Keita Tsuzuki

Flux pinning properties in a GdBa2Cu3O7−δ bulk superconductor with the addition of magnetic alloy particles

Pinning stability and the introduction of pinning centers are essential problems in the application of high temperature superconductors. A study of the role of the addition of a variety of oxides into Gd-123 bulk superconductors was pursued with respect to the effect of magnetic pinning centers on enhancement of critical current density. We have found the addition of magnetic alloy particles into Gd-123 bulk contributes to the enhancement of the critical current density under a magnetic field. The magnetic particles of Fe–B alloy with small amounts of Cu–Nb–Si–Cr were added into the Gd123 matrix and grown into single-domain bulk in air. The effects of Fe–B alloy magnetic particles on the superconducting pinning properties of Gd123 bulk are reported. The experimental results showed that the Tc of the bulk was suppressed significantly and the transition width became larger with an increase in the doping content of the magnetic particles. This was consistent with magnetic impurities suppressing superconductivity. The critical current density deduced from the magnetization as a function of applied external magnetic field was enhanced under low magnetic field and high magnetic field, which is different from the bulk doped with paramagnetic particles. Possible mechanisms and the role of magnetic materials from the viewpoint of increased flux pinning inside the bulk are discussed.

Trapped Flux and Levitation Properties of Multiseeded YBCO Bulks for HTS Magnetic Device Applications—Part II: Practical and Achievable Performance

After clarifying the essential characteristics of the multiseeded YBCO bulk in the previous work, that is, the existence of strongly connected or coupled grain boundaries between two grains by means of intergrain super current with a big flowing loop size, this paper proceeds to report its achievable performance in actual excitation conditions by working as trapped-field magnets or levitation devices. Besides evaluation of the trapped-flux characteristics by two most popular excitation methods, namely, static field-cooling magnetization (FCM) and pulsed-field magnetization in liquid nitrogen (LN2), the trapped-flux capability at lower temperatures up to 30 K, an interesting temperature for present superconducting machine applications, is also reported. From LN2 temperature to 30 K, the trapped flux with over six times improvement from 0.64 to 3.91 T was observed by FCM, at the same time along with a much decreased relaxation rate from 13.7% to 0.9% in 1-h measurements. In the aspect of levitation applications, the magnetic force density (both levitation force and guidance force, two most important performance parameters for maglev) above a permanent magnet guideway with Halbach style is measured and found to be able to promote the loading and stability performance of the first high-temperature superconducting maglev test vehicle simultaneously. As the multiseeding technique of the top-seeded melt-growth process has provided a promising way to fabricate bulk superconductors with large size, time saving and cost reduction, the presented trapped-flux and magnetic force results may help to comprehend, refer, and finally make better use of the multiseeded bulks in the large-scale superconducting magnetic devices.

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.

Development of a Cryogenic Helium-Neon Gas Mixture Cooling System for Use in a Gd-Bulk HTS Synchronous Motor

Temperature Superconductors (HTS) applied to rotating machines require an efficient cooling system. It is necessary to increase the maximum trapped flux density in the bulk HTS magnets and decrease the overall cooling time. In this paper, we added a gaseous helium phase to a condensed-neon closed-cycle thermosyphon. The latent heat of neon-film cooling is combined with heliums high thermal conductivity. Different mixture proportions were evaluated in terms of resistance to variable heat loads. More helium decreased the temperature variation of the evaporator. The mixture was then used to cool down a 30 kW-grade gadolinium-bulk HTS synchronous motor. The eight bulk HTS conductors of the rotor were cooled to 40 K in less than six hours. The application of this thermosyphon is envisioned for larger rotating machines.

Influence of AC Magnetic Field on a Rotating Machine With Gd-Bulk HTS Field-Pole Magnets

A synchronous rotation study with eight field rotor pole of Gd123 bulks was performed with the axial-type motor. The field pole of Gd123 packs with and without doped amorphous magnetic particles Fe-B-Si-Nb-Cr-Cu (MP) were employed. Before and after the synchronous operation, the change of the trapped magnetic flux on the bulk was investigated at the operating temperature of 40 K. The observed decay of the trapped integrated flux on the surface of Gd123 packs after 5 hours synchronous rotation were 7.2% for the Gd123 pack without MP doping and 4.1% for the Gd123 bulk doped with MP. The present result indicates that the lowering operation temperature may be beneficial to avoid the AC loss. This also leads to another complementally conclusion that the employment of the condensed neon gas cooling system is a suitable choice from the viewpoint of practical applications of the HTS bulks in contrast to liquid nitrogen cooling. In addition, the Gd123 doped with MP alloy particles provide a practical magnetic flux trapping function at 40 K.

