38 research outputs found
Strongly Enhanced Current Densities in Superconducting Coated Conductors of YBa2Cu3O7-x + BaZrO3
There are numerous potential applications for superconducting tapes, based on
YBa2Cu3O7-x (YBCO) films coated onto metallic substrates. A long established
goal of more than 15 years has been to understand the magnetic flux pinning
mechanisms which allow films to maintain high current densities out to high
magnetic fields. In fact, films carry 1-2 orders of magnitude higher current
densities than any other form of the material. For this reason, the idea of
further improving pinning has received little attention. Now that
commercialisation of conductors is much closer, for both better performance and
lower fabrication costs, an important goal is to achieve enhanced pinning in a
practical way. In this work, we demonstrate a simple and industrially scaleable
route which yields a 1.5 to 5-fold improvement in the in-field current
densities of already-high-quality conductors
Effect of two bands on critical fields in MgB2 thin films with various resistivity values
Upper critical fields of four MgB2 thin films were measured up to 28 Tesla at
Grenoble High Magnetic Field Laboratory. The films were grown by Pulsed Laser
Deposition and showed critical temperatures ranging between 29.5 and 38.8 K and
resistivities at 40 K varying from 5 to 50 mWcm. The critical fields in the
perpendicular direction turned out to be in the 13-24 T range while they were
estimated to be in 42-57 T the range in ab-planes. In contrast to the
prediction of the BCS theory, we did not observe any saturation at low
temperatures: a linear temperature dependence is exhibited even at lowest
temperatures at which we made the measurements. Moreover, the critical field
values seemed not to depend on the normal state resistivity value. In this
paper, we analyze these data considering the multiband nature of
superconductivity in MgB2 We will show how the scattering mechanisms that
determine critical fields and resistivity can be different.Comment: 17 pages, 3 figure
Oxygen tracer diffusion and surface exchange kinetics in Ba0.5Sr0.5Co0.8Fe0.2O3 - δ
The oxygen tracer diffusion coefficient, Db∗, and the oxygen tracer surface exchange coefficient, k, were measured in Ba0.5Sr0.5Co0.8Fe0.2O3 - δ (BSCF5582) over the temperature range of 310-800 °C and the oxygen partial pressure range of 1.3 × 10- 3-0.21 bar. Several measurement techniques were used: isotope exchange followed by depth profiling (IEDP) within individual single grains or line scanning (IELS) along the sample cross-section and gas-phase analysis (GPA). Surface exchange kinetics was initially found to be slow and presumably inhibited by the formation of a passivating layer on the sample surface. High temperature pre-anneals (900-950 °C) changed the nature of this layer and enhanced surface exchange. Fast bulk oxygen diffusion and surface exchange kinetics were observed after high temperature pre-anneals within the temperature range studied. The activation energies for 18O tracer diffusion and surface exchange at 0.21 bar were 0.72 ± 0.05 and 1.10 ± 0.15 eV, respectively. The tracer diffusion coefficient showed weak dependence upon oxygen partial pressure, whereas the surface exchange coefficient exhibited strong oxygen partial pressure dependence. The microstructure of the samples (the porosity and grain size) had a profound effect on the measured tracer diffusion coefficient. © 2014 Elsevier B.V
In situ epitaxial MgB2 thin films for superconducting electronics
A thin film technology compatible with multilayer device fabrication is
critical for exploring the potential of the 39-K superconductor magnesium
diboride for superconducting electronics. Using a Hybrid Physical-Chemical
Vapor Deposition (HPCVD) process, it is shown that the high Mg vapor pressure
necessary to keep the MgB phase thermodynamically stable can be achieved
for the {\it in situ} growth of MgB thin films. The films grow epitaxially
on (0001) sapphire and (0001) 4H-SiC substrates and show a bulk-like of
39 K, a (4.2K) of A/cm in zero field, and a
of 29.2 T in parallel magnetic field. The surface is smooth with a
root-mean-square roughness of 2.5 nm for MgB films on SiC. This deposition
method opens tremendous opportunities for superconducting electronics using
MgB
The structure of the superconducting gap in MgB2 from point-contact spectroscopy
We have studied the structure of the superconducting gap in MgB2 thin films
by means of point-contact spectroscopy using a gold tip. The films were
produced by depositing pure boron on a sapphire substrate, using e-beam
evaporation, followed by reaction with magnesium vapour. The films have a Tc of
38.6 +- 0.3 K and resistivity of about 20 microOhm cm at 40 K. The
point-contact spectra prove directly the existence of a multi-valued order
parameter in MgB2, with two distinct values of the gap, DELTA1=2.3+-0.3 meV and
DELTA2=6.2+-0.7 meV at 4.2 K. Analysis of the spectra in terms of the
Blonder-Tinkham-Klapwijk model reveals that both gaps close simultaneously at
the Tc of the film. Possible mechanisms that can explain the intrinsic
co-existence of two values of the gap are discussed.Comment: 9 pages, 9 figure
In situ annealing of superconducting MgB2 films prepared by pulsed laser deposition
The in situ annealing conditions of pulsed laser deposited MgB2 films were
studied. The precursor films were deposited at 250 C from a stoichiometric MgB2
target in a 120mTorr Ar atmosphere. The films were then in situ annealed at a
temperature from 450 C to 800 C and an annealing time from 1 minute to 10
minutes. We found that the superconducting properties depend in a crucial way
on the annealing conditions: temperature, heating rate and time. The best film
with a thickness of ~600nm was obtained under the following annealing
conditions: Tanneal=680-690 C, tanneal=1 min, heating rate= 38 C/min. The Tc
onset of the film is 28K with a transition width of ~10K. The hysteresis loop
of magnetic moment of the film indicates weak field dependence in high fields.
Magneto-optical imaging of the film showed quite homogeneous magnetic flux
penetration, indicating structural homogeneity. The films without annealing
showed no superconductivity.Comment: 12 pages, 6 figure
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