Critical Current Density Jc Research Articles

Preparation of coated conductor architectures on Ni composite tapes

An epitaxial Y2O3/YSZ/CeO2 buffer architecture has been prepared using pulsed laser deposition on cube textureNi–3 at.%W/Ni–10 at.%Cr–1.5 at.%Al and Ni–5 at.% W/Ni–10 at.% W composite tapes. The buffer layer growth was studied in detail andrevealed good texture transfer throughout the structure, similar to standardNi–5 at.% Wsubstrates. YBCO layers were successfully deposited on the buffered tapes and compared to films grownon SrTiO3 single crystals. A superconducting transition temperature of about 90 K with a smalltransition width was observed on all metal substrates. A critical current densityJc ofabout 1 MA cm−2 was measured at 77 K in a self-field. It was found thatJc ismainly limited by the grain boundary network in magnetic fields below 4 T, whereas similar values ason SrTiO3 single crystals were measured at higher fields.

Accurate numerical calculation of shielding current density in HTS thin film: application to noncontact measurement method of critical current density

The numerical method has been developed for calculating the shielding current density in a high temperature superconducting thin film. After formulated by use of the current-vector-potential method, the behavior of the shielding current density is expressed as an integral–differential equation. When the initial-boundary-value problem of the equation is discretized with respect to time and space, improper integrals appear as coefficients of the nonlinear system. In order to evaluate the coefficients accurately, the double exponential formula is applied to the improper integrals. Consequently, the shielding current density can be determined accurately. As an application of the method, two types of the noncontact methods for measuring the critical current density jC have been simulated numerically. The results of computations show that the maximum repulsive force is roughly proportional to jC in the permanent magnet method. In addition, it is found that an error of the inductive method is not more than 3% if jC exceeds a certain lower limit.

An internal Mg–Li alloy composite process for the fabrication ofMgB2 wire

This paper proposes a composite process for the fabrication ofMgB2 wire utilizing the excellent room temperature workability of Mg–Li alloy (LA141) as one ofthe promising processes for scale-up production. The starting composite is a Fe sheath tubewith a coaxial internal Mg–Li alloy rod and powder mixture of B and nano-SiC filling thespace between the inner wall of the Fe tube and the Mg–Li alloy rod. The composite can bedeformed uniformly into the final wire with large cross-sectional area reduction and nointermediate annealing. During the subsequent heat treatment, Mg and Li in the liquidstate infiltrate into the B layer and react with B forming a reacted layer along the innerwall of the Fe tube and a hole at the centre. The reacted layer is composed of aMgB2 matrix composed of equi-axed grains with sub-micron size. Li is little incorporated into theMgB2 phaseand does not degrade the superconducting properties. The superconducting transition temperatureTc is 38 K and the transport critical current densityJc(core) for thecore is 3 × 104 A cm−2 at 4.2 K and 4 T.

Enhancement in the high-Tc phase of BSCCO superconductors by Nb addition

Pure and Nb2O5 added Bi1.6Pb0.4NbxSr2Ca2Cu3Oδ superconductors (x varies up to 0.30) were synthesized by the solid state reaction method. Structural andsuperconducting properties of the samples were investigated. The fraction of thehigh-Tc phase (2223) increases with Nb addition up tox = 0.10, for which a nearly single 2223 phase was obtained. The criticaltemperature of the samples increases from 96 to 104 K with doping up tox = 0.20 and a further increasein Nb decreases the Tc sharply. Similarly, there is a considerable enhancement in the critical current density(jc) of the samples in the same doping range. Both AC susceptibility andtransport measurements showed that the optimum Nb concentration isx = 0.20. The out-of-phase component of the AC susceptibility measurements showed thatinter-grain coupling was enhanced up to the same doping level. Nb additions abovex = 0.30 decrease thefraction of the high-Tc phase and degrade the superconducting properties. Present measurements indicate thatx = 0.20 is the optimum Nb concentration in the BSCCO superconductor toenhance phase purity, inter-grain coupling and superconducting parameters(Tc and jc).

Finite-element modelling of YBCO fault current limiter with temperature dependentparameters

In this paper, we present a numerical model which takes into account both the thermal andthe electromagnetic aspects of the over-critical current regime for high-temperaturesuperconducting (HTS) materials. The electromagnetic and thermal equations have beenimplemented in finite-element method (FEM) software in order to obtain a novelmodel for investigating the behaviour of the materials when the current exceedsIc and the superconductor material goes to the normal state. The thermaldependence of the electrical parameters, such as the critical current densityJc, has been introduced. This model has been used to analyse the behaviour of strip lines of aYBCO/Au fault current limiter (FCL) on a sapphire substrate. Simulations with currents exceedingIc have been performed, showing that the current limitation phase can be correctlyreproduced. Such a model can be used to study the influence of the geometry onthe performance of the FCL. It can replace experiments with currents exceedingIc which may damage or destroy HTS samples and devices.

