Takuya Hoshina

Preparation of nm-Sized Barium Titanate Fine Particles and Their Powder Dielectric Properties

Barium titanate (BaTiO3) crystallites with various particle sizes from 17 to 100 nm were prepared by the 2-step thermal decomposition method of barium titanyl oxalate (BaTiO(C2O4)24H2O). The crystal structure of these BaTiO3 particles was assigned to cubic m-3m by a X-ray diffraction (XRD) measurement while it was assigned to tetragonal 4mm by a Raman scattering measurement. Investigation of impurity in these particles using both TG-DTA and FT-IR measurements revealed that no impurity was detected in the BaTiO3 lattice while hydroxyl and carbonate groups were detected only on the surface. The dielectric constants of these powders were measured using suspensions by a modified powder dielectric measurement method. As a result, the dielectric constant of BaTiO3 particles with a size of around 70 nm exhibited a maximum of over 15,000. This study revealed that BaTiO3 particles with a size around 70 nm were the most desirable for capacitor application.

Composite structure and size effect of barium titanate nanoparticles

Almost impurity and defect-free barium titanate (BaTiO3) nanoparticles with various sizes from 20 to 430 nm were prepared using 2-step thermal decomposition method. The nano-structures of these particles were analyzed using a synchrotron radiation X-ray diffraction (XRD). As a result, it was found that the BaTiO3 nanoparticles had composite structure consisted of (a) internal tetragonal layer, (b) Gradient-Lattice-Strain Layer (GLSL) and (c) surface cubic layer.

Size effect on the crystal structure of barium titanate nanoparticles

A size effect on crystal structure has been investigated for barium titanate (BaTiO3) nanoparticles of 40-, 140-, and 430-nm sizes, by means of neutron and high-resolution synchrotron x-ray powder-diffraction and Raman-scattering techniques. These samples were prepared by a modified two-step thermal decomposition method from barium titanyl oxalate, resulting in very few lattice impurities. Rietveld analysis of the neutron-diffraction data for the 430-nm- and 140-nm-sized BaTiO3 particles was performed assuming a single phase of tetragonal (P4mm) structure. The axial ratio c∕a of tetragonal BaTiO3 decreases with a decrease in particle size from 430 to 140 nm. Barium titanate particles with a size of 40 nm consist of (1) tetragonal crystals (83 wt %) with a large cell volume and an axial ratio of unity c∕a=1.000(5) and of (2) a hexagonal phase (P63mmc, 17 wt %) with a large unit-cell volume. Rietveld and maximum-entropy method analyses suggest that there exist atomic displacements from the ideal site of a c...

Size and temperature induced phase transition behaviors of barium titanate nanoparticles

High density, almost impurity-free and defect-free barium titanate (BaTiO3) fine particles with various sizes from 20to1000nm were prepared by the two-step thermal decomposition of barium titanyl oxalate and postheating treatment. The crystal structures of these particles were investigated as a function of the size and the temperature using synchrotron radiation x-ray diffraction (XRD) measurement. As a result, the size-induced ferroelectric (tetragonal-cubic) phase transition observed at around 30nm. Moreover, the temperature dependence of the crystal structures revealed that one phase transition at 135°C separated into two kinds of phase transition behaviors with decreased particle sizes, i.e., the tetragonal-cubic phase transition temperature was constant at 135°C despite particle sizes while the cell volume expansion temperature shifted to low temperature with decreasing particle sizes. Moreover, the temperature dependence of Raman scattering spectra clarified that the temperature at a discontinuous c...

Domain Size Effect on Dielectric Properties of Barium Titanate Ceramics

Barium titanate (BaTiO3) ceramics with various grain sizes were prepared by a conventional sintering method and a two-step sintering method. The permittivity of the ceramics increased with decreasing the grain size down to 1.1 µm on average. The BaTiO3 ceramics with an average grain size of 1.1 µm had a high permittivity of 7,700. The transmission electron microscopy (TEM) observation revealed that the 90° domain density increased with decreasing the grain size. The domain size of the ceramics with the highest permittivity of 7,700 was approximately 100 nm. From an ultra wide range dielectric spectroscopy, it was found that the high domain density enhanced the orientational polarizability due to the domain-wall vibration and the ionic polarizability due to the lattice vibration. It was clarified that the increase of the permittivity with decreasing the grain size was due to the domain size effect.

Ultrawide range dielectric spectroscopy of BaTiO3-based perovskite dielectrics

Ultrawide range dielectric spectra from the kilohertz to terahertz range of BaTiO3 (BT), Ba(Zr0.25Ti0.75)O3 (BZT), (Ba0.6Sr0.4)TiO3 (BST), and SrTiO3 ceramics were presented by analyzing dielectric permittivity and IR reflectivity data. It was found that the permittivity of the ST was determined only by the ionic polarization while that of the BT was determined by the ionic polarization as well as the dipole polarization due to the domain contribution. The high permittivity of the BZT ceramics was attributed to the dipole polarization of polar nanoregions in the relaxors. The dipole and ionic polarizations overlapped in the BST.

Preparation of nm-sized BaO3 particles using a new 2-step thermal decomposition of barium titanyl oxalate

To obtain inpurity-free and nm-sized barium titanate (BaTiO3) particles, a new 2-step thermal decomposition method from barium titanyl oxalates (BaTiO(C2O4)2 · 4H2O) was proposed. At the 1st step, BaTiO(C2O4)2 · 4H2O was annealed at 400°C for 1 h in the O2 flow. The annealing temperature of 400°C was chosen for the following reasons; (1) no formation of BaCO3 and TiO2 and (2) the complete removal of H2O and other carbon species. This compound obtained at 400°C was amorphous phase, and its chemical composition was BaCO3-TiO2. When this compound was annealed in air at higher temperatures, the large BaTiO3 particles were prepared with by-products such as BaCO3. Thus, at the 2nd step, to prevent the crystal growth and the formation of BaCO3, this compound was annealed above 600°C in vacuum. Finally, the BaTiO3 single crystals with a size with 16.5 nm were prepared around 620°C. These BaTiO3 fine particles were characterized using various methods to investigate defects and impurities in the particles. As a result, it was confirmed that there was no impurity in the BaTiO3 lattices.

A Shear-Mode Ultrasonic Motor Using Potassium Sodium Niobate-Based Ceramics with High Mechanical Quality Factor

(K,Na)NbO3–LiNbO3–CuO lead-free piezoelectric ceramics that show a high mechanical quality factor Qm were synthesized and used as a drive element of an ultrasonic motor. The Qm of the (K,Na)NbO3 ceramic could be enhanced by chemical modification using Li and Cu as well as microstructure control to obtain ceramics with fine grains. The grain size dependence of the Qm was consistent with a model based on the formation of internal bias field to stabilize the domain structure. A shear mode was used to drive the ultrasonic motor because the piezoelectric d31 and d33 constants of the ceramics were not sufficient for the motor applications. A shear-mode motor driven with four piezoelectric ceramic plates was developed using the lead-free ceramics with a high Qm of 1400, a high d15 of 207 pC/N, and a high k15 of 0.72. The highest revolution speed of 486 rpm was achieved at 34.5 kHz with the input voltage of approximately 180 Vp–p (peak to peak).

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.

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.