Anisotropy Of Critical Current Density Research Articles

Greatly decreased critical current density anisotropy in YBa2Cu3O6+x thin films ablated from nanocrystalline and BaZrO3-doped nanocrystalline targets

The anisotropy of the critical current density Jc was determined from angular critical current measurements for YBa2Cu3O6+x (YBCO) thin films laser deposited from a nanograined and a BaZrO3-doped nanograined target. In quantitative analysis using Blatter scaling the effective Jc anisotropy was found to be much smaller than for standard YBCO films.

Substrate-induced current anisotropy in YBa 2Cu 3O 7− δ thin films

The defect structure in thin epitaxial films of YBa 2Cu 3O 7− δ is strongly dependent on the substrate. Using vicinal cut substrates can lead to an anisotropic critical current density in the film plane. Temperature-dependent measurements using quantitative magneto-optics make it possible to investigate current densities along different directions independently. By applying a step-wise power-law to the data, different current limiting mechanisms could be identified.

Critical Current Studies on Nb-Ti Deformed-Strands

The Nb-Ti hard conductors used in LHC dipole and quadrupole magnets are Rutherford cables composed of several tens of strands. During the cabling process, the strands are severely compacted especially at the thin edge of the cable. In order to assess, on the whole wire length, the deformation effect on the transport current of the wires, LHC-type Nb-Ti superconducting strands of various types were flattened by means of rollers. The critical current was then measured as a function of deformation and applied magnetic field at both 4.3 K and 1.9 K. The measurements were performed for both orientations (flat face perpendicular or parallel to magnetic field). The critical current density anisotropy of such deformed strands and the correlation with magnetization effects are discussed. This study permits to better understand and to quantify the critical current degradation of few percent observed in strands due to cabling. Comparisons with wires extracted from Rutherford cables are presented

Critical state model with anisotropic critical current density

Analytical solutions of Bean’s critical state model with critical current density Jcbeing anisotropic are obtained for superconducting cylindrical samples ofarbitrary cross section in a parallel geometry. We present a method forcalculating the flux fronts and magnetization curves. Results are presented forcylinders with elliptical cross section with a specific form of the anisotropy.We find that over a certain range of the anisotropy parameter the fluxfronts have shapes similar to those for an isotropic sample. However, ingeneral, the presence of anisotropy significantly modifies the shape ofthe flux fronts. The field for full flux penetration also depends on theanisotropy parameter. The method is extended to the case of anisotropic Jc that also dependson the local field B,and magnetization hysteresis curves are presented for typical values ofthe anisotropy parameter for the case of |Jc |that decreases exponentially with |B|.

Electrodynamic features of anisotropic hard superconductors

The low-frequency electromagnetic response of superconducting plates characterized by strong anisotropy of current-carrying capacity in the plane of the sample is studied experimentally and theoretically. Measurements are made on polycrystalline textured plates of the Y-123 system with the c axis lying in the plane of the sample and on a single crystal with a single preferred direction of twinning boundaries. It is shown that the shape of the curves describing the dependence of the relative losses q on the ac field amplitude h0 is quite sensitive to the orientation of vector h0 in the sample plane. As in the case of isotropic samples, the q(h0) dependence is characterized by a single size peak if vector h0 is oriented along one of the principal symmetry directions of the anisotropic critical current density. If h0 deviates considerably from principal directions, two size peaks are observed on the q(h0) curve. A detailed analysis of the evolution of the q(h0) curves upon a rotation of vector h0 in the sample plane is carried out.

Comparative study of the vortex motion between in-fan and across-fan in YBa 2Cu 3O 7 single crystals with crossed columnar defects

We have studied vortex motions along the across-fan and in-fan directions in YBa 2Cu 3O 7 single crystals with crossed columnar defects using micro-Hall probe array. We find anisotropic critical current density, J c, depending on the relative orientation of the vortex motion with respect to the defect configuration. J c was higher for the across-fan motion. On the other hand, the normalized relaxation rate, S, and the activation energy, U, did not show a clear anisotropy except at 30 K. The lack of obvious directional dependence in S and U compared to J c implies that the observed J c anisotropy is mainly due to the anisotropy in the flux bundle volume. In addition, during the flux entry below the full penetration we find the largest relaxation takes place at the advancing flux front.

