HEAD WAVES IN A PIEZOELECTRIC HALF-SPACE

Head waves are an interesting phenomenon that is often ignored in harm onic piezoelectric wave study. They present a noise source in SAW devi ces, but carry useful information in seismology and ultrasonic NDE. In this work, a model based on exact Newtonian and Maxwell's equations o f motion is used instead of the commonly used quasi-static approximati on, 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 electromagne tic acoustic (EMAC) head waves, is new and unique to piezoelectric mat erials, The directivity of a head wave depends only on its critical an gles, 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 fou nd that a metallized surface reduces the number of head waves in a pie zoelectric 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 e lectrical sources, and with both free and metallized surfaces.