Bulk high-Tc Superconductors Research Articles

Influence of the Trapped Field on the Levitation Performance of the Magnetized Bulk High-T c Superconductor

A magnetized bulk high-Tc superconductor (HTSC) magnet is a good candidate to improve the levitation performance of the high-Tc superconducting (HTS) maglev system. Compared with the unmagnetized bulk HTSC, the magnetized bulk HTSC magnet can supply stronger levitation or guidance force above a permanent magnet guideway (PMG). Different from the permanent magnet, the magnetic field of a magnetized bulk HTSC magnet is sustained by the induced superconducting current produced during the magnetizing process. Given that the induced superconducting current within the magnetized bulk HTSC magnet is very sensitive to the magnetic field, the levitation performance of the magnetized bulk HTSC magnet is directly related to its own trapped field and the magnetic field of the PMG. This article discusses the influence of trapped and external magnetic fields on the levitation performance of a magnetized bulk HTSC magnet by experiments, and the Critical State Model is used to analyze the test results. The analyses and conclusions of this article are useful for the application of magnetized bulk HTSC magnet in practical HTS maglev systems.

Study of High-Tc Superconducting Bulk Moving With Different Speeds in Applied Magnetic Fields

The force-speed relationship of a bulk YBCO moving above a permanent magnetic guideway was studied. The experiments show that the levitation forces at the lowest position Fmax increase with the increment of descending speed and tend to approach a certain levitation force FS.max. Calculations based on a simplified superconducting ring and flux flow model show that the force variations can be ascribed to the resistive electric field produced by flux motion. Further calculation reveals that, by increasing the descending speed, the thermo-dissipation inside the superconducting ring immediately after the descending process exponentially declines, approaching zero while the speed approaches infinity, and Fmax reaches FS.max. Therefore, the thermo-dissipation inside the bulk YBCO can be defined as a parameter for evaluating levitation force losses. Those results might be useful in pragmatic design of high-Tc superconducting levitation devices and magnetization of type-II superconductors.

Numerical Investigation of the Lateral Movement Influence on the Levitation Force of the Bulk HTS Based on a 3-D Model

On the basis of a 3-D model of the high- superconducting maglev, the characteristics of the levitation force (LF) of a bulk high-Tc superconductor (HTS) under continuous lateral movement was numerically investigated. The Functional rule of the influencing factors, such as field-cooling height (FCH), levitation height (LH), maximum lateral displacement (MLD), and material property, was demonstrated. The results indicate that, for the case with a shorter MLD, and better material property, the LF decay due to lateral movement is more remarkable. Finally, the Pre-Loading, i.e., make the bulk HTS trap more fluxes before working, was also simulated and its validation to reduce the LF decay is confirmed.

Influence of Re-magnetized HTS Bulk Samples on the Levitation Performances of a Maglev System

High-Tc superconducting (HTS) bulk samples used for a maglev system can be re-magnetized after being subjected to another applied magnetic field which may influence the levitation performance of a HTS bulk over the permanent magnetic guideway (PMG). In order to clarify the relationship between the HTS bulk’s re-magnetization and its levitation performance, a YBCO bulk was re-magnetized at different values and its levitation performance studied experimentally above a permanent guideway (PMG) system. The results show that the increase of trapped magnetic flux inside the HTS bulk is closely related to the magnitude and direction of the applied field during the re-magnetization process. Furthermore, the levitation force changes on the HTS-PMG system affected by the re-magnetized HTS bulk are discussed allowing for optimization improvements to the maglev system load capabilities.

A Method to Increase the Lateral Reversible Region of Bulk YBCO Above a Permanent Magnet Guideway

Lateral reversible region of the bulk high-Tc superconductor (HTSC) above the permanent magnet guideway (PMG) is a key parameter for the high-Tc superconducting (HTS) maglev system. The lateral stability and guidance force are proportional to the width of the lateral reversible region. When the lateral displacement of the bulk HTSC exceeds the reversible region, the maglev system will return to a new lateral equilibrium position. The method with the use of pre-displacement can successfully increases the lateral reversible region of the bulk HTSC. By the correct choice of the maximum lateral offset and a new starting point, the instability of flux motion can be suppressed. Unsuitable usage of this method will cause the intersection of guidance force and reduce the reversible region. Analyses based on the magnetizing history and pinning metamorphoses well explain the experimental results. Difference between this method and the pre-load method is also discussed in this article.

