Improved critical current densities in bulk FeSe superconductor using ball milled powders and high temperature sintering

The present study is investigating the effect of high temperature sintering combined with ball milled powders for the preparation of FeSe material via solid state sintering technique. The commercial powders of Fe (99.9% purity) and Se (99.9% purity) were mixed in a nominal ratio Fe:Se = 1:1 and thoroughly ground and ball-milled in a glove box during 6 h. Then, the powder mixture was pressed into pellets of 5 mm in diameter and 2 mm thickness using an uniaxial pressure of 100 MPa. The samples were sealed in quartz tubes and sintered at 600 °C for 24 h. Then, the pellets were again thoroughly ground and ball-milled in the glove box and pressed into pellets, and the final sintering was performed at two different temperatures, namely at 900 °C for 24 h and at 950 °C for 24 h. X-ray diffraction results confirmed that both samples showed mainly of the β-FeSe with tetragonal structure. The temperature dependence of magnetization (M-T) curves revealed a sharp superconducting transition T_{c, onset} = 8.16 K for the sample sintered at 900 °C. Further, scanning electron microscopy observations proved that samples sintered at 900 °C show a platelike grain structure with high density. As a result, improved irreversibility fields around 5 T and the critical current density (J_c) values of 6252 A cm⁻² at 5 K and self-field are obtained. Furthermore, the normalized volume pinning force versus the reduced field plots indicated a peak position at 0.4 for the sample sintered at 900 °C. Improved flux pinning and the high J_c values are attributed to the textured microstructure of the material, produced by a combination of high temperature sintering and ball milling.