Tadashi Shiosaki

BaTiO3–(Bi1∕2Na1∕2)TiO3 solid-solution semiconducting ceramics with Tc>130°C

As a candidate of a lead-free positive temperature coefficient of resistivity (PTCR) material, barium titanate (BaTiO3)-based solid-solution ceramics, Ba1−x(Bi1∕2Na1∕2)xTiO3 with x=0.05 (BBNT5), have been synthesized by an ordinary sintering technique. Temperature dependences of the dielectric and ferroelectric properties show ferroelectric and paraelectric phase transition of the ceramics with the Curie temperature of Tc=170°C, which is higher than that of BaTiO3 (130°C). The La-doped BBNT5 ceramics display low ρ values of 102–103Ωcm at room temperature and their abrupt changes of 102–104 at about 220 °C. The reduced BBNT5 ceramics show small PTCR characteristics compared to the La-doped ones.

High Tc lead-free BaTiO3–(Bi1∕2Na1∕2)TiO3 positive temperature coefficient of resistivity ceramics with electrically heterogeneous structure

A distinct positive temperature coefficient of resistivity effect has been observed in BaTiO3–(Bi1∕2Na1∕2)TiO3 (BT-BNT) ceramics sintered in a N2 flow with low O2 concentration. With the addition of BNT, the samples exhibit resistivity jumps of ∼103–105 starting at ∼190–210°C. X-ray diffraction results indicate that the BNT phase and BT phase formed a solid solution during sintering. An electrically heterogeneous structure, consisting of the grain interiors, outer grain shells, and grain boundaries, is revealed by the complex impedance analyses. The observed dc resistivity jump is attributed to the rapid resistivity rise in both grain boundaries and grain shells.

Effective substitution of aluminum for gallium in langasite-type crystals for a pressure sensor use at high temperature

Aluminum-substituted La3Ga5SiO14 (LGS)-type crystals, La3Ga5-xAlxSiO14 (LGAS(x)) and La3Ta0.5Ga5.5-xAlxSiO14 (LTGA(x)), were successfully grown by the Czochralski technique. These crystals were 18-23 mm in diameter and 70-100 mm in length. The piezoelectric modulus d11 values (6.21 and 6.92 pC/N) of the LGAS(x=0.9) and LTGA(x=0.5) crystals were slightly larger than that of the pure LGS and La3Ta0.5Ga5.5O14 (LTG) ones (6.08 and 6.89 pC/N), respectively. The LTGA(x=0.5) crystals showed the lowest temperature dependence of d11. The electric resistivity ρ values of the Al-substituted crystals at 400 o C were significantly higher than those of the pure LGS and LTG crystals grown in an oxygen-containing atmosphere. The aluminum substitution is effective for improving the electrical properties of the LGS and LTG, and the LTGA(x=0.5) crystals are suitable for combustion pressure sensor materials.

Single-ionized-oxygen-vacancy-related dielectric relaxation in Bi3.25La0.75Ti3O12 ferroelectric films

The dielectric and conductive properties of Bi3.25La0.75TiO3 ferroelectric films were investigated in the temperature range of 25°C–600°C. A dielectric peak with a relaxation-type characteristic was observed around 400°C. This peak can be greatly suppressed or eliminated by high sintering temperature or annealing in an oxygen atmosphere (O2), and induced again by annealing in a reducing atmosphere (N2). The activation energy of dielectric relaxation and the characteristic relaxation time are estimated to be 1.89eV and 1.08×10−14s, respectively. In the corresponding temperature region, activation energy of conductivity is 0.65eV. The mechanism of this dielectric anomaly is discussed.

Characterization of manganese-doped BaTiO3–(Bi1∕2Na1∕2)TiO3 positive temperature coefficient of resistivity ceramics using impedance spectroscopy

Small amount of manganese (Mn)-doped BaTiO3–(Bi1∕2Na1∕2)TiO3 (BT-BNT) positive temperature coefficient of resistivity ceramics are investigated by impedance analyses. The impedance/modulus spectroscopic plots reveal that a third resistance-capacitance (RC) response besides grains and grain boundaries is exhibited in the Mn-free BT-BNT ceramic with 4mol% BNT, but is not observed in the Mn-doped samples. The third RC element can be attributed to a barium vacancy-rich layer in the outer grain region. The evidence of impedance spectroscopy indicates that the highly Bi donor is partially compensated by the Mn acceptor and the predominant charge compensation defect shifts from barium vacancies to electrons with doping small amount of Mn dopant.

