3,933 research outputs found
Improvement of critical current in MgB2/Fe wires by a ferromagnetic sheath
Transport critical current (Ic) was measured for Fe-sheathed MgB2 round
wires. A critical current density of 5.3 x 10^4 A/cm^2 was obtained at 32K.
Strong magnetic shielding by the iron sheath was observed, resulting in a
decrease in Ic by only 15% in a field of 0.6T at 32K. In addition to shielding,
interaction between the iron sheath and the superconductor resulted in a
constant Ic between 0.2 and 0.6T. This was well beyond the maximum field for
effective shielding of 0.2T. This effect can be used to substantially improve
the field performance of MgB2/Fe wires at fields at least 3 times higher than
the range allowed by mere magnetic shielding by the iron sheath. The dependence
of Ic on the angle between field and current showed that the transport current
does not flow straight across the wire, but meanders between the grains
Comparison of small-field behavior in MgB2, Low- and high-temperature superconductors
Different types of superconductors have been investigated at small magnetic fields (Ba) over wide temperature (T) ranges at different Ba orientations. It has been shown that the temperature dependence of the characteristic field (B*), separating the Ba-independent critical current density (Jc) plateau (single vortex pinning regime) and the region with Jc(Ba) (collective pinning), can be attributed either to the temperature dependence of the magnetic penetration depth for Nb-film and MgB2 bulk superconductors, or to thermally activated processes for Bi-based superconductors and YBa2Cu3O7−8 superconducting films. In both cases the vortex pinning influence appears to have a secondary role, affecting the effective vortex depinning radius. An exception in such B*(T) behavior is considered for Nb film when the magnetic field has its considerable component applied perpendicular to the main surface of the film
Properties of superconducting MgB_2 wires: "in-situ" versus "ex-situ" reaction technique
We have fabricated a series of iron-sheathed superconducting wires prepared
by the powder-in-tube technique from (MgB_2)_{1-x}:(Mg+2B)_x initial powder
mixtures taken with different proportions, so that x varies from 0 to 1. It
turned out that "ex-situ" prepared wire (x = 0) has considerable disadvantages
compared to all the other wires in which "in-situ" assisted (0 < x < 1) or pure
"in-situ" (x = 1) preparation was used due to weaker inter-grain connectivity.
As a result, higher critical current densities J_c were measured over the
entire range of applied magnetic fields B_a for all the samples with x > 0.
Pinning of vortices in MgB_2 wires is shown to be due to grain boundaries.
J_c(B_a) behavior is governed by an interplay between the transparency of grain
boundaries and the amount of "pinning" grain boundaries. Differences between
thermo-magnetic flux-jump instabilities in the samples and a possible threat to
practical applications are also discussed.Comment: To be published in Supercond. Sci. Technol. (2003), in pres
The effect of sample size on field dependence of Jc for MgB2 superconductor
A strong effect of sample size on magnetic Jc(H) was observed for bulk MgB2
when Jc is obtained directly from the critical state model. Thus obtained
zero-field Jc (Jc0) decreases strongly with the sample size, attaining a
constant value for the samples larger than a few millimetres. On the other
hand, the irreversibility field (Hirr) defined at Jc = 100 A/cm2 increases with
the sample size. The decrease of Jc0 is described in terms of voids in the bulk
MgB2 samples and superconducting screening around the cells of superconducting
material between these voids (35 micro-m), because of concentration of the
current in the narrow bridges connecting the cells. For samples larger than a
few millimetres, the value of magnetic Jc is in agreement with the transport Jc
and it is restricted by the voids. The critical state model is not suitable for
obtaining Jc for small bulk MgB2. The increase of Hirr with the sample size is
an artefact of defining Hirr by the value of Jc at which an additional
superconducting screening on 1mm scale dominates Dm
Transport critical current of Solenoidal MgB2/Cu Coils Fabricated Using a Wind-Reaction In-situ Technique
In this letter, we report the results of transport Jc of solenoid coils upto
100 turns fabricated with Cu-sheathed MgB2 wires using a wind-reaction in-situ
technique. Despite the low density of single core and some reaction between Mg
and Cu-sheath, our results demonstrate the decrease in transport Jc with
increasing length of MgB2 wires is insignificant. Solenoid coils with diameter
as small as 10 mm can be readily fabricated using a wind-reaction in-situ
technique. The Jc of coils is essentially the same as in the form of straight
wires. A Jc of 133,000 A/cm2 and 125,000 A/cm2 at 4 K and self field has been
achieved for a small coil wound using Cu-sheathed tape and Cu-sheathed wire
respectively. These results indicate that the MgB2 wires have a great potential
for lage scale applicationsComment: 6 pages, 4 figures, 1 tabl
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