[show abstract][hide abstract] ABSTRACT: The c-axis textured Sr1-xKxFe2As2 tapes produced by cold rolling and
post-annealing, could carry a high super-current over 2*104 A/cm2. However, the
magnitude is far from its maximum, because of the current obstacles associated
with various defects in the material. To predict the maximal transport critical
current density, we modeled the current paths in a c-axis textured polycrystal
as a three-dimensional flow network, and calculated the maximum flow with the
Ford-Fulkerson algorithm. It indicates that a much higher super-current of
about 2*105 A/cm2 could be achieved in an ideal c-axis textured K-doped 122
polycrystal. The dependences of transport Jc on density, content of invalid
boundary and grain size and shape were also studied. The results imply that,
over 30% of the grain boundaries in the reported c-axis textured Sr1-xKxFe2As2
tapes may act as current obstacles, and the large ratio of width to thickness
was expected to be the most favorable grain shape for high transport Jc in
c-axis textured 122 superconducting tapes.
Physica C Superconductivity 11/2012; 485. · 0.72 Impact Factor
[show abstract][hide abstract] ABSTRACT: Rationale: We and others have demonstrated that anthocyanins have antiatherogenic capability. Because intact anthocyanins are absorbed very poorly, the low level of circulating parent anthocyanins may not fully account for their beneficial effect. We found recently that protocatechuic acid (PCA), a metabolite of cyanidin-3 to 0-β-glucoside (Cy-3-G), has a remarkable antiatherogenic effect. Objective: To investigate whether mouse gut microbiota metabolizes Cy-3-G into PCA and to determine whether and how PCA contributes to the antiatherogenic potency of its precursor, Cy-3-G. Methods and Results: PCA was determined as a gut microbiota metabolite of Cy-3-G in ApoE(-/-) mice, verified by the utilization of antibiotics to eliminate gut microbiota and further microbiota acquisition. PCA but not Cy-3-G at physiologically reachable concentrations promoted cholesterol efflux from macrophages and macrophage ABCA1 and ABCG1 expression. By conducting a miRNA microarray screening, we revealed that expression of miRNA-10b in macrophages can be reduced by PCA. Functional analyses demonstrated that miRNA-10b directly represses ABCA1 and ABCG1 and negatively regulates cholesterol efflux from murine- and human-derived macrophages. Further in vitro and ex vivo analyses verified that PCA accelerates macrophage cholesterol efflux, correlating with the regulation of miRNA-10b-ABCA1/ABCG1 cascade, whereas Cy-3-G consumption promoted macrophage RCT and regressed atherosclerotic lesion in a gut microbiotaendependent manner. Conclusions: PCA, as the gut microbiota metabolite of Cy-3-G, exerts the antiatherogenic effect partially through this newly defined miRNA-10b-ABCA1/ABCG1-cholesterol efflux signaling cascade. Thus, gut microbiota is a potential novel target for atherosclerosis prevention and treatment.
Circulation Research 07/2012; 111(8):967-81. · 11.86 Impact Factor
[show abstract][hide abstract] ABSTRACT: In this paper we report the superconductivity above 40 K in the electron doping single crystals Ca1−xRxFe2As2 (R = La, Ce, Pr). The x-ray diffraction patterns indicate high crystalline quality and c-axis orientation. The resistivity anomaly in the parent compound CaFe2As2 is completely suppressed by partial replacement of Ca with rare-earth elements and the superconducting transition reaches as high as 46 K, which is higher than the value in electron doping FeAs-122 compounds formed by substituting Fe ions with transition metal, and even surpasses the highest value observed in hole doping systems with a transition temperature up to 38 K. The upper critical fields have been determined with the magnetic field along the ab-plane and c-axis, yielding an anisotropy of 2–3. Hall effect measurements indicate that the conduction in this material is dominated by electron-like charge carriers. Our results confirm the feasibility of inducing superconductivity in Ca122 compounds via electron doping using aliovalent rare-earth substitution into the alkaline earth site, which should add more ingredients to the underlying physics of the iron-based superconductors.
Superconductor Science and Technology 02/2012; 25(4):045007. · 2.76 Impact Factor
[show abstract][hide abstract] ABSTRACT: Fe-clad BaxK1-xFe2As2 superconducting tapes were fabricated by the ex situ
powder-in-tube method combined with a short high-temperature annealing
technique. The effect of annealing time and different dopants on the transport
properties of the BaxK1-xFe2As2 tapes were systematically studied. By co-doping
with Ag and Pb, the transport critical current density Jc of BaxK1-xFe2As2
tapes was significantly improved in whole field region and the highest
transport Jc was up to 1.4x10^4 A/cm^2 (Ic=100 A) at 4.2K in self field. It is
proposed that the superior Jc in the co-doped samples are due to the combine
effect of Pb doping at low fields and Ag doping at high fields.
