Bulk High-temperature Superconductors Research Articles

Theoretical characterization and its application of electromagnetic anisotropy in high-temperature superconducting bulks under rotated postures

The electromagnetic anisotropy of high-temperature superconducting (HTS) bulks limits their levitation ability in the applied magnetic fields from the permanent magnet guideway (PMG), thus impeding the enhancement of load-carrying capacity in HTS pinning maglev systems. Developing a suitable matching scheme between bulk orientation and magnetic field direction is a valuable way to relieve this restriction. In this paper, a method for characterizing bulk anisotropy in a rotating coordinate system is proposed to explore the best bulk orientation. The method is based on the concept of equivalent resistivity tensor and its eigenvectors, and includes an extended description of two types of anisotropy: conductivity anisotropy and magnetic field angle dependence. It provides a theoretical foundation for simulating anisotropic bulks under any rotated posture. Experimental investigations on the levitation force distribution of cylindrical bulks with different c-axis orientation were conducted, through which the accuracy of the characterization method and calculated results were validated. Analysis of current distribution reveals that aligning the c-axis parallel to the external magnetic field helps achieve the best match between the bulk and the PMG. Additionally, considering that the two types of anisotropy have opposite effects on levitation force distribution trends, prioritizing conductivity anisotropy when analyzing anisotropic bulk is recommended. This research not only offers a theoretical framework for simulating the anisotropy of rotated HTS bulks but also provides guidance for matching the optimal bulk orientation in applied magnetic fields.

Optimizing superconducting magnetic bearings of HTS flywheel systems based on 3D H-[formula omitted] formulation

The superconducting flywheel system exploiting the magnetic coupling between the bulk high temperature superconductors (HTSs) and permanent magnets (PMs) exhibits excellent performance of self-stable levitation, and is promising for power applications. In this paper, we use the H-ϕ formulation combined with moving mesh to establish a full 3D model for the thrust type and journal type bearings in the HTS flywheel system. We then proposed different Halbach schemes to enhance the magnetic flux density of the rotor and thus the coupling, and investigated the levitation force, relaxation characteristics, electromagnetic transient distribution, and temperature characteristics of the bearings. Results show that, under axial zero field-cooled (ZFC) condition, the optimized PM rotor scheme can significantly improve the maximum levitation force and stiffness by 4.0 and 2.3 times respectively for the thrust type bearing, and the maximum levitation force of journal type bearing can be improved by a factor of 5.5. Under the radial ZFC condition, the maximum levitation force and stiffness of the journal type bearing have been increased by 4.9 times and 2.9 times. For the relaxation of both bearings during operation, the optimized PM rotors lead to relatively greater attenuation of levitation force. The proximity of the optimized PM rotors intensifies the magnetic flux movement of the HTS bulks but only brings about a limited temperature rise, and the superconductors still maintain a good low-temperature working environment. This study provides an effective methodology for analyzing the HTS bearing systems and good references for the optimal design of compact HTS flywheel energy storage systems (FESSs).

An FEM framework for modeling forces between NdFeB magnets and HTS bulks

Purpose This paper aims to introduce an alternative for modeling levitation forces between NdFeB magnets and bulks of high-temperature superconductors (HTS). The presented approach should be evaluated through two different formulations and compared with experimental results. Design/methodology/approach The T-A and H-ϕ formulations are among the most efficient approaches for modeling superconducting materials. COMSOL Multiphysics was used to apply them to magnetic levitation models and predict the forces involved.The permanent magnet movement is modeled by combining moving meshes and magnetic field identity pairs in both 2D and 3D studies. Findings It is shown that it is possible to use the homogenization technique for the T-A formulation in 3D models combined with mixed formulation boundaries and moving meshes to simulate the whole device’s geometry. Research limitations/implications The case studies are limited to the formulations’ implementation and a brief assessment regarding degrees of freedom. The intent is to make the simulation straightforward rather than establish a benchmark. Originality/value The H-ϕ formulation considers the HTS bulk domain as isotropic, whereas the T-A formulation homogenization approach treats it as anisotropic. The originality of the paper lies in contrasting these different modeling approaches while incorporating the external magnetic field movement by means of the Lagrangian–Eulerian method.

