49 research outputs found
Effect of high-intensity ultrasound on superconducting properties of polycrystalline YBCO
High intensity ultrasonic irradiation (sonication) of alkane slurries of
polycrystalline \ybco leads to a significant modification of the grain
morphology and, if performed with enforced oxygen flow, results in the increase
of the superconducting transition temperature. Sonication with added Fe(CO)
produces magnetic \fe2o3 nanoparticles deposited on the surface of \ybco (YBCO)
granules. Upon sintering these nanoparticles should act as efficient pinning
centers utilizing both condensation and magnetic contributions to the free
energy. The developed method could become a major technique to produce
practically useful high-pinning nanocomposite materials based on \ybco
superconductor.Comment: to be presented at LT-25 conferenc
Bulk high-Tc superconductors with drilled holes: how to arrange the holes to maximize the trapped magnetic flux ?
Drilling holes in a bulk high-Tc superconductor enhances the oxygen annealing
and the heat exchange with the cooling liquid. However, drilling holes also
reduces the amount of magnetic flux that can be trapped in the sample. In this
paper, we use the Bean model to study the magnetization and the current line
distribution in drilled samples, as a function of the hole positions. A single
hole perturbs the critical current flow over an extended region that is bounded
by a discontinuity line, where the direction of the current density changes
abruptly. We demonstrate that the trapped magnetic flux is maximized if the
center of each hole is positioned on one of the discontinuity lines produced by
the neighbouring holes. For a cylindrical sample, we construct a polar
triangular hole pattern that exploits this principle; in such a lattice, the
trapped field is ~20% higher than in a squared lattice, for which the holes do
not lie on discontinuity lines. This result indicates that one can
simultaneously enhance the oxygen annealing, the heat transfer, and maximize
the trapped field
Pulsed-field magnetization of drilled bulk high-temperature superconductors: flux front propagation in the volume and on the surface
We present a method for characterizing the propagation of the magnetic flux
in an artificially drilled bulk high-temperature superconductor (HTS) during a
pulsed-field magnetization. As the magnetic pulse penetrates the cylindrical
sample, the magnetic flux density is measured simultaneously in 16 holes by
means of microcoils that are placed across the median plane, i.e. at an equal
distance from the top and bottom surfaces, and close to the surface of the
sample. We discuss the time evolution of the magnetic flux density in the holes
during a pulse and measure the time taken by the external magnetic flux to
reach each hole. Our data show that the flux front moves faster in the median
plane than on the surface when penetrating the sample edge; it then proceeds
faster along the surface than in the bulk as it penetrates the sample further.
Once the pulse is over, the trapped flux density inside the central hole is
found to be about twice as large in the median plane than on the surface. This
ratio is confirmed by modelling
Measurement of the magnetic field inside the holes of a drilled bulk high-Tc superconductor
We use macroscopic holes drilled in a bulk YBCO superconductor to probe its
magnetic properties in the volume of the sample. The sample is subjected to an
AC magnetic flux with a density ranging from 30mT to 130mT and the flux in the
superconductor is probed by miniature coils inserted in the holes. In a given
hole, three different penetration regimes can be observed: (i) the shielded
regime, where no magnetic flux threads the hole; (ii) the gradual penetration
regime, where the waveform of the magnetic field has a clipped sine shape whose
fundamental component scales with the applied field; and (iii) the flux
concentration regime, where the waveform of the magnetic field is nearly a sine
wave, with an amplitude exceeding that of the applied field by up to a factor
of two. The distribution of the penetration regimes in the holes is compared
with that of the magnetic flux density at the top and bottom surfaces of the
sample, and is interpreted with the help of optical polarized light micrographs
of these surfaces. We show that the measurement of the magnetic field inside
the holes can be used as a local characterization of the bulk magnetic
properties of the sample
Influence of crossed fields in structures combining large grain, bulk (RE)BCO superconductors and soft ferromagnetic discs
peer reviewedBulk (RE)BCO superconductors are able to trap record magnetic fields and can be
used as powerful permanent magnets in various engineering applications such as rotating
machines and magnetic bearings. When such superconducting (SC) “trapped field magnets” are
combined to a ferromagnetic (FM) disc, the total magnetic moment is increased with respect to
that of the superconductor alone. In the present work, we study experimentally the magnetic
behaviour of such hybrid FM/SC structures when they are subjected to cycles of applied field
that are orthogonal to their permanent magnetization, i.e. a “crossed-field” configuration.
Experimental results show that the usual “crossed-field demagnetization” caused by the cycles
of transverse field is strongly reduced in the presence of the ferromagnet.ARC 11/16-0