M. J. Qin

High-transport critical current density above 30 K in pure Fe-clad MgB2 tape

Abstract Fe-clad MgB 2 long tapes have been fabricated using a powder-in-tube technique. An Mg+2B mixture was used as the central conductor core and reacted in situ to form MgB 2 . The tapes were sintered in pure Ar at 800°C for 1 h at ambient pressure. SEM shows a highly dense core with a large grain size of 100 μm. The Fe-clad tape shows a sharp transition with transition width of ΔT c of 0.2 K and a T c0 of 37.5 K. We have achieved the highest transport critical current reported so far at 1.7×10 4 A/cm 2 for both 29.5 K in 1 T and 33 K in zero applied field. Resistivity temperature dependence and transport critical current were also measured in magnetic fields applied perpendicular and parallel to the tape plane. Not only is the use of an Fe sheath necessary for the successful processing of in situ reacted powder-in-tube MgB 2 , it confers on the finished wire the additional benefit of magnetic screening.

Very fast formation of superconducting MgB2/Fe wires with high Jc

Abstract In this paper, we have investigated the effects of sintering time and temperature on the formation and critical current densities of Fe-clad MgB 2 wires. MgB 2 wires were fabricated using the powder-in-tube process and sintered for different periods of time at predetermined temperatures. All the samples were examined using XRD, SEM, and magnetisation measurements. In contrast to the common practice of sintering for several hours, the present results show that there is no need for prolonged heat treatment in the fabrication of Fe-clad MgB 2 wires. A total time in the furnace of several minutes is more than enough to form nearly pure MgB 2 with high performance characteristics. The results from T c , J c and H irr show convincingly that the samples which were sintered for 3 min above 800°C are as good as those sintered for longer times. In fact, the J c field performance for the most rapidly sintered sample is slightly better than for all other samples. J c of 4.5×10 5 A/cm 2 in zero field and above 10 5 A/cm 2 in 2 T at 15 K has been achieved for the best Fe-clad MgB 2 wires. As a result of such a short sintering there is no need for using high purity argon protection and it is possible to carry out the heat treatment in a much less protective atmosphere or in air. These findings substantially simplify the fabrication process, making it possible to have a continuous process for fabrication and reducing the costs for large-scale production of MgB 2 wires.

The doping effect of multiwall carbon nanotube on MgB2/Fe superconductor wire

We evaluated the doping effect of two types of multiwall carbon nanotubes (CNTs) with different aspect ratios on MgB2∕Fe monofilament wires. Relationships between microstructure, magnetic critical current density (Jc), critical temperature (Tc), upper critical field (Hc2), and irreversibility field (Hirr) for pure and CNT doped wires were systematically studied for sintering temperature from 650to1000°C. As the sintering temperature increased, Tc for short CNT doped sample slightly decreased, while Tc for long CNT doped sample increased. This indicates better reactivity between MgB2 and short CNT due to its small aspect ratio, and substitution of carbon (C) from short CNT for boron (B) occurs. In addition, short CNT doped samples sintered at high temperatures of 900 and 1000°C exhibited excellent Jc, and this value was approximately 104A∕cm2 in fields up to 8T at 5K. This suggests that short CNT is a promising carbon source for MgB2 superconductor with excellent Jc. In particular, inclusion of nanosized M...

Significant enhancement of flux pinning in MgB2 superconductor through nano-Si addition

Polycrystalline MgB2 samples with 10 wt.% silicon powder addition were prepared by an in situ reaction process. Two different Si powders, one with coarse (44 μm) and the other with nano-size (<100 nm) particles were used for making samples. The phases, microstructures, and flux pinning were characterized by XRD, TEM, and magnetic measurements. It was observed that the samples doped with nano-sized Si powder showed a significantly improved field dependence of the critical current over a wide temperature range compared with both undoped samples and samples with coarse-Si added. Jc is as high as 3000 A/cm2 in 8 T at 5 K, one order of magnitude higher than for the undoped MgB2. X-ray diffraction results indicated that Si had reacted with Mg to form Mg2Si. Nano-particle inclusions and substitution, both observed by transmission electron microscopy, are proposed to be responsible for the enhancement of flux pinning in high fields. However, the samples made with the coarse-Si powders had a poorer pinning than the undoped MgB2.

Enhancement of in-field Jc in MgB2 /Fe wire using single- and multiwalled carbon nanotubes

The authors investigated the doping effects of single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs) on the Tc, lattice parameters, Jc(B), microstructure, and Hc2 of MgB2∕Fe wire. These effects systematically showed the following sequence for Tc and the a axis: the SWCNT doped wire the MWshortCNT doped wire>the MWlongCNT doped wire>undoped wire. A dominating mechanism behind all these findings is the level of C substitution for B in the lattice.

Effect of boron powder purity on superconducting properties of MgB2

The effect of the properties of starting boron powders on the superconducting properties of MgB2 has been studied. The 92% and 96% pure powders produce lower surface reactivity and larger particle size than the 99% boron powder, as can be seen from Brunauer?Emmett?Teller (BET) and scanning electron microscopy (SEM) results, indicating that the low purity powders cannot be used to archive the same superconducting properties as those of samples made from pure 99% boron powder. However, the purity of 92% and 96% boron powders can be improved by using a simple chemical process, leading to enhanced magnetic critical current densities Jc. From x-ray diffraction (XRD) measurement, oxide impurity has been observed, which might be originated from the B2O3 phase in the boron powders. In order to get high performance MgB2, it is obviously important to control the phase composition and microstructure of amorphous boron starting powders and solid reaction conditions.

Sample-size dependence of the magnetic critical current density in MgB2 superconductors

Sample size dependent magnetic critical current density has been observed in magnesium diboride superconductors. At high fields, larger samples provide higher critical current densities, while at low fields, larger samples give rise to lower critical current densities. The explanation for this surprising result is proposed in this study based on the electric field generated in the superconductors. The dependence of the current density on the sample size has been derived as a power law

Dependence of the flux-creep activation energy on current density and magnetic field for the MgB2 superconductor

j\ensuremath{\propto}{R}^{1/n}

Effects of precursor powders and sintering processes on the superconducting properties of MgB2

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