Transport Critical Current Measurements Research Articles

Pinning Landscape Analysis in YBCO Films With Epitaxial and/or Non-Coherent BZO Nanoparticles

The effect of different kind of BaZrO3 (BZO) nanoparticles on the pinning landscape of YBCO thin films have been investigated. We have correlated the microstrain of the YBCO nanocomposites with the presence of non-coherent and epitaxial BZO nanoparticles. Isotropic and anisotropic pinning contributions have been analyzed by means of angular transport critical current measurements. We show that very high enhancement of isotropic Jc is obtained due to the presence of non-coherent BZO nanoparticles, which can be correlated with an enhancement of the microstrain and a reduction of the anisotropy. Moreover, the contribution of epitaxial interfacial nanoparticles, which enhances the anisotropic Jc component along the c-axis have been also identified.

Effect of Nano-Sized ZnO on the Physical Properties of (Cu0.5Tl0.25Pb0.25)Ba2Ca2Cu3O10−δ

High-temperature superconductor phase of (Cu0.5Tl0.25Pb0.25)-1223 was synthesized by solid-state reaction technique and characterized using X-ray powder diffraction (XRD). XRD analysis revealed that the prepared sample was nearly monophase and exhibited tetragonal structure with space group P4/mmm. Nano-zinc-oxide, prepared by Co-precipitation method, was added to the sample. ZnO-concentrations y varied from 0.0 to 2.0 wt.% of the sample’s mass. The prepared samples were investigated through XRD, scanning electron microscope (SEM), energy dispersive X-ray (EDX), particle size analyzer (PSA), differential scanning calorimeter (DSC), electrical resistivity and transport critical current density measurement. X-ray data analysis showed that the nano-Zn addition does not affect the tetragonal structure of (Cu0.5Tl0.25Pb0.25)-1223 phase, whereas the lattice parameters showed an insignificant variation. The results of the superconducting transition temperature, the transport critical current density and melting point of the prepared samples were found to depend on nano-ZnO concentrations.

Diffusion coefficient, activation energy and the effect of Ag-doping on some physical properties of silver doped Bi 1.8Pb 0.4Ca 2.2Sr 2Cu 3O x

The aim of the present study is to investigate the effect of diffusion-doped silver on some physical properties of Bi 1.8Pb 0.4Ca 2.2Sr 2Cu 3O x superconducting samples and to calculate the diffusion coefficient and the activation energy of silver. The present work consists of three parts: (a) optimization of annealing temperature, (b) the effect of Ag diffusion-doped on microstructure and superconducting properties of Bi(Pb)CaSrCuO, and (c) calculation of diffusion coefficient of silver. First, to investigate the optimum annealing temperature we prepared Bi 1.8Pb 0.4Ca 2.2Sr 2Cu 3O x ceramic superconductors annealed at 830, 835, 840, 845, and 850 °C using the solid state reaction method. The investigations consist of XRD, SEM, dc resistivity, and transport critical current density measurements. The highest T c and J c values were observed for the sample annealed at 840 °C for 48 h (B840). Large grain size, denser surface, and high volume fraction of the high- T c phase were obtained for the sample B840. The diffusion doping of Bi 1.8Pb 0.4Ca 2.2Sr 2Cu 3O x by silver increased the critical current density from 123 to 696 A/cm 2 and the transition critical temperature by about 3 K compared with the undoped sample. Ag-doping increased the amount of high- T c phase and improved the surface morphology. It also caused an increase of the lattice parameter c by an amount of 0.15%. The temperature dependence of the silver diffusion coefficient in the range 600–800 °C is described by D=2.9×10 −4exp(−1.05 eV/ k B T). Possible reasons for the observed improvement in the structural and superconducting properties of the samples due to silver diffusion are discussed.

