Extrinsic two-dimensional flux pinning centers, via graphene-encapsulated boron powder as precursors, have been introduced into MgB2 superconductors by means of in situ and diffusion sintering methods. Uniform graphene encapsulation of the boron powders was achieved by the hydrothermal method with highly dispersed graphene oxide as the precursor. The graphene coating layers induce remaining graphene layers and other defects acting as flux pinning centers in the matrix as well as improved connectivity in between grains. The increased critical current density ( Jc) is attributed to the enhanced flux pinning force and improved connectivity. Two-dimensional flux pinning centers provided by thin graphene layers and grain boundaries in MgB2 possess high flux pinning efficiency without suppressing the connectivity of the MgB2 superconductor.
Keywords: MgB2 superconductor; carbon substitution; connectivity; critical current density; graphene coating on boron; two-dimensional flux pinning.