MODELING 1-3 COMPOSITE PIEZOELECTRICS - THICKNESS-MODE OSCILLATIONS

A simple physical model of 1-3 composite piezoelectrics is advanced for the material properties that are relevant to thickness-mode oscillations. This model is valid when the lateral spatial scale of the composite is sufficiently fine that the composite can be treated as an effective homogeneous medium. Expressions for the composite's material parameters in terms of the volume fraction of piezoelectric ceramic and the properties of the consituent piezoelectric ceramic and passive polymer are derived. A number of examples illustrate the implications of using piezocomposites in medical ultrasonic imaging transducers. While most material properties of the composite roughly interpolate between their values for pure polymer and pure ceramic, the composite's thickness-mode electromechanical coupling can exceed that of the component ceramic. This enhanced electromechanical coupling stems from partially freeeing the lateral clamping of the ceramic in the composite structure. Their higher coupling and lower acoustic impedence commend composites for medical ultrasonic imaging transducers. Our model also reveals that the composite's material properties cannot all be optimized simultaneoulsy; trade-offs must be made. Of most significance is the trade-off between the desired lower acoustic impedance and the undesired smaller electromechanical coupling that occurs as the volume fraction of piezoceramic is reduced.