Microstructure and critical current density in MgB₂ bulk made of 4.5 wt% carbon-coated boron

Superconducting performance and its uniformity was studied in the single-step sintered MgB₂ bulk prepared with 4.5 wt% of carbon in the carbon-encapsulated boron. The 20 mm in diameter MgB₂ pellet was cut into several pieces from bottom to top and the microstructure, superconducting transition temperature (T_c onset), and critical current density at 20 K were studied. DC magnetization measurements showed a sharp superconducting transition with onset T_c at around 35.5 K in all positions. SEM analysis indicated a dispersion of grains between 200 and 300 nm in size, as the main pinning medium in this MgB₂ superconductors. The critical current density at 20 K was quite uniform, around 330 kA/cm² and 200 kA/cm² at self-field and 1 T, respectively, for all measured positions. The results indicate that the carbon-encapsulated boron is very promising for production of high quality bulk MgB₂ material for various industrial applications.