| Popis: |
Head waves are an interesting phenomenon that is often ignored in harmonic piezoelectric wave study. They present a noise source in SAW devices, but carry useful information in seismology and ultrasonic NDE. In this work, a model based on exact Newtonian and Maxwell's equations of motion is used instead of the commonly used quasi-static approximation, because the former behaves better analytically than the latter and correctly describes certain waves that the latter cannot even predict, We identify three types of head waves. One type, called electromagnetic acoustic (EMAC) head waves, is new and unique to piezoelectric materials. The directivity of a head wave depends only on its critical angles, while the intensity changes with the kind of source that excites it. The polarization, on the other hand, is constant, independent of the observation angle and the kind of the excitation source. It is found that a metallized surface reduces the number of head waves in a piezoelectric medium and affects intensities of existing head waves, but does not change their propagation characteristics. Numerical examples are given for all three types of head waves, for both mechanical and electrical sources, and with both free and metallized surfaces. > |