Temperature dependence of power transfer efficiency and power handling capability of wireless power transfer system using superconducting bulk coil

We developed a cryocooler system to reveal the temperature dependence of the quality (Q) factor of high-temperature superconducting (HTS) bulk coil and the power transfer efficiency (PTE) and power handling capability (PHC) of a wireless power transfer (WPT) system using the HTS bulk coil. The measured Q factors of the HTS bulk coil were 9051 at 77 K and 14 050 at 25 K. The measured PTEs and PHCs of the WPT system were 44.2% and 4.48 W at 77 K, and 50.8% and 25.2 W at 25 K.

R-z plane spatial critical current inhomogeneity-induced mechanical response of GdBCO superconducting bulks during field cooling magnetization

Benefiting from the high critical current density (J c), single-grain (RE)BCO (where RE = rare earth or Gd) bulks are capable of trapping over 17.6 T magnetic field which is crucial for the application of bulk superconductors. Nevertheless, during field cooling magnetization (FCM), the large mechanical stress induced by Lorentz forces may lead to fracture behavior in the brittle ceramic nature of (RE)BCO materials. Most previous numerical models that adopted simplified homogeneous J c had difficulty reflecting the real stress/strain situation in high temperature superconductor (HTS) bulks. Based on the proposed modified Jirsa model considering r-z plane J c inhomogeneity, we investigate the mechanical response of GdBCO bulks manufactured by top-seeded melt growth (TSMG) process. A 2D axisymmetric electromagnetic-thermal-mechanical coupled model is implemented to take into account the dependence of J c upon mechanical deformation. The simulation results show the electromagnetic-thermal-mechanical response of the r-z plane inhomogeneous J c model is lower than that obtained by the homogeneous J c model. This confirms Takahashi’s speculation (K Takahashi 2019 Supercond. Sci. Technol. 32 015007) about the mismatch between experimental data and the simulation results of homogeneous J c model, and suggests the stress levels in the bottom plane of HTS bulk are overestimated by the previous homogeneous J c model. On top of that, the overall stress level of GdBCO bulk is strongly determined by the magnitude and position of the Lorentz force load, and the stress distribution of inhomogeneous J c model is mainly concentrated in high J c regions near top surface, instead of being symmetrically distributed along the z-axis as in homogeneous J c model. The mechanical response of stainless steel reinforced GdBCO bulk was aslo simulated and analyzed. Finally, the coupling effect between the fracture strength variability caused by defects and cracks and the trapped field in GdBCO bulks with r-z plane J c inhomogeneity is further studied. This study may provide a relatively realistic mechanical response of HTS bulk during FCM, and a novel design consideration for its mechanical reinforcement.

Electromechanical behavior of a rectangular bulk superconductor with an inhomogeneous critical current density under pulsed-field magnetization

Bulk high-temperature superconductors are widely used in various superconducting devices for their high critical current density and the ability to trap large magnetic field. In some applications, bulk superconductors can be arranged in an array structure to increase the magnetic field strength. However, rectangular bulk superconductors can be arranged more compactly than commonly used cylindrical bulk superconductors. The unique shape of rectangular bulk superconductors results in different distributions of electromagnetic fields, temperature, and stress under a pulsed-field magnetization (PFM) than for cylindrical bulk superconductors. In bulk superconductors, growth sector boundaries and growth sector regions have different critical current density, resulting in an uneven critical current density. Therefore, in this study, the electromagnetic and mechanical behavior of non-uniform rectangular bulk superconductors during the PFM process is investigated. The corresponding distribution and variation in the magnetic field, current, temperature, and stress in the bulk are calculated and analyzed. The influence of rectangle aspect ratio on the electromagnetic and mechanical behavior is discussed. The calculation results show that the magnetic flux jumps occur accompanied by sudden changes in the temperature and pressure. The effect of the aspect ratio of rectangular bulk superconductors on their electromagnetic and mechanical properties is analyzed. The influence of preexisting cracks in a non-uniform rectangular bulk superconductor on the simulation results is discussed. The numerical results indicate that the presence of an edge crack exacerbates the magnetic flux jump as well as the jumps in the temperature and stress. However, a central crack has a relatively small effect on the stability of a bulk superconductor.

