Microstructural and crystallographic imperfections of MgB₂ superconducting wire and their correlation with the critical current density

A comprehensive study of the effects of structural imperfections in MgB₂ superconducting wire has been conducted. As the sintering temperature becomes lower, the structural imperfections of the MgB₂ material are increased, as reflected by detailed X-ray refinement and the normal state resistivity. The crystalline imperfections, caused by lattice disorder, directly affect the impurity scattering between the π and σ bands of MgB₂, resulting in a larger upper critical field. In addition, low sintering temperature keeps the grain size small, which leads to a strong enhancement of pinning, and thereby, enhanced critical current density. Owing to both the impurity scattering and the grain boundary pinning, the critical current density, irreversibility field, and upper critical field are enhanced. Residual voids or porosities obviously remain in the MgB₂, however, even at low sintering temperature, and thus block current transport paths.