Enhancement of the upper critical field in sub-micron carbon-dopedMgB2 by two-step reaction method

The two-step reaction method was proposed to prepare sub-micron carbon-dopedMgB2 superconductors. The sample with a nominal composition ofMg(B0.95C0.05)4 wassintered at 1000 °C for 0.5 h. An appropriate amount of Mg was then added to get the stoichiometry ofMg(B0.95C0.05)2, which was sinteredat 750 °C for 2 h. The uppercritical field of Hc2 for thesub-micron carbon-doped MgB2 by a two-step reaction reached 10.5 T at 20 K. The critical current densityJc is 7.8 × 104 A cm−2 at 20 K and 1 T. The results indicate that the two-step reaction methodcould effectively introduce the doping element of carbon into theMgB2 crystalline lattice, and therefore improve the upper critical field.

Composite process for the fabrication ofMgB2 wire using ductile Mg–Li alloy

This paper proposes a composite process for the fabrication ofMgB2 wire utilizing the excellent room temperature workability of Mg–Li alloy (LA141)as one of the promising processes for scale-up production. Starting compositeis a LA141 alloy tube filled with B powder or a mixture of B powder and SiCnanoparticles, which is sheathed with a pure Fe tube. The composite can be deformeduniformly into the final tape or wire with large cross-sectional area reductionand no intermediate annealing. During the subsequent heat treatment above themelting point of Mg–Li alloy, Mg in liquid state reacts with B to form a layer ofMgB2 single phase with dense structure. Li is little incorporated into theMgB2 phase and precipitates as Li compounds, forming a separate region from theMgB2 layer. Superconducting transition temperatureTc is 37–38 K and the highest transport critical current densityJc (core) obtainedso far is 7 × 103 A cm−2 at 4.2 K and 4 T.

Thickness-dependent pinning in a superconductor thin film

The dependence of critical current density jc on film thickness (d) is calculated for the simple configuration of a semi-infinite finite-thickness superconductor film and a single vortex. This film-thickness dependence reflects the impact of vortex broadening in a thin film on its pinning by the film’s edge or an internal surface. The calculated film-thickness variation of jc in the range d∕λ<2, where λ denotes the penetration depth, is found to be proportional to d−1∕2. A parameter-free estimate of the ensuing critical current is within a factor of 3 of observed values in YBa2Cu3O7 coated conductors. It is argued that this type of film-thickness scaling applies beyond the simple configuration considered here.

Coexistence of critical and normal state magnetostrictions in type II superconductors: A model exploration

Exploiting the simple formula proposed by Ikuta et al. [Phy. Rev. Lett. 70, 2166 (1993)] to describe the magnetostriction of high TC superconductors and introducing a B2 dependent magnetic flux density component, as proposed by Koziol and Dunlap [J. Appl. Phys. 79, 4662 (1996)], we reproduce families of magnetostriction curves measured by Nabialek et al. [Physica B 319, 286 (2002)] and de Visser et al. [Phys. Rev. B 45, 2962 (1992)], using a simple expression for the dependence of the critical current density JC on the magnetic field in each analysis.

Observations of a hysteresis loss valley in yttrium barium copper oxide superconductors

We present measurements of the ac susceptibility of sintered yttrium barium copper oxide pellets prepared by a melt processed solid state reaction method, with different amounts of ZnO added. The measurements were performed versus temperature T for several different ac magnetic field amplitudes Hac in the presence of static bias magnetic fields Hb directed along Hac. For all our samples, for each choice of Hac, the imaginary part of the ac susceptibility versus T traces a peak, denoted by χ″max, centered at a temperature denoted by Tmax.We also find that plots of the values of χ″max vs Hac and of χ″max vs Tmax generate curves that display a valley. Calculations of hysteresis losses, exploiting a critical-state model where the critical current density jc=α(T)∕Bn, reproduce the experimental data quite well.

Properties of MgB2 thick film on silicon carbide substrate

We have successfully synthesizedMgB2 thick films on 4H-SiC substrate by hybrid physical–chemicaldeposition (HPCVD). The films have transition temperatureTc above 40 K. X-ray diffraction (XRD) shows thec-axis orientedstructure of MgB2, with Mg and small MgO impurities. The critical current densityJc, estimated using the measured magnetic hysteresis loop and the Bean model, is6 MA cm−2 in self-field at 10 K.

