High-temperature Superconductor Samples Research Articles

Improvement of magnetic circuit in levitation system using HTS and soft magnetic material

This paper presents improvement of a novel levitation system in which soft magnetic material can be levitated by high-temperature superconductor (HTS). The levitation system consists of two permanent magnets, HTS samples of Dy/sub 1/Ba/sub 2/Cu/sub 3/Oy (DBCO), and movable yoke with cylindrical parts to which trapped flux in the HTS is gathered and produces an attractive force. The attractive force generally increases with decreasing air gap, but it decreases in a certain air gap called "positive stiffness air gap," which causes positive stiffness to achieve stable levitation of the movable yoke. In this work, we propose closed magnetic circuit using ferromagnetic yoke to enhance the magnetic force and investigate the relationship between attractive force and air gap with different thicknesses of the HTS and different diameters of cylindrical parts. By our proposing closed magnetic circuit, we obtain maximum levitation force of 85 N and stiffness of 4.5 N/mm. The tradeoff between the levitation force and positive stiffness air gap with the thickness of the HTS indicates that there is optimal thickness of HTS for the practical levitation system.

A current-density scale for characterizing nonlinearity in high-T/sub c/ superconductors

In this paper, we characterize microwave nonlinearity in a high temperature superconducting (HTS) thin-film by measuring a geometry-independent current-density scale j/sub o/. The quantity j/sub o/ specifies the strength of a material-dependent nonlinearity, and can be used to calculate the nonlinear microwave response of planar superconducting transmission-line devices. Our procedure for determining j/sub o/ involves microwave measurements on superconducting coplanar waveguide devices patterned onto the HTS sample. In a YBa/sub 2/Cu/sub 3/O/sub 7-x/ (YBCO) sample, we obtained a maximum experimental value for j/sub o/ of approximately 3.26/spl times/10/sup 8/ A/cm/sup 2/ at 25 K. From the measured temperature-dependence of j/sub o/ in our YBCO sample, and assuming a theoretical pair-breaking current density j/sub c/ equal to 3/spl times/10/sup 8/ A/cm/sup 2/, we calculated the temperature dependence of the quasiparticle factor b(T). The curve of the experimentally obtained b(T) matched well with a theoretically-predicted behavior in the scenario of d-wave symmetry in the superconducting order parameter.

A Simple Measurement Technique for Critical Current Density by Using a Permanent Magnet

We developed a novel nondestructive and contactless method for measurements of critical current density (J/sub c/) in high-temperature superconductor (HTS) thin films by using a permanent magnet (Sm/sub 2/Co/sub 17/). This measurement technique is based on a repulsive force (F/sub r/) and attractive force (F/sub a/) between the magnet and HTS film caused by the shielding currents and pinning forces. The F/sub r/ and F/sub a/ were measured by using a high-resolution load sensor changing the distance (L) between the magnet and HTS film. An effective maximum repulsive force (F/sub m,eff/) could be determined from an extrapolated value of the F/sub r/ vs. L curve for L=0 mm. We investigated the relationship of the J/sub c/ to the F/sub m,eff/ in many HTS (YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// and ErBa/sub 2/Cu/sub 3/O/sub 7-/spl delta//) samples. The standard J/sub c/ of HTS films was measured by using an inductive measurement system produced from THEVA GmbH. We found that the J/sub c/ is almost proportional to the two-thirds power of the F/sub m,eff/ normalized by the film thickness. This result indicates that the simple permanent magnet technique can be easily estimate the J/sub c/ of the HTS films. This system has good reproducibility for the J/sub c/ measurements and is useful to provide a rapid, routine characterization of the superconductivity in the HTS materials.

Hall sensor magnetometer for ac characterization of high temperature superconducting tapes

During the last five years, the ac performance of high temperature superconductor (HTS) tapes has been under intense development due to the need of practical applications. In order to measure and understand the current density distributions inside these tapes, several methods have already been developed. However, no system has been able to give the time dependent, spatial distribution of the magnetic flux density at a selected distance above the HTS tape carrying any cyclic current signal. In this paper, such a system operating at a frequency range of 0–400 Hz is presented, and possible error sources are extensively investigated. Post-processing of the measured data is explained in detail and error analysis of several example measurements is given. Measurements with HTS and copper samples are compared in order to demonstrate the usefulness of the system.

