Innovations in piezoelectric materials for ultrasound transducers

Piezoelectric materials lie at the heart of ultrasonic transducers. For transducers used in medical imaging (3?7 MHz), PZT-5H ceramics offer high electromechanical coupling (k33 ? 75%), resulting in good bandwidth and sensitivity. As transducer arrays become smaller with increasing frequency, the development of high permittivity ( ?RT > 7,000 vs. 3,400 for PZT-5H), piezoelectrics based on polycrystalline PMN-PT, provide improved electrical impedance matching. Advanced medical diagnostic techniques, including contrast and harmonic imaging, have taken advantage of the recent development in single crystal Relaxor-PTs that offer coupling k33?s > 90% and subsequently, significant increases in bandwidth. For small animal, ophthalmology and cellular imaging, higher resolution is demanded, thus requiring transducers operational in the range of 20?100 MHz. Advancements in ceramic processing include pore-free and fine-grain (?1 micron) piezoelectric ceramics of PT and PZT, being an ?enabling? technology, allowing the fabrication of high frequency single element and annular arrays. Innovations in the fabrication of high frequency arrays (? 30 MHz) include tape casting and sol-gel molding techniques. Of particular significance, DRIE (deep reaction ion etching), has demonstrated the ability to mill out ultrafine features, allowing 1?3 crystal-polymer composites operational at frequencies ? 60 MHz, far beyond that achieved by current state-of-the-art dicing.