Characteristics of a quasi-sh (shear horizontal) acoustic wave in Z-X lithium niobate plates

A SH (Shear Horizontal) type acoustic wave is one in which the particle displacement lies in the propagation surface, and is at right angles to the direction of wave propagation. SH waves have recently attracted much attention for use in measuring properties of liquids and in liquid-phase-based chemical and biological sensors. This paper shows, for the first time, that a quasi-SH acoustic wave can exist in thin plates of Z-X lithium niobate. The wave is actually a Lamb wave (The A ₁ Lamb wave mode) in which all three components of particle displacement are present. The shear horizontal component, however, becomes dominant if the ratio h/λ (h=plate thickness, λ=acoustic wavelength) becomes less than about 0.5. Attractive properties of this quasi-SH wave include: (a) phase velocity nearly constant for all values of h/λ, (b) ability to propagate in contact with a liquid medium, and (c) electromechanical coupling coefficient, K² that reaches a maximum value as high as 0.15. These properties make the wave attractive for use in a variety of sensor and signal processing applications. Experimental measurements of phase velocity and coupling coefficient of the mode are found to be in fair agreement with theoretical calculations. An example of sensor applications is illustrated by using the wave to measure conductivity of liquids (aqueous KCl solution). The frequency of a 12 MHz quasi-SH mode oscillator fabricated on a 0.48 λ thick plate is found to vary by more than 80 kHz for variation in KCl concentration from 0 to 0.15%