RE123 Bulk Research Articles

Magnetization behavior of RE123 bulk magnets bearing twin seed-crystals in pulsed field magnetization processes

Melt-textured Y-Ba-Cu-O high temperature superconducting bulk magnets were fabricated by the cold seeding method with using single or twin-seed crystals composed of Nd-Ba-Cu-O thin films on MgO substrates. The behavior of the magnetic flux penetration into anisotropic-grown bulk magnets thus fabricated was precisely evaluated during and after the pulsed field magnetization operated at 35 K. These seed crystals were put on the top surfaces of the precursors to grow large grains during the melt-processes. Although we know the magnetic flux motion is restricted by the enhanced pinning effect in temperature ranges lower than 77 K, we observed that flux invasion occurred at applied fields of 3.3 T when the twin seeds were used. This is definitely lower than those of 3.7 T when the single-seeds were employed. This means that the magnetic fluxes are capable of invading into twin-seeded bulk magnets more easily than single-seeded ones. The twin seeds form the different grain growth regions, the narrow-GSR (growth sector region) and wide-GSR, according to the different grain growth directions which are parallel and normal to the rows of seed crystals, respectively. The invading flux measurements revealed that the magnetic flux invades the sample from the wide-GSR prior to the narrow-GSR. It suggests that such anisotropic grain growth leads to different distributions of pinning centers, variations of Jc values, and the formation of preferential paths for the invading magnetic fluxes. Using lower applied fields definitely contributed to lowering the heat generation during the PFM process, which, in turn, led to enhanced trapped magnetic fluxes.

Simple die pressing for making artificial holes in single-grain Gd1.5Ba2Cu3O7−y superconductors

The presence of artificial holes in single-grain REBa2Cu3O7−y (RE123, RE: rare-earth elements) bulk superconductors can facilitate oxygen diffusion into superconducting grains through the increased surface area. In addition to the enhancement of oxygen diffusion, the mechanical properties and thermal conductivity of bulk superconductors can be improved by filling holes with a metallic conductive phase. This study presents a new and simple hole-making process for single-grain RE123 bulk superconductors. Artificial holes 3 or 5 mm in diameter were made for Gd1.5Ba2Cu3O7−y (Gd1.5) powder compacts prior to the sintering/melt growth process using specially designed pressing dies. The die pressing neither induced cracking in powder compacts nor influenced thermal procedures for the Gd123 growth. Single-grain Gd1.5 bulk superconductors with holes were successfully fabricated by a top-seeded melt growth (TSMG) process using Gd1.5 powder compacts with holes. The die pressing was proven as a time-saving process in comparison with the conventional method which makes holes in sintered or melt-processed hard bodies by mechanical drilling. The detailed fabrication process of single-grain Gd1.5 bulk superconductor with holes, magnetic levitation forces, and magnetic flux density, estimated for the single-grain Gd123 bulk superconductors with holes, are reported.

Bi-axial magnetic orientation in a twinnedErBa2Cu3Oy superconductor by controlling grain size

Bi-axial magnetic orientation inErBa2Cu3Oy (Er123) superconductor with a twinned microstructure was demonstrated.A ball-milling process effectively improved the degree of the in-plane andc-axis orientation of Er123 powder samples oriented in a modulated rotating magnetic fieldof 10 T. From the relationship between the particle size and the degree of orientation, it isconcluded that pulverization induced the appearance of the in-plane magnetic anisotropy ofEr123 grains through inhomogenization of two different types of domains in their grains.The present study presents a production technique of RE123 bulk magnets and coatedconductors without epitaxy technology.

Evaluation of magnetic force distribution on a pair of superconducting bulk magnets

We study the construction of superconducting permanent magnets by RE123 bulk materials and the investigation of these industrial applications such as a magnetic separation. A bulk magnet can generate strong magnetic fields exceeding 2T, which is the limit of ordinary iron-cored electromagnets, in a compact device with a low running cost. A magnetic field distribution of the bulk magnet is a cone shape, and it contributes to an increase of magnetic force which is proportional to the product of a magnetic field and its gradient. It is important to evaluate magnetic force when the application of the bulk magnet is discussed. In this paper, two Gd123 bulk materials of 65mm in diameter were magnetized using a pair of superconducting bulk magnet system and three-axis components of magnetic flux density (Bx,⋅By, and Bz) in an open space between the magnetic poles were scanned with pitch of 2mm in each direction. From these measured data, the axial and radial components of magnetic force factor, Bz⋅dBz/dz and Br⋅dBr/dr, were calculated. At 10mm gap, the Bz⋅dBz/dz value reached 180.6T2/m for a field of 2.33T, which is comparable to Bz=6.76T for a common 10T–100mm∅ superconducting magnet.

