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 constituent 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 freeing the lateral clamping of the ceramic in the composite structure. Their higher coupling and lower acoustic impedance recommend composites for medical ultrasonic imaging transducers. The model also reveals that the composite's material properties cannot be optimized simultaneously; tradeoffs must be made. Of most significance is the tradeoff between the desired lower acoustic impedance and the undesired smaller electromechanical coupling that occurs as the volume fraction of piezoceramic is reduced.

Transducers for medical ultrasonic imaging have been made from composite piezoelectric materials. This paper describes a simple physical model for the material properties which govern the thickness-mode oscillations in thin plates of PZT-rod/polymer composites. We consider the case where the lateral periodicity of the rods is much smaller than all relevant acoustic wavelengths. Expressions are derived for the effective material parameters in terms of the properties of the constituents. The composites' properties can then be tailored to device requirements by choosing appropriate components and by varying the volume fraction of piezoceramic. Our analysis reveals the need for a trade-off between the desired lower acoustic impedance and the undesired smaller electromechanical coupling that occurs as the volume fraction of piezoceramic is reduced. The predictions of this model are in good agreement with measurements made on rod-composite plates containing 5 to 35 percent PZT.