Selecting passive and active materials for 1.3 composite power transducers C. Richard * , L. Goujon, D. Guyomar, H.S. Lee, G. Grange Laboratoire de G enie Electrique et Ferro electricit e, INSA Batiment G. Ferri e, 20 Avenue A. Einstein, 69621 Villeurbanne Cedex, France Abstract 1.3 PZT-polymer composites were fabricated using the dice and fill method with various PZT types and volume fractions. These composites were evaluated for power underwater transducer applications with an air backed and no matching layer configuration. Electrical input and acoustical output powers were monitored as a function of the drive level. Total acoustic power densities of 30 W/cm 2 were obtained with a P189/epoxy piezocomposite vibrating at 350 kHz with a low duty cycle (1–5%) and with a 90% effi- ciency. Power densities up to 20 W/cm 2 were measured with a 50% duty cycle. Evolution and destruction of the transducers were monitored versus increasing averaged power. It was observed that better efficiencies were obtained with low volume fraction configurations allowing natural acoustic impedance matching to water. It was found that hard PZT type (Navy III) are optimal compositions even for piezocomposite transducers. It is shown that, unlike a common belief, the polymer mechanical losses are comparable to those of the active ceramic justifying that 1.3 piezocomposites are suited for low-cost power applications. In fact, the main limitation induced by the polymer phase is a strong thermal breakdown when the temperature of the transducer approaches the glass transition region of the polymer. Measurements of the polymer losses as a function of the temperature were obtained confirming this point and offering interesting new alternatives for future composite power transducers. Ó 2002 Elsevier Science B.V. All rights reserved. Keywords: Piezoelectric composite; Ultrasonic transducer; Efficiency; Thermal modeling 1. Introduction A 1.3 connectivity piezocomposite consists of several PZT rods embedded in a polymer matrix [1]. This kind of structure allows to associate the electromechanical properties of the ceramic to the low acoustic impedance of the polymer for improving the transducer acoustic properties that can be tuned to the desired application by adjusting the composite phases. They have been widely used for medical imaging and non-destructive testing. This paper deals with the study of piezocomposite for underwater power applications in the 300–400 kHz fre- quency range. The objective of this work is to determine among different PZT types and volume fractions, the suitable configuration for power emission in the case of a transducer with no matching layer and an air backed configuration. In Section 2, the experimental transducers and the measurement conditions will be described. Then the re- sults for low electrical input will be presented in Section 3 and compared with those obtained theoretically with the KLM model [2]. The one dimensional character of the vibration of the composite with regard to the mas- sive case will especially be discussed. The measurement results under high electrical input and low duty cycle (up to 40 W/cm 2 at 1% duty cycle) will be presented in Section 4. They show that hard PZT type based piezocomposites present the lowest mechan- ical losses and that best efficiencies are obtained with low to medium volume fractions of PZT. Piezocom- posites appear to be more promising than bulk trans- ducers for high power applications in water. An interpretation of this adequation to high power sources will be given using a KLM modeling of the transducer with losses. The results obtained when the duty cycle is increased, while keeping constant the drive level are presented in Section 5. The role of the polymer in the destruction of the composites by excessive heating will be pointed out. Ultrasonics 40 (2002) 895–901 www.elsevier.com/locate/ultras * Corresponding author. Tel.: +33-472-43-82-86; fax: +33-472-43- 85-13. E-mail address: crichard@ge-serveur.insa-lyon.fr (C. Richard). 0041-624X/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII:S0041-624X(02)00221-4