Improvement of microstructure and mechanical properties of large single domain GdBaCuO superconductor with low void density

We fabricated large single domain of GdBaCuO (Gd-123) densified superconductor with a seeding and temperature gradient method in oxygen at the melt stage then in air at the growth stage for future applications such as a superconducting flywheel for energy storage system, magnetic separation, motor, current lead, and a bulk magnet. The innovative melt-textured single domain of Gd-123 system superconductor with low void density was about 46mm in diameter and 15mm in thickness. Gd-123 densified superconductors are expected to have high Jc and trapped magnetic field properties with higher mechanical strength. In this paper, we discuss processing, superconducting and mechanical properties of the densified Gd-123 bulk for improving the microstructure and strength of the RE-123 bulk. The relation between the microstructure and the flexural strength of the Gd-123 bulk was clearly shown.

A Strong Magnetic Field Generation by Superconducting Bulk Magnets With the Same Pole Arrangement

We study on the construction of strong magnetic field by superconducting bulk magnets with RE123 bulk materials and the investigation of these industrial applications such as a magnetic separation. In this paper, we try to generate a strong magnetic field in the direction parallel to the bulk magnet. Two Gd123 bulk materials of 65 mm diameter are magnetized in the same pole (N-N) using a face-to-face superconducting bulk magnet system. The radial component of magnetic field, Bx, is measured in an open space between the magnetic poles by varying the gap (g=20-50 mm). The strong magnetic field area of g=20 mm is 15.8 times as large as that of g=50 mm and the maximum field reaches 1.53 T.

超伝導体における磁束ピンニング[7

The flux pinning properties of RE-123 bulk superconductors, RE-123 coated conductors, Bi-2223 tapes and MgB2 superconductors are reviewed. In bulk superconductors, the pinning mechanism of the lower Tc region, such as the Ba sites substituted by RE elements or twin boundaries with oxygen deficiency, is considered to be kinetic energy interaction under a proximity effect with the superconducting matrix. The dependence of the irreversibility field on the superconducting layer thickness is very complicated for RE-123 coated conductors. This is partly attributed to the thickness dependence of the critical current density that arises from the deterioration of the superconducting layer structure in thicker films and partly to the thickness dependence of the flux creep. The reason for the dramatic improvement in the critical current properties is also discussed for Bi-2223 tapes fabricated using the over-pressure sintering technique. Theoretical analysis using the percolation theory clarifies that the low critical current density in conventional MgB2 in situ wires is attributed to low electrical connectivity, which is due to the voids and wetting insulating layers between grains. It is clarified that the flux pinning strength of the grain boundaries in MgB2 is significantly strong, and there is a room for a drastic increase in the flux pinning strength by dirtying the MgB2 through C-doping. Finally, the flux pinning mechanism of columnar defects nucleated by heavy ion irradiation is discussed. It is shown that the columnar defects larger than the coherence length are desirable to increase the probability of flux lines being easily captured by the defects.

Evaluation of magnetic field distribution of superconducting bulk magnets with the same pole arrangement

We study on the construction of superconducting permanent magnets by RE123 bulk materials and the investigation of these industrial applications such as a magnetic separation. The bulk magnets can generate strong magnetic field in comparison with common permanent magnets and iron-cored electromagnets. In order to propagate an industrial application of bulk magnet in the feature, it is necessary to form various shape of magnetic field, as well as its strength is enlarged. In this paper, the strong magnetic field in the radial direction has been constructed by a pair of bulk magnets. Two Gd123 bulk materials are magnetized by the IMRA method and these are arranged face-to-face with the same pole facing each other. When the radial component of magnetic fields, B x , is measured in the open space between the magnetic poles with a gap of 50 mm, the area of | B x | > 0.5 T increases to 148% compared with the single pole.

