Katsuya Yamagiwa

High critical current density YBa2Cu3O7−δ films on surface-oxidized metallic substrates

Abstract YBa 2 Cu 3 O 7−δ (YBCO) films with high critical current density ( J c ) were fabricated on nickel tapes buffered with bi-axially textured NiO prepared by surface-oxidation epitaxy (SOE). The effects of oxide cap layers, such as YSZ, CeO 2 and MgO, on the SOE-grown NiO were investigated to improve the superconducting properties of the YBCO films on NiO. By inserting a thin MgO cap layer between NiO layer and YBCO film, a J c of 3×10 5 A/cm 2 (77 K, 0 T) was achieved. This result indicates the potentiality of the SOE method. In this paper, the application of the NiO/Ni substrate to non-vacuum processings, such as metal-organic deposition (MOD) and liquid phase epitaxy (LPE) will be also introduced.

Fabrication of YBa2Cu3O7−x film by metalorganic deposition method using trifluoroacetates and its process conditions

Abstract Fabrication of YBa 2 Cu 3 O 7− x (YBCO) superconductors by the metalorganic deposition (MOD) method has many advantages: precise controllability of metal components, wide flexibility to coating objects, and low-cost non-vacuum approach. We succeeded in the preparation of a high-quality coating solution that enables us to obtain YBCO films having high critical currents ( I c ). We also investigated processing conditions: proper control of water vapor in furnace, suppression of the reaction between substrate and fluoride during heat treatments, and other process conditions. By their optimization, we can successfully fabricate the film, which has critical current density ( J c ) of 4.6 MA/cm 2 at 77 K, 0 T. The results show the potentiality of the metalorganic deposition method using trifluoroacetates (TFA-MOD method) for high-temperature superconductor applications.

Processing of Piezoelectric (Li,Na,K)NbO3 Porous Ceramics and (Li,Na,K)NbO3/KNbO3 Composites

Porous Li0.06(Na0.5K0.5)0.94NbO3 (LNKN-6) ceramics with different pore volumes have been prepared using preceramic powder and phenol resin fiber (KynolTM) as a pore former. It was confirmed that the porous ceramics synthesized by the “two-stage firing method” suppressed the loss of alkali elements from the porous body during heat treatment. The porous LNKN-6 ceramics were then converted to LNKN-6/KNbO3 composites through soaking and heat treatment using a sol–gel precursor source composed of KNbO3 to form 3–3-type composites. The microstructure, dielectric, and piezoelectric properties of the porous LNKN-6 ceramics and LNKN-6/KNbO3 composites were characterized and compared. The LNKN-6/KNbO3 composites had a hollow structure whose pores in the region near the surface were filled and coated with KNbO3 precipitates; however, a large amount of residual air was trapped in the pores inside the composites. As a result, the LNKN-6/KNbO3 composites fabricated using 30 vol % KynolTM showed an enhanced piezoelectric voltage output coefficient (g33) of 63.0×10-3 V·m/N, compared with monolithic LNKN-6 ceramics having a g33 of 30.2×10-3 V·m/N.

Orientation Behavior of REBa2Cu3O7-y (RE = Rare Earth and Y) Films Prepared by Chemical Solution Deposition.

We have succeeded in preparing REBa2Cu3O7-y (RE123; RE = rare-earth elements and Y) films by chemical solution deposition (CSD) processing using metal naphthenates under low oxygen partial pressure. Few reports have been published on the preparation of RE123 by CSD processing, therefore, we prepared various RE123 films on different substrates systematically and examined the difference in the appearance of a/b-axis oriented grains and their in-plane alignment for each RE123 film as a function of heat-treatment parameters. Processing temperature and substrate dependencies of these films, grown by solid phase epitaxy, were observed to be the same as in the case of vapor phase epitaxy. Dependencies on ionic radius of RE elements are also observed and it is explained by lattice misfit.

Origin of high electrical conductivity in alkaline-earth doped LaCoO3

Alkaline-earth doped LaCoO3 has been found to be promising as conductive oxides instead of standard metals for use as electrodes. Electrical conductivity has a maximum, σ = 4.4 × 103 S cm−1, in La0.6Sr0.4CoO3. We believe that the increase in mobility was caused by the decrease in the Co–O bond length.

