The current status of large-grained RE-Ba-Cu-O (RE: Y or rare earth elements) bulk superconductors with excellent superconducting properties is described. Gd-Ba-Cu-O bulk superconductors can trap a very high magnetic field even if they are melt-processed in air. Although the electromagnetic force caused by the trapped field is larger for a larger sample and may break the sample, a large sample of Gd-Ba-Cu-O 46 mm in diameter has the potential of trapped magnetic fields greater than 10 T at around 40 K. In addition, single-grained bulk superconductors as large as 150 mm can be obtained using the RE compositional gradient method. Dy-Ba-Cu-O is an ideal material for current leads because it has low thermal conductivity and high critical current density at 77 K in high magnetic fields. Eu-Ba-Cu-O has low magnetic permeability, and is therefore suitable for bulk NMR applications. Progress in machining technology has made possible various bulk superconductors with complicated shapes such as coils, leading to small and strong electromagnets by stacking several coil-shaped bulk superconductors together.
The applications of REBaCuO superconducting bulks (RE: rare earth element or Y) have been investigated recently because of the enhancement of the superconducting characteristics such as critical current density Jc and the trapped field BT. A superconducting bulk can trap higher BT of over 17 T via conventional field-cooled magnetization (FCM) and BT=5.2 T via pulsed field magnetization (PFM), which has been intensively studied because a superconducting magnet is not used. This review article summarizes the magnetizing mechanism of the superconductors, the recent activities of PFM conducted experimentally and numerically, and the practical applications of several superconducting bulk magnet devices.
In this article, the development progress of our GdBaCuO 30-kW-grade bulk HTS motor is presented. Two main features will be described here: the addition of the bulk HTS field poles using magnetic particles, and the amelioration of the condensed-neon cooling system through the addition of a gaseous helium phase. Owing to this new cooling procedure, the cooling time of the motor was reduced by more than 30% and the flux decay following the in-situ magnetization of the bulk HTS was halved. The addition of magnetic particles into the Gd-123 bulks allowed a 1.1- to 1.4-fold trapped flux density and an additional reduction of the flux decay from 7% to approximately 4% after five hours of synchronous operation under an AC field.
Magnetic levitation systems using HTS bulks, such as magnetic bearing devices for flywheel energy storage systems, transporters, linear actuators, magnetic gears and so on, have been developed by many research groups. Most of the HTS magnetic levitation systems are composed of HTS bulks and permanent magnets. "Stable magnetic levitation without any other control system" is a specific characteristic of the HTS bulks. A non-contact spin processor and a magnetic-levitation seismic isolation device are introduced as magnetic levitation systems that are composed of HTS bulks and permanent magnets. Furthermore, an active magnetic levitation system using spherical HTS bulks for inertial nuclear fusion and a three-dimensional HTS actuator are introduced as magnetic levitation systems with HTS bulks and electromagnets. Levitation force, levitation gap and stability are closely related to the size and shape of the HTS bulk, the magnetic field distribution around the HTS bulk, the conditions in the field-cooling process of the HTS bulk and so on. Achieving large levitation force, large levitation gap and highly stable levitation is very important for practical use of the HTS magnetic levitation system. Theoretical analysis using a numerical simulation code for understanding electromagnetic behaviors within the HTS bulk is useful for improving the levitation force, levitation gap and stability. Finally, numerical techniques based on the finite element method are introduced in this paper.
A magnetic drug delivery system (MDDS) has recently been developed enabling magnetic seeded drugs to be navigated around the diseased parts of the human body. To improve the magnetic drug delivery performance, a portable superconducting bulk magnet system with strong magnetic fields has also been developed. This magnet system primarily consists of small bulk high-temperature superconductors and a compact Stirling-cycle cryocooler. The materials used in the high-temperature superconductors are rare earth 123 single domain compounds (GdBaCuO). The main body of the magnet is 9.5 kg and 740 mm in length. In this study, a bulk magnet was successfully activated using field-cooling magnetization under a superconducting solenoid magnet. The magnetic flux densities at the surface of the vacuum chambers that contain the bulk magnets reached 5.07 T and 6.76 T under the static fields of 6 T and 10 T, respectively. We clarified that the magnetic gradient was approximately 10 T/m at a position 50 mm from the surface of the vacuum chambers. This operating bulk magnet system is portable and can be easily transported via car over long distances.
A demagnetized Nd-Fe-B permanent magnet was scanned just above the magnetic pole containing the HTS bulk magnet, generating a magnetic field of 3.27 T. The magnet sample was subsequently found to be fully magnetized in the open space of the static magnetic fields. We examined the magnetic field distributions when the magnetic poles were scanned twice to activate the magnetic plates inversely with various overlap distances between the tracks of the HTS bulk magnet. The magnetic field of the "rewritten" magnet reached the values of the magnetically saturated region of the material, showing steep gradients at the border of each magnetic pole. As a replacement for conventional pulse field magnetizing methods, this technique is proposed to expand the degree of freedom in the design of electromagnetic devices, and is proposed as a novel practical method for magnetizing rare-earth magnets, which have excellent magnetic performance and require intense fields of more than 3 T to be activated.
