Doping Effect of CNT and Nano-Carbon in Magnesium Diboride Bulk

摘要

We fabricated carbon nanotube (CNT)- and nano-carbon (NC)-doped MgB_2 using an in-situ process in order to improve the critical current density (J_c) at high magnetic field. We then evaluated the effects of the doped carbon content on phase formation, microstructure, and critical properties. CNT had a diameter and length of 5-10 nm and 0.5-1 μm, respectively, and NC was a sphere with a diameter of 5-30 nm. The bulk MgB_(2_x)C_x samples with x = 0, 0.05, and 0.1 for NC and CNT were fabricated by pressing into pellets and then sintered at 900℃ for 30 min. NC was more effective than CNT for carbon doping at the B site in MgB_2 and, therefore, the NC-doped MgB_2 samples had a lower critical temperature (T_c) of 35.0-34.7 K than that of the CNT-doped samples (36.4-36.1 K). In addition, the J_c(B) behavior was improved when NC and CNT were doped due to doping effect. Microstructural observation suggested that the nano-sized and unreacted NC particles and the nanodomain MgB_2 acted as effective flux pinning centers for the NC- and CNT-doped MgB_2, respectively.