Effect of charcoal doping on the superconducting properties of MgB2 bulk

The effect of charcoal doping on the superconducting properties of in situ processed MgB2 bulk samples was investigated. To understand the size effect of the dopant the charcoal powder was attrition milled for 1 h, 3 h and 6 h using ZrO2 balls. The milled charcoal powders were mixed with magnesium and boron powders to a nominal composition of Mg(B0.975C0.025)2. The Mg(B0.975C0.025)2 compacts were heat-treated at 900 °C for 0.5 h in flowing Ar atmosphere. Magnetic susceptibility for the samples showed that the superconducting transition temperature (Tc) decreased as the size of the charcoal powder decreased. The critical current density (Jc) of Mg(B0.975C0.025)2 prepared using large size charcoal powder was lower than that of the undoped MgB2. However, a crossover of Jc value was observed at high magnetic fields of about 4 T in Mg(B0.975C0.025)2 prepared using small size charcoal powder. Carbon diffusion into the boron site was easier and gave the Jc increase effect when the small size charcoal was used as a dopant.