A convergent beam of ultrasonic waves at very high frequencies can be used in an acoustic microscope to achieve resolutions of the order of 1 μm at depths of up to 1 mm. One application of the acoustic microscope are welding tests (e.g. the Figure which shows an Si/Mo structure welded at 475°C).
We have studied the relationship between the acoustic properties of a near-field acoustic sensor and the properties of a liquid. An acoustic stepped horn is driven by two piezoelectric elements. The generated acoustic waves cause the solid horn to resonate and an acoustic load at the tip modifies these oscillation modes. This leads to a change in electric impedance at the piezoelectric elements. This sensitivity to acoustic load allows a study of semi-infinite liquids. We found a relation between the resonance frequency and the liquid density and a relation between the electric impedance and the viscosity.
ß We report in this paper some results on ultrasonic absorption and velocity of selected low loss liquids and mixtures which may improve the resolving power of the scanning acoustic microscope. We have investigated the effect of electrolytes as a solvent in water which serve to lower the value of a/f 2 of water.We also report on our results with hydrogen peroxide and carbon disulfide for these two liquids should allow us to improve the resolution by more than a factor of 2. Subject Classification: [43 ] 35.24
Nondestructive investigations of elastic properties of materials via acoustic material signature, using reflection acoustic microscopy systems, are becoming more and more attractive. The necessity of using coupling liquids in such systems leads to the generation of generalized surface leaky waves such as Rayleigh waves. The V(z) response and hence the postion of its Fourier transform peaks change according to the liquid impedance due to a fundamental phenomenon known as mass loading. This effect, neglected up till now, has been studied in details on several materials (tungsten, stainless steel, and SiO2) with water, mercury, and other coupling liquids having different densities assuming the same velocity. For example, it is found that skimming shear velocity can be determined with a better precision when heavy coupling liquids are used. However, skimming longitudinal velocity seems to be independent of the coupling liquid density, whereas its efficiency gets larger with higher liquid densities.
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