Optimization of Ferroelectret Transducers for Pulse-Echo Water Immersion Operation

The adaptation and optimization of polypropylene ferroelectret (FE) films as the main piezoelectric element for water immersion ultrasonic transducers is studied. One of its main features is that they present a very wide band response so they can be an alternative for certain medical applications, especially those demanding intermediate frequencies and reduced size. The main problems of using this kind of materials are the poor sensitivity and the weak bonding of the metallization to the FE film that is easily degraded when entre in contact with water. The objective of this work is to optimize FE transducers for pulse-echo water immersion, while preserving the bandwidth, by: i) protecting the surface and ii) improving the impedance matching to the water.Metallization degradation produced by water has been quantified, then, main materials requirements to produce matching layers for this application are reviewed and potential materials to produce them proposed. A procedure to reinforce the metallization while preserving the electromechanical film response is proposed (based on Au sputtering) and first prototypes of transducers with a single matching layer are proposed, built and characterized.