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Effect of boron milling on phase formation and critical current density of MgB2 bulk superconductors
Ist Teil von
한국초전도.저온논문지, 2019, 21(1), , pp.18-24
Ort / Verlag
한국초전도저온학회
Erscheinungsjahr
2019
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This study was carried out to investigate the effect of milling of boron (B), which is one of raw materials of MgB2, on the critical current density (Jc) of MgB2. B powder used in this study is semi-amorphous B (Pavezyum, Turkey, 97% purity, 1 micron). The size of B powder was reduced by planetary milling using ZrO2 balls (a diameter of 2 mm). The B powder and balls with a ratio of 1:20 were charged in a ceramic jar and then the jar was filled with toluene. The milling time was varied from 0 to 8 h. The milled B powders were mixed with Mg powder in the composition of (Mg+2B), and the powder mixtures were uniaxially pressed at 3 tons. The powder compacts were heat-treated at 700℃ for 1 h in flowing argon gas. Powder X-ray diffraction and FWHM (Full width at half maximum) were used to analyze the phase formation and crystallinity of MgB2. The superconducting transition temperature (Tc) and Jc of MgB2 were measured using a magnetic property measurement system (MPMS). It was found that B2O3 was formed by B milling and the subsequent drying process, and the volume fraction of B2O3 increased as milling time increased. The Tc of MgB2 decreased with increasing milling time, which was explained in terms of the decreased volume fraction of MgB2, the line broadening of MgB2 peaks and the formation of B2O3. The Jc at 5 K increased with increasing milling time. The Jc increase is more remarkable at the magnetic field higher than 3 T. The Jc at 5 K and 4 T was the highest as 4.37×104 A/cm2 when milling time was 2 h. The Jc at 20 K also increased with increasing milling time. However, The Jc of the samples with the prolonged milling for 6 and 8 h were lower than that of the non-milled sample.