603 research outputs found
Definition of procedures for coil electrical testing and preparation for PF transient analysis
Development and test of a 35 kA - HTS CroCo cable demonstrator
The answer to energy-efficient electric power transfer of high currents in the range of several tens of kA can be given by high temperature superconducting (HTS) cables. BSCCO and MgB2 have been used widely for such cables, reaching maximum currents of about 20 kA. REBCO coated conductors are promising for future HTS cables beyond 20 kA and allow the operation based on subcooled liquid nitrogen. Several cabling concepts based on REBCO tapes were developed world-wide to realize such cables. Using the stacked-Tape concept, a scalable semi-industrial process was developed by KIT, called HTS CrossConductor (HTS CroCo). Key aspects of the conceptual design of high-current HTS cables are discussed and the design of a 35 kA DC cable demonstrator made from HTS CroCo strands is presented. Aspects regarding joints, current redistribution between individual strands and electrical stabilization are highlighted. The performance of this demonstrator cable was tested, reaching the envisaged current
Caracterização edafoclimática do assentamento Itamarati, MS, e análise socioeconômica regional.
bitstream/item/38272/1/DOC200253.pd
HTS CroCo - A Strand for High Direct Current Applications
High temperature superconductors (HTS) are discussed as energy-efficient solutions for applications needing high direct currents beyond 10 kA e.g. for large high-field magnets or bus bar systems in industrial electrolysis plants. A number of high-current cable concepts based on REBCO tapes were developed such as the Roebel cable, co-axially wound tapes and several stacked-tape arrangements, among them the HTS CrossConductor (HTS CroCo), a stacked-tape conductor with high current density developed at KIT. In this manuscript, the experimental test of a high DC demonstrator, termed Supra-DC-Cable, made from twelve HTS CroCo strands is presented. The demonstrator was tested successfully at T = 77 K, reaching the expected critical current of 33 kA at 77 K and even for a constant-current operation at 36 kA for more than 30 minutes limited by the copper connections, not the superconducting cable. Currents and voltages were measured in all twelve strands individually during the parallel operation in the cable. These measured data allow the experimental validation of the modelled current distribution, based on the individual characterization of the twelve strands
Anomalously large oxygen-ordering contribution to the thermal expansion of untwinned YBa2Cu3O6.95 single crystals: a glass-like transition near room temperature
We present high-resolution capacitance dilatometry studies from 5 - 500 K of
untwinned YBa2Cu3Ox (Y123) single crystals for x ~ 6.95 and x = 7.0. Large
contributions to the thermal expansivities due to O-ordering are found for x ~
6.95, which disappear below a kinetic glass-like transition near room
temperature. The kinetics at this glass transition is governed by an energy
barrier of 0.98 +- 0.07 eV, in very good agreement with other O-ordering
studies. Using thermodynamic arguments, we show that O-ordering in the Y123
system is particularly sensitive to uniaxial pressure (stress) along the chain
axis and that the lack of well-ordered chains in Nd123 and La123 is most likely
a consequence of a chemical-pressure effect.Comment: 4 pages, 3 figures, submitted to PR
Relaxation Effects in the Transition Temperature of Superconducting HgBa2CuO4+delta
In previous studies on a number of under- and overdoped high temperature
superconductors, including YBa_{2}Cu_{3}O_{7-y} and Tl_{2}Ba_{2}CuO_{6+\delta},
the transition temperature T_c has been found to change with time in a manner
which depends on the sample's detailed temperature and pressure history. This
relaxation behavior in T_c is believed to originate from rearrangements within
the oxygen sublattice. In the present high-pressure studies on
HgBa_{2}CuO_{4+\delta} to 0.8 GPa we find clear evidence for weak relaxation
effects in strongly under- and overdoped samples () with
an activation energy . For overdoped
HgBa_{2}CuO_{4+\delta} E_{A} increases under pressure more rapidly than
previously observed for YBa_{2}Cu_{3}O_{6.41}, yielding an activation volume of
+11 \pm 5 cm^{3}; the dependence of T_c on pressure is markedly nonlinear, an
anomalous result for high-T_c superconductors in the present pressure range,
giving evidence for a change in the electronic and/or structural properties
near 0.4 GPa
Stable and Metastable vortex states and the first order transition across the peak effect region in weakly pinned 2H-NbSe_2
The peak effect in weakly pinned superconductors is accompanied by metastable
vortex states. Each metastable vortex configuration is characterized by a
different critical current density J_c, which mainly depends on the past
thermomagnetic history of the superconductor. A recent model [G. Ravikumar, et
al, Phys. Rev. B 61, R6479 (2000)] proposed to explain the history dependent
J_c postulates a stable state of vortex lattice with a critical current density
J_c^{st}, determined uniquely by the field and temperature. In this paper, we
present evidence for the existence of the stable state of the vortex lattice in
the peak effect region of 2H-NbSe_2. It is shown that this stable state can be
reached from any metastable vortex state by cycling the applied field by a
small amplitude. The minor magnetization loops obtained by repeated field
cycling allow us to determine the pinning and "equilibrium" properties of the
stable state of the vortex lattice at a given field and temperature
unambiguously. The data imply the occurence of a first order phase transition
from an ordered phase to a disordered vortex phase across the peak effect.Comment: 20 pages, 10 figures. Corresponding author: S. Ramakrishna
Core pinning by intragranular nanoprecipitates in polycrystalline MgCNi_3
The nanostructure and magnetic properties of polycrystalline MgCNi_3 were
studied by x-ray diffraction, electron microscopy, and vibrating sample
magnetometry. While the bulk flux-pinning force curve F_p(H) indicates the
expected grain-boundary pinning mechanism just below T_c = 7.2 K, a systematic
change to pinning by a nanometer-scale distribution of core pinning sites is
indicated by a shift of F_p(H) with decreasing temperature. The lack of scaling
of F_p(H) suggests the presence of 10 to 20% of nonsuperconducting regions
inside the grains, which are smaller than the diameter of fluxon cores 2xi at
high temperature and become effective with decreasing temperature when xi(T)
approaches the nanostructural scale. Transmission electron microscopy revealed
cubic and graphite nanoprecipitates with 2 to 5 nm size, consistent with the
above hypothesis since xi(0) = 6 nm. High critical current densities, more than
10^6 A/cm^2 at 1 T and 4.2 K, were obtained for grain colonies separated by
carbon. Dirty-limit behavior seen in previous studies may be tied to electron
scattering by the precipitates, indicating the possibility that strong core
pinning might be combined with a technologically useful upper critical field if
versions of MgCNi_3 with higher T_c can be found.Comment: 5 pages, 6 figures, submitted to PR
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