S. Nariki

Melt-processed Gd–Ba–Cu–O superconductor with trapped field of 3 T at 77 K

We fabricated a single-domain Gd?Ba?Cu?O bulk superconductor 65?mm in diameter and studied the microstructure, superconducting and field-trapping properties. Melt-processing was performed under a controlled oxygen partial pressure of 1.0% using a precursor containing Gd123 and Gd211 powders in a molar ratio of 2:1, with 0.5?wt% of Pt and 20?wt% of Ag2O added. The distribution of Ag and Gd211 particles was almost homogeneous. The addition of Ag was very effective in reducing the amount of cracking in the sample. The maximum trapped magnetic field recorded was 3.05?T at 77?K. We also measured the trapped field between two Gd?Ba?Cu?O bulk samples in order to minimize the demagnetizing effect and found that the trapped field reached 4.3?T at 77?K.

Influence of the size of Gd211 starting powder on the critical current density of Gd-Ba-Cu-O bulk superconductor

The relationship between the particle size of Gd211 powder in the precursor and the particle size of Gd211 inclusions in Gd-Ba-Cu-O bulk has been investigated. Gd211 starting powders with various diameters were prepared by the calcination of Gd2O3, BaO2 and CuO powders at different temperatures between 800 and 1000 °C. The particle size of Gd211 in the melt-grown bulk was proportional to the particle size of the initial Gd211 powder. In conclusion, the employment of fine Gd211 powder led to a size reduction of 211 particles in the bulk, while largely enhancing the Jc values in low magnetic fields. A large Gd-Ba-Cu-O/Ag bulk sample, 32 mm in diameter, could also be fabricated by the hot-seeding method. The maximum trapped field value revealed 1.5 T at 77 K.

High critical current density in Y-Ba-Cu-O bulk superconductors with very fine Y211 particles

We have succeeded in enhancing the critical current density (Jc) of melt-textured Y?Ba?Cu?O superconductors by employing ultra-fine Y211 powder as precursor. The Y211 starting powders were reduced down to 110?nm in diameter with ball-milling. Single-grain Y?Ba?Cu?O samples up?to?33?mm in diameter were fabricated from the precursors containing finely milled Y211 powders. The Y211 inclusions in the melt-textured samples could be refined by decreasing the particle size of the Y211 starting powders. The maximum value of Jc at 77?K was 1.1 ? 105?A?cm?2 in a self-field. However, the size reduction of Y211 led to an inhomogeneous distribution of Y211 particles, in which the volume fraction of the Y211 phase greatly decreased near a seed crystal. An Ag-added bulk sample 33?mm in diameter exhibited a high trapped magnetic field of 1.6?T at 77?K.

Processing of high-performance Gd-Ba-Cu-O bulk superconductor with Ag addition

We fabricated large-grain c-axis-oriented Gd–Ba–Cu–O bulk superconductors with very fine 211 particles and studied their field-trapping properties. Single domain Gd–Ba–Cu–O/Ag bulk samples with different molar ratios of Gd-123:Gd-211 = 10:x (x = 2 to 6) were melt-textured under controlled oxygen partial pressure of 1.0% by employing Gd-211 starting powders with a particle size of 1.0 μm. The maximum trapped magnetic field of 2.0 T was recorded at 77 K in the bulk 48 mm in diameter with the composition of x = 5. It is notable that the trapped field in the space of two Gd–Ba–Cu–O bulks reached an extremely high value of 3.3 T at 77 K. Furthermore, the field-trapping performance of the bulk sample was improved by applying the ultra-fine 211 powders of a size of 0.2 μm. The single-bulk Gd–Ba–Cu–O 50 mm in diameter exhibited a trapped field of 2.6 T at 77 K.

Fabrication of large melt-textured GdBaCuO superconductor with Ag addition

Abstract Large c -axis oriented GdBaCuO bulk superconductors with Ag addition were melt-textured under controlled oxygen partial pressure of 1.0%. Nd123 was used as a seed crystal for the growth of large grain bulk. Single grain GdBaCuO samples of diameters 32 mm and 48 mm were successfully fabricated with no macro-sized cracks. Using fine Gd211 powder as a starting material remarkably reduced the size of 211 particles dispersed in the bulk. As a result, high critical current density ( J c ) of 6000 – 70000 A/cm 2 in a selffield was achieved at 77K. The trapped magnetic fields of the samples 32 mm and 48 mm in diameter reached 1.5T and 1.8T at 77K, respectively. These values are comparable to that of a melt-textured SmBaCuO bulk in similar size, and exceeds those of an YBaCuO bulk and the previous GdBaCuO bulk.

