Critical Current Measurements Research Articles

Inverted-parabolic and weak strain dependencies on the critical current in practical <110> and <100> oriented REBCO tapes

In the commercial coated conductor tapes, the twinned structure of REBCO (REBa2Cu3O7-d, RE = Y and Gd) is characterized as either <100> or <110> orientation based on the tape length direction. In this study, we investigate the effects of the two different twinned structures on the critical current Ic of the REBCO tapes by combining; transport critical current and synchrotron radiation diffraction measurements. For the tapes with <100> oriented twins, we observed the inverted parabolic strain behavior on the uniaxial strain dependence of Ic. In contrast, the ones with <110> oriented twins showed a weak strain behavior without any maximum appeared in the strain dependence. Such a different uniaxial strain dependence was analyzed by using the one-dimensional twin model with different fractional lengths of A-domains and B-domains of REBCO twins. This model explains the essential features of the different uniaxial strain dependence we observed.

Polymer-assisted surface decoration for critical current enhancement in YBa2Cu3O7−x films

In this study, chemically decorated surfaces are employed as tools for Artificial Pinning Centres (APC) introduction in superconducting YBa2Cu3O7−x (YBCO) films. Self-assembled oxide nanostructures are grown on crystalline substrates via a very versatile and low-cost chemical technique called Polymer Assisted Deposition (PAD). Such decorated templates are used, in a second step, as substrates for YBCO film growth via Metal Organic Decomposition (MOD) or Pulsed Laser Deposition (PLD). The aim of the structures is to produce in the superconducting matrix strain and/or defects that should act as APCs and therefore enhance the transport properties of the samples. The oxides selected for the decoration are commonly used for the introduction of artificial pinning centres in YBCO: BaZrO3 and ZrO2. The decorated surfaces are analyzed via Atomic Force Microscopy (AFM) and Scanning Electron Microscopy (SEM). YBCO films have been characterized via SEM, X-Ray Diffraction (XRD), Transmission Electron Microscopy (TEM), DC resistivity and critical current measurements. The results show that the nanostructures are stable and that they can efficiently introduce strain and stacking faults in the film. This causes an increase in the value of the critical current density Jc and better in-field behaviour in all the investigated field range, compared to the analogous film grown on a standard substrate, especially for chemically derived YBCO. Also Jc angular dependence measurements, Jc(θ), show an isotropic enhancement of the critical current for every angle of application of the field.

Effect of Nano-Sized Nickel Ferrite Addition on Superconducting Properties of (Tl<sub>0.85</sub>Cr<sub>0.15</sub>)Sr<sub>2</sub>CaCu<sub>2</sub>O<sub>7-δ</sub> Superconductors

The effect of nanosized nickel ferrite (NiFe2O4) addition on (Tl0.85Cr0.15)Sr2CaCu2O7-δ superconductors has been investigated. (Tl0.85Cr0.15)Sr2CaCu2O7-δ high temperature superconductors was prepared via solid-state reaction method using high purity oxide powders. Nanosized NiFe2O4 was added to the compound at composition x = 0 - 0.003 wt.%. Samples were studied using scanning electron microscopy (SEM), X-ray diffraction method (XRD), electrical resistance versus temperature, transport critical current density Jc and AC susceptibility measurements. SEM micrographs showed close-packed microstructure with low porosity. XRD patterns showed a dominant phase of Tl-1212 with a minor phase of Tl-1201 for all samples. All of the samples displayed metallic normal state behavior before the onset transition temperature (Tc-onset) which was between 102 and 104 K. The transition temperature (Tcχ’) from AC susceptibility measurement also showed the similar results between 101 and 103 K. Tc-zero was slightly suppressed with the addition of nanosized NiFe2O4. The peak temperature Tp from the imaginary part of the susceptibility χ” was found between 77 - 91 K for all samples. The highest transport critical current density (Jc) at 77 K was shown by sample x = 0.001 wt.% at 2213 mAcm-2. Nanosized NiFe2O4 has significantly enhanced the Jc of (Tl0.85Cr0.15)Sr2CaCu2O7-δ superconductor.

