S. Yoshizawa

Preparation of Bi-2223 bulk composed with silver-alloy wire

Abstract We prepared Bi-2223 sintered bulk composed with silver alloy wires to improve simultaneously both the superconductivity and the mechanical properties. When 24 Ag alloy-wires of 0.4 mm in diameter were composed, the critical current density ( J c ) at 77.3 K in the self-field was obtained 700 A/cm 2 . Scanning electron microscope observation results showed that dense structure of Bi-2223 plate-like grains was formed around the interfacial region between the Bi-2223 oxide and metal Ag. The mechanical property was estimated by three point bending test. The bending stress of 80 MPa at the rupture was obtained for Bi-2223 AgMg alloy composite bulk irrespective of the number of the wires. In the AgZr alloy-wires composite bulks, the bending stress decreased to the half value of that in Bi-2223 bulk without the wires. After the maximum bending stress the sample fractured separately in Bi-2223 bulk without alloy-wires, while the composite did not fracture but only fine cracks were induced.

Analysis of fluctuation conductivity in melt-textured DyBa2Cu3Oy superconductors

Abstract We evaluated the superconducting properties of DyBa 2 Cu 3 O x (Dy123) with different amounts of Dy 2 BaCuO 5 (Dy211) phase. The samples were melt-textured DyBaCuO superconductors, which were made from the mixtures in molar ratios of Dy123:Dy211=1: x ( x =0.03, 0.05, 0.08 and 0.1). The temperature dependence of resistance of DyBaCuO superconductors on different amounts of 211 phase was measured. The data of fluctuation amplitude and the dimension of conduction were obtained from analyzing the fluctuation. The results show that coherence length obtained from this analysis becomes larger as amounts of 211 phase increase. The evidence is found that the pinning force in the sample is well correlated with the characteristic length of superconductivity.

Large domain growth of Ag-doped YBaCuO-system superconductor

Abstract The effect of Ag addition on the phase diagram, solidification conditions and fabrication of large domain YBaCuO-system has been studied. Calcined powder of composition Y1.8Ba2.4Cu3.4O9-x with 0.5 wt.% Pt and 0, 1, 5, 10, 20 and 30 wt.% Ag was prepared. The melting point of each powder was measured by differential thermal analysis (DTA). A large domain (45 mm diameter and 20 mm thick) of the Ag-doped YBaCuO-system superconductor was prepared for each composition by a seeding and unidirectional growth technique which involved controlling the furnace temperature gradient. In order to assess the quality of the crystal orientation, the distribution of trapped flux density was measured. The entire sample was magnetized uniformly and the trapped magnetic field observed to form a single dome shape.

Improvement of Jc and mechanical property in Bi-2223/Ag wires composite bulk

Abstract We studied on improvement of J c and mechanical properties of oxide bulk by composing Ag wires. As results, J c values of the Ag wires composite bulk were improved in proportional to the pieces of Ag wires. When 42 pieces of Ag wires were composed, the J c value of the composite bulk was estimated to be 800 A/cm 2 at 4.2 K and self-field. This J c value was about twice higher than that of no Ag wire bulk. We found that the magnetic field dependence of the J c value of the composite bulk was smaller than that of no Ag wire bulk. The good alignment and dense structure of (Bi,Pb) 2 Sr 2 Ca 2 Cu 3 O x (Bi-2223) phase was observed near the interface between Bi-2223 oxide and Ag wires. It is mentioned that this structure brings about improvement of J c property and the magnetic field dependence of the J c value for the composite bulk. It is also found that mechanical property of Bi-2223 bulk is improved with increasing pieces of composed Ag wires from the results of a three point bending test.

Bi-2223 large sized magnetic shield prepared by jointing sintered bulks

Abstract In order to fabricate large sized superconductor plate shielding magnetic field above several mT, small pieces of sintered bulk were jointed with superconductor paste prepared with mixture of calcined powder and organic vehicle. Thirteen Bi-2223 four-cornered bulks were jointed superconductively to 40 mm 2 and 4 mm thick plate. The magnetic shielding property was measured at 77 K. Uniform magnetic field was applied perpendicular to the plate surface. Two-dimensional distribution of the magnetic flux density over the sample surface was measured by a Hall prove parallel to the sample surface. When magnetic field of 15 mT was applied to the sample, the magnetic flux penetrated the sample along the jointed line. Magnetic shielding effect of the sample was observed uniformly with the external magnetic field of 10 mT.