超電導材料特性計測技術―マッピング技術の進展を中心として―永久磁石を用いた非接触型臨界電流密度測定システム

We developed a novel non-destructive and non-contact method for the measurement of critical current density (Jc) using a permanent magnet. A high-temperature superconductor (HTS) thin-film was fixed on the stage of the system and a small cylindrical permanent magnet (Sm2Co17) was set above the HTS film. The repulsive force (Fr) and attractive force between the small magnet and HTS film were measured by a high-resolution load sensor changing the distance (L) between the HTS film and the magnet. The maximum repulsive force (Fm) could be estimated from an experimental result of the Fr vs. L curve. We investigated the relationship of Jc to Fm in several HTS samples. The standard Jc of the HTS films was determined using the inductive measurement system made from THEVA GmbH. We found that the Jc is approximately proportional to the Fm normalized by the film thickness. This result indicates that the permanent magnet technique can easily estimate the Jc of HTS films. We also found that the system can measure Jc with sufficient reproducibility and in a short time. Moreover, it was checked to confirm that Jc mapping of a YBCO thin film could be also measured.

High-field ac susceptometer using Helmholtz coils as a magnetizer

An ac susceptometer is designed for measuring ac susceptibility of disc, film or tape samples of high-temperature superconductors at 77 K in an applied field as high and uniform as possible. It is equipped with a pair of magnetizing Helmholtz coils and a pair of coaxial concentric measuring–compensating coils. For this, the field uniformity and impedance of the magnetizing Helmholtz coils with square coil cross-section and the flux linkage in the measuring–compensating coils produced by the sample moment are systematically and accurately computed based on the analytical formulae for the mutual inductance between two loops and the self-inductance of a thin solenoid. Problems related to the susceptometer calibration and the susceptibility measurements are discussed.

Magnetic imaging of pearl vortices in artificially layered (Ba0.9Nd0.1CuO2+x)m/(CaCuO2)n systems.

We have used scanning SQUID magnetometry to image vortices in ultrathin (Ba0.9Nd0.1CuO2+x)(m)/(CaCuO2)(n) high temperature superconductor samples, with as few as three superconducting CuO2 planes. The Pearl lengths (Lambda=2lambda(2)(L)/d, lambda(L) the London penetration depth, d the superconducting film thickness) in these samples, as determined by fits to the vortex images, agree with those by local susceptibility measurements, and can be as long as 1 mm. The in-plane penetration depths lambda(ab) inferred from the Pearl lengths are longer than many bulk cuprates with comparable critical temperatures. We speculate on the causes of the long penetration depths, and on the possibility of exploiting the unique properties of these superconductors for basic experiments.

Design and Nanofabrication of Superconductor Ceramic Strands and Customized Leads

We made possible an engineering application of the 18‐year‐old Nobel Prize‐winning discovery of High Temperature Superconductor (HTS) ceramic particles. Guided by certain theories of physics and nano‐science, we created the ceramic‐silicone nanofabrication method where a silicone additive controls both the nanostructural evolution of the magnetically oriented HTS ceramic nano‐grains by a 3D polymeric matrix scaffold and thermally induced nano‐phase transformation. These result in the sintered granular superconductor ceramic composite material with the desirable superconducting nano‐phase composition and 3D nanostructure including vortex‐pinning network. The prototype HTS samples were nanofabricated in the form of adhesion substrate coated strands or surfaces or variously shaped bulk leads.

Superconducting magnetic bearings for a 2 MW/10 kW h class energy storage flywheelsystem

Single-domain crack-free CeO2-doped YBCO monoliths synthesized in a conventional box furnace using atop-seeded-melt-growth process with an optimized time–temperature profile exhibittrapped magnetic fields of up to 1.4 T and self-field critical current densities at 77 K of up to1.3 × 105 A cm−2. The observed magnetic properties of the samples correlate with the microstructure, whichwas studied using optical and scanning electron microscopy. With these high quality hightemperature superconductor samples a superconducting magnetic bearing for a2 MW/10 kW h class flywheel energy storage system was designed and has been tested in amodel arrangement.

