PROPAGATION OF ULTRASONIC LAMB WAVES IN PIEZOELECTRIC PLATES

The propagation of the lowest-order symmetric (S0) and antisymmetric (A0) Lamb wave modes in a piezoelectric plate is investigated both theoretically as well as experimentally. The characteristics of the modes and their relation to the surface acoustic wave is modified by electrical boundary conditions on the plate surfaces. It is found that when both surfaces are either metallized or unmetallized, the surface acoustic wave is obtained by a linear superposition of the A0 and S0 modes. On the other hand, if only one surface is metallized, then the surface acoustic wave is equivalent to just one of modes (either the A0 or S0 mode, depending on which surface is metallized). The beating phenomenon, whereby wave energy launched on one surface transfers periodically back and forth between opposite plate surfaces, is present when both surfaces are either metallized or unmetallized, but is absent if only one surface is metallized. Experimental measurements performed on Lamb wave devices fabricated on Y-cut, Z-propagating lithium niobate plates are found to be in good agreement with theoretical calculations.