CHARACTERISTICS AND APPLICATIONS OF QUASI-SHEAR-HORIZONTAL ACOUSTIC-WAVES

It is shown that a wave which is nearly polarized in the shear horizon tal (SH) direction can exist in thin plates of arbitrary piezoelectric materials. The wave is actually a Lamb wave (the A(1) Lamb wave mode) in which all three components of particle displacement are present. T he shear horizontal component, however, becomes dominant if the ratio h/lambda (h = plate thickness, lambda = acoustic wavelength) becomes l ess than about 0.5. Attractive properties of this quasi-SH wave includ e: (a) phase velocity nearly constant for all values of h/lambda; (b) ability to propagate in contact with a liquid medium; and (c) very hig h value of K-2, the electromechanical coupling coefficient. These prop erties make the wave attractive for use in a variety of sensor and sig nal processing applications. Experimental measurements of the phase ve locity and coupling coefficient of the mode are found to be in fair ag reement with theoretical calculations. An example of sensor applicatio ns is illustrated by using the wave to measure conductivity of liquids (aqueous KCl solution). The frequency of a 12 MHz quasi-SH mode oscil lator fabricated on a 0.48 lambda thick Z-X lithium niobate plate is f ound to vary by more than 80 kHz for a variation in KCl concentration from 0 to 0.15%.