Optimization Possibilities for DC Current Transformer

The paper analyzes the consumption performance of DC current transformers based on linear class B, and half bridge classD compensation amplifiers, as well as self-oscillating flux gate current transformers based on push-pull and source–sink output stages.Compensation amplifiers, push-pull and source-sink output stages are used to generate feedback current in order to compensatemagnetic flux in the magnetic concentrator core, produced by electrical current flowing through a conductor under measurement. Thefocus of the analysis is the investigation of conditions for appearance of the bus-pumping effect to which all switching versions of DCcurrent transformers are prone. All sufficient conditions for bus pumping effect appearance are explained for the most critical case ofDC current measurement; the main issues are analyzed, giving better insight to power consumption, and possible hazards to DCcurrent transformer circuitry. It is highlighted the existence of particular exploitation conditions that lead to the state where the class Dbased DC transformers are inferior to conventional class B based counterparts. Finally, when a linear compensating amplifier isreplaced with a switching counterpart, it has been demonstrated that it is possible to expect a feasible improvement in energyconsumption in the middle of the ideal half-class D class and classical B class realisations.

AC Losses Analysis of HTS Composite Conductors With Different Current Ramp Rates

High-temperature superconducting (HTS) composite conductors with high carry-current abilities become a preferable choice for superconducting devices to meet the requirements of the power system. In actual operation conditions of superconducting power devices, it is always accompanied by variational current ramp rates, which may cause the ac loss in the conductors. Therefore, this work carries out research on the ac losses of a stacked conductor and a twisted stacked-tape cable (TSTC) with different current ramp rates by changing their structural parameters. Critical current measurements at multiple rates of HTS tapes are initially investigated. Furthermore, the influence of the number of stacked tapes and the twist pitch on the critical current of the conductors is studied. Based on the experimental results, the ac losses of the samples are calculated in COMSOL multiphysics by using a T–A method. This work provides variation trend references of the critical current in HTS tapes and conductors under several rates. Moreover, the ac losses of a stacked-tape conductor and a TSTC with different structural characteristics have been presented in detail.

Magnetotransport Experiments on Fully Metallic Superconducting Dayem-Bridge Field-Effect Transistors

In the last 60 years conventional solid and electrolyte gating allowed sizable modulations of the surface carrier concentration in metallic superconductors resulting in tuning their conductivity and changing their critical temperature. Recent conventional gating experiments on superconducting metal nano-structures showed full suppression of the critical current without variations of the normal state resistance and the critical temperature. These results still miss a microscopic explanation. In this article, we show a complete set of gating experiments on Ti-based superconducting Dayem bridges and a suggested classical thermodynamic model which seems to account for several of our experimental findings. In particular, zero-bias resistance and critical current IC measurements highlight the following: the suppression of IC with both polarities of gate voltage, the surface nature of the effect, the critical temperature independence from the electric field and the gate-induced growth of a sub-gap dissipative component. In addition, the temperature dependence of the Josephson critical current seems to show the transition from the ballistic Kulik-Omelyanchuck behavior to the Ambegaokar-Baratoff tunnel-like characteristic by increasing the electric field. Furthermore, the IC suppression persists in the presence of sizeable perpendicular-to-plane magnetic fields. We propose a classical thermodynamic model able to describe some of the experimental observations of the present and previous works. Above all, the model grabs the bipolar electric field induced suppression of IC and the emergence of a sub-gap dissipative component near full suppression of the supercurrent. Finally, applications employing the discussed effect are proposed.