[show abstract][hide abstract] ABSTRACT: In this paper we report superconductivity in single crystals of the new iron-pnictide system CaFe2−xIrxAs2 (x=0, 0.05, 0.10, 0.13) grown by a self-flux solution method and characterized via X-ray, transport and magnetic measurements. The magnetic ordering associated with a structural transition at 160 K present in CaFe2As2 is completely suppressed by substitution of Fe with Ir and superconductivity is induced at a critical temperature Tc=22 K in the sample with x=0.13. We also determined the upper critical fields along the ab-plane and c-axis. The anisotropy of the CaFe2−xIrxAs2 superconductors determined by the ratio of Habc2 and Hcc2 is estimated to be 1.5–2.2. Our results suggest that superconductivity can also be easily induced in the CaFe2As2 family by substituting the Fe with Ir, at almost the similar maximum transition temperatures as for Co, Ni and Rh doping.
[show abstract][hide abstract] ABSTRACT: We report the realization of grain alignment in Sn-added Sr1-xKxFe2As2
superconducting tapes prepared by ex-situ powder-in-tube method. At 4.2 K, high
transport critical current densities Jc of 2.5x10^4 A/cm^2 (Ic = 180 A) in
self-field and 3.5x10^3 A/cm^2 (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
[show abstract][hide abstract] ABSTRACT: 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.
[show abstract][hide abstract] ABSTRACT: 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/cm<sup>2</sup> (I<sub>c</sub>=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.
IEEE Transactions on Applied Superconductivity 07/2011; · 1.20 Impact Factor
[show abstract][hide abstract] ABSTRACT: We report here an atomic resolution study of the structure and composition of
the grain boundaries in polycrystalline Sr0.6K0.4Fe2As2 superconductor. A large
fraction of grain boundaries contain amorphous layers larger than the coherence
length, while some others contain nanometer-scale particles sandwiched in
between amorphous layers. We also find that there is significant oxygen
enrichment at the grain boundaries. Such results explain the relatively low
transport critical current density (Jc) of polycrystalline samples with respect
to that of bicrystal films.
[show abstract][hide abstract] ABSTRACT: Superconducting FeSe in the form of wires and tapes were successfully
fabricated using a novel gas diffusion procedure. Structural analysis by mean
of x-ray diffraction shows that themain phase of tetragonal PbO-type FeSe was
obtained by this synthesis method. The zero resistivity transition temperature
of the FeSe was confirmed to be 9.3 K. The critical current density as high as
137 A/cm^2 (4 K, self field) has been observed. The results suggest that the
diffusion procedure is promising in preparing high-quality FeSe wires and
Superconductor Science and Technology 03/2011; 24(6). · 2.76 Impact Factor
[show abstract][hide abstract] ABSTRACT: 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.
Superconductor Science and Technology 03/2011; 24(5):055016. · 2.76 Impact Factor
[show abstract][hide abstract] ABSTRACT: Single crystals of K0.58Fe1.56Se2 were successfully synthesized by a new
single step process with the onset superconducting transition temperatures 31.9
K. The x-ray diffraction patterns suggest that they have high crystalline
quality and c-axis orientation. A possible modulation structure of Fe-vacancy
along c axis was observed. The upper critical field has been determined with
the magnetic field along ab-plane and c-axis, yielding an anisotropy of about
3.3. It has also been shown that the critical current density of the
K0.58Fe1.56Se2 is about 1.7x10^4 A/cm^2 at 5 K.
[show abstract][hide abstract] ABSTRACT: We fabricated the c axis and ab-plane biaxially aligned Sr0.6K0.4Fe2As2
superconductor using a two-step magnetic field procedure. The effect of
magnetic fields on the structure and superconducting properties of
Sr0.6K0.4Fe2As2 has been investigated by using X-ray diffraction and magnetic
measurements. The degree of orientation of the samples was about 0.39 for the c
axis and 0.51 for ab-plane orientation, as evaluated from the Lotgering factor
of X-ray diffraction. This technology might be useful in a variety of potential
applications, including preparing iron based superconducting bulks and wires
with high critical currents.
[show abstract][hide abstract] ABSTRACT: Single crystals of Ba0.6K0.4Fe2As2 with excellent quality have been
successfully grown without fluxing agent through a simple one-step method for
the first time. X-ray diffraction patterns demonstrate that the samples have
high crystalline quality and c-axis orientation. The onset transition
temperature is up to 38 K with the zero resistivity temperature about 36.7 K.
Both the R-T and M-T data show a very sharp superconducting transition with
transition width 0.4 K. We also found that the samples possess very large
current carrying ability and high upper critical fields, indicating potential
applications requiring very high field. The above simple and safe one-step
technique of single crystal growth can be effective in other systems of
Superconductor Science and Technology 11/2010; 24(6). · 2.76 Impact Factor
[show abstract][hide abstract] ABSTRACT: In this paper we report the synthesis of iron-based superconductors
CaFe2-xRhxAs2 using one-step solid state reaction method, which crystallizes in
the ThCr2Si2-type structure with a space group I4/mmm. The systematic evolution
of the lattice constants demonstrates that the Fe ions are successfully
replaced by the Rh. By increasing the doping content of Rh, the
spin-density-wave (SDW) transition in the parent compound is suppressed and
superconductivity emerges. The maximum superconducting transition temperature
is found at 18.5 K with the doping level of x = 0.15. The temperature
dependence of DC magnetization confirms superconducting transitions at around
15 K. The general phase diagram was obtained and found to be similar to the
case of Rh-doping Sr122 system. Our results explicitly demonstrate the
feasibility of inducing superconductivity in Ca122 compounds by higher
d-orbital electrons doping, however, different Rh-doping effect between FeAs122
compounds and FeAs1111 systems still remains an open question.