Measurement of levitation force of high-temperature superconducting maglev under high-speed operation condition

High-temperature superconducting (HTS) pinning magnetic levitation (maglev) has garnered significant attention in high-speed maglev transportation due to its inherent self-stability, low energy consumption, and absence of mechanical friction. Ensuring the safe and stable operation of HTS pinning maglev systems necessitates a dedicated focus on the performance and stability of HTS bulks levitated above the permanent magnetic guideway (PMG). Previous research has indicated that variations in the temperature within the HTS bulk can impact the levitation performance of the system. This temperature-related phenomenon occurs when the external magnetic field applied to the HTS bulk changes. However, it is noteworthy that previous levitation force tests for HTS magnetic levitation systems have been limited to quasi-static or low-speed studies. The exploration of dynamic levitation forces, particularly at high speeds, has remained constrained due to the associated high costs. Therefore, the objective of this study is to investigate dynamic levitation forces while the HTS pinning maglev system is in motion at high speeds, utilizing a self-developed ultra-high-speed maglev test rig. Initially, the relationship between the levitation force and the vertical displacement of the HTS pinning maglev system is examined based on quasi-static experiments. Subsequently, comparative studies are conducted to measure levitation forces at varying speeds. Finally, the correlation between running speed and dynamic levitation force is discussed. The investigation reveals that the levitation force experiences only a marginal decrease as the running speed increases. At a running speed of 240 km/h, the attenuation rate of the levitation force is approximately 2.478 %, demonstrating the commendable stability of HTS pinning maglev systems. The article concludes by presenting the dynamic levitation characteristics and their attenuation trends to speed. These findings can serve as valuable references for future design and practical implementation of HTS pinning maglev systems.

Numerical simulation of bulk superconductor during pulsed field magnetization based on multi-field coupled analysis

High-temperature bulk superconductors are widely used in various superconducting devices due to their high critical current density, which enables them to trap high magnetic fields. However, the critical current density of bulk superconductors is influenced by magnetic field, temperature and stress, and both increase of stress and temperature can lead to a decrease in the critical current density. And the influence of magnetic field, current, stress are interactional. In this paper, the multi-field coupled analysis is employed to investigate the electromagnetic and mechanical behavior on bulk superconductor during pulsed field magnetization. The simulation results were compared with the present experimental results. The results showed a good agreement between the simulation and experimental results, further validating the reliability of the numerical simulation method. The magnetic field, current, temperature, and stress distribution and variation within the bulk sample during the pulsed magnetization process were calculated and analyzed. Additionally, the influence of the central crack and edge crack in the bulk superconductor was investigated. The results indicate that the region near the crack experiences a variation in current flow direction, resulting in local temperature rise and significant electromagnetic forces. The tip of the crack exhibits stress concentration, leading to a decrease in critical current density. Bulk supercondutor with edge crack case exhibits greater stress concentration compared to those with central crack case. The impact of crack length on maximum stress was analyzed, revealing that as the crack length increases, both the maximum hoop stress and radial stress within the bulk sample rise.

Flux jump and mechanical response in inhomogeneous high-temperature superconductor under the pulsed-field magnetization

The high-temperature bulk superconductors with high critical current density are brittle, and can be damaged by large Lorentz forces and thermal stress during magnetization. Several studies have reported the failure of bulk superconductors during flux jumps. In this study, we analyzed the magnetization characteristics and mechanical response of the HTS bulk with inhomogeneous current density along the c-axis. The numerical simulation was consistent with the experimental results presented in the reference. Moreover, a flux jump occurred near the area of the pre-arrangement flux during the second pulsed field magnetization. The maximum temperature is lower than the critical temperature during the flux jump. In the mechanical analysis, the flux jump led to an abrupt change in the maximum stress of the bulk, and the maximum radial stress was significantly higher than the maximum hoop stress during the flux jump. The maximum radial stress increased with decreasing ambient temperature during the flux jump, and the maximum stress area was always near the seeded plane. Subsequently, the magnetization characteristics and mechanical response were studied for different locations of the seeded surface, two concentric superconducting bulks, and non-uniform fields.