In this study, we propose a new undulator structure based on a bulk high-temperature superconductor stacked array for insertion devices in synchrotron radiation facilities and free-electron laser devices. A strong periodic magnetic field can be generated in a short period from induced currents in the stacked bulk high-temperature superconductors. This study describes the principle of operation and features of the undulator, and shows the results of generation and control of the periodic magnetic field. We have also developed a numerical model for estimating the performance of the undulator. Excellent performance is expected at approximately 20 K.
Two new approaches to fabricate Y123 melt-solidified bulks with excellent superconducting properties have been developed. One is a new method of crystal growth from a seed crystal with its c-axis parallel to the top surface, and the other is a high-temperature annealing process under a moderately reducing atmosphere after crystal growth. In the present study, the former method increased the area of high-Jc regions, and the latter enhanced Jc, particularly at positions far from the seed crystal at low temperatures through suppression of yttrium substitution for barium. The field-trapping property was also improved through introduction of a reductive annealing process prior to oxygen annealing at low temperatures.
In order to develop a compact cryogen-free 4.7-T (200 MHz) NMR magnet, we investigated methods of obtaining a homogeneous magnetic field distribution inside REBCO bulks. In this study we successfully fabricated large single-domain annular GdBCO bulk superconductors 60 mm in diameter and up to 60 mm thick. We also fabricated a large EuBCO bulk 60 mm in diameter, and evaluated the performance of these annular bulks in terms of the NMR magnet. It was found that enhancing Jc-B, increasing bulk thickness and employing an Eu element showing low magnetic permeability were effective in increasing the homogeneity of the magnetic field distribution. Furthermore, it was confirmed that a homogeneous area of magnetic distribution was obtained even at a higher field magnetization of 7 T (300 MHz) at 40 K for stacked GdBCO bulk superconductors 60 mm in diameter and 60 mm thick. Finally, several issues relating to further increasing the trapped magnetic field were discussed.
A compact cryogen-free NMR magnet was developed using annular bulk superconductors. The superconducting bulk magnet is energized using a field-cooling (FC) method operated at 4.7 T. Two magnets with different configurations are proposed and investigated. One magnet consists of three GdBaCuO and two SmBaCuO bulks with outer diameters (OD) of 60 mm, inner diameters (ID) of 16 mm and a total height of 60 mm. The GdBaCuO bulks that have higher relative magnetic permeability (1.01) are sandwiched between the SmBaCuO bulks that have lower permeability (1.0003). The other magnet consists of four EuBaCuO bulks with a permeability of 1.001, OD of 60 mm, ID of 16 mm and height of 70 mm. Using these magnets, we observed a chemical shift of toluene 1H NMR spectra and confirmed that the bulk magnets generated a magnetic field with homogeneity less than 0.5 ppm from the full width at half the maximum (FWHM) of a methyl proton signal of toluene. Magnetic field stability of less than 0.02 μT/hour was achieved in the EuBaCuO bulk magnet.
Cryogenic slush fluids such as slush hydrogen and slush nitrogen are two-phase, single-component fluids containing solid particles in a liquid. Since their density and refrigerant capacity are greater than those of liquid-state fluid alone, there are high expectations for the use of slush fluids in various applications such as clean-energy fuels, fuels for spacecraft to improve the efficiency of transportation and storage, and as refrigerants for high-temperature superconducting equipment. Experimental tests were performed using slush nitrogen to obtain the flow and heat-transfer characteristics in two different types of horizontal circular pipes with inner diameters of 10 and 15 mm. One of the primary objectives for the study was to investigate the effect of pipe diameter on the pressure-drop reduction and heat-transfer deterioration of slush nitrogen according to changes in the pipe flow velocity, solid fraction and heat flux. In the case of an inner diameter of 15 mm, pressure drop was reduced and heat-transfer characteristics deteriorated when the pipe flow velocity was higher than 3.6 m/s. On the other hand, in the case of an inner diameter of 10 mm, pressure drop was reduced and heat-transfer characteristics deteriorated when the pipe flow velocity was higher than 2.0 m/s. From these results, it can be seen that a larger pipe diameter produces a higher onset velocity for reducing pressure drop and deteriorating heat-transfer characteristics. Furthermore, based on observations using a high-speed video camera, it was confirmed that pressure drop was reduced and heat-transfer characteristics deteriorated when the solid particles migrated to the center of the pipe and the flow pattern of the solid particles inside the pipe was pseudo-homogeneous.