Tuning of superconductivity of bulk MgB2 by ball milling and sieving the boron precursor

Abstract

This study aims to explore the impact of sieving the crystalline boron precursor applied after its ball milling on the critical current density (Jc) of MgB2 produced by the in-situ reaction. Various analytical techniques, such as X-ray diffraction, magnetization measurements, and microstructure analysis were employed to systematically investigate the bulk polycrystalline MgB2 samples. SEM revealed that grains of ball-milled B powder had a nanoscale size. Sieving made the powder more uniform in size. The purity of the refined powder was tested using X-ray diffraction, and four different precursor combinations were evaluated. MgB2 bulk had a homogeneous, dense microstructure composed of MgB2 nanocrystals, in contrast to the bulk MgB2 prepared of as-purchased B. The latter exhibited uneven microstructure containing both crystalline and porous non-crystalline portions. The improved inter-grain connectivity and microstructural homogeneity after ball milling led to an increase of self-field Jc at 20 K from 71.2 kA/cm2 to 144.6 kA/cm2. After sieving the B precursor before and after ball milling, a further increase of Jc by 18% and 8% was achieved.