Optimization of High-Energy Ball Milling Aided Sintering Process for FeSe Superconductors

Superconducting FeSe bulks with high superconducting content of PbO-type tetragonal β-FeSe phase were prepared with high-energy ball milling (HEBM) aided sintering process. During the HEBM process, different ball-to-powder ratios of 6:1, 8:1, and 10:1 were adopted. The influences of ball-to-powder ratio on the phase composition of both ball milled powders and sintered bulks, morphology, and superconducting properties of final bulks were systematically investigated. It was noticed that, with the increase of ball-to-powder ratio, the energy input into the ball milling system increased, and the high-energy collision times increased. Therefore, more Fe atoms entered into the Fe-Se matrix, and the ball milling products changed from Fe
₃
Se
₄
to Fe
₇
Se
₈
. On the other hand, the decreasing total collision times led to the increase of final particle size. After the same sintering process, FeSe bulks with the major phase of β-FeSe were all obtained. However, with the increasing ball-to-powder ratio, the contents of both hexagonal δ-FeSe phase and residual Fe contents increased. Due to the different Fe/Se ratio in the final β-FeSe phase, the superconducting transition behavior varied obviously. Higher critical temperature of 8.0 K and larger superconducting phase content were achieved with the optimal ball to powder ratio of 6:1.