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S. X. Dou

Publications and source records attributed to S. X. Dou.

At least 55 records · Page 3Linked to original sources
arXiv2003

The origin of paramagnetic magnetization in field-cooled YBa2Cu3O7 films

Temperature dependences of the magnetic moment have been measured in YBa_2Cu_3O_{7-δ} thin films over a wide magnetic field range (5 <= H <= 10^4 Oe). In these films a paramagnetic signal known as the paramagnetic Meissner effect has been observed. The experimental data in the films, which have strong pinning and high critical current densities (J_c ~ 2 \times 10^6 A/cm^2 at 77 K), are quantitatively shown to be highly consistent with the theoretical model proposed by Koshelev and Larkin [Phys. Rev. B 52, 13559 (1995)]. This finding indicates that the origin of the paramagnetic effect is ultimately associated with nucleation and inhomogeneous spatial redistribution of magnetic vortices in a sample which is cooled down in a magnetic field. It is also shown that the distribution of vortices is extremely sensitive to the interplay of film properties and the real experimental conditions of the measurements.

cond-mat.supr-con↗
arXiv2003

On the sample size dependence of the critical current density in MgB$_2$ superconductors

Sample size dependent 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 $j\propto R^{1/n}$ ($n$ is the $n$ factor characterizing $E-j$ curve $E=E_c(j/j_c)^n$). This dependence provides one with a new method to derive the $n$ factor and can also be used to determine the dependence of the activation energy on the current density.

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arXiv2003

Effects of Fission-Fragment Damage on Vortex Dimensionality in Silver-sheathed Bi2Sr2Ca2Cu3Ox Tapes

We report on the vortex dimensionality of uranium-doped Ag/Bi2223 tapes, before and after irradiation to a thermal-neutron fluence. The effective activation energies as a function of current density and applied field were calculated from dynamic magnetisation relaxation measurements. A dimensional crossover from a 3D elastic creep regime to 2D plastic creep was observed in the nonirradiated tape at an applied magnetic field Hcr = 0.37 T, with an associated change in the flux hop velocity and temperature dependence of the activation energy. After introduction of the fission-fragment damage by irradiation, a shift in the crossover to Hcr = 0.65 T was observed. These results indicate an enhancement of the c-axis vortex coherence by the introduced splayed columnar defects, explaining the greater pinning efficiency of the uranium-fission method in Bi2223 rather than the less anisotropic Y123. Conflicting results obtained for the irradiated tape in the absence of any temperature scaling of the activation energies demonstrate the importance of the inclusion of scaling in the magnetisation relaxation analysis.

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arXiv2003

Uranium Doping and Thermal Neutron Irradiation Flux Pinning Effects in MgB2

The U/n method is a well-established means of improving flux pinning and critical current performance in cuprate superconductors. The method involves the doping of the superconductor with 235U followed by irradiation with thermal neutrons to promote fission. The resultant columnar damage tracks produced by the energetic fission products pin flux vortices and improve critical current performance in magnetic fields. No such improvement could be observed when the U/n method was applied to MgB2 superconductor. No fission tracks could be observed in TEM, even for samples that were irradiated at the highest fluence. Gamma-ray spectroscopy indicated that fission had occurred in the expected way.

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arXiv2003

Effect of nano-carbon particle doping on the flux pinning properties of MgB2 superconductor

Polycrystalline MgB2-xCx samples with x=0.05, 0.1, 0.2, 0.3, 0.4 nano-particle carbon powder were prepared using an in-situ reaction method under well controlled conditions to limit the extent of C substitution. The phases, lattice parameters, microstructures, superconductivity and flux pinning were characterized by XRD, TEM, and magnetic measurements. It was found that both the a-axis lattice parameter and the Tc decreased monotonically with increasing doping level. For the sample doped with the highest nominal composition of x=0.4 the Tc dropped only 2.7K. The nano-C-doped samples showed an improved field dependence of the Jc compared with the undoped sample over a wide temperature range. The enhancement by C-doping is similar to that of Si-doping but not as strong as for nano-SiC doped MgB2. X-ray diffraction results indicate that C reacted with Mg to form nano-size Mg2C3 and MgB2C2 particles. 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.

cond-mat.supr-con↗
arXiv2002

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 mm) 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.

