Anisotropy Of Critical Current Density Research Articles

Flux dynamics, anisotropy in Jc and vortex phase diagram of H+-intercalated FeSe single crystal

This paper investigates the superconductivity, pinning, and vortex dynamics of FeSe before and after protonation. The study shows that, after being protonated, FeSe's superconducting critical temperature Tc has been increased to more than 40 K and the critical current density Jc has also been increased from 2.1 × 104 A/cm2 to 1.2 × 106 A/cm2, with significantly larger magnetic relaxation rates (Q). Meanwhile, the critical current density anisotropy (γ) decreases substantially while the upper critical field Hc2 increases from 13 T to 91 T after protonation. In addition, the reasons for the absence of the second peak effect in Hx-FeSe were explored. This investigation also reveals the existence of the vortex phase transition associated with the pinning behavior in protonated FeSe, demonstrated by the foot-like characteristics of resistivity. Based on these results, the vortex phase diagrams of FeSe before and after protonation were established. These findings provide important insights for exploring new high-temperature superconductors and their magnetic flux dynamics through protonation.

Flux creep and flux pinning behavior in Cu-doped FeSe0.4Te0.6 single crystals

This paper presents the preparation and comparative study of the superconducting properties and magnetic flux dynamics of FeSe and Fe1-xCuxTe0.6Se0.4 (x = 0, 0.005, 0.01, 0.015) single crystals, with the aim of investigating the origin of the second peak in FeTe0.6Se0.4. The Cu element significantly suppresses the superconducting transition temperature (Tc) of Fe1-xCuxTe0.6Se0.4 from 14.5 K at x = 0 to 9.23 K at x = 0.015, which is close to the 9 K of FeSe, while also suppressing the second peak effect. Additionally, the critical current density anisotropy (γ) is reduced. Dynamic relaxation rates (Q) indicate a crossover from rapid single-vortex pinning near the second peak to collective pinning, linking fast and slow magnetic flux relaxation with the second peak effect. By comparing the pinning properties of FeSe and Fe1-xCuxTe0.6Se0.4 (x = 0, 0.005, 0.01, 0.015), it is found that the second peak effect is closely related to the transition from δl - pinning to δTc - pinning in Fe1-xCuxSe0.4Te0.6 (x = 0, 0.005, 0.01, 0.015). While surface pinning centers dominate in FeSe, the pinning in Fe1-xCuxSe0.4Te0.6 (x = 0, 0.005, 0.01, 0.015) originates from the interaction between the core and normal centers. Furthermore, the electronic transport properties of the FeSe and Fe1-xCuxSe0.4Te0.6 (x = 0, 0.005, 0.01, 0.015) series are also studied. Te substitution suppresses the nematic transition in FeSe, while Cu substitution suppresses the superconducting transition in Fe1-xCuxSe0.4Te0.6 (x = 0, 0.005, 0.01, 0.015) and reduces the residual resistivity. Additionally, the upper critical field is reduced from 150 T in FeSe0.4Te0.6 to 41 T in Fe0.085Cu0.015Se0.4Te0.6, but still higher than the 12.5 T observed in FeSe.

Anisotropic Critical Current Densities in Ba0.6K0.4Fe2As2 with Splayed Columnar Defects

Enhancement and non-monotonic field dependence of critical current density (Jc ), called anomalous peak effect, were observed in Ba1−x K x Fe2As2 with splayed columnar defects. In principle, anisotropy of in-plane Jc should be present in this superconductor since the introduced splayed columnar defects introduce anisotropy in the plane. This study reports in-plane anisotropy of Jc observed in 2.6 GeV U irradiated Ba0.6K0.4Fe2As2 with splay angles of ±15° and ±20°. Results show that Jc component parallel to the splay plane is larger than the perpendicular component, and the parallel component contribute to the anomalous peak effect at around B Φ/3 more.

Improvement of the value and anisotropy of critical current density in GdBa2Cu3O7-δ coated conductors with self-assembled 3-dimensional BaZrO3 nanostructure

In this study, we investigate the GdBa2Cu3O7–x (Gd123) films with several types of self-assembled BaZrO3 (BZO) nanostructure on the ion-beam-assisted deposited MgO substrate (I-BAD) buffered with (La, Sr)MnO3. With additional in-situ annealing between the BZO-doped Gd123 and BZO bilayer, the BZO-doped-Gd123/BZO multilayer sample reveals a three-dimensional (3D) BZO nanostructure network in the Gd123 matrix, which significantly enhances the critical current density JC and flux pinning force (Fp) under high magnetic fields both along the c-axis and ab-plane. The studies on the resistance near transition temperature and the flux dynamic demonstrate that the flux pinning in the tape with a 3D BZO nanostructure network is more 3D-like. With the 3D BZO nanostructure network, our superconductor-coated conductor’s performance in JC (extracted from magnetic and transport data) and angular dependence at high magnetic field is better than the existing reported results, and it could be further improved by optimizing BZO nanostructure parameters.

