Yun Zhang

Improved Jc of MgB2 superconductor by ball milling using different media

In this paper, the effects of ball milling boron (B) powders using different media, such as acetone, ethanol, and toluene, on the superconducting properties of MgB2 have been studied. It was observed that toluene medium was the most effective of them all for enhancing Jc. Jc was estimated to be 5 ? 103?A?cm?2 at 8?T and 5?K. This value is much higher than that of pure MgB2 that was not ball milled, by a factor of 20. It was considered that ball milling B using toluene leads to smaller MgB2 grains, resulting in enhanced Jc at low operating temperature and high field.

Improving Superconducting Properties of MgB

We report the synthesis and characterization of MgB₂ made from nano-boron and doped with graphene in the following mole percentages, x = 0, 3.0 and 12.0. The effect of graphene doping on the normal state resistivity (ρ), superconducting transition temperature (T_c), irreversibility and upper critical fields (H_irr and H_c2), and critical current density (J_c), as well as the pinning force (F_p) were evaluated. We found that the graphene doping has a positive impact on the above mentioned properties. In the case of the optimally doped (x = 3.0%) sample, the critical current density at 5 K corresponds to 1.4 × 10⁵ A/cm² for 2 T field, whereas the undoped sample showed 9.6 × 10⁴ A/cm² for the same field, i.e., 1.5 times improvement. Furthermore, the optimally doped sample showed a J_c of nearly 1 × 10⁴ A/cm² at 5 K, 8 T, which is a significantly high value. The upper critical field has been enhanced to 13 T at 20 K for the optimal doping level. The flux pinning behavior has been evaluated from the curve of flux pinning force against applied magnetic field, and it reveals that the maximum pinning has been improved by nearly 1.2 times at 20 K, due to the graphene doping.

_{2}

In this work, sucrose was doped into MgB2 samples to act as a carbon source. The sintering temperature varied from 850 to 1050??C. The effects of sucrose doping and sintering temperature on the lattice parameters, microstrain, critical temperature (Tc), resistivity, and upper critical field (Hc2) have been investigated in detail. It has been found that sucrose doping results in a small depression in Tc and high resistivity, while the Hc2 performance is improved. The best performance was shown in the sucrose-doped sample sintered at 850??C. The reason for the enhancement of Hc2 is likely to be disorder caused by C substitution for B and/or diffusion of C atoms in the MgB2 lattice as interstitial atoms.

by Graphene Doping

The particle size, purity and form of the starting boron powders play an important role in the superconducting properties of MgB2. In this paper high-purity (99.999%) boron powders with crystalline phase that were processed by ball milling have been used as the precursor powders. It was observed that the critical current density (Jc) value was 9.1 ? 103?A?cm?2 at 5?K and 8?T, which is two times higher than for a sample with ball-milled amorphous boron powders, and a factor of 40 higher than a sample made from as-supplied amorphous boron powders. Significant Jc improvement at high fields is attributed to the Hc2 enhancement caused by the increased disorder.

The effect of carbon doping on the upper critical field (Hc2) and resistivity of MgB2 by using sucrose (C12H22O11) as the carbon source

The superconductivity of carbon nanotube (CNT) doped MgB2 sintered in pulsed magnetic field (PMF) was investigated with Raman scattering measurements and Raman spectral fit analysis. Although the carbon (C) substitution for the boron (B) is increased for the sample sintered in PMF, its superconductivity is advanced comparing with the sample sintered without PMF. The improved critical transition temperature, Tc, is attributed to the strengthening of the electron-phonon coupling (EPC) in MgB2, as reflected by the broadened E 2g mode in the Raman spectra, because the carbon atoms are homogeneously distributed in the boron planes.

Significant improvement of Jc in MgB2 bulk superconductor using ball-milled high-purity crystalline boron

The influences of microstructure, connectivity, and disorder on the critical current density, <i>J</i>_c, are discussed to clarify the different mechanisms of <i>Jc</i>(<i>H</i>) in different magnetic field ranges for in-situ and combined in-situ/ex-situ MgB₂/Fe wires. Sintering temperature plays a very important role in the electromagnetic properties at different temperatures and under various magnetic fields. Connectivity, upper critical field, <i>H</i>_c2 , and irreversibility field, <i>Hirr</i>, are studied to demonstrate the mechanism of J_c dependence on magnetic field. The combined in-situ/ex-situ process is proved to be a promising technique for fabrication of practical MgB₂ wires.

Increased Superconductivity for CNT Doped

Pure MgB2 bulk samples were sintered in different atmospheres: high purity Ar, high purity Ar with 0.01%xa0O2, high purity Ar with 0.03%xa0O2 and high purity Ar with 5%xa0H2. The superconductivity and microstructure of these samples were studied. It was found that in the range of this study the impurity gas in Ar had strong effects on aspects of the superconductivity such as critical current density, Jc, the irreversibility field, Hirr, and the upper critical field Hc2. When the samples were sintered in Ar with O2 impurity, their low field Jc decreased as the O2 content increased. The high field Jc, in contrast, was improved as the concentration of the O2 impurity increased. For the sample sintered in Ar with H2 impurity, its Jc is significantly higher than that of the sample sintered in high purity Ar. The smaller grain size and crystallinity degradation caused by the impure atmosphere is proposed to be the reason for the Jc improvement in both the O2 and the H2 added to the Ar gas sintering cases.

{\hbox {MgB}}_{2}

The superconductivity of nano-SiC doped MgB2 sintered in pulsed magnetic field (PMF) was investigated with Raman scattering measurements and Raman spectral fit analysis. The critical transition temperature, Tc, for the sample sintered in 5T PMF is improved compared with that of the sample sintered without PMF. The high Tc is attributed to the strengthening of the electron-phonon coupling (EPC) in MgB2, as reflected by the broadened E2g mode in the Raman spectra. The EPC constants are estimated as 0.876 and 0.874, with the electron-E2g coupling contribution 2.30 and 2.25, respectively. Magnetic field processing technology has been proved to be a powerful tool to improve the superconducting properties of SiC-doped MgB2 superconductor.

Sintered in 5T Pulsed Magnetic Field

MgB2/Fe wire samples were prepared by using ball-milled 96% boron (B) powder with strong semi-crystalline phase. We observed samples that contained ball-milled 96% B in comparison with one made from as-supplied commercial 96% B, with the results showing a significant enhancement in the high field transport critical current density (J ct) due to small grain size and better reactivity. However, the inter-grain connectivity became worse, which could lead to poor J ct in low field and an increased level of disorder.

Evolution of Electromagnetic Properties and Microstructure With Sintering Temperature for

In this paper bulk magnesium diboride (Mg_xB₂) with x varying from 0.9 to 1.3 was prepared by solid state reaction. The Mg:B mixing ratio for ideal MgB₂ is 1:2. The samples were sintered at 800degC for 60 h. Quantitative X-ray diffraction (XRD) analysis was performed to obtain the lattice parameters, the microstrain, and the weight fraction of impurities using the Rietveld refinement method. It has been found that the fraction of pure MgB₂ phase increases from x = 0.9 to x = 1.1, and then decreases with further increases in x. Mg_1.1B₂ exhibits the highest critical current density, J_c, over other samples in both low fields and high fields. A direct correlation between J_c and connectivity indicates that better connectivity, caused by smaller amounts of impurities, results in the best J_c in Mg_1.1B₂.