Tomoaki Karaki

Lead-Free Piezoelectric Ceramics with Large Dielectric and Piezoelectric Constants Manufactured from BaTiO3 Nano-Powder

Barium titanate (BaTiO3) ceramics with a density of more than 98% of the theoretical value were fabricated by two-step sintering method from hydrothermally synthesized BaTiO3 nano-particles of 100 nm. The average grain size was around 1.6 µm and the biggest one was controlled less than 3 µm. Dielectric constant er33T of the poled samples was 5000 and electromechanical coupling factor kp was 42%. Large piezoelectric constants d33 = 460 pC/N and d31 = -185 pC/N were measured by a d33-meter and the resonance–antiresonance method, respectively. A high Poissons ratio σ= 0.38 was determined from the ratio of overtone frequency and resonant frequency in the planar mode. The high Poissons ratio and the large dielectric constants are most likely the origin of the high d33 of the ceramics. The discovery of high d33 in non-lead-based BaTiO3 ceramics with low cost process has important practical consequences in addition to scientific interest.

Barium Titanate Piezoelectric Ceramics Manufactured by Two-Step Sintering

Two-step sintering was applied to manufacture fine-grain barium titanate (BaTiO3) piezoelectric ceramics from hydrothermally synthesized 100 nm particles. Scanning electron microscopy (SEM) revealed a small and irregular domain structure in the samples. The sintering condition dependences of density, dielectric constants, and piezoelectric properties were investigated. Under the optimal conditions, dense specimens had an average grain size of approximately 1.6 µm, and showed large dielectric constants and excellent piezoelectric properties. The large piezoelectric constant of d33=460 pC/N was measured using a d33 meter. The Curie and orthorhombic-to-tetragonal phase transition temperatures were 126 and 24 °C, respectively. These results indicated the possibility of applying non-lead-based BaTiO3 ceramics manufactured by a low-cost process to ultrasonic generators, actuators, piezoelectric vibrators, and sensors working at room temperature.

Piezoelectric Properties of Ca3NbGa3Si2O14 Single Crystal

Langasite-type single crystal Ca3NbGa3Si2O14 (CNGS) was grown by the Czochralski technique. Dielectric, elastic and piezoelectric constants of CNGS were measured by the resonance-antiresonance method. At room temperature, dielectric constants e11T/e0 and e33T/e0 were 17.8 and 27.9, respectively. Electromechanical coupling coefficients k12, k25 and k26 were also determined as 10.9, 17.3 and 11.9%, respectively. The measurements were carried out in a temperature range from -30 to 80°C. Temperature coefficients of the dielectric, elastic and piezoelectric constants were obtained. The line-focus-beam and plane-wave ultrasonic material characterization system was employed for measuring bulk acoustic velocities, and longitudinal and transverse wave velocities of 7408.4 m/s and 3136.2 m/s, respectively, in the c-direction uncoupled with piezoelectricity at 23°C were obtained. This was in good agreement with the results determined by the resonance-antiresonance method. The density of CNGS was 4125 kg/m3. All the parameters of the CNGS crystal for bulk and surface acoustic wave applications were determined in this research.

Hydrothermal Synthesis of (K,Na)NbO3 Particles

(K,Na)NbO3 (KNN) particles were successfully prepared by hydrothermal synthesis. The results showed that Na+ reacted more readily with Nb to form NaNbO3 than K+. For the purpose of obtaining KNN particles with K/Na=1, a mixed alkaline solution with K+/Na+ ratios ranging from 3.5/1 to 4/1 was required as a starting solution. The morphology and size of KNN particles synthesized strongly depended on K/Na ratio in the KNN particles. The KNN particles synthesized from the starting alkaline solution with K+/Na+=3.5/1 were the smallest with a pelletlike morphology affected by NaNbO3- and KNbO3-based particles. Surfactants such as sodium dodecylbenzenesulfonate (SDBS) and sodium hexametaphosphate (SH) were used to synthesize well dispersed and small KNN particles. Platelike KNN particles with 100 nm thickness and 1.5 µm width were obtained in this study.

Subgrain Microstructure in High-Performance BaTiO3 Piezoelectric Ceramics

We have successfully observed subgrains of 1 µm average size in large grains (1 to 10 µm) in high-performance barium titanate (BaTiO3) piezoelectric ceramics manufactured by two-step sintering. The BaTiO3 ceramics fabricated from hydrothermally synthesized BaTiO3 with an average particle size of 0.1 µm had a density = 5.93 g/cm3, an electromechanical coupling factor kp = 50%, and a piezoelectric constant d33 = 500 pC/N, which are the highest values reported thus far. Subgrain boundaries were considered to have a lower energy than main-grain boundaries. The subgrain structure restricted ferroelectric domain growth resulting in a nanosized domain structure, a large kp and a large d33. These features have important practical implications in addition to the scientific interest they generate in the fabrication of lead-free high-performance piezoelectric ceramics.

