The project conducted by NEDO for developing a high-temperature superconducting flywheel energy storage system is introduced; the two test results of fundamental studies are described. One is the measurement of levitation force and rotation loss of superconducting magnetic bearings composed of oxide superconducting bulks and permanent magnet composite. Two types of superconducting magnetic bearings, axial and radial types, were fabricated and tested. The other test was the fabrication and testing of two functional models. A small-sized superconducting flywheel model of the 0.5kWh class was fabricated and tested. A medium-sized rotating functional model of the 10kWh class was fabricated as well.
Superconducting magnets made of high-Tc superconductors are promising for industrial applications. It is well known that REBa2Cu3O7-x superconductors prepared by melt processes have a high critical current density, Jc, at 77K and high magnetic fields. The materials are very prospective for high magnetic field application as a superconducting permanent/bulk magnet with liquid-nitrogen refrigeration. LREBaCuO bulks, compared with REBaCuO bulks, exhibit a larger Jc in high magnetic fields and a much improved irreversibility field, Hirr, at 77K. In this study, we discuss the possibility and trapped field properties of a superconducting bulk magnet, as well as the melt processing for bulk superconductors and their characteristic superconducting and mechanical properties. One of the applications is a superconducting bulk magnet for future magnetically levitated (Maglev) trains.
Irradiation of superconducting material with particles, such as electrons, protons, neutrons, ions etc., is known as a valuable tool to introduce pinning centers in a controlled way. Due to its extremely large penetration range, neutron irradiation is thought to be most suitable for the production of randomly distributed defects in bulk materials. Fast neutrons produce spherical cascade defects with a diameter of approximately 5-6nm. The trapped field of a Y-Ba-Cu-O bulk sample was found to be increased up to 3.7T at 77K by fast neutron irradiation, which suggests the practical importance of this technique. On the other hand, by using thermal neutron irradiation, we can create fission tracks in the sample doped with uranium, which causes a dramatic enhancement of Jc.
Large single-grain bulk rare earth element (RE)-Ba-Cu-O superconductors can be used for various applications such as magnetic bearings, load transport and trapped field magnets. The magnetic field generated by bulk RE-Ba-Cu-O superconductors is proportional to its radius, however, the growth of a large single-grain bulk with good quality is difficult due to contamination from the substrate or the crucible and also due to liquid loss. Such problems can be solved by growing RE-Ba-Cu-O bulk in a microgravity environment, where the bulk can be supported by a seed crystal alone during crystal growth. Such experiments will be conducted in the Unmanned Space Experiment Recovery System (USERS) project. In this paper, the experiment plan and the present status of the system development are reported.
The current status of large-diameter Y-Ba-Cu-O QMG® bulk superconductors is described. 75mm-diameter QMG® samples with fairly concentric trapped-magnetic-flux-density distributions have been successfully grown. In addition, a record-high value of the magnetic levitation force, 171kgf, measured at 77K using a 90mm-diameter Nd-Fe-B magnet has been obtained for a 100mm-diameter sample. The crystal orientation of subgrains in the sample was determined using Electron Back-Scattering Diffraction as well as using a conventional optical apparatus including a He-Ne laser. For the sample grown under normal process conditions, the maximum value of the -axis misorientation angles at the subgrain-boundaries we obtained in the c-substructure region is 2.9°. However, in the a/b-substructure regions, the misorientation angles are larger, and the maximum value obtained is 6.6°, exceeding the critical angle at which a significant reduction of the critical current density occurs in Y123 thin films due to the weak-link problem.
We have studied the quasi-binary phase diagram along the line of NdBa2Cu3Oy and BaCuO2 in 1%O2+Ar. In this study, the nucleation and melt of NdBa2Cu3Oy in Ba-rich solvent were observed in situ using a laser-scanning microscope under 1%O2+Ar. The liquidus and solidus lines of nearly stoichiometric Nd123 were constructed. Phase identification has been performed on the samples quenched from the reaction temperatures determined by differential thermal analyses through X-ray powder diffraction measurements and scanning electron microscopy.
