Fabrication of $\hbox {FeTe}_{0.5}\hbox {Se}_{0.5}$ Superconducting Wires and Tapes by a Chemical-Transformation PIT Process

We have fabricated single core superconducting FeTe_0.5Se_0.5 wires and tapes using a process based on a chemical transformation from hexagonal Fe(Te_0.5Se_0.5)_1+d(nonsuperconducting) to tetragonal FeTe_0.5Se_0.5 (superconducting) via an optimal supply of Fe from the Fe sheath by annealing. This process enhanced a packing density of wire core inside the sheath, owing to an expansion of the lattice volume via a chemical transformation from high-density hexagonal Fe(Te_0.5Se_0.5)_1+d to low-density tetragonal FeTe_0.5Se_0.5. An obvious correlation between annealing temperature (T_a) and phase transformation was observed. Transformation of wire core from hexagonal to tetragonal was observed above 500°C. Finally, the hexagonal phase completely transformed into the tetragonal phase by annealing at 700°C. The obtained superconducting wire showed superconductivity below ~13 K. The magnetic J_c value of 1.9 ×10³ A/cm² at 4.2 K was obtained for the sample annealed at 500°C.