New Journal of Physics 05/2010; 13(3). · 4.06 Impact Factor
[show abstract][hide abstract] ABSTRACT: We report a study on the effect of sintering temperature on the microstructure and superconducting properties of Sr0.6K0.4Fe2As2 bulk samples, which were synthesized by the one-step solid reaction method. The heat treatment temperatures were varied from 700 to 900 °C in an argon atmosphere. It was found that the annealing temperature had little influence on the critical temperature Tc. However, the irreversibility field Hirr and critical current density Jc were significantly affected by the sintering temperature; for instance, the Jc at both 5 and 20 K increased with increasing the synthesis temperatures up to 850 °C, and then further increasing the temperature hardly affected the Jc improvement. In addition, samples heat treated at temperatures over 850 °C exhibited a very weak Jc-field dependence, especially at 20 K. Characterizations revealed that high temperature sintering resulted in large grains with fewer impurities.
Superconductor Science and Technology 05/2010; 23(6):065009. · 2.76 Impact Factor
[show abstract][hide abstract] ABSTRACT: The superconducting properties of polycrystalline Sr0.6K0.4Fe2As2 were strongly influenced by Ag doping (Supercond. Sci. Technol. 23 (2010) 025027). Ag addition is mainly dominated by silver diffusing, so the annealing process is one of the essential factors to achieve
high quality Ag doped Sr0.6K0.4Fe2As2. In this paper, the optimal annealing conditions were studied for Ag doped Sr0.6K0.4Fe2As2 bulks prepared by a one-step solid reaction method. It is found that the annealing temperature has a strong influence on
the superconducting properties, especially on the critical current density J
c. As a result, higher heat treatment temperature (∼900°C) is helpful in diffusing Ag and reducing the impurity phase gathered
together to improve the grain connectivity. In contrast, low-temperature sintering is counterproductive for Ag doped samples.
These results clearly suggest that annealing at ∼900°C is necessary for obtaining high J
c Ag-doped samples.
-iron-based superconductor-heat treatment
[show abstract][hide abstract] ABSTRACT: The multiferroic composites with nominal stoichiometry Bi <sub>0.8</sub> Dy <sub>0.2</sub> Fe <sub>1-y</sub> Ti <sub>y</sub> O <sub>3+δ</sub> were fabricated by adding Ti to Bi <sub>0.8</sub> Dy <sub>0.2</sub> FeO <sub>3</sub> through a two-stage solid-state reaction method. In the composites obtained, the major phase was isostructural to Bi <sub>0.8</sub> Dy <sub>0.2</sub> FeO <sub>3</sub> , and the secondary phase of Dy <sub>3</sub> Fe <sub>5</sub> O <sub>12</sub> was also detected. Most interestingly, it was found that both the magnetization and electrical polarization were encouraged by the secondary phase Dy <sub>3</sub> Fe <sub>5</sub> O <sub>12</sub> . In addition to the improved ferroelectric property, a large saturation magnetization of 2.07 emu/g was reached for y≥0.03 samples. The simultaneously enhanced ferroelectric and magnetic properties achieved in the composites due to the existence of ferrimagnetic insulated Dy <sub>3</sub> Fe <sub>5</sub> O <sub>12</sub> may have implications for further studies on BiFeO <sub>3</sub> based materials.
Journal of Applied Physics 04/2010; · 2.21 Impact Factor
[show abstract][hide abstract] ABSTRACT: The discovery of iron-based superconductors, the first non-cuprate family of superconductors with Tc above 40 K, has stimulated enormous interest in the field of superconductivity since last year. This remarkable discovery not only offers the opportunity to study the origin of superconductivity, but also opens up new possibilities of application. One of the most fascinating and useful properties of superconductors is the ability to carry electrical current with zero resistance. Here, we report the successful fabrication of dense Sr0.6K0.4Fe2As2 superconducting wires using the ex situ powder-in-tube (PIT) method and demonstrate a transport Jc of 3750 A cm − 2 at 4.2 K. The connectivity of grains was improved upon doping (Ag or Pb) and the transport property of Sr0.6K0.4Fe2As2 wires was enhanced for a lead-doped sample, especially in low fields, to a best Ic of 37.5 A. Our results suggest that grain boundary properties require much greater attention when looking for applications of the high-Tc iron-based superconductors.
Superconductor Science and Technology 03/2010; 23(5):055009. · 2.76 Impact Factor