Bulk High-temperature Superconductors Research Articles

Crack nucleation and propagation of electromagneto-thermo-mechanical fracture in bulk superconductors during magnetization

Bulk high-temperature superconductors (HTS) like REBCO (RE: rare earth element or Y) have been widely adopted in a variety of engineering applications. With high magnetic flux applied in a short time of milliseconds, significant tensile stresses are induced during pulsed field magnetization by the Lorentz electromagnetic forces and Joule heating thermal expansions. Consequently, potential damage and fracture may occur in bulk HTS, leading to degradation of the capacity in trapping the magnetic field. In this multi-physical problem, due to the complex stress distribution even in the elastic stage and the intensive redistribution once crack nucleation occurs, the resulting fracture pattern is very complicated. Accordingly, it is a challenging issue to model crack nucleation and propagation of such electromagneto-thermo-mechanical fracture involving both the Lorentz electromagnetic forces and the Joule heating. In this work, the mechanical stresses and multi-physical fracture in bulk HTS during magnetization are studied systematically by the phase-field cohesive zone model (PF-CZM). The governing equations and the involved constitutive relations for the electromagnetic, thermal and cracking-mechanical sub-problems are given together with the numerical implementation. The elastic semi-analytical solutions for the Lorentz forces and thermal expansions induced stresses in an infinitely long cylindrical bulk HTS during magnetization are derived for the determination of crack nucleation. Numerical results are then discussed in details regarding the whole fracture process in bulk HTS under isothermal and non-isothermal cases. It is found that the first crack nucleation occurs at a certain interior position along the circumferential direction after which a couple of cracks nucleate and propagate outwards along the radial direction. The effects of Joule heating and cracking on the electromagnetic performances are quantified, and those of the inhomogeneity in the critical current density are also briefly discussed. Moreover, extension of the proposed PF-CZM to more complex cases is straightforward, making it a promising tool in optimizing the fabrication and practical application of bulk HTS.

Dynamic guidance performance of HTS bulk under varying external magnetic field

With the unique flux pinning characteristics, high temperature superconducting (HTS) bulk can stably levitate above the permanent magnet guideway (PMG). Moreover, owing to its advantages of environmental protection, frictionless and pollution-free, the HTS maglev has shown significant potential in high-speed rail transit. At present, HTS maglev technology is at a critical moment from laboratory research to engineering application. Among the numerous parameters related to its engineering application, the curve negotiation ability determined by the guidance performance of the HTS bulk superconductors is crucial to the safety and stability of its operation. Thus, in this paper, we experimentally investigated the dynamic guidance force of the HTS bulks under different operational conditions including different rotational speeds, maximum lateral displacement (MLD), field cooling height (FCH), and working height (WH) based on a customized HTS maglev dynamic measurement system (SCML-03). In addition, the dynamic guidance force of HTS bulk under long-time operation was studied. It is concluded that FCH, WH, and MLD are intimately associated with the dynamic guidance performance of the HTS bulk, while the rotational speed of the circular PMG has little influence on it. Namely, to improve the dynamic guidance performance of the HTS bulk, reducing the FCH and increasing the WH properly can be regarded as an effective method. Eventually, this research also verifies the long-term lateral stability of the HTS maglev system during dynamic operation.

Loading capacity, rotation loss and torsional oscillation research on an Evershed-type hybrid superconducting bearing used for micro-thrust measurements

Characterized by their very low rotational drag, applications of high-temperature superconductor (HTS) bearings have been expanded in some high-precision instruments. We have developed a sensitive magnetic suspension stand based on an Evershed-type hybrid HTS bearing to measure μN-level thrust. The hybrid HTS bearing uses the strong attractive force of a permanent magnet (PM)-biased bearing to support the main load in the bearing, while instability in the PM-biased bearing is compensated by the magnetic stability from a HTS bearing. Compared with a single PM/HTS bearing, the hybrid HTS bearing is intended not only to support larger loads but also to suppress rotation loss and levitation drift. Loading capacity, loss and damping torque were explored for different design parameters such as bearing gaps L 1, L 2 and the field cooling height (FCH) of bulk HTSs. Spin-down testing suggested that the loss of a hybrid HTS bearing can be reduced by increasing the HTS bearing gap L 2 or reducing the FCH. The hybrid HTS bearing showed a typical oscillation behavior at extreme low frequencies, and torsional pendulum testing suggested that the spring constant k of the hybrid HTS bearing can be as low as 36.39 μN m deg–1. A micro-thrust stand based on the hybrid HTS bearing was established to test an electrospray thruster in a vacuum chamber, and the measurement result of 26.61 μN shows the ability of the micro-thrust stand to achieve μN level testing.

