Hiroaki Takeda

Proton transport polarization and depolarization of hydroxyapatite ceramics

Polarization of sintered hydroxyapatite (HAp) ceramics by application of an external dc field at higher temperature was analyzed by thermally stimulated depolarization current (TSDC) measurements. The mechanisms for the polarization and depolarization of HAp were discussed in relation to the instability of the protons in the hydroxide groups. The TSDC spectra consisted of broad peaks, while the ferroelectric substances such as the BaTiO3 ceramics exhibited a sharp peak. Although the maximum current density of 7.87 nA cm−2 for the HAp polarized at 400 °C under 1.0 kV cm−1 was approximately 1/12 lower than that of BaTiO3, the polarization charge of 14.9 μC cm−2 was almost twice as large as that of BaTiO3. Considering the activation energy of 0.72–0.89 eV for the depolarization, it was revealed that the polarization of HAp was ascribed to the migration of protons in the columnar OH− channels with a micrometer-order distance. It was also found that the polarization charge was large and long enough to enhance ...

Growth and characterization of lanthanum gallium silicate La3Ga5SiO14 single crystals for piezoelectric applications

Abstract We have successfully grown a new piezoelectric La 3 Ga 5 SiO 14 (LGS) single crystal, with dimensions of 1 or 2 inch in diameter, using the Czochralski technique. Coloring of the grown crystals was discussed. The device properties of LGS resonators and filters were demonstrated. The LGS single crystal showed superior properties for piezoelectric applications.

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.

Crystal growth and structural characterization of new piezoelectric material La3Ta0.5Ga5.5O14

New piezoelectric La3Ta0.5Ga5.5O14 bulk single crystals were grown using the Czochralski technique. The crystal structure (space group P321, a=8.228(2), c=5.124(2) A) has been refined using single-crystal X-ray diffraction data with a precision corresponding to an R index of 0.04. Ta atoms were found to be ordered in the octahedral site coordinated by six oxygen atoms.

Synthesis and characterization of Sr3TaGa3Si2O14 single crystals

New piezoelectric Sr3TaGa3Si2O14 (STGS) single crystals were grown using the Czochralski technique. The crystal structure of STGS is isostructural to that of Ca3Ga2Ge4O14, which has the trigonal space group P321 and Z = 1. The lattice parameters were a = 8.293(2) and c = 5.078(2) A. The final R-factors were R = 0.047 and Rw = 0.046 for 1406 unique reflections. The distribution of each cation was found to be ordered in each site. Some piezoelectric properties of STGS are reported.

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.

Polarization behavior in diffuse phase transition of BaxSr1−xTiO3 ceramics

Ultrawide band dielectric spectroscopy analysis of barium strontium titanate [BaxSr1−xTiO3 (BST)] ceramics at different temperatures disclosed a contribution of the dipole and the ionic polarizations to their diffuse phase transition (DPT). In the BST ceramics, thermal behavior of the ionic polarization governs the DPT. The dielectric maximum temperature (Tm) is in agreement with the maximum temperature of the permittivity determined by the ionic polarization. The maximum of the permittivity determined by the ionic polarization at the Tm is caused from the decrement of the vibration frequency of the soft mode. The dipole polarization gives small contribution to the DPT. The gradual increase in the permittivity determined by the dipole polarization with decreasing temperature could be explained by the increase in the size of the polar nanoregions.

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.

Size Effect of Nanograined BaTiO3 Ceramics Fabricated by Aerosol Deposition Method

We demonstrated the size effect of nanograined BaTiO3 ceramics using freestanding BaTiO3 thick films fabricated by the aerosol deposition (AD) method. Dense BaTiO3 thick films fabricated by the AD method were crystallized and detached from the SrTiO3 substrate by annealing treatment at 600 °C, and then the grain size was controlled by 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 from 170 to 24 nm. The decrease in permittivity was due to the decreases in ferroelectricity and domain-wall contributions with decreasing grain size.

Growth and Characterization of Lanthanum Calcium Oxoborate LaCa4O(BO3)3 Single Crystals

Lanthanum calcium oxoborate LaCa4O(BO3)3 (LaCOB) bulk single crystals were grown by the Czochralski technique. The crystals have almost constant transmittances between 320 and 2500 nm. The relationship between the growth habit and crystallographic abc - rectangular XYZ axes was clarified. Some dielectric and piezoelectric constants as well as elastic compliance constants of the LaCOB crystals were evaluated. The surface acoustic wave (SAW) and pseudo-SAW (PSAW) parameters of the LaCOB crystals were also measured. For the SAW, the largest electromechanical coupling coefficient of approximately 0.26% and a velocity of 3550 m/s were obtained for the YX orientation. For the PSAW, these parameters were approximately 0.6% and 4000 m/s for the ZY orientation.