Progress in Superconductivity and Cryogenics (한국초전도ㆍ저온공학회논문지)

The Korean Society of Superconductivity and Cryogenics (한국초전도저온학회)
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Effect of boron milling on phase formation and critical current density of MgB2 bulk superconductors

  • Kang, M.O. (Korea Atomic Energy Research Institute) ;
  • Joo, J. (Sungkyunkwan University) ;
  • Jun, B.H. (Korea Atomic Energy Research Institute) ;
  • Park, S.D. (Korea Atomic Energy Research Institute) ;
  • Kim, C.S. (Korea Research Institute of Standards and Science) ;
  • Kim, C.J. (Korea Atomic Energy Research Institute)
  • Received : 2019.03.14
  • Accepted : 2019.03.28
  • Published : 2019.03.31
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Abstract

This study was carried out to investigate the effect of milling of boron (B), which is one of raw materials of $MgB_2$, on the critical current density ($J_c$) of $MgB_2$. 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 $ZrO_2$ 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 $700^{\circ}C$ 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 $MgB_2$. The superconducting transition temperature ($T_c$) and $J_c$ of $MgB_2$ were measured using a magnetic property measurement system (MPMS). It was found that $B_2O_3$ was formed by B milling and the subsequent drying process, and the volume fraction of $B_2O_3$ increased as milling time increased. The $T_c$ of $MgB_2$ decreased with increasing milling time, which was explained in terms of the decreased volume fraction of $MgB_2$, the line broadening of $MgB_2$ peaks and the formation of $B_2O_3$. The $J_c$ at 5 K increased with increasing milling time. The $J_c$ increase is more remarkable at the magnetic field higher than 3 T. The $J_c$ at 5 K and 4 T was the highest as $4.37{\times}10^4A/cm^2$ when milling time was 2 h. The $J_c$ at 20 K also increased with increasing milling time. However, The $J_c$ of the samples with the prolonged milling for 6 and 8 h were lower than that of the non-milled sample.

Keywords

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Fig. 1. FE-SEM image of (a) B and (b) Mg powders used as raw materials.

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Fig. 2. FE-SEM images of the B powders milled for (a) 2 h, (b) 4 h, (c) 6 h, and (d) 8 h.

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Fig. 3. Variation of X-ray diffraction patterns of the B powders as a function of milling time.

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Fig. 4. Histogram of an apparent density before/after the heat treatment as a function of milling time.

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Fig. 5. Variation of an apparent density of the heat-treated sample as a function of milling time.

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Fig. 6. SEM micrograph of the MgB2 sample after heat treatment.

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Fig. 7. Powder X-ray diffraction patterns of the samples heat-treated using the non-milled and milled B powders.

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Fig. 8. Volume fraction of MgB2, Mg and MgO as a function of milling time.

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Fig. 9. M-T curves of MgB2 prepared using non-milled and milled B powders.

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Fig. 10. Jc-B curves at 5 K and 20 K of MgB2 prepared using non-milled and milled B powders.

TABLE I FWHM OF (002) AND (110) PEAK AND CALCULATED GRAIN SIZE AND MICROSTRAIN AS A FUNCTION OF MILLING TIME.

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TABLE II SUPERCONDUCTING TRANSITION TEMPERATURES AND TRANSITION WIDTH OF THE SAMPLES PREPARED USING THE NON-MILLED AND MILLED POWDER.

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