Effects of Ce, Cr and Er Doping and Annealing Conditions on the Microstructural Features and Electrical Properties of PbZrO3 Thin Films Prepared by Sol–Gel Process

Lead zirconate (PbZrO3) thin films doped with Ce, Cr and Er were grown on Pt(111)/Ti/SiO2/Si(100) substrates by sol–gel spin coating process. Polycrystalline films with a phase-pure perovskite structure and a random orientation were obtained regardless of doping materials. Microstructural examinations of the films revealed a two-phase microstructure consisting of large rosette-type perovskite regions with a nanocrystalline phase located in between. Perovskite rosettes were found to be of polycrystalline in nature with grain size ranging between 50–250 nm. Increasing number of annealing cycles and temperature were found to improve the microstructure, crystallinity and electrical properties. Doping elements were also found to increase electrical properties with a saturation polarization (Psat) reaching 65 ×10-6 C/cm2 in Ce- and Cr-doped coatings compared to 39 ×10-6 C/cm2 for undoped PZ films.

Effect of aluminum substitution in La3Ga5SiO14 crystals on their structure and piezoelectricity

We report La3Ga5−xAlxSiO14 (0⩽x⩽1) crystals, with a Ca3Ga2Ge4O14-type structure, suitable for piezoelectric application. A single-crystal x-ray structure analysis reveals that Al atoms are distributed in all cation sites except for the decahedral one occupied by La, by rather favoring the smallest tetrahedral one. The piezoelectric modulus |d11| increased ∼1.3% and d14 decreased ∼7.7% with the increasing Al content, x, up to 1.0. We discuss the possible correlation between the atomic displacement and the observed compositional dependence of the piezoelectric modulus |d11| by referring to other isomorphs. The optimal solid-solution compositions and the use of Al substitution for obtaining the maximum electromechanical coupling factors (k12, k25, and k26) are advanced.

Annealing effects on the characteristics of high Tc lead-free barium titanate-based positive temperature coefficient of resistivity ceramics

Annealing effects on the positive temperature coefficient of resistivity (PTCR) behavior of (Bi1/2Na1/2)TiO3 and (Bi1/2K1/2)TiO3 modified BaTiO3 semiconducting ceramics are investigated. The annealing treatments result in the occurrence of the PTCR effect and the increased resistivity jump. The impedance spectra reveal that the resistivity of grain interior of annealed sample is little influenced by the annealing process, indicating that the increase in the overall resistivity is entirely a grain-boundary effect. A higher annealing temperature leads to an increase in acceptor state density, potential barrier height and barrier layer width, which contributes to the improved PTCR effect in the annealed samples.

Growth and Electric Properties of Al-Substituted Langasite-Type La3Ta0.5Ga5.5O14 Crystals at High Temperature

We report an effective substitution of aluminum for gallium in langasite-type La3Ta0.5Ga5.5O14 (LTG) crystals for use in a pressure sensor at high temperature. Al-substituted LTG (La3Ta0.5Ga5.5-xAlxO14; LTGAx) single crystals up to the solubility limit of x=0.5 have been grown by the conventional Czochralski technique. The electric properties of the LTGAx crystals were investigated and compared with those of LTG. By Al substitution, the piezoelectric constant d11 and the electromechanical coupling factors (k12) became slightly larger. The LTGAx crystals showed a lower temperature dependence of d11 and a higher electric resistivity ρ than those of the LTG crystals.

Fabrication of Ferroelectric Photonic Crystals

ABSTRACT Ferroelectric photonic crystals consisted of a PLZT matrix and 800 nm-pitch air-holes with a diameter of 560 nm were successfully fabricated by the electron-beam-induced patterning process. Hexagonal holes inscribed to the circles with a diameter of 400 nm in a precursor matrix changed to circular holes with a diameter of 560 nm after heat-treatment due to shrinkage of the matrix. The optical properties of CSD-derived PLZT films were also evaluated. The PLZT films showed the refractive index of 2.3 and the Kerr constant of 0.4 × 10− 16 m2/V2. Therefore, it is expected that an applied voltage of 9.7 V to 1 μ m-thick PLZT photonic crystals induces 1%-decrease in refractive index and 40°-swinging of light with a wavelength of 1.55 μ m.