Maximum Trapped Magnetic Field Research Articles

Magnetic-thermal-mechanical coupling behavior simulation for a porous high-temperature superconductor based on fractal derivatives

In this paper, the magnetic-thermal-mechanical coupling behavior for a porous high-temperature superconductor placed in a pulsed field is studied numerically within the framework of finite element analysis and based on fractal derivatives. Firstly, as a kind of oxide ceramic material, the porous properties of high-temperature superconductors (SCs) are characterized through fractal methods. Then, we obtained the Maxwell equation and heat conduction equation in the form of fractal derivatives, and thus the difficulties brought by the multi connectivity of materials to finite element (FEM) modeling can be overcome. The FEM simulation results indicate that the porous properties have a significant impact on the maximum trapped magnetic field, surface temperature of superconductors, and maximum stress inside superconductors. The presented method can also provide a more efficient solution for the multi-field coupling simulation of other porous electromagnetic media.

Investigating the flux jump behaviour during single waveform control pulsed field magnetization of GdBaCuO superconducting bulk

We aimed to improve the trapped magnetic flux density obtained with a waveform control pulsed field magnetization (WCPM) by using a negative feedback control of the magnetic flux density on the growth sector boundary of a GdBaCuO superconducting bulk sample. The WCPM method with negative feedback control has previously shown that it could help increase the trapped magnetic flux density, compared to conventional passive pulsed flux magnetization, if the magnetic field penetrates the bulk centre substantially using a flux jump. The flux jump sometimes greatly changes the magnetic and thermal state of the bulk, which limits the maximum trapped magnetic field. The active control method of the applied magnetic field helps to overcome this limit if the control conditions are appropriate. While searching for the ideal control conditions of the WCPM, the magnetization characteristics have been investigated. We found that the flux jump, which assists the flux in penetrating the centre of the bulk, can be “slowed down” thanks to the negative-feedback WCPM. This operation helped to decrease the heat generated by the moving flux inside the bulk and reduced the temperature rise, which contributed to the increase of the trapped magnetic flux density.

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.

Distribution of the superconducting critical current density within a Gd–Ba–Cu–O single grain

The magnitude of the maximum trapped magnetic field in a bulk, single-grain superconductor is a key performance figure of merit. This is determined, generally, by the magnitude of the critical current density, Jc, and the length scale over which it flows. As with all type-II superconductors, Jc is related closely to the microstructure of the superconducting material and, in the case of RE–Ba–Cu–O [(RE)BCO, where RE is a rare-earth element or yttrium] single grains, RE2BaCuO5 (RE-211) inclusions in the superconducting REBa2Cu3O7−δ (RE-123) phase matrix are key microstructural features that act effectively as flux pinning centres. Although the distribution of RE-211 in single-grain bulk superconductors has been studied extensively, the variation of Jc within a given sample has been much investigated much less thoroughly. A detailed experimental understanding of the variation of Jc in these technologically important materials, therefore, is required given the growing popularity and significance of numerical techniques for modelling the behaviour of type-II bulk superconductors. Here we report a systematic investigation of the correlation between Gd-211 particle density and sample porosity, which are microstructural features, and Tc and Jc in a Gd–Ba–Cu–O bulk, single grain fabricated using a buffer layer and a supply of additional liquid phase. This was performed by cutting the sample into numerous sub-specimens of approximate dimensions 1.8 × 2.8 × 1.5 mm3. We observe that Jc decreases with distance from the seed, although more strongly with distance along the c-axis than along the a–b plane. In contrast to what might be expected given the assumed contribution of RE-211 inclusions to flux pinning, we find no evidence of a clear correlation between the local RE-211 precipitate density and local critical current on a length scale of mm. We observe that the porosity of the sample is a more dominant factor in determining the distribution of Jc within a single grain.

Effect of an Elliptical Inclusion on Critical Current Density of a Long Cylindrical High-T c Superconductor

An analytical investigation is presented to display the distribution of critical current flow and trapped magnetic field around an elliptical nonsuperconducting inclusion within a long cylindrical superconductor. The current streamlines, the critical current density, and the trapped field around the inclusion in the superconductor without deformation are obtained based on the Bean model and the method of conformal mapping. The results show that the critical current density of a superconductor will be decreased dramatically due to a macroscopic nonsuperconducting inclusion. Besides, the maximum trapped magnetic field is limited by the inclusion.

