Characteristics of quasi-shear-horizontal (QSH) acoustic waves in thin piezoelectric plates

This paper presents theoretical and experimental study of QSH waves propagating in thin plates of lithium niobate. The electromechanical coupling coefficient K² has been calculated as a function of plate thickness and propagation direction for all main crystallographic cuts. Y-cut and X-propagation for which K² may be more than 30% were chosen for our experiments. We have measured K ² as a function of plate thickness and have obtained a good agreement with theory. The extremely large value of K² opens possibility of realizing devices whose parameters can be controlled by electrical boundary conditions. We have studied SH waves propagating in the presence of a conducting electrode. It is found that by varying the gap between plate and electrode, the phase shift for a 3.2 MHz delay line can be varied by as much as 240°. We have also studied the influence of a thin layer of arbitrary conductivity on the velocity and attenuation of the QSH wave. Analysis show that the variations in these parameters can be as high as 18% and 4 dB/λ for a change of surface conductivity from 10^-7 to 10^-5 S