Papers by Author: Hiroaki Takeda

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 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: The piezoelectric shear response of 94.0(Ka0.5, Na0.5)NbO3 (KNN) + 5.0LiNbO3 (LN) + 0.5SrTiO3 (ST) + 0.5BiFeO3 (BF) ceramics was investigated by Laser Doppler Vibrometry (LDV) and resonance-antiresonance method. From resonance-antiresonance method, the piezoelectric d15 constant was obtained to be 273 pC/N. The shear strain obtained by LDV　at the frequency of 150kHz showed strong nonlinearity. This suggested that the domain contribution to piezoelectric response in shear mode of KNN-LN-ST-BF ceramics existed at the operational frequency for the shear mode divices.

Abstract: Barium titanate (BaTiO3) ceramics with various grain sizes from 0.7 to 13 μm on average were prepared by a conventional sintering method, a two-step sintering method and a rate controlled two-step sintering method. The permittivity of the ceramics was increased with decreasing grain size to 1.1 μm on average. However, the permittivity of the ceramics was decreased when the grain size was below 1 μm. The field emission scanning electron microscope (FE-SEM) observations revealed that the 90º domain width decreased with decreasing the grain size. By ultrawide range dielectric spectra from kHz to THz range of the BaTiO3 ceramics, the domain contribution to the permittivity was investigated. For the BaTiO3 ceramics with grain sizes over 1 μm, the dipole polarizability and the ionic polarizability were enhanced by high domain-wall density. In contrast, for the BaTiO3 ceramics with grain sizes below 1 μm, these polarizabilities were weakened.