163 research outputs found
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
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
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
Application prospects of MgB in view of its basic properties
Seven years after the discovery of superconductivity in magnesium diboride,
the fundamental superconducting properties of this compound are well known and
the peculiar current transport in polycrystalline materials is essentially
understood. Based on this knowledge the ultimate performance of wires or tapes
at high magnetic fields will be predicted and compared to state-of-the-art
materials and to other superconductors. The key parameter for high field
applications is the upper critical field, which can be strongly enhanced by
impurity scattering. This fundamental property might be further optimized in
bulk materials, since higher values were reported for thin films. The MgB
grains are usually very small, if prepared by the in-situ technique. The
resulting high density of grain boundaries leads to strong pinning, close to
the theoretical limit. On the other hand, the connectivity between the grains
is still rather poor and strongly reduces the achievable critical currents,
thus leaving room for further improvements
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