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 (ε_R^T > 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.