Visco-elastic properties of materials can be evaluated by measuring ultrasonic wave properties. In this paper, we introduce an ultrasonic measurement apparatus to observe wave properties in the MHz region as a function of temperature. Using this apparatus, we have measured ultrasonic wave velocities with accuracy of 0.1 . The apparent wave attenuation had a tendency to increase gradually due to the temperature increase. This could be compensated for by considering the thermal expansion of the apparatus. As one application of this apparatus, properties of ultrasonic wave propagation was investigated during the gel-sol transformation of a micellar gel at a constant heating rate. The transformation of soft materials is known to be difficult to measure by longitudinal waves because the change in the compressibility is very small. A subtle dip in the velocity of the ultrasonic waves, however, was measured during the process of the gel-sol transformation. The attenuation per wavelength increased rapidly during the transformation. The temperature of the gel-sol transformation depended on the heating rate. We have confirmed the possibility of precise non-destructive evaluation of visco-elastic properties using the ultrasonic apparatus. This kind of investigation at high frequencies is difficult to achieve by conventional mechanical techniques.
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