Surface Acoustic Wave Properties of La3Ga5SiO14 (LANGASITE) Single Crystals

Theoretical and experimental results have shown the potential usefulness of La3Ga5SiO14 (LGS) for surface acoustic wave (SAW) applications. The existence of orientations with zero-temperature coefficient of delay (TCD) have been calculated by the computer analysis. A simulation of the temperature-compensated Rayleigh SAWs located around both (0°, 140°, 25°) and (15°, 151°, 40.5°) with the Rayleigh SAW velocity v of 2762 and 2860 m/s, coupling coefficient k2 of 0.38 and 0.41%, and power flow angle PFA of 2 and 0°, has been carried out. The Rayleigh SAW velocity v is lower than those of other SAW materials, which enables miniaturization of SAW design.

Surface Acoustic Wave Properties of La3Ta0.5Ga5.5O14 Single Crystals

The propagation characteristics of Rayleigh waves on a La3Ta0.5Ga5.5O14 (LTG) substrate were theoretically investigated for all cuts and propagation directions. The material constants, such as the elastic stiffness, piezoelectric constants and their thermal coefficients, were measured by the resonant and antiresonant frequency method. The existence of orientations with zero-temperature coefficient of delay (TCD) was calculated by computer analysis. The electromechanical coupling coefficient (k2) is about three times larger than that of ST-cut quartz. The phase velocity is approximately 2690 m/s. These results indicate a large possibility for application to wide-band intermediate-frequency surface acoustic wave (SAW) filter devices.

Morphotropic Phase Boundary Slope of (K,Na,Li)NbO3–BaZrO3 Binary System Adjusted Using Third Component (Bi,Na)TiO3 Additive

In the binary composition-temperature phase diagram, the slope of the phase boundary is related to the temperature stability of the phases and/or properties. The slope of the tetragonal–rhombohedral morphotropic phase boundary (MPB) in the BaZrO3–(K,Na,Li)NbO3 (BZ–KNLN) binary system has been adjusted using the third component (Bi,Na)TiO3 (BNT). Piezoceramics were synthesized by a conventional solid-state reaction method, and their crystal structures as well as their MPB were determined from X-ray diffraction patterns measured from room temperature to 300 °C. With increasing BNT content, the slope of the MPB changed from negative to positive. An MPB with temperature-independent behavior, the so called vertical MPB, was successfully discovered in this lead-free piezoceramic system. The MPB composition was determined to be 0.075BZ–0.915KNLN–0.01BNT and the Curie temperature was found to be about 270 °C. The results obtained in this work could be used to develop high-performance lead-free piezoceramics compatible with lead-based ones. Furthermore, this is the first time a vertical MPB has been artificially formed by adjusting the MPB slope. The research method presented in this work has important practical implications as well as scientific interest for searching for vertical MPBs in lead-free piezoceramic systems.

Electrical Properties of Epitaxial (Pb,Sr)TiO3 Thin Films Prepared by RF Magnetron Sputtering.

(Pb,Sr)TiO3 (PST) thin films were prepared by rf magnetron sputtering using a multi-element target on (111)Ir/SiO2/Si, (100)SrTiO3:La and (100)Ir/(100)SrTiO3 substrates. Polycrystalline films were obtained on the (111)Ir/SiO2/Si substrate and epitaxial thin films were obtained on (100)SrTiO3:La and (100)Ir/(100)SrTiO3 substrates. Their dielectric properties were measured with and without DC bias field. The electric field dependence of the dielectric constant with a value of 70% was measured under a bias of 200 kV/cm at room temperature from the epitaxial thin films.

Growth and Optical Properties of Ferroelectric K3Li2Nb5O15 Single Crystals

Potassium lithium niobate (KLN) single crystals were grown from a melt with potassium and lithium enriched composition using the rf-heating Czochralski technique. The growth conditions were discussed. A crystal with the composition of 0.315K2O–0.15Li2O–0.535Nb2O5 was grown from a melt of 0.348K2O–0.187Li2O–0.465Nb2O5. The transparent crystal was pale-yellow with an absorption edge at 376 nm, and showed a very small optical absorption in the 420–900 nm region. The electrooptic and nonlinear optic coefficients were measured. The deviation of composition in the crystals was investigated by measuring the change of the Curie temperature and phase matching angle of the second harmonic generation.