ASA 124th Meeting New Orleans 1992 October

1aPA5. Ultrasonic flux imaging in anisotropic solids.

Richard L. Weaver

Dept. of Theor. and Appl. Mech., Univ. of Illinois, Urbana, IL 61801

Matt R. Hauser

James P. Wolfe

Dept. of Physics, University of Illinois

A new method is discussed for characterizing acoustic flux propagation in anisotropic media. The technique utilizes a pair of water-immersion focused acoustic transducers as a point source and point detector. Raster scanning of either the source or detector over a slab-like specimen of an anisotropic solid produces a transmission pattern that exhibits the anisotropies in acoustic flux previously known as ``phonon focusing.'' Unlike phonon focusing images, however, the pattern is modulated by ``internal diffraction'' fringes arising from interference between sheets on the acoustic wave surface. In addition to the diffraction effects, the images reveal interesting critical-cone structures associated with the water/solid interface. These images may be understood theoretically by taking into account the full anisotropy of the medium, the boundary conditions between the solid and the water, and the pressure fields produced by the immersion transducers. The theoretical predictions agree well with the experimental observations of UFI in silicon and a number of other materials, which include single-crystal metals, insulators, and semiconductors. As a new method, UFI holds promise for examining anisotropies in the vibrational properties, and, possibly, electron--phonon coupling in metals and superconductors. The principles and techniques may also have application in the nondestructive characterization of textured polycrystalline and composite materials. [Work supported by MRL Grant NSF DMR-89-20538 and by NSF MSS-91-14360.]