Teruo Izumi

Growth Mechanism During Directional Solidification on Y-System Superconductors

The unidirectional solidification, which is a zone melt method, was performed on Y-system superconductors. The condition for continuous growth of YBa2Cu3Oy crystals was confirmed to be larger G/R than 300°Ch/cm2 where G is temperature gradient and R is growth rate. The faceted interface was recognized in the sample grown at 1 and 2mm/h. The necessary solute diffusion for peritectic reaction was clarified to be provided through liquid. The solidification model based on the liquid diffusion control was developed and it agrees qualitatively with the experimental results.

Partial Dissolution Mechanism of YBa2Cu3Ox Seed Films on MgO Substrate during Liquid Phase Epitaxy

We have investigated the relationship of the grain alignment between the seed films and the LPE films. It was found by XRD analysis that four fold symmetrical LPE films on the surface of MgO substrates were grown not only from a four fold symmetrical seed film but from an eight fold symmetrical one. These results could be explained by means of the coarsening mechanism using the difference in interface energies between the Y123 grains and the MgO substrate with different orientations.

Initial Growth Mechanism of Sm1+xBa2-xCu3Oy Film Prepared by Liquid Phase Epitaxy

Sm1+xBa2-xCu3Oy(Sm123) films on MgO substrates with seed films were prepared by the Liquid Phase Epitaxy(LPE) process. The initial growth mechanism of Sm123 LPE films was investigated by vertical dipping. In the initial growth stage of LPE, the seed films partially dissolved into BaO-CuO melt, and then island crystals of Sm123 were grown epitaxially. The relative angle of θ=45° between the [100] direction of the Sm123 island crystal and the [100] direction of the MgO substrate was preferentially grown with dissolving of the other island crystal with θ=0°. It is found that the preferential dissolution phenomenon could be explained by the difference bonding at the interface between Sm123 and MgO. It was also found that the growth rate in the Sm system was higher than that of the Y system under the same undercooling.

Single Crystals of Nd1+x(Ba1-ySry)2-xCu3O7±δ Solid Solution Grown by Crystal Pulling

Series of single crystals of Nd1+x(Bal-ySry)2-xCu3O7±o (Nd1113ss) solid solution were grown by crystal pulling. The phase relationship was investigated, which could lead to controlling cation stoichiometric ratio. The Sr substitution on Ba sites can be controlled by increasing the Sr content in the liquid. On the other hand, the Nd substitution on the Ba sites can be depressed by using the liquid composition with a high Ba/Cu ratio.

Fabrication process of twin-free orthorhombic Nd123 single crystal under uni-axial compression

A fabrication process of twin-free orthorhombic NdBa2Cu3O7-δ (Nd123) single crystals was studied, in which the as-grown tetragonal single crystals were directly annealed under uni-axial compression in order to fabricate the complete twin-free single crystals. Almost twin-free orthorhombic Nd123 single crystals could be obtained by modifying the annealing conditions from the viewpoint of the distribution of the oxygen contents and the release of the stress in the crystals during annealing. The critical current density (Jc)-H curves for the twin-free, the detwinned, and the not-detwinned Nd123 single crystals, which were affected by the Nd/Ba ratio fluctuation appearing in these crystals during annealing, showed a quite different tendency from those for YBa2Cu3O7-δ(Y123) single crystals.

Growth Anisotropy of YBCO Superconductors

The unidirectional solidification by a zone-melting method was performed in order to investigate the growth mechanism of YBa2Cu3Ox (123) superconductive oxides from the partial molten state. The samples grown from polycrystalline rods show the large faceted interfaces at the growth rate lower than 2 mm/h. The crystallographic faces toward the growth direction are estimated by a pole-figure analysis and are in the range from {223} to {225}. The samples grown from the SmBa2Cu3Ox seed crystal show the same crystal orientation as that of the seed crystal.

Alumina-Doping Effect on Solidification Mechanism of Y-System Superconductors

Alumina-doping effects on properties for solidification (melting point; TM, maximum undercooling region; ΔTmax and growth rate; R) of the YBa2Cu3Oy (123) phase by the undercooling method[1]. The melting point of the 123 phase decreases from 1010°C to 1004°C, due to 0.5 wt% of alumina-doping. ΔTmax increases from 4°C to 8°C due to the doping, when maximum temperature for partial melting (Tmax) is controlled at 1050°C. A large difference was not recognized in the relation between ΔT and R due to the doping. This means that a higher growth rate can be obtained in the case of alumina-doping than in that without doping, when the Tmax is 1050°C, which is relatively low compared to processes such as the MPMG method. Additionally, this low temperature (1050°C) for the partial melting state makes the Y2BaCuO5 (211) particles finer.

Crystal Growth of Oxide Superconductors Prepared by the Bridgman Method

The effects of temperature gradient (G) and growth rate (R) on the microstructure and characteristics of Y-based superconductors grown from the partially molten state by the horizontal Bridgman method were studied. The structure showed highly aligned YBa2Cu3Oy crystals containing non-aligned finely dispersed Y2BaCuO5 crystals for high G/R ratio. Coarsening of the YBa2Cu3Oy crystals was found to occur as GR was decreased. A sample grown under conditions of high G/R ratio (482 Kh/cm2) and low GR (5.8 K/h) had a critical current density of 19000 A/cm2 at 77K in a magnetic field of 1T.