Papers by Author: Toshio Ogawa

Abstract: Sound velocities were measured in relaxor single-crystal plates, included in piezoelectric transducers for medical uses, using an ultrasonic precision thickness gauge with high-frequency pulse generation. The velocities were compared with the ones of piezoelectric ceramics in order to clarify characteristics of the single crystals. Estimating the difference in the sound velocities and elastic constants in the single crystals and ceramics, it was possible to evaluate effects of domain and grain boundaries on elastic constants. Existence of domain boundaries in single crystal affected the decrease in Young’s modulus, rigidity, Poisson’s ratio and bulk modulus. While existence of grain boundaries affected the decrease in Young’s modulus and rigidity, Poisson’s ratio and bulk modulus increased. It was thought these phinomina come from domain alignment by DC poling, and both the boundaries act as to absorb mechanical stress by defects due to the boundaries. In addition, the origin of piezoelectricity in single crystals is caused by low bulk modulus and Poisson’s ratio, and high Young’s modulus and rigidity in comparison with ceramics. On the contrary, the origin of piezoelectricity in ceramics is caused by high Poisson’s ratio by high bulk modulus, and furthermore, low Young’s modulus and rigidity due to domain alignment.

Abstract: Material research and development on piezoelectric ceramics, especially lead-free ceramics, was proposed from a viewpoint of relationships between piezoelectricity and elastic constants such as Young’s modulus and Poisson’s ratio. We developed a method to be convenient to measure acoustic wave velocities by an ultrasonic thickness gauge with high-frequency. From the change in longitudinal and transvers wave velocities before and after DC poling, it was found that the ceramic bulk density was important to improve the piezoelectricity in lead-free ceramics. As a result, the candidates of lead-free ceramic compositions with higher piezoelectricity were proposed. Furthermore, the ratio of transvers wave velocity to longitudinal wave velocity was clarified to estimate compositions with higher piezoelectricity. The measurement of sound velocities was an effective method for researching and developing piezoelectric materials, and it was possible to design the material compositions of lead-free piezoelectric ceramics as well as lead-containing ceramics by the novel measuring method.

Abstract: Poling characteristics, especially DC poling field (E) dependence of dielectric and piezoelectric properties were investigated to evaluate the domain behavior in lead-free ceramics such as alkali niobate and alkali bismuth titanate ceramics. The domain switching was confirmed by the E to realize minimum dielectric constant (εr), minimum electromechanical coupling factor (k) and maximum frequency constant (fc), because of domain clamping. On the other hand, the domain rotation occurred at the E to obtain maximum εr and minimum fc. These phenomena are observed in the lead-free ceramics as well as PZT ceramics.