Electrical, Structural, and Thermal Properties of Ferrite/Superconductor (Ni0.5Zn0.5Fe2O4)x/YBa2Cu3O7-δ) Nanocomposite Materials

The superconductor YBa2Cu3O7-δ (Y123) is prepared by the solid-state route, while the nanosized Ni0.5Zn0.5Fe2O4 (Ni-Zn ferrite) is synthesized via citric acid sol-gel technique. The composite material of the type (Ni0.5Zn0.5Fe2O4)x/YBa2Cu3O7-δ, where x = 0.00, 0.03, 0.10, and 0.50 wt%, is prepared by a modified solid-state method to study the effect of doping Ni-Zn nanosized ferrite on the superconducting, structure, and thermal properties of Y123. The prepared samples show Meissner effect which confirmed the presence of superconducting phase at liquid nitrogen temperature (77 K). The prepared samples have been characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersion of X-ray (EDX), infra-red (IR) spectroscopy, thermogravimetric analysis (TGA), vibrating sample magnetometer (VSM), DC resistance by four-probe, and critical current density measurements. It was found that by increasing Ni0.5Zn0.5Fe2O4 nanoparticles in the Y123 compound, the unit cell volume of the orthorhombic phase decreases, but the transition temperature Tc and critical current density Jc decrease for low doping (x = 0.03 wt%) then increase (x = 0.10 and 0.50 wt%) for Ni0.5Zn0.5Fe2O4 doped in Y123. The transport properties of the (Ni0.5Zn0.5Fe2O4)x/YBa2Cu3O7-δ composite as electrical resistance R, Tc, and Jc are interpreted according to Bean’s critical state model and charge-vortex interaction combined with vortex pinning in Y123 high-Tc mixed-state type-II superconductor. The critical current density Jc decreased for low doping of nanosized Ni-Zn ferrite into Y123 with ratio of 0.03 wt% and increased by increasing doping ratio to 0.50 wt%. The physical properties are improved by the coexistence of the superconductor Y123 and nanosized Ni-Zn ferrites in nanocomposite material.

Investigation of the structural, magnetic and electrical properties of the Au doped YBCO superconductors

In this study, a systematic investigation of Au-added YBa2Cu3O7−x (YBCO) samples has been undertaken mainly to understand their structural, magnetic as well as electrical behavior. In order to get high quality of superconducting bulk samples, optimum preparation conditions have been provided. The bulk samples were produced by conventional solid state reaction method at 930 °C under the low oxygen atmosphere. The bulk samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), electron dispersive X-ray, dc electrical resistivity (ρ-T), critical current density and ac magnetic susceptibility measurements. The samples were carried out by means of powder X-ray diffraction for phase analyses, and lattice parameters, SEM for microstructure examinations. From XRD and SEM results, an increase in the intensity of main peaks and average grain size but a increase in the lattice parameter c with addition of Au in comparison with the unadded sample. In addition to this, onset-offset critical transition temperature ( $${T}_{c}^{on}, {T}_{c}^{off}$$ ) was determined from dc electrical resistivity ( $$\rho -T$$ ) as a function of temperature measurements. Moreover, the transition temperature (Tc) and the critical current density (Jc) are enhanced upon Au addition when compared with unadded YBCO superconductor sample. The increment of Au content decreased the transition temperature and the critical current density values. From the AC susceptibility measurements, inter-grain, intra-grain peak temperature and superconducting transition temperature were determined. The possible reasons for the observed improvements in structural, magnetic and electrical properties due to the Au addition in the YBCO samples were discussed by comparison with other studies about addition of transition metals in high temperature superconductors.

Mechanical—electromagnetic property of stainless sheet laminated BSCCO–2223 wires

In order to elucidate the effect of applied tensile stress/strain on the critical current of DI-BSCCO-2223 wires, the critical current measurement and the tensile test were carried out at 77 K. Neutron diffraction measurements were also performed at 77 K to get direct information about the local strain exerted on the BSCCO superconducting filaments. It was reported that the critical current of BSCCO wires decreases linearly in a small range of tensile stress/strain and that the critical current returns reversibly on reducing the stress/strain. The gradual decrease of critical current was observed within the reversible region. When the tensile stress /strain increased beyond a characteristic value, the critical current decreased rapidly due to the brittle fracture of BSCCO filaments. It is proposed that the reversible stress/strain limit of critical current (Rrec and Arec) can be defined experimentally as the 99% Ic recovery stress (R99%rec) and strain (A99%rec). The reversible strain limit is proposed to consist mainly of three factors of pre-strain, thermal strain and tensile fracture strain of SC filaments themselves. The calculated relation between the Rrec and Arec could well explain quantitatively the observed relation between 99% Ic recovery stress and strain.