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arXiv2002

Magnetic shielding in MgB2/Fe superconducting wires

Transport critical current (Ic) was measured for MgB2/Fe round wires, with magnetic field oriented perpendicular to the wire and parallel to it. Measurements were made on a wire with pure MgB2 core and another wire where MgB2 core was doped with nano-size SiC. This doping strongly improved the vortex pinning in MgB2. The field dependence of Ic was strongly improved due to the presence of the iron sheath. At 30K, Ic did not depend on the field for fields between 0.09 and 0.7T. At lower temperatures, Ic increased with the field, after initial decrease, resembling a "peak effect". This effect was extended to higher fields as the temperature went down: at 10K the peak appeared at 3.5T. This improvement was not due to mere magnetic shielding by iron, but more likely to an interaction between the iron sheath and superconductor. Improvement of vortex pinning did not affect the range of fields within which this effect was observed.

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arXiv2002

Enhancement of the critical current density and flux pinning of MgB2 superconductor by nanoparticle SiC doping

Doping of MgB2 by nano-SiC and its potential for improvement of flux pinning was studied for MgB2-x(SiC)x/2 with x = 0, 0.2 and 0.3 and a 10wt% nano-SiC doped MgB2 samples. Co-substitution of B by Si and C counterbalanced the effects of single-element doping, decreasing Tc by only 1.5K, introducing pinning centres effective at high fields and temperatures and enhancing Jc and Hirr significantly. Compared to the non-doped sample, Jc for the 10wt% doped sample increased by a factor of 32 at 5K and 8T, 42 at 20K and 5T, and 14 at 30K and 2T. At 20K, which is considered to be a benchmark operating temperature for MgB2, the best Jc for the doped sample was 2.4x10^5A/cm2 at 2T, which is comparable to Jc of the best Ag/Bi-2223 tapes. At 20K and 4T, Jc was 36,000A/cm2, which was twice as high as for the best MgB2 thin films and an order of magnitude higher than for the best Fe/MgB2 tapes. Because of such high performance, it is anticipated that the future MgB2 conductors will be made using the formula of MgBxSiyCz instead of the pure MgB2.

cond-mat.supr-con↗
arXiv2002

Superconductivity, critical current density, and flux pinning in MgB_{2-x}(SiC)_{x/2} superconductor after SiC nanoparticle doping

We investigated the effect of SiC nano-particle doping on the crystal lattice structure, critical temperature T_c, critical current density J_c, and flux pinning in MgB_2 superconductor. A series of MgB_{2-x}(SiC)_{x/2} samples with x = 0 to 1.0 were fabricated using in-situ reaction process. The contraction of the lattice and depression of T_c with increasing SiC doping level remained rather small due to the counter-balanced effect of Si and C co-doping. The high level Si and C co-doping allowed the creation of intra-grain defects and highly dispersed nano-inclusions within the grains which can act as effective pinning centers for vortices, improving J_c behavior as a function of the applied magnetic field. The enhanced pinning is mainly attributable to the substitution-induced defects and a local structure fluctuations within grains. A pinning mechanism is proposed to account for different contributions of different defects in MgB_{2-x}(SiC)_{x/2} superconductors.

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arXiv2002

Substitution induced pinning in MgB_2 superconductor doped with SiC nano-particles

By doping MgB_2 superconductor with SiC nano-particles, we have successfully introduced pinning sites directly into the crystal lattice of MgB_2 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 MgB_2(SiC)_x having x = 0.34, the highest doping level prepared, dropped only by 2.6 K.

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arXiv2002

Transport critical current of Solenoidal MgB2/Cu Coils Fabricated Using a Wind-Reaction In-situ Technique

In this letter, we report the results of transport Jc of solenoid coils upto 100 turns fabricated with Cu-sheathed MgB2 wires using a wind-reaction in-situ technique. Despite the low density of single core and some reaction between Mg and Cu-sheath, our results demonstrate the decrease in transport Jc with increasing length of MgB2 wires is insignificant. Solenoid coils with diameter as small as 10 mm can be readily fabricated using a wind-reaction in-situ technique. The Jc of coils is essentially the same as in the form of straight wires. A Jc of 133,000 A/cm2 and 125,000 A/cm2 at 4 K and self field has been achieved for a small coil wound using Cu-sheathed tape and Cu-sheathed wire respectively. These results indicate that the MgB2 wires have a great potential for lage scale applications

cond-mat.supr-con↗
arXiv2002

Improvement of critical current density in the Cu/MgB2 and Ag/MgB2 superconducting wires using the fast formation method