Dynamics of magnetic flux propagation in bulk, single grain superconducting rings during pulsed field magnetisation

When used as trapped field magnets (TFMs), single grain, bulk high-temperature superconducting (HTS) rings are promising candidates for the generation of strong, uniform magnetic fields for nuclear magnetic resonance. The pulsed field magnetisation (PFM) technique provides a low cost, compact and portable method to magnetise these samples as TFMs; however it has proven difficult to achieve high trapped fields in HTS rings using PFM. To date, a record field of only 0.60 T has been achieved for rings magnetised by single-pulse PFM—compared with over 4 T for disc-shaped HTS—and the reasons for this discrepancy are poorly understood. In this work, we use the finite element method to model the propagation of magnetic flux into HTS rings under quasi-static zero field cooled magnetisation and PFM, and validate the results analytically and experimentally. Magnetic flux is found to penetrate finite HTS rings from both the inner and outer surfaces, inducing a negative field at the inner face of the ring. This field is reversed as the applied field increases past the point of full penetration, locally dissipating magnetic energy and heating the sample. HTS rings are therefore more susceptible to local instabilities that severely limit their ability to trap a useful magnetic field. Consequently, thermomagnetic stability of HTS rings during single-pulse PFM can only be ensured by taking careful consideration of reducing flux movement through the bulk around the point at which the field is reversed. This may require more advanced PFM techniques like waveform control or multi-pulse stepwise-cooling to reduce local heating and increase the trapped field.

Efficiency Improvement of an Electric-Grid Transformer Using the Diamagnetism Characteristics of a Bulk Superconductor

An innovative method to improve the efficiency of a single-phase electric-grid 125 kVA, 50 Hz shell type and distribution transformer is presented. The diamagnetism characteristic of a bulk high-temperature superconductor (HTS), designed in a specific dimension, is used to construct a magnetic shield around the air gaps that form between the core joints and among the coils of the transformer. Consequently, the shielded flux engages the core area and increases the flux density in the core, resulting in an increase in the output power, and hence an improved transformer efficiency. The transformer was designed and simulated using advanced electromagnetic software. Simulation results indicate that the width and thickness of the HTS material, as its precise location placed on the air gaps around the core and the coils, can be a substantial factor in generating a magnetic shield that results in an efficiency improvement, superior compared to conventional transformers. The most enhanced performance was received for HTS thickness of 2.6 mm, around 2.4% output power improvement compared with a conventional transformer model. In a transformer of this type that efficiency improvement can lead to great energy savings, around 10,000 kWh for half a year of working under load.

Review on thermal-related measurement methods for superconducting devices and prospect for high-speed maglev transportation application

High-temperature superconducting (HTS) bulks can not only be self-stable when levitated above a permanent magnet (PM) but also can be used as quasi PM with higher magnetic energy product due to their magnetic flux pinning characteristics. Therefore, HTS bulks have wide application potentials in maglev trains, maglev bearings, flywheel energy storage, drug delivery, and high field magnets. In the external magnetic field of common application scenarios, HTS bulks have no external input current, so it is difficult to achieve the overall quench. However, local quenching in the bulk is still possible in the harsh fluctuating external field environment. Although it is difficult to reach the total quench, its critical parameters like Jc will inevitably deteriorate, which may collapse the application system. Therefore, in contrast to superconducting wires and tapes that are more concerned with quench detection, HTS bulks with a 3D volume effect are more focused on internal sensitive temperature locations, the impacts of volume and scale, and the coupling influence on application parameters such as magnetism and force. Therefore, for efficient thermal-related measurement of HTS bulk applications, this paper investigates and discusses 12 commonly-used temperature measurement or quench detection methods in all superconducting application fields. These methods primarily refer to the current quench detection technologies used in HTS tapes and wires. From the standpoint of practical temperature measurement requirements of HTS bulks and technological limitations of maglev application scenarios, working characteristics and service conditions of the 12 methods, and 4 temperature detection methods are selected through a comprehensive understanding and comparison of basic principles. They are expected to be used in real-time monitoring and early warning schemes for onboard superconducting levitation devices of HTS maglev transportation or other applications in the future.

