301 research outputs found
Significant enhancement of irreversibility field in clean-limit bulk MgB2
Low resistivity ("clean") MgB2 bulk samples annealed in Mg vapor show an
increase in upper critical field Hc2(T) and irreversibility field Hirr(T) by a
factor of 2 in both transport and magnetic measurements. The best sample
displayed Hirr above 14 T at 4.2 K and 6 T at 20 K. These changes were
accompanied by an increase of the 40 K resistivity from 1.0 to 18 microohm-cm
and a lowering of the resistivity ratio from 15 to 3, while the critical
temperature Tc decreased by only 1-2 K. These results point the way to make
prepare MgB2 attractive for magnet applications.Comment: 3 pages, 4 figures, submitted to Applied Physics Letter
Influence of growth temperature on the vortex pinning properties of pulsed laser deposited YBa(2)Cu(3)O(7-x) thin films
Epitaxial high-temperature superconducting YBa(2)Cu(3)O(7-x) thin films grown on 2 degrees miscut (001) (LaAlO(3))(0.3)-(SrAl(0.5)Ta(0.5)O(3))(0.7) substrates by pulsed laser deposition show significant and systematic changes in flux pinning properties on changing the substrate temperature from 730 to 820 degrees C. The bulk pinning force is highest for the 760 degrees C growth, rising to a maximum of 4.4 GN/m(3) at 77 K, though there are indications that vortex pinning strength is even higher for the 730 degrees C growth once allowance for the current-blocking effects of a-axis oriented grains is made. Cross-sectional transmission electron microscope images show that the density of antiphase boundaries, stacking faults, and edge dislocations increases strongly with decreasing growth temperature, and is highest at 730 degrees C. In spite of the enhanced density of the pinning defects mentioned above, their vortex pinning effect is still much smaller than for insulating nanoparticles of high density and optimum size, where pinning forces can be four to five times higher.open121
Local Measurement of Current Density by Magneto-Optical Current Reconstruction in Normally and Overpressure Processed Bi-2223 Tapes
Magneto-optical current reconstruction has been used for detailed analysis of
the local critical current density (Jc) variation in monocore Bi-2223 tapes. We
find, even in high quality tapes with bulk transport Jc ~ 40 kA/cm^2 (77K, 0T),
that there exist local regions which possess current densities of more than 200
kA/cm^2. Overpressure processing at 148 bar significantly improved Jc to 48
kA/cm^2 by improving the connectivity. For the overpressure-processed sample we
find that the current distribution is more uniform and that the maximum local
current density at 77 K is increased almost to 300 kA/cm^2.Comment: Presented at Applied Superconductivity Conference, Houston, August
4th -9th, 200
Development of high critical current density in multifilamentary round-wire Bi2Sr2CaCu2O8+x by strong overdoping
Bi2Sr2CaCu2O8+x is the only cuprate superconductor that can be made into a
round-wire conductor form with a high enough critical current density Jc for
applications. Here we show that the Jc(5 T,4.2 K) of such Ag-sheathed
filamentary wires can be doubled to more than 1.4x10^5 A/cm^2 by low
temperature oxygenation. Careful analysis shows that the improved performance
is associated with a 12 K reduction in transition temperature Tc to 80 K and a
significant enhancement in intergranular connectivity. In spite of the
macroscopically untextured nature of the wire, overdoping is highly effective
in producing high Jc values.Comment: 4 figure
Effect of grain refinement on enhancing critical current density and upper critical field in undoped MgB2 ex-situ tapes
Ex-situ Powder-In-Tube MgB2 tapes prepared with ball-milled, undoped powders
showed a strong enhancement of the irreversibility field H*, the upper critical
field Hc2 and the critical current density Jc(H) together with the suppression
of the anisotropy of all of these quantities. Jc reached 104 A/cm2 at 4.2 K and
10 T, with an irreversibility field of about 14 T at 4.2 K, and Hc2 of 9 T at
25 K, high values for not-doped MgB2. The enhanced Jc and H* values are
associated with significant grain refinement produced by milling of the MgB2
powder, which enhances grain boundary pinning, although at the same time also
reducing the connectivity from about 12% to 8%. Although enhanced pinning and
diminished connectivity are in opposition, the overall influence of ball
milling on Jc is positive because the increased density of grains with a size
comparable with the mean free path produces strong electron scattering that
substantially increases Hc2, especially Hc2 perpendicular to the Mg and B
planes.Comment: 26 pages, 9 figures, submitted to J. Appl. Phy
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