Texture development during the Bi(2223)/Ag multifilamentary tape processing

We have developed a method that allows, by a simple set of magnetic measurements, to study the texturing of the grains inside Ag-sheathed (Bi,Pb)/sub 2/Sr/sub 2/Ca/sub 2/Cu/sub 3/O/sub 10+x/ tapes by means of two representative angles, /spl theta//sub L/ and /spl theta//sub T/. They identify the mean grain misalignment with respect to the tape surface respectively in longitudinal (i.e. the rolling direction) and transverse direction. Here we present the texture evolution of the superconducting grains inside silver sheathed multifilamentary tapes prepared in different ways. We have started our investigation from the initial steps of the mechanical deformation up to the final thermal treatment stage. We will compare the results obtained by the magnetic method to more traditional ones, i.e. X-ray diffraction and critical current density anisotropy measurements. Finally the advantages and the limits of the magnetic method will be briefly discussed.

Size effect in anisotropic hard superconductors

The amplitude dependence of the relative AC losses q in anisotropic superconducting plate-like samples has been studied experimentally and theoretically. Contrary to the isotropic case where there exists one maximum in the dependence of q= χ ′′ on the amplitude h 0 of the AC magnetic field, two maxima in the dependence q( h 0) were observed. Both maxima are found to be related to the penetration of the AC field up to the middle of a sample. This size effect occurs if the direction of the AC magnetic field differs significantly from the principal axes of symmetry. Such an unusual manifestation of the size effect can be understood under the assumption that two components of the AC field along the principal axes of symmetry are independently shielded by the two principal components of the anisotropic critical current density. The obtained results are interpreted within the framework of the critical state model generalized to the anisotropic case.

STUDY OF THE TEXTURING IN BSCCO-AG TAPES THROUGH MAGNETIC MEASUREMENTS

We have developed a method that allows, by a simple set of magnetic measurements, to study the texturing of the grains inside a BSCCO-Ag tape. Because the texture is anisotropic we define the angle ϑ L that identifies the mean grain misalignment angle with respect to the tape surface in longitudinal direction (i.e. rolling direction) and the angle ϑ T in transverse direction. The technique is based on the assumption that, because of the very high anisotropy of the critical current density in BSCCO superconducting compounds, the magnetic moment is essentially generated by the current circulating in the a-b planes of the BSCCO grains. The different magnetisation cycles, measured when the orientation of the magnetic field with respect to the tape surface is changed, depend only on the grain orientation inside the tape, which determines the effective magnetic field component normal to the a-b planes of the grains. Here we present the texture evolution of the BSCCO grains inside silver sheated multifilamentary tape starting from the initial steps of the mechanical deformation up to the final heating stage. The results obtained from the magnetic method are compared with those obtained with other methods, i.e. X-ray diffraction and critical current density anisotropy. Also results obtained on samples prepared in different way will be presented.

Vortex pinning and anisotropy in Ag/(Bi, Pb)2Sr2Ca2Cu3O10 tapes by means of surface columnar defects

Abstract Surface columnar defects (SCDs) are produced in high-quality Ag/(Bi, Pb)2Sr2Ca2Cu3O10 tapes by irradiating them with 0.25 GeV gold ions only on a top layer up to about 10% of the fuli volume. The ion beam is orthogonal to the tape piane. Such defects seem to be able to confine the vortices inside ‘tubes’ having a diameter determined by the SCD average distance. This confinement shows itself in the shift of irreversibility lines (ILs) towards high temperatures and magnetic fields, in the enhancement of critical current densities and in the strong decrease in the dependence of ILs on freauency. In this paper we concentrate on the shutdown of the IL anisotropy (low-current regime) as well as on the decrease in the critical current density anisotropy (high-current regime). IL anisotropy is also studied as a function of the angle between the applied magnetic field and the ion tracks. The following results should be emphasized, because they are peculiar to the SCD topology. Firstly, the enhancement of ...

Vortex Pinning due to Strong Quasiparticle Scattering at Antiphase Boundaries inYBa2Cu3O7−δ

Using a recently developed quantitative magneto-optical method we investigated the vortex pinning mechanism in ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ films. Parallel oriented planar antiphase boundaries (APB's), generated in epitaxial thin films grown on vicinal ${\mathrm{SrTiO}}_{3}$ (001), serve as well defined pinning sites, yielding extraordinary high pinning forces and an anisotropic critical current density ${j}_{c}$ up to $\ensuremath{\approx}30%$ of the depairing current. The measured angular and temperature dependence as well as the magnitude of ${j}_{c}$ prove that pinning at APB's is dominated by quasiparticle scattering induced variation of condensation energy, characteristic of superconductors with anisotropic order parameter.