Single-grained bulk superconductors of Gd-Ba-Cu-O 32mm in diameter were grown along the c-axis by melt processing in air. In addition to the stoichiometric 123 composition, the Ba-rich starting compositions of Gd/Ba ratio were also investigated. For comparison, a sample was fabricated in a reduced oxygen atmosphere. In contrast to Sm123 and Nd123, the properties of Gd123 bulk superconductors were not degraded even after fabrication in air. Gd0.95Ba2.05Cu3Oy crystal grown along the c-axis in air showed a Tc of about 93K and Jc of about 3×108(A/m2) at 77K and low applied field. The trapped magnetic field of this sample exhibited about 0.6T at 77K.
Sm123 precursors with various RE211 (RE=Y, Gd and Sm) and Nd422 were melt-textured in a reduced oxygen atmosphere. We have studied the relationship between the size of the 211 phase and critical current density (Jc). When Y211 and Gd211 were added to a starting material, the size of the 211 phase in the bulk sample was remarkably reduced. As a result, the Jc value of these samples exceeded that of Sm-Ba-Cu-O bulk with Sm211 particles. A large c-axis-oriented bulk sample with a 32mm diameter was also fabricated from Sm123-Gd211 precursor with 20wt% Ag2O addition. The trapped magnetic field of this bulk exhibited 1.0T at 77K. This value was higher than that of an Ag doped Sm-Ba-Cu-O bulk of similar size.
Unlike Y-Ba-Cu-O, the Nd-Ba-Cu-O system forms a Nd1+xBa2-xCu3O7-δ-type solid solution (Nd123ss). An understanding of PO2 and temperature dependence of the solubility limit will be the key to obtaining high Tc and Jc Nd123ss materials. Therefore, we carefully studied the solubility limit of Nd123ss as a function of PO2 and temperature. Through X-ray diffraction and differential thermal analysis measurements, we have found that Ba can substitute for Nd site under low PO2 at high temperatures. Furthermore, DC magnetization measurements showed that the superconducting transition temperature of Ba-rich Nd123ss (x<0) is higher than that of Nd rich Nd123ss (x≥0).
We systematically investigated the peritectic decomposition temperatures (Tp) of (Sm1-xGdx) Ba2Cu3Oy solid solutions and superconducting properties (Tc and Jc-B) of binary mixed (Sm1-xGdx)-Ba-Cu-O bulk superconductors melt-processed in a 1%O2-Ar atmosphere. The Tp of (Sm1-xGdx) 123 linearly decreases from 1, 023 to 1, 003°C with increasing x (i.e., the amount of Gd substitution on Sm). The cooling rate of x=0 and 1 needed to fabricate a single-domain bulk is higher than that of x=0.25, 0.50 and 0.75. Although the Tc (93.5-94K) of melt-textured (Sm1-xGdx) BCO is independent of x and the Pt dopant, Jc values increase with Pt doping since the (Sm1-xGdx)2Ba1Cu1Oy is refined.
We have studied the mechanical properties of melt-processed single-grain Sm-Ba-Cu-O bulks with and without silver doping. The tensile strength of the silver-doped sample was 37.4MPa and about 1.5 times larger than that of the undoped sample. Dimension effects can be estimated from the Weibull distribution function and the effective volume of the samples. It was shown that the strengths, which were obtained in the samples with different dimensions or with different measurement methods like bending strength, can be converted into relative strengths by considering this effect. The bending strength of the silver-doped sample was estimated to be 70-120MPa.
Large single-grain bulk rare earth element (RE)-Ba-Cu-O superconductors can trap large fields exceeding several teslas and thus can function as very strong quasi-permanent magnets. however, the maximum trapped field is essentially limited by the mechanical strength of the bulk superconductors. The stress produced by refrigeration sometimes causes cracking. A large electromagnetic force also acts on superconductors when they trap large magnetic fields, and this occasionally leads to device failure. We have recently found that epoxy resin can penetrate into bulk superconductors under certain conditions. Microstructural observation revealed that microcracks as well as porosities can be impregnated with epoxy resin, which greatly improves the mechanical properties of bulk RE-Ba-Cu-O and thus results in the improvement of field trapping capability.
