新型铁基超导带材的连接技术与机理研究

So far, the critical current density of new iron-based superconductor is around 10e5A/cm2 (4.2 K, 10 T) for Sr(Ba)0.6K0.4Fe2As2 (Sr(Ba)-122) tapes, which achieves the level desired for practical applications. Long Sr(Ba)-122 tapes and their practical usages are expected in the near future. However, in large scale usages, it is very common to connect several pieces of long superconductive tapes. Furthermore, in nuclear magnetic resonance (NMR) and many scientific magnet systems, persistent current (PC) working is important for achieving high stability of the magnetic field; this also requires very low joint resistance. Hence, making superconducting joints is very important in large scale usages for the merits of lowering the total heating generation and PC operation. However, in the case of iron-based superconductor, Sr(Ba)-122 filaments are brittle and low-reactivity, thus direct connection of two filaments are difficult. It is to be mentioned here that there were no reports on the method of joining of Sr(Ba)-122 tapes or wires. This project intends to apply cold pressing welding, soldering, diffusion welding and hot pressing welding to prepare the superconducting joints for Sr(Ba)-122 tapes. These jointing methods were proposed following the systematic study on its synthesizing, micro-structures and grain boundaries at the jointing area. The relationships between the electromagnetic properties, mechanical properties and microstructures were systematically studied as functions of the pressing and heating conditions. Using the optimized joints preparation technologies, the short samples, one-turn close circuit and test coils will be fabricated and investigated for Sr(Ba)-122 tapes. Finally, the developed jointing techniques of Sr(Ba)-122 tapes will be well established and can satisfy the need of the persistent current operational superconducting magnets.

自新型铁基超导体发现以来，Sr(Ba)0.6K0.4Fe2As2型超导带材的临界传输电流已达到了实用化水平，其相应的超导长线研制工作也已经开展。但是，在超导电工应用中，一方面需要通过连接多根超导带材来满足使用长度的需求；另一方面，在磁场均匀度要求很高的核磁共振谱仪等装置中超导磁体是闭环运行，需要接头电阻尽可能低以满足可持续电流运行。因此，在新型铁基超导线带材广泛应用之前非常有必要对其进行超导连接技术研究。然而，铁基超导材料为高温脆相、化学活性低、连接性较差，所以超导接头制作难度比较大，而且至今尚没有进行过系统研究。本课题拟采用冷压焊、低温钎焊、扩散焊接及热压烧结等焊接方法连接新型铁基超导接头，通过阐明超导接头微观结构、电磁性能、机械性能与连接工艺之间的联系和内在机制，探索出实现铁基线带材超导连接的原理及可行方法，解决铁基超导材料在电工应用中无法避免的超导连接难题。

自新型铁基超导体发现以来，Sr(Ba)0.6K0.4Fe2As2 (Sr(Ba)-122)型超导带材的临界传输电流已达到了实用化水平，其相应的超导长线研制工作也已经开展。但是，在超导电工应用中，一方面需要通过连接多根超导带材来满足使用长度的需求；另一方面，在磁场均匀度要求很高的核磁共振谱仪等装置中超导磁体是闭环运行，需要接头电阻尽可能低以满足可持续电流运行。在新型铁基超导线带材广泛应用之前非常有必要对其进行超导连接技术研究。然而，铁基超导材料为高温脆相、化学活性低、连接性较差，所以超导接头制作难度比较大，而且之前尚没有进行过系统研究。本课题采用冷压、钎焊、扩散焊接及热压烧结等焊接方法连接新型铁基超导接头，通过阐明超导接头微观结构、电磁性能与连接工艺之间的联系和内在机制，探索出实现铁基线带材超导连接的原理及可行方法。研究发现要制备出高传输效率的铁基超导接头，需要使接头连接区域同时满足：1）带材与带材之间接触紧密，没有明显的微观裂纹；2）带材内部没有宏观裂纹，尤其是垂直于带材长度方向的宏观裂纹；3）各个元素没有明显的损失，尤其是钾元素。通过采用冷压焊接可以制备高传输电流的铁基超导接头，当压强为1.67GPa时所制得的接头样品的临界电流在10T和4.2K下达到了105A，相应的传输效率达到了95%，这是目前国际上的最高水平。此外，采用冷压焊接工艺制备出了闭环铁基超导线圈并进行了闭合环路电流运行。通过本项目的研究，解决铁基超导材料在电工应用中无法避免的超导连接难题。