Doping effect of CNT and nano-carbon in magnesium diboride bulk

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, samples with x = 0, 0.05, and 0.1 for NC and CNT were fabricated by pressing into pellets and then sintered at 900 °C 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.