Effects of the Final Heat-Treatment Conditions on Microstructures of YbBa2Cu3O7- δ Superconducting Final Films Deposited on LaAlO3(001) Substrates by the Dipping—Pyrolysis Process

We investigated by transmission electron microscopy the relationship between the initial heat-treatment conditions and the microstructures of YbBa2Cu3O7-δ superconducting final films deposited by a dipping-pyrolysis process on SrTiO3(001) single crystal substrates. An amorphous precursor film was prepared by heating rapidly up to 425°C. By heat-treating at 725°C, this film became a c-axis oriented YbBa2Cu3O7-δ film with a thickness of 100 nm, and showed a sharp resistive transition around the critical temperature (TC). On the other hand, a polycrystalline precursor film was produced by heating slowly. This film did not give rise to an epitaxial film, but to a YbBa2Cu3O7-δ polycrystalline film with a grain size of about 200 nm, and exhibited a broad transition. Therefore, the initial heat-treatment conditions and the microstructures of the precursor films are important for the epitaxial growth and superconducting properties of the final YbBa2Cu3O7-δfilms.

Electronic transport properties of the perovskite-type oxides La1−xSrxCoO3±δ

La1−xSrxCoO3 is a leading candidate oxide electrode material for replacing standard metal electrodes. Here we investigate the origin of its high electronic conductivity by means of Rietveld analysis, Hall effect measurements, and band-structure calculations. A high electrical conductivity at room temperature, σ = 4.4 × 103 S cm−1 (which is the largest value for oxides that do not contain noble metals), was achieved by the substitution of 40–50 mol% Sr into the La sites, with a high carrier concentration of 4.5 × 1021 cm−3 and a carrier mobility of 5.8 cm2 V−1 s−1. The carrier concentration results from a trade-off between hole generation via Sr substitution and electron generation via oxygen deficiency. The high mobility arises from the small effective mass originating in the transition to a higher spin state.

Transmission Electron Microscopic Studies of YbBa2Cu3O7-δ-Precursor Films Formed on SrTiO3 by Dipping-Pyrolysis Process

We prepared precursor films of YbBa2Cu3O7-δ by coating SrTiO3(001) single crystal substrate with metalorganic compounds solution and by heating at 425°C. Then, microstructures of the precursor films were observed by transmission electron microscopy. The film heated rapidly up to 425°C was amorphous. On the other hand, the film heated gradually was polycrystalline. After final annealing at 725°C, the former sample showed a sharp transition of the electric resistance around critical temperature (TC). The latter one showed a broad transition. Therefore, we conclude that a high heating rate at the pre-treatment is important for achieving a sharp resistive transition of the final film.

Effect of Pore Shape on the Electrical Property of Porous LNKN Piezoceramics

Porous Li 0.06 (Na 0.5 K 0.5 ) 0.94 NbO 3 (LNKN-6) piezoceramics with different pore shapes and volumes have been fabricated by sintering a mixture of calcinated LNKN-6 powder and volatilizable pore formers of phenol resin fibers (Kynol™) or plastic spheres [poly(methyl methacrylate)] (PMMA). Kynol™ fiber was shown to more effective for inducing open-pore formation in the ceramics than PMMA sphere. The porous LNKN-6 ceramics prepared using Kynol™ fiber demonstrated lower dielectric loss, higher piezoelectric constant (d 33 ), and higher piezoelectric voltage coefficient (g 33 ) than those prepared using PMMA sphere. The effect of pore shape on the electrical property was investigated.

Preparation of c-axis oriented RE123 films by metalorganic deposition process

We have succeeded in preparing c-axis oriented REBa2Cu3O7–y (RE123; RE = Yb, Gd, Sm, Nd, La) films by metalorganic deposition process. A homogeneous coating solution having a molar ratio, RE:Ba:Cu=l:2:3, was prepared by dissolving metal naphthenates in toluene. These solutions were spin-coated onto various substrates. The precursor films calcined at 425°C were fired at various temperatures under low oxygen partial pressure condition. In the case of RE123/MgO, RE123 films crystallize from amorphous under ambient oxygen pressure and give strong (00n) peaks which correspond to c-axis orientation, however, the films were not in-plane aligned at the lower temperature region. In-plane aligned RE123 films can be obtained at the higher temperatures. On the other hand, in the case of using SrTiO3, NdGaO3 or LaA1O3 as substrate, RE123 films showed not only (00n) peaks but (n00) or (0n0) peaks which correspond to a/b-axes orientation. In-plane aligned c-axis oriented films appears at the higher temperature region.