Preparation and Mechanical Properties of Large Single Domain DyBaCuO Superconductor

We prepared large single domain of DyBaCuO (Dy-123) superconductor for future applications such as a superconducting flywheel for energy storage system, magnetic separation, and a bulk magnet. Dy-123 superconductors are expected to have high and trapped field properties. Melt-textured single domain of Dy-123 system superconductor about 48 mm in diameter and 10 mm in thickness was fabricated with a seeding and temperature gradient method in air. Dy-123 and Dy-211 powders were mixed in a molar ratio of 1:0.3, and of 10 wt% and Pt of 0.5 wt% were added to the mixture. The maximum trapped magnetic field over the bulk was approximately 0.2 T at 77.3 K. We also discussed mechanical properties such as the bending stress, the hardness and the surface roughness of the Dy-123 bulk. Mechanical property results showed important information for evaluating Dy-123 bulks.

Trapped field in large single-domain Dy-123 system superconductor

Since melt-textured RE-system bulk can trap strong magnetic flux, it acts as a quasipermanent magnet stronger than conventional permanent magnets. Large single domain Dy-123 system superconductors about 48 mm in diameter and 15 mm in thickness were prepared with a seeding and temperature gradient method in air. DyBa2Cu3Ox (Dy-123) and Dy2BaCuOy (Dy-211) powders were mixed in molar ratios of 1:0.3. Ag2O of 10 wt% and Pt of 0.5 wt% were added to the mixture. The pellets were partially melted at 1070 °C for 4 hour, and rapidly cooled to 980 °C for one hour. When the top temperature of the sample reached 980 °C, the ab-plane of Nd-123 seed crystal was placed in contact with the top surface of the bulk. After the seeding, the samples were cooled to 930 °C with a cooling rate of 0.07-0.08 °C/hr, and then the bulks were cooled to room temperature with furnace cooling. Finally, the samples were annealed in pure oxygen atmosphere from 500 °C to 250 °C for 500 hours. Two samples of the melt-textured single domain were fabricated. Homogeneous morphology was obtained in the samples. When magnetic field of 1.5 T was applied for the samples, about 0.7 T was trapped at 77.3 K.

Effect of CIP process on superconducting properties of Bi-2223/Ag wires composite bulk

Abstract In order to improve superconducting property of Bi-2223 superconductor bulk, Ag wires were composed in the bulk. The composite was made by stacking alternately Ag wires and the calcined powder. The sample was sintered at 840 °C for 50 h in air. After treatment with a cold isostatic press as an intermediate pressing, the sample was sintered again. The maximum J c at 4.2 K and self-field of the sample composed with 24 Ag wires of 0.4 mm in diameter was obtained to be 2000 A/cm 2 . Scanning electron microscope and electron probe microanalyzer observation results showed that highly c -axis oriented and densely structured Bi-2223 plate-like grains could be formed around the interfacial region between the superconducting oxide and the metal Ag.

Fabrication of 10,000A/cm2 Class Bi-2223 Sintered Bulk

Many kinds of preparation processes of calcined powder have been proposed for producing sintered bulk Bi-2223. We adopted a coprecipitatcd method that is able to form homogeneous and fine powders that lead to facilitating synthesis in the calcination and sintering reaction. However, calcination condition of coprccipitated powder for obtaining high Jc sintered bulk is delicate. Oxide superconducting calcined powder adsorbs firstly atmospheric moisture, then reacts with carbon dioxide, which aim deteriorate the superconducting properties of the oxide superconductor. Carbon that is included in the calcined powder segregates in crystal grain boundaries during sintering process. This segregation decreases Jc, by cause in weak links, preventing superconducting current from flowing across grain boundaries. We reconsidered the calcined powder preparation process. By improving a solvent removal method after a wet pulverizing method where moisture is easily adsorbed, it was possible to reduce the carbon content to under 0.03wt%. The calcined powder was pressed into 20 mm diameter pellets and then sintered at 850 °C for 50hr in air with an intermediate pressing by the CIP (cold isostatic press) method. The Jc of the sintered bulk was increased to be 10,000A/cm2, reproducibly.

Microstructure and critical current density of the YBaCuO superconductor prepared by melt process

Abstract The critical current density (Jc) value increases along the growth direction from the top to the bottom of the YBaCuO samples prepared by a melt process with a unidirectional growth technique. The volume fraction of Y2BaCuO5 (211) particle is constant all over the sample, while the average size decreases along the growth direction from the top to the bottom. The dependence of Jc on the volume fraction and the particle size agrees with the theoretical result. It seems that an increase of Jc along the growth direction from the top to the bottom can be attributed to the decrease of the 211 average size. The cause of the inhomogeneity for the 211 particle size is discussed.