On narrowed coated conductors: granular field-hysteresis effect on transport critical current

Superconducting film coated onto flexible metallic tapes -- so-called coated conductors -- which are being developed for second-generation power transmission lines, are long polycrystals subject to effects of the film's granularity. Although intergranular misorientations have been effectively minimized, local higher-angle misorientations remain. One earmark of such weak links is the hysteresis of the critical current density $J_{c}$ with respect to applied field $H$, brought about when large circulating currents trapped within adjacent grains produce a focused field within the grain \textit{boundaries }(GB's) which can partially cancel out $H$ when applied field is \textit{decreasing}. This shifts the maximum $J_{c}$ from zero to a finite $H$ where the local field at the GB is at minimum. This effect has been seen in measurements of \textit{induced }(magnetization) currents, but has not been documented using \textit{transport }(applied) currents that can be percolative. However, in applications where the coated conductor is \textit{patterned} into conduits 100 $\mu $m wide or less, it is possible to effectively channel the percolation across a single GB; the field-focusing effect is well known in transport currents across \textit{single }GB's. This study shows that, indeed, the effect is clearly manifested in coated conductors narrowed to a few grains wide. Systematic measurements, analyses, and ramifications will be discussed.