Optimization of the non-linear polarization process of piezoelectric ceramics for ultrasonic transducers

The objective of this work was to optimize the process of non-linear polarization (apodization) of piezoelectric ceramic disks. The apodization of the piezoelectric ceramic was performed to reduce the diffraction in the acoustic field generated by ultrasonic transducers. We looked for the better temperature of the insulating mineral oil that surrounds the piezoelectric ceramic, and how long the poling electric field should be applied to the ceramic to achieve a good apodization (piezoelectric coupling coefficient k_t>0.4 for the thickness mode of vibration). We have tested different initial oil bath temperatures and time interval of the poling field application. The results showed that if we raised the oil bath temperature above 200°C we needed to apply the poling field for a shorter time. Temperatures bellow 160°C were efficient only when the poling field was applied until the oil were cooled down to 35°C (what did not take less than 4 hours). We concluded that using higher temperatures, but still bellow Curie Point, it was possible to reduce the poling process, with satisfactory results, from 4-5 hours to 5 minutes.