Characteristic of the Guidance Force in the High-T c Superconducting Levitation System with Pre-Load Process

The stable levitation above permanent magnet is an important characteristic of the bulk high-Tc superconductor (HTS). When an external force pushes the bulk HTS up, down or sideways, or tries to tilt it, a restoring force can return it to its initial position. The HTS Maglev relied on this characteristic can overcome the external force from wind or pass the curve lines successfully. The change of guidance force (GF) during many times lateral movement is studied. Experiments show that GF increases during the lateral movement, no matter what kind of PMG or HTS is used, and the change of the GF slows down after 5 times lateral movement. The pre-load method can reduce the levitation force decay during lateral movement. So the influence of GF by the pre-load method is needed to be studied. It is found that the pre-load method can increase GF and reduce the change of the GF during lateral movement. The Halbach permanent magnetic guideway (PMG) can offer much more GF but the change is larger just as the levitation force decays. The GF of cylindrical bulk HTS increases more than of the rectangular bulk HTS in the pre-load case. The characteristics of the GF during the lateral movement are explained. These results are important for further HTS Maglev vehicle system designs.

Effect of the repulsive force in the HTSC–permanent magnet hybrid bearing system

Magnetic levitation using the pinning force of the YBaCuO high- T c bulk superconductor (HTSC) materials has an advantage to achieve stable levitation without control. To increase levitation force, the HTSC–permanent magnet hybrid magnetic bearing system is introduced. A circular shaped three phase Nd–Fe–B permanent magnet is installed on the rotor, and HTSC bulk superconductor is set on the stator. The additional permanent magnet is installed under the HTSC. Repulsive force of the permanent magnet is used for levitation, and pinning force between the HTSC and permanent magnet is used for guidance force of the bearing. In this system, relationship between permanent magnet and the HTSC is important. When repulsive force of the permanent magnet is large, pinning force of superconductor is used to keep the rotor position. As a result, stability for the lateral direction is decreased with hybrid system. For levitation force, effect of the hybrid system is not observed with column HTSC. Compared with the ring HTSC results, the following thing is considered. Because there is no space that flux of one permanent magnet acts on the other one with the column HTSC configuration, interaction between two permanent magnets becomes small.

Measurement of the magnetic field inside the holes of a drilled bulkhigh-Tc superconductor

We use macroscopic holes drilled in a bulk YBCO superconductor to probe its magnetic propertiesin the volume of the sample. The sample is subjected to an AC magnetic flux with a density rangingfrom 30 to 130 mT and the flux in the superconductor is probed by miniature coils inserted in theholes. In a given hole, three different penetration regimes can be observed: (i) theshielded regime, where no magnetic flux threads the hole; (ii) the gradual penetrationregime, where the waveform of the magnetic field has a clipped sine shape whosefundamental component scales with the applied field; and (iii) the flux concentrationregime, where the waveform of the magnetic field is nearly a sine wave, withan amplitude exceeding that of the applied field by up to a factor of two. Thedistribution of the penetration regimes in the holes is compared with that ofthe magnetic flux density at the top and bottom surfaces of the sample, and isinterpreted with the help of optical polarized light micrographs of these surfaces.We show that the measurement of the magnetic field inside the holes can beused as a local characterization of the bulk magnetic properties of the sample.

Magnetic torque transferring study for bulk High-Tc superconductors and permanent magnets

The torque transferring mechanism taking place in a superconducting mixer design has been studied. Several coupling magnetic arrangements were investigated for more details in the engineering design. A bulk superconductor sample was used to study the torque forces for various cooling gaps, and the twist angle dependence was also monitored for the rotational stiffness in stability. The experimental data with four permanent magnet configurations have been studied in the present work. The maximum torque forces are summarized for usage of engineering design with various gaps. The torque/gap characteristics for four configurations were also measured for the optimisation of the torque at a designed operating gap.

Levitation performance of YBCO bulk in different applied magnetic fields

The maglev performance of bulk high-Tc superconductor (HTS) is investigated above three different types of permanent magnet guideways (PMGs). The main difference among these PMGs is the method used to concentrate the magnetic flux. The experimental results indicate that the levitation force depends only in part on the peak value of the magnetic field. The variation of the vertical component of the magnetic field (Bz), and the structure of the magnetic field are also responsible for the levitation force. These results imply that the permanent magnet with high coercive force is better at concentrating flux than iron. The conclusions contribute in a very helpful way to the design and optimization of PMGs for HTS maglev systems.