Influence of Ag Doping on Thermal Conductivity and Magnetic Levitation of Single Grain YBCO Superconductors for High-Temperature Superconducting Maglev

Ag doping in (RE)Ba2Cu3O7−δ [(RE)BCO] bulk high-temperature superconductors has been recognized as an effective way to improve the mechanical and superconducting properties of REBCO bulk superconductors for practical applications such as high-temperature superconducting (HTS) Maglev. Ag2O nanoparticles doped YBCO samples with different doping amounts (x, x varies from 0∼0.5 wt%) were fabricated by Top-Seeded Melt-Growth method (TSMG). The samples’ steady-state thermal conductivity (λ) and the magnetic levitation force (FL) were measured by a testing platform under a similar condition to the HTS Maglev system. The effects of Ag2O doping on the λ and FL of single domain YBCO bulks have been systematically studied. The λ of the c-axis and the ab-plane were calculated through the steady-state heat flow method. We observed that λ increases distinctly when x ≥0.3 wt%. The doping amount of 0.3 wt% has been evidenced to be the optimal value, of which the λ of the c-axis and ab-plane are approximately 52% and 48% higher than the undoped one respectively. According to the results of the FL measurement, we found FL will increase while the relaxation rate of the FL will decrease by adding trace amounts of Ag2O nanoparticles in comparison to the undoped sample. This observation suggests that Ag-doping is an effective way to enhance thermal conductivity as well as the superconducting properties of YBCO superconductors for practical applications.

Unified EOS incorporating the finite strain theory for explaining thermo elastic properties of high temperature superconductors, nanomaterials and bulk metallic glasses

In the present research paper, we have proposed a novel EOS, which is based on the concepts of finite strain theories for predicting the thermo elastic properties of various materials viz. bulk metallic glasses, nanomaterials, and high temperature superconductors. Development of the present EOS involved deriving an innovative mathematical framework that considers the effects of finite strain, enabling the prediction of essential properties like pressure and bulk modulus under different compression ratios. For validity and applicability of the proposed EOS, we have conducted an extensive analysis by comparing the calculated thermo elastic properties with existing theoretical models or equation of sates and experimental data available in the literature. The present comparison revealed a remarkable agreement between our predictions and the observed values for a wide range of compression of nanomaterials, bulk metallic glasses, and superconductors. Our research not only introduces a comprehensive equation of state for these advanced materials but also demonstrates its effectiveness in accurately capturing their unique thermodynamic behavior. This present derived EOS can serve as a valuable tool to predict and understand the response of nanomaterials, bulk metallic glasses, and superconductors to varying pressure conditions, aiding the design and development of innovative applications in fields such as materials science, nanotechnology, and condensed matter physics.

Investigation of saturation levitation force and optimization of permanent magnet guideway based on multi-seeded superconducting bulks model

After more than 20 years of research, the superconducting maglev has advanced to the engineering research stage. In engineering applications, multi-seeded superconducting bulks have been widely used in high-temperature superconducting (HTS) maglev systems, but there remains a deficiency in terms of the theoretical model description of multi-seeded superconducting bulks. In this paper, the fully coupled, partially coupled, and uncoupled models of the multi-seeded superconductor are presented to simulate HTS multi-seeded bulk. Then, the levitation performance and trapped field of three kinds of HTS bulks were experimentally tested, revealing a saturation levitation performance on the permanent magnet guideway (PMG). Therefore, the levitation force and trapped field of HTS bulks with different critical current density are further studied by 3D simulation. Taking the 90% and 95% of the maximum levitation force as the benchmarks, the corresponding critical current density of HTS bulks are respectively 4.6 × 108 A m−2 and 6.1 × 108 A m−2. Further, a mapping coupled boundary method was adopted to automatic optimize the PMG to increase the levitation force above the PMG. After optimization, the levitation force under a unit of the permanent magnet is increased by 19%, while the maximum levitation force above multi-period Halbach PMG is increased by 9.23%. This study provides a general multi-seeded superconducting bulk simulation model and levitation force optimization method, which can serve as a reference for selecting suitable superconducting bulk.

