Kozo Yoshikawa

Preparation of Pb-Doped Bi–Sr–Ca–Cu–O Superconductors by a Hot-Press Method

Synthetic conditions of Pb-doped Bi-Sr-Ca-Cu-O oxide superconductors produced by a technique of a uniaxial hot-press method were studied. The quality of the samples was measured by means of X-ray diffraction analysis, secondary electron microscope measurement and density measurement. The hot-pressed sample showed a single 2223-phase with density close to the ideal value.

Properties of Pb-Doped Bi-Sr-Ca-Cu-O Superconductors Prepared by a Hot-Press Method

Pressure effect during sintering (hot pressing) on the superconducting properties, focusing particularly on the critical current density Jc, has been studied for the high-Tc-phase Pb-doped Bi-Sr-Ca-Cu-O system. The properties are compared with those for the samples before the pressing or with intermediate-pressing at room temperature. The transport Jc observed by resistive transition measurement at 77.3 K was 7,000 A/cm2 at H=10 Oe for the hot-press sample, 6,260 A/cm2 for the intermediate pressed sample at H=0 and around 500 A/cm2 for the nonpressed one at H=0. The magnetic hysteresis loops were measured at 5 K and 77.3 K, and the hot-pressed sample showed larger hysteresis than that for the intermediate-pressed one.

Substitution effect of Ba for Sr in Bi2223 superconductors

Abstract We studied the effect of Ba substitution of Sr on magnetic J c properties and irreversibility line of single phase (Bi,Pb) 2 Sr 2 Ca 2 Cu 3 O y superconductors. We found from field dependence of the magnetic J c that the Ba substitution of 10% Sr resulted in maximal improvement on magnetic J c and irreversibility line.

Flux-pinning potential and doping effect in (Bi,Pb)2Sr2Ca2Cu3Oy

Abstract Distribution of the activation energies of flux pinning measured for non-doped Bi(2:2:2:3) samples showed a single peak around 30 meV, while a sub-peak was observed around 150 meV for Gd- or La-doped samples.

Flux pinning potential in Bi-2223 tapes irradiated by Xe ion

Abstract We have investigated the flux pinning potential of Ag–Cu alloy sheathed Bi-2223 tapes with columnar defects parallel to the c -axis introduced by Xe-ion irradiation. We measured the magnetic relaxation of the remanent. The sample was cooled in zero field and the field above H *, where the flux front penetrated at the center of the sample. The time decay of the remanent moment was measured for about 10 5 s. The relaxation of the remanent moment showed the linear ln( t ) dependence in all observed temperature range. The change in the distribution of the activation energies with irradiation was obtained. Since the deeper pinning center were increased, it is expected that the columnar defects will act as effective pinning centers.

Microstructures and Pinning Forces of Ag-Cu Alloy Sheathed Tapes

We measured high-resolution transmission electron microscope images (HRTEM) for the Hf-doped Ag-Cu sheathed (Bi,Pb)2Sr2Ca2Cu3Oy (Bi-2223) tape and Ba-doped Bi-2223 bulk sample. The HRTEM images of the crystal a-c plane indicate that the several atoms in the SrO layers have different brightness. This suggests the possibility that Hf or Ba atoms are substituted for Sr atoms. The influence of the microstructures on the critical current was studied by the magnetic measurement. The result indicates the improvement of the magnetic Jc for the doped samples, suggesting that a small amount of the doping acts as the effective pinning centers of the fluxes in the both samples. The Hf or Ba doping effect is discussed based on the relation between the volume pinning force and the microstructures.

Transport and magnetic properties of superconducting MgB2

Abstract We have investigated the transport and magnetic properties of the recently discovered superconducting materials MgB 2 . We obtained irreversibility temperature T r ( H ) and irreversibility field H r ( T ) from magnetic measurements. The irreversibility temperature T r ( H ) was determined from the merging temperature of the magnetic moment observed in the zero field cooling process in to the one in field cooling process. We also defined the transport irreversibility temperature B c2 from resistivity measurements at zero-resistivity in the presence of magnetic field. Those results of the irreversibility lines are plotted in H – T plane. The magnetic irreversibility lines are consistent with each other, while the transport one locates at the higher temperature side.

Effect of heavy ion irradiation on flux pinning properties of Ag-Cu alloy sheathed Bi-2223 tapes

We investigated the co-existing effects of columnar and point defects in Hf-doped Ag-Cu alloy sheathed Bi-2223 tapes. The irreversibility lines are appreciably improved compared to the individual effect by Hf doping or columnar defects.

Magnetic Properties of Bi-2223 Tapes Irradiated by Xe Ion

We have investigated the magnetic properties of Ag-Cu alloy sheathed Bi-2223 tapes where Hf impurities are included in the pristine alloy We introduced columnar defects in the tapes parallel to the aligned c-axis (normal to the tape surface) by Xe-ion irradiation. We found that the irreversibility line in the heavy-ion irradiated tapes showed the shift toward higher fields. Moreover, the irreversibility curve of the irradiated sample is located in the higher temperature side of the irradiated Ag-sheathed tape (without Hf impurities in the tape) even in the higher magnetic field than the matching field. In addition, the improved irreversibility line is observed in the configuration of the field parallel to the tape (i.e., perpendicular to the columnar defects).

Extraction of Ag diffusion effect in hot-pressed Bi-2223

We have investigated the annealing effects of Bi-2223 hot-pressed samples on the transport and magnetic properties and the Ag-diffusion effect of those on microscopic property. The behavior of the resistivity as a function of temperature and that of the magnetic J C as a function of magnet field were hardly changed with the annealing temperature. Comparison of the results of the present annealing measurements with those of the previous Ag-diffusion measurements indicated that characteristic changes were induced by dominant Ag-diffusion effect. On microscopic observations of a surface with Ag, we found Ag agglomerates that would be reflected in the diffusive behavior.