Head-to-head comparison of acoustic properties of lead-free and PZT based HIFU transducers operating at 12 MHz

A direct comparison of performance and acoustic properties of high intensity focused ultrasonic transducers utilizing lead-free (Sodium Bismuth Titanate - NBT) and lead-based (Lead Zirconate Titanate - PZT) piezoceramics is discussed. All transducers operate at 12 MHz at third harmonic frequency, having an outer diameter of 20 mm, a central hole of 5 mm in diameter and a radius of curvature of 15 mm. The electro-acoustic efficiency determined by a radiation force balance is evaluated in a range of input power levels up to 15 W. Schlieren tomography as well as hydrophone measurements are used for evaluation of the acoustic field distribution. It is found that the average electro-acoustic efficiency of NBT-based transducers is approximately 40% while it is around 80% in the PZT-based devices. NBT devices show significantly higher inhomogeneity of the acoustic field under schlieren tomography compared to PZT devices. From pressure measurements in the pre-focal plane, it was found that the inhomogeneity could be attributed to depoling of significant areas of the NBT piezo-component during the fabrication process. In conclusion, PZT-based devices performed significantly better than those using lead-free material. However, the NBT devices show promise for this application and their electro-acoustic efficiency as well as the uniformity of the acoustic field could be improved by employing a low-temperature fabrication process or repoling after processing.