High coupling KNbO3 width-extensional vibrators with a polar multidomain structure

The longitudinal piezoelectric coupling factor for the width-extensional mode of a thin KNbO₃ plate, k₁₁´, has theoretically been predicted to be as high as 82.4% for the 43.5° rotated Z-cut about the Y-axis. To confirm the predicted value experimentally, we fabricated a width-extensional vibrator using a 45° rotated Z-cut plate about the Y-axis, which is close to the maximum k₁₁´ cut. However, the measured value of k₁₁´ was 71.3%, appreciably lower than the predicted value. It is clarified that the lower coupling factor can be ascribed to the tilt of the electric field vector from the major face of the plate, which results from an off-diagonal component of the dielectric tensor. To prevent the electric field tilt, a polar multidomain structure consisting of two kinds of 90° domains is introduced into the width-extensional vibrator, instead of a single domain structure. It is demonstrated by FEM simulations that the coupling factor approaches the predicted value as the number of domains formed in the vibrator increases. The measured coupling factor of a width-extensional vibrator with a polar multidomain structure is as high as 79.4%, which is close to the predicted value. An additional advantage of using this domain structure is its lower spurious response.