Extrinsic Two-Dimensional Flux Pinning Centers in MgB2 Superconductors Induced by Graphene-Coated Boron

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 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 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.

MgB2 Superconducting Joint Architecture with the Functionality to Screen External Magnetic Fields for MRI Magnet Applications

A superconducting joint architecture to join unreacted carbon-doped multifilament magnesium diboride (MgB₂) wires with the functionality to screen external magnetic fields for magnetic resonance imaging (MRI) magnet applications is proposed. The intrinsic diamagnetic property of a superconducting MgB₂ bulk was exploited to produce a magnetic field screening effect around the current transfer path within the joint. Unprecedentedly, the joint fabricated using this novel architecture was able to screen magnetic fields up to 1.5 T at 20 K and up to 2 T at 15 K and thereby almost nullified the effect of the applied magnetic field by maintaining a constant critical current (I_c). The joint showed an I_c of 30.8 A in 1.5 T at 20 K and an ultralow resistance of about 3.32 × 10^-14 Ω at 20 K in a self-field. The magnetic field screening effect shown by the MgB₂ joint is expected to be extremely valuable for MRI magnet applications, where the I_c of the joints is lower than the I_c of the connected MgB₂ wires in a given magnetic field and temperature.

Superconducting Joining Concept for Internal Magnesium Diffusion-Processed Magnesium Diboride Wires

A superconducting joint of unreacted monofilament internal magnesium diffusion-processed magnesium diboride (MgB₂) wires was fabricated by exploiting the phenomenon of magnesium diffusion into the boron layer inside the superconducting joint. Unprecedentedly, the joint was able to carry an almost identical transport current compared to the bare wire in a 2-7 T magnetic field at 20 K. The joint also exhibited very low joint resistance of 2.01 × 10^-13 Ω in self-field at 20 K. Among commercially available superconductors, this work is the first to successfully realize a superconducting joint that is capable of transferring current from one conductor to another without any notable degradation under strong magnetic fields. This work demonstrates great potential to apply MgB₂ in a range of practical applications, where superconducting joints are essential.

Carbon-Coating Layers on Boron Generated High Critical Current Density in MgB2 Superconductor

Boron particles with a homogeneous carbon-coating layer were employed as the precursor to fabricate MgB₂ superconductors to generate artificial two-dimensional (2D) flux-pinning centers. Systematic microstructure investigation reveals that the carbon layers are well-distributed in the MgB₂ matrix without agglomeration. The thickness of the carbon layers is smaller than the MgB₂ coherent length, which makes them transparent to supercurrent. The critical current density is increased because of the strong flux-pinning effects of the 2D carbon layers in the superconductor as highly efficient flux-pinning centers and the increased irreversibility field due to the carbon-doping effects.