EFFECTS OF FILAMENT SIZE ON SUPERCONDUCTIVITY IN MULTIFILAMENTARY WIRES

为了实现50-60赫兹交流电源工作的超导器件，具有更细细丝和高电阻率CuNi阻挡层的多芯超导导线的开发正在进行中。在这项工作中，我们从实验和理论的角度研究了细丝尺寸对超导电性的影响，特别是由直径小于或等于0.1微米的细丝组成的超细多丝NbTi丝，这是一种很有前途的交流用线材。所取得的成果总结如下：1.与传统方法制备的直径在1微米以上的NbTi丝相比，超细多丝丝具有更高的临界电流密度j_c。然而，它们的场依赖性很强，并且临界电流密度随着温度的升高而显著降低。这些结果表明，这些导线的应用性能受温度和磁场的影响很大。大多数被研究的线材在临界温度T_c附近表现出反常的高J_c性质，这表明磁通钉扎机制随温度的不同而不同。在具有不同德拜频率的两层交替叠加的超导超晶格中，平行于层和垂直于层平面的上临界场B_&lt；c2&gt；的值明显不同。在具有不同BCS耦合常数的超晶格中也观察到了类似的结果。必须阐明多芯超导线中的钉扎机制，并改善其温度和场的相关性。

To realize superconducting devices operated by AC power sources of 50-60Hz, the development of multifilamentary superconducting wires with finer filaments and CuNi barrier of high resistivity is in progress. In this work, we have investigated the effects of filament size on superconductivity from experimental and theoretical standpoints, especially, in ultra-fine multifilament NbTi wires consisting of filaments of 0.1 mum or below in diameter, which are promising wires for AC use. Obtained results are summarized as follows.1. Ultra-fine multifilament wires show high critical current density j_c compared to conventionally fabricated NbTi wires with filaments of 1 mum or above in diameter. Their field dependence, however, is highly strong and the critical current density is appreciably lowered with increasing temperature. These results suggest that the performance of applications being constructed of these wires is considerably affected by temperature and magnetic field.2. Most of investigated wires show anomalous high-J_c nature in temperatures near critical temperature T_c suggesting that the mechanism of flux-pinning varies with temperature.3. In superconducting superlattices where two layrs with different Debye frequencies alternately pile up, values of upper critical field B_<c2> parallel and perpendicular to the layr plane are appreciably different. Similar results are seen in superlattices with different BCS coupling constants.The pinning mechanism in multifiamentary superconducting wires must be clarified and their temperature and field dependence should be improved.