Dependence Of Critical Current Research Articles

Low-field vortex pinning model for undoped sinteredMgB2 powders

Sintered MgB2 powders constitute a porous ensemble of irregularly shaped agglomerates of tightly packedgrains. The low-field critical current density in such powders was experimentallyobserved to scale with the inverse of the average agglomerate size. Motivated bythis observation we consider a flux pinning model which accounts for theMgB2 powder porosity by focusing on a single finite-size agglomerate size. According to themodel the observed critical current density dependence on the agglomerate sizereflects the outward pull exerted on a vortex that is pinned in proximity to theagglomerate edges. The calculated critical current density replicates the observedscaling within agglomerate-size bounds. Implications of the model are discussed.

A predictive model of the temperature dependence of AC transport losses in(Bi, Pb)2Sr2Ca2Cu3Ox tapes

Critical currents and AC losses of(Bi, Pb)2Sr2Ca2Cu3Ox superconducting tapes were measured in self-field as a function of temperature. Theexperimental data of the temperature dependence of critical current were compared withcalculated results. An approach to calculating AC losses as a function of temperature wasdeveloped and the calculated AC losses were compared with the measured data. The studyshows that AC losses at any temperature can be estimated using the model from thecritical parameters or from the measured AC loss factor at a certain temperature, such as77 K.

Surface Decoration Combined With Second Phase of ${\hbox {BaTiO}}_{3}$ for YBCO Film by Chemical Solution Deposition

The decoration of substrate surfaces is a simple technique for inducing growth defects in high temperature superconductor films in order to artificially introduce nanostructures as flux pinning centers, which can effectively improve the critical current density and magnetic field dependence. In this paper, we attempted to develop a novel technology of surface decoration induced second phase flux pinning centers. BaTiO3 decoration has been applied to LaAlO3 substrate surfaces by using chemical solution deposition. Different heat-treatment processes were used to anneal amorphous films, and the effects of the annealing conditions on the shape, size, and density of the BaTiO3 particles have been systematically investigated. YBa2Cu3O7 (YBCO) film with BaTiO3 second phase was deposited on the top of the decorated substrate.

Electromechanical Characterization of Bi-2212 Strands

The uniaxial strain dependence of critical current was measured both in tension and compression in Bi2Sr2CaCu2O8+x (Bi-2212) high-temperature superconducting round wires. Permanent damage to the critical current easily occurred due to strain. To improve the electromechanical properties of Bi-2212 wires, development of stronger sheathing materials is needed. Ideal materials would be not only mechanically strong, but also chemically compatible with Bi-2212 during the final heat treatment. To identify such materials, we measured stress-strain properties of some new Ag alloys and extracted their respective Young's modulus values and yield strength. The database may be useful for development of new Bi-2212 strands for fabricating high-field superconducting magnets above 20 T.

Magnetic interaction effect on the critical switching current in vortex arrays

In this paper, we investigate the influence of the dipolar magnetic interaction on the current-driven vortex dynamics in a two-dimensional array of nanodisks. A general formula is established for the prediction of the correlation between the restoring force of the target nanodisk and the polarities and circulations of the neighboring vortices. Large numbers of possible magnetization configuration combinations among the target and the surrounding neighbors are reduced into four groups under an extreme condition. The maximum and minimum restoring forces are analytically derived, which can be used to predict the critical switching current dependence on the magnetic configuration combinations of vortices.

Peculiarities of magnetization of second generation high-temperature superconducting tapes in a wide temperature range

We present the results of study of magnetization and critical current of coated conductors with magnetic and nonmagnetic substrates. The measurements of magnetization curves were done in a wide temperature range from 4,2 to 100 K and magnetic field up to 14 T. To determine the dependence of transport critical current on the magnetic field we measured a set of current-voltage characteristics in the range of magnetic field from 0 to 8 T at T = 77 K with perpendicular to the tape field orientation. It was obtained that the substrates magnetism dramatically changes the form of magnetization curves but not influence the value of critical current. Comparison of field dependence of critical current, obtained by contact and contactless method at T = 77 K shows that for both samples is observed coincidence of the curves at low fields and a strong divergence at H> 1 Tesla.

