Kunihiko Egawa

Irreversibility line in superconducting Bi-2212 single grain with fine normal particles

Abstract The irreversibility line was obtained in terms of the imaginary a.c. susceptibility measurement for a melt-processed superconducting Bi-2212 single grain with fine non-superconducting particles. This line showed different temperature dependences in high and low temperature regions. The one in the low temperature region agrees approximately with the prediction from the flux creep theory with experimentally observed pinning parameters. On the other hand, the one in the high temperature region is close to the well known characteristics in Y-based superconductors, suggesting flux pinning by boundaries such as the surfaces of non-superconducting particles. A quantitative discussion is also given on flux bundle size.

Isotropic Jc‐B properties of YBa2Cu3O7−x thin films containing fine precipitates by metalorganic chemical vapor deposition

Y1Ba2Cu3O7−x superconducting thin films which have isotropic Jc‐B properties were prepared onto SrTiO3 (100) substrates at 700 °C by metalorganic chemical vapor deposition. The Jc did not depend on directions of the field either perpendicular or parallel to the c axis of the films and the films had a high Jc of 6.1×104 and 7.1×104 A/cm2 at 77 K and 5 T perpendicular and parallel to the c axis, respectively. Transmission electron microscopy observation and x‐ray energy dispersive spectroscopy analysis indicated that the films contained many fine Y2BaCuO5‐like precipitates with diameter of ∼10 nm and they were distributed uniformly with the density Np as high as 1023/m3.

Critical Current Density and Flux Creep in Melt-Processed Bi-Pb-Sr-Ca-Cu-O Superconductors

We investigated the critical current density and the flux creep in melt-processed Bi-Pb-Sr-Ca-Cu-O for the purpose of studying the existence of strong flux pinning forces due to normal conducting precipitates. We fabricated samples with a similar microstructure to melt-processed Y-Ba-Cu-O and Tl-Sr-Ca-Cu-O having a strong pinning force. When the matrix was (Bi, Pb)2Sr2Ca1Cu2Oy (2212) phase, the irreversibility field Birr was expressed as Birr\propt(1-T/Tc)3/2, which suggested that the expected flux pinning was due to normal conducting precipitates. A part of the matrix was transformed to (2223) phase by the postannealing of the sample; however, the superconducting properties were inferior to those observed in (2223) polycrystals.

Absorption and Desorption of Oxygen in Y1Ba2Cu3O7-δ

The absorption and desorption velocities of oxygen in a Y1Ba2Cu3O7-δ sintered sample were measured by a thermogravimetric analysis when the atmospheric gas of nitrogen was exchanged for oxygen and oxygen for nitrogen. The weight change of the sample could be expressed by an exponential function of time in each process. In the absorption process of oxygen, the time constant was a complex function of the temperature, while in the desorption process, it was the monotonous temperature dependence. The diffusion constant and the activation energy were estimated at the temperature range of the tetragonal phase.

Low resistivity contacts to YBa2Cu3O7−y superconductors using Ag2O powder

Abstract Low resistivity contacts to bulk specimens of YBa 2 Cu 3 O 7− y with Ag have been developed 1 . In order to obtain homogeneous mixtures of YBa 2 Cu 3 O 7− ity and Ag we used Ag 2 O powder. The contact resistivity of 0.19 μΩ cm 2 was obtained by annealing the specimen at high temperature. The current versus voltage characteristic of the contact was found to be ohmic. The contact was made to the specimen which was then exposed to air for 110 days; the contact resistivity after this period had elapsed was almost the same as the contact made to the fresh specimen. Using physical and chemical analysis it was found that all the Ag 2 O in YBa 2 Cu 3 O 7− y was separated to Ag and that Ag existed in all voids.

Microstructure and superconducting properties of Bi-Sr-Ca-Cu-O system prepared by a melt process

The microstructure and superconducting properties for samples with the nominal composition of Bi2Sr2Ca1+2xCu2+xOy (x=0, 0.2, 1.0) prepared by a melt process have been examined. The magnetic field dependence of the intragrain critical current density was improved by enriching the Ca and Cu contents. Such samples included two kinds of precipitates, (Ca, Sr)2CuOy and Sr-Pt-O compounds. The latter precipitates were much smaller in size than the former, and this suggests that they may be more effective in enhancing the pinning force.

Superconducting Properties of

An internal-tin route Nb₃Sn superconducting wire that has both remarkably low hysteresis loss (Q_h) and high critical current density (J_c) was developed according to a new design idea. The wire was constructed by arranging the filaments in a radial layout, enlarging the outer filaments along the radial direction, intentionally narrowing the filament spacing in the radial direction, and enlarging the filament spacing in the tangential direction. Thus, the electromagnetic coupling among the filaments in the tangential direction due to the bridging and/or proximity effect was suppressed without decreasing the volume fraction of Nb. As a result, excellent properties such as J_c(12 T)=1.15times10³ A/mm² Q_h=301 mJ/cm³ (for 1 cycle of B=plusmn3 T), T_c=17.3 K, and B_c2=24.1 T were obtained. Scanning electron microscope observations confirmed that the cause of experimental Q_h values being lower than expected is that the majority of the filaments in the second and outermost rows were not completely combined. Electron probe micro-analyses confirmed that the good T_c and B_c2 values resulted from the qualitative improvement of the Nb₃Sn compound. This wire is promising for use with conduction-cooled high-field magnets and for the strands of fusion coils

rm Nb_3rm Sn

Abstract To strengthen the pinning force by introducing extrinsic pinning centers, Bi 2 Sr 2 Ca 1+2n Cu 2+n O y superconductors which contained dispersed precipitates with high density were synthesized by using the melt process. (Ca, Sr) 2 Cu 1 O y and PtSrO seemed to be candidates for the pinning centers through XRD and EPMA observations. Critical currents flowing perpendicular to the c-axis were evaluated through magnetization measurements. Those at 4.2K were scarcely influenced by the existence of such precipitates. But, with the n =0.15 those at 20K were five times higher in the magnetic field below 0.1 teslas, and three hundred times higher above 3 teslas, and the degradation by increasing the magnetic field from 0.1 to 3 teslas was only one unit.

Wires With Radially Arranged Filaments

The Bi‐based (2212) single grains including a large number of normal particles, (Ca,Sr)2Cu1O3, were synthesized by a melt process. To characterize the flux creep behavior in such a superconductor, magnetization measurements were performed at various temperatures and magnetic fields. The flux creep rate exhibited a maximum at 20 K in both magnetic fields parallel and normal to c axis. The magnetic field dependence of the pinning potential, U0, evaluated from the residual magnetization was not represented by the expression, B−α, in Y1Ba2Cu3O7 superconductors. The relation between U0 and the critical current density is discussed.

Introduction of pinning center in Bi-based oxides prepared using melt process

The long size superconducting plates for current lead were made via a few times of an intermediate pressing to (Bi,Pb)2Sr2Ca2Cu3Oy (2223) bulk samples. The critical current density was improved by the process, which was over 103A/cm2 at 77K, OT. The minimum contact surface resistivity was performed by sintering silver — films coated on the superconductor surface as the contact pads, which was about 10−6Ωcm2 at 77K, OT. We developed 500A — class current lead using 2223 bulk plates with 80K thermal anchor, and we could show that the thermal load from the lead drastically decreased under 20% in comparison with optimum designed metal leads with helium vapor cooling.