Orientation dependence of electromechanical coupling factors in KNbO3

This paper is concerned with the electromechanical coupling factors for bulk waves in KNbO3 crystal. The dependence of coupling factors on the orientation of vibrators for various types of vibration modes has been calculated. As a result, it has been found that most of these coupling factors are very large at certain orientations of vibrators. Especially, the maximum coupling factor of the thickness-extensional mode excited with a perpendicular field, k(t), is as high as 69% for the rotated X-cut by an angle 49.5 degrees about the Y-axis; that of the thickness-sheer mode, k(s), is 88% for the X-cut. To the best of our knowledge, these coupling factors are the highest among those of known piezoelectrics. It has also been found that the coupling factor of the width-extensional mode of a thin finite-width plate with electrodes on its edges, k(ww)', is 82% for the rotated Z-cut by an angle 43.5 degrees about the Y-axis. This vibrator would be promising as elements of phased array ultrasonic probes.