Modification of Critical Current Density Anisotropy in High-Tc Superconductors by Using Heavy-Ion Irradiations

The critical current density Jc, which is a maximum value of zero-resistivity current density, is required to exhibit not only larger value but also lower anisotropy in a magnetic field B for applications of high-Tc superconductors. Heavy-ion irradiation introduces nanometer-scale irradiation tracks, i.e., columnar defects (CDs) into high-Tc superconducting materials, which can modify both the absolute value and the anisotropy of Jc in a controlled manner: the unique structures of CDs, which significantly affect the Jc properties, are engineered by adjusting the irradiation conditions such as the irradiation energy and the incident direction. This paper reviews the modifications of the Jc anisotropy in high-Tc superconductors using CDs installed by heavy-ion irradiations. The direction-dispersion of CDs, which is tuned by the combination of the plural irradiation directions, can provide a variety of the magnetic field angular variations of Jc in high-Tc superconductors: CDs crossing at ±θi relative to the c-axis of YBa2Cu3Oy films induce a broad peak of Jc centered at B || c for θi < ±45°, whereas the crossing angle of θi ≥ ±45° cause not a Jc peak centered at B || c but two peaks of Jc at the irradiation angles. The anisotropy of Jc can also modified by tuning the continuity of CDs: short segmented CDs formed by heavy-ion irradiation with relatively low energy are more effective to improve Jc in a wide magnetic field angular region. The modifications of the Jc anisotropy are discussed on the basis of both structures of CDs and flux line structures depending on the magnetic field directions.

Comparison of Linear Superconducting Magnetic Bearings Using Isotropic and Anisotropic Materials

Critical current density anisotropy is a common property of high-temperature superconducting materials. We clarify here how it could impact the performances of superconducting magnetic bearings by comparing linear bearings using isotropic and anisotropic materials. To be fair, the comparison considers optimized designs. An H-formulation finite element model is used to obtain the levitation and guidance forces of the bearing for a given moving sequence. It is coupled with a stochastic optimization algorithm. For the considered bearing topology (single bulk above PM Halbach array), it was found that for applications requiring only levitation force, both isotropic and anisotropic materials are suitable. But for applications requiring both levitation and guidance forces, isotropic materials are more suitable than anisotropic ones since they can provide a stable guidance force for a given minimal levitation force at the smallest cost. This could serve as general design guidelines for future engineering applications.

T-A Formulation for the Design and AC Loss Calculation of a Superconducting Generator for a 10 MW Wind Turbine

The current capacity of high-temperature superconductors (HTS) has encouraged several applications of these materials in electric power systems. These applications in electrical machines represent a promising solution for more compact and efficient designs. Despite no measurable resistance in the superconducting state, HTS could experience losses under time changing transport current or magnetic field. Therefore, loss estimation is a key input for the design. Maxwell’s equations in the T-A formulation form can be used to model and estimate losses in the superconducting tapes of an electrical machine. This formulation requires the current as a function of time in each superconducting tape as an input. A methodology to calculate this current distribution is presented in this article. The procedure introduces a previous step in the building model process and allows a better connection of the machine design with the estimation of losses in the superconductor in order to get a more efficient machine. The approach is applied to a 10 MW superconducting generator, where over one thousand tapes cross-sections are modelled in 2D. The superconductor’s non-linear behaviour and critical current density anisotropy are considered. Losses are estimated for different designs and a sensitivity analysis is presented for different temperatures and frequencies, in addition to other alternatives to reduce losses.

Quench Simulation of REBCO Cable-in-Conduit Conductor With Twisted Stacked-Tape Cable

Quench protection is a critical issue for magnet using high-temperature superconducting (HTS) materials such as REBCO. For cable-in-conduit conductor (CICC) with twisted REBCO tapes, which is considered for high-current HTS conductor for fusion magnet, the anisotropic critical current density (Jc) of tape and complex conductor layout will further complicate the quench behavior. The CryoSoft code THEA is used to simulate the quench behavior of various subscale CICCs in view of a quench experiment in SULTAN facility. The CICCs are composed of twisted tape stack assembled into copper shell (strand). It has been found that neglecting the anisotropy of Jc in quench simulation will significantly underestimate the hot spot temperature by about 40 K. Therefore, it could leave potential risk to damage the magnet. It is also shown that the thermal contact resistance between strand and the steel jacket (the conduit) plays a very important role on quench behavior. The hot spot temperature can be reduced by more than 100 K if the thermal resistance is reduced by two order of magnitude. On the other hand, variations of interstrand electrical and/or thermal resistance have only very little influence (few kelvin).

