Chengduo Wang

High transport critical current densities in textured Fe-sheathed Sr1−xKxFe2As2+Sn superconducting tapes

We report the realization of grain alignment in Sn-added Sr1−xKxFe2As2 superconducting tapes with Fe sheath prepared by ex-situ powder-in-tube method. At 4.2 K, high transport critical current densities Jc of 2.5 × 104 A/cm2 (Ic = 180 A) in self-field and 3.5 × 103 A/cm2 (Ic = 25.5 A) in 10 T have been measured. These values are the highest ever reported so far for Fe-based superconducting wires and tapes. We believe the superior Jc in our tape samples are due to well textured grains and strengthened intergrain coupling achieved by Sn addition. Our results demonstrated an encouraging prospect for application of iron based superconductors.

Effect of high-energy ball milling time on superconducting properties of MgB2 with low purity boron powder

MgB2 bulks and tapes were fabricated using ball-milled precursor powders. The selected B powders have the ?-rhombohedral crystalline structure as a main phase, with a small amount of MgO impurity. Milling time was tested from 0.5 to 120?h to get an optimized value. It is found that ball milling can facilitate MgB2 phase formation, refine the MgB2 grains and increase the defect density. Moreover, the active area, Bc2 and flux pinning are improved, which is of benefit to the enhancement of Jc performance. The 80 h milled MgB2 bulk sample has a Jc value of 2.8???104?A?cm?2 at 5?K and 6?T, 41 times larger than that of the 0.5?h sample. At the same time, the transport Jc of the 80?h MgB2 tape reaches 2.3???103?A?cm?2 at 4.2?K and 9?T, 32 times larger than that of the 0.5?h one. Due to the increase of impurities and the release of lattice deformation and strain, the Jc was decreased for the sample milled up to 120?h.

The effects of chemical substitutions on the (Pb, Cd)-1212 superconducting cuprates

The effects of different substitutions on (Pb0.5Cd0.5)Sr2(Y0.5Ca0.5)Cu2O7 have been investigated. For (Pb0.5Cd0.5)Sr2(Y1-xCax)Cu2O7 (0.0 less-than-or-equal-to x less-than-or-equal-to 1.0), an anomalous variation of lattice constants with x is observed, and a sharp increase in T(c) occurs when x increases from 0.4 to 0.5. For (Pb0.5Cd0.5)Sr2-y(Y0.5+yCa0.5)Cu2O, (-0.2 less-than-or-equal-to y less-than-or-equal-to 0.3), as yttrium content increases the phase purity increases, and both the a and c lattice constants decrease. The superconducting properties deteriorate as y deviates from 0.0. For (Pb0.5Cd0.5)Sr2-z(Y0.5Ca0.5+z)Cu2O7 (-0.2 less-than-or-equal-to z less-than-or-equal-to 0.2), both the a and c lattice constants decrease as z increases. The superconducting properties deteriorate as z deviates from 0.0, although the effect is very small. From the T(c) results obtained in the three series, the conclusion can be reached that the yttrium content in the (Y, Ca) sites is a key factor in controlling the superconducting properties of the (Pb, Cd)- 1 212 phase.

Enhancement of Jc–B properties for binary and carbon-doped MgB2 tapes by hot pressing

The effect of the hot pressing method on in situ powder-in-tube-processed MgB2/Fe tapes was studied. The tapes doped with and without nanosized C are hot-pressed at 100?MPa and 650?800??C for 1?5?h in a flowing Ar gas atmosphere. Compared to the non-hot-pressed tapes, the Jc?B property (at 4.2?K and 10?T) of pure and C-doped samples hot-pressed at 650??C for 5?h was enhanced by a factor of 2.5 and 1.4, respectively. For hot-pressed tapes, the optimized sintering temperature is below 700??C at 100?MPa. The enhancement of Jc performance for hot-pressed MgB2 samples is mainly attributed to the improved grain connectivity.

Effect of starting materials on the superconducting properties of SmFeAsO1−xFx tapes

SmFeAsO1?xFx tapes were prepared using three kinds of starting materials. This showed that the starting materials have an obvious effect on the impurity phases in the final superconducting tapes. Compared with the other samples, the samples fabricated with SmAs, FeO, Fe2As, and SmF3 have the smallest arsenide impurity phases and voids. As a result, these samples possess much denser structures and better grain connectivities. Moreover, among the three kinds of sample fabricated in this work, this kind of sample has the highest zero resistivity temperature, ?40?K, and the largest critical current density, ?4600?A?cm?2, in self-field at 4.2?K. This is the highest Jc value reported so far for SmFeAsO1?xFx wires and tapes.