Optimization of a condensed-neon cooling system for a HTS synchronous motor with Gd-bulk HTS field-pole magnets

The axial-gap synchronous machine developed in our laboratory is based on Gd-bulk HTS field-pole magnets, able to trap a part of the magnetic flux they are submitted to when cooled down below Tc. At the liquid nitrogen temperature, by the Pulsed-Field Magnetization (PFM), 1.04 T was trapped in 60 mm-diameter and 20 mm-thickness magnets, leading to an output power of the motor of 10 kW at 720 rpm. To enhance this performance, we have to increase the total amount of trapped flux in the bulk, the shortest way being to decrease the temperature of the bulk HTS. Thus, we focused on the improvement of the condensed-neon cooling system, a closed-cycle thermosyphon, so that it provided enough cooling power to lead the rotor plate enclosing the magnets to a low temperature. The present study implied coming out with a new fin-oriented design of the condensation chamber; hence, the numeric calculations and FEM software (ANSYS) heat transfer simulations were conducted for various shapes and positions of the fins. The trapezoidal design offering the best efficiency was then manufactured for testing in a heat-load test configuration, leading to cooling times divided by three and a maximum heat load endured of 55 W.

Pulsed Field Magnetization Properties of Bulk RE-Ba-Cu-O as Pole-Field Magnets for HTS Rotating Machines

Aiming at exploiting the high trapped field ability of bulk high-temperature RE-Ba-Cu-O (RE: rare-earth elements) superconductors, a brushless axial-gap type superconducting synchronous motor has been successfully developed as a technology demonstrator using eight melt-textured GdBa2Cu3Oy (Gd-123) bulks as pole-field magnets. An output power of 10 kW at 720 rpm was realized by an average trapped field peak of 0.56 T reaching the armature coils, under a pulsed field excitation at the liquid nitrogen temperature. For such motors, to improve the trapped flux of bulks is a direct way to enhance the motor performance. In order to obtain a high trapped flux density with a regular shape for a high performance motor utilization, the pulsed field magnetization (PFM) properties of two kinds of bulk superconductors have been investigated. By a reasonable multi-PFM process with a pair of vortex-type pulsed coils, it was found that the multi-seed Y-Ba-Cu-O bulk can bring a high average trapped field, while the single-grain Gd-Ba-Cu-O bulk under a size-adjustable pulsed coil can obtain a very regular trapped field distribution. These results will be useful to the following optimization of the rotating machine system.

A 100-W grade closed-cycle thermosyphon cooling system used in HTS rotating machines

The cooling systems used for rotating High-Temperature Superconducting (HTS) machines need a cooling power high enough to ensure a low temperature during various utilization states. Radiation, torque tube or current leads represent hundreds of watts of invasive heat. The architecture also has to allow the rotation of the refrigerant. In this paper, a free-convection thermosyphon using two Gifford-McMahon (GM) cryocoolers is presented. The cryogen is mainly neon but helium can be added for an increase of the heat transfer coefficient. The design of the heat exchangers was first optimized with FEM thermal analysis. After manufacture, they were assembled for preliminary experiments and the necessity of annealing was studied for the copper parts. A single evaporator was installed to evaluate the thermal properties of such a heat syphon. The maximum bearable static heat load was also investigated, but was not reached even at 150 W of load. Finally, this cooling system was tested in the cooling down of a 100-kW...

Helium-Neon Gas Mixture Thermosyphon Cooling and Stability for Large Scale HTS Synchronous Motors

Commercial high-temperature superconducting (HTS) wires restrain us to an operation temperature ranging from 20 to 40 K for field-pole magnets applied to rotating machinery. We have proposed to employ a mixture of helium and neon gas in a closed-cycle thermosyphon based on a GM cryo-refrigerator. In this paper, we discuss the temperature stability of the evaporator created by a helium-neon mixture cooling coupled with a closed-cycle thermosyphon, upon an external heat load on a 100-kW-grade HTS synchronous machine. A home-made cooling system, including the condenser, was designed. The evaporator was assembled within the motor in accordance with thermal and mechanical analysis. The cooling of the evaporator with the 100-kW-grade HTS synchronous motor was applied against a variable heat load equivalent with the actual HTS rotor field poles. We report these results and propose its potential application to a large-scale ship propulsion motor.