Influence of Critical Current Density Distribution on Transport AC Losses in Superconducting Wire in a DC Magnetic Field

In typical application-like conditions, the inhomogeneous distribution and anisotropy of critical current density must be considered simultaneously in transport AC loss calculation. In this paper, we derive the analytical formulas of transport AC losses for high-temperature superconductors (HTSs) with linear and quadratic distribution of critical current density under applied DC magnetic field. The influence of the inhomogeneous distribution and anisotropy of critical current density has been analyzed. The results show that the impact of the distribution form on transport AC loss is more obvious under applied DC magnetic field. And the influence of applied DC magnetic field will increase as the distribution form becomes steeper.

Isotropic enhancement in the critical current density of YBCO thin films incorporating nanoscale Y2BaCuO5 inclusions

The effect of incorporation of nanoscale Y2BaCuO5 (Y211) inclusions on the vortex pinning properties of YBa2Cu3O7-δ (YBCO or Y123) superconducting thin films is investigated in detail on the basis of variation of critical current density (JC) with applied magnetic field and also with the orientation of the applied magnetic field at two different temperatures: 77 K and 65 K. Surface modified target approach is employed to incorporate nanoscale Y211 inclusions into the superconducting YBCO matrix. The efficiency of Y211 nanoinclusions in reducing the angular anisotropy of critical current density is found to be significant. The observed angular dependence of the critical current density is discussed on the basis of mutually occupied volume by a vortex and spherical and/or planar defect. A dip in JC near the ab-plane is also observed which has been analyzed on the basis of variation of pinning potential corresponding to a spherical (3-D) or planar (2-D) pinning center and has been attributed to a reduced interaction volume of the vortices with a pinning center and competing nature of the potentials due to spherical and planar defects.

Vortex-lattice pinning and critical current density in anisotropic high-temperature superconductors

The anisotropy of critical current density is an impressive manifestation in the physics of high-temperature superconductors. We develop an analytical characterization of anisotropic flux-lattice pinning and critical current density in a system of random point defects. The effect of superconducting anisotropy on the pinning force and critical current density is formulated. The in-plane/out-of-plane anisotropy and microscopic characteristic lengths are incorporated in the field and angular dependence of the critical current density. This is helpful in understanding the physical essence of the scaling behavior in the experiments for critical current anisotropy. We discuss the role of strong and weak point defects in the anisotropic flux-lattice pinning. Relevance of the theory to the critical-state model is dictated as well.

Hybrid artificial pinning centers of elongated-nanorods and segmented-nanorods in YBa2Cu3O7 films

To control the anisotropy of critical current density (Jc), hybrid artificial pinning centers (APCs) of elongated-nanorods and segmented-nanorods were incorporated into YBa2Cu3O7 films. The elongated-nanorods and segmented-nanorods were formed by fabricating multilayer films using YBa2Cu3O7+BaSnO3 targets with a different BaSnO3 content. According to the elastic calculation, the BaSnO3-free YBa2Cu3O7 regions between BaSnO3 segmented-nanorods were highly strained, resulting in their alignment along the c-axis. Pinning of the vortex kinks and straight vortices by the nanorod ends improved Jc in a wide range around B//ab. The angular dependence of Jc systematically varied with the multilayer structure of layer thickness and BSO content. Jc depended on the layer thickness even with keeping the constant average BSO content, showing that the BaSnO3 distribution, as well as the average BaSnO3 content, affected the Jc. The hybrid pinning effect of elongated-nanorods and nanorod ends improved the Jc anisotropy although the effect was not so large in the present films. The control of strain and interface is expected to lead to further improvement of Jc.

Modeling and Comparison of In-Field Critical Current Density Anisotropy in High-Temperature Superconducting (HTS) Coated Conductors

The development of high-temperature superconducting (HTS) wires is now at a stage where long lengths of high quality are commercially available, and of these, (Re)BCO coated conductors show the most promise for practical applications. One of the most crucial aspects of coil and device modeling is providing accurate data for the anisotropy of the critical current density J c (B, θ) of the superconductor. In this paper, the in-field critical current density characteristics J c (B, θ) of two commercial HTS coated conductor samples are experimentally measured, and based on these data, an engineering formula is introduced to represent this electromagnetic behavior as the input data for numerical modeling. However, due to the complex nature of this behavior and the large number of variables involved, the computational speed of the model can be extremely slow. Therefore, a two-variable direct interpolation method is introduced, which completely avoids any complex data fitting for J c (B, θ) and expresses the anisotropic behavior in the model directly and accurately with a significant improvement in computational speed. The two techniques are validated and compared using numerical models based on the H-formulation by calculating the self-field and in-field dc critical currents and the ac loss for a single coated conductor.