Influence of the ferromagnetic layer on the pair breaking and low alternating current field magnetic response in superconductor/ferromagnet bilayer

We investigate the influence of the ferromagnetic layer on the magnetic and transport properties of YBa2Cu3O7−δ in YBa2Cu3O7−δ (YBCO)/La0.7Sr0.3MnO3 (LSMO) bilayers. The temperature dependent dc magnetization study reveals the presence of magnetic anisotropy in YBCO/LSMO bilayer as compared to the pure YBCO layer. The ac susceptibility study on YBCO/LSMO bilayers reveals stronger pinning and the temperature dependent critical current is found to be less prone to temperature. Besides, the current (I) dependent electrical transport studies on YBCO/LSMO exhibit a significant reduction in the superconducting Tc with increase in I and it follows I2/3 dependence in accord with the pair breaking effect. The higher reduction of superconducting Tc in YBCO/LSMO is believed to be due to the enhanced pair-breaking induced by the spin polarized carriers being injected into the superconductor.

Effect of ZnO and Zn0,95Mn0,05O nano-particle inclusions on YBCO polycrystalline pinning properties

The effect of nano-size ZnO or Zn0.95Mn0.05O (ZnMnO) addition on the microstructure and pinning properties of polycrystalline YBa2Cu3Oy (YBCO) was analyzed. Samples with nano-particles ZnO and Zn0.95Mn005O addition were synthesized by solid state reaction. Microstructure investigation by transmission electron microscopy (TEM) coupled with energy dispersive X-ray spectroscopy (EDS), critical current density dependence on applied magnetic field Jc (B) and electrical resistivity as a function of temperature ρ(T) were carried out to evaluate the relative performance of samples. Using the right amount of ZnO or Zn0.95Mn0.05O addition (0.2 wt.%), the flux pinning ability of YBCO has been enhanced. The best flux pinning was observed for Zn0.95Mn0.05O added sample. The flux pinning enhancement suggests magnetic interaction of pinning centers via vortices.

Improving the numerical model for high temperature coated conductors using the Hall-probe measurement

Electric power applications of coated conductor tapes (e. g. power transmission cables, transformers, fault current limiters) are developed nowadays. Optimal design of these applications requires a realistic model of the electromagnetic behaviour of a single tape. We compare here four models that could represent the tape in electromagnetic calculations. The simplest model assumes a rectangular cross-section of superconducting layer and constant critical current density. Models that are more complex consider the critical current density depending on magnetic field and its orientation. Furthermore, one could introduce a non-uniformity of the superconducting layer across the tape width. We checked the predictive power of these models by comparing the agreement between the calculated values of magnetic field above the tape and the experimental data obtained by Hall-probe experiments. In the case of commercial coated conductor tape on non-magnetic substrate the most representative model incorporated the critical current density dependence on magnetic field as well as the assumption of worsening the layer properties towards the tape edges. The latter conclusion was confirmed by the experiment in which longitudinal strips cut from the tape were tested.

Critical current characteristics and history dependence in superconducting SmFeAsOF bulk

The superconducting SmFeAsO1−xFx (x=0.2) polycrystalline bulks were prepared by the powder-in-tube (PIT) method. The magnetic field and temperature dependences of critical current densities in the samples were investigated by resistive and ac inductive (Campbell's) methods. It was found that a fairly large shielding current density over 109 A/m2, which is considered to correspond to the local critical current density, flows locally with the perimeter size similar to the average grain size of the bulk samples, while an extremely low transport current density of about 105 A/m2 corresponding to the global critical current density flows through the whole sample. Furthermore, a unique history dependence of global critical current density was observed, i.e., it shows a smaller value in the increasing-field process than that in the decreasing-field process. The history dependence of global critical current characteristic in our case can be ascribed to the existence of the weak-link property between the grains in SmFeAsO1−xFx bulk.

Prediction of variation in critical current with applied tensile/bending strain of Bi2223 composite tape from tensile stress-strain curve

An approach to predict the variation in critical current with applied tensile/bending strain of Bi2223/Ag/Ag alloy composite tapes from the tensile stress-strain curves was presented. Three different fabrication-route samples were used to examine the applicability of the present approach. The damage strain parameter, referring to the difference between the tensile fracture strain and residual strain of Bi2223 filaments along the sample length direction (current transport direction), was estimated from the variation in the slope of the tensile stress-strain curve. With the estimated damage strain parameter, the irreversible tensile strain for critical current was predicted, which agreed well with the experimental result in all samples. Also by substituting the estimated damage strain parameter into the core shape—incorporated model, the critical current-bending strain curve was predicted, which described satisfactorily the experimental result in all samples. The present approach could be a useful tool for prediction of critical current-dependence on applied tensile/bending strain.

