Yousuke Yanagi

Melt-processed Sm - Ba - Cu - O superconductors trapping strong magnetic field

Large c-axis-oriented Sm - Ba - Cu - O bulk superconductors were prepared using the melt-process technique in a reduced oxygen atmosphere. Single-domain samples up to 36 mm in diameter were successfully and reproducibly synthesized with no severe weak-link within the samples. The trapped magnetic field at the open surface of the sample was recorded as 8.0 T at 40 K when measured immediately after the external field was swept down to zero. This value far exceeds that of a melt-processed Y - Ba - Cu - O with a similar size.

High-Temperature Superconducting Motor Using Y-Ba-Cu-O Bulk Magnets

A DC motor equipped with melt-processed Y-Ba-Cu-O (YBCO) superconductors as field magnets was constructed. The YBCO bulks were magnetized at temperatures of 65-77 K in a liquid nitrogen vessel by applying a pulsed magnetic field. The maximum output power was 940 W when the YBCO bulks were cooled to 65 K. Continuous running tests were carried out for up to 6 h. We found that the trapped magnetic fluxes are hardly affected by the magnetic field of 7500 A/m generated by the armature current and that cooling of the YBCO bulks to low temperatures is very effective for holding the trapped field at a high level and, hence, substantially elevating the performance of a high-temperature superconducting (HTSC) motor.

Very high trapped field in melt-processed Sm-Ba-Cu-O

We prepared c-axis oriented Sm-Ba-Cu-O bulk-superconductors by the melt-process technique in a reduced oxygen atmosphere. We particularly studied the effect of Ag addition to the composite in the present work, because Ag was found to greatly suppress the formation of macro-sized cracks in our earlier works. The samples were evaluated by measuring the residual magnetic flux density after field-cooling the samples. The trapped-field distributions mapped at 77 K show that samples with 10 or more wt.% Ag/sub 2/O consist of a single domain with no severe weak-links. The trapped magnetic flux density at the open surface of a sample with 20 wt.% Ag/sub 2/O and 30 mm in diameter was 8.0 T at 40 K. It was also found that the addition of Ag affects the microstructure of the sample and the distribution of the Sm/sub 2/BaCuO/sub 5/ phase was more homogeneous in samples with larger amount of Ag.

Applications of superconducting permanent magnets driven by static and pulsed fields

By making full use of an excellent trapped field performance of the c-axis oriented single-domain Ag-bearing Sm123 superconductors with a diameter of 36 mm, we have developed several superconducting permanent magnet systems driven by static and pulsed fields. The Sm123 superconductor can trap a magnetic field of 2.1 at 77 K and can increase its maximum trapped field to 9 T at 25 K in the static field cooling magnetization. A Nd-Fe-B plate (50×50×5 mm3) can be magnetized to the same level as that due to the uniform static magnetization by scanning it over the Sm123 superconducting permanent magnet. The construction of a pulse field-driven face-to-face-type magnetic field generator capable of producing 1.8 T in an open space with a gap of 12 mm is also presented. We also suggest hollowed Sm123 superconductors to serve as a permanent magnet providing a uniform magnetic field exceeding 2 T over the range up to 10 mm.

Melt-processed RE-Ba-Cu-O (RE=Sm, Nd) superconductors for quasi-permanent magnets

The microstructures of melt-processed Nd-BCO/Ag samples were investigated. A rather homogeneous distribution of Nd422 particles was found, which can be attributed to the fast growing speed of NdBCO and the large size of Nd422 particles. The dispersion of Ag depended on the content of Ag/sub 2/O, and the phase relation of the Nd-BCO/Ag system is discussed. We also studied the pulsed field magnetization technique, and found that the amount of magnetic fluxes trapped by SmBCO significantly increased by sandwiching the sample with ferromagnetic yokes and repeatedly applying magnetic pulses with decreasing amplitude.

