A new technique for the design of acoustic matching layers for piezocomposite transducers

At present a lot of piezoelectric broad band ultrasonic transducers for both, medical and NDE purposes, use as active material piezoelectric ceramic composites. A lot of scientific and technological research has been devoted to the optimisation of piezocomposite materials, in order to increase the transducer band and efficiency. In a typical transducer based on piezocomposites the active material is mounted on a soft lossy backing and one matching layer is placed on the front, radiating face of the transducer with the aim to match the acoustic impedance of the medium and to enlarge the bandwidth. In this paper an optimisation work is shown to demonstrate that a composite configuration can be used in the matching layer, in order to improve the efficiency and the band of the transducer. An approximated two-dimensional analytical model has been used to optimise the design of a composite-structured matching layer in the case of 2-2 composites, obtaining different results for the polymer and piezoceramic composite phases; a design technique is suggested in order to improve the transducer performance. With the aim to verify the proposed design criterion, a transducer prototype, based on a 2-2 piezocomposite, with a composite matching layer was realised. On this sample we measured the electrical input impedance and the insertion loss and we compared the obtained results with those of a transducer classically matched to the load. The obtained results confirm the computed improvements in the transducer performance and justify the proposed design approach.