Papers by Author: Takuya Hoshina

Abstract: An effect of addition of CaO on electrical resistivity in BaTiO₃-(Bi_1/2Na_1/2)TiO₃ (BT-BNT) solid solution ceramics was investigated. Three kinds of CaO adding processes were applied. Extra CaO additive against stoichiometric composition is effective to give semiconductive property into the BT-BNT ceramics. However, the ceramics with CaO-inclusive composition such as Ba_0.9(Bi_0.5Na_0.15Ca_0.35)_0.1TiO₃ had a high resistivity. From chemical composition analysis and structure analysis, it was concluded that Ca²⁺ dominantly substituted for Na⁺ in the A site of perovskite-type ABO₃ structure and then that residual Bi³⁺ acts a donor.

Abstract: In order to clarify the origin of the elastic nonlinearity in piezoelectric ceramics, the effect of Mn substitution into Pb (Zr,Ti)O₃-based ceramics on the nonlinear elastic stiffness was investigated by analyzing the transient response after burst excitation. The real part of 3rd-order elastic stiffness drastically decreased with increasing Mn content. It was also found that the appearance of the elastic nonlinearity enhanced the mechanical loss. From the results, the irreversible domain wall motion overcoming the potential barrier due to defect complexes was considered as a possible origin of the elastic nonlinearity.

Abstract: Aluminum substituted La₃Ta_0.5Ga_5.5O₁₄ (LTGA) single crystals were grown via a floating zone (FZ) method in N₂ or O₂ gas flows. Also, LTGA single crystals that were grown using the Czochralski method were annealed in N₂ or O₂ gas flows. The resistivity of these crystals was measured in a temperature range from 300 to 700°C. The LTGA crystals that were grown/annealed in the N₂ atmosphere showed higher resistivity by 1 order of magnitude as compared to those crystals grown/annealed in the O₂ atmosphere. Based on these results, we suggest that the difference in resistivity is due to the concentration of oxygen vacancy and interstitial oxygen ions.

Abstract: The relation between the dielectric tunability and the electro-optic (EO) effect of barium strontium titanate (Ba_0.5Sr_0.5TiO₃) thin film was discussed. The tunability of dielectric permittivity calculated from the complex admittance with planer electrodes reached to 53.1%, and the tunability of birefringence by EO effect was 0.6%. The birefringence change from EO effect was much lower than the tunability of dielectric permittivity. The materials with high tunability do not always exhibit high EO effect. This is concluded to arise due to the ionic mass in the EO materials.

Abstract: Pure Barium titanate (BaTiO₃) ceramics with grain sizes of 0.6, 1.5, and 18 μm on average were prepared by using a two-step sintering method. The wideband dielectric spectra in the range from kHz to THz were measured at 25 and 150 °C to understand the polarization mechanism of fine-grained BaTiO₃ ceramics. It was revealed that the dipole and ionic polarizations decreased with decreasing grain size at 25 and 150 °C when the grain size was smaller than 1 μm. These mechanisms were interpreted as a grain boundary effect.

Abstract: BaTiO₃–(Bi_1/2Na1/2)TiO₃ (abbreviated as BT-BNT) solid solution ceramics, as a lead-free PTC (Positive Temperature Coefficient of resistivity) thermistor material usable over 130°C, were synthesized in air atmosphere. By conventional processes, La-doping was not effective in giving semiconducting property to the BT-BNT ceramics because of volatilization of sodium and bismuth during sintering. By preventing the volatilization by lowering the sintering temperature using very fine TiO₂ powder and by adding calcium oxide to calcined BT-BNT powders, we obtained BT-BNT semiconducting ceramics which shows a resistivity change near Curie temperature.

Abstract: BaTiO₃–(Bi_1/2K_1/2)TiO₃ (abbreviated as BT-BKT) solid solution ceramics, as a lead-free PTC (positive temperature coefficient of resistivity) thermistor material usable over 130°C, has been synthesized by sintering in N₂ atmosphere and after annealing in air over 1200°C. In the BT-BKT ceramics with PTC property, the impedance/modulus spectroscopic plots have revealed that a third resistance-capacitance (RC) response besides grains and grain boundaries. Using the remote electron beam induced current (REBIC) configuration, imaging has revealed EBIC contrast consistent with the presence of negatively charged electrostatic grain boundary barriers in the BT-BKT semiconducting ceramics.

Abstract: An estimation method of the influence of interfaces on properties in perovskite-type oxide thin-film capacitors is presented. We proposed a modified Schottky model that can be employed to explain the electric properties of metal/perovskite-type oxide junctions. The modified model considers the electric field dependence of permittivity and the flow of electrons from metal to defect states located in the band gap of the perovskite oxide. The simulation based on this model could successfully describe the results of capacitance-voltage measurements of junctions between metals (Pt, Au, and Ag) and Nb-doped SrTiO₃, which were not explained by the conventional Schottky model. Additionally, a simulation for a thin-film capacitor with hysteretic behavior could depict an asymmetric capacitance-voltage curve.

Abstract: BaTiO₃ films were fabricated on Si substrate using inkjet printing technique. To achieve low-temperature fabrication of BaTiO₃ films, Ba-Ti-alkoxide ink was prepared. The mixing ratio and sequence of raw materials were optimized for controlling the chemical stability and viscosity of alkoxide ink. The inkjet printing films on Si substrate were annealed at 700 °C, and it resulted in the formation of BaTiO₃. It was found that the film quality was improved by adding BaTiO₃ nanoparticles into the Ba-Ti-alkoxide ink.

Abstract: Freestanding BaTiO₃ ceramics films were fabricated using the aerosol deposition (AD) method and the size effect of nanograined BaTiO₃ ceramics was demonstrated. Dense BaTiO₃ thick film fabricated by the AD method was crystallized and detached from substrate by an annealing treatment at 600 °C, and then the grain size was controlled by a reannealing treatment at various temperatures. As a result, freestanding BaTiO₃ thick films with various grain sizes from 24 to 170 nm were successfully obtained. Polarization–electric field (P–E) measurement revealed that BaTiO₃ ceramics with grain sizes of more than 58 nm showed ferroelectricity, whereas BaTiO₃ ceramics with an average grain size of 24 nm showed paraelectricity at room temperature. Dielectric measurement indicated that the permittivity decreased with decreasing grain size in the range of 170 to 24 nm.