Shogo Hara

Trapped Flux and Levitation Properties of Multiseeded YBCO Bulks for HTS Magnetic Device Applications—Part I: Grain and Current Features

The strongly connected or coupled grain boundaries (GBs) between adjacent grains and their macroembodiment as flowing intergrain supercurrents crossing the GBs inside multi- seeded bulk high-temperature superconductors were elucidated by trapped-flux evaluation. Trapped-fleld measurements, after cutting and polishing two multiseeded YBCO bulk samples, were conducted to present the existence of coupled GBs and their distribution along the c-axis growth. The intensive trapped-flux density observed near the GB areas inside the whole sample is direct evidence of a strongly connected or coupled GB. The relatively strong trapped flux near the GB areas and the significant improvement of the total trapped flux compared with the isolated single-grain bulks were ascribed to the intergrain supercurrent flowing across the GBs in large macroscopic loops with coordination of the intragrain supercurrent circulating in each grain of the multiseeded bulk. Based on the experimental results, a simplified simulation model that incorporates two forms of the intra- and inter supercurrents flowing inside the multiseeded bulk is introduced to reproduce the trapped-flux density features, and qualitative agreement is obtained by comparison with the experimental ones.

MgO buffer-layer-induced texture growth of RE–Ba–Cu–O bulk

A novel cold-seeding approach of top-seeded melt growth of RE–Ba–Cu–O bulk was studied by employing a MgO crystal seed and a buffer pellet. The growth process is divided into two steps. The MgO seed is used to texture-grow the small RE–Ba–Cu–O pellet with high melting point (Tp) and the textured pellet induces the texture growth of the bulk at lower temperature. Undercooling and RE211 content of the pellet are adjusted to avoid the misorientation caused by lattice mismatch between MgO and the RE–Ba–Cu–O matrix. Bulk samples prepared with this method show good growth sections and superconducting performance. One of the promising advantages of this method is to process high Tp RE–Ba–Cu–O bulks with a cold-seeding method, for example Nd–Ba–Cu–O bulk.

Flux pinning properties of Gd–Ba–Cu–O trapped field magnets grown by a modified top-seeded melt growth

We report a modified top-seeded melt-growth technology for processing high performance Gd–Ba–Cu–O trapped field magnets (TFMs) by providing an additional liquid source during the melt-textured growth. The enriched liquid source allows sufficient growth in the a−b plane with an increased growth rate, and depresses the accumulation of Gd2BaCuO5 (Gd211) particles, which largely suppresses the formation of sub-grain boundaries in the a-growth sectors. An average trapped magnet field of 1.2 T is achieved for the 32 mm Gd–Ba–Cu–O TFMs at 77 K. These Gd–Bd–Cu–O TFMs exhibit superior in-field flux trapping performance, thanks to the significant second peak effect of the critical current density (Jc). The additional liquid source intensifies the Gd–Ba substitution, and therefore increases the in-field Jc, especially at the temperature range of 30–70 K, which is meaningful for practical applications.

Enhancement of the Critical Current Densities and Trapped Flux of Gd-Ba-Cu-O Bulk HTS Doped With Magnetic Particles

For bulk HTS rotating machines, enhancement of the trapped flux is a crucial task to achieve practical applications with high torque density. The increase of critical current density Jc using artificial pinning centers is an efficient technique for the enhancement of the flux trapping properties. We attempted to enhance both Jc and the trapped flux in bulk HTS with magnetic/ferromagnetic particles additions. Fe-B-Si-Nb-Cr-Cu amorphous alloy, Fe2O3 and CoO particles were introduced into the Gd123 matrix. The melt growth of the single-domain bulks with different magnetic particles was performed in air. Enhancement of the critical current density Jc at 77 K was derived in the bulks with Fe2O3 and Fe-B-Si-Nb-Cr-Cu additions, while the superconducting transition temperature of 93 K was not degraded significantly. The experiment of the magnetic flux trapping was then conducted under static magnetic field magnetization with liquid nitrogen cooling. In the bulk with 0.4 mol% of Fe-B-Si-Nb-Cr-Cu, the integrated trapped flux exceeds over 35% compared to the one without magnetic particle addition. On the other hand, the addition of CoO particles resulted in a reduction of both Jc and trapped magnetic flux. Present results indicate that the introduction of magnetic particles gives a significant effect to the flux pinning performance.

Process Technology and Superconducting Properties of Bulk HTS With Multi-RE Elements

A series of REBa₂Cu₃O_y (RE123)/Ag bulk superconductors with multi-RE elements at the RE site, were prepared by top-seeded melt growth. By employing the differential thermal analysis, we carefully adjusted the peritectic decomposition temperature (<i>T</i>_p) by changing the ratio of the two RE123 compositions to meet the requirements of the cold-seeding process. The superconducting properties, <i>T</i>_c, <i>J</i>_c and the flux trapping performance were evaluated after achieving the growth of the single grains. The microstructure was also observed. A relatively low growth temperature range depressed the LRE-Ba substitution, which was effective to suppress the probable RE211 coarsening. By employing ZrO₂ chemical doping, the flux pinning property was further improved under both zero-field and in-field, and the <i>J</i>_c was enhanced without reducing <i>T</i>_c.

