Sihai Zhou

Carbohydrate doping to enhance electromagnetic properties of MgB2 superconductors

The effect of carbohydrate doping on lattice parameters, microstructure, Tc, Jc, Hirr, and Hc2 of MgB2 has been studied. In this work the authors used malic acid as an example of carbohydrates as an additive to MgB2. The advantages of carbohydrate doping include homogeneous mixing of precursor powders, avoidance of expansive nanoadditives, production of highly reactive C, and significant enhancement in Jc, Hirr, and Hc2 of MgB2, compared to undoped samples. The Jc for MgB2+30wt% C4H6O5 sample was increased by a factor of 21 at 5K and 8T without degradation of self-field Jc.

Substitution-induced pinning in MgB2 superconductor doped with SiC nano-particles

By doping MgB2 superconductor with SiC nano-particles, we have successfully introduced pinning sites directly into the crystal lattice of MgB2 grains (intra-grain pinning). It became possible due to the combination of counter-balanced Si and C co-substitution for B, leading to a large number of intra-granular dislocations and the dispersed nano-size impurities induced by the substitution. The magnetic field dependence of the critical current density was significantly improved in a wide temperature range, whereas the transition temperature in the sample MgB2(SiC)x having x = 0.34, the highest doping level prepared, dropped only by 2.6 K.

Superconductivity, critical current density, and flux pinning in MgB2–x(SiC)x/2 superconductor after SiC nanoparticle doping

We investigated the effect of SiC nanoparticle doping on the crystal lattice structure, critical temperature Tc, critical current density Jc, and flux pinning in MgB2 superconductor. A series of MgB2−x(SiC)x/2 samples with x=0–1.0 were fabricated using an in situ reaction process. The contraction of the lattice and depression of Tc with increasing SiC doping level remained rather small most likely due to the counterbalancing effect of Si and C co-doping. The high level Si and C co-doping allowed the creation of intragrain defects and highly dispersed nanoinclusions within the grains which can act as effective pinning centers for vortices, improving Jc behavior as a function of the applied magnetic field. The enhanced pinning is mainly attributable to the substitution-induced defects and local structure fluctuations within grains. A pinning mechanism is proposed to account for different contributions of different defects in MgB2−x(SiC)x/2 superconductors.

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.

Properties of superconducting MgB2 wires: in situ versus ex situ reaction technique

We have fabricated a series of iron-sheathed superconducting wires prepared by the powder-in-tube technique from (MgB2)1−x:(Mg+2B)x initial powder mixtures taken with different proportions, so that x varies from 0 to 1. It turned out that ex situ prepared wire (x = 0) has considerable disadvantages compared to all the other wires in which in situ assisted (0 0. Pinning of vortices in MgB2 wires is shown to be due to grain boundaries. Jc(Ba) behaviour is governed by an interplay between the transparency of grain boundaries and the amount of ‘pinning’ grain boundaries. Differences between thermo-magnetic flux-jump instabilities in the samples and a possible threat to practical applications are also discussed. (Some figures in this article are in colour only in the electronic version)

Influence of Ag, Cu and Fe sheaths on MgB2 superconducting tapes

Copper, iron and silver MgB2 sheathed tapes have been manufactured under different conditions. It has been found that copper-sheathed tapes can show a higher critical current density than iron-sheathed tapes if heat-treated at temperatures below 850 °C. The influence of different overall mechanical deformation rates has been studied for tapes sheathed by all three types of metals. By increasing the deformation rate the critical current density was improved by about an order of magnitude in the case of the copper-sheathed tapes, while the critical current density of the iron-sheathed tapes remained constant.

Effect of the processing parameters of MgB1.8(SiC)0.1/Fe tapes on the critical current density

Abstract Nano-SiC doping into MgB 2 has induced extraordinary enhancement in critical current density ( J c ) and flux pinning as a result of Si and C co-substitution into the lattice and the formation of nano-inclusions within the grains. In this work, a systematic study of the effect of the preparation parameters for processing MgB 1.8 (SiC) 0.1 /Fe tapes on critical temperature ( T c ), critical current density ( J c ) and microstructures was carried out. A mixture of Mg and B was doped with nano-SiC particle powder, and the SiC-doped samples were fabricated with the reaction in situ method. The samples were characterised using X-ray diffraction, scanning electronic microscopy and magnetisation measurements. It was found that the sintering temperature had a strong effect on both T c and J c . T c was increased with sintering temperature, while J c of the tapes sintered at 750 and 850 °C is higher than that of the tapes sintered at 680 and 950 °C. A longer sintering time was found to be beneficial to T c but has negligible effects on J c . Slow cooling improves the J c field performance at 20 and 30 K. Uniaxial pressure (250 and 680 MPa) applied to tapes before sintering has almost no effect on J c .

Single- and multi-filamentary Fe-sheathed MgB2 wires

Abstract A novel method is proposed to increase the MgB 2 core density and, as a consequence, the critical current density of Fe-sheathed MgB 2 wires. This method involves two additional stages compared to the usual wire fabrication: post-annealing mechanical deformation and a consecutive heat-treatment. Using this method we obtained denser core structures in 7-filament wire having J c values larger by a factor of about 4.3 than that found in the initial “in situ” prepared mono-core wire. Independent of the manufacturing procedure, the wires showed a transport property degradation effect after being exposed to the air for 4 months.

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

Different precursor powders and procedures were used to fabricate MgB2 superconductor. It was found that the purity of the B powder had strong effects on the Tc and Jc of MgB2. The Jc of the sample made from 90% B powder decreased 40 times at 3 T and 20 K compared with the 99% B powder samples. The impurity phases might be the reason for the suppression of the Tc and the Jc. Variation of the Mg:B ratio also influenced the MgB2 properties. Jc decreased with fluctuations from the normal rate (Mg:B = 1:2). It was found that the effects of the oxidization of the Mg powder on Jc were not strong when the Mg powder was only slightly oxidized. However, when the Mg powder was severely oxidized, the Jc decreased significantly. A two-step sintering process was adopted to make MgB2. In this process, the first sintering stage was at a lower temperature. It was found that this pre-sintering did not have a good effect on the properties, despite the fact that the mass density was improved. The phase composition and microstructure were analysed to explain the different effects.

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

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.