Bulk High-temperature Superconductors Research Articles

A 3-D Strong-Coupled Electromagnetic-Thermal Model for HTS Bulk and Its Uses to Study the Dynamic Characteristics of a Linear HTS Maglev Bearing

Thermal effect will greatly affect the engineering performance of high-temperature superconductors (HTSs) due to its strong dependence of electromagnetic parameters upon the local temperature. To advance the understanding of such thermal effects, a validated 3-D strong-coupled electromagnetic-thermal model for HTS bulk was established in commercial finite-element software COMSOL, which ensures the easy access and universality of the model. Jc(B,T) was employed to reflect both magnetic field and thermal field dependences of HTS in this model. In addition, the thermal transient equation and convective boundary condition were employed with experimentally measured HTS thermal conductivity and heat capacity to describe the thermal flux exchange between HTS and cryogenic medium. As an example of application, the established electromagnetic-thermal model was tailored to study the dynamic characteristics of a linear HTS magnetic levitation (maglev) bearing. The methodologies to numerically study the dynamic response of the linear HTS maglev bearing under free vibration state and typical operating excitations, e.g., earthquake, track irregularity, and crosswind, were put forward in this article. The influences of field cooling height, preload, and ambient temperature, were also studied, and promising methods to improve the system stability were put forward according to the obtained conclusions. The above-mentioned results are reasonable and keep in concert with former experimental and theoretical studies. Moreover, some results which are inaccessible in the 2-D models, for instance, the thermal field distribution inside HTS bulk, can also be obtained due to the versatility of the 3-D model. To conclude, the established HTS electromagnetic-thermal model could serve as a flexible and extensible simulation tool to study various applications of HTS bulk. Besides the application in linear HTS maglev bearing, which was systematically studied in this article, other potential applications, such as thermal analysis of HTS bulk in pulse magnetization process and HTS bulk-based electrical machines, can also be expected in future work.

Electromagnetic performance analysis of linear vernier machine with PM and HTS-Bulk

Today, superconductor materials are widely used in electric machines because of their unique properties. This has been always a question for designers and researchers whether super-conductors can be a good substitute for Permanent Magnet (PM) or not. In this paper, the performance of a dual stator linear vernier machine under identical conditions in the presence of High-Temperature Superconductor Bulk (HTS-Bulk) and PM are analyzed and compared. HTS-Bulk and PM are located between the slot of the stator to illustrate the advantages and disadvantages of the superconductor. The performance of both machines is based on the magnetic gear effect which due to the existence of numerous magnetic pole pairs is able to generate relatively high thrust force at very low speed. The main parameters of both machines such as: air gap flux density, PM flux, back-EMF, self-inductance, mutual-inductance, power factor, detent force and thrust force have been compared to each other at the same geometry by finite element method.

Levitation performance of an onboard high-temperature superconducting bulk unit with cryocooler direct cooling

High-temperature superconducting (HTS) maglev systems show the intrinsic advantage of self-stabilizing levitation without external control, therefore, they have great potential to be a new type of rail transit. At present, the cryogenic condition (77 K) of HTS bulks is maintained by pouring liquid nitrogen (LN2) into the dewar and supplementing LN2 at regular intervals, which cannot be maintained for a long period and in a controllable cryogenic environment. This cooling method is easy to realize but not the best approach for future commercial high-speed maglev. It is therefore necessary to try other cooling methods. In this paper, we try the direct cooling method. A cryocooler was used to directly cool a cryogenic unit with eight bulk superconductors which are designed to be close to the practical application. The levitation performance of the cryogenic unit with superconductors is tested on SCML-01. The temperature stability of the cryogenic unit was verified and measurements of levitation force versus temperature and levitation force versus field cooling heights were performed at temperatures from 50 to 90 K. The experimental results show that the HTS bulk unit can obtain a stable and controllable cryogenic environment. By analyzing the experimental data, it is found that decreasing temperature is beneficial to the levitation performance of HTS bulks and as the temperature decreases, the rate of the levitation force increase steps down. Moreover, according to the application environment of the HTS maglev cryogenic unit, 60 K is recommended as its operating temperature. The experimental results provide valuable references for the design of HTS maglev units in the future.

