Zhaoshun Gao

Impact of annealing on morphology and ferromagnetism of ZnO nanorods

Room temperature ferromagnetism has been observed in ZnO nanorods prepared by hydrothermal method. Saturation magnetization of ∼0.004emu∕g was measured in the nanorods with diameters of ∼10nm and lengths of below 100nm, and the magnetization reduced with the increase of the size. Annealing of the samples at 900°C in air completely transformed the nanorods into twinning structures and weakened the magnetizations. The mechanism of morphology transformation was discussed. Analysis indicates that the interstitial zinc at the surface may contribute to the ferromagnetism in ZnO nanorods.

Achievement of practical level critical current densities in Ba1−xKxFe2As2/Ag tapes by conventional cold mechanical deformation

The recently discovered iron-based superconductors are potential candidates for high-field magnet applications. However, the critical current densities (Jc) of iron-based superconducting wires remain far below the level needed for practical applications. Here, we show that the transport Jc of Ba1−xKxFe2As2/Ag tapes is significantly enhanced by the combination process of cold flat rolling and uniaxial pressing. At 4.2 K, Jc exceeds the practical level of 105 A/cm2 in magnetic fields up to 6 T. The Jc-H curve shows extremely small magnetic field dependence and maintains a high value of 8.6 × 104 A/cm2 in 10 T. These are the highest values reported so far for iron-based superconducting wires. Hardness measurements and microstructure investigations reveal that the superior Jc in our samples is due to the high core density, more textured grains, and a change in the microcrack structure. These results indicate that iron-based superconductors are very promising for high magnetic field applications.

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.

The role of silver addition on the structural and superconducting properties of polycrystalline Sr0.6K0.4Fe2As2

The effect of Ag addition (0–20 wt%) on polycrystalline Sr0.6K0.4Fe2As2 superconductor has been investigated. It is found that the critical transition temperature Tc was not depressed, and the irreversibility field Hirr and hysteresis magnetization were significantly enhanced upon Ag addition. A characterization study reveals that larger grains are observed in the Ag-added samples. Moreover, the formation of a glassy phase as well as an amorphous layer, which are present in almost all the grain edges and boundaries in pure samples, are suppressed by Ag addition. The improvement of superconducting properties in Ag-added samples may originate from the enlargement of grains as well as better connections between grains

Superconductivity at 34.7 K in the iron arsenide Eu0.7Na0.3Fe2As2

EuFe2As2 is a member of the ternary iron arsenide family. Similar to BaFe2As2 and SrFe2As2, EuFe2As2 exhibits a clear anomaly in resistivity near 200 K. Here, we report the discovery of superconductivity in Eu0.7Na0.3Fe2As2 by partial substitution of the europium site with sodium. ThCr2Si2 tetragonal structure, as expected for EuFe2As2, is formed as the main phase for the composition Eu0.7Na0.3Fe2As2. Resistivity measurements reveal that the transition temperature Tc as high as 34.7 K is observed in this compound. The rate of Tc suppression with the applied magnetic field is 3.87 T K−1, giving an extrapolated zero-temperature upper critical field of 100 T. It demonstrates a very encouraging application of the new superconductors.

High critical current density and low anisotropy in textured Sr1−xKxFe2As2 tapes for high field applications

From the application point of view, large critical current densities Jc (H) for superconducting wires are required, preferably for magnetic fields higher than 5 T. Here we show that strong c-axis textured Sr1−xKxFe2As2 tapes with nearly isotropic transport Jc were fabricated by an ex-situ powder-in-tube (PIT) process. At 4.2 K, the Jc values show extremely weak magnetic field dependence and reach high values of 1.7×104 A/cm2 at 10 T and 1.4×104 A/cm2 at 14 T, respectively, these values are by far the highest ever reported for iron based wires and approach the Jc level desired for practical applications. Transmission electron microscopy investigations revealed that amorphous oxide layers at grain boundaries were significantly reduced by Sn addition which resulted in greatly improved intergranular connectivity. Our results demonstrated the strong potential of using iron based superconductors for high field applications.

Development of Powder-in-Tube Processed Iron Pnictide Wires and Tapes

The development of PIT fabrication process of iron pnictide superconducting wires and tapes has been reviewed. Silver was found to be the best sheath material, since no reaction layer was observed between the silver sheath and the superconducting core. The grain connectivity of iron pnictide wires and tapes has been markedly improved by employing Ag or Pb as dopants. At present, critical current densities in excess of 3750 A/cm2 (Ic=37.5A) at 4.2 K have been achieved in Ag-sheathed SrKFeAs wires prepared with the above techniques, which is the highest value obtained in iron-based wires and tapes so far. Moreover, Ag-sheathed Sm-1111 superconducting tapes were successfully prepared by PIT method at temperatures as low as ~900°C, instead of commonly used temperatures of ~1200°C. These results demonstrate the feasibility of producing superconducting pnictide composite wires, even grain boundary properties require much more attention.

Synthesis and properties of La-doped CaFe2As2 single crystals with Tc=42.7 K

Large single crystals of La-doped CaFe2As2 were successfully synthesized by the FeAs self-flux method. The X-ray diffraction patterns suggest high crystalline quality and c-axis orientation. By substitution of trivalent La3+ ions for divalent Ca2+, the resistivity anomaly in the parent compound CaFe2As2 is completely suppressed and the superconducting transition temperature reaches the value of 42.7 K, which is higher than that of about 30 K reported in Saha S. R. et al., arXiv:1105.4798v1. The upper critical field has been determined with the magnetic field along the ab-plane and c-axis, yielding an anisotropy of about 3.3.

Strongly enhanced critical current density in MgB2/Fe tapes by stearic acid and stearate doping

Fe-sheathed MgB2 tapes with cheap stearic acid, Mg stearate and Zn stearate doping were prepared through the in situ powder-in-tube method. It is found that Jc, Hirr and Hc2 were significantly enhanced by doping. Compared to the pure tapes, Jc for all the doped samples was improved by more than an order of magnitude in a high-field regime. At 4.2?K, the transport Jc reached 2.02 ? 104?A?cm?2 at 10?T for the Zn stearate doped tapes and 3.72 ? 103?A?cm?2 at 14?T for the stearic acid doped samples, respectively. Moreover, at 20?K, Hirr for the Zn stearate doped tape achieved 10?T, which is comparable to that of the commercial NbTi at 4.2?K. The improvement of Jc?H properties in doped samples can be attributed to the increase of Hc2 resulting from the incorporation of C atoms into the MgB2 crystal lattice as well as a high density of flux-pinning centres.

Enhancement of the critical current density and the irreversibility field in maleic anhydride doped MgB2 based tapes

A significant enhancement of Jc, Hirr, and Hc2 in MgB2 tapes has been achieved by using maleic anhydride (C4H2O3) as dopants. MgB2 tape samples with 0 up to 30wt% C4H2O3 added were prepared by an in situ processed powder-in-tube method. Compared to the pure tapes, Jc for the 10wt% C4H2O3 doped samples was improved by more than an order of magnitude in high fields up to 18T. At 4.2K, the transport Jc for the 10wt% doped samples reached 1.08×104, 5.42×103, and 2.18×103A∕cm2 at 12, 14, and 16T, respectively, and were better than those of the best MgB2∕Fe tapes reported by far. Furthermore, the Hirr for the 10wt% doped sample reached 9.6T at 20K, which was comparable to that of NbTi at 4.2K. The improvement of the superconducting properties in doped tapes can be attributed to the increase of Hc2 and the grain size refinement by C4H2O3 doping.