Critical Current and Microstructure of FeSe Wires and Tapes Prepared by PIT Method

The FeSe wires and tapes with Fe (S22 steel) and Cu/Nb sheath were fabricated by various modifications of the powder-in-tube (PIT) method. The superconducting critical parameters (upper critical field, critical current, critical temperature) depending on heat treatment time were investigated using resistive R (T) and transport measurements I (V) in magnetic fields up to 9T. The low-temperature annealing up to 72 h improve superconducting properties of superconducting wire with steel sheath. However, the annealing is the cause of superconducting wires degradation due to deterioration of contacts between the steel shell and the FeSe core. We show that industrial PIT technology of manufacturing $\text{Nb}_3\text{Sn}$ wires and tapes might be adopted for iron-based superconductors. Moreover, industrial PIT is a promising technique for fabricating large scale and high-quality superconducting wire and tapes. In addition, we show that fine grinding of the FeSe bulks leads mainly to a phase transition from the tetragonal to the hexagonal crystal structure that exhibits no superconductivity.