Study of microstructure and magneto-transport properties in Bi1.6Pb0.4Sr2Ca3GdxCu4Oδ superconducting systems

The role of Gd addition in the Bi1.6Pb0.4Sr2Ca3GdxCu4Oy (x = 0.0, 0.1, 0.2, 0.3 and 0.4) system was examined by X-ray powder diffraction, scanning electron microscopy, critical current density (Jc) and magneto-transport measurements. The samples were fabricated by the conventional solid state reaction method. The analysis showed that the critical temperature (Tc) and hole number (p) of the materials decreased while the room temperature resistances increased with the addition of Gd. The resistance–temperature dependence for the low-resistance regions of the transition can be determined by the thermally activated flux flow model. The upper critical field [Hc2(T)] values calculated by using this model decreased with the addition of Gd. Furthermore, it was observed that the activation energy (U0) decreased both with the increase in the amount of Gd and applied magnetic field. The degradation of the superconducting properties of the samples may be related to the alteration of the structural, electronic and magnetic properties of the materials depending on the presence of the Gd ions. As a result, the decrease in the number of holes and the magnetic properties of Gd ions are thought to be the fundamental reasons for the deterioration of the superconducting properties of the materials. It has been observed that Gd additions from the microstructure studies of the materials affected the surface morphology negatively.

Instrumentation, cooling, and initial testing of a large, conduction-cooled, react-and-wind MgB2 coil segment for MRI applications

A react-and-wind MgB2 coil segment for a conduction-cooled magnetic resonance imaging (MRI) machine has been fabricated and tested. The coil was developed as part of a collaborative effort on a conduction-cooled, MgB2-based, whole-body MRI image guided radiation therapy device. This study focuses on the fabrication, winding, instrumentation, cooling, and initial critical current (Ic) testing of this near-full-size MgB2 segment coil. The coil was 0.9 m in diameter; the winding pack, 44.0 mm wide × 50.6 mm high, used 1.7 km of an 18 filament MRI-style conductor with Nb chemical barriers, Cu interfilamentary matrices, and an outer monel sheath. The conductor was insulated and reacted before winding onto a stainless steel former. The coil was instrumented with Cernox and E-type thermocouple temperature sensors, strain sensors, and voltage taps. The conduction-cooled coil was mounted in a cryostat capable of accepting coils of up to 0.9 m in diameter and 0.5 m in height. Critical current measurements were performed as a function of temperature during a controlled heating of the coil. The operational target was I = 200 A at 13 K. The full magnet was designed to produce 0.75 T in the imaging area (at I = 200 A), with a maximum field of 1.93 T in the winding. The single segment coil measured here exceeded this operation specification, with an Ic of 280 A at 15 K and a maximum field 1.93 T in the winding. The coil was modeled using a finite element method, and a load line plot showed that 100% of short sample was reached at 21.5 K and above. These measurements demonstrate the viability of conduction-cooled MgB2 background coils as replacements for liquid helium cooled NbTi background coils in future MRI devices.

Polar projections for big data analysis in applied superconductivity

There is a growing problem to represent and analyse large experimental datasets in many emerging fields of science aside of traditional big data-based disciplines, i.e., elementary particles, genetics/genomics and geoscience. One of these emerging fields is applied superconductivity where recently a large, regularly up-dated, public database of critical currents of commercial superconductors was established. The size, dimensionality and resolution of this data makes current methods of display and analysis inadequate. As is often the case in physics and materials science, when dealing with any anisotropic properties, one measures the effects of rotations around a low symmetry axis, this is also the case in critical current measurements as found in applied superconductivity. In this paper we propose the use of polar projected images to map these much larger data sets into useful visualizations for analysis. Where we suggest the radial coordinate and the colour represent amplitudes of two measured parameters, and sample rotation angle is naturally mapped to the polar coordinate. We demonstrate the advantage of this projection for analysing, otherwise unwieldy large, critical current datasets, and naturally recover previously used empirical relations.