Thermal conductivities of OCMG-processed (Nd, Eu, Gd)Ba2Cu3Oy [NEG123] bulk samples containing 10, 20 and 30mol% second phase [SP:NEG211:(Nd, Eu, Gd)2BaCuO5] have been studied between 10-20K, and 100-150K and up to 14T. Thermal conductivity κ of the sample with SP 10mol% shows a clear enhancement below Tc, which is a common feature of high-Tc oxide superconductors. The enhancement becomes smaller as the SP amount increases. The magnitude of the thermal conductivity decreases with the increase in magnetic field, but the magnetic field dependence is very small in all samples. There is a strong correlation between κ(peak)/κ(100K) and the average SP size. It turns out from these results that phonon thermal conductivity is dominated in these samples, and is strongly affected from the scattering by SP.
The flux pinning properties of Nd4Ba2Cu2O10 (Nd422) inclusions in Nd-Ba-Cu-O bulk superconductors has been studied. The temperature and field dependence of critical current density as well as magnetization relaxation have been studied for various Nd-Ba-Cu-O samples. Nd422 inclusions are effective not only in increasing critical current density in a low-field region, but also in enhancing the irreversibility field (Birr) and reducing flux creep even in relatively high fields.
The levitation of bulk Y-Ba-Cu-O superconductors can be controlled using a Bi-Sr-Ca-Cu-O (Bi2223) superconducting electromagnet. It was found that stable levitation without tilting could be obtained only when the sample trapped a certain amount of fields, the minimum of which depended on the external field and sample dimensions. We employed a novel analysis method for levitation based on the total energy balance, which is much simpler than the force method and could be applied to understanding general levitation behavior. Numerical analyses thus developed showed that stable levitation of superconductors with large dimensions can only be achieved when the induced currents can flow with three-dimensional freedom.
This paper describes a new “electromalev” system, in which multiple Y-Ba-Cu-O bulk superconductors are levitated on a DC magnet. In this system, we have succeeded in controlling the levitation height and force like the case of a single bulk system. We also simulated the total levitation forces for multiple bulks, which were in good agreement with the experimental results.
Strong magnetic fields can be generated for magnetization of the high-Tc superconductor (HTS) with pulsed magnetic fields using a small exciting coil. In the present study, magnetization using the pulsed magnetic field is performed on HTS bulk samples in the zero-field cooled state. In order to saturate the magnetization, it was necessary to optimize the time constant of the magnetic flux invasion and the flux flow during the increasing and decreasing processes of the pulsed magnetic field. Therefore, we measured the magnetic flux density on the pulsed field magnetization process and explained the relation between the pulsed magnetic field and the time constant of the magnetic flux invasion and flux flow. An empirical model of the pulsed field magnetization process was also proposed and optimum conditions were studied for pulsed field magnetization of the HTS. From this study, it was confirmed that an optimum pulsed waveform existed for pulsed field magnetization.
A superconducting permanent magnet system capable of generating magnetic fields up to 2T has been constructed by using a c-axis-oriented Sm-based large bulk superconductor fabricated as a trapped-field magnet through the melt processing. A high magnetic field has been produced by using the pulsed-field magnetization (PFM) technique at temperatures below 80K, attained by a cryocooler and vacuum pumps. Detailed analysis of the motion of magnetic fluxes caused by the PFM revealed that the method of iteratively magnetizing pulsed-field operation with reducing amplitude (IMRA) is very effective and inevitable for magnetizing high Jc bulk superconductors at temperatures near 30K.
