Development of Directional Point-Focus-Beam Acoustic Microscope System

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A practical system of directional point-focus-beam(PFB) acoustic microscope with two functions of imaging and quantitative measurements has been developed as a new research tool to characterize anisotropic materials on a microscopic scale in the VHF and UHF ranges. Directional PFB acoustic probes in the frequency range of 200-400 MHz have been developed by introducing directionality into a PFB acoustic probe, limiting excitation of leaky surface acoustic waves(LSAWs) within a narrow angle with a rectangular transducer instead of a circular transducer.It has been revealed that it is essential to take into consideration the effect of beam steering of LSAWs due to the elastic anisotropy on the transducer output of the acoustic probe in order to realize a directional PFB acoustic microscope, and to limit the region of defocusing distance near the focal point for the V(z) curve analysis.It is necessary to chose arbitrarily an imaging area and an LSAW propagation direction, for characterizing the elastic properties of anisotropic materials on a microscopic area in imaging and quantitative measurements. The mechanical and electrical systems have been developed, and the experimental procedures in measurements have been established.The system has been demonstrated for investigation of a polycrystalline sample of Mn-Zn ferrite with an average grain size of 100mum. Directional acoustic images were obtained at 225 and 375 MHz for several different propagation directions of LSAWs. Anisotropy measurements of LSAW velocity in some grains of the sample have been performed through V(z) curve analysis. Anisotropic elastic properties of each grain have been characterized both in imaging and quantitative measurements.