Transport Critical Current of Filamentary Zr-Doped Gd-Ba-Cu-O Superconductors in High Magnetic Fields

Filamentary Gd-Ba-Cu-O superconductors doped with nominal Zr concentrations between 0 and 0.5 at% relative to Gdl23 were prepared by a solution spinning method. Samples were partially melted in flowing 20% O2 + Ar atmosphere gas at various partial melting temperatures and oxygenated in pure oxygen gas. High J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> value of 2 times 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sup> A/cm at 77 K and 0 T were obtained for the Gd123 + 0.1at%Zr sample optimally processed. In the case of doping levels less than 0.5 at %, T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> values, which were about 92 K with sharp transitions, were hardly influenced by Zr concentration. From transport critical current measurements in magnetic fields, the J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> value of the pure Gd123 filament sharply decreased with increasing applied field. Gd123 with 0.5 at%Zr doping processed under optimum conditions showed little deterioration of J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> values in low magnetic fields and J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> values higher than 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> A/cm was maintained up to 14 T. It was also found that J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> s in high magnetic field and the irreversibility field were improved by a small amount of Zr doping.

The influence of Gd addition on microstructure and transport properties of Bi-2223

We have investigated the effect of addition of Gd in Bi 1.8Pb 0.35Sr 1.9Ca 2.1Cu 3Gd x O y superconductor with x=0, 0.1, 0.2, 0.3, 0.4 and 0.5. The samples were prepared by standard solid-state reaction methods. The investigation consisted of X-ray diffraction (XRD), scanning electron microscopy (SEM), dc electrical resistivity, hole concentration and transport critical current density measurements. Transport measurements indicated that the superconducting transition temperature, transport critical current density and hole concentration values of the samples strongly depend on the Gd addition. The values of T c, and J c of the samples decreased with the increase in Gd addition. When Gd addition ratio was increased, surface morphology and grain connectivity of the samples were observed to degrade from SEM investigations and phase ratio of the high- T c (Bi-2223) phase to the low- T c (Bi-2212) phase decreased (XRD measurements). The possible reasons for the observed degradation in superconducting and microstructure properties of Bi-2223 due to Gd addition were discussed.

EDXRF measurements on gold diffusion-doped Bi 1.8Pb 0.35Sr 1.9Ca 2.1Cu 3O y

Gold (Au) diffusion in superconducting Bi 1.8Pb 0.35Sr 1.9Ca 2.1Cu 3O y was investigated over the temperature range 500–800 °C by the energy dispersive X-ray fluorescence (EDXRF) technique. It is found that the Au diffusion coefficient decreases as the diffusion-annealing temperature decreases. The temperature dependences of Au diffusion coefficient in grains and over grain boundaries are described by the relations D 1=6.7×10 −5exp(−1.19 eV/k BT) and D 2=9.7×10 −4exp(−1.09 eV/k BT), respectively. The diffusion doping of Bi-2223 by Au causes a significant increase of the lattice parameter c by about 0.19%. For the Au-diffused samples, dc electrical resistivity and transport critical current density measurements indicated the critical transition temperature increased from 100 to 104 K and the critical current density increased from 40 to 125 A cm −2, in comparison with those of undoped samples. From scanning electron microscope (SEM) and X-ray diffraction (XRD) measurements it is observed that Au doping of the sample also improved the surface morphology and increased the ratio of the high- T c phase to the low- T c phase. The possible reasons for the observed improvement in microstructure and superconducting properties of the samples due to Au diffusion are also discussed.

Electromagnetic characterization ofYBa2Cu3O7−δ thin films with calcium doping for bi-crystal grain boundary conductivity enhancement

The objective of this study was to examine the transport properties of twoYBa2Cu3O7−δ thin filmswith (Y0.9Ca0.1)2BaCuO5 additionsdeposited on vicinal SrTiO3 6° bi-crystal substrates and to investigate the possible correlations between spatialcalcium distribution and local electromagnetic properties across bi-crystal grainboundaries using evanescent microwave microscopy (EMM) and atomic forcemicroscopy (AFM). The samples under consideration differed in transport criticalcurrent measurements by a factor of two although they were deposited on thesame type of bi-crystal substrate. A near-field evanescent microwave microscopebased on a coaxial transmission line resonator with an end-wall aperture wasused to measure changes in conductivity local to the bi-crystal boundary ofYBa2Cu3O7−δ thin films below (79.2 K) and above (room temperature) the superconducting transitiontemperature. Atomic concentration measurements by electron microprobe analysiswere performed in the same regions, and a clear correlation between calciumdistribution and conductivity at 79.2 K (as represented by the change in quality factor)was found. Surface potential imaging (SPI) and quality factor scans in the areaof the bi-crystal grain boundaries were performed at room temperature usingAFM and EMM, respectively, to evaluate local electromagnetic properties in thenormal state and investigate their correlation with superconducting properties.