Evaluation of Trapped Magnetic Field Properties in Superconducting MgB2 Bulk Magnets of Various Shapes by Finite Element Method

The trapped magnetic field properties of superconducting MgB2 bulk magnets with various shapes such as a triangular, a quadrangular, a hexangular bulk were calculated by the Finite Elements Method (FEM). The effect for the combination of several numbers of bulks was also investigated for several kinds of shapes to obtain large area of bulk surface in spite of one large bulk. In this calculation, the simple magnetization process replaced by the field-cool magnetization was used to obtain the equivalent distribution of the magnetic field, and the thermal equation in FEM was omitted. The trapped magnetic field for the triangular bulk by FEM was compared with the experimental result. It was found that the calculated results agreed well with the experimental result. The maximum trapped magnetic field was obtained in the cylindrical shape among several kinds of shapes. The trapped magnetic field was increased by the combination of multi-bulks. It was confirmed that the trapped magnetic field of the multi-bulks was larger than that of the single bulk. The trapped magnetic field increases with increasing the number of the bulks.

Magnetization and Demagnetization Studies of an HTS Bulk in an Iron Core

High-temperature superconductors (HTSs) are promising materials in a variety of practical applications due to their ability to act as powerful permanent magnets. Thus, in this paper, we have studied the influence of some pulsed and pulsating magnetic fields applied to a magnetized HTS bulk sample. The bulk sample, the iron core, the two inductors, and the current sources used to magnetize and demagnetize the superconducting bulk in the iron core are presented. Then, we detail various magnetizing and demagnetizing processes and their purposes. For the magnetization studies, we have shown that there is no significant difference between the successive process and the direct process of magnetization. The maximum trapped magnetic field (TMF) is about 1 T, 10 min after the magnetization process. For the demagnetization studies, we propose many results as a hysteresis loop corresponding to the pulsed demagnetizing field influence. Various time evolutions of the TMF are also shown, i.e., when the demagnetizing ac field is applied a few minutes after or several days after the magnetization. This leads to completely different results, from 5%/min to 1%/(6 h) of reduction of the TMF. The results presented here give many practical information to users of superconducting bulk magnets. We assume that decreasing the temperature will increase the capabilities of trapping the magnetic field. We conclude that a strong interest must be kept on the applications of HTS bulks in electrical engineering, preferably at lower temperature than 77 K.

Evaluation of Trapped Magnetic Field Properties in Superconducting MgB2 Bulk Magnets by Finite Element Method

The trapped magnetic field properties in superconducting MgB2 bulk magnets with various kinds of shape such as a disk, a ring and a pair of disks were calculated by the finite element method (FEM). For simplicity, field cool magnetization was replaced by a simple magnetization process at constant temperature to obtain equivalent distribution of magnetic field, and the thermal equation in FEM was omitted. It was confirmed that the result of FEM agreed well with the result by analytical method in infinite long cylinder. We compared the trapped magnetic field property between FEM result and experimental result in reference in order to research the simple evaluation method of the trapped magnetic field of MgB2 bulk magnet. It was found that the result of FEM agreed with the experimental result and it can explain the distribution of trapped magnetic field of superconducting MgB2 bulk magnet. From these results, it was found that it was possible to be calculated in various kinds of shape with using simple evaluation by FEM. Therefore, the optimization of the maximum trapped magnetic field in superconducting MgB2 bulk magnet can be discussed.

Preparation and Mechanical Properties of Large Single Domain DyBaCuO Superconductor

We prepared large single domain of DyBaCuO (Dy-123) superconductor for future applications such as a superconducting flywheel for energy storage system, magnetic separation, and a bulk magnet. Dy-123 superconductors are expected to have high and trapped field properties. Melt-textured single domain of Dy-123 system superconductor about 48 mm in diameter and 10 mm in thickness was fabricated with a seeding and temperature gradient method in air. Dy-123 and Dy-211 powders were mixed in a molar ratio of 1:0.3, and of 10 wt% and Pt of 0.5 wt% were added to the mixture. The maximum trapped magnetic field over the bulk was approximately 0.2 T at 77.3 K. We also discussed mechanical properties such as the bending stress, the hardness and the surface roughness of the Dy-123 bulk. Mechanical property results showed important information for evaluating Dy-123 bulks.

The microstructure and properties of single grain bulk Ag-doped Y–Ba–Cu–O fabricated by seeded infiltration and growth

Single grain, Ag-doped Y–Ba–Cu–O bulk superconductors of up to 30 mm in diameter have been fabricated successfully by a seeded infiltration and growth (IG) technique. These samples exhibit low porosity compared with samples grown by conventional top seeded melt growth (TSMG). The density of voids within as-processed IG samples can be reduced further by adding metallic Ag, rather than Ag 2O, to the Y 2BaCuO 5 (Y-211) pre-form. An inhomogeneous distribution of Y-211 has been observed in these samples. Finally, a maximum trapped magnetic field of 0.5 T was observed for Ag-doped Y–Ba–Cu–O of diameter 30 mm at liquid nitrogen temperature (77 K).