Doping effects of Nd2O3 on the superconducting properties of powder-in-tube MgB2 tapes

Nd2O3 doped Fe-sheathed MgB2 tapes with different doping levels were fabricated by the in?situ powder-in-tube method, and their superconducting properties were studied systematically. The x-ray diffraction (XRD) patterns showed the main phase of MgB2 along with the impurity phases such as MgO and NdB6 in the Nd2O3 doped samples. Compared to the undoped tapes, the critical current density Jc of all the doped samples was improved with only a slight suppression of the critical temperature Tc, and the 5?at.% Nd2O3 doped tapes achieved the highest Jc. Through the transmission electron microscope (TEM) study, nanoscale impurity particles were found in the 5?at.% Nd2O3 doped samples, and they were thought to be responsible for the enhanced pinning behavior and thus the increased critical current density.

THE CRYSTAL-STRUCTURE OF (PB0.5CD0.5)SR2(Y1-XCAX)CU2O7

The crystal structures of (Pb0.5Cd0.5)Sr2YCu2O7, (Pb0.5Cd0.5)Sr2(Y0.6Ca0.4)Cu2O7 and (Pb0.5Cd0.5)Sr2(Y0.5Ca0.5)Cu2O7 have been refined by the Rietveld method for the X-ray diffraction data. The refinement results indicate that both Pb and Cd atoms in the (Pb, Cd)O layers and O(3) atoms are displaced from their ideal sites, and that there is a complicated occupation in the (Pb, Cd)O layers, i.e., other cations, such as Sr2+, Cu2+ and Ca2+, can occupy the (Pb, Cd) site for these samples. This may be the reason why the lattice constants do not vary monotonically with the calcium content, and why there exist broad superconducting transitions in the system (Pb0.5Cd0.5)Sr2(Y1-xCax)Cu2O7. The refinement results also indicate that, as the calcium content increases, the O(2) atoms move close to the CuO2 planes and far from the (Pb, Cd)O layers. Thus, the increase of calcium content results in charge transfer from the charge reservoir layer (Pb, Cd)O to the CuO2 planes. This result is consistent with the bond valence sums of the Cu ions in the CuO2 planes.

Superconductivity and upper fields in Na-doped iron arsenides Eu1?xNaxFe2As2

In this paper, we report a systematic investigation of the crystal structure and superconducting properties of iron-based superconductors Eu1?xNaxFe2As2 (x?=?0?0.5). X-ray diffraction patterns indicate that the compounds form the ThCr2Si2-type structure with space group I4/mmm. The systematic evolution of the lattice constant demonstrates that the Eu ions are successfully replaced by Na. The use of alkali metal substitution into the Eu site allows us to suppress the magnetic/structural phase transition in the parent compounds and superconductivity reaches as high as 35?K with a doping level of x?=?0.5. In addition, single crystals of Eu1?xNaxFe2As2 (x?=?0, 0.5) have been successfully synthesized using the self-flux method. The upper critical fields have been determined with the magnetic field along the ab-plane and the c-axis, yielding an anisotropy of 1.7. The high upper critical fields and the low superconducting anisotropy of the Na-doped EuFe2As2 compounds indicate a potential for applications such as the generation of high magnetic fields.

Enhanced Jc–B properties of MgB2 tapes by yttrium acetate doping

We report the effect of yttrium acetate addition on in situ powder-in-tube processed MgB2/Fe tapes. The amount of yttrium acetate was varied from 0 to 20?wt% and sintering was performed at 700?850??C for 1?h in a flowing Ar atmosphere. We found that a significant Jc?B enhancement was easily achieved in high field by yttrium acetate doping. At 4.2?K, the transport Jc for the best yttrium-acetate-added tapes (10?wt%) reached almost 104?A?cm ? 2 at 12?T. The improvement of Jc performance in the yttrium-acetate-doped samples can be attributed to the combination of the carbon substitution for B and stronger flux pinning.

The pinning process of GdBa2Cu3O7−x films

Abstract We measured the pinning force density F p in high quality GdBa 2Cu 3 O 7−x films as a function of temperature ranging 63–78 K and magnetic field up to 7 T. The results indicate that the process of pinning of a -axis oriented platelets in GdBa 2 Cu 3 O 7−x films can be successfully explained by a transition from individual pinning. The anisotropic pinning possibly results from a two-dimensional pinning center, and flux-lattice melting for H ∥ C below and for H ⊥ C at H c 2 and an anisotropic vortex lattice.