Enhanced Flux Pinning and Microstructural Study of Single-Domain Gd-Ba-Cu-O Bulk Superconductors With the Addition of Fe-Containing Alloy Particles

Introducing secondary-phase inclusions, especially magnetic particles, into the superconducting matrix is regard as an effective way to enhance the performance of high-temperature superconductors. The effect of Fe-B alloy additives on the microstructure and superconducting properties of top-seeded melt growth processed Gd-Ba-Cu-O bulk superconductors has been investigated systematically. Gd-Ba-Cu-O bulks 17 mm in diameter were melt-textured for various amounts of Fe-B additives in the range of 0-1.4 mol%. With the observations of scanning and transmission electron microscopy, the Fe-B alloy additives were found to decompose into Fe-containing particles with submicrometer in diameter. The refinement and homogeneous distribution of Gd2BaCuO5 inclusions seems to be improved remarkably. It leads to enhance flux pinning, flux trapping capability, and critical current density of Gd-Ba-Cu-O bulks, correspondingly. This phenomenon might be related to the magnetic interactions between the Fe-containing particles and the surrounding components, which may be the origin of the improvement of Gd2BaCuO5 nucleation process.

Doping Effect of Fe-Ni Alloy Particles on Flux Pinning in Gd-Ba-Cu-O Bulk Superconductor

Enhancement of the critical current density (<i>J</i>_C) and trapped flux density (<i>B</i>_T) of bulk high-temperature superconductor are essential for large-scale applications. Introducing artificial pinning centers into the superconducting matrix has been proved to be an effective way to enhance <i>J</i>_C and <i>B</i>_T. It has been suggested that the ferromagnetic particles doping may provide higher pinning potential. The doping effects of Fe-B soft magnetic particles for GdBa₂Cu₃O_{7 - σ} (Gd123) bulk were reported previously. In the present work, we reported the improvements in both <i>J</i>_C and <i>B</i>_T of Gd123 bulk doped by Fe-Ni alloy particles. To make sure a uniform distribution of the dopant, Fe-Ni alloy particles, with amount varied from 0 to 20 mol%, were initially added to prepare Gd123 precursor powders for the bulk processing. X-ray diffraction showed a tetragonal-orthorhombic transition of Gd123 crystal structure with increasing doping amount. The in-field <i>J</i>_C was greatly enhanced by the Fe-Ni particles doping and the optimal doping amount was found to be 0.5 mol%. The microstructure of the bulk samples were also investigated by scanning electron microscopy. The great improvement of trapped flux by Fe-Ni doping was considered to be benefit for practical applications.

NiFe alloy particles doping effect of Gd–Ba–Cu–O bulks processed by a new cold-seeding technology

Abstract The process of cold seeding melt growth of GdBa2Cu3Oy (Gd123) bulk superconductors using NdBa2Cu3Oy (Nd123) thin films was reported. In addition, a novel cold seeding concept of combining MgO crystal and buffer pellet was also introduced. The misorientation caused by the lattice mismatch between MgO and Gd123 melt was overcome by choosing suitable heat treatment program and Gd2BaCuO5 (Gd211) content of the buffer pellet. The doping effect of soft ferromagnetic NiFe alloy particles was also reported. The bulk sample with 0.4% (mole fraction) doping amount shows the best performance on the flux trapping. The critical current density is largely enhanced under the external field of 1–2 T, which is promising for large-scale applications. This effect is originated from the substitution of Fe and Ni ions for the Cu sites contributing to magnetic flux pinning.

The Effect of Recrystallization of Fine Inclusions on the Enhancement of Flux Pinning in Textured Gd-Ba-Cu-O Bulk Superconductors

Well textured GdBa2Cu3O7-δ single domain bulks have been prepared successfully. Based on these samples, dilute impurities doping for the enhancement of flux pinning has been investigated. To achieve the fine inclusion particles, except ball milling process, a possible recrystallization mechanism is proposed. BaTiO3 powder was synthesized as dopant. BaO2 doping and mixed BaO2 and TiO2 doping were performed as references. The recrystallization mechanism was studied by means of the evolution of flux pinning behaviors along the growth direction of the successfully textured single domain bulks. BaTiO3 retains a semi-melting state in the harsh ambient of the melting process. The state was assumed like a colloidal droplet, which can be divided into more than one smaller fragment by the anisotropic growth of matrix. This hypothesis was studied in the sample, which was grown at a lower initial growth temperature. The evolution of flux pinning behaviors along the c-axis shows a remarkable increase of the critical current density under low field together with the decreased peak effect. The increased low-field pinning strength is partially attributed to the recrystallization of the BaTiO3 doping related fine precipitations.