Electromagnetic and Thermal Properties of HTS Bulk Combined With Different Ferromagnetic Structures Under Pulsed Field Magnetization

Pulsed field magnetization (PFM) is considered as the most effective method for magnetizing high temperature superconducting (HTS) bulks. Applying ferromagnetic materials around HTS bulks during PFM can increase their trapped field. This work proposes four ferromagnetic structures (disk, bottom ring, side ring, and bowl types) combined with HTS bulk. The trapped field and temperature are calculated in the HTS bulk with different ferromagnetic structures. The results show that the bowl type has the most significant ability to capture the magnetic field, while the bottom ring type has the smallest one. Compared with the trapped field of single bulk, the trapped field of the bowl type is increased by about 24% under the same PFM condition, and the temperature rise is also significantly decreased. In addition, the effects of different thicknesses of the ferromagnet on the trapped field and maximum temperature of bulk are also investigated. As the thickness of the ferromagnetic structure enlarges, the maximum temperatures of the disk and bottom ring increase, while the side ring and the bowl decrease.

Simulation of Remote Forces Generated by High-Temperature Superconducting Bulks for Magnetic Drug Delivery

The strong background fields and field gradients required for the magnetic delivery of chemotherapeutic drugs in cancer treatments has proven to be challenging to achieve at the scale of the human body. Although several magnetic drug delivery (MDD) methods have been proposed to generate adequate forces in deep tissues, current technologies lack in either force strength, directional changes and/or duty cycle of the treatment. The current MDD system capable of generating the strongest forces, dipole field navigation, uses ferromagnetic (FM) cores in the strong uniform field of an MRI to steer the drug-loaded particles. Hence, considering the nearly ten-fold increase in magnetization of high-temperature superconducting (HTS) bulks in comparison to the strongest FM, the forces obtained in dipole field navigation could potentially be increased by replacing the FM cores with HTS bulks. In this work, we evaluate two different methods for generating magnetic forces using an HTS bulk in a magnetic resonance imaging (MRI) scanner by using finite element method simulations. First, the HTS pellet is zero field cooled and inserted in the uniform field of an MRI, such that the pellet is magnetized by the fringe field of the MRI. Secondly, a field cooled HTS pellet is rotated in the uniform field of an MRI in order to generate magnetic field gradients. The magnetic forces produced by both methods are compared with those obtained by dipole field navigation. In both methods evaluated, we found that the forces generated by the HTS pellet is either complementary to or stronger than those obtained with FM cores. Therefore, this work shows that HTS bulks could potentially be used to navigate magnetic particles in the deep tissues of the human vascular network more efficiently.

A Linear Motor Launcher System Integrating a Multisurface Superconducting Magnetic Bearing

In this study, a linear motor launcher system equipped with superconducting magnetic bearings has been constructed and tested under various conditions. The guide rail of the launcher features permanent magnets that support superconducting levitation and interaction between square prism-shaped bulk superconductors on various surfaces. This results in increased stiffness of a single superconductor along any given axis and comprehensive support of lateral forces. The study investigates the use of a superconducting magnetic bearing in a linear launcher system as a novel approach to a multi-surface levitation structure. The experimental system integrates the high-temperature superconductor (HTS) bulk material-based carriage system and an open cryostat structure. The carriage system levitates on the permanent magnet (PM) rail by moving it using an integrated DC linear motor. The study examines the levitation force relationship and performance of the system under different selected speeds and acceleration profiles. The design of the system allows for an adjustable slope of the permanent magnet rail, demonstrating the potential for use as a linear launcher pad. Additionally, the proposed system has the potential for application in larger systems, such as maglev vehicles.

A Flux-Switching Permanent Magnet Machine with HTS Bulks and Radially Magnetized PMs Arranged

In order to improve the torque density of flux switching permanent magnet machine (FSPMM), a novel multi-tooth fault-tolerant FSPMM with high temperature superconducting (HTS) bulks is proposed in this paper. The HTS bulks and radially magnetized PMs are arranged inside in the stator. And due to the flux bridges at both ends of the permanent magnet, 6 stator teeth are connected as a whole. Then the optimized machine parameters are obtained based on multi-objective genetic algorithm (MOGA). Finally, the topology performance is assessed and verified by finite element method (FEM) Compared with the conventional model, the torque density of the proposed model is increased by 30.39%, and the coupling coefficient ratio are reduced by 14.62%.