Effect of physical and geometrical parameters on vertical magnetic stiffness when a permanent magnet moves vertically above a high-temperature superconductor

Magnetic stiffness is as important as vertical and lateral forces to achieve stable levitation in high-temperature superconductor (HTS) levitation systems. To date, research on magnetic stiffness has mainly focused on a few aspects, but the understanding of its comprehensive characteristics has not been included. This study is focused on the quantitative properties of the physical and geometrical parameters that affect the vertical stiffness of HTS levitation systems using a numerical approach when a permanent magnet (PM) above the HTS bulk moves vertically under both zero-field cooling and field cooling (FC) conditions. The results show that the magnetic field of the PM, the critical current density (Jc), and the PM to HTS area ratio (α) significantly affect the vertical stiffness, while the PM to HTS thickness ratio has little effect. In addition, some unexpected phenomena were discovered. At some large vertical distances, the vertical stiffness first decreased and then increased as the external magnetic field increased under FC. The phenomenon of sign switching for the vertical stiffness appears to be related to several parameters, such as the external magnetic field, Jc, and α. At most vertical distances (except positions at which the saturation characteristic of vertical stiffness appears), the vertical stiffness increases with α. These findings are different from the results of the vertical force dependence with the same parameters.

Theoretical Study of the Effect of Magnetic Field Sweep Rate and Temperature on Shielding Efficiency in a Bulk Tube-Shaped HTS

In this paper, we have studied the shielding properties of a high temperature superconducting (HTS) bulk tube-shaped cylinder in varying magnetic fields. Within this scope, we have simulated the hollow cylinder geometry in axis-symmetric finite element method (FEM) in which the electromagnetic properties of a HTS are described by a nonlinear electric field-current density (E − J) power law including a critical current density depending on magnetic field. In the first stage, we have analyzed temperature dependence of shielding factor (SF), which is described as the ratio of the applied magnetic field to the magnetic field in the center of the hollow cylinder. In the second stage, the effect of magnetic field’s sweep rate (dBapp/dt) on the magnetic shielding was numerically examined. In the final stage, using the certain lines over the superconducting tube, we have worked out the shielding field (Bsz) of the HTS tube by subtracting the z-component of magnetic field (Bz) from applied magnetic field (Bapp). Our numerical investigations indicate that the superconducting tube can provide field and temperature-controlled magnetic shielding.

Levitation Force Improvement by Effective Magnetic Interaction in Two Magnetic Levitation Systems With HTS Coil and Bulks

In this article, we studied a magnetic levitation system which was composed of a high-temperature superconducting (HTS) coil, HTS bulks, and a ferromagnetic material (rail). We set two magnetic levitation systems in one ferromagnetic rail. We analyzed the levitation force depending on changing the distance between HTS coils of each direction of current flowing. As a result of comparing two poles that has current flowing through its coils in one direction and two poles that has current flowing through its coils in different directions, the model which had the coils in one direction was better. A reason for this result is discussed comparing with the magnetic flux density and the stress tensor in a ferromagnetic rail. In addition, the distance between coils was also important for the magnetic interaction. We show the tendency of levitation force obtained by changing the distance between coils in each direction of current flowing and explain the reason. In this article, it is found that a greater force can be obtained by using magnetic field interaction effectively. We optimized arrangement and magnetic pole combination and we succeeded to improve the levitation property.

