INVESTIGATION TO RETAIN HIGH CRITICAL CURRENT OF COMPOSITE SUPERCONDUCTORS FROM MECHANICAL VIEWPOINT

在制造和缠绕期间，施加由组分之间的热膨胀差异引起的热应力和拉伸/弯曲应力。在操作期间也施加洛伦兹力。这些应力是静态和周期性地施加到复合超导体上的。因此，揭示复合材料的力学行为及其对超导性能的影响，并在此基础上阐明在外加应力下保持高超导性能的条件是最重要的研究课题之一。本文试图了解多相Bi_2Sr_2Ca_2Cu_3O_x/Ag（以下简称Bi_（2223）/Ag）、Nb_3Al/Cu和Nb-Ti/Cu超导复合线材和带材的应力损伤的产生、生长和积累与临界电流的关系，并获得保持原始临界电流的力学条件。主要研究结果如下： ...更多信息 在拉伸应力作用下，Bi_（2223）、Nb_3Sn、Nb_3Al和Nb-Ti合金丝的断裂应变分别为0.09- 0.13，0.5 - 1.2，0.6 -1.0和2%。保持原始临界电流的粗略条件是使用低于这种应变水平的复合超导体。重要的是，可以通过引入残余应力来延迟长丝的断裂。对于断裂应变较低的Bi 2223复合材料，残余应力的引入是非常有效的。基于实验和分析，所施加的应变允许高达约0.25%，其是最小断裂应变和残余应力的总和，并且所施加的弯曲应变高达约0.4%。在循环应力作用下，疲劳裂纹在铜包层中萌生，当应力循环达到疲劳寿命的70-90%时，疲劳裂纹扩展到芯部区域。当出现这种情况时，临界电流减小。为了保持原来的临界电流，它是有效的，采用复合超导体低于这样的应力周期。少

Thermal stresses arising from the difference in thermal expansion among the constituents and tensile/bending stresses are exerted during fabrication and winding. Also Lorentz force is exerted during operation. These stresses are statically and cyclically given to the composite superconductors. Therefore, it is one of the most important research subjects to reveal the mechanical behavior of the composites and its influence on superconducting properties, and, based on such results, to clarify the condition to retain high superconducting properties under applied stresses. The present work attempted to understand the relation of the initiation, growth and accumulation of the stress-induced damages to critical current and to obtain the mechanical condition to retain the original critical current for the multifilamentary Bi_2Sr_2Ca_2Cu_3O_x/Ag (hereafter noted as Bi2223/Ag), Nb_3Al/Cu and Nb-Ti/Cu superconducting composite wires and tapes. Main results are summarized as follows.Under the sta … More tically applied tensile stresses, the fracture strains of the Bi2223,Nb_3Sn,Nb_3Al and Nb-Ti filaments were around 0.09-0.13,0.5-1.2,0.6-1.0 and 2%, respectively. The rough condition to retain the original critical current is to use the composite superconductors below such strain levels. It is important that the fracture of the filaments can be delayed by introducing residual stresses. In the case of Bi2223 composite with low fracture strain, the introduction of residual stress is very effective. Based on the experiments and analysis, the applied strain is allowable up to around 0.25% which is the sum of the minimum fracture strain and residual stress and applied bending strain up to around 0.4%. Under the cyclic stresses, the fatigue crack initiates in the clad cupper and it propagates to the core region when the stress cycles reaches around 70-90% of the fatigue life. When such situation arises, the critical current is reduced. To retain the original critical current, it is effective to employ the composite superconductors below such stress cycles. Less