Formation mechanism of BaZrO3 nanoparticles in Y1−xSmxBa2Cu3Oy-coated conductors derived from trifluoroacetate metal–organic deposition

Grains of BaZrO3 (BZO) in REBa2Cu3Oy (REBCO) films exhibit microstructural differences, depending on whether they weredeposited by pulsed-laser deposition (PLD) or metal–organic deposition (MOD). In orderto understand the origins of these differences, we examined the formation mechanism ofBZO nanoparticles in the MOD process with detailed observations of the quenchedfilms by transmission electron microscopy. The BZO nanoparticle was found toform in the precursor during the temperature ramp prior to the formation of theY1−xSmxBa2Cu3Oy (YSmBCO) crystals. The YSmBCO grew layer by layer while entrapping the BZOparticles, which resulted in random dispersion of the BZO particles in the YSmBCO layer.Consequently, uniformly dispersed BZO nanoparticles were formed in the YSmBCO matrixderived from the TFA-MOD process. These findings indicate that a key factor in achievingfine dispersion of BZO nanoparticles in the superconducting matrix is strongly related tonucleation of the BZO phase crystals in the precursor before growth of the YSmBCO layer,which is unique to the MOD process. In subsequent testing, YSmBCO-coatedconductors with uniformly dispersed and densely concentrated BZO nanoparticlesshowed striking isotropic magnetic-field angular dependence of critical currents.

Simulation by Finite Difference Numerical Method of ${\rm Nb}_{3}{\rm Sn}$ Strand Under Bending Strain

We report on the simulation of the current distribution in Nb3Sn strand subjected to pure bending strain, obtained by resolving the implicit diffusion equations with finite difference algorithm in Mathworks environment. The critical current dependence on bending, temperature, and magnetic field is modeled by the Improved Deviatoric Scaling Law and is used in the power law electric field dependence across the superconductor. The strand is discretized in elements representing groups of twisted filaments embedded in the stabilization matrix and a distributed constant circuit model is applied for current transfer among filament bundles. The code is preliminarily validated by comparison with analytical solutions for different simplified situations, each one corresponding to a proper boundary condition. Transverse matrix resistivity and twist-pitch values are crucial elements for matching numerical results with experimentally measured critical currents.

Flux Pinning by Barium Stannate Nanoparticles in MOD YBCO Coated Conductors

Tin-based nanoparticles, proposed to be barium stannate, have been formed in YBCO films fabricated by metal-organic deposition, through modification of the precursor solution. These randomly-oriented 30-50 nm particles are dispersed throughout the film thickness, providing an enhancement in the isotropic flux pinning relative to undoped YBCO. For a low level of nanoparticle addition of 2 vol%, a flux-pinning force enhancement of up to 32% is achieved. We report field and field-angle dependence of the transport critical current, transmission electron microscopy, and X-ray diffraction of our YBCO films on RABiTS substrates, and compare with similar barium zirconate additions.

Performance of High-Sensitivity Nano-SQUIDs Based on Niobium Dayem Bridges

Measurements and noise performances of integrated nano superconducting quantum interference devices (nano-SQUIDs) are presented. The nano-sensors are based on nanometric niobium constrictions (Dayem bridges) inserted in a square loop having a side length of 200 nm. Integrated micrometric niobium coils located close to the sensor allow both an easy tuning/calibration and the optimization of the working point. Improved measurements of voltage-flux characteristic, flux to voltage transfer factor and noise performances are reported. In small signal mode, the sensors have shown a magnetic flux noise spectral density of 1.5 muPhi <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> /Hz <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1/2</sup> . Furthermore, preliminary measurements concerning the critical current vs. temperature dependence and the switching probability of the critical current are shown.

