Piezoelectric properties of KNbO/sub 3/ crystals for extensional modes

The theoretical calculation of orientation dependence of the electromechanical coupling factor for the thickness extensional mode, k₁, in KNbO3 crystal has shown that it is as high as 69% for the 49.5° rotated X-cut about the Y-axis, which is the highest among known piezoelectricS. To experimentally confirm this, the immpedence responses of the thickness-extensional mode vibrators are measured and the related constants are evaluated. The measured coupling factor is as high as 70%. The strain vs electric-field behaviors of the pseudocubic (001) cut, which is close to the 49.5° rotated X-cut, are measured. The strain vs electric field curves exhibit large hysteresis in a high electric field range for as-cut single domain crystals, whereas for etched single domain crystals they exhibit good linearity even in the high electric field range. Similar experiments are performed for multidomain (001) cut crystals poled along the <001> direction and the results are cornjmed with those of single domain crystals. Based on the results, a possibility that the enhanced piemelectric properties in engineered domain structures of KNb03, PZN-PT, and BaTiO₃ are mainly due to the orientation dependence of d₃₃ is suggested.