292 research outputs found
Consequences of the peculiar intrinsic properties of MgB2 on its macroscopic current flow
The influence of two important features of magnesium diboride on the
macroscopic transport properties of polycrystalline MgB2 is discussed in the
framework of a percolation model. While two band superconductivity does not
have significant consequences in the field and temperature range of possible
power applications, the opposite is true for the anisotropy of the upper
critical field. The field dependence of the critical current densities strongly
increases and the macroscopic supercurrents disappear well below the apparent
upper critical field. The common scaling laws for the field dependence of the
volume pinning force are altered and Kramer's plot is no longer linear,
although grain boundary pinning dominates in nearly all polycrystalline MgB2
conductors. In contrast to the conventional superconductors NbTi and Nb3Sn, a
significant critical current anisotropy can be induced by the preparation
technique of MgB2 tapes
Influence of disorder on Hc2-anisotropy and flux pinning in MgB2
The upper critical field and flux pinning in MgB2 single crystals were
investigated. The implications of these properties for technical applications
are discussed and compared with transport properties of polycrystalline bulk
samples and wires. In these untextured materials current percolation is
important, especially at high magnetic fields. It is shown that the anisotropy
of the upper critical field influences the "irreversibility line" and that the
application range of MgB2 is limited by the smallest upper critical field
(i.e., for the field direction perpendicular to the boron planes). Disorder,
introduced by irradiation with neutrons, enhances the upper critical field,
reduces the anisotropy and drastically changes flux pinning. While the enhanced
Hc2 and the reduced anisotropy generally improve the transport properties of
the polycrystalline samples, the contribution of the radiation-induced defects
to flux pinning is small compared to the as-grown defect structure (grain
boundary pinning).Comment: 7 pages, 13 figure
Influence of the upper critical field anisotropy on the transport properties of polycrystalline MgB
The intrinsic properties of MgB form the basis for all applications of
this superconductor. We wish to emphasize that the application range of
polycrystalline MgB is limited by the upper critical field H and its
anisotropy. In wires or tapes, the MgB grains are randomly oriented or only
slightly textured and the anisotropy of the upper critical field leads to
different transport properties in different grains, if a magnetic field is
applied and the current transport becomes percolative. The irreversibility line
is caused by the disappearance of a continuous superconducting current path and
not by depinning as in high temperature superconductors. Based on a percolation
model, we demonstrate how changes of the upper critical field and its
anisotropy and how changes of flux pinning will influence the critical currents
of a wire or a tape. These predictions are compared to results of neutron
irradiation experiments, where these parameters were changed systematically
Neutron irradiation of coated conductors
Various commercial coated conductors were irradiated with fast neutrons in
order to introduce randomly distributed, uncorrelated defects which increase
the critical current density, Jc, in a wide temperature and field range. The
Jc-anisotropy is significantly reduced and the angular dependence of Jc does
not obey the anisotropic scaling approach. These defects enhance the
irreversibility line in not fully optimized tapes, but they do not in
state-of-the-art conductors. Neutron irradiation provides a clear distinction
between the low field region, where Jc is limited by the grain boundaries, and
the high field region, where depinning leads to dissipation
Anisotropic critical currents in FeSe0.5Te0.5 films and the influence of neutron irradiation
We report on measurements of the superconducting properties of FeSe05Te05
thin films grown on lanthanum aluminate. The films have high transition
temperatures (above 19 K) and sharp resistive transitions in fields up to 15 T.
The temperature dependence of the upper critical field and the irreversibility
lines are steep and anisotropic, as recently reported for single crystals. The
critical current densities, assessed by magnetization measurements in a vector
VSM, were found to be well above 10^9 Am-2 at low temperatures. In all samples,
the critical current as a function of field orientation has a maximum, when the
field is oriented parallel to the film surface. The maximum indicates the
presence of correlated pinning centers. A minimum occurs in three films, when
the field is applied perpendicular to the film plane. In the forth film,
instead, a local maximum caused by c-axis correlated pinning centers was found
at this orientation. The irradiation of two films with fast neutrons did not
change the properties drastically, where a maximum enhancement of the critical
current by a factor of two was found
Critical currents in weakly textured MgB2: Nonlinear transport in anisotropic heterogeneous media
A model for highly non-linear transport in heterogeneous media consisting of
anisotropic particles with a preferred orientation is proposed and applied to
the current transport in weakly textured magnesium diboride, MgB2. It
essentially explains why, unlike in conventional superconductors, a significant
macroscopic anisotropy of the critical currents can be induced by the
preparation of MgB2 tapes. The field and angular dependence of the critical
current is calculated for various degrees of texture and compared to
experimental data
Disorder effects on the superconducting properties of BaFeCoAs single crystals
Single crystals of superconducting BaFeCoAs were exposed
to neutron irradiation in a fission reactor. The introduced defects decrease
the superconducting transition temperature (by about 0.3 K) and the upper
critical field anisotropy (e.g. from 2.8 to 2.5 at 22 K) and enhance the
critical current densities by a factor of up to about 3. These changes are
discussed in the context of similar experiments on other superconducting
materials
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