Investigation of $J_{c}$ -Suppressing Factors in Flat-Rolled $\hbox{Sr}_{0.6}\hbox{K}_{0.4}\hbox{Fe}_{2}\hbox{As}_{2}/\hbox{Fe}$ Tapes Via Microstructure Analysis

Pnictide superconductors will be very promising for applications if wires with high critical current density $J_{c}$ can allow reel-to-reel large-scale fabrication at low costs. To understand the mechanism(s) that limited $J_{c}$ in flat-rolled $hbox{Sr}_{0.6}hbox{K}_{0.4}hbox{Fe}_{2}hbox{As}_{2}$ (Sr122) tapes, microstructure analysis has been considered the most direct and efficient way. Here, we report on high-resolution microstructure imaging and analysis on Fe-sheathed flat-rolled Sr122 tapes, which have a $J_{c}$ as high as $2.3times 10^{4} hbox{A/cm}^{2}$ at 10 T and 4.2 K. The overlapping nature of the Sr122 plates was clearly observed. Transmission electron microscopy/scanning transmission electron microscopy analysis showed that, besides the cracks formed during the fabrication process, the $hbox{SrO}_{2}$ phase and cavities caused by the inhomogeneously dispersed Sr and K are the other important factors suppressing $J_{c}$ . The wetting phase FeAs at the grain boundaries can be partially substituted by Sn in Sn-added samples. Our findings provide insights that pave the way to further enhance the critical current of the rolled 122 tapes up to the practical level.