Optimization of Dual-Flux-Modulator Magnetic Gear With HTS Bulks and Uneven Segment Based on GA

Spoke structure can increase the torque density of coaxial magnetic gear (CMG). However, the outer rotor can produce a large number of magnetic leakages. To improve the utility of PMs, a novel magnetic gear was proposed in this paper. The structure of the auxiliary flux modulator is a slotted iron yoke, and the slot angle is same as the stationary ring. Among, the permanent magnets (PMs) of inner rotor are adopted uneven block structure to improve air gap flux density. At the same time, the stationary ring adopts high-temperature superconducting (HTS) bulks, replacing the epoxy resin. In addition, Genetic algorithm (GA) is applied to calculate the optimal parameters of the proposed CMG. Finally, the air gap flux density and output torque of the magnetic gear a compared by the finite element method. The results show that the output torque of the proposed magnetic gear is effectively improved.

A Double-Stator Single-Rotor Magnetic Field Modulated Motor With HTS Bulks

Double stator single rotor permanent magnet synchronous motor (DSSR-PMSM) has the disadvantages of large torque ripple and abundant back-EMF harmonic content. To improve the electromagnetic performance of the motor, a double-stator single-rotor magnetic field modulation motor (DSSR-MFMM) with high-temperature superconductor (HTS) bulks is proposed in this paper. The epoxy resin is replaced by superconducting materials, and the auxiliary slot is opened in the outer stator of the motor. Some parameters of the motor are optimized by genetic algorithm. Compared with DSSR-PMSM, the proposed model has the characteristics of large back-EMF amplitude and high sinusoidal degree. The output torque is increased by 4.36% and the torque ripple is reduced by 5.12%.

Numerical Studies of Dynamic Characteristics of a Stack-Type HTS Maglev System Based on H-Formulation

High temperature superconducting (HTS) magnetic levitation (maglev) is a promising technology for future high-speed transit systems. Superconducting magnetic bearings are made of a permanent magnet guideway and HTS bulks (bulk-type) or stacks of 2G HTS tapes (stack-type). If the dynamic behavior of bulk-type HTS maglev systems has been well reported, there is little literature about the dynamics of stack-type HTS maglev system. To fill the gap, we built here an electromagnetic-mechanical model of a stack-type bearing. We used a segregated <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">H</i> -formulation finite element model to account for the nonlinear resistivity. The multi-layer structure of the HTS tape is either homogenized or modeled in detail. We studied the bearing dynamic response when subjected to different time-varying external magnetic fields mimicking the guideway irregularity and the vehicle speed. The results show that the induced current in the copper layer negatively impacts the stability. By extension, since it cannot account for the copper layer, the homogenized model is not suitable for such analysis. This work contributes to finding viable measures for improving the stability of stack-type HTS maglev systems.

Levitation Characteristics of High-Temperature Superconducting Bulks of Different Orientations and Arrays

High-temperature superconducting (HTS) maglev has good application prospects due to its passive stability. There are a lot of researches on the levitation characteristics of a single HTS bulk in the applied magnetic field. However, the bulks are usually arranged in an array in practical applications. In addition, experiments proved that the levitation force and guidance force of the system with multiple bulks are not equal to their linear superposition. Therefore, it is interesting to further study the law of interaction between bulks quantitatively. In this paper, firstly, the method of how the bulk's inhomogeneities are described in the electromagnetic model is introduced. Secondly, the levitation and guidance force of bulks of different orientations and combinations are calculated using a three-dimensional (3D) simulation model considering the inhomogeneities. A Qualitative Rapid Analysis Method (QRAM) to predict the effect of the combinations is proposed. The accuracy of the prediction method has been verified for different working conditions. Finally, the effect of the shape of bulks on the levitation characteristics is investigated. The result shows a weak “aggregation advantage”. This paper provides some reference for the design and prediction of the levitation and guidance performance of the HTS maglev system with a combination of multiple HTS bulks.

The Simulation on Thermal-Electromagnetism of High-Tc Superconducting Bulks Under Stochastic Excitations

High temperature superconducting (HTS) maglev utilizes the magnetic flux pinning characteristics of vehicle's high temperature superconducting bulk to achieve the stable suspension. During the operation, ac losses will occur inside the HTS bulks due to the irregularity of the permanent magnetic guideway (PMG), causing the change of the temperature, which directly affects the stability of the suspension system. In this article, the magnetic-thermal-guideway coupled model of two-dimensional HTS bulks YBCO, liquid nitrogen (LN) and PMG is established by using COMSOL Multiphysics. The test track irregularity spectrum of high-speed railway is introduced and added to the PMG to simulate the magnetic field fluctuation caused by its arrangement irregularity. The characteristics of the HTS bulks’ power loss, the average current density distribution over the cross section and the temperature rise of the bulk, LN, and suspension gap (SG) are studied respectively under four conditions of excitations: lateral coupled with vertical excitations, lateral excitations, vertical excitations, and nonexcitations. Meanwhile, by comparing and analyzing with the model without considering the temperature, the influence of the temperature on the current density is studied. These results provide certain references for the safe and stable operation of HTS suspension system.

