PROPAGATION OF A QUASI-SHEAR HORIZONTAL ACOUSTIC-WAVE IN Z-X LITHIUM-NIOBATE PLATES

It is found that an acoustic wave which is nearly polarized in the she ar horizontal (SH) direction can propagate along the X axis of a Z-cut lithium niobate plate if the ratio h/lambda, where h = plate thicknes s and X = acoustic wavelength, is less than about 0.5. Attractive prop erties of this quasi-SH wave include: 1) phase velocity nearly constan t for all values of h/lambda; 2) ability to propagate in contact with a liquid medium; and 3) electromechanical coupling coefficient as high as 0.15. These properties make the wave attractive for use in a varie ty of sensor and signal processing applications, 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 wavelength thick Z-X lithium niob ate plate is found to vary by more than 80 kHz for variation in KCl co ncentration from 0 to 0.15%.