Microscopic unravelling of nano-carbon doping in MgB₂ superconductors fabricated by diffusion method

Highlights: • First report on nano-carbon doped MgB₂ superconductors synthesized by diffusion method. • Microstructure and superconducting properties of the superconductors are discussed. • B₄C region blocks the Mg from reacting with B in the 10% nano-carbon doped sample. • MgB₂ with 2.5% nano-carbon doped showed the highest J_c, ≈10⁴ A/cm² for 20 K at 4 T. - Abstract: We investigated the effects of nano-carbon doping as the intrinsic (B-site nano-carbon substitution) and extrinsic (nano-carbon derivatives) pinning by diffusion method. The contraction of the in-plane lattice confirmed the presence of disorder in boron sublattice caused by carbon substitution. The increasing value in full width half maximum (FWHM) in the X-ray diffraction (XRD) patterns with each increment in the doping level reveal smaller grains and imperfect MgB₂ crystalline. The strain increased across the doping level due to the carbon substitution in the MgB₂ matrix. The broadening of the T_c curves from low to high doping showed suppression of the connectivity of the bulk samples with progressive dirtying. At high doping, the presence of B₄C region blocked the Mg from reacting with crystalline B thus hampering the formation of MgB₂. Furthermore, the unreacted Mg acted as a current blocking phase in lowering down the grain connectivity hence depressing the J_c of the 10% nano-carbon doped MgB₂ bulk superconductor