Trapped magnetic field of 48 mm diameter single-domain Dy-123 system superconductor

Since melt-textured rare earth (RE)-system superconducting bulk can trap high magnetic field, it acts as a quasi-permanent magnet stronger than conventional permanent magnets. Large-sized single-domain DyBa2Cu3Ox (Dy-123) system bulks about 48 mm in diameter and 15 mm in thickness were prepared with a seeding and temperature gradient method in air. Dy-123 and Dy2BaCuO5 (Dy-211) powders were mixed in molar ratios of 1:0.3. The pellets were partially melted at 1,070°C. When the top temperature of the sample reached 980°C, Nd-123 seed crystal was placed on the top surface of the bulk. Then, the samples were cooled from 980°C to 930°C, and then with furnace cooling. Finally, the samples were annealed in pure oxygen atmosphere. When magnetic field of 1.5 T was applied to the samples, the maximum trapped magnetic field of 0.7 T was trapped at 77.3 K in the samples prepared with the cooling time for 600 hr and 700 hr from 980°C to 930°C. Only 0.2 T was trapped in the sample prepared with the cooling time for 1,000 hr, whose bottom part was decomposed due to long time heating.

Influence of High Frequency AC Magnetic Field on Trapped Magnetic Field

The trapped magnetic field in an HTS bulk is decayed and even erased by the application of an AC magnetic field whose amplitude is much smaller than the maximum trapped magnetic field. In previous experiments, the decay of the trapped magnetic field was shown to be due to the temperature rise of the bulk caused by the AC losses. Especially, in a high frequency magnetic field, AC losses are larger and the decay of the trapped magnetic field is accelerated by this AC loss increase. We investigated the time relationships of the AC loss of the HTS bulk in a high frequency magnetic field using the thermal equilibrium equation.

Magnetization of Ferromagnetic-SC Heterostructures for Trapped Field Low Power Motors

The applicability of bulk based superconducting parts as magnetic elements for small sized motors is limited because the fact that the maximum flux density they can trap depends on the size. Also, the long time stability of the flux trapped in superconducting parts depends on the working interacting field which modifies the magnetization of pellets. In this work, we analyze the magnetic behavior of structures made by an iron yoke and a bulk-based ring around the yoke when subjected to an external and variable magnetic field. The in-field Hall mapping technique is used to determine how the iron improves the maximum trapped magnetic field. The stability of the flux trapped in the structure when a non-aligned external field is applied, and the application to low power motors is discussed.

Magnetic field distributions of stacked large single domain Gd–Ba–Cu–O bulk superconductors exceeding 140 mm in diameter

We have studied the magnetic field distribution of the stacked two single domain Gd–Ba–Cu–O large bulk superconductors exceeding 140mm in diameter, and compared with the distribution of one single domain bulk. The maximum trapped magnetic field of the stacked bulk at 65K was 4.3T and 5T at the surface and at the center of two large bulks respectively, when the bulks were field-cooled in 5T. It was found that the magnetic field could reach a longer distance and wider area by increasing the thickness. Therefore, larger and thicker bulk superconductor shows several benefits in terms of practical applications.

Magnetic properties of melt-processed large single domain Gd–Ba–Cu–O bulk superconductor 140 mm in diameter

We have studied magnetic properties of a single grain Gd–Ba–Cu–O bulk superconductor 140mm in diameter. The distribution of the trapped magnetic field showed that the sample was a single domain without serious weak links. The maximum trapped magnetic field at 65K was 3.66T, when the bulk was field-cooled in 4T. From the distribution measurements of the trapped magnetic field, it was confirmed that a large amount of flux could be trapped in a large grain superconductor. In addition, the magnetic field could reach a longer distance in a large bulk compared to smaller samples. The repulsive force measurements also showed that the repulsive force density of the larger bulk was much larger than those of smaller samples.