Different directional random excitation's influence on the thermal-electromagnetism of HTS bulks with the real-time coupling method

High-temperature superconducting (HTS) maglev, owing to its unique characteristic, has a wide range of engineering application. Researches on the dynamic response, mostly affected by the irregularity of permanent magnetic guideway (PMG), is the top priority. In order to detailly find out the influence of the irregularity of PMG in lateral/vertical direction on the dynamic response of the HTS bulks respectively, this paper introduces the different directional random excitation and employs a real-time mechanic-electromagnetic-thermal coupling method to study the HTS bulks’ dynamic response. The results show that the different directional excitation has a more significant effect than the temperature does. This study makes a detailed understanding of the HTS bulks’ dynamic response under the random excitation in different directions, providing an effective reference for the HTS bulks’ dynamic analysis.

Optimizing trapped field in superconductors with perforations

The use of artificial holes can improve the performance of high-temperature bulk superconductors. We report the results of Monte Carlo simulations of the trapped magnetic flux in superconducting samples with different configurations of perforated holes. Vortex trapping and emission at the hole boundary were modeled to account for flux pinning on the holes. It was found that the trapped flux can reach its maximum value, which does not depend on the number of holes. The dependence of the trapped flux on the diameter and number of holes is explored and described by the suggested relations that account for pinning on the holes and the effective hole area. Recommendations are provided to attain the maximum trapped flux for a fixed number or diameter of holes.

Dynamic guidance performance of GdBaCuO and YBaCuO bulk single grain superconductors under a varying external magnetic field

High temperature superconducting (HTS) maglev technology has attracted considerable attention from researchers around the world. It provides a new direction of development for urban rail and high-speed rail transit due to its considerable advantages of frictionless transportation, low noise, and environmental protection. HTS bulk materials are able to provide a significant performance advantage to maglev technology and are a key component to achieving both levitation and vehicle guidance. HTS bulk samples with enhanced flux pinning characteristics can enable improved curve negotiating ability and lateral stability for the maglev vehicle. This paper reports the results of experiments on the dynamic guidance force of single grain Y–Ba–Cu–O (YBCO) and Gd–Ba–Cu–O (GdBCO) bulk HTS single grains in order to explore their potential for enhanced maglev guidance performance. A customized HTS maglev dynamic measurement system (SCML-03) with a rotating, circular permanent magnet guideway (PMG) was employed to simulate the dynamic operation of the HTS vehicle above the PMG at different translational speeds. It was observed from the experimental results that the GdBCO bulk superconductor is more able to resist the attenuation of the dynamic guidance force compared to YBCO under the same operational conditions. In addition, the GdBCO bulk single grain is more able to return to the original equilibrium position following exposure to an external displacement. It is concluded that the GdBCO bulk single grain offers greater potential than YBCO for practical application in HTS maglev given its enhanced guidance performance. The results of these tests provide an important benchmark for the future design of HTS maglev transportation systems.

Double Armature HTS Bulk Synchronous Machine for Contra-Rotating Turbine Generator

Many high-temperature superconducting (HTS) rotors of the radial flux type mainly use the magnetic field from single sided surface of the field pole. However, field poles using HTS bulk have the advantage of capturing high magnetic fields on both sides of the field poles, which can be effectively used to generate electricity. Therefore, we have developed a double armature structure in which a field stator is sandwiched between both the inner and outer armature rotors of the HTS synchronous generator. Due to the high magnetic field captured by the HTS bulk, it is also possible to directly join a counter-rotating turbine. In this paper, we designed the developed model and performed electromagnetic field analysis. The analytical speed was calculated at a low speed depending on the design of the 500kW reverse rotation turbine. Using the HTS bulk as the boundary pole, the torque consistency with the input mechanical torque of the turbine was confirmed, and it was obtained from the compatibility analysis with the 500kW class tidal current power generation. We also analyzed the model with the smallest torque ripple from multiple models with different HTS bulk placements and found the model with the best placement.

