Sanemasa Kawabata

Direct Observations of Melting and Solidification of Bi–Pb–Sr–Ca–Cu–O Compound

Melting and solidification of Bi–Pb–Sr–Ca–Cu–O compound were directly observed by means of high-temperature optical microscopy. Crystals which appeared in heating and cooling processes were analyzed by means of micro-X-ray diffraction analysis and electron probe microanalysis. In heating, a pressed powder of the high-Tc phase, (Bi, Pb)2 Sr2Ca2Cu3Oy, started melting at about 910°C and the melt was separated into two different liquid phases on a sapphire plate at about 1000°C. In cooling, crystals of the Ca2CuO3 phase and the low-Tc phase, (Bi, Pb)2Sr2CaCu2Oy, were obtained from these two liquid phases and no single crystals of the high-Tc phase appeared.

Ternary Phase Diagram of the YO1.5-BaO-CuO System and the In Situ Observation of Crystal Growth

The ternary phase diagram of the YO1.5-BaO-CuO system was proposed based on the in situ observation of the transformation structures and their reaction temperatures in the growth process of YBa2Cu3O7-y crystal from self-flux of various compositions. This phase diagram enables us to explain many phenomena in the growth process of YBa2Cu3O7-y single crystals, such as flux flow and concentration deviation.

High-Pressure Synthesis of a Bi-Pb-Sr-Ca-Cu-O Superconductor

Nearly perfect high-Tc single-phase powder of Bi-Pb-Sr-Ca-Cu-O was prepared by a modified solid-phase reaction method. This powder was then synthesized under 5.4 GPa to form a high-density block. While the pressed sample exhibited semiconducting behaviour, the post annealed sample showed a superconducting transition with Tc,onset=113 K, Tc,end=105 K and Jc (at 77 K)=508 A/cm2.

Resputtering effect on Y1Ba2Cu3O7-d thin films prepared by RF-magnetron sputtering

Y1Ba2Cu3O7-δ thin films were prepared on MgO(001) substrates by rf-magnetron sputtering. The influence of oxygen partial pressure on film preparation has been investigated quantitatively. The oxygen flow rate near the substrate was controlled by an oxygen inlet ring. As-deposited films in an appropriate condition of oxygen flow rate show a superconducting transition at 83 K. When too much oxygen gas is introduced into a chamber, as-deposited films are damaged by resputtering and their superconducting property disappears. The growth mechanism of the thin films is discussed in terms of the resputtering effect.

Direct Observations of the Process of Growth of YBa2Cu3O7-δ Crystals from the Melt by Means of Optical Microscopy

The melting and growth process of Y–Ba–Cu–O crystals was directly observed by means of optical microscopy for the first time, and after the solidification, typical regions were analyzed by means of micro-X-ray diffraction analysis and electron probe microanalysis. The observed results not only had a good correspondence to the phase diagrams suggested by the previous workers but also yielded much information about the place, shape, and velocity in the crystal growth process.

High Pressure Hot-Pressing of Y1Ba2Cu3O7-δ Superconductor

An almost theoretical density of Y1Ba2Cu3O7-δ was achieved under a high pressure of 5.4 GPa. While the pressed sample showed a semiconducting behaviour, the postannealed sample showed a superconducting transition with Tc, onset=92 K, Tc, end=90 K. The best critical current density, Jc at 77 K was 1650 A/cm2. The crystal structure of Y1Ba2Cu3O7-δ was distorted by the high pressure but recovered through annealing.

Preparation of Bulky Bi(Pb)-Sr-Ca-Cu-O Superconductor by Magnetized Twin-Roll

A highly oriented (Bi, Pb)2Sr2Ca2Cu3Ox bulk superconductor has been prepared by magnetized twin-roll processing. In these bulks, plate-like crystal grains were highly oriented by a magnetic and mechanical force. The grain c-axes were parallel to the magnetic field and pressing directions. In fact, both critical current density (Jc) and orientation degree of the sample rolled under 2 T were higher than those of the sample rolled with no magnetic field. The magnetized twin-roll processing is effective not only in enhancing grain-orientation but also in packing to improve Jc.

Preparation and Characterization of Grain Boundary Josephson Junction from Amorphous Thin Film

To prepare the grain boundary Josephson junction, Y-Ba-Cu-O amorphous thin film was first etched into a bridge shape and then annealed at a high temperature, 990°C, to form grain boundaries. The bridge thus prepared showed clear Shapiro steps under the irradiation of microwaves at 4.2 K. Grains of the films sensitively varied their structures, sizes and electric properties with annealing temperature. The grains annealed at a lower temperature, 960°C, were as small as a few micrometers or less. Large grains appeared when the annealing temperature became higher, and the higher the annealing temperature, the greater the number of large grains, untill 990°C. The bridge of 5 µm in width that had grains over 10 µm in size exhibited a.c. Josephson effect, but the effect was not observed with the bridge which consisted of only small grains of a few µm.