Enhanced Flux Pinning of an Nd‐Added (Bi,Pb)‐2212 Superconductor

The flux pinning properties of a (Bi,Pb)‐2212 superconductor by the addition of a rare‐earth element Nd with varying concentrations were studied. The Nd content of the samples was varied from x=0.0 to 0.5 on a general stoichiometry of Bi1.7Pb0.4Sr2.0Ca1.1Cu2.1NdxOy. The samples were characterized by powder X‐ray diffraction, scanning electron microscopy, energy‐dispersive X‐ray analysis, and transport critical current measurements at 64 K. Both self‐field JC and JC in the presence of an applied magnetic field (JC–B) of Nd‐added samples were found to be much better than those of the pure sample. Also, the peak values of the bulk pinning force density (FPmax) of Nd‐added samples shift toward higher magnetic fields, indicative of the enhanced flux pinning strength of the samples. The results are explained based on the replacement of cations such as Sr2+, Ca2+, and Bi3+ by Nd3+ ions and the associated distortion of the lattice and change in hole concentration in the system. The replacement produced point‐like defects due to the difference in the ionic radii of Sr2+ (1.12 Å), Ca2+ (0.99 Å), Bi3+ (0.96 Å), and Nd3+ (1.04 Å). The nanoscale secondary phase precipitates produced due to Nd addition may also act as flux pinners.

Current-Voltage Measurements in a 2G YBCO Coil

The Oak Ridge National Laboratory in collaboration with American Superconductor Corporation and Cryomagnetics Inc. has designed, fabricated, and tested an HTS coil wound with second-generation (2G) YBCO coated conductor tape. The purpose of the HTS coil project was to study the quench characteristics in 2G YBCO coils at 77 K and lower temperatures (~30-45 K). These quench characteristics were investigated in both a pool boiling LN2 environment and in a conduction cooled configuration at ~30 K and 45 K. Transport critical current (Ic) measurements taken on the very first thermal cycle of the YBCO coil in pool boiling LN2 showed an Ic~31 A corresponding to a central magnetic field of 0.32 T. The measured Ic value was consistent with the calculated value using the calculated maximum perpendicular B-field component and the measured short sample Ic at 77 K. Subsequent Ic measurements taken in the conduction cooling configuration at 34 K and 45 K, showed a steady-state Ic~45-49 A and 38-44 A, respectively. These Ic values were significantly lower than the calculated value assuming a literature derived temperature dependent Ic of the 2G YBCO tape. A steady degradation was observed in the Ic of the coil with each successive thermal cycle. In addition, the coil was also pulse tested up to 1-T in non-steady state transient conditions and for ramp rates varying between 0.01 and 5 A/s. The issues and limitations encountered during testing of this new type of 2G coil are briefly discussed.

Development of a Low-Temperature Electro-Mechanical Testing Device

Understanding the strain-sensitivity of high temperature superconductors is important for the development of applications. Conductors for magnets experience mechanical loads during all stages of manufacturing and thermal cycling, as well as Lorentz force induced loads during operation. Thus, it is important to study the effects of mechanical loads on HTS conductors in the presence of magnetic field. Here we report on the development of a tensile testing device that was designed to characterize the in-field electromechanical behavior of HTS conductors. The device is capable of applying tension or compression, controlled fatigue cycles, and in-situ transport critical current measurements. We report on the development and capabilities of the device, as well as the initial stress-strain results at room temperature.

Tensile stress applied to NbTi, Nb3Sn, Bi-2223 and MgB2 composite superconductors at room temperature

Axial tensile stress was applied to four different composite wires containing the superconductors NbTi,Nb3Sn, Bi-2223and MgB2 at room temperature. The basic mechanical properties obtained from the stress-straincharacteristics of these wires are compared and discussed. The wire samples were thentested by transport critical current measurements at 4.2 and 77 K. The results obtainedshow that the critical current degradation of all composite wires occurs at the stressvalue corresponding to the beginning of the plastic deformation. These data haveimportant implications for the technological application of such superconducting wires.

