Superconducting joints are one of the key components needed to make Ag-alloy clad Bi2Sr2CaCu2O8+x (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 I c 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 I c of the open-ended short conductor samples with identical 1 bar HT, the I c 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 5×10-12 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.

中文翻译：

---

Bi-2212/Ag-合金复丝圆线之间持久超导接头的开发

超导接头是使 Ag 合金包覆 Bi2Sr2CaCu2O8+x (Bi-2212) 超导圆线 (RW) 成功用于高场、高均匀性磁体应用，尤其是核磁共振 (NMR) 磁体所需的关键部件之一其中非常需要持续电流模式 (PCM) 操作。在这项研究中，开发了一种在线圈反应期间制造 Bi-2212 圆线之间的超导接头的程序。通过差热分析 (DTA) 研究了具有不同 Ag 添加量的 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。测试接头直径为 1.3 毫米。使用典型的部分熔化 Bi-2212 热处理 (HT) 在 1 bar 流动氧气中热处理的导线在 4.2 K 和自场下具有 ~900 A 的传输临界电流 I c，在 4.2 K 时以 0.1 μV/cm 评估，在 14 T 时降至 ~480 A。与具有相同 1 bar HT 的开端短导体样品的 I c 相比，超导接头的 I c 值小 ~20%比在平行场中测量的导体样品大，但比在垂直场中测量的导体样品大 20%。通过扫描电子显微镜 (SEM) 检测的微观结构清楚地表明在两条 Bi-2212 圆线之间形成了超导 Bi-2212 界面。此外，具有超导接头热处理的 Bi-2212 RW 闭环螺线管在 1 bar 流动氧气中进行了热处理，根据其场衰减率，估计接头电阻低于 5×10-12 Ω。该值足够低，足以证明大电感 Bi-2212 线圈持续运行的潜力。