Superconductivity of nano-TiO₂-added MgB₂

We report on the synthesis, phase formation, microstructure, and magnetization, of nano(n)-TiO₂-added MgB₂ polycrystalline compounds. The added n-TiO₂ amounts are varied from 1% to 15% in weight (wt). All the studied samples are near single phase with small amounts of un-reacted Mg/MgO up to 10 wt%. The 15 wt% n-TiO₂ added sample seems to be a multi-phase compound with unusual broadening of the main MgB₂ reflection and additional unidentified lines in its X-ray diffraction (XRD) pattern. The superconducting transition temperature (T_c), as measured by magnetization experiments, decreases marginally with n-TiO₂ addition, for example the T_cs are at 37.5 K and 35.5 K, respectively, for pristine and 10 wt% n-TiO₂-added samples. This indicates that Ti has not significantly substituted into the host MgB₂ lattice. The grain morphology of these compounds reveals porous regions and does not change much with TiO₂ addition. High resolution transmission electron microscopy (HRTEM) studies revealed the presence of n-TiO₂ in these samples. The critical current density (J_c) of the MgB₂-n-TiO₂ samples, as estimated using the Bean model, shows better performance under magnetic fields above 3 T than pristine MgB₂ for up to 4 wt% of addition, and decreases rapidly for additions above 6 wt%. We conclude that n-TiO₂ helps in enhancing the flux pinning centers in MgB₂ superconductor and hence improves the J_c(H) performance for additions up to 4 wt% in fields above 3 T.