Artificial pinning centre Nb–Ti superconducting wire for AC applications

The present work presents the steps for the manufacture of an Nb–Ti artificialpinning centre (APC) multifilament wire for AC applications. The wire productionstarts from a Ti bar inserted in a Nb tube. The superconductor Nb–Ti alloywas obtained using heat treatments during the wire production steps throughthe growth of the diffusion layer between the pure Nb and Ti regions. The finalwire is composed of many filaments dispersed in a Cu–Ni matrix. The transportcritical current density was measured in samples for different conditions of heattreatment, followed by wire diameter reduction. Magnetization hysteresis loops weremeasured to extract the hysteretic losses in wires with filament diameters of 76 and92 µm and compared to the results in commercial wires with comparable filamentdiameters. We obtained an Nb–Ti APC wire with hysteretic losses of the sameorder of magnitude as the commercial wires. However, critical current densitiesJc are higher for fields lower than 2 T, where AC applications are most important. TheJc results confirmed a previous result that the decrease of normal region sizes is a lessimportant pinning source in this material than the pinning from variations in thecomposition of the diffusion layer.

Enhancement of critical current density ofMgB2 thin films irradiated with 200 MeV Au ions

The effect of 200 MeV Au ion irradiation on the temperature and field dependence of the critical current densityJc of highquality MgB2 thinfilms on Al2O3(0001) substrate is reported. The films were irradiated by 200 MeV Au ions with fluences5 × 1010 and 1 × 1012 ions cm−2. Substantial enhancement of the critical current densityJc with increasingfluence was observed. Jc was also increased when the measurements were made at 10–5 K.Columnar defects are expected because of high electronic energy loss(23 keV nm−1) for the present ion energy combination; these act as pinning centres to enhanceJc. Decrease in thetransition temperature Tc resulted after irradiation with a dose of5 × 1010 ions cm−2; however, at thehigher dose of 1 × 1012 ions cm−2, Tc increased marginally.

Conductance of Atom-Sized Zn Contacts

We have produced atom-sized Zn contacts and measured their high-bias conductance at room temperature. Contrary to previous experiments at and below 4.2 K, no 0.8G0 single-atom peak is observed in our conductance histograms. This result suggests that Zn single-atom contacts are too unstable at room temperature to produce a well defined peak in conductance histograms. We have also investigated the break conductance of Zn nanocontacts and showed that ther current-induced break of such nanocontacts exhibits the same characteristics as those observed on nanocontacts of other metals. Assuming the Sharvin conductance for Zn nanocontacts, the critical current density jc, at which Zn nanocontacts rupture with a maximum likelihood, is estimated to be jc∼2.8×1010 A/cm2. Comparing this jc with those of other metals shows a linear correlation between jc and the melting point of metals.

Mechanisms of weak thickness dependence of the critical currentdensity in strong-pinning ex situ metal–organic-deposition-routeYBa2Cu3O7−x coated conductors

We report on the thickness dependence of the superconducting characteristics including critical currentIc, critical current densityJc, transition temperatureTc, irreversibility fieldHirr, bulk pinning forceplot Fp(H), and normalstate resistivity curve ρ(T) measured after successive ion milling of∼1 µm thickhigh-Ic YBa2Cu3O7−x films made by an ex situ metal–organic deposition process on Ni–W rolling-assisted biaxially texturedsubstrates (RABiTSTM). In contrast to many recent data, mostly on in situ pulsedlaser deposition (PLD) films, which show strong depression ofJc with increasingfilm thickness t, our films exhibit only a weak dependence ofJc on t. The two better textured samples had full cross-section averageJc,avg (77 K, 0 T)∼4 MA cm−2 near the bufferlayer interface and ∼3 MA cm−2 at full thickness, despite significant current blocking due to∼30% porosity in the film. Taking account of the thickness dependence of the porosity, we estimate thatthe local, vortex-pinning current density is essentially independent of thickness, while accountingfor the additional current-blocking effects of grain boundaries leads to local, vortex-pinningJc values wellabove 5 MA cm−2. Suchhigh local Jc values are produced by strong three-dimensional vortex pinning which subdivides vortexlines into weakly coupled segments much shorter than the film thickness.

Effect of surface modification ofCeO2 bufferlayers on Jc and defect microstructures of large-area YBCO thin films

High-quality CeO2 buffer layers are requisite for the successful growth of YBCO thin films with excellent propertieson sapphire substrates. In this study, we evaluated the effect of surface modification of theCeO2 layers on the properties of the YBCO thin films prepared by large-areapulsed laser deposition (PLD), in particular the critical current densityJc and defect microstructure. High-temperature annealing(1050 °C) has been found to significantly smoothen the very rough and granular surfaces of the as-grownCeO2 layers (surfaceroughness rms∼5–10 nm)to atomic flatness (rms∼0.5 nm). However, a rather unique characteristic of theCeO2 layers deposited by large-area PLD is the development of pores when subjected to prolongedhigh-temperature annealing. For very short annealing periods (10–20 min), the surface morphologybecomes atomically flat, along with the appearance of a high density of ‘nanopores’ that are∼40–100 nm indiameter and ∼3–5 nm in depth. Extending the annealing period to 60 min or moreresults in the development of a surface subtended with enlarged pores∼0.2–0.5 µm in diameter. Compared with the YBCO thin films deposited on as-grownCeO2, YBCO thin films onannealed CeO2 exhibitedbetter homogeneity of Jc and better crystalline texture. Among the YBCO thin films deposited on annealedCeO2, higher self-fieldand in-field Jc was obtained for YBCO thin films deposited onCeO2 with smooth surfaces but interspersed with nanopores. Investigation of the defectmicrostructure via the etch pit method in conjunction with atomic force microscopy (AFM)of the YBCO thin films revealed a high density of linear defects in the form of screw andedge dislocations, which correlated well with a high density of nanopores on annealedCeO2. Transmission electron microscopy (TEM) further confirmed the presence of threadingdislocations clearly emanating from the nanopore sites. Angular dependence ofJc revealed enhanced flux pinning for YBCO thin films deposited onCeO2 containing a high density of nanopores.

