Novel ultra-sonic boron refinement in distilled water for cost-efficient fabrication of MgB2 bulk ceramic superconductors

Abstract

Cheap commercial boron was refined ultra-sonically in distilled water medium for various durations. X-ray diffraction pattern (XRD) indicated impurity-free nature of ultra-sonically refined boron. Field emission scanning electron microscope (FE-SEM) showed refinement of coarse boron to nanoscale. Rietveld phase fraction analysis on XRD pattern of sintered bulk prepared from ultra-sonically refined boron and Mg powder revealed a complete MgB2 phase formation, with a few impurities of MgO. FWHM (Full width at half maximum) is used to estimate crystallite size and microstrain. Tc,onset was around 38.5 K, with a sharp transition (ΔTc ∼ 0.5 K). Critical current density (Jc) of bulks prepared with B ultra-sonicated in distilled water for 15 and 30 min reached 295 and 305 kAcm−2 at 20 K/self-field, respectively. This is 35% increase in Jc showing the cost efficiency of this novel technique. Transmission electron microscopy of bulk MgB2 revealed a high density of nano-sized grains in the bulk prepared of B ultra-sonicated for 30 min. Flux pining diagrams with peak position at b = 0.2 points to a strong grain boundary pinning. The results prove that ultra-sonic treatment in distilled water results in boron refinement and Jc increase. The role of ultra-sonication medium viscosity in the process of MgB2 grain size refinement and Jc enhancement is elucidated.