Anisotropy Improvement of Critical Current Density in Bi-based Superconductor by Mg Impurity Doping

We investigated effects of Mg impurity doping on Bi 2 Sr 2 CaCu 2 O y high-temperature superconductor (Bi-2212 HTS). We used Bi-2212 HTS whisker as a sample and tried to improve its anisotropy of critical current density (J c ). For the Mg-doping, we adopted thermal diffusion process. In the process, nonsuperconducting phase is expected to appear on the surface of the whisker. From experimental results, we found that the anisotropy parameter of J c can be improved from 13.8 to 1.89 by doping Mg impurity with 400 °C annealing.

Influence of the shielding currents lengthscale and anisotropy effects on the magnetic flux profiles of high-temperature superconductors

The so-called "magnetic flux profile" AC inductive technique is a powerful method for determining the critical current density Jc of bulk superconductors. In this work we aim at reporting analytical expressions for magnetic flux profiles of superconducting rectangular samples exhibiting a critical current density anisotropy. The results are used for examining the error resulting from approximating a rectangular cross-section by an "infinite cylinder" or "infinite slab" geometry. It is found that such approximations can lead to an artificial curvature of the flux profiles and errors of 10%–20% in the determination of Jc. Next, the effects of how planar defects (cracks, platelet boundaries,...) affect the magnetic flux profile signal are discussed. It is found that the magnetic flux profiles are much sensitive to the lengthscale of shielding currents, thereby providing means of investigation of the typical size of induced current loops in bulk superconductors. Finally some illustrative flux profile data measured on a bulk, large grain melt-processed YBCO single domain exhibiting Jc anisotropy are presented and discussed in relation with theoretical predictions.

Anisotropy of critical current density in superconducting networks

We have fabricated Nb square superconducting networks and measured critical current density with current flowing along the direction of 0° and 45° tilted from the lattice of the networks. The temperature and magnetic field dependences of the critical current density for both directions are measured and compared with the flux penetration pattern in those samples. It is found that the anisotropy of the critical current density reflects the flux penetration pattern in the superconducting network samples. Near Tc at zero field, anomalous enhancements of voltage for 0° are observed.

AC Loss Properties of Laser-Scribed Multi-Filamentary GdBCO Coated Conductors with Artificial Pinning Centres

RE 1 Ba 2 Cu 3 O 7-∂ (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 J c 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 J C, 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 J C by introducing artificial pinning centers.

Influence of hot-pressing on MgB2/Nb/Monel wires

Uniaxial hot-pressing was applied as an alternative way to further improve the grain connectivity of both un-doped and carbon doped MgB2/Nb/Monel wires. In this study, hot-pressing of 100MPa resulted in the improvement of the mass density, the critical current density, and the grain connectivity of the wires. However, this also caused additional critical current density anisotropy, which is associated with the texture of a- and b-axes oriented grains. In particular, the anisotropy factors of critical current density for the un-doped and the carbon doped conductors were estimated to be 2.6 and 1.7, respectively, under an external magnetic field of 14T.

Magneto-Optical Observation of Anisotropic Critical Current Density in Nb Films with Artificial Asymmetric Pinning Potential

We investigated the anisotropic motion of vortices in superconducting Nb films by theoretical analysis and magneto-optical (MO) imaging. To realize the anisotropic motion of vortices, we fabricated an array of step-like grooves with a periodicity of 5.0 µm and a length of 1.0 mm on 0.5-µm-thick Nb films. The asymmetric pinning potential of the vortices was generated by the array of step-like grooves. Theoretical analysis of the pinning potential around the steps revealed anisotropies in the elementary pinning force and critical current density. The MO images revealed an asymmetric flux distribution, thereby indicating an anisotropic critical current density, and the ratio of the maximum critical current density to the minimum critical current density was found to be 1.3.

Evaluation of anisotropy of critical current density in stoichiometric Bi-2212 single crystals

The anisotropy of the critical current density was estimated for stoichiometric Bi 2 Sr 2 CaCu 2 O y (Bi-2212) and Bi 2.1 Sr 1.8 Ca 1.0 Cu 2.0 O y single crystals. For this purpose, the critical current density in the a– b plane J c ab and that along the c-axis J c c were discriminated from measurement of DC magnetic moments for specimens with different aspect ratio. The magnetic moment of specimen was measured by a SQUID magnetometer in a magnetic field parallel to the wide surface which is normal to the c-axis. Both of J c ab and J c c are larger in the stoichiometric specimen. In addition, the anisotropy, J c ab / J c c , is found to be significantly reduced in the stoichiometric specimen.