Influence of sample size on ultrasonic phase velocity measurements in piezoelectric ceramics

Phase velocities of ultrasound are used for characterizing elastic properties of solids. One of the basic requirements is that the sample area should be much larger than the size of the transducer so that plane wave approximation will hold. This geometric requirement may not be possible to realize for some materials that can only be made in very small size. Using poled and unpoled lead zirconate titanate (PZT-5H) ceramics as examples, we have analyzed experimentally the sample size influence to the ultrasonic measurements. The smallest dimension that is in contact with the transducer is only 14% of the diameter of the transducer. We found that the phase velocity increases when the contact area becomes smaller. The velocity increase is 1.4% and 0.9%, respectively, in the unpoled and poled PZT-5H for the smallest dimension sample compared to bulk values.