Synthesis of Three-Dimensional Carbon Nanosheets and Its Flux Pinning Mechanisms in C-Doped MgB2 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. MgB2 bulks doped with 3D-CNS with molar ratio composition of MgB2-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 MgB2 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 MgB2 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 MgB2 grain boundaries such as released extra C without substitution and MgO, which is considered to further deteriorate the grain connectivity.

Automated summary

Three-dimensional carbon nanosheets (3D-CNS) were used to solve the agglomeration issue of 2D nanocarbon. The doping of 3D-CNS in MgB2 not only refined the grains but also provided effective flux pinning centers. However, the positive effect on critical current density at low fields was offset by the concentration of impurities.