Preparation and properties of OCMG-processed Gd–Ba–Cu–O bulk superconductors with very fine Gd211 particles

Abstract We have succeeded in enhancing Jc values of bulk Gd–Ba–Cu–O by employing fine Gd2BaCuO5 (Gd211) powders as precursor material. The mean diameter of Gd211 particles dispersed in the GdBa2Cu3Oy matrix could be reduced to 0.3 μm when we used ball-milled Gd211 starting powders

Development of Gd-Ba-Cu-O bulk magnets with very high trapped magnetic field

Abstract The field-trapping characteristics of recently developed Gd–Ba–Cu–O bulk superconductors will be reported. Large Gd–Ba–Cu–O bulk superconductors with Ag addition were melt-textured under controlled oxygen partial pressure of 1.0%. The single domain Gd–Ba–Cu–O bulk samples 24–50 mm in diameter with submicron-sized Gd2BaCuO5 inclusions could be fabricated without cracks. The field trapping abilities of the bulk samples could dramatically be enhanced through the size reduction of Gd2BaCuO5 particles and the enlargement of grain size. The trapped magnetic field of the bulk Gd–Ba–Cu–O superconductor 50 mm in diameter and 30 mm in thickness reached an extremely high value of 2.7 T at 77 K.

Superconducting properties of Dy-Ba-Cu-O bulk superconductors

We report on the superconducting properties of melt-textured Dy123 bulk superconductors containing 5 to 40 mol% Dy211. Dy?Ba?Cu?O bulk samples were fabricated with the cold-seeding method in air using a Nd123 seed crystal. The superconducting properties of the bulk samples were strongly affected by the Dy211 content and the oxygen-annealing temperature. In the Jc?B curve of the Dy123 sample with 5 mol% of Dy211, the secondary peak was clearly observed at the applied fields of 0.5?2.0 T. The peak Jc values were 32 000?37 000 A cm?2 at 77 K. The peak position shifted to lower fields with increasing oxygen-annealing temperature, presumably due to an increase in oxygen vacancies. We believe that the peak effect in Dy?Ba?Cu?O is ascribed to oxygen deficiency, since RE/Ba substitution is not present in the system. This idea was supported by the fact that the secondary peak decreased with increasing Dy211 contents and disappeared in the Dy123 sample with 40 mol% of Dy211.

Strong pinning in ternary (Nd–Sm–Gd)Ba2Cu3Oy superconductors

We have studied the flux pinning in melt-textured (Nd0.33Sm0.33Gd0.33)Ba2Cu3Oy NSG-123 superconductors with various numbers of Gd2BaCuO5 (Gd-211) particles. Transmission electron microscopy (TEM) showed that submicron Gd-211 particles are uniformly distributed in the superconductive matrix. Dark-field TEM observations further showed that a high density of RE rich RE1+xBa2−xCu3Oy (RE-123ss) clusters 3–10 nm in size were distributed in the NSG-123 matrix. A strongly developed fishtail was observed in the magnetization hysteresis loops of all the samples. A critical current density of 100 kA/cm2 (77 K) was achieved at the secondary peak field of 2 T for the H∥ c axis in the NSG-123 sample with 10 mol % Gd-211. Large grain NSG-123 pellets with 30 mol % Gd-211 and 20 wt % AgO2, 30 mm in diameter and 15 mm in height, exhibited a single-cone profile with a peak value of 1.2 T at 77 K. A higher trapped-field value of 1.5 T was recorded at 2 T, reflecting the secondary peak effect.

Processing of GdBa2Cu3O7-y bulk superconductor and its trapped magnetic field

Abstract Large c -axis oriented GdBa 2 Cu 3 O 7− y (Gd-123) bulk superconductors, which contain Gd 2 BaCuO 5 (Gd-211) secondary phase and Ag, were melt textured under controlled oxygen partial pressure of 1.0%. Nd-123 was used as a seed crystal for the growth of large-grain bulk. Single-domain Gd-123 samples 32 mm in diameter were successfully fabricated with no macro-sized cracks. The employment of fine starting Gd-211 powders led to a reduction in the mean diameter of Gd-211 particles dispersed in the bulk, which resulted in an improvement in both J c and field-trapping capability. The trapped magnetic field of the bulk Gd–Ba–Cu–O/Ag made from Gd-211 particles 0.8 μm in diameter reached 1.5 T at 77 K. Furthermore, the trapped magnetic field could be improved by the enlargement of bulk diameter and the optimization of Gd-211 content. The maximum trapped field of 2.0 T at 77 K was achieved for the Gd–Ba–Cu–O/Ag bulk sample 48 mm in diameter with the composition of Gd-123:Gd-211=10:5.