One of the applications of a bulk magnet in which flux is trapped in oxide superconducting bulks is a superconducting magnet for Maglev systems. We are investigating the possibility of using bulk magnets in Maglev systems. It is believed that bulk magnets far Maglev systems must be composed of superconducting bulks arranged in rows and columns for every pole, because a superconductor made of melt-processed Y-Ba-Cu-O with a high critical current density is smaller in size than the pole. Therefore, it is necessary to examine the flux-trapping characteristics of superconducting bulks in an array. Flux-trapping experiments are performed using melt-processed Y-Ba-Cu-O superconducting bulks arranged in rows and columns, in which each bulk has a magnetizing coil (an individual magnetization method) or all bulks have one magnetizing coil centered at outer bulks in an array (unified magnetization method), and the bulks are magnetized by field cooling. The magnetic field on the superconducting bulks arranged in an array is smaller than that of the single bulk arrangement because the magnetic field generated by the surrounding superconducting bulks acts to weaken the field of the surrounded suerconducting bulk. Furthermore, in the case of the individual magnetization method, the magnetic field applied is decreased during field cooling so that the trapped flux is smaller too. The rate of decrease in the individual magnetization method is larger than the rate of decrease in the unified magnetization method. It is clear that the unified magnetization method is more suitable than individual magnetization for magnetizing bulk magnets for aglev systems, which require a high magnetic field and large-scale system.
The electromagnetic force (EMF) that is exhibited by the interaction between bulk superconductors and permanent magnets is used for magnetic levitation devices. To facilitate such applications, it is necessary to enhance the EMF. We measured the EMF for Y-Ba-Cu-O superconductors with different shapes and dimensions by varying the structure of disk magnets such as their thickness and the number of poles. The EMF is strongly dependent on the magnet structure as long as permanent magnets and good quality bulk superconductors are used. Thus, for the design of practical devices, it is important to optimize the magnet structure.
In order to clarify the rotation loss and levitation force reduction characteristics of two kinds of radial and axial-type superconducting magnetic bearings (SMB) consisting of a ring-shaped YBCO and a permanent magnet composite, we measured rotation losses and levitation forces of each SMB with a new rotation-loss measuring device using active magnetic bearings. The rotation loss of the SMB increased with increased initial load of the SMB. The levitation force o the SMB decreased remarkably just after activating the initial load to the SMB and during acceleration of the rotor suspended by the SMB. The reduction in levitation force was improved by means of applying a pre-load, that means a temporary load, before the initial load against the SMB. But the rotation loss of the SMB was increased as pre-load was increased. When the YBCO was cooled down from 77 to 66K, the rotation loss of the SMB decreased as the temperature of the SC decreased.
In the design of a large-scale flywheel, load capacity and bearing constants (i.e. spring and damping constants) should be accurately calculated. In this resport, a newly developed analysis method for radial-type superconducting magnetic bearings (SMBs) composed of several couples of magnet rings and magnetic material spacers is described. The analysis method is based both on electromagnetic FEM of the magnetic field and the 2-dimensional Bean model for analysis of the magnetization of type-II superconductors. To odtain accurate magnetization hysteresis that reflects the complex magnetic fields, a superconductor is meshed into cells and then the electromagnetic force between the magnetic fields of magnets and the magnetization of the superconductor are calculated. Recently, computer programs which can calculate the axial load capacity of radial-type SMBs have been developed. Furthermore, programs which can calculate bearing constants are close to being completed. Calculated results on axial load capacity showed good agreement with the experimental results.
The V-I properties of YBCO bulk superconductor (QMG®: YBa2Cu3O7-x single-crystal containing Y2BaCuO5 particles) were measured using a single rectangular pulse current at 77 and 87K to obtain basic data for super-normal transition. Because of its high Jc property, QMG is a promising bulk material for resistive-type HTC-FCL application. I-shaped QMG rods having cross sections of 0.5×1.0mm were prepared to measure voltage due to pulse current. The plots of ln V vs ln I yielded a straight line with a gradient ranging from 5 to 7. By changing the amplitude and length of pulses, the time from the start of sending electricity till the occurrence of quench was obtained for various amplitudes of pulse current. In this experiment, it is considered that quenching did not occur in the whole rod but only in a small part of it.