Trapped field potential of commercial Y-Ba-Cu-O bulk superconductors designed for applications

Bulk high-temperature superconductors (HTSs) that act as a trap field magnet establish a novel type of magnetic field source, which is significantly different from that of a permanent magnet or solenoid; therefore, they can be potentially used in magnetic-force-based applications. However, the primary issues related to the commercial use of the bulk HTS technology are established on the enhancement of trapped magnetic fields, as well as their reliability and reproducibility at temperatures achievable with off-the-shelf cryocoolers. This study presents experimental investigations on the strong trapped magnetic fields observed in commercial Y-Ba-Cu-O bulk HTSs produced as a double-sample stack, a cylindrical bulk, and a ring-shaped bulk. Consequently, a reliable and reproducible magnetic field of 9.5 T at 50 K was trapped in commercial Y-Ba-Cu-O assembled as a double-sample stack. In this case, shrink-fit encapsulation with either aluminum or stainless steel tube ensured equally effective reinforcement. Higher magnetization, although accompanied with partial flux jumps, yielded a trapped field of 16.85 T at 30 K, which is comparable to the reported record trapped fields. Finally, a maximum trapped field of 9.78 T - the strongest trapped field reported to date - was attained in the 6 mm hollow space of the ring-shaped bulk HTS.

Flux pinning and microstructure of a bulk MgB2 doped with diverse additives

In bulk high-temperature superconductors, Ag is usually used as an additive thus improving mechanical performance. In MgB2 doped with Ag, the Ag reacts with Mg, forming Mg–Ag phases acting as a vortex pinning medium. In this work, we analyze the electromagnetic and pinning properties of bulk MgB2 doped with 1 wt% MgB4, 4 wt% Ag, and 1 wt% Dy2O3, prepared at the Shibaura Institute of Technology (SIT), Tokyo. In three compounds of MgxB2 + 4 wt% Ag with x = 1, 1.075, and 1.1, the effect of Mg excess was studied. The magnetic moment was measured by a vibrating sample magnetometer (VSM). Pinning was studied in terms of a pinning diagram, i.e. the field dependence of the normalized pinning force density, F n = F/F max. In all studied samples, the peak of the F n (b) dependence (b= B/B irr, B irr being the irreversibility field) was observed at around b= 0.2, indicating a prevailing flux pinning at grain boundaries. A slight shift of the peak with decreasing temperature indicated a defect size distribution in the pinning landscape. Transmission electron microscopy (TEM) showed a granular structure of all samples composed of MgB2 grains of about 230 nm (average size), with ensembles of small grains (22 nm in average) of Ag3Mg, AgMg, Ag, Dy2O3, and MgB4. While the large MgB2 grains control the main pinning mechanism, the small precipitates seem to determine details of the current flow through the grain boundaries.

Electromagnetic-thermal behaviors of bulk superconductor with cuboid slot under moving magnetic field

We established a 3D coupled model to study the electromagnetic-thermal behaviors of a levitation system containing a bulk high-temperature superconductor (HTS) with a cuboid slot under the nonuniform magnetic field due to the movement of a permanent magnet (PM), analysing the influences of the geometrical parameters of the slot and inclination angle on the electromagnetic-thermal behaviors, giving the dynamic distribution of the global electromagnetic force density in the HTS. The results show that the longer the length of the slot, the smaller the levitation force, and the greater the loss. As its length takes up 7/10 of the HTS diameter, the loss is 4 times that of the HTS without damage. Due to the homodromous induced current appearing near the slot, the maximum guidance force increases with the length. The levitation force decreases with the increase of the slot depth. Its curve near the maximum becomes blunt and the loss increases significantly. As the depth is 3/5-4/5 of the HTS height, the loss is 300–400 times of that without damage. The guidance force and loss increase with the depth. As the inclination angle rises, the levitation and guidance force both drop. At 30 °, their maximums decrease by 70 % and 90 %, respectively. The lateral stability of the system thus becomes poorer, indicating the inclination angle is one of the important factors for instability. As the PM moves vertically, the guidance force density symmetrically distributes near the both wide sides of the slot. Its side faces with the wide sides will bear the shear force, resulting in that the slot has the tendency to tear along these faces. As the PM reciprocates laterally, the slot is subjected to the lateral shear force, proportional to the displacement.

