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 (J c) 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.

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The mixed-state superconducting properties of bulk MgB 2 + 2 at.% TiO 2 and + 8 at.% SiC, prepared by the in situ solid state reaction, have been investigated. The analysis on the mixed-state parameters, such as the upper critical field, the coherence length and the Ginzburg-Landau parameter, proves that the MgB 2 + + 2 at.% TiO 2 is a high-k type-II superconductor in the dirty limit while the MgB 2 + 8 at.% SiC corresponds to that in the moderately clean limit. It was shown that the anisotropic grain-boundary pinning is realized in the fine-grained doped MgB 2 polycrystals rather than the electron scattering one. The field-cooled temperature dependences of magnetic moment exhibit a transition of the samples to the paramagnetic state at certain applied magnetic fields, which is treated as manifestation of the paramagnetic Meissner effect. The experimental results are discussed on the base of modern theoretical approaches.

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