Kouichi Nakashima

Enhanced piezoelectric response of BaTiO3–KNbO3 composites

The piezoelectric response of solvothermally synthesized BaTiO3 (BT)–KNbO3 (KN) composites (the nominal BT/KN ratio was 1) with distinct interfaces was investigated. The x-ray diffraction pattern showed two distinct peaks began to merge into a singular broad peak at a two-theta position between (200) and (002) tetragonal-related peaks of BT. The transmission electron microscopy observation showed a heteroepitaxial interface region between BT single-crystal particles and deposited KN crystals. The large-field piezoelectric constant was 136 pC/N, which was three times larger than that of a sintered 0.5BT–0.5KN composite. The enhanced piezoelectric response was attributed to the strained epitaxial interface region.

Electric field induced lattice strain in pseudocubic Bi(Mg1/2Ti1/2)O3-modified BaTiO3-BiFeO3 piezoelectric ceramics

Contributions to the piezoelectric response in pseudocubic 0.3BaTiO3-0.1Bi(Mg1/2Ti1/2)O3-0.6BiFeO3 ceramics were investigated by synchrotron X-ray diffraction under electric fields. All of the lattice strain determined from the 110, 111, and 200 pseudocubic diffraction peaks showed similar lattice strain hysteresis that was comparable to the bulk butterfly-like strain curve. It was suggested that the hysteresis of the lattice strain and the lack of anisotropy were related to the complex domain structure and the phase boundary composition.

Preparation of Barium Titanate–Potassium Niobate Nanostructured Ceramics with Artificial Morphotropic Phase Boundary Structure By Solvothermal Method

Barium titanate (BaTiO3, BT)–potassium niobate (KNbO3, KN) (BT–KN) nanostructured ceramics with a distorted interface region, i.e., artificial morphotropic phase boundary (MPB) structure, were prepared by the solvothermal method. The results of the optimization of reaction conditions showed that the metastable region of only KN crystal growth was obtained using a mixture of KOH and K2CO3 as the K source and Nb2O5 as the Nb source in ethanol. Moreover, KN formation under the metastable region using a particle compact composed of a mixture of BT and Nb2O5 particles as substrates resulted in the successful preparation of BT–KN nanostructured ceramics with an artificial MPB region and a porosity of around 35%. This is the first report on the preparation of ceramics with a heteroepitaxial interface between BT and KN below 230 °C.

Crystal Structure of BaTiO3?KNbO3 Nanocomposite Ceramics: Relationship between Dielectric Property and Structure of Heteroepitaxial Interface

High-energy synchrotron radiation powder diffraction experiments have been carried out to investigate the crystal structure of solvothermally synthesized KNbO3 (KN)/BaTiO3 (BT) nanocomposite ceramics in which a ceramic grain consists of a BT nanoparticle thinly coated with KN crystals through the heteroepitaxial interface. Rietveld analysis reveals that the ceramic grain has the core/multishell structure consisting of a BT core and distorted BT and KN multishells. BT is gradually distorted in the large region to form the interface with KN from the tetragonal structure at the core toward the cubic structure at the boundary between BT and KN. The variations of the volume of the distorted interface region of BT and the dielectric property of the ceramics show similar trends to the variation of KN/BT molar ratio, which suggests that the electrically soft interface of BT nanoparticles governs the dielectric properties of the ceramics.

Relaxor Characteristics of BaTiO3-Bi(Mg1/2Ti1/2)O3 Ceramics

Relaxor characteristics of (1-x)BaTiO3-xBi(Mg1/2Ti1/2)O3 (x=0.1–0.7) ceramics were investigated. Microstructural observation showed second phases and no domain structure for the sample with x=0.6. Deviation from the Curie-Weiss behavior was found in temperature dependence of the inverse permittivity for all the samples. The stronger dielectric dispersion was found for x=0.6 and 0.7 and they were described by the Vogel-Fulcher relationship. The temperature dependence of the remanent polarization and coercive field indicated the freezing temperature was 100~150°C for x=0.6. The strong dielectric dispersion of x=0.6 is believed to be induced by the structural disorder due to the second phases.