AC losses in high-temperature superconductors: revisiting the fundamentals of the loss modelling

The fundamentals of the electromagnetic modelling of high-temperature superconductors are discussed. Special attention is paid to intrinsic features of high-temperature superconductors different to those of low-temperature superconductors. Examples of those features are strong thermal fluctuations, which results in enhanced flux creep and slanted E( J)-characteristics, anisotropy of critical current density and material resistivity, and the granularity of the material. Having established the fundamental principles for the loss modelling, the influence of thermal fluctuations, anisotropy and granularity on the AC losses are considered.

High-pressure-high-temperature effect on the structure of YBaCuO- and NdBaCuO-based superconductive ceramics

The structure of dense (99%) high-pressure (2-5 GPa)-high-temperature treated single-phase and melt-textured YBaCuO and NdBaCuO ceramics has been investigated. High-pressure-high-temperature (HP-HT) treatment under 5 GPa in contact with annealed zirconia allows us to prevent oxygen losses from Nd123 and Y123 structures even when heated up to 1100 and , respectively, and thus, keep the superconductive characteristics. For MT-YBaCuO ceramics, a more than twofold decrease in the anisotropy of critical current density occurs because of doubling critical current density in the direction of the c-axis of Y123 crystallites (in the perpendicular direction it remains almost unchanged) may be explained by the increase in dislocation density in the (001) plane (from up to ) and by narrowing of twin domains (from 150 down to 20 nm). For MT-NdBaCuO, narrowing of spacing between the twins has been observed in the polarized optical microscope.

Flux penetration into flat rectangular superconductors with anisotropic critical current

Superconductors with anisotropic critical-current density ${j}_{c}$ exhibit characteristic anisotropic flux-density patterns during penetration of magnetic flux. We investigate this anisotropic flux penetration in detail by observations using the magneto-optical Faraday effect and by first-principles calculations which describe the superconductor as a nonlinear anisotropic conductor. Our samples are thin plates of DyBa${}_{2}$Cu${}_{3}$O${}_{7\ensuremath{-}\ensuremath{\delta}}$ into which anisotropic pinning is introduced by oblique irradiation with 340-MeV Xe ions creating linear defects. Excellent agreement between experiment and theory is obtained. In particular, we find that in rectangular plates with ${j}_{c}$ anisotropy equal to the side ratio, the intrinsic and shape anisotropies may compensate such that the flux pattern looks like that in an isotropic square stretched to the rectangular shape. This means the current streamlines are concentric rectangles which shrink to a point rather than to a line, and the discontinuity lines where the current bends sharply, coincide with the diagonals of the rectangle rather than forming the usual double-Y structure.

Critical current anisotropy in conventional and artificial pinning center round wire Nb-Ti superconductors

Critical current density anisotropy was detected in conventional and artificial pinning center (APC) Nb-Ti round wires by means of a relevant increase of the magnetization M as the wire aspect ratio A/sub w/ (length L/filament diameter d/sub f/) was increased. However, contrary to what is observed in APC wires, the critical current density J/sub /spl par// calculated from the SQUID measured saturated-magnetization in conventional wires using an anisotropic Bean model (BM), is significantly different from the transport critical current density J/sub ct/. We suggest that the mismatch between SQUID measured J/sub /spl par// and J/sub ct/ is due to a combination of the discontinuity of the ribbons along the wire length in conventional Nb-Ti wires and also to the low electric fields generated in SQUID measurements. The low electric fields allows the magnetization-induced currents to redistribute inside the wire due to the local inhomogeneities associated with the presence of /spl alpha/-Ti ribbons. In this case anisotropic BM cannot be used to extract J/sub c/ from M because it will be function of the intrinsic lengths of these ribbons.

