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 εr33T 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 Poisson's ratio σ= 0.38 was determined from the ratio of overtone frequency and resonant frequency in the planar mode. The high Poisson's 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.

Two-step sintering was applied to manufacture fine-grain barium titanate (BaTiO 3 ) 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 mm, and showed large dielectric constants and excellent piezoelectric properties. The large piezoelectric constant of d 33 ¼ 460 pC/N was measured using a d 33 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 BaTiO 3 ceramics manufactured by a low-cost process to ultrasonic generators, actuators, piezoelectric vibrators, and sensors working at room temperature.

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 BaZrO 3 -(K,Na,Li)NbO 3 (BZ-KNLN) binary system has been adjusted using the third component (Bi,Na)TiO 3 (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. #

(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.

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 ε11 T/ε0 and ε33 T/ε0 were 17.8 and 27.9, respectively. Electromechanical coupling coefficients k 12, k 25 and k 26 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.