A new low frequency piezoelectric composite transducer

Piezoelectric composite transducers have many recognised advantages for medium frequency (0.2 MHz-20 MHz) ultrasound applications. However, the extension to lower frequency bands has not been straightforward, especially with regard to active (i.e. transmission) behaviour. Manufacture from single ceramic blocks is problematic, due to polarisation limitations and inevitably, the low capacitance compromises transmit sensitivity. Alternative configurations, based on multilayered stacks can overcome most of these problems but introduce additional complexities with device manufacture and operational robustness. This paper presents a different method for manufacture of low frequency composites, utilising the fundamental symmetric Lamb mode (S0) in a conventional thickness drive piezoelectric plate. A composite plate, with electrodes positioned on the major faces, is driven at the fundamental frequency corresponding to the plate length dimension. This is shown to correspond with the S0 mode and demonstrates low loss, longitudinal wave propagation, with uniform surface displacement at the end faces that are normal to the direction of wave travel. A combination of experiment and finite element modelling using PZFlex is used to demonstrate the validity of this approach for low frequency (10 kHz-100kHz) 2-2 piezoelectric composite arrays. Measured coupling coefficients of approximately 0.5 for pzt5h ceramic and 0.8 for single crystal pmn-pt are shown to provide good agreement with theory as do laser scans of the radiating surface profile. The simulated TVR is superior to ceramic based tonpilz configurations of a similar frequenc