Preparation of Barium Titanate/Strontium Titanate Multilayered Nanoparticles

Nucleation and particle growth conditions of barium titanate (BaTiO3, BT) were investigated for preparation of the BT/strontium titanate (SrTiO3, ST) multilayered nanoparticles. The conditions with and without BT nucleation were clarified. Epitaxial growth of the BT layer on the ST substrate particles was studied using both conditions. The formation of the BT layer on the ST substrate particles was confirmed using the condition with BT nucleation.

Microstructure Control of Barium Titanate – Potassium Niobate Solid Solution System Ceramics by MPB Engineering and their Piezoelectric Properties

Effect of the microstructural homogeneity of 0.5 BaTiO3 - 0.5 KNbO3 (0.5BT-0.5KN) solid solution ceramics on the dielectric and piezoelectric properties was investigated. Microstructure of a sample prepared by a conventional sintering method was homogenous, and the room temperature crystal structure was assigned to cubic Pm3m symmetry and therefore the sample was paraelectric. On the other hand, microstructure of samples prepared by a two-step sintering method was inhomogeneous, that is, it was made of BT and KN grains. The large electric field piezoelectric constant d33* increased with increasing interface area.

Nanostructure Control of Barium Titanate--Potassium Niobate Nanocomplex Ceramics and Their Enhanced Ferroelectric Properties

Barium titanate (BaTiO3,BT)–potassium niobate (KNbO3,KN) nanocomplex ceramics with various KN/BT molar ratios were prepared by a solvothermal method. From a transmission electron microscopy (TEM) observation, it was confirmed that the KN layer thickness on BT particles was controlled from 5 to 40 nm by controlling KN/BT molar ratios. Their dielectric constants were measured at room temperature and 1 MHz, and the maximum dielectric constant of 370 was measured for the BT–KN nanocomplex ceramics with a KN thickness of 22 nm. TEM observation revealed that at a KN thickness below 22 nm, the BT/KN heteroepitaxial interface was assigned as a strained interface, while at 40 nm, the interface was assigned as a relaxed one. These results suggested that the strained heteroepitaxial interface could be responsible for the enhanced dielectric properties.

Role of structure gradient region on dielectric properties in Ba(Zr,Ti)O3–KNbO3 nanocomposite ceramics

Crystal structures of KNbO3 (KN)/BaZrxTi1−xO3 [BZT, (0.1 ≤ x ≤ 1)] nanocomposite ceramics have been investigated using synchrotron radiation X-ray powder diffraction, where BZT nanoparticles thinly coated with KN crystals through the heteroepitaxial interface are sintered. The Rietveld analysis based on the multicomponent model reveals that the ceramic grain has the core/multishell structure consisting of a BZT core, distorted BZT and KN multishells. The variations of the volume ratio of the distorted BZT shell region corresponding to the structure gradient interface region and the dielectric property of the ceramics show similar trends as a function of x. From these results, we propose that the structure gradient region is electrically soft, and provides a crucial contribution to the dielectric properties of the nanocomposite ceramics.

Preparation of Barium Titanate Grain-Oriented Ceramics by Electrophoresis Deposition Method under High Magnetic Field Using Single-Domain Nanoparticles

Barium titanate (BaTiO3, BT) grain-oriented ceramics were prepared by electrophoresis deposition (EPD) method under high magnetic field (HM-EPD) of 12 T. For this objective, the BT single-domain nanoparticles with high c/a ratio of 1.008 and size of 103 nm were prepared by two-step thermal decomposition method. Using the BT nanoparticle slurry, BT nanoparticle accumulations were prepared by EPD or slipcasting (SC) methods with/without high magnetic field. After binder burnout, these accumulations were sintered at 1350 °C and it was revealed that only the BT ceramics prepared by the HM-EPD method was assigned to grain-oriented ceramics with weak preferential crystallographic orientation along [11 direction.