Novel magnetizing technique using high temperature superconducting bulk magnets for permanent magnets in interior permanent magnet rotors

A novel magnetizing technique for permanent magnets (PMs) embedded in interior permanent magnet (IPM) motors has been proposed as a safer and more effective process than the conventional processes using copper pulse-coils. In the new process, the high temperature superconducting (HTS) bulk magnet, emitting an intense magnetic field of over 3 T, was employed to activate the rotor and PMs inside. A magnetic pole containing the HTS bulk magnet was scanned on the surface of the IPM rotor, resisting against the extremely strong attractive force between the magnet and rotor. Following experiments in the past, numerical simulation studies have been conducted to evaluate the magnetic field-trapping performances of the PMs. In this study, the numerical analysis of the magnetization behaviours was conducted for the PMs embedded in the rotor of the commercial air-conditioner motor in a hybrid car. The simulation results showed us that the demagnetized Nd-Fe-B plates in the rotor were fully magnetized in the intense static magnetic fields on the magnetic pole containing the HTS bulk magnet. The magnetizing behaviour in this peculiar magnetizing operation was made understandable by estimating the anisotropic easy-direction of the PM plates embedded in the rotor. The novel activation method for PM is characterized as a no-shock and no-heating method, which would promote the degree of freedom in the IPM motor design.

Vertical and lateral drift behavior of a linear superconducting magnetic bearing system under multi-operating modes

Since the discovery of high temperature superconducting (HTS) material, HTS maglev with the characteristics of self-stabilization and zero magnetic drag, has shown excellent potential for future transportation applications. However, this stability characteristic of HTS maglev is difficult to fully ensure in a spatial position, because of the magnetic flux flow and creep of the HTS bulk. Hence, to study the drift phenomenon of the HTS maglev vehicle excited by this unguaranteed stability characteristic, the linear superconducting magnetic bearing system, composed of a ring permanent magnetic guideway line (double track) and a HTS maglev vehicle, the ‘Super-Maglev’, was utilized. This system aims to explore the drift phenomenon in the vertical and lateral directions of the whole maglev vehicle under various operating conditions: different running speeds, unbalanced loading and cyclic loading situations. As an illustration induced by the experimental results, the lateral drift (LD) at a faster running speed (15 km h−1) is twice than that at 10 km h−1. Also, compared to the constant load condition, the vertical drift (VD) under cyclic load is larger. Moreover, with time going by, both LD and VD tend to be stable. Specifically, for the LD curve, it can be fitted by an exponential function. Finally, all the above experimental results support references for the future design and study of linear SMB systems.

Comparison of fast computing methods for ZFC High-temperature superconductors bulks

With the development of high-temperature superconducting (HTS) materials, several HTS applications are been proposed and developed. Due to the HTS non-linear and non-homogeneous electromagnetic characteristics, the optimization process of an HTS system is a complex task requiring high computational time and resources. Within this context, this paper focuses on the comparison of HTS models for a fast estimation of the magnetization level of zero-field cooled HTS bulks. Four HTS models are analyzed: Kim’s model, Bean’s model, a simplified analytical model considering J = Jc and an equivalent permeability model. As a case study, these models are used to simulate the levitation forces of a horizontal axis HTS levitation system. A 3D Finite Element model is used as a base tool for the implementation of each model. Results are compared with the experimental ones. The simplified analytical model (J = Jc) and the equivalent permeability model present accurate enough results with a very low computational time when compared with Kim’s and Bean’s models. For the 3D simulation of the levitation system, a computational time between 10 and 24 h is required for the Kim’s and Bean’s models, while only between 10 and 30 s for the simplified analytical and equivalent permeability models. Besides, the simplified analytical model (J = Jc) also provides information about the eddy-current losses inside the HTS bulks.

Static Levitation Force and Its Thickness Dependence in HTS Tape Stack System

High-temperature superconductors (HTS) have great potential for magnetic levitation applications. Nowadays, coated conductor (CC) tapes are playing an increasingly important role in this field. Compared with HTS bulk materials, CC tapes have a higher critical current density and can be mass-produced, easy to shape, and have other important advantages for magnetic levitation applications. In order to gain an in-depth understanding of the relevant levitation characteristics of CC tapes, we studied the static magnetic levitation force between the permanent magnet and HTS tape stack, with the number of the CC tapes in the stack being 10 to 160. The experimental results show that the magnetic levitation force increases steadily as the thickness of the CC tape stack increases, but gradually reaches saturation.