Quantitative Description of Critical Current Density in YBCO Films and Multilayers

The vortex pinning model based on the presence of the large number of edge dislocations in high quality YBa <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Cu <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">7</sub> (YBCO) films and multilayers has been refined. By introducing the pinning potential of a chain of individual edge dislocations, we have been able not only to describe the critical current density dependence on the applied magnetic field over its entire range, but also to extract the microstructural parameters in the films, such as inter-dislocation spacing and average domain size, without employing sophisticated microstructural analysis. The model applicability and its results have been verified with the help of microstructural characterization combined with magneto-optical imaging in YBCO films and multilayers with different properties.

Role of the Grain Oxidation in Improving the In-Field Behavior of ${\rm MgB}_{2}$Ex-Situ Tapes

The grain size and the grain boundaries play an important role in determining the pinning properties and the current carrying capability of MgB2. Here we present an experimental study on the effect of grain oxidation on the superconducting properties of MgB2 ex-situ Powder-in-Tube (PIT) tapes. We fabricated Fe-sheathed monofilamentary tapes using a variety of powders synthesized in inert (IA) or oxidizing (OA) atmosphere. In order to understand the role of magnesium oxide (MgO) we studied the effect of the presence of the oxidation layer on the field dependence of the critical current density. To this purpose, we developed a process for the powder synthesis and tube filling and sealing in controlled atmosphere to reduce the oxygen contamination; with this process the connectivity of the grains in the final tapes proved to be increased. Then, we introduced a controlled oxidation step. We will show how the presence of the MgO layer enhances the critical current field dependence. High energy X-ray diffraction measurements were performed on the samples, and the grain size of the unsintered and sintered powders in the tapes was measured by means of Scanning Electron Microscopy (SEM). The pinning force was studied, and different mechanisms were found for the samples prepared in IA and OA. The different behavior shown by the two different typologies of tapes was correlated with the presence or absence of the MgO layer.

AC Loss Characteristics of Copper-Stabilized YBCO Coated Conductor

Influence of copper layer and mechanical stress on AC losses in copper stabilized YBCO coated conductor (CC) was investigated. The uni-axial tensile stress was applied to the sample CC in liquid nitrogen at atmospheric pressure, and the AC losses (transport, magnetization and total AC losses) were measured by an electric method. Influence of the copper layer was investigated by investigating frequency dependence of the AC losses. The experimental results show that there are some influences of the copper layer. However, the influence of the copper layer is not significant from the stand point of practical applications. As to the stress dependences, experimental results show that, when stress dependences of the transport current and magnetization losses are expressed by properly normalized values using stress dependent critical currents, the loss curves fall close to unified curves, respectively and that the stress dependence of the AC losses can be estimated by knowing stress dependent critical current and AC losses in CCs without stresses.

Effect of nano-size ZrO2 addition on the flux pinning properties of (Bi, Pb)-2223 superconductor

We investigated the microstructure and flux pinning capability of the bulk(Bi,Pb)2Sr2Ca2Cu3Oy system (denoted as (Bi, Pb)-2223) with 0.0–0.3 wt% nano-sizeZrO2 particles (10–20 nm) addition synthesized by solid state reaction. Phase analysis byx-ray diffraction (XRD), granular structure examination by scanning electronmicroscopy (SEM), microstructure investigation by transmission electronmicroscopy (TEM) equipped with energy dispersive x-ray spectrometer(EDS), critical current density dependence on the applied magnetic fieldJc(H) and electricalresistivity ρ(T,H) were carried out. The results show that activation energies of vortex flux motionU(H), criticalcurrent density Jc(H) behavior in applied magnetic field and volume pinning force densityFp are enhanced by theaddition of 0.1 wt% ZrO2. The pinning force analysis and microstructure observations indicated that theimprovement of flux pinning is suggested to originate from the introduction of extrinsicpinning centers due to the formation of normal nano-sized Zr-rich phase.

Nonsinusoidal current-phase relation in strongly ferromagnetic and moderately disordered SFS junctions

We study the Josephson current in a junction comprising two superconductors linked by a strong ferromagnet in presence of impurities. We focus on a regime where the electron (and hole) motion is ballistic over the exchange length and diffusive on the scale of the weak link length. The first and the second harmonics of the current-phase relation are obtained for both two- and three-dimensional ferromagnetic weak links. In the clean limit, the possibility of temperature-induced $0\text{\ensuremath{-}}\ensuremath{\pi}$ transitions is demonstrated. The corresponding critical current versus temperature dependence is also studied.