Scaling behavior of the critical current density inMgCNi3microfibers

We present transport critical current measurements on microfibers consisting of a $80\text{\ensuremath{-}}\mathrm{nm}$ thick layer of polycrystalline $\mathrm{Mg}\mathrm{C}{\mathrm{Ni}}_{3}$ synthesized directly onto $7\text{\ensuremath{-}}\ensuremath{\mu}\mathrm{m}$ diameter carbon fibers. Near the transition temperature ${T}_{c}$, the critical current density ${J}_{c}$ is well described by the power law form ${[1\ensuremath{-}{(T∕{T}_{c})}^{2}]}^{\ensuremath{\alpha}}$, where $\ensuremath{\alpha}=2$ with no crossover to the Ginzburg-Landau exponent $\ensuremath{\alpha}=1.5$. We extrapolate ${J}_{c}(0)\ensuremath{\approx}4\ifmmode\times\else\texttimes\fi{}{10}^{7}\phantom{\rule{0.3em}{0ex}}\mathrm{A}∕{\mathrm{cm}}^{2}$, which is an order of magnitude greater than estimates obtained from magnetization measurements of polycrystalline powders. The field dependence is purely exponential ${J}_{c}(T,H)={J}_{c}(T)\mathrm{exp}(\ensuremath{-}H∕{H}_{0})$ over the entire field range of $0\phantom{\rule{0.5em}{0ex}}\text{to}\phantom{\rule{0.5em}{0ex}}9\phantom{\rule{0.3em}{0ex}}\mathrm{T}$. The unconventional scaling behavior of the critical current may be rooted in an anomalous temperature dependence of the London penetration depth.

Transport critical current measurement of Y–Ba–Cu–O thin film fabricated by coating pyrolysis process

The coating pyrolysis or metal organic deposition process is expected to be an attractive method offering the possibility of cost reduction and ease of large-scale fabrication of YBCO thin films. This process could realize a high critical current density of YBCO thin films. However the critical current density was measured only by the induced current method. Therefore, we compared the results of the induced current method with those of the transport current method. According to the measurement results, these values were almost the same as 2 MA/cm 2, and a agreement between them was evident. The reaction between a YBCO film and indium contact and the change in quality of the YBCO film at the current contact point were examined. Because our experiments focused on large current applications such as the fault current limiter, reliable contact between the copper current lead and the YBCO film was essential. The difficulty of the large current measurement of the YBCO thin film was shown by our research.

Critical current of YBa(2)Cu(3)O(7-delta) low-angle grain boundaries.

Transport critical current measurements have been performed on 5 degrees [001]-tilt thin film YBa(2)Cu(3)O(7-delta) single grain boundaries with the magnetic field rotated in the plane of the film, phi. The variation of the critical current has been determined as a function of the angle between the magnetic field and the grain boundary plane. In applied fields above 1 T the critical current j(c) is found to be strongly suppressed only when the magnetic field is within an angle phi(k) of the grain boundary. Outside this angular range the behavior of the artificial grain boundary is dominated by the critical current of the grains. We show that the phi dependence of j(c) in the suppressed region is well described by a flux cutting model.

High quality weld of melt textured YBCO using Ag doped YBCO junctions

The addition of silver to YBa 2Cu 3O 7− δ (Y123) is well known to decrease its decomposition temperature. This study demonstrates the success in welding large Y123 textured samples with Ag doped Y123 junctions. Attempts to weld two melt textured domains of Y123 by a 1 mm thick layer of Ag doped Y123 are reported. The crucial importance of the initial Y211 excess content in the spacer material is pointed out. Microstructural observations, trapped field measurements and transport critical current density measurements prove that for a given Y211 amount a very high joint quality can be obtained. A transport critical current density as high as 10 kA/cm 2 is registered through the junction at 77 K and 5 T when the applied field is parallel to the ( a, b) planes.

