Tsutomu Minami

Crystallization kinetics for quenched Bi‐Ca‐Sr‐Cu‐O glasses

The kinetics of crystallization in the BixCaSrCu2Ow (x=1.5, 2.7) glasses prepared by twin‐roller and metal‐plate quenching methods have been studied by a nonisothermal method using differential scanning calorimetry. Scanning electron microscopy studies suggest surface crystallization as the dominant mechanism. The activation energy for crystal growth calculated from modified Kissinger plots assuming surface nucleation decreased with an increase in the Bi content. Such a composition dependence is consistent with that of the activation energy for viscous flow in the glass transformation range.

Instantaneous preparation of superconducting thick films through melts in the Bi-Ca-Sr-Cu-O system

Thick films of Bi-Ca-Sr-Cu-O superconductors were instantaneously prepared by crystallization through melts on a MgO single-crystal substrate. It took only half an hour in annealing to obtain superconducting thick films with Tc (zero) of 93 K and Tc (onset) of 98 K. From the X-ray diffraction patterns of these films, it was found that the superconducting phase was strongly oriented along the c-axis perpendicular to the substrate.

Temperature‐time‐transformation diagrams for crystallization process of rapidly quenched Bi‐Pb‐Ca‐Sr‐Cu‐O glasses

The formation process of the superconducting phases from the rapidly quenched Bi2−xPbxCa2Sr2Cu3Owu2009(0≤x≤0.4) glasses has been studied as a function of both heat treatment temperature and time. The temperature‐time‐transformation (T‐T‐T) diagrams constructed for the glasses greatly depend on the Pb content. The formation region of Ca2PbO4, which may play an important role in producing the 110 K phase, is very wide in the T‐T‐T diagrams of Pb‐rich samples.

Transition range viscosity of rapidly quenched Bi-Ca-Sr-Cu-O glasses

As a precursor of high Tc superconductors, glasses in the system BixCaSrCu2Ow (x=1.5,2.7) have been prepared using twin‐roller and metal‐plate quenching procedures. Transition range viscosities have been measured on samples as thin as 0.02 mm using the beam‐bending method with a technical modification. From the viscosity data, the system Bi‐Ca‐Sr‐Cu‐O can be characterized as a ‘‘fragile’’ liquid like the ZrF4‐based fluorides. The possibility of drawing fibers of high Tc superconductor precursors is discussed on the basis of the temperature dependence of viscosity.

Direct Preparation of Fine Powders of the 80 K Superconducting Phase in the Bi–Ca–Sr–Cu–O System by Spray Pyrolysis

Submicron powders of the 80 K superconducting phase in the Bi-Ca-Sr-Cu-O system have been directly prepared by the spray pyrolysis of aqueous solutions of corresponding metal nitrates, Bi:Ca:Sr:Cu=1:1:1:2. The powders obtained were spheres with uniformly distributed diameters below 1 µm. The crystalline phase of these powders was found to greatly depend on the oxygen partial pressure in the carrier gas as well as the decomposition temperature. The preparation conditions for the 80 K superconducting phase were examined.

Crystalline Phases Precipitated by Heat-Treatment of Rapidly-Quenched Glasses in the Bi-Ca-Sr-Cu-O System

The precipitation process of the superconducting phases from the rapidly-quenched glasses has been studied as a function of both heat-treatment temperature and time for the three main compositions in the Bi-Ca-Sr-Cu-O system: Bi2Ca2Sr2Cu3Ox, BiCaSrCu2Ox, and Bi2CaSr2Cu2Ox. The temperature-time-transformation diagram for each composition has been constructed and a difference has been found between those for the former two compositions and that for the last one.

Formation process of high Tc superconducting Bi-Pb-Ca-Sr-Cu-O thick films via melt solidification

High Tc superconducting thick films Bi1.6Pb0.4Ca2Sr2Cu3Ox were prepared by the melt solidification process. The preparation conditions were optimized to obtain films with higher Tc. Rapid cooling of the molten samples to the heat treatment temperature of 855u2009°C produced superconducting films with a maximum Tc (zero) of 104 K. Slow cooling of the melt declined the superconducting properties of the films because of the formation of Ca2CuO3, which may prevent the formation of the 110 K phase, during cooling through the temperature range around 900u2009°C.

Preparation of Bi-Pb-Ca-Sr-Cu-O Superconducting Thick Films Using Spray-Pyrolyzed Fine Powders

High-Tc superconducting thick films were prepared using fine powders obtained by spray pyrolysis. The 110 K phase was predominantly formed in the films heat-treated at 855°C. The films prepared using fine powders with an average particle size of 1.4 µm showed a higher Tc(zero) of 105 K compared with those using powders of 0.8 µm.

Critical current density of Bi‐Pb‐Ca‐Sr‐Cu‐O high Tc superconductors via rapidly quenched glass precursors

Two‐ and three‐step heat treatment processes of Bi1.6Pb0.4Ca2Sr2Cu3Ox rapidly quenched precursor glasses were examined to produce dense high Tc superconducting glass ceramics with a high critical current density Jc. The heat treatment at around the glass transition, which was expected to accelerate the nucleation rate, contributed to enhance the Jc values. The maximum Jc value of 1800 Au2009cm−2 was obtained through the three‐step heat treatment performed firstly at 350u2009°C, for 1 h, then at 700u2009°C for 1 h, and finally at 855u2009°C for 20 h.

Viscosity and crystallization of quenched Bi-Ca-Sr-Cu-O glasses as high-Tc superconductor precursors

Abstract The possible procedure for the fabrication of high- T c superconductor glass-ceramic fibers is proposed on the ground of the transition-range viscosity studies of the Bi-Ca-Sr-Cu-O glasses. The possibility of fiber drawing in the glasses is discussed on the basis of the composition dependence of viscous and thermal properties of the glasses.