SENSITIVITY OF PHASE-VELOCITY OF A COMPOSITE ZNO PLATE TO HUMIDITY

The propagation characteristics and sensitivity of the Lamb wave of a polyimide/ZnO/silicon nitride plate to humidity have been theoretically and experimentally studied. The measured values of phase velocity and the electromechanical coupling constant are 672 m/s and 0.87%, respectively, which almost correspond to the theoretical values. Comparing the theoretical sensitivity lines with the measured curves, it is found that in the low humidity range the fractional change of phase velocity would be mainly due to that of the density of the polyimide and in the middle humidity range it would be mainly due to that of the stiffness of the polyimide. By normalizing with the same phase velocity and polyimide thickness the sensitivity of phase velocity of the Lamb wave to humidity is about six times as large as that of the Rayleigh wave.