Mechanism for high critical current density in in situ MgB2 wire with large area-reduction ratio

A comparative study of in situ MgB2 wire and MgB2 bulk was carried out to clarify the mechanism for the high critical current density, J(c), in the practical in situ MgB2 wires. The in situ MgB2 wire was manufactured with an area-reduction ratio of 99.93%, which was one of the highest values in MgB2 superconducting wires previously reported. The electrical connectivity, K, and the flux pinning strength, F-p, which are important factors in explaining the behavior of J(c), could be determined in the same manner as those for the bulk sample; K was well understood with the three-dimensional percolation model, and F-p was effectively explained by the electron scattering mechanism by grain boundaries. On the other hand, the area-reduction process dramatically enhanced the value of K, leading to an increase in the value of J(c). The respective values of K and J(c)(20 K, similar to 0 T) reached 0.24-0.34 and 6.0 x 10(3) -8.4 x 10(3) A mm(-2), which were twice or three times higher than those of typical in situ bulks. This is because the plastic deformation of magnesium particles increased the packing factor of raw powders through a repetitive drawing process.