High-performance PBT7-Th:PC70BM polymer photodiode with transferred charge blocking layers

Abstract We demonstrated a high performance low-bandgap organic polymer photodiode (OPD), with broadband spectrum, with transferred charge blocking layers. Besides the popular P3HT:PCBM bulk heterojucnction (BHJ) system, we investigated the OPD performance of PTB7-Th:PC70BM BHJ active layer with transfer printed charge blocking layer. To achieve higher performance OPD with lower dark current and higher responsivity, various device structures employing hole blocking layer (HBL) or and electron blocking layer (EBL) were investigated. A very low dark current of ∼5 nA/cm2 at reverse bias 0.5 V was achieved with BHJ active layer sandwiched between both HBL and EBL resulting in high specific detectivity of 1.48 × 1013 Jones at 700 nm. In addition, critical noise current measurement of the OPD was done to evaluate more accurately Noise Equivalent Power (NEP) value. It turned out that the shot noise is dominant over the thermal noise in the OPD verified by the in-depth noise analysis. More deliberate design of the device structure can deliver higher performance OPD cell with developed transfer printing method for use in organic photodetectors image sensors.

International Round Robin Test for Critical Current Measurement of RE-Ba-Cu-O Superconducting Tapes

VAMAS TWA 16 promoted an international round robin test (RRT) on critical current (I c ) measurement of RE-BaCu-O (REBCO; RE = rare earth) superconductors to establish the transport I c measurement method in liquid nitrogen bath without an external magnetic field. Ten laboratories from five countries participated in the RRT and tested commercially available REBCO superconductors. I c and measurement conditions were reported. In parallel, intrinsic I c nonuniformity of the superconductors were evaluated by reel-to-reel scanning Hall-probe microscopy for 10-m conductors. Statistical analyses of I c provided uncertainty of the measurement method. Combined standard uncertainty was analyzed considering the intrinsic nonuniformity of I c . The analysis indicated that the intrinsic nonuniformity has the largest contribution to the uncertainty of I c .

High Current Probe for Ic(B,T) Measurements With ±0.01 K Precision: HTS Current Leads and Active Temperature Stabilization System

The current carrying capabilities of high critical temperature ( Tc ) technical conductors have considerably improved in recent years, and this has practical consequences on the requirements of the measurement equipment. We have designed, constructed, and tested a probe for high-current, high-precision critical current measurements in liquid helium and in gas flow at different temperatures. To minimize the ohmic heating and the thermal conduction losses, the probe employs high-temperature superconductors (HTS) current leads rated at 2000 A, which are cooled by the exhaust helium gas at temperatures below 70 K. An active temperature stabilization system keeps the sample temperature constant at the target during current runs with a precision of ±0.01 K. It consists of a proportional–integral–differential loop that controls heaters close to the sample taking the current-induced heating effects into account. We present the design of the probe and data collected on HTS from different manufacturers, which highlight the performance of the system.

Production of Aluminum Stabilized Superconducting Cable for the Mu2e Transport Solenoid

The Fermilab Mu2e experiment [1] , [2] , currently under construction at Fermilab, has the goal of measuring the rare process of direct muon to electron conversion in the field of a nucleus. The experiment features three large superconducting solenoids: the production solenoid (PS), the transport solenoid (TS), and the detector solenoid (DS). The TS is an “S-shaped” solenoid that sits in between the PS and the DS producing a magnetic field ranging between 2.5 and 2.0 T. This paper describes the various steps that led to the successful procurement of over 740 km of superconducting wire and 44 km of Al-stabilized Rutherford cable needed to build the 52 coils that constitute the Mu2e TS cold mass. The main cable properties and results of electrical and mechanical test campaigns are summarized and discussed. Critical current measurements of the Al-stabilized cables are presented and compared to expected critical current values as measured on extracted strands from the final cables after chemical etching of the aluminum stabilizer. A robust and reliable approach to cable welding is presented, and the effect of cable bending on the transport current is also investigated and presented.

