New low acoustic impedance piezoelectric material for broadband transducer applications

This paper describes a new ceramic-air piezocomposite material, ie. porous piezoceramic based on a soft PZT matrix. First, the microstructure is analyzed and the electromechanical parameters are measured, in particular dielectric constant, acoustic wave velocity and mechanical losses, acoustical impedance and thickness mode electromechanical coupling factor. Among these parameters, the frequency dependence is investigated experimentally using complex impedance measurements at fundamental and harmonic frequencies of the free thickness resonance mode. A significant increase of mechanical losses with frequency is observed, as well as a decrease of thickness coupling factor. The composite is then modeled using unit-cell approaches, and the theoretical results are compared to measurements. The connectivity of the composite is investigated through the use of both 3-3 and 3-0 connectivity models, and by microstructure image analysis. Theoretical results are given as a function of porosity volume fraction and then compared to measurements on a few samples. It is shown that the air trapped in the ceramic matrix allows a decrease of acoustic impedance by a factor 2 to be achieved in comparison with classical bulk PZT, the electromechanical coupling factors and dielectric constants being close to those of commercial piezoceramics used for ultrasonic transducer applications, namely thickness coupling factor around 50%. Porosity values around 25% are found to offer a good compromise in terms of functional properties and ease of fabrication. Finally, transducer designs are considered for NDE and medical diagnostic applications. The overall performance, i.e. sensitivity and bandwidth of transducers based on this new material are compared to those of bulk PZT based devices and a frequency limit is determined.