Tasuku Kitamura

Densification effect on the microstructure and critical current density in (Bi,Pb)2Sr2Ca2Cu3OxAg sheathed tape

The density of oxide core in (Bi,Pb)2Sr2Ca2Cu3OxAg sheathed tape was estimated from Vickers hardness and found to be increased to as high as 93% of the theoretical density with increasing the degree of cold working. During heat treatment, this densification before heat treatment promoted the formation of a strong linked microstructure which is characterized by dense microstructure, a high degree of orientation of oxide crystal and smooth interface between the Ag sheath and oxide core. Consequently, we obtained high values of critical current density: 66 000 A/cm2 at 0 T and 14 500 A/cm2 at 1 T for 77 K.

Critical currents of YBa2Cu3Oy thick films prepared by liquid phase epitaxial growth

We have succeeded in preparing c‐axis oriented high‐Jc YBa2Cu3Oy films on MgO(100) substrate by the liquid phase epitaxy (LPE) technique. The growth rate was typically about 2 μm/min, which was 10–102 times larger than that by ordinary vapor growth techniques. The film thickness ranged 10–50 μm by choosing the dipping time. The Tc of the best film exceeded 88 K after oxygen annealing, and the transport Jc was 1.1×105 A/cm2 at 77 K and 0 T. In‐field Jc’s at 77 K and 1.5 T were 2.8×104 A/cm2 and 2.0×104 A/cm2 for the B⊥ab plane and B∥ab plane, respectively. In addition, the peak effect of Jc was observed at several tesla for B∥ab plane geometry. Based on the microstructure observed by high resolution transmission electron microscopy, the relevant peak effect is considered to be caused by stacking faults which act as a field induced pinning center.

CRYSTALLINE ORIENTATION OF YBA2CU3OY FILM PREPARED BY LIQUID-PHASE EPITAXIAL GROWTH ON NDGAO3 SUBSTRATE

Superconducting YBa2Cu3Oy films were grown by liquid‐phase epitaxy on NdGaO3(110) single‐crystalline substrates by a modified top‐seeded solution growth method. Two different melt compositions were used. In the case of Cu‐poor melt compositions (Ba:Cu=3.0:5.0), only c‐axis oriented growth was observed. However, if we used Cu‐rich melt compositions (Ba:Cu=3.0:6.5), both c‐axis and a‐axis oriented films were obtained. In the latter case the crystalline orientations of the YBa2Cu3Oy films were dependent on the temperature of the surface of the melt. At lower temperatures, c‐axis oriented films were obtained, whereas the films showed a‐axis orientation at higher temperatures. The width of the temperature region over which the orientation changes is less than 3 °C, which is very narrow compared to the width observed in conventional vapor phase depositions (typically 50 °C). We propose that changes in orientation during the liquid‐phase epitaxial growth is due to the unique properties of the phase diagram of th...

Initial growth mechanism of a/b‐axis oriented YBa2Cu3O7−y film prepared by liquid phase epitaxy

The a/b‐axis oriented YBa2Cu3O7−y (YBCO) films were fabricated by liquid phase epitaxy (LPE) on NdGaO3 (110) single‐crystalline substrates using a modified top‐seeded solution growth method. The x‐ray diffraction measurement, and transmission electron microscopic observation showed the epitaxial growth of YBCO film on the substrate. Scanning electron microscopic observation revealed the epitaxy of YBCO on the substrate and an initial growth mechanism of a/b‐axis oriented YBCO film by LPE. The film growth consists of the following stages; (1) nucleation and island growth, (2) lateral growth of island, and (3) layer formation due to coalescence of island.

FIELD-INDUCED PINNING CENTERS OF YBA2CU3O7-Y SUPERCONDUCTING THICK FILM PREPARED BY LIQUID PHASE EPITAXY

Abstract Superconducting YBa 2 Cu 3 O 7− y films were fabricated by liquid phase epitaxy (LPE) on MgO (100) and NdGaO 3 (110) single-crystalline substrates. The film of 10 μm thickness on the MgO substrate shows a high critical current density over 10 5 A/cm 2 at 77.3 K. This film includes the stacking faults that were introduced due to the large lattice mismatch between the YBCO film and the MgO substrate. We measured J c − B characteristics of this high J c and found the peak effect: enhancing J c with increasing field. On the other hand, J c of the films on the NdGaO 3 substrate is comparably low, and we have a microstructure without stacking faults. These results suggest that the stacking faults in the films on MgO substrates behave as field-induced pinning centers at 77.3 K.

