Fabrication and Characterization of the ${\rm MgB}_{2}$ Bulk Superconductors Doped by Carbon Nanotubes

Carbon nanotube (CNT) doped MgB_1.9 - 0.1 at% CNT bulk superconductors were fabricated using an in-situ technique to improve the critical current density (J_c) in a high magnetic field. The effects of doping and the sintering temperature on the phase formation, microstructure, and critical properties were evaluated. Two types of the CNT were used as dopants: the conventional CNT (C_CNT) with a mixture of large and small diameter (5 and 20 nm) and the small CNT (S_CNT) with 5 nm diameters. For both CNT-doped samples, the a-axis lattice parameter decreased but its reduction was more significant at 900degC than at 800degC. The decrease in T_c was consistent with the change in the a-axis lattice parameter, which is probably due to the effect of C substitution for the B sites in MgB₂. The J_c of the doped samples decreased more slowly with increasing magnetic field than that of the undoped samples. In addition, the S_CNT-doped sample exhibited superior J_c(B) behavior than the C_CNT-doped sample. This was partly attributed to the higher doping level and the presence of finer CNTs, which acts as an effective pinning center.