Synthesis and electrical resistivity studies of Tl1−xPrxBa2Ca2Cu3O9−δ superconductors

Several high-temperature superconductor samples of the general composition Tl1−xPrxBa2Ca2Cu3O9−δ, with 0 ⩽ x ⩽ 0.3, have been synthesized by a single solid-state reaction. Within a narrow range of composition, x ≅ 0.2, the samples are almost single phase possessing a tetragonal unit cell similar to the Tl-1223 phase. The electrical resistivity exhibits a metallic behaviour for 0 ⩽ x ⩽ 0.2 above the transition temperature whereas it exhibits a semiconductor-like behaviour for x > 0.2. The normal electrical resistivity ρn is enhanced as the Pr content increases and the resistivity data, in the semiconductor regime, are well fitted according to the variable range hopping model. The transition temperature, determined from electrical resistivity measurements, decreases monotonically with x and the superconductivity disappears at around xc = 0.29. The depression in the transition temperature as Pr content increases is due to two different mechanisms involving hole filling or Cooper pairs breaking.

The application of microbalance technique in study of the high-temperature superconductors

Many physical-chemical properties of oxide superconductors depend on oxygen content and its distribution in the sample. Investigations of the thermal decomposition of the oxide superconductors in dynamic vacuum are valuable, particularly in view of the high oxygen diffusion coefficient in these cuprates. Dynamic vacuum protects against the readsorption of oxygen from the environment. Samples of the high-temperature superconductors were synthesised from stoichiometric mixtures of high-purity oxides and carbonates.

Magnetic moments of welded HTS samples: their dependence on the current flowing through the welds

We present a method to calculate the magnetic moments of the high-temperature superconducting (HTS) samples which consist of several welded HTS parts. The approach is generalized for the samples of various geometrical shapes and an arbitrary number of welds. The relations obtained between the sample moment and the density of the critical current that flows through the welds allow the magnetization loops to be used for a quantitative characterization of the weld quality in a wide range of temperatures and/or magnetic fields.

Superconducting properties of Tl1−xYbxBa2Ca2Cu3O9−δ

A series of high temperature superconductor samples of type Tl1−xYbxBa2Ca2Cu3O9−δ with x ranging from 0 to 0.5 have been synthesized at 860 °C for 4 h. The x-ray powder diffraction and the electrical resistivity measurements for the prepared samples have been reported. X-ray data indicate that the solubility limit xs between Tl3+ and Yb3+ ions in Tl1−xYbxBa2Ca2Cu3O9−δ phase is about 0.3. The transition temperature Tc, determined from the electrical resistivity data, shows a depression in its value with increasing Yb-content. The critical concentration xc, at which the superconductivity disappeared, is estimated to be 0.58. The effect of external magnetic field on the electrical resistivity behaviour has been studied. The activation energy, calculated from these data, shows a decrease in its value as the applied magnetic field and Yb-content increase.

Interface roughness and proximity effect on ac-axis Josephson junction betweens-wave andd-wave superconductors

The scanning superconducting quantum interference device microscope on tri-crystal high-temperature superconductor (HTSC) samples unambiguously identifies the d-wave pairing symmetry as a predominant component. This fact was also seen clearly from the current phase relation (CPR) for an in-plane junction between HTSC's, where both $\ensuremath{\pi}$ periodicity and $2\ensuremath{\pi}$ periodicity are observed, depending on the relative crystal orientation. However, for c-axis junctions between HTSC's and conventional superconductor, ac Josephson effect shows that the main Shapino steps occur at $V=nhf/2e$ $(n$ is integer) and thus a significant s-wave component is indicated. To understand the experimental measurements, we have studied interface roughness and proximity effect on CPR of such junctions. The order parameter profiles and current phase relation are computed self-consistently using the quasiclassical theory and rough interface model. Our results suggest that the existence of a minor surface s-wave component stemming from a repulsive s-channel pairing potential in the d-wave superconductor is able to give a coherent picture.

Phenomenological model for the anomalous peaks in the microwave surface resistance of high temperature superconductors

The paper presents a simple model to describe the observed anomalous peaks below transition temperature in the microwave surface resistance in single crystal and bulk samples of high temperature superconductors. The peaks observed in YBCO single crystals and BPSCCO bulk samples are due to the suppression in the quasi-particle scattering rate, which are affected by the presence of impurities in the samples. The two key parameters of the present phenomenological model are the power law factor ` γ' of extended two fluid model and the residual to intrinsic resistivity ratio ` r' deciding the impurity level in the sample. These two empirical parameters of the model are able to explain the temperature dependent amplitude and position of the surface resistance peaks. Results of the theoretical model are fitted with the published experimental data on ordered YBCO single crystal to disordered bulk BPSCCO bulk samples.

