Effects of heating condition and Sn addition on the microstructure and superconducting properties of Sr0.6K0.4Fe2As2 tapes

• A systematic study on the effects of sintering temperature and Sn addition.
• Field independence of Jc for the samples sintered at 850 °C is optimal.
• High transport Jc values have been achieved in all Sn-added tapes.
• Sn addition involves in the Sr-122 phase formation and affects its composition.
• Jc improvement is related to well connected grains, growth steps and dislocations.

We report a systematic investigation on the effects of sintering temperature and Sn addition on the phase formation, resistivity transition, critical current density and microstructure of Sr-122 tapes. With increasing sintering temperature from 800 to 950 °C, the a-lattice parameter of Sr122 + Sn samples slightly elongates while c-lattice parameter shrinks. The composition of superconducting phase was affected by different sintering conditions with Sn addition. It is found that the grain connectivity of Sr122 + Sn samples is greatly enhanced while higher content of FeSn impurity and structure defects are introduced at higher sintering temperature. The samples sintered at 900 °C show a maximal transport Jc value in self-field whereas the field independence of Jc for the samples sintered at 850 °C is optimal. On the other hand, comparing with the pure samples, the Sn-added samples present well connected cake-like grains, growth steps and a large number of dislocations, resulting in high transport Jc properties. It is proposed that molten Sn enters the rim of Sr-122 phase and greatly accelerates the dissolution of misfit at the edges of grains within a short sintering time, and subsequently reduces the interfacial energy. Our results indicate that moderate Sn addition in combination with optimized heating conditions is very promising for high Jc-H performance.