Optimization of sintering parameters for fabrication of Ag-sheathed FeSe superconducting wires

Ag-sheathed Fe_1.05Se superconducting wires were fabricated with ex-situ powder in tube (PIT) process. Fe and Se powders with molar ratio of 1.05 were firstly mixed and sintered under 600 °C for 12 h to form precursor powders. Owing to the complex Fe–Se binary phase diagram, both superconducting tetragonal FeSe and non-superconducting hexagonal FeSe could be formed simultaneously during sintering. Aiming at the reduction of hexagonal FeSe phase content and higher superconducting phase volume, the influences of key parameters, including sintering time, cooling rate and heating rate, on the phase composition of sintered wires were systematically studied. Optimal sintering parameters are obtained, and the maximum tetragonal FeSe phase content of ~ 97% is achieved. Meanwhile, the effects of packing density of precursor powders on the phase composition of final wires were also discussed. Owing to the shorter length of diffusion path, more tetragonal FeSe was formed with higher packing density. The superconducting transition signal with critical temperature of ~ 7.5 K was obtained, which proved the effectiveness of our optimal sintering process.