T. Izumi

Phase separation of Nd1+xBa2−xCu3O6+δ during annealing processing

Abstract In order to investigate phase separation in solid, Nd 1+ x Ba 2− x Cu 3 O 6+ δ powders with different compositions were annealed under different annealing conditions. The temperature – substitution ratio – oxygen content phase diagram of Nd 1+ x Ba 2− x Cu 3 O 6+ δ solid solution (Nd123ss) was constructed. The condition to occur Nd123ss phase separation was clarified by investigating the compositional fluctuation, XRD pattern, the time dependences of T c properties and oxygen content. Consequently, only orthorhombic phase with small x can separate into two orthorhombic phases with larger and smaller x by the spinodal decomposition mechanism.

Influence of the second buffer layer for alignment of seed film on Ni tape with NiO buffer layer

Abstract In order to investigate the influence of the buffer layer for the seed film quality which is important to obtain high quality liquid phase epitaxy film, we prepared several different kinds of SmBa 2 Cu 3 O y (Sm123) seed films on SOE-NiO buffered biaxially textured Ni tapes with and without the second buffer layer of BaZrO 3 by the pulsed laser deposition method. As a consequence of the investigation, it was found that BaZrO 3 is a suitable material as a buffer layer to obtain highly in-plane aligned Sm123 seed films on SOE-NiO buffered biaxially textured Ni tapes.

Superconducting transition temperature of NdBCO liquid phase epitaxy film on MgO substrate and effect of Mg diffusion

Abstract NdBCO thick films were grown by liquid phase epitaxy (LPE) for the electronic device substrates. To optimize the process for achieving high critical transition temperature ( T c ), different oxygenation conditions were applied. At 250°C held for 200 h in the oxygen gas atmosphere, the high T co of NdBCO LPE film was achieved at 91 K with a sharp transition, which is lower than the reported maximum value of nearly 95 K for the NdBCO system. The contamination of Mg from the MgO substrate to the RE123 film plays a significant role in the T c reduction. The Mg diffusion coefficient D Mg in NdBCO and its relation with temperature were deduced.

Mechanism of NdBCO film growth on YBCO-seeded MgO substrate using liquid phase epitaxy

Abstract YBa 2 Cu 3 O 7− x (YBCO) seed films were used to grow NdBa 2 Cu 3 O 7− x (NdBCO) superconducting films on MgO substrates by liquid phase epitaxy. The film microstructures were characterized using optical and electron microscopies to clarify the growth mechanism. It was found that most of the seed grains decomposed at the high processing temperature and dissolved when they touched the solution. The NdBCO grains were formed first by the quasi-homo-epitaxial growth of NdBCO units on the few surviving YBCO seed grains and then grew pendently to cover the large area of the bare MgO substrate surface region quickly by lateral overgrowth. A micrometer-thick melt layer was entrapped between the film and the substrate. Through the few links provided by the surviving seed grains, a stable film/substrate orientation relationship could still be maintained.

Superconducting properties of Ca-doped YBCO thick film by liquid phase epitaxy

Abstract Liquid phase epitaxial (LPE) Y1−xCaxBa2Cu3O7−δ (Ca–YBCO) was successfully grown in the fabrication of thick films with high Tc values (over 90 K) even with a small thickness of 0.5 μm. ICP analyses reveal the Ca distribution coefficient (solid composition/liquid composition) is higher than unity, which may imply a Ca-poor grain boundary in Ca-doped LPE thick films. TEM-EDX analyses reveal Mg is non-detectable in a Ca-doped sample, while it is detected to be higher than 1 at.% in a non-doped YBCO thick film, suggesting that the interaction parameter between Ca and Mg (eCaMg) is positively high and leads to suppressing the Mg contamination. Jc decreases less in the Ca-doped film than in the non-doped film with increase of the applied magnetic field.

Fabrication of coated conductor using LPE process

Abstract We tried to apply the LPE process to fabricate RE123 coated conductors. We have been investigating the construction using a BaZrO 3 buffer layer, called “non-reactive buffer type” on the textured Ni substrate. It was confirmed that this construction is stable for the LPE process, because the BaZrO 3 is stable to the RE–Ba–Cu–O solution at the process temperature. First, Y123 was grown on the MgO single crystal substrate with the BaZrO 3 buffer layer in order to confirm the potential of this combination. We confirmed that the BaZrO 3 layer could be effective to prevent from chemical diffusion of Ni and a high superconducting performance cloud be obtained as T c =92 K and J c =2×10 5 A/cm 2 . We prepared with the Sm123 seeded substrate on the BaZrO 3 /SOE/textured Ni, and confirmed that the seed layer and buffer layer showed the fourfold symmetry in the crystal in-plane alignment. Y123 was tried to grow on this sample and it was confirmed that the BaZrO 3 buffer layer prevented from the reaction between the Ni substrate and the solution. Consequently, the Y123 layer with a good in-plane alignment was obtained and it showed high T c value of 90 K.

Growth mechanism of RE-Ba–Cu–O film on MgO substrate by liquid phase epitaxy

Abstract Hetero-epitaxial growth of YBa2Cu3Oy (YBCO) crystals on MgO substrates has been investigated by comparing with the homo-epitaxial growth on a YBCO substrate, in order to clarify the initial growth mechanism of the liquid phase epitaxy (LPE). It was found that the slope angle of the growth grain varied with growing in the initial stage of the LPE growth. In the case of the homo-epitaxial growth, the slope angle decreased with growing the crystal. On the other hand, the slope angle increased with growing the crystal in the hetero-epitaxial growth. These phenomena could be explained by considering the difference in step-advancing rates between on the hetero- and homo-epitaxial interfaces. It was found that the hetero-epitaxial interface strongly affected the quality of the LPE film. The guideline for the high quality LPE film by the hetero-epitaxial growth could be obtained.

Fabrication of RE123 coated conductor using LPE process

Abstract We have tried to apply the liquid phase epitaxy (LPE) process to fabricate RE123 coated conductors. We have developed the architecture using a BaZrO 3 buffer layer, called “non-reactive buffer type” on a textured Ni substrate. In the previous study, RE123/BaZrO 3 /NiO/Ni sample was shown that T c value was about 90 K, however the J c value was lower than 10 3 A/cm 2 . In order to increase J c of the film, the perfect coverage on of the BaZrO 3 on the substrate has to be attained. Then, we tried to grow a BaZrO 3 buffer layer directly on a textured Ni substrate without formation of the NiO layer to maintain good in-plane alignment. The alignments of the BaZrO 3 layer and the seed layer become better in this architecture, and the linkage of LPE grains was improved. Consequently, the sample of Y–Yb/BaZrO 3 /Ni revealed the T c value of about 90 K and the I c value of 12 A.

Phase-field analyses for Y123 crystal growth

Abstract A phase-field evolution equation (a kind of time dependent Ginzburg–Landau equation), in which order parameter ( φ ) transition in width restricted within a unit grid, was solved using an implicit finite difference method. The direct implicit scheme would have an ability to correspond to a long-range order parameter correlation, which appeared around a phase transition. When the transition width is restricted within a unit grid, a skew directional finite difference is necessary to obtain an isotropic growth rate. Using these schemes, an isotropic growth shape was obtained for a single composition system with a latent heat free condition. In the case of a large latent heat condition, which is the case for Y123 crystal growth, we could obtain a faceted growth shape without using any anisotropic parameters.