Nb₃SnおよびNb₃Al線材の臨界電流特性に及ぼす横方向圧縮応力効果

抄録

The effects of transverse compressive stress on the critical current, I_c, of two different superconductors, bronze processed Nb₃Sn single-core wires and Nb-tube processed Nb₃Al multifilamentary wires, have been measured. It has been found that both type of wires show basically the same stress dependence of I_c. I_c's decrease monotonically with externally applied stress and I_c-degradations become larger with increasing magnetic field. The process of I_c-degradations can be divided into 2 stages. In the first stage the I_c-degradation rate with stress is relatively small and lowered I_c's due to applied stress completely recover to the unstressed value when stress is removed, while in the second stage the I_c-degradation rate with stress is large and permanent I_c-degradations remain after removal of load. In the first stage of I_c-degradations due to stress, from an analysis based on the elasticity theory of the stress state of superconducting filaments of the multifilamentary wire under transverse compressive stress, it is suggested that the sensitivity of the critical current to transverse stress is substantially the same as that to axial stress. On the other hand, in the second stage it can be speculated that nonuniform deformation of the matrix may cause local bends or kinks in the superconducting filaments and these damages to the filaments may be chiefly responsible for the I_c-degradations. The stress beyond which the second stage starts are 150MPa and 700MPa for Nb₃Sn and Nb₃Al wires, respectively. This stress seems to correspond to the yield stress of the matrix material of the wires where superconducting filaments are embedded.