Enhancement of critical current density in MgB₂ bulk with CNT-coated al addition

Bulk MgB₂ sample with carbon nanotube (CNT)-coated Al addition was prepared by conventional solid-state reaction at 900°C for 30 min. We investigated the effects of Al and C co-doping on the lattice parameter, the microstructure, and the critical current density of MgB₂. The substitution of Al and C atoms for the sites of Mg and B in the MgB₂ lattice resulted in dislocations in the MgB₂ grains, which makes great contributions, along with the nanoscale oxide particles, to the enhancement of critical current density at high field (10³ A cm ^-2, 7 T, 5 K) in the co-doped sample. These results contrasted significantly with the measured values of the pure MgB₂ and Al- and C-doped samples. Co-doping introduced more electrons into MgB₂ and decreased both the parameters c and a of MgB₂ lattice, and the used coating technique delayed and shortened the oxidation process of Mg and Al, leading to the decrease in the size and the content of the oxide. These advantages should be responsible for the enhancement of the critical current density as well.