In this study, we report on a further improvement of the critical current density of the sintered bulk MgB2 material utilizing the optimized sintering temperature combined with a varying content of carbon in carbon-encapsulated boron. The MgB2 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 at around 38.5 K, decreasing with increasing carbon content. For 7.3 wt.% of carbon, the bulk MgB2 reached the superconducting transition at around 33 K. Scanning electron microscopy of the fractured bulk MgB2 cross section showed a dispersion of 100-200-nm large grains. Due to the carbon doping and optimized processing, the critical current density (J) in bulk MgB2 samples with the carbon-coated boron was improved both in low and high magnetic fields. The highest J values at 20 K, of 375 and 220 kA/cm2, in the self-field and 1 T, respectively, were achieved in the MgB2 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 performance of the bulk MgB2 material.
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