive, Tallahassee, FL 32310, USA.
Department of Mechanical Engineering, FAMU-FSU College of Engineering, Florida State University, 2525 Pottsdamer Street, Tallahassee, FL 32310, USA.
Supercond Sci Technol. 2017 Feb;30(2):025020. doi: 10.1088/1361-6668/30/2/025020. Epub 2016 Dec 29.
Superconducting joints are one of the key components needed to make Ag-alloy clad BiSrCaCuO (Bi-2212) superconducting round wire (RW) successful for high-field, high-homogeneity magnet applications, especially for nuclear magnetic resonance (NMR) magnets in which persistent current mode (PCM) operation is highly desired. In this study, a procedure for fabricating superconducting joints between Bi-2212 round wires during coil reaction was developed. Melting temperatures of Bi-2212 powder with different amounts of Ag addition were investigated by differential thermal analysis (DTA) so as to provide information for selecting the proper joint matrix. Test joints of 1.3 mm dia. wires heat treated in 1 bar flowing oxygen using the typical partial melt Bi-2212 heat treatment (HT) had transport critical currents of ~900 A at 4.2 K and self-field, decreasing to ~480 A at 14 T evaluated at 0.1 μV/cm at 4.2 K. Compared to the of the open-ended short conductor samples with identical 1 bar HT, the values of the superconducting joint are ~20% smaller than that of conductor samples measured in parallel field but ~20% larger than conductor samples measured in perpendicular field. Microstructures examined by scanning electron microscopy (SEM) clearly showed the formation of a superconducting Bi-2212 interface between the two Bi-2212 round wires. Furthermore, a Bi-2212 RW closed-loop solenoid with a superconducting joint heat treated in 1 bar flowing oxygen showed an estimated joint resistance below 5×10 Ω based on its field decay rate. This value is sufficiently low to demonstrate the potential for persistent operation of large inductance Bi-2212 coils.
超导接头是使银合金包覆的铋锶钙铜氧（Bi-2212）超导圆线（RW）成功应用于高场、高均匀性磁体的关键部件之一，特别是对于极需持续电流模式（PCM）运行的核磁共振（NMR）磁体。在本研究中，开发了一种在绕线反应过程中制造Bi-2212圆线之间超导接头的工艺。通过差示热分析（DTA）研究了添加不同量银的Bi-2212粉末的熔化温度，以便为选择合适的接头基体提供信息。使用典型的部分熔化Bi-2212热处理（HT）在1巴流动氧气中对直径1.3毫米的线进行测试接头热处理，在4.2 K和自场下的传输临界电流约为900 A，在14 T、4.2 K下以0.1 μV/cm评估时降至约480 A。与具有相同1巴HT的开口端短导体样品相比，超导接头的临界电流值比在平行场中测量的导体样品小约20%，但比在垂直场中测量的导体样品大约20%。通过扫描电子显微镜（SEM）检查的微观结构清楚地显示了两根Bi-2212圆线之间形成了超导Bi-2212界面。此外，一个带有在1巴流动氧气中热处理的超导接头的Bi-2212 RW闭环螺线管，根据其场衰减率估计接头电阻低于5×10Ω。该值足够低，以证明大电感Bi-2212线圈持续运行的潜力。