Levitation Force Characteristics of High-Temperature Superconducting Bulks in a High Magnetic Field

The levitation force of high temperature superconducting (HTS) bulks can be enhanced by raising the strength of the external magnetic field. As to the field of HTS magnetic levitation (maglev), many researchers have also obtained this conclusion by levitation experiments above a permanent magnet guideway (PMG). However, the experimental results have limitations because the magnetic field above the PMG is relatively low, around 0.4 T at the 15 mm working height. In order to obtain more comprehensive levitation force characteristics of HTS bulks, it is necessary to carry out more experiments in high magnetic field. In this paper, the effects of magnetization angle and external field gradient on the levitation force of an HTS YBCO bulk which was manufactured with a top-seeded multi-seeded melt-textured process were studied. We set up a levitation force measurement system based on a 5-T Cryogen-Free Magnet (CRY-CFM-5 T-300-H3, Cryogenic Limited). The HTS bulk was cooled by liquid nitrogen in zero-field-cooling condition, and the external magnetic field was enhanced from 0 to 3 T. The HTS bulk was rotated 30° each time and finally rotated to 90°. Experiments show that the levitation force decreases from θ = 0° to 90°, where θ is the angle between the upper surface of the HTS bulk and the horizontal plane. The conclusion that the levitation force increases with the increase of the external magnetic field gradient in high magnetic field was also further confirmed.

Improvement of trapped magnetic field in GdBCO by waveform control pulse magnetization at 70 K

We attempted to trap a strong magnetic field with a single pulsed magnetic field by applying waveform control pulse magnetization (WCPM) method to a high-temperature superconductor (HTS) bulk sandwiched between two vortex-type copper coils at less than 77 K. The GM cryocooler conductively cooled the GdBCO bulk to 70 K in an arrangement similar to the internal structure of our axial gap type HTS motor. The HTS bulk showed no flux jump, while the duty ratio of WCPM was small. When we increased the duty ratio, we found a flux jump without looking at any prior signs and the trapped magnetic flux density increased twice. Hence we attempted a negative feedback WCPM with the control target at the highest trapped magnetic flux density while no flux jump was found. After trial and error of control conditions, the HTS bulk sample trapped a maximum magnetic flux density of 1.79 T by a single pulse magnetic field. If the WCPM technique is used, a single pulse magnetic field will cause the HTS bulk to obtain a higher trapped field at lower temperatures.

Comparison of properties of a bulk HTS and a stack of HTS tapes after FC and ZFC

This paper presents the results of the analysis of properties of a high-temperature superconducting (HTS) material in two forms - HTS bulks and a stack made of a number of HTS tape layers. Stack of HTS tape is a new type of application of superconductors, which can be used instead of a convention bulk material. In this work, we study the levitation properties of HTS stack and compare it with the same characteristics of HTS bulks to find the effective way of modelling such type of HTS. Experimental measurements of the force interaction between a permanent magnet and HTS material, as well as a "trapped" magnetic field, are performed for both types of a superconductor. The HTS samples are analysed in the case of Field Cooling (FC) and Zero Field Cooling (ZFC). The aim of the research is to analyse experimental results to compose and justify mathematical models for calculation magnetic system with such types of superconductor.

Magnetizing Technique for Permanent Magnets in IPM Motor Rotors Using HTS Bulk Magnet

A unique activation technique for permanent Nd-Fe-B magnets which were embedded in the rotors of interior permanent magnet (IPM) motors has been developed as a magnetizing tool with use of high temperature superconducting (HTS) bulk magnet. The experimental and numerical simulation studies were conducted to evaluate the magnetic field-trapping performances of the rotor of air-conditioner compressor in the hybrid-type automobiles. The sample rotor containing six demagnetized permanent magnet plates were exposed in the intense static magnetic fields above the magnetic pole which contained the bulk magnet generating over 4 T. As a result, the sample magnets were perfectly magnetized in the static magnetic field. The precise simulation on the flux distribution in the rotor clarified that it is important to make the flux direction in the rotor core and the easy magnetization axis of PM identical. We convinced this activation technique should enable us to promote the degrees of freedom of motor designing and its processing.