Growth mechanism of thick c-axis oriented YBa2Cu3O7 − y films prepared by liquid phase epitaxy

Abstract c -axis oriented YBa 2 Cu 3 O 7 − y (Y123) thick films were grown on NdGaO 3 (110) single crystalline substrates by liquid phase epitaxy (LPE). The growth mode of the film changes from a multi-nucleation growth to a spiral-step growth. The growth of the initial stage occurs due to the local supersaturation at the growth interface by the temperature difference between the substrate and the surface of the melt and the later stage growth is driven by the supersaturation from the transport of the solute through convection of the melt. The change of growth mode is also described as the change from the transient mode LPE (TMLE) growth to the steady state growth.

Peak effect, oxygen deficiency, and scaling behavior of magnetization for YBa2Cu3O7−δ crystals prepared by top-seeded solution growth

Abstract We present the magnetic properties of YBa 2 Cu 3 O 7−δ crystals prepared by top-seeded solution growth (TSSG) [1]. The crystals showed the peculiar peak effect (fish-tail) for fields parallel to the c -axis, for which the hysteresis width in the magnetic isotherm M ( H ) showed a single maximum at | H p | > 0. This M ( H ) was observed up to 85 K, just below the maximum T c = 86 K, and was found to be insensitive to various oxygen annealings. On the other hand, oxygen reduction, which resulted in a decrease in T c by up to 4 K, showed a scaling behavior of the normalized ( M · H ) for both temperatures and oxygen contents, indicating a single pinning mechanism. This implies that the oxygen vacancies or twin planes are unlikely to account for this peak effect. We also discuss the effects of defects such as twisted boundaries, localized stacking faults, and impurities on the peak effect. Together with the fish-tail associated with oxygen vacancies, we suggest that the peak effect is not related to specific ions, but to material processing.

Growth mechanism and crystalline orientation of liquid-phase epitaxially grown YBa2Cu3O7-y films

Superconducting YBa 2 C 3 O 7-y , (Y123) films were grown by liquid-phase epitaxy (LPE) on a NdGaO 3 (110) single-crystal substrate by a modified top-seeded crystal-growth method. The crystalline orientation of the epitaxially grown Y123 films depends on the growth temperature and the composition of the melt. In the case of copper-poor melt compositions (Ba:Cu = 3.0:5.0 and 6.0), only c-axis oriented growth was observed, while both c-axis and a/b-axis oriented films were obtained using the melts of copper-rich composition (Ba:Cu = 3.0:6.5 and 7.0). In the latter case, the crystalline orientation of Y123 films was dependent on the temperature of the surface of the melt. At lower temperatures, c-axis orientation films were obtained, while the films showed a/b-axis orientation at higher temperatures. The mechanism of the orientation of epitaxial growth of Y123 film is explained by the change of the supersaturation of the solute at the growth interface dominated by the solubility line of the Y 2 BaCuO 5 (Y211)-BaO + CuO system.

Superconducting properties of Nd-Ba-Cu-O thick films prepared by liquid phase epitaxial growth

C-axis oriented Nd/sub 1+x/Ba/sub 2-x/Cu/sub 3/O/sub y/ thick films were grown by the liquid phase epitaxy (LPE) technique onto MgO (100) single crystal substrates in Ar gas atmosphere containing 0.5 at.% O/sub 2/. The superconducting transition temperature T/sub c/ was very broad, when the film thickness was 10 /spl mu/m but became very sharp when it was increased to 100 /spl mu/m. The T/sub c/ value for the 100 /spl mu/m thick film could be increased up to 92 K by post-annealing at 300/spl deg/C for 336 hrs in O/sub 2/ atmosphere. Moreover, the peak effect, which is characterized by an increase in magnetisation with increasing magnetic field up to 15000 G at 50 K, was observed in the 100 /spl mu/m thick film. A substantial improvement in the superconducting properties when the film thickness is increased to 100 /spl mu/m, can be attributed to the stable persistence of the steady state growth mode instead of the transient mode responsible for the first 10 /spl mu/m thick deposition.

FORMATION MECHANISM OF GRAIN BOUNDARIES IN YBA2CU3O7-Y SUPERCONDUCTING THICK FILM BY LIQUID PHASE EPITAXY

Abstract Superconducting YBa 2 Cu 3 O 7 − y thick films were fabricated by liquid phase epitaxy (LPE) on the MgO (100) single crystalline substrate. The transport superconducting characteristics of the films depend on the crystalline in-plane alignment of the film. The basal plane grain boundaries with large tilted angle of 36° and 45° were found by transmission electron microscope observation in the film of the low critical current density. The large tilt angle grain boundaries behave as a resistive type grain boundary and lower transport J c of the YBCO film. These boundaries were formed by the coalescence of island grown YBCO layer due to the multi-nucleation and growth mode of the hetero-epitaxial growth on the MgO (100) substrate.