Refinement and carbon incorporation effects on the superconducting properties of MgB2 through wet milling process of low purity boron powder

Abstract

The wet milling effects of boron (B) powder on the full width at half maximum (FWHM) value of X-ray reflexes, grain size, carbon (C) substitution, critical temperature (Tc), critical current density (Jc), irreversibility field (Hirr), and upper critical field (Hc2) have been investigated for MgB2 bulk superconductors. The semi-crystalline B powder with a low purity of 95–97% was refined with different milling times of 1 to 15h using a toluene medium. As a result of B powder milling, the particle size of the B powder decreased and the semi-crystalline phase became an amorphous phase. As the milling time increased, Tc of MgB2 bulk decreased by the deterioration of the crystallinity and Jc increased over the applied magnetic fields. With the refinement of the B powder, the grain boundary pinning increased due to smaller MgB2 grain size. In addition, it was found that a C substitution for the B site occurred from a toluene medium during wet milling process. The increased electron scattering due to an increase of the defects like grain boundary and lattice disorder by C substitution led to enhance the superconducting properties with an increase of the FWHM value of the MgB2 peaks and a decrease of the Tc. The MgB2 prepared from B powder mechanically milled for 7h showed the highest magnetic Jc of approximately 104A/cm2 at 5K and 8T. The Hc2 and Hirr were also improved due to the increase of grain boundary pinning and lattice disorder as a result of milling in toluene.