A NEW ULTRASONIC COMPOSITE TRANSDUCER IMPLEMENTED WITH THICK-FILM TECHNOLOGY

A new ultrasonic transducer operating in air and fabricated by thick f ilm technology is presented. It consists of a composite mechanical str ucture in which three active elements, made with planar alumina substr ates supporting PZT-based piezoelectric films, behave as elastic guide s. The active elements have a constant rectangular cross-section and a re radially placed, 120 degrees apart. They are mounted between a rigi d base (operating as the acoustic backing) and an alumina disk. On bot h sides of each substrate a piezoelectric layer las interdigital thick -film electrodes embedded in layer itself, so that it can be polarized parallel to the long side of the active element; consequently, it all ows the creation of periodic surface stresses responsible for symmetri c extensional strains in the alumina plates. The elements, synchronous ly driven in phase, form, with the disk, a vibrating resonant structur e, which radiates acoustic energy perpendicular to the disk surface. T he assembly of active elements, acoustic backing and radiating disk fo rms a composite ultrasonic transducer operating in air without the nee d of any acoustic matching layer. Among the prominent features offered by the transducer design are: a transduction efficiency of 25% at an exiting voltage of 100 V-p and a high directivity of the emitted beam.