Advanced Coated Conductors

The application of high temperature superconductors in cables or coils with a high current transport capability requires materials with a strong crystallographic texture to reduce the detrimental effect of high angle grain boundaries. To realize such properties, the superconducting layers are epitaxially grown on highly textured, metal-based templates. Nowadays, such textured templates are routinely produced in long length using different approaches. Beside the strong crystallographic texture, such coated conductors reveal a characteristic grain boundary network, which depends on the template used as well as on the preparation route of the superconductor itself. In any case, this grain boundary network determines the global current transport in a decisive way. Therefore, it is crucial to understand the correlation between the local grain structure of the superconductor and the local current transport in order to optimize the performance of the coated conductors. The detailed knowledge will finally enable a more cost efficient fabrication of such conductors in order to open new application prospectives or to replace existing materials, which require an expensive Helium cooling. Therefore, the major aim of this joint project is to correlate the properties of the grain boundary network in the superconductor itself with the local current transport. To realize this goal, advanced high- resolution techniques will be applied to determine the current flow on a grain level. These methods allow identifying limiting areas in these materials, which reduce the global current transport. In particular, it is planned to study the influence of incorporating artificial secondary phases as well as the use of novel liquid-assisted deposition methods on the local current transport in such grain boundary networks. Simultaneously, recently developed Fe-based superconductor layers will be investigated in order to evaluate, if the improved transport behavior of their grain boundaries results in advantages for such coated conductor architectures if compared to the typically used cuprate materials. Finally, the identified correlations between the microstructure and the local current flow will be used to model the global current transport in these coated conductors, which enables a further optimization for the respective applications.

The application of high temperature superconductors in cables or coils with a high current transport capability requires materials with a strong crystallographic texture to reduce the detrimental effect of high angle grain boundaries. To realize such properties, the superconducting layers are epitaxially grown on highly textured, metal-based templates. Nowadays, such textured templates are routinely produced in long length using different approaches. Beside the strong crystallographic texture, such coated conductors reveal a characteristic grain boundary network, which depends on the template used as well as on the preparation route of the superconductor itself. In any case, this grain boundary network determines the global current transport in a decisive way. Therefore, it is crucial to understand the correlation between the local grain structure of the superconductor and the local current transport in order to optimize the performance of the coated conductors. The detailed knowledge will finally enable a more cost efficient fabrication of such conductors in order to open new application prospectives or to replace existing materials, which require an expensive Helium cooling. The properties of the grain boundary network and the influence of the local texutre in superconductors were statistically evaluated and related with the local current transport properties. Advanced high-resolution techniques were applied to determine the current flow on a grain level and accross grain boundaries. It was found that not only the misalignement between neighbouring grains (which was expected before) but also the local tilt of the crystallographic c-axis are detrimental for the performance of the superconductor. The texture could be improved by doping with barium hafnium oxide. No significant differences between cuprate and iron-based coated condcutors in terms of texture requirements could be identified, despite the weaker influence of the misalignemnet angle on the grain boundary currents known from previous experiments with films on bicrystalline substrates.