Double-side superconducting MgB2 films for microwave and electronic applications

Double-side superconducting MgB2 films have been grown on double-side polished sapphire substrates by using hybrid physical-chemical vapor deposition method. The zero resistance temperatures TC(0)&amp;gt;37K and the critical current densities JC(5K,0T)&amp;gt;3×106A∕cm2 have been achieved for both sides. In the first 8.73G MgB2 microstrip resonator fabricated by a double-side film, an unloaded Q value of 3400 at 11K was obtained corresponding to surface resistance of 1.2mΩ for the MgB2∕Al2O3 interface. By simply using a sapphire substrate with two polished holes, superconducting connection has been achieved for the double-side MgB2 film. The results show potential applications for the superconducting MgB2 films in microwave and electronic devices.

Flux pinning properties of ErBa2Cu3Oy thin films with BaZrO3 nanorods

ErBa2Cu3Oy (ErBCO) thinfilms with BaZrO3 (BZO) nanorods were prepared by a PLD method for an enhancement of the critical current densityJc. Thevalues of Jc for the ErBCO thin film containing 1.5 wt% BZO (Er15) in magnetic fields are higher thanthose for the ErBCO thin film containing 0.5 wt% BZO (Er05). The peaks ofJc have been observed in the angular dependence ofJc in both the films when the magnetic field is applied parallel to thec-axis.It has been found that the peak is attributed to the flux pinning by BZO nanorods oriented parallel tothe c-axis. The vortex glass–liquid transition temperatureTg and the pinningparameter m were derived by fitting observed electric transport properties to thetheoretical expression based on the percolation transition model. The value ofTg of Er15 is higher than that of Er05. This result indicates that the vortex glass phaseextends to a higher temperature region on increasing the fraction of BZO. The peak ofm has been found in the magnetic field dependence. This fact is probably due to matching thedensity of BZO nanorods with that of fluxoids, which was confirmed by TEM observations.

Numerical investigations on nondestructive and contactless method for measuring critical current density by permanent magnet method

The nondestructive and contactless method for measuring the critical current density jC has been investigated by means of the numerical simulation. In the method, a permanent magnet is brought closer to a high-temperature superconducting film and the electromagnetic force is measured simultaneously. According to the experiment, the maximum repulsive force is proportional to jC. For the purpose of reproducing the experimental result numerically, the behavior of the shielding current density is formulated by use of the current-vector-potential method. The numerical code has been developed for solving an initial-boundary-value problem of the resulting integral–differential equation and, by means of the code, the relation between the maximum repulsive force and the critical current density is determined. The results of computations show that there is direct proportionality between them. This tendency validates the nondestructive and contactless measurement method of jC.

Radiation linewidth of the flux-flow oscillator with integrated self-field coil

The FFO is a long Josephson junction in which the flow of magnetic flux quanta, and thus the oscillator frequency is controlled by the DC bias current IB, and the DC current ICL producing the magnetic field at the junction ends. We have experimentally studied the properties of the Nb/AlOx/Nb and Nb/AlN/Nb trilayer FFOs when IB also contributes to the magnetic field. A number of samples were fabricated, each containing an FFO integrated with a bias current loop designed so that the bias current can generate approximately the same positive or negative contribution to the magnetic fields at both ends of the FFO. The magnetic field is proportional to ICL + Lp*IB, where Lp is given by the loop design. We have made a study of both the DC parameters and the radiation linewidth for FFOs with different integrated loops and compared our results to that for the FFO without any loop. It was found that loops with positive contribution (Lp > 0) make the FFO linewidth smaller (although the differential resistance of the FFO on the bias current becomes larger), while loops with negative contribution (Lp < 0) result in broadening of the FFO radiation linewidth. The magnitude of this effect strongly depends on the critical current density JC ∼ (Rn*S)−1. The Rn*S product was varied in our experiments from 13 to 35 Ohm*µm2. We propose a possible explanation of this effect.