Synthesis of Three-Dimensional Carbon Nanosheets and Its Flux Pinning Mechanisms in C-Doped MgB(2) Superconductors

Three-dimensional carbon nanosheets (3D-CNS) were synthesized by salt template spray-drying method in order to solve the agglomeration of 2D nanocarbon by a traditional mixing method. MgB(2) bulks doped with 3D-CNS with molar ratio composition of MgB(2−x)(3D-CNS)(x) (x = 0, 0.1 and 0.2) have been prepared by in situ sintering process. The microstructure, critical current density and flux pinning of the sintered samples have been investigated. Differing from the structure in previous studies, the 3D-CNS doping is more efficient for the refinement of the MgB(2) grains due to the 3D network structures. The results clearly show that more active C releasing from 3D-CNS at high temperature can provide effective flux pinning centers by the substitution of C for B in MgB(2) lattice. Furthermore, the lattice distortion and increased grain boundaries should be responsible for the enhancement of critical current density (J(c)) at high magnetic fields as well as the increased irreversible magnetic field (H(irr)). However, the positive action in J(c) at low field has been extremely offset by the concentration of impurities at MgB(2) grain boundaries such as released extra C without substitution and MgO, which is considered to further deteriorate the grain connectivity.