Method to reduce levitation force decay of the bulk HTSC above the NdFeB guideway dueto lateral movement

A magnetic levitation vehicle using bulkhigh-Tc superconductors (HTSC) is considered as a promising transportation type thanks to itslateral inherent stability, but previous studies have found that the levitation force (LF)decays due to lateral movement. In this paper, a pre-load method is presented to reduce theLF decay, and the experimental results indicate that this method is very applicable insupressing this decay in spite of the applied field and material property of the bulkHTSC, and this effect can be ascribed to the reduction of the hysteresis loss inthe bulk HTSC, i.e. more trapped magnetic flux after the pre-load case. In theend, experimental results indicate that the Halbach PMG has an advantage toreduce the cost of the PMG, but its rate of LF decay is also larger due to lateralmovement

Bulk high-Tc superconductors with drilled holes: how to arrange the holes to maximize the trappedmagnetic flux?

Drilling holes in a bulk high-Tc superconductor enhances the oxygen annealing and the heat exchange with the coolingliquid. However, drilling holes also reduces the amount of magnetic flux that can betrapped in the sample. In this paper, we use the Bean model to study the magnetizationand the current line distribution in drilled samples, as a function of the hole positions.A single hole perturbs the critical current flow over an extended region that isbounded by a discontinuity line, where the direction of the current density changesabruptly. We demonstrate that the trapped magnetic flux is maximized if thecentre of each hole is positioned on one of the discontinuity lines produced by theneighbouring holes. For a cylindrical sample, we construct a polar triangularhole pattern that exploits this principle; in such a lattice, the trapped field is∼20% higher than in a squared lattice, for which the holes do not lie on discontinuity lines. Thisresult indicates that one can simultaneously enhance the oxygen annealing, the heattransfer and maximize the trapped field.

A novel propulsion method for high-Tc superconducting maglev vehicle

High-Tc superconducting (HTS) maglev is considered as a perfect transportation type because of its unique inherent stability. A direct current (DC) linear motor using the permanent magnet guideway (PMG) as the stator and the on-board coil as the rotor instead of the present inductive or synchronous alternate current (AC) linear motor which has an economic disadvantage due to the necessity to lay primary coil along the guideway is proposed in this paper. In order to modulate the magnetic field under the PMG, an inverse E shape ferromagnetic device (IESFD) core is designed. The possible winding method for the on-board coil is listed, and the analytical result shows that a considerable net ampere force and thus the propulsion force can be generated by this special structure. The influence of the concentrated effect of the IESFD on the maglev performance of HTS bulk is studied by a numerical program, and the results show that the levitation force with the IESFD is 90% of that without. It is also indicated that the load capability and lateral performance of the maglev vehicle combined this propulsion method can be improved thanks to the attractive effect between the IESFD and PMG. The cost of the HTS maglev vehicle will be remarkably reduced and then shorten the distance to practical application with this propulsion method.

Maximum trapped field of a ring bulk superconductor by low pulsed field magnetization

Pulsed field magnetization is an important technique for a bulk superconducting magnet, which is one practical application of a bulk high Tc superconductor (HTS). Full magnetization of a ring HTS is effective to obtain large trapped field for low pulsed field magnetization. In this study, trapped field in a ring bulk superconductor by the low pulsed field magnetization is numerically analyzed under assumption of variable shielding current by the temperature control. Differences between one-dimensional Bean model and axisymmetric three-dimensional numerical solution are discussed through the analysis. There is maximum trapped field in the axisymmetric three-dimensional model because of finite thickness of the ring HTS. The shielding current density and inner radius of the ring HTS are discussed to obtain the maximum trapped field.

Stability of the Maglev Vehicle Model Using Bulk High Tc Superconductors at Low Speed

Based on the study of static properties of the high temperature superconducting (HTS) Maglev vehicle, the dynamic characteristics and stability are investigated in the paper. A Maglev vehicle model using 86 bulk high Tc superconductors (HTSCs) is made at 1/4 scale of the first man-loading HTS Maglev test vehicle. Dynamic characteristics of the low-speed HTS vehicle in three directions are studied by measuring vibration signals of six essential point of the vehicle model. The natural frequency is analysed. 30 mm height is suggested as a reasonable field cooling height (FCH). Lower FCH brings the operation stability of the HTS Maglev system over permanent magnetic guideway (PMG) at low speed. Dynamic stability dependence on the speed of the HTS vehicle is quite different from that of the conventional Maglev vehicle system. The effect of speed on the unsafe motions can be suppressed by decreasing FCH in the operation of the vehicle model over the PMG to a great extent.

