A correlation between transport current density and grain connectivity in MgB2/Fe wire made from ball-milled boron

Studies of the magnetic field dependence of the transport critical current density (J(ct)) and the grain connectivity of MgB2/Fe wires fabricated from ball-milled boron have been conducted in detail, and strong correlations have been found, as evidenced by differences in grain size, critical transition temperature, and resistivity. It was observed that the samples fabricated by ball milling had relatively small grain sizes, resulting in a strong field dependence of the J(ct) in the high field region. On the other hand, the ball-milled boron was associated with poor connectivity between adjacent grains. It is clearly shown that the observed reduction in low field J(ct) is related to the reduction in the superconducting area fraction (A(F)) that is reflected by the connectivity factor. Even if high temperature sintering in all the samples can compensate for the degradation of the J(ct) in the low field region, the subsequent grain growth is mainly responsible for the degradation of J(ct) in the high field region.