Yukio Kubo

Superconducting glass‐ceramic rods in BiCaSrCu2Ox prepared by crystallization under a temperature gradient

Superconducting BiCaSrCu2Ox glass rods and coils were successfully prepared by pumping up the melt at 1150 °C into SiO2 or Pyrex glass tubes. After the outer glass tubes were removed, the resultant glass rods up to 30 cm long were crystallized under a temperature gradient of 30 °C/mm without deforming and cracking. The Tc (onset) and Tc (zero) of the resulting glass‐ceramic rods were ∼80 and 62 K, respectively.

Effect of Al 2 O 3 additives on the formation of superconducting whiskers (2212 phase) in melt-quenched BiSrCaCu 2 O x

Whiskers of 2212 phase Bi 2 Sr 2 Ca 1 Cu 2 O x were prepared by heating in air the powder-compacted pellets of melt-quenched glassy products in BiSrCaCu 2 O x containing a small amount of Al 2 O 3 . The dissolved and/or physically mixed Al 2 O 3 in the powder-compacted specimen is found to enhance the whisker formation remarkably.

Preparation of High Jc Bi-Sr-Ca-Cu-O Bulk Sample by Floating Zone Method

Bi-Sr-Ca-Cu-O bulk samples with three different nominal compositions of Bi2Sr2CaCu2Oy, Bi2Sr2Ca2Cu3Oy and Bi0.7Pb0.3SrCaCu1.8Oy were prepared by the floating zone method at growth rates ranging from 2 mm/h to 70 mm/h. The sample grown at 2 mm/h with the nominal composition of Bi2Sr2CaCu2Oy possessed high density, highly aligned morphology, and a transport critical current density higher than 1490 A/cm2 at 77 K under a zero magnetic field without any postannealing.

Preparation of a Ag-Doped Bi-Sr-Ca-Cu-O Bulk Sample by the Floating-Zone Method

Bulk samples with nominal composition of Bi2Sr2CaCu2Oy doped with 0, 10 and 20 weight % Ag were prepared by the floating-zone method at growth rates of 2 mm/h and 5 mm/h. Ag-doping seems to slightly enhance Jc, while annealing is very effective for Jc enhancement. From preliminary ac susceptibility measurements, the Jc enhancement by annealing is considered to be due to improvement of the weak link between superconducting grains. The 10% Ag-doped sample grown at 2 mm/h possessed Jc of 5360 A/cm2 at 77 K under zero magnetic field after annealing.

Contact Resistance and V-I Characteristics in a Ag-Doped Bi-Sr-Ca-Cu-O Superconductor

Contact resistance and V-I characteristics were investigated in Ag-doped and undoped Bi-Sr-Ca-Cu-O bulk samples prepared by the floating-zone method. In undoped samples, with increasing current pulse width, rapidity of voltage rise in V-I characteristics increases and Jc decreases. The contact resistance is nonohmic and temperature dependence is semiconductorlike. In a Ag-doped sample, rapidity of voltage rise and Jc are not influenced by pulse width, and deviation of Jc among samples is small. The ohmic contact resistance has metallike temperature dependence, and its value is less than 1/500 of that in an undoped sample.

Magnetic measurements of superconducting glass‐ceramic fine rods in Bi1Ca1Sr1Cu2Al0.5Ox prepared under a temperature gradient

It is shown that the crystallization of the glass precursor under a temperature gradient is very effective for preparing the superconducting glass ceramics in the Bi‐Ca‐Sr‐Cu‐O system. The magnetization measurements show that the specimen prepared under a temperature gradient has a magnetization hysteresis several times larger at 4.2 K than that prepared under no temperature gradient; the coupling between superconducting grains of the former is stronger than that of the latter.

Superconducting Glass-Ceramic Fine Rods in Bi1Ca1Sr1Cu2Al0.5Ox Prepared under a Temperature Gradient –Tc and the Texture of Specimen–

Fine glass rods (dia. 1~2 mm) in Bi1Ca1Sr1Cu2Al0.5Ox prepared by pumping up the melt at 1150°C into SiO2 glass tubes were reheated to 845°C according to a given procedure under a temperature gradient (≈30°C/mm) or under no temperature gradient. The incorporated Al enhanced the glassification. In the former specimen crystals grew well to become much larger, and impurity Al was excluded from the superconducting phases on crystallization under a temperature gradient to form the aluminous fibers; the Tc(zero) is ≈80 K and a shoulder appears around 110 K.

Textured Bi-Sr-Ca-Cu-O bulk sample

A textured structure with the c-axes of the 2212 crystal nearly parallel to each other was obtained in a bulk superconducting sample of Bi2.2Sr1.8CaCu2Ox doped with 10 wt%Ag. When Ag-doping was not performed or a Bi2Sr2CaCu2Oy composition was used, the textured structure was not obtained. The textured sample has an Ic value and magnetic field dependence superior to those of the untextured sample. Quenching experiments were carried out to investigate the formation mechanism of the textured structure and it was revealed that the morphology of the interface between solid and melt in the textured sample was different from that in the untextured sample.

Ag-Doped Bi-Sr-Ca-Cu-O Superconductor Prepared by Floating Zone Method

Ag-doped Bi2Sr2CaCu2Oy bulk samples were prepared by floating zone (FZ) method aiming at enhancement of critical current density (Jc) and lowering of contact resistance at current terminal. It was found that Ag particles tend to be elongated along the growth direction, and be trapped in the oxide grains. Jc value was not significantly affected by Ag-doping up to 10%, but the contact resistance at current terminal was lowered to about 1/1000 that of undoped sample.

Electrical properties of bulk BSCCO/IN interfaces

Abstract The influence of Ag-doping and Ag sputtering on electrical properties of bulk BSCCO/In interfaces were examined. In Ag-doped and undoped samples, Rc were ∼ 10 −6 ω cm 2 and ∼ 10 −3 ω cm 2 , respectively. While Rc of Ag-doped samples showed positive dependency on magnetic field, there was no dependency in undoped samples. Ar ion beams ethcing, Ag sputtering with moderate film thickness and anneal treatments to an undoped sample significantly decreased Rc (∼ 10 −6 ω cm 2 ).