Optimization of FeSe superconductors with the high-energy ball milling aided sintering process

FeSe Superconducting bulks with high superconducting phase ratio were prepared with high energy ball milling (HEBM) aided sintering process. During the HEBM process, the solid solution of Fe–Se (Fe(Se)SS) can be successfully obtained, which not only assured the atomic uniformity of precursor powders, but greatly shortened the diffusion distance between Fe and Se atoms. Therefore tetragonal β-FeSe superconducting phase can be formed directly with one step sintering process and the formation of hexagonal δ-FeSe non-superconducting phase can be avoided. The influences of Fe/Se ratio and sintering temperature on the phase transition process and the superconducting properties were systematically studied. The change of Fe/Se ratio showed great influences on both the superconducting volume and critical temperature of final bulks. By increasing the sintering temperature to 700 °C, FeSe bulk with higher texture, better crystallization and optimized chemical composition was achieved. The FeSe superconducting bulk with the critical temperature Tc(onset) of 9.0 K was obtained with the Fe/Se ratio of 1.20, HEBM time of 6 h, and sintering temperature of 700 °C.