Yuh Shiohara

Large Levitation Force due to Flux Pinning in YBaCuO Superconductors Fabricated by Melt-Powder-Melt-Growth Process

An extremely large levitation force of as high as 30 N at a height of 1 mm was achieved in Ag-doped YBaCuO fabricated by the Melt-Powder-Melt-Growth process using a repulsive force against a 0.4 T rare-earth magnet at 77 K. The combination of a large Jc value and large shielding current loop is the source of such a large levitation force.

Microstructures of the melt-powder-melt-growth processed YBaCuO

Microstructures of two MPMG processed YBaCuO materials with and without Y{sub 2}BaCuO{sub 5} (211) inclusions were investigated by transmission electron microscopy. Using the MPMG process, it is possible to change the quantity of the 211 inclusions in the YBa{sub 2}Cu{sub 3}O{sub 7} (123) matrix. We prepared two YBaCuO samples with 0 and 30 vol. % 211 and with respective critical current density values of 2000 and 30 000 A/cm{sup 2} at 77 K and 1 T (magnetic field parallel to the c-axis). As possible pinning centers, we found stacking faults in the 123 matrix. However, we observed no appreciable change in their number and structure by introducing the 211 inclusions. Therefore, the difference in {ital J}{sub {ital c}} values can be attributed to the 211 inclusion itself.

Fracture Toughness of YBaCuO Prepared by MPMG Process

The fracture toughness of melt-powder-melt-growth(MPMG)-processed YBaCuO samples was measured and the beneficial effect of the dispersed 211 particles in the 123 superconducting phase on toughening the material was confirmed. Preferred fracture planes were (100), (010) and (001) planes, and the fracture toughness of the (001) plane was lower than that of the (100) and (010) planes.

Transmission electron microscopic studies on growth mechanism of YBa 2 Cu 3 O 7− y films formed by advanced trifluoroacetates metalorganic deposition process

Microstructure evolution of YBa 2 Cu 3 O 7− y (YBCO) films during the two heat-treatments in the advanced trifluoroacetates metalorganic deposition (TFA-MOD) process has been investigated by means of transmission electron microscopy. In the calcination process, precursor films including nanopores were formed through the shrinkage of the film after a remarkable increase of the thickness due to the thermal decomposition of metalorganic salts in the starting solution. During the crystallization process, the densification and shrinkage of the film occurred after agglomeration of nanopores and coarsening of unreacted phase particles such as Y 2 Cu 2 O 5 , CuO, and Ba–O–F in the precursor films. The YBCO films were then epitaxially grown with the remaining unreacted phase particles in the film, finally pores were generated again by a reaction of these unreacted particles to form YBCO accompanied by the volume reduction. It is important to control the densification of precursor films and coarsening of the unreacted phase particles in the crystallization process, to fabricate YBCO final films with fine crystallinity and high critical current values.

Growth Mechanism of YBa2Cu3Ox Films Grown from Seed Films by Liquid Phase Epitaxy

The initial stage of YBa2Cu3Ox (Y123) films grown on (100) MgO substrates by liquid phase epitaxy (LPE) was investigated. Y123 films could not be grown on MgO substrates by LPE directly but it was grown successfully on the MgO substrates with Y123 thin films deposited by vapor phase growth methods. In the initial stage Y123 seed film partially dissolved into BaO-CuO solution, and Y123 islands were epitaxially formed on an MgO substrate within one second. It is found that the dissolution phenomenon could be explained by means of the Ostwald ripening mechanism.

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.

Fabrication of YBCO Thick Films on Ceramic Substrates by the Gas Deposition Method Using Fine Powders

Superconducting thick films in 1mm width were fabricated on ceramic substrates (SrTiO3) by the gas deposition method using fine powders of Y1Ba2Cu3O7−x with diameter less than 1µm. After heat treatment in an O2 gas atmosphere, Tc(onset) value of 93K and Tc(end) of 75K were obtained. In the case of mixed fine powders of Y123 and Ag, Tc(end) increased to 88K.

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.

Bi-based Superconducting Films Synthesized by a Combination of Metalorganic Deposition (MOD) and a Diffusion Process

Bi-based superconducting thin films have been prepared by a combination of metalorganic deposition(MOD) and a diffusion process. The films derived from a layer of Bi/Sr/Ca/Cu=2/2/1/3 and a layer of Ca/Pb=2/1, after firing for only 4 h, yielded a nearly single phase layer of the 2223(high Tc) phase. It was found that Ca2PbO4 played a very important role in formation of the 2223 phase in short firing times, acting as a souce of Ca ions to react with the 2212 phase and CuO.

Superconducting Textured Crystal Produced by a Partial Melt Process

A partial melt process is one of the processes useful for obtaining textured crystals. The sample was pressed at above the solidus temperature (where solid and liquid phases coexist). When the solid phase is a superconductive one, superconducting textured crystals can be obtained by squeezing out the liquid phase through the partial melt process. The texture, phase, critical temperature (Tc) and critical current (Jc) of the specimens were investigated. As an example, a sample pressed at 850°C with nominal composition of Bi:Sr:Ca:Cu:O=2:1.6:0.8:1.9:y is in a highly textured manner. Three phases, Bi2212 (low Tc Phase), Bi2201, and CuO were observed in the sample. The values of Tc and Jc of this sample were 60K and 740A/cm2 at 30K, respectively.