Since the discovery of high-Tc cuprate superconductors 30 years ago, their practical applications particularly for saving and storing energy have been expected. However, high-Tc superconducting materials had required very long time before their extensive applications, because of many intrinsic difficulties to achieve enough high critical current performance originated in large anisotropic characteristics, short coherence length and large nonstoichiometry in chemical compositions. Fabrication of polycrystalline superconducting tapes having highly grain-aligned microstructure and well controlled chemical composition is crucially important for deriving potentials of superconductors. Even now, eager efforts have been made to improve synthesis processes for producing long length tapes with high uniformity, high reliability and low cost.
In this review paper, current status of high-Tc superconducting materials and their practical applications mainly for Bi2223 superconducting tapes are shown after brief introduction of superconductors, metallic superconducting materials and characteristic features of cuprate superconductors.
REBa2Cu3O7-δ (REBCO, RE: rare earth elements) superconductors have been considered as the candidate for electric power industries such as power cable and high magnet coil application, because of their high critical current with zero resistance at temperature beyond liquid nitrogen temperature. REBCO wires consisted of superconducting thin films with highly crystal grain alignment and high critical current density in high external magnetic fields are required for the electric power applications. In this report, the growth techniques for REBCO superconductor single crystals and REBCO thin films are described. The Solute-Rich-Liquid Crystal Pulling (SRL-CP) method is a good technique for the fabrication of high quality large REBCO single crystals. The REBCO single crystals grown by this method have been applied to the scientific materials characterization. On the other hand, highly oriented REBCO thin films have been fabricated using a metal organic deposition (MOD) technique. Flux pinning centers are introduced artificially into the films through MOD process to enhance critical current density in magnetic fields.
Magnetic alignment using static magnetic field (SF) and modulated rotating magnetic field (MRF) enables formation of uni-axial and tri-axial grain arrangement at room temperature for substances with magnetic anisotropy, respectively. In this review article, we introduce two topics to show high potential of combination of magnetic alignment and colloid processes. The former topic is uni-axial orientation of a layered thermoelectric (TE) cobaltite by electrophoretic deposition in SFs. Using this technique, the fabrication of multilayered TE module containing c-axis-oriented cobaltite layer is demonstrated. The latter topic is the fabrication of tri-axially grain-aligned ErBa2Cu4O8 (Er124) green pellets by slip-casting under MRFs. The degrees of tri-axial orientation are sensitive to viscosities of suspensions. So far, an Er124 green pellet with the degree of inplane orientation with ～7° has been successfully fabricated by control of viscosity of suspensions and introduction of oscillation type of MRF.
Photoelectrochemical (PEC) water splitting using semiconductor photoelectrodes have been regarded as one of the promising mean to convert solar energy to chemical energy, hydrogen, directly. Cu (In, Ga) Se2 (CIGS) based photoelectrode shows hydrogen evolution from water under simulated sunlight with half-cell solar to hydrogen conversion efficiency of 8.5% at 0.38 VRHE. BaTaO2N photoelectrode was prepared by particle transfer method with using Ta, and Ti as contact, and conductor layer, respectively, and the photoelectrode showed relatively large anodic photocurrent of 4.3 mAcm-2 at 1.23 VRHE. A PEC cell consisting of the combination of Pt loaded Al-LTC0.9A0.1 and Co loaded BaTaO2N fabricated by particle transfer method placed side by side showed spontaneous overall water splitting under visible light.
We have been refining the chemistry in the flux growth of single-crystal for the development of cutting-edge battery materials. Flux growth is based on the flow of source materials to the growing crystals in molten salts (flux) at high temperature. Main challenging of this method is selecting suitable fluxes to achieve shape-controlled crystal growth. In this paper, we shortly summarize our recent activities on the strategic studies of flux growth for battery materials, including detailing how to select fluxes.
We have developed novel scintillation crystals based on Ce-doped (Gd，La)2Si2O7 grown by the floating zone and Czochralski methods for oil well logging. The light output of 35,000 photons/MeV or more was kept in the temperature range from 25 to 150 ºC for these crystals. Ce: (Gd, La)2Si2O7 single crystals can be applied to oil well logging and other radiation detection application under high temperature conditions.