Mechanical properties of DyBaCuO superconducting bulks

Melt-processed REBaCuO (RE: rare earth) superconductors have a high Jc at 77K and a high magnetic field, which are expected to be used for high field applications such as superconducting permanent magnets with liquid nitrogen refrigeration, flywheels, current leads and so on. Mechanical properties such as flexural strength, fracture toughness and ductility are very crucial as well as the superconducting properties: Tc, Jc, and Hirr for industrial applications of high-Tc oxide superconductors. However, oxide superconductors have the intrinsic brittleness of the perovskite structure, thus, the strength and the fracture toughness of REBaCuO superconductors have been reported to be low and anisotropic. Therefore, we should investigate and improve mechanical properties to achieve structural reliability for applications.Large single domain of melt-processed REBaCuO (Dy-123) superconductors with Dy2BaCuO5 (Dy-211) particles and Ag2O of 10wt% was fabricated with a seeding and temperature gradient method in air. In this study, we discuss mechanical properties such as the hardness and the surface roughness, and the flexural strength of the RE-123 bulk, measured at RT. The results of Vickers hardness, surface roughness and the flexural strength showed very important information for evaluating characteristics of RE-123 bulks.

Evaluation of Young’s modulus of RE123 bulk superconductors by three point bending tests

In order to investigate the evaluation method of the Young’s modulus of RE123 single-grain bulk superconductors, 3-point bending tests have been carried out at room and liquid nitrogen temperatures. An attempt to evaluate it from the measurements of deflection of the specimen with 3mm thickness was made. The consistent stiffness of the test machine including the jig was obtained by using a dummy specimen. For the test of the bulk, increasing repeated load within a level less than the fracture load was applied to stabilize the loading condition. The Young’s modulus calculated by using the deflection of the bulk specimen obtained subtracting the machine stiffness coincided with that obtained by the conventional method, where the strain gage was adhered to the specimen. The effects of specimen thickness and the oxygen annealing sequence with respect to specimen cutting were also investigated.

Microstructure and superconducting properties of meltprocessed RE-Ba-Cu-O with dispersed barium cerate particles

We studied microstructure and superconducting properties of melt-processed RE123 (RE = Y and Dy) bulk superconductors with a large amount of BaCeO3 addition. The melttextured bulk samples with compositions of Y123 + xBaCeO3 (x = 0.05-0.4) and 0.5 wt% of Pt were fabricated in air. The BaCeO3 particles with the diameter of 1-2 µm were trapped in the melt-textured samples. The critical current density (Jc) in a low field region increased with the amount of BaCeO3 inclusions. This suggests the BaCeO3 particles act as effective flux pinning centers, as well as Y211 particles in conventional melt-textured Y-Ba-Cu-O bulk materials. We found that the size of BaCeO3 particles could be reduced by substituting Zr on Ce site. The Y123 bulk sample with 30 mol% of Ba(Ce0.75Zr0.25)O3 and 0.5 wt% Pt showed a high Jc value of 110 kA/cm2 at 77 K. The Dy123 bulk samples dispersed BaCeO3 and Ba(Ce0.75Zr0.25)O3 particles also exhibited excellent Jc-B properties.

Temperature measurement of RE123 bulk superconductors on magnetizing process

We study on the magnetization behavior of to magnetize RE123 bulk superconductors to apply it as strong magnets. Through magnetizing process, the temperature of bulk superconductors is raised by pinning loss caused by the magnetic fluxes motion (e.g. flux jump of flux flow), and the trapped field is decreased. This paper presents the measurement of temperature changes of Sm123 bulk superconductors during the exciting process by iteratively magnetizing pulsed-field operation with reducing amplitudes (IMRA) method. Five thermocouples are put on the surface of Sm123 bulk superconductor of 46 mm in diameter. The temperatures at the center, on the growth sector boundary (GSB) line and in the sector region surrounded by GSB's line (inter-GSB region) are monitored. The temperature at a cold stage is also measured. A Hall sensor is attached near the center thermocouple to measure the trapped field. After a bulk superconductor is cooled by the GM type refrigerator until 40 K, iterative pulsed-fields of 2.32–5.42 T are applied by a magnetizing coil. When high magnetic field of 5.42 T is applied, a temperature of bulk superconductor reaches to 72.4 K and the magnetic field distribution has C form with which a part of circle is dented, and then, a trapped field is 2.28 T. When a lower magnetic field of 4.64 T is applied, a maximum temperature is 68.3 K and a trapped field is raised to 2.70 T, and moreover, the distribution becomes round shape like field-cooling method (FC). We showed clearly that heat generation by pinning loss was related to the mechanism of magnetic field capture.

Subgrain structures and their effects on critical current properties in large single-domain RE123 bulk superconductors

We have studied the microstructures of subgrains in large single-domain RE123 (RE=Y, Sm) bulk superconductors through microscopic observation and X-ray pole figure analyses. We have found that misorientation of the subgrains increases along with the crystal growth, and also that the misorientation has positive relation to the size and concentration of the second-phase particles. Such subgrains are considered to affect current carrying capabilities in bulk materials, so that we performed magnetic and electric measurements for several samples having different subgrain structures. The relationships between obtained J c– B or AC transport characteristics and subgrain geometries are discussed.