Fast device for large transport critical current measurements

In this work we describe some modifications that improve an equipment for the fast measurement of transport critical currents in superconducting materials having a critical current of hundreds of amperes. It is based on the appliance of an electrical current for a very short period of time, rapid enough to preserve the integrity of the current leads and minimize the Joule effect. Power is applied to the wire-sample ensemble and the voltage drop is measured within seconds, with a resolution that can reach the order of 10 nV. The hardware is composed of three parts: the current pulse generator, a low noise-quick response differential voltage amplifier and a PC with a digital to analog converter—analog to digital converter card. The data acquisition is achieved via an Assembler program.

Microstructure and growth of joins in melt-textured YBa2Cu3O7−δ

Joining of melt-textured YBa2Cu3O7-δ (Y123) grains has been achieved without use of an external agent. The technique uses barium-cuprate liquid phase released from platelet boundaries to mediate the growth of Y123 at the interface between two grains. The epitaxial nature and high quality of the growth was determined by optical and transmission electron microscopy. The composition of Ba–Cu–O phases found in some parts of the joins was determined by electron probe microanalysis. A clean low-angle join was found to consist of a grain boundary with dislocation networks and facets. Transport critical current measurements on this type of join revealed strongly coupled behavior. The technique shows promise for the joining of melt-textured material for power engineering applications.

Hysteresis in transport critical-current measurements of oxide superconductors

We have investigated hysteresis in transport critical-current (I/sub c/) measurements of Ag-matrix (Bi,Pb)/sub 2/Sr/sub 2/-Ca/sub 2/Cu/sub 3/O/sub 10-x/ (Bi-2223) and AgMg-matrix Bi/sub 2/Sr/sub 2/CaCu/sub 2/O/sub 8+x/ (Bi-2212) tapes. I/sub c/ hysteresis causes measurements with field, angle, and temperature sweeps to be multi-valued. Which value is correct is addressed in the context that the proper sequence of measurement conditions reflects the application conditions. Hysteresis in angle-sweep and temperature-sweep data is related to field-sweep hysteresis.

Bi:2212/Ag-based Rutherford cables: production, processing and properties

The fabrication of Bi:2212/Ag-based Rutherford-type accelerator cables, their transport critical current testing and ac loss measurement are described. Multifilamentary strands were used to form several cables with 18-19 strands and a lay pitch, (half the transposition pitch) of 27.5 mm. The inclusion of a metallic alloy core was proposed for mechanical strength, the limiting of cable winding damage and ac loss/residual magnetization mitigation. Single-strand measurements of `poisoning' from several candidate core materials are reported, as well as the influences of winding pitch and thermal shock degradation on . Nichrome 80 was selected as a core material on the basis of ready availability in strip form and its initially observed inertness from a poisoning standpoint. In-cable single-strand studies indicated that winding damage could degrade edge-measured by about 16%. Full cable measurements demonstrated that core-induced poisoning could reduce by a further 35-40%. Ac loss measurements on a series of specially designed cables showed that the core (either bare or coated) effectively insulated the strands against crossover contact. This, together with the fact that the cable had been only lightly compacted (thereby ensuring moderate but not strong side-by-side contact) allowed the effective interstrand contact resistance of a projected Bi:2212/Ag-wound core-type LHC Rutherford cable to fall close to the acceptability range for the windings of accelerator magnets.

Thick highly textured (Bi, Pb)-2223 ceramics

Dense ceramics textured by sinter-forging exhibit superconducting properties among the best for polycrystalline bulk 2223 materials. Investigations showed that the deformation during the sinter-forging process is mainly responsible for the grain alignment of the ceramics. A sharp texture can only be obtained after a strong deformation from the starting cold-pressed powder to the final thin forged disc. To obtain thick ceramics, several thin and highly identically textured discs were linked together by an additional short sinter-forging step. Microscopic observations showed that the interface between the discs disappeared after the hot treatment. Transport critical current measurements performed at 77 K through bars of different sizes confirm that, through an equal texture, the critical current density () is nearly constant with the shape of the bar section: thin or thick, narrow or wide. A ceramic composed of many stacked and sinter-forged discs allowed resistivity and to be measured along the sinter-forging axis. Low anisotropic ratios confirmed the strong link between the thin discs and a very homogeneous whole ceramic. This shape processing of bulk 2223 ceramics is thus attractive for the fabrication of current leads and limiters with high capabilities.