Comparison of the Effect of Graphene and Graphene Oxide Doping on YBCO

Weak-links in high temperature superconductors (HTS) are a result of an incorrect stoichiometric ratio found in grain boundaries and misaligned planes. These weak-links severely decrease the critical transport current of the bulk HTS material (such as YBCO) and increase resistance in the normal state. Bulk YBCO was doped with graphene oxide (GO) and reduced graphene oxide (rGO) in the following dopant concentrations: 0, 0.1, 0.5, 0.7, 0.9 and 1 % wt. Through mechanical hardness testing, it was observed that the hardness of the samples increased as the GO and rGO concentration increased. The 1 % wt. doped GO and rGO samples was observed to be hardest, with a true hardness of 2.630 GPa and 2.405 GPa respectively, compared to the control sample with a true hardness of 1.499 GPa. Critical transition temperature (TC ) of the GO doped YBCO samples are greater than rGO doped samples and is presumably related to the higher oxygen content found in GO. The 0.7 % wt. doping for GO and rGO samples had the highest critical transition temperature of 98 K and 91 K, respectively. Both GO and rGO samples showed a decrease in normal resistance up to 0.7 % wt. doping, compared to the control sample. The weak-link effect was most improved in the 0.7 % wt. doped samples for GO and rGO.

Strategies to improve the dynamic levitation performance of superconducting maglevs against force decay and disturbance

In the design of maglev systems, the levitation force determines the levitation height and the dynamic stability associated with potential vibrations, especially the offset of the levitation point relative to the working point. However, such two key parameters are often antagonistic: a relatively low dynamic stability comes with a high levitation force, whereas a relatively low levitation force can come with a high dynamic stability. In this paper, we will discuss several strategies to deal with this problem by means of a two-dimensional numerical model based on Newton's second law and Maxwell's equations together with a power-law constitutive relation. The dynamics of maglev systems consisting of a bulk high-temperature superconductor and a Halbach-type permanent-magnet guideway with soft ferromagnets are analyzed. The results show that the drift phenomenon occurs in both vertical and lateral directions triggered by a transverse disturbance, and preloading can alleviate such a phenomenon, but this will lead to a reduction in the levitation force. Improved preloading is effective in enhancing the levitation force without sacrificing the dynamic stability. In some systems, the levitation force and dynamic stability can be further improved by adjusting the soft ferromagnets to an appropriate location in the guideway. Moreover, some guidelines on how the superconducting part should be designed are provided in order to overcome the technical difficulty and reduce the material consumption while at the same time maintaining the dynamic levitation performance.

A simple, reliable and robust reinforcement method for the fabrication of (RE)–Ba–Cu–O bulk superconductors

Bulk high temperature superconductors (HTS) based on the rare-earth barium cuprates [(RE)BCO] have the potential to be applied in a variety of engineering and technological applications such as trapped field magnets, rotating electrical machines, magnetic bearings and flywheel energy storage systems. The key materials figure of merit for most practical applications of bulk superconductors is simply the product of the maximum current density that can be supported, which correlates directly with the maximum achievable trapped magnetic field, and the physical length scale over which the current flows. Unfortunately, however, bulk (RE)BCO superconductors exhibit relatively poor mechanical properties due to their inherent ceramic nature. Consequently, the performance of these materials as trapped field magnets is limited significantly by their tensile strength, rather than critical current and size, given that the relatively large Lorentz forces produced in the generation of large magnetic fields can lead to catastrophic mechanical failure. In the present work, we describe a simple, but effective and reliable reinforcement methodology to enhance the mechanical properties of (RE)BCO bulk superconductors by incorporating hybrid SiC fibres consisting of a tungsten core with SiC cladding within the bulk microstructure. An improvement in tensile strength by up to 40% has been achieved via this process and, significantly, without compromising the superconducting performance of the bulk material.