Sn addition effects on CaKFe4As4 superconductors.

The effects of adding low-melting point metals (Pb, Sn, In and Ge) to polycrystalline CaKFe₄As₄ (CaK1144) superconductors were investigated. We found that Sn is the most stable with CaK1144 among these low-melting point metals. Synthesis conditions as well as the doping quantity of Sn were optimized, and phase stability, microstructures and superconducting properties were investigated. At the optimized sintering temperature of 700 °C, Sn was effectively solidified in grain boundaries (GBs) of the CaK1144 bulk samples and secondary phase formations were not serious in optimized samples. As the Sn amount increased, filling of Sn in the GBs increased up to 30 wt% with homogeneous dispersion. The magnetic field dependence of critical-current density (J_c) was significantly enhanced by 30 wt% Sn addition and decreased with further increase of Sn addition. The filling up of 1144 grain boundaries by the optimal amount of Sn enhanced the J_c from 11.7 kA cm⁻² to 21.7 kA cm⁻² in a self-field and at 4.2 K. Sn addition was effective both to suppress the impurity phases as well as increase J_c of CaK1144. [ABSTRACT FROM AUTHOR]