Investigation of structure and transport properties ofGd1-x-zPrxCazBa2Cu3O7-δ system

The granular Gd1-x-zPrxCazBa2Cu3O7-δ (GdPrCa-123)high-temperature cuprate superconductor samples were prepared andcharacterized. The electrical resistivity of Pr-free samples show a reductionin Tc with increasing Ca content. For these samples, the Tc(z) curve is nonlinear and aplateau appears at z≈0.05-0.15. With increasing Ca concentration,the lattice parameter a increases. Moreover, it is observed that the in-planeCu(2)-O(2) bond distance increases with increasing Ca content. On theother hand, the Cu(2)-O(2)-Cu(2) angle increases and the CuO2 plane becomes flatterwith successive Ca doping, which causes the suppression of superconductivity.The same bond distances in Pr-Ca substituted samples decrease slowly withincreasing Ca doping, but increase with increasing Ca concentrationafter an optimum value. In addition, the electrical resistivity of Pr-Ca-dopedsamples shows an increase of Tc with Ca doping up to an optimum value, whereafter itbegins to decrease. We suggest that the competition of two effects, the numberof holes and the flattening of CuO2 planes by Ca doping, results in a nonlineardecrement in the Tc(z) curve.

Neutron diffraction study of anisotropy of crystal lattice compression in irradiated of fast neutron high-T c superconductors YBa2Cu3O6·95 and La1·83Sr0·17CuO4 under pressure

The structural changes in the high temperature superconductors (HTSC) samples of the compounds YBa2Cu3O6·95 and La1·83Sr0·17CuO4 under pressure being unirradiated and irradiated by fast neutrons have been studied by the neutron diffraction method. The different anisotropy of crystal lattice compression was found for these compounds in the initial and irradiated states.

Laterally resolved microwave surface-resistance measurement of high-T/sub c/ superconductor samples by cavity substitution technique

Microwave surface impedance measurement of a high-temperature superconductor (HTS) is a sensitive probe to test its quality, particularly if a microwave device is to be fabricated. Most microwave characterization employs resonance techniques in which the components of the surface impedance are extracted from the measured Q value and the shift in the resonance frequency. In this paper, we present a modification of the widely used complete end-plate substitution technique to measure the surface resistance of samples having dimensions smaller than the dimension of the end plate at 20 GHz, as well as to facilitate the laterally resolved surface resistance measurement of large-area HTS samples. From the knowledge of the electromagnetic-field configuration in a TE/sub 011/-mode cylindrical cavity, the loss contribution from the HTS sample is analyzed theoretically and measured experimentally in the temperature range of 20-100 K. The design of the cavity is discussed to optimize the sensitivity of the measurement by the placement of the sample and to maximize the difference in the measured Q value.

Microbalance and Related Studies of High-temperature Superconductors

Many properties of oxide superconductors depend on the oxygen concentration and its distribution in the samples. The microgravimetric method is very useful to study of oxide superconductors, as it allows investigations in vacuum and controlled environments in classical applications: thermogravimetric analysis for the study of solid-state reactions, determinations of oxygen contents in redox reactions and the combined measurement of mass and additional parameters, e.g. evolved gas analysis etc. Single-phase samples of high-temperature superconductors were synthesized from stoichiometric mixtures of high-purity oxides and carbonates. Appropriate amounts of the precursor powders were homogenized manually or by a mechanical ball mill and subsequently calcined at temperatures in the 800 to 950°C range with intermediate grinding to ensure homogenous reaction. The lattice parameters of all preparations were controlled, in both initial and final experiments, by the X-ray powder method (CuKα radiation), using a Stadi P (Stoe) diffractometer with a position-sensitive detector. It is well known that temperature and mass can be measured with an accuracy higher by orders of magnitude than it is still possible for the thermoanalyst to determine the transformation temperatures or the mass changes due to overlapping partial reactions. Applications of Cahn microbalance to study of high-temperature superconductors are presented.

HTS wire irradiation test with 8 GeV protons

The radiation level at High Energy Particle Accelerators (HEPA) can be high enough to damage electronic devices. Any active component near the accelerator must be radiation resistant. Since High Temperature Superconductors (HTS) have great potential to be used in HEPAs (e.g., in superconducting magnets, current leads, RF cavities), it is important to understand the radiation hardness of these materials. A radiation test of HTS wire (BSCCO-2223) was performed at Fermilab. The HTS sample was irradiated with 8 GeV protons and the critical current, I/sub c/, was measured during the irradiation. The total radiation dose was 10.8/spl plusmn/1.0 Mrad. No change in I/sub c/ was observed.