History effect and anisotropy of critical current density in screen-printed (Bi, Pb) 2Sr 2Ca 2Cu 3O x/Ag tape

Transport critical current density J C( T, B, θ) of screen-printed, c-axis-oriented (Bi,Pb) 2Sr 2Ca 2Cu 3O x /Ag composite tape with a ceramics mono-layer core was investigated between 4.5 and 77 K in magnetic fields up to 8 T as a function of the field angle θ between B and the c axis. At low temperatures T ⪕ 30 K , a shift of a peak angle from θ = 90° (i.e., B ⊥ c axis) occurs in the J C versus θ curve, accompanied by an additional asymmetry. Such anomalies are explained by the history effect in the J C values on magnetic field for B∥ c axis ascribed to the grain-boundary weak links, on the assumption that J C is dominated by the field component B ∥ = Bcos θ parallel to the c axis. At elevated temperatures, disappearance of the history effect results in a perfectly symmetrical angular dependence of J C with a peak at θ = 90°.

Electrical contacts to CRT Bi-2212 superconductors: preparation, characterization and analysis

The application of bulk high-temperature superconductors (HTSs) in power engineering systems requires the development of reliable contact preparation technology and a thorough understanding of the current transfer and voltage characteristics of the HTS - normal metal interface. This article describes a technique for preparing in situ Au electrical contacts to (Bi-2212) bulk superconductors and a method for measuring the spatial dependence of voltage and current in a contact. Currents of approximately 70 A (the of the sample) transfer from the contact metal to Bi-2212 over a surface area of . An expression for the spatial dependence of voltage and current in a contact is derived and discussed in the context of semiconducting devices, LTSs and HTSs. The surface resistivity of the Au - Bi-2212 interface is measured at and it is demonstrated that an anisotropic critical current density significantly increases the apparent surface resistivity by as much as a factor of 10 in the region where current first transfers to the HTS. Modified expressions for the spatial dependence of voltage and current in contacts are given which account for surface and flux flow resistive components.

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RE1Ba2Cu3O7-∂ (RE: Rare Earth, Gd, Y and so on) coated conductors have a large aspect ratio and also anisotropy in critical current density. For practical application of REBCO tapes, we evaluated Jc and ac loss properties in a wide range of temperature and magnetic field. The induced ac losses due to the perpendicular magnetic field to a tape face are considerably huge. Therefore, in case that REBCO coated conductors are applied electric power machines and devices, ac loss induced in the windings should amount to a great part of the total heat load. We proposed an ac loss reduction method, which is combined with a laser scribing technique and a special winding process into a multi–filamentary structure. Additionally, we are trying to enhance critical current density JC, by doping artificial pinning centre. In this study we investigated the ac loss properties of multi-filament structural GdBCO tapes with BZO nano–lods as artificial pinning centers. We measured ac losses of 5, 10, 20-filament GdBCO tapes by using a saddle-shaped pickup coil. Consequently we confirmed the ac loss decreased in inverse proportion to the number of filaments even in the GdBCO tapes with enhanced JC by introducing artificial pinning centers.

Anisotropy of critical current density in c -axis oriented epitaxial YBa 2 Cu 3 O 7-δ films

The dependence of critical current density Jc on the angle α between the directions of the applied magnetic field H (which was rotated in the c-axis-I plane) and the in-plane current I was measured on a c-axis oriented epitaxial YBa2Cu3O7-δ films at 81 K, with the magnetic field strength up to 6T. Analysis of the experimental results on the basis of the classical scaling law of pinning force shows that there exist simultaneously planar-pinning and volume-pinning mechanisms, and the contribution of volume pinning increases with decreasing α while that of the planar pinning decreases. We propose that the decrease of Lorentz-force-independent critical current density with increasing H for H//I results from the suppression of superconductivity by the magnetic field. The fact that the contribution of volume pinning increases with decreasing α also arises from the suppression of superconductivity in CuO2 plane by the magnetic field.

Anisotropic critical current densities of silver sheathed BSCCO-2223 tape

A novel experimental arrangement is described in which current is constrained to flow perpendicular to the faces of (Bi, Pb) 2Sr 2Ca 2Cu 3O x silver clad tape produced by the standard powder in tube technique. This configuration with “perpendicular ” current flow shows that significant voltages are developed at current densities less than 1% of the conventional, longitudinal, critical current density. The effect of applied magnetic fields on the perpendicular critical current density are also reported, with fields perpendicular to the plane of the tape being most effective at reducing the critical current density. From this data it is calculated that the average critical current of each grain boundary is 0.5 mA and < 10 μA for the longitudinal and perpendicular flows respectively. This difference is attributed to the perpendicular current probing the whole of the superconductor core, whilst the longitudinal current may flow predominantly in a higher critical current density region near the silver sheath. Implications for the current distribution and critical current density measurement in short samples of BSCCO-2223 tape are considered.