Multi-Objective Optimization Analysis of Magnetic Gear With HTS Bulks and Uneven Halbach Arrays

To improve the output torque of coaxial magnetic gears, a novel coaxial magnetic gear (CMG) with high temperature superconducting (HTS) bulks and uneven Halbach arrays is proposed in this paper. The convex structure is used in the middle of inner rotor permanent magnets (PMs). The PMs of the outer rotor are arranged in a low-high-low arrangement, epoxy resin is added between the PMs of outer rotor and the outer iron yoke. At the same time, the HTS bulks replace the epoxy resin to reduce magnetic flux leakage. In addition, according to the sensitivity analysis, the structural parameters can be divided into different levels, the multi-objective optimization analysis is carried out. Finally, the optimal CMG topology with 4 inner and 17 outer pole pairs is established. The results show that the amount of PM is reduced and the output torque is effectively increased by finite element analysis (FEA).

Noncontact Evaluation of HTS Cylinder for Compact Cryogen-Free NMR

Herein, a compact cryogen-free nuclear magnetic resonance (NMR) system is developed using a stacked high-temperature superconducting (HTS) bulk system. To compensate for magnetic field uniformity, an HTS cylinder is installed inside the measurement space. The cylinder is wound with REBCO tapes placed on the surface of a copper cylinder. The uniformity of its critical current density is important for NMR signal detection. Such an operation requires an examination of the quality of the critical-current-density distribution of the cylinder. A noncontact measurement method is required for such an examination. When a magnetic field is applied, the superconductor exhibits a demagnetization effect. Moreover, if any defect exists in any part of the superconducting tape, that particular part would contain no demagnetization, thus disturbing the relevant magnetic field distribution. In this study, this phenomenon is investigated using DC and AC magnetic fields. The magnets and sensor are placed inside the HTS cylinder, and magnetic fields are applied to the HTS cylinder surface. The results confirm that the proposed method is effective and applicable for evaluating the critical-current distribution.

Working temperature threshold analysis for HTS maglev system based the novel thermal-dynamic levitation force coupling measurement device

The previous researches show that the temperature rises inside the high temperature superconducting (HTS) bulk will affect the levitation performance of the HTS bulk. In order to further explore the relationship between the internal temperature rise and the levitation force of the HTS bulk, a thermal-dynamic levitation force coupling measurement device is reformed, and the Halbach PM wheel is used to generate alternating MF at field cooling height (FCH) 30/40 mm, working height (WH) 8/12 mm and speed from 500 rpm to 3500 rpm with intervals of 500 rpm, to study the influence of MF fluctuation on the temperature rise and levitation force of the HTS bulk. The temperature values are obtained when the levitation force decays greatly (82 K) and fails (87 K). The experimental results can provide theoretical suggestions for the irregularity standard of PMG and reference for the protection of the HTS pinning maglev system.

Optimisation of stacked, bulk high temperature superconductors for trapped-field magnet applications

It is necessary to fabricate (RE)BCO bulk high temperature superconductors in the form of individual single grains in order to maximise the length scale over which current flows, and hence the trapped magnetic field. However, inherent difficulties in the grain growth process place limitations on the diameter and height of the single grain that may be achieved by existing melt processes. A practical approach to increase the height of the sample and the trapped field at its surface is by assembling individual single grains in a stack formation with their ab planes aligned parallel, primarily to avoid the expensive process of fabricating large, individual monoliths. The trapped fields observed at the top and bottom surfaces of a single grain sample are frequently different since both the superconducting and physical properties of single grain (RE)BCO samples are generally non-uniform. This leads to challenges in determining how to spatially arrange stacks of single grain samples to generate the largest and most uniform trapped field overall. In this study, we report the optimisation of two-stack configurations involving a total of 8 individual GdBCO/Ag single grains. The samples were arranged in four pairs and configured with different surfaces in contact in the assembly of the stack. The primary superconducting properties for trapped field and total flux distributions were measured at 77 K and compared for each stack arrangement. The initial results indicate that surfaces with inferior flux trapping properties (measured in terms of the overall trapped field value) of a two-sample stack should be positioned at the middle of the assembly to achieve the best overall trapped field and higher total flux at the external, and therefore, usable surface of the stack sample. A numerical modelling method that incorporates different Jc-B characteristics for the top and bottom layers of a single grain to take account of the variability in physical properties and spatial non-uniformity confirmed the optimised experimental arrangement of the stacked bulk samples. Furthermore, the optimisation of single grains of ring geometry to achieve a longer and wider uniform magnetic field zone inside the bore was also performed.