Optimization of Carbon Encapsulated Boron Doping for High-Performance Bulk Sintered MgB₂

Carbon has been a standard doping agent in MgB₂ for long time. It, however, has also participated in a non-uniform distribution of the constituents throughout the bulk MgB₂. To address this issue, carbon encapsulated boron (CEB) was used instead of the manually added mixture of boron and carbon. The previous studies confirmed that only low concentrations of carbon in CEB were effective for synthesis of a high-performance bulk MgB₂. Here, a further step in optimization carbon content in CEB is reported. Carbon content in CEB varied as 1, 1.1, 1.35, 1.5, and 1.9 wt%. X-ray diffraction (XRD) results depict a slight shift in peaks corresponding to a–b plane, indicating carbon substitution into the lattice. High superconducting critical current density in self-field, such as 660, 550, and 435 kA cm⁻², was observed in the samples with 1.5 wt% CEB at 10, 15, and 20 K, respectively. In addition, J_c of 75 kA cm⁻² at 2 T and 20 K was observed in the 1.5 wt% CEB sample, which is thrice the value observed in the pure sample, with a minute tradeoff in T_c (around 37.5 K). Scanning electron microscope (SEM) images reveal that small particles of size ranging from 50 to 200 nm contribute to J_c improvement. Energy-dispersive X-ray (EDX) results show carbon uniformly distributed throughout the bulk.