Evaluation of Magnetic Field Hysteresis and Flux Creep of a QMG Coil Magnet

This paper presents an evaluation of the magnetic hysteresis of a coil type QMG magnet for a high field application of oxide bulk high Tc superconductors (HTS). The QMG is a high Jc bulk material, which consists of single crystalline 123 with finely dispersed 211. The bulk coil is made from a slice of Ag doped Gd based QMG by using precise machining techniques. In the present study, magnetic field profile on the QMG coil in experiment is discussed from shielding current distribution. Magnetic field hysteresis in experiments is numerically analyzed with an axisymmetric model, where the superconductive characteristic is evaluated with a critical state model. Flux creep phenomenon in rapid magnetization with rectangular applied current is also evaluated by the numerical simulation.

Fabrication and properties of QMG coil magnets

QMG is a high Jc bulk material, which consists of single crystalline 123 with finely dispersed 211. A QMG coil magnet is proposed as a high field application of oxide bulk high Tc superconductors (HTS). Since the QMG is produced in the form of a single crystal bulk, it has been considered difficult to make a coil shaped magnet by drawing it into wire and winding it into a coil like the Nb–Ti superconducting wire now commercially available. However, the latest machining techniques have made it possible to slice a QMG into a wafer whose thickness is 1mm or less and form it into a spiral coil. We have been developing the magnets by stacking the QMG coils fabricated through such machining work and soldering them at the inner and outer ends. It can be excited by applying electric current. Presently, we have succeeded to fabricate the split-type QMG magnet that can generate a magnetic flux density of 2.3T at 54K. The fabrication and properties of the QMG coil magnet are reported in this paper.

Substrate Contribution to the Surface Impedance of HTS Films on Si

We study the effects of semiconductor substrates on the surface impedance of high-T c Superconductor (HTS) YBa2Cu3O7−δ (YBCO) films. The characteristic impedance of silicon (Si) (for different doping levels and for different charge carrier scattering times) is evaluated. In particular, the most relevant features of Si electrodynamics are highlighted by the introduction of suitable normalized quantities. The effective surface impedance of the YBCO films on Si substrates is then calculated and discussed for different temperatures and frequencies in the microwave range, comparing the obtained results to their limiting expressions for bulk and thin-film HTS. Our analysis shows how the widely used thin-film approximation for the surface impedance can fail, critically highlighting the conditions it requires to be correctly used. We show that substrate contributions can heavily influence the overall response.

Advanced tensile testing methods for bulk superconductors at cryogenic temperatures

Tensile tests of bulk high Tc superconductors at room temperature have been generally performed by gluing the bulk specimens to Al-alloy rods. Because of the difference in the coefficient of thermal expansion, thermal stresses were induced at cryogenic temperatures especially near the interface between the specimen and the rods. In this study, tensile testing methods with minimized effect of the thermal stress were tried by using specimens cut from Dy–Ba–Cu–O superconductors. These were: (1) The rod material of Al-alloy was replaced with Ti-alloy, which has the coefficient close to the bulk. (2) The interlayer made of the identical bulk superconductor was inserted between the specimen and the Ti-alloy rod. The nominal tensile strength at the liquid nitrogen temperature (LNT) of the specimen glued to the Ti-alloy rods was significantly higher than that glued to the Al-alloy rods. The application of the interlayers increased the strength significantly. The FEM analysis showed that the thermal tensile stress component in the direction of loading axis within the specimen at LNT is markedly reduced by the method (1) and substantially eliminated in the method (2).

Running Performance of a Pinning-Type Superconducting Magnetic Levitation Guide

A pinning-type superconducting magnetic levitation guide with bulk high-Tc superconductors was studied for use as a goods transportation system, an energy storage system, etc. A superconducting magnetic levitation running test apparatus with a circular track of ca. 38 m length, 12 m diameter, which comprises the magnetic rail constituted by Nd-B-Fe rare-earth permanent magnets and steel plates, was manufactured to examine loss and high-speed performance of the magnetic levitation guide. Running tests were conducted in air. These tests clarify that a vehicle supported by a superconducting magnetic levitation guide runs stably at speeds greater than 42 km/h above the circular track.