Clarification of current transport properties in YBCO coated conductors as a fundamental technology for power applications

研究了YBCO涂层高温超导带材的电流输运特性。特别是，通过研制新型高场型低温激光扫描显微镜系统(HF-LTLSM)，我们首次成功地显示了高达5T强磁场下的磁通流动耗散。结果表明，随着偏置电流密度的增加，局域电阻区向样品内部渗透。我们还开发了目前基于扫描鱿鱼显微镜(SSM)磁成像的可视化技术。变温SSM的高灵敏度磁成像使我们能够估计超导层中的薄层电流密度。结合这些测量结果，可以深入了解镀膜导体中的限流机制。我们的结果表明，在宏观长度尺度上观察到的临界电流IC是由空间分布的障碍物引起的电流不均匀造成的，而每个YBCO颗粒中的颗粒连通性不是限制因素。这些障碍的典型周期是几十μm到数百μm。还研究了局部IC分布与全球输运特性的关系。基于局部测量和理论考虑，我们建立了一个模型来解析地描述导体中的全局电流-电压特性。它使我们能够预测在广泛的温度、磁场和电场条件下的载流特性。该外推的准确性已在30T左右的磁场中得到了实验验证。这些结果对于高温超导器件的实际功率应用具有重要的基础技术意义。

Current transport properties in YBCO coated high Tc superconducting (HTS) tapes have been investigated. Especially, we have succeeded for the first time to visualize flux flow dissipation under high magnetic field up to 5 T by developing novel high field type low temperature laser scanning microscope system (HF-LTLSM). It has been shown that the localized resistive region percolates into the sample as the bias current density is increased. We have also developed current visualization technique based on magnetic imaging by scanning SQUID microscopy (SSM). High sensitive magnetic imaging by variable temperature SSM allows us to estimate sheet current density in the superconducting layer. Combination of those measurements leads deep insights into current limiting mechanism in the coated conductor. Our results show that non-uniform current flow due to spatially distributed obstacles is responsible for the critical current, Ic, observed in macroscopic length scale, whereas grain connectivity in each YBCO grains is not the limiting factor. Typical period of those obstacles is several tens μm to hundreds μm. The relationship between local Ic distribution and global transport properties has also been investigated. Based on the local measurements as well as theoretical consideration, we developed a model to describe analytically the global current-voltage characteristics in the conductors. It allows us to predict the current carrying properties over broad conditions of temperature, magnetic field and electric field. The accuracy of the extrapolation has been verified experimentally in magnetic fields up to around 30 T. Those results are relevant as fundamental technologies for practical power applications of HTS based devices.