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

点击翻译按钮获取中文摘要

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