Improvement of Current-Carrying Capacity and In Situ Control of the Superconducting Fraction of MgB2-MgO Composites

Superconducting magnesium diboride-magnesium oxide (MgB ₂-MgO) composite attracts our attention due to its relatively high superconducting transition temperature close to the one of a pure MgB ₂ bulk and metallic transport behavior with low resistivity in the normal state even at a high fraction of insulating MgO. In this paper, we report that the current-carrying capacities of MgB ₂-MgO composites were enhanced by improving the replacement reaction process, and the superconducting MgB ₂ fraction can be in situ controlled by using magnesium (Mg), boron trioxide (B ₂ O ₃), and boron (B) as the raw materials. The composites with mass fractions of superconducting MgB ₂ from 27 to 80 % show that the zero-resistance transition temperatures are above 36.7 K, and the self-field critical current densities are from 0.2 ×10⁶ to 4 ×10⁶ A/cm ² at 10 K. The high J _cS of the superconducting composites under the applied fields prove that there is no appreciable difference between a perfect MgB ₂ sample and one with MgO doping, but the critical current density and flux pinning are improved. These results are meaningful for studying further the flux-pinning mechanism in MgB ₂.