Subgrains in large-grain RE123 bulk superconductors

We have investigated the microstructures of large-grain RE123 (RE = Y, Sm) bulk superconductors with an emphasis placed on the effects of subgrains on superconducting properties. For the samples grown by slow-cooling, the misorientation angles of the subgrains were larger at the later stage of the growth. For isothermally grown samples, subgrains are likely to form under large undercooling. Based on the results of micro-area x-ray pole figure analyses, the possible mechanism of subgrain formation was concluded to be the accumulation of dislocations formed as a result of entrapment of RE211 inclusions during crystal growth. Both magnetization and transport measurements showed that the subgrains act as weak-links. Hence the amount of subgrains should be minimized for the fabrication of high performance large-grain RE123 materials.

ＲＥ１２３バルク超電導体のサブグレイン構造と超電導特性

We have studied the microstructures of subgrains in large-grain RE123 (RE=Y, Nd, Sm) bulk superconductors through microscopic observation and micro-area x-ray pole figure analyses. We have found that misorientation of the subgrains increases along with the crystal growth, and also that the misorientation has a positive relation to the size and concentration of the second-phase particles. An electron backscatter diffraction technique was successfully utilized for characterizing the subgrain boundaries. We have also performed magnetic and electric measurements for several samples having different subgrain structures. The results of AC transport measurements indicated that the subgrains act as weak links, especially in a high magnetic field region.

超電導バルク材料開発の現状

The critical current density and trapped field of large grain RE123 bulk superconductors have drastically increased in the last few years. High-quality bulk superconducting magnets can trap a very high magnetic field of 3T at 77K and above 15T at 20K. The recent progress in bulk superconducting materials has led to many applications such as superconducting flywheel systems, current leads, superconducting motors, magnetic separation systems, magnetron sputtering machines. In this paper, we review recent research and development of the processing, critical current density, trapped field and mechanical properties of melt-textured bulk superconducting materials.

Comparison of different approaches to modelling the fishtail shape in RE-123 bulk superconductors

We analyse and compare properties of different models proposed so far for modelling the shapes of F( B) and J( B) curves measured in RE-Ba 2Cu 3O 7− δ (RE-123, RE=rare earth) samples. The formulas following from the discussed models allow for a direct fit of experimental data in various representations. The models divide in two categories, according to the pinning potential used, namely the power-law and the logarithmic potential. We show that none of the pinning regimes proposed for low- T c superconductors accounts for the fishtail effect (FE) observed in high- T c materials. The conventional expressions allow a good fit of high- T c experimental data, however, the resulting fitting parameters are much higher than those predicted by the theory. The logarithmic pinning potential justified in high- T c materials by both magnetic and transport experiments leads to F( B) and J( B) functions that decay exponentially at high fields. The fit of experiments on a wide range of RE-123 samples, using only one free parameter, is nearly perfect. The role of free parameters is discussed in all the models.

The seleggtor : Factory Management and Maintenance, 94, 68

We study on the magnetization behavior of to magnetize RE123 bulk superconductors to apply it as strong magnets. Through magnetizing process, the temperature of bulk superconductors is raised by pinning loss caused by the magnetic fluxes motion (e.g. flux jump of flux flow), and the trapped field is decreased. This paper presents the measurement of temperature changes of Sm123 bulk superconductors during the exciting process by iteratively magnetizing pulsed-field operation with reducing amplitudes (IMRA) method. Five thermocouples are put on the surface of Sm123 bulk superconductor of 46 mm in diameter. The temperatures at the center, on the growth sector boundary (GSB) line and in the sector region surrounded by GSB's line (inter-GSB region) are monitored. The temperature at a cold stage is also measured. A Hall sensor is attached near the center thermocouple to measure the trapped field. After a bulk superconductor is cooled by the GM type refrigerator until 40 K, iterative pulsed-fields of 2.32–5.42 T are applied by a magnetizing coil. When high magnetic field of 5.42 T is applied, a temperature of bulk superconductor reaches to 72.4 K and the magnetic field distribution has C form with which a part of circle is dented, and then, a trapped field is 2.28 T. When a lower magnetic field of 4.64 T is applied, a maximum temperature is 68.3 K and a trapped field is raised to 2.70 T, and moreover, the distribution becomes round shape like field-cooling method (FC). We showed clearly that heat generation by pinning loss was related to the mechanism of magnetic field capture.