Record critical current density in bulk MgB₂ using carbon-coated amorphous boron with optimum sintering conditions

We report on the synthesis and characterization of a sintered bulk MgB₂ material produced at an optimized sintering temperature with a varying content of carbon-encapsulated amorphous boron. A series of MgB₂ bulks was prepared with 0%, 1.5%, 2.8%, 7.3%, 12% and 16.5% of carbon-encapsulated boron. In the samples with 12% of carbon-encapsulated boron, Mg and MgB₂C₂ formation was observed. T_c was around 38.4 K for the pure MgB₂ and decreased with increasing carbon content up to 25 K for 16.5 % of carbon-encapsulated boron. The highest J_c values of 470 kA/cm² and 310 kA/cm², in the self-field and 1 T, respectively, were achieved at 20 K, in the MgB₂ sample with 1.5% of carbon-encapsulated boron. It proved that the optimized sintering conditions together with the appropriate amount of the carbon-coated boron were able to bring critical current performance of bulk MgB₂ material up to the level necessary for real technical applications.