The SCML-05 Developed for Studying the 3D Force and PMG Irregularities Characteristics of HTS Maglev

In order to promote the application of high temperature superconducting (HTS) maglev vehicle, the series of superconducting maglev (SCML) measurement devices — 01, 02 & 03 have been developed in Southwest Jiaotong University. During the course of research, the importance of three-dimensional (3D) study of the interaction between HTS bulks and permanent magnet guideways (PMGs) has become evident. More specifically, when the HTS vehicle is moving at a high speed, small irregularities of the PMG's magnetic field become the source of vibration. Therefore, in order to study the 3D force and the characteristics of HTS maglev under PMG irregularities, a new measurement device called SCML-05 was designed, constructed, and validated. This newly developed measurement device consists of two major parts. The first one is a closed-loop sliding platform providing 3D motion. The second part is measurement and data acquisition system with the software developed for this purpose. In addition, a laser displacement sensor is used to measure the geometry irregularities of the PMG. Overall, the SCML-05 is flexible and light. The force characteristics of the HTS bulks placed above the PMG can be precisely investigated by means of this system. Additionally, the PMG irregularities including geometry and magnetic field irregularities can be measured. These parameters are required to analyze the relationship between the PMG irregularities and the force of HTS maglev vehicle. The test process and typical results obtained by this system are presented in this paper.

Equivalent Models of Critical Current and Magnetization for Calculating the Magnetic Field in Simply Connected Structures Made of HTS Bulk and HTS Tape

This paper proposes a way to represent the anisotropic nonlinear superconducting properties using a discrete model for magnetization with the corresponding critical parameters, equivalent to the Power Law model for currents. This simplified model makes it possible to analyze magnetic systems with superconducting elements in a stationary mode, which reduces the calculation time. Using of proposed model is demonstrated by the calculation of force interactions between high-temperature superconducting (HTS) samples and permanent magnet in zero-field-cooling (ZFC) and field-cooling (FC) modes. HTS bulks in the form of a prism and stack made of HTS tape are considered. The calculation of the electromagnetic field was carried out using the method of integral equations for field sources. The data of calculations are compared with the results of experimental studies.

Waveform Control Pulse Magnetization of GdBaCuO Bulk Near Operating Temperature of Our Superconducting Rotating Machine

We have tested our waveform control pulse magnetization with negative feedback (WCPM-NFB) method on a GdBa <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> C <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">7-δ</sub> (GdBaCuO) high temperature superconducting (HTS) bulk sample, near operating temperatures of superconducting machines. This advanced single pulse field magnetization (PFM) method was demonstrated to significantly increase the trapped magnetic flux density in the center of the bulk compared to more conventional PFM techniques. The WCPM-NFB method has been further enhanced by modifying the control of the applied pulsed magnetic field. The aim was to adapt the control method to make use of the flux jump in a more efficient way while limiting the heat generation in the bulk. The target magnetic flux density of the PID controller was changed during the pulsed magnetization, allowing the applied magnetic field to be adapted consequently to the change of condition inside the bulk due to the flux jump. A magnetic flux density of about 3 T has been trapped at 50 K in a single pulse using our improved WCPM-NFB method.

Magnetic Float Liquid Level Detection Method for High-Temperature Superconducting Flux-Pinning Maglev System

High temperature superconducting (HTS) flux-pinning maglev system uses the flux-pinning characteristics of HTS bulk to achieve self-stable levitation, but the superconductivity of HTS bulk depends on low temperature maintained by liquid nitrogen. It can be seen that the real-time detection of liquid nitrogen level is an essential safety detection in the HTS flux-pinning maglev system. However, the liquid nitrogen level measurement of Dewar is limited by many conditions, such as small available size of Dewar, small liquid nitrogen capacity, low temperature characteristics of liquid nitrogen, etc. These conditions require the liquid level detection system to be able to withstand low temperature, have sufficient identification accuracy and have no significant impact on the cold insulation performance of Dewar. All of these bring difficulties to the design of real-time liquid level detection in the maglev system. Based on the structural characteristics of Dewar, a new type of magnetic float liquid level detection device is studied in this paper. The device can realize liquid nitrogen level detection function and low-level alarm function. Its technical principle is to use Hall sensor array to detect the position of the magnetic float in real time, and transmit the data to the onboard computer for data storage and data monitoring. The experimental results show that the proposed liquid nitrogen level detection method can meet the requirements of liquid nitrogen level detection and alarm with certain accuracy, and will not bring additional heat leakage to affect the Dewar performance.

Simulation of the Waveform Control Pulse Magnetization of a High-Temperature Superconducting Bulk With Negative Feedback

Simulation of the Waveform Control Pulse Magnetization of a High-Temperature Superconducting Bulk With Negative Feedback