Preparation and properties of large single domain DyBaCuO superconductor

We prepared large single domain of DyBaCuO (Dy-123) superconductor for future applications such as a superconducting flywheel for energy storage system, magnetic separation, and a bulk magnet. Dy-123 superconductors are expected to have high Jc and trapped field properties. Melt-textured single domain of Dy-123 system superconductor about 48mm in diameter and 10mm in thickness was fabricated with a seeding and temperature gradient method in air. Dy-123 and Dy-211 powders were mixed in a molar ratio of 1:0.3, and Ag2O of 10wt% and Pt of 0.5wt% were added to the mixture. The maximum trapped magnetic field over the bulk was approximately 0.2T at 77.3K. We also discussed mechanical properties such as the hardness and the surface roughness of the Dy-123 bulk. Vickers hardness and surface roughness results showed some basic and useful information for evaluating Dy-123 bulks.

Field trapping property of Gd–Ba–Cu–O bulk superconductor 140 mm in diameter

We have studied field trapping ability of single grain Gd–Ba–Cu–O bulk superconductor 140mm in diameter, which was fabricated by using Gd2BaO4 and Ba–Cu–O phase as raw materials. The distribution of the trapped magnetic field showed single domain without serious weak links, when the bulk was field-cooled in 1T at liquid nitrogen temperature. The maximum trapped magnetic field at 77K and 65K were 2.30T and 3.66T respectively, when a field of 4T was applied. From the distribution measurements of magnetic field, it was confirmed that a large amount of flux can be trapped in this large bulk. It was also confirmed that magnetic field can reach a longer distance in a larger bulk than smaller diameter samples.

AC Losses in HTS Bulk at Various Temperatures

HTS bulks applied to electric machines are exposed to AC magnetic field perturbations which cause AC losses in the bulks. AC losses affect the efficiency of the machines and decay the trapped magnetic field in an HTS bulk. Critical current density and the maximum trapped magnetic field of a bulk are enhanced as temperature is decreased. Thereby, the lower temperature operation of the bulk is preferable. The AC losses in the bulk and their influence on the trapped magnetic field are dependent on the temperature. Therefore, to estimate the efficiency and decay of the trapped magnetic field at various operation temperatures, it is necessary to know the temperature dependence of the AC losses. We investigated AC loss characteristics of a bulk at various temperatures. In the work, AC losses were measured for the amplitude of the AC external magnetic field up to 0.08 T and frequency 17.3 Hz/spl sim/62.2 Hz at the temperature 65 K/spl sim/100 K. It is also shown that the temperature dependences of the AC losses are well estimated by a theoretical model based on the Bean model.

Melt process of Sm–Ba–Cu–O bulk superconductors by thin film cold seeding

Abstract We discuss Sm123 bulks melt-processed in air and their characteristic superconducting properties for improving superconducting properties and producing a larger bulk. Isothermal undercooling growth in air with oxygen annealing and Nd123/MgO thin film cold seeding technique were applied in SmBaCuOy/Ag system to seek the high-efficiency of process, homogeneity of composition, and feasibility of batch production. We investigated process conditions such as heat treatment temperatures, compositions, seeding methods, and atmosphere. Single-domain growth of superconducting phases of a square larger than 10 mm on a side and 5 mm in thickness was achieved using this technique. T c,onset and T c,zero are 94 and 90 K, and J c is 3 × 10 4 A/cm 2 at around 2 T at 77 K with a typical peak effect in the LRE system. In the case of Sm211 = 10 and 40 mol% addition, the maximum trapped magnetic field of the bulks is 1000 and 2100 G, respectively. The maximum magnetic field increases as Sm211 volume fractions increase. The result implies that melt-processed in air applying isothermal method and thin film seeding in Sm system is feasible for producing larger bulks in large scale applications.

Processing of high-performance Gd–Ba–Cu–O bulk superconductor with Ag addition

We fabricated large-grain c-axis-oriented Gd–Ba–Cu–O bulk superconductors with very fine 211 particles and studied their field-trapping properties. Single domain Gd–Ba–Cu–O/Ag bulk samples with different molar ratios of Gd-123:Gd-211 = 10:x (x = 2 to 6) were melt-textured under controlled oxygen partial pressure of 1.0% by employing Gd-211 starting powders with a particle size of 1.0 μm. The maximum trapped magnetic field of 2.0 T was recorded at 77 K in the bulk 48 mm in diameter with the composition of x = 5. It is notable that the trapped field in the space of two Gd–Ba–Cu–O bulks reached an extremely high value of 3.3 T at 77 K. Furthermore, the field-trapping performance of the bulk sample was improved by applying the ultra-fine 211 powders of a size of 0.2 μm. The single-bulk Gd–Ba–Cu–O 50 mm in diameter exhibited a trapped field of 2.6 T at 77 K.