Improved Critical Current Densities of Bulk MgB2 Using Carbon-Coated Amorphous Boron

In this study, we report on a further improvement of the critical current density of the sintered bulk MgB
₂
material utilizing the optimized sintering temperature combined with a varying content of carbon in carbon-encapsulated boron. The MgB
₂
bulk was prepared from high-purity commercial powder of Mg metal and a carbon-encapsulated boron with 0 wt.%, 2.8 wt.%, 4.5 wt.%, and 7.3 wt.% of carbon, using a single-step solid-state reaction at 805°C for 3 h in pure argon atmosphere. The magnetization measurements confirmed a sharp superconducting transition with onset T
_c
at around 38.5 K, decreasing with increasing carbon content. For 7.3 wt.% of carbon, the bulk MgB
₂
reached the superconducting transition at around 33 K. Scanning electron microscopy of the fractured bulk MgB
₂
cross section showed a dispersion of 100-200 nm large grains. Due to the carbon doping and optimized processing, the critical current density (J
_c
) in bulk MgB
₂
samples with the carbon-coated boron was improved both in low and high magnetic fields. The highest J
_c
values at 20 K, of 375 and 220 kA/cm
²
, in the self-field and 1 T, respectively, were achieved in the MgB
₂
sample with 2.8 wt.% of carbon in the carbon-encapsulated boron. The present results clearly demonstrate that the optimized sintering temperature combined with the appropriate amount of carbon in carbon-coated boron is able to improve the entire J
_c
performance of the bulk MgB
₂
material.