119 research outputs found
Development of Powder-in-Tube Processed Iron Pnictide Wires and Tapes
The development of the PIT fabrication process of iron pnictide
superconducting wires and tapes has been carried out in order to enhance their
transport properties. 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^2 (Ic = 37.5 A) at 4.2 K have been achieved on
Ag-sheathed SrKFeAs wires prepared with the above techniques, which is the
highest 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 900C, instead of commonly used temperatures of 1200C. These results
demonstrate the feasibility of producing superconducting pnictide composite
wires, even grain boundary properties require much more attention.Comment: 4 pages, 6 figures. Submitted to ASC2010 proceeding
Enhanced critical current properties in Ba0.6K0.4+xFe2As2 superconductor by over-doping of potassium
Phase-pure polycrystalline Ba0.6K0.4+xFe2As2 with were prepared using a
one-step solid-state reaction method. We found that over-doping of potassium
can improve critical current density (Jc). High-field Jc for samples with x =
0.1 is three times higher than that for samples with x = 0. Over-doping of K
has minimal effect on the critical transition temperature (Tc). Less than 0.5 K
degradations in Tc was measured for samples with x = 0.1. TEM revealed high
concentration of dislocations in samples with x = 0.1, resulting in enhanced
flux pining. Further analyses on magnetization loops for powder samples confirm
that K over-doping can promote intra-grain Jc. Our results indicate that slight
excess of K in Ba0.6K0.4Fe2As2 samples is beneficial to high-field
applications.Comment: 13 pages, 4 figure
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 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
superconductors.Comment: 14 pages, 4 figure
Superconductivity induced by doping Ru in SrFe2-xRuxAs2
Using one-step solid state reaction method, we have successfully synthesized
the superconductor SrFe1-xRuxAs. X-ray diffraction indicates that the material
has formed 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 Ru. By increasing the doping content of Ru, the
spin-density-wave (SDW) transition in the parent compound is suppressed and
superconductivity emerges. The maximum superconducting transition temperature
is found at 13.5 K with the doping level of x = 0.7. The temperature dependence
of DC magnetization confirms superconducting transitions at around 12 K. Our
results indicate that similar to non-isoelectronic substitution, isoelectronic
substitution contributes to changes in both the carrier concentration and
internal pressure, and superconductivity could be induced by isoelectronic
substitution.Comment: 14 pages, 4 figure
Direct observation of nanometer-scale amorphous layers and oxide crystallites at grain boundaries in polycrystalline Sr1-xKxFe2As2 superconductors
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.Comment: 12 pages, 4 figure
Fabrication and characterization of iron pnictide wires and bulk materials through the powder-in-tube method
The recent discovery of superconductivity in the iron based superconductors
with very high upper critical fields presents a new possibility for practical
applications, but fabricating fine-wire is a challenge because of mechanically
hard and brittle powders and the toxicity and volatility of arsenic. In this
paper, we report the synthesis and the physical characterization of iron
pnictide wires and bulks prepared by the powder-in-tube method (PIT). A new
class of high-Tc iron pnictide composite wires, such as LaFeAsO1-xFx,
SmFeAsO1-xFx and Sr1-xKxFeAs, has been fabricated by the in situ PIT technique
using Fe, Ta and Nb tubes. Microscopy and x-ray analysis show that the
superconducting core is continuous, and retains phase composition after wire
drawing and heat treatment. Furthermore, the wires exhibit a very weak Jc-field
dependence behavior even at high temperatures. The upper critical field Hc2(0)
value can exceed 100 T, surpassing those of MgB2 and all the low temperature
superconductors and indicating a strong potential for applications requiring
very high field. These results demonstrate the feasibility of producing
superconducting pnictide composite wire. We also applied the one step PIT
method to synthesize the iron-based bulks, due to its convenience and safety.
In fact, by using this technique, we have successfully discovered
superconductivity at 35 K and 15 K in Eu0.7Na0.3Fe2As2 and SmCoFeAsO compounds,
respectively. These clearly suggest that the one-step PIT technique is unique
and versatile and hence can be tailored easily for other rare earth derivatives
of novel iron-based superconductors.Comment: Review for the special issue of Physica C on iron-based pnictide
superconductor
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