Winding Technology and Experimental Study on 500 kV Superconductive Fault Current Limiter

500 kV saturated iron core superconductive fault current limiter (SFCL) has been built and tested successfully. The superconducting magnet consists of 88 high-temperature superconducting (HTS) double pancake coils with an inner diameter of 1940 mm and an outer diameter of 2040 mm. Double pancake coil plays an important role in 500 kV SFCL, the structure and the winding process have great influence on current carrying capability and reliability of the coil. In this paper, the key technology of winding process of the world's largest HTS coil was introduced, which included bending effect on critical current measurement of HTS tapes, insulation treatment of pancake coil and coil winding process. The V-I curve of the HTS coil was also investigated under the condition of liquid nitrogen in the self-magnetic field. The experimental results show that not only the pancake coil but also the superconducting magnet has good properties and the process is reliable and suitable for manufacturing 500 kV SFCL. The study will not only provide important references for large HTS coils fabrication but also accumulate experience and data for other similar engineering practices.

Impact of planetary ball milling parameters on the microstructure and pinning properties of polycrystalline superconductor Y3Ba5Cu8Oy

We have reported the influence of planetary high energy ball milling parameters on morphology, microstructure and flux pinning capability of polycrystalline Y3Ba5Cu8Oy. Samples were prepared through the standard solid-state reaction by using two different milling methods, ball milling in a planetary crusher and hand grinding in a mortar. Phase analysis by X-ray diffraction (XRD) method, microstructural examination by scanning electron microscope (SEM), electrical resistivity, the global and intra-granular critical current densities measurements are done to characterize the samples. The processing parameters of the planetary milling have a considerable impact on the final product properties. SEM observations show the presence of nanoscale entities submerged within the Y3Ba5Cu8Oy crystallites. The results show that the fine grain microstructure of the Y3Ba5Cu8Oy bulk induced by ball milling process contributes to critical currents density enhancement in the magnetic field and promotes an optimized flux pinning ability.

An Investigation of Y3Ba5Cu8O18 Doping with Ag Nanoparticles and Its Application as Superconductor

Y3Ba5Cu8O18 superconductors were prepared through a standard solid-state reaction and the structural properties of the samples were studied through XRD and the Rietveld refinement method. The effect of silver nanoparticles doping on the Y3Ba5Cu8O18 superconductors was studied as well. It is known that the size of nanoparticles is increased during aging. Therefore, two batches of samples with 1 and 2 wt% of Ag nanoparticles and the size range of 30, 200, 500, 700, 800, and 1000 nm were prepared. After preparing the samples and observing the Meissner effect, the crystallography, critical current density, critical temperature, magnetic susceptibility, SEM, and EDX experiments of the samples were carried out. The results of the critical current density measurements showed that the sample with 2 wt% Ag nanoparticles and the size of 700 nm has the maximum current density. In both batches of samples, increasing the nanoparticle size to 700 nm led to an increase in the critical current density. The crystallography studies showed that silver nanoparticles do not insert into the superconductor’s frame. Actually, they are placed in the samples as a distinct phase.

Influence of B4C-doping and high-energy ball milling on phase formation and critical current density of (Bi,Pb)-2223 HTS

In this paper, the combined effects of B4C-doping and planetary ball milling on the phase evolution, microstructure and transport properties of Bi1.7Pb0.3Sr2Ca2Cu3Oy(B4C)x HTS with x = 0 ÷ 0.125 were studied through X-ray diffraction (XRD), scanning electron microscopy (SEM), resistivity and critical current density measurements. Obtained results have shown that B4C additive leads to the strong acceleration of high-Tc phase formation and substantial enhancement in Jc. High-energy ball milling seems to produce a more homogeneous distribution of refined doped particles in the (Bi,Pb)-2223 HTS which results in an improved intergranular flux pinning and better self-field Jc performance.