Pulse field magnetization of melt-processed Sm-Ba-Cu-O

We studied the dynamical motion of flux lines in melt-processed Sm-Ba-Cu-O (SmBCO) bulk-superconductors driven by a magnetic pulse at 77 K. The total amount of trapped magnetic flux (ΦT) as a function of the magnitude of pulse exhibited a peak; the decrease of ΦT for large pulses is attributed to the increase in temperature of the sample due to the resistive force exerted on moving flux lines. Compared to melt-processed Y-Ba-Cu-O, it is found that the penetration of flux lines into the sample requires a larger pulse, reflecting the strong pinning effect of SmBCO.

Trapped Field Distribution on Sm-Ba-Cu-O Bulk Superconductor by Pulsed-Field Magnetization

A melt-processed Sm-Ba-Cu-O was magnetized by pulsed-fields at 30–77 K. The trapped field distribution on the sample was measured by scanning a Hall sensor after magnetization. We found that flux lines penetrated into the sample by flux jump below 70 K. As a result, the trapped field of the sample reached more than 70% of the applied field, which can not be expected in the static zero field cooling. The maximum trapped field of 3.8 Tesla was obtained by the improved IMRA (Iteratively Magnetizing pulsed-field operation with Reducing Amplitude) method.

Development of a superconducting bulk magnet for NMR and MRI.

A superconducting bulk magnet composed of six vertically stacked annular single-domain c-axis-oriented Eu-Ba-Cu-O crystals was energized to 4.74 T using a conventional superconducting magnet for high-resolution NMR spectroscopy. Shim coils, gradient coils, and radio frequency coils for high resolution NMR and MRI were installed in the 23 mm-diameter room-temperature bore of the bulk magnet. A 6.9 ppm peak-to-peak homogeneous region suitable for MRI was achieved in the central cylindrical region (6.2 mm diameter, 9.1 mm length) of the bulk magnet by using a single layer shim coil. A 21 Hz spectral resolution that can be used for high resolution NMR spectroscopy was obtained in the central cylindrical region (1.3 mm diameter, 4 mm length) of the bulk magnet by using a multichannel shim coil. A clear 3D MR image dataset of a chemically fixed mouse fetus with (50 μm)(3) voxel resolution was obtained in 5.5 h. We therefore concluded that the cryogen-free superconducting bulk magnet developed in this study is useful for high-resolution desktop NMR, MRI and mobile NMR device.

Extracting the utmost from the high performance of Sm–Ba–Cu–O bulk superconductors by pulse field magnetization

We have studied the magnetization of a melt-processed Sm–Ba–Cu–O bulk superconductor by magnetic pulses at 77 K. The amount of trapped magnetic flux (ΦT) as a function of the pulse amplitude exhibited a peak, which is attributable to the heating of the sample caused by the fast motion of flux lines in the presence of resistive forces. The peak value of ΦT was about half the amount of magnetic flux that was trapped when the sample was field cooled to 77 K, where the field-cooling (FC) value corresponds to the full capability of trapping magnetic flux by the sample. We found that sandwiching the sample with ferromagnetic materials and iteratively applying magnetic pulses while progressively decreasing the amplitude increases the amount of trapped magnetic flux. By combining these two techniques, ΦT increased to about 90% of that of FC magnetization.

Synthesis of c-axis oriented single-domain Sm123 superconductors capable of trapping 9 Tesla at 25 K and its application to a strong magnetic field generator

Abstract The c -axis oriented single-domain Sm123 bulk superconductors containing 10–20 wt.% Ag 2 O have been synthesized under the Ar gas flowing atmosphere by using Nd123 as a seed crystal in the melt-texturing process. Samples synthesized were either 30 or 36 mm in diameter and 12–18 mm in thickness. The maximum trapped flux density at 77 K has reached 2.1 T at the center of the free surface of the sample containing 10 wt.% Ag 2 O. The highest trapped flux density of 9.0 T was achieved by lowering temperature to 25 K. A compact magnetic field generator capable of producing higher than 2 Tesla in free space has been constructed by magnetizing the Sm123 superconductor by means of the pulse field technique.