The powder in tube method has been used to fabricate Ag and Cu clad MgB2 wires using an in-situ reaction method. The effects of short time sintering on the critical current densities of Ag and Cu clad MgB2 wires were studied. All the samples were examined using XRD, SEM, and magnetization measurements. For Ag clad wire Jc is improved by more than two times after the short time sintering process. Jc values of 1.2x10^5 A/cm2 in zero field and above 10^4 A/cm2 in 2T at 20 K have been achieved for Ag clad MgB2 wire which is only sintered for 6 minutes at 800oC. However, a remarkable degree of reaction has been found between the superconducting cores and the sheath materials, leading to the formation of Cu2Mg and Ag3Mg for copper and silver clad wires, respectively. The results from Tc, Jc and Hirr convincingly show that the short sintering causes less reaction between the magnesium and the sheath materials and markedly improves the critical current density. Our result shows that Iron is still the best sheath material because of the lack of reaction between Fe and the superconducting MgB2 material.

cond-mat.supr-con↗
arXiv2001

Enhancement of Flux Pinning in Neutron Irradiated MgB$_{2}$ Superconductor

m-H loops for virgin and neutron irradiated bulk and powder samples of MgB_{2} were measured in the temperature range 5-30 K in magnetic field B<= 1 T. The irradiation at thermal neutron fluences 9*10^{13} and 4.5*10^{14} cm^{-2} caused very small enhancement of m-H loops at lower temperatures (T<20 K), whereas the effect at high temperatures was unclear due to difficulty in achieving exactly the same measurement temperature prior and after irradiation. However, the irradiation at 4.5*10^{15} cm^{-2} produced clear enhancement of m-H loops (hence J_{c}) at all investigated temperatures, which provides the evidence for the enhancement of flux pinning in MgB_{2} due to ion tracks resulting from n+^{10}B reaction. The potential of this technique for the enhancement of flux pinning in high temperature superconductors is briefly discussed.

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arXiv2001

Calculation of levitation forces in permanent-superconductor systems

The magnetic levitation forces of a superconducting disk (SC) levitated by a cylindrical permanent magnet (PM) have been calculated from first principles for superconductors with finite thickness. The current $j(ρ,z)$ and field $B(ρ,z)$ profiles in the SC in the non-uniform magnetic field generated by the PM have been derived. The levitation force is observed to depend non-linearly on the critical current density $j_c$ and on the thickness of the SC. The flux creep is described by a current-voltage law $E(j)=E_c(j/j_c)^n$, from which we show that the levitation force depends on the speed at which the PM approaches or recedes from the SC, which accounts for the experimentally observed force creep phenomenon. The stiffness of the system has been derived by calculating the minor force loops. The numerical results reproduce many of the features exhibited by experiments.

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arXiv2001

Improvement of critical current in MgB2/Fe wires by a ferromagnetic sheath

Transport critical current (Ic) was measured for Fe-sheathed MgB2 round wires. A critical current density of 5.3 x 10^4 A/cm^2 was obtained at 32K. Strong magnetic shielding by the iron sheath was observed, resulting in a decrease in Ic by only 15% in a field of 0.6T at 32K. In addition to shielding, interaction between the iron sheath and the superconductor resulted in a constant Ic between 0.2 and 0.6T. This was well beyond the maximum field for effective shielding of 0.2T. This effect can be used to substantially improve the field performance of MgB2/Fe wires at fields at least 3 times higher than the range allowed by mere magnetic shielding by the iron sheath. The dependence of Ic on the angle between field and current showed that the transport current does not flow straight across the wire, but meanders between the grains.

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arXiv2001

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

In this paper we have investigated the effects of sintering time and temperature on the formation and critical current densities of Fe-clad MgB2 wires. MgB2 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 MgB2 wires. A total time in the furnace of several minutes is more than enough to form nearly pure MgB2 with high performance characteristics. The results from Tc, Jc and Hirr show convincingly that the samples which were sintered for 3 minutes above 800 oC are as good as those sintered for longer times. In fact, the Jc field performance for the most rapidly sintered sample is slightly better than for all other samples. Jc of 4.5 times 10 ^5 A/cm2 in zero field and above 10 ^5 A/cm2 in 2T at 15 K has been achieved for the best Fe-clad MgB2 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 MgB2 wires.

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arXiv2001

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

Fe-clad MgB2 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 MgB2. The tapes were sintered in pure Ar at 800 ^(o) C for 1 h at ambient pressure. SEM shows a highly dense core with a large grain size of 100 micron. The Fe clad tape shows a sharp transition with transition width of 0.2 K and Tc0 at 37.5 K. We have achieved the highest transport critical current reported so far at 1.6 times 10^(4) A/cm^2 for both 29.5 K in 1 Tesla and 33 K in null field. R-T and critical current were also measured for fields perpendicular and parallel to the tape plane. The iron cladding shielded on the core from the applied external field, with the shielding being less effective for the field in the tape plane. Fe cladding may be advantageous for some applications as it could reduce the effects of both the self-field and external fields.

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