Limits to the enhancement of piezoelectric transducers achievable by materials engineering

The case of a piezoelectric material with ∞m axial symmetry (applicable to piezoceramics and most piezocomposites) is analyzed, and formulae for the resulting structures are derived. In terms of transducer design, the constraints on the elastic and dielectric constants provide no blockage to achieving acoustic impedance matching to the medium and electric impedance matching to the electronics. Electromechanical coupling is shown to be limited by the material´s maximal coupling factors. A simple formula is derived which facilitates evaluating the maximal electromechanical coupling factor when the electric field is applied along the polar axis from the planar coupling factor and the thickness coupling factor, which can be readily evaluated from the lateral and thickness resonances of a disk