High frequency piezo-composite transducer with hexagonal pillars

Developing a high-frequency piezo-composite material is a challenge due to the extremely small pillar dimensions. The high frequency composites made by conventional dicing saw techniques will most likely have a low ceramic volume fraction and a large pillar width to height aspect ratio, because of the limitation of making narrow kerfs and small pitches. Large aspect ratio pillars in a low ceramic volume ratio composite will cause a dramatically decrease of both electromechanical coupling and effective velocity. In this work, we investigated a new composite geometry with hexagonal pillars. The performances of the composites have been simulated by using a finite element analysis tool (PZFlex). The simulation results show that the composites with hexagonal pillars provide a significant improvement of performance over the composites with other pillar shapes, at a low volume ratio and high aspect ratio. A hexagonal pillar composite, with a volume ratio of 0.32 and an aspect ratio of 0.9, can maintain an effective electromechanical coupling coefficient of 0.6 with a drop in effective velocity of 20 percent, while secondary pulses, due to lateral resonances, are about 20 dB below the main pulse. To verify the simulation findings, hexagonal pillar composites with low ceramic volume ratio of 0.32 have been fabricated by the dice-and-fill technique. The composites were finished at different thicknesses to vary the aspect ratios. Each was mounted on an SMA connector. The electrical impedances of the transducers were measured to compare with the simulations. The electromechanical coupling coefficients and effective velocities were calculated from the resonance and anti-resonance frequencies. The measured electromechanical coupling shows an improvement of more than 50% over previous geometry composites. The experimental results agree well with the simulations. It suggests that this hexagonal geometry is a promising structure for fabrication of high frequency composite transducer- - s and arrays.