Single crystals vs. PZT ceramics for medical ultrasound applications

The major advantages of single crystals (PMN-PT and PZN-PT) over conventional PZT 5H are their high piezoelectric and electro-mechanical coupling constants, which are attractive for high performance transducers. Other properties that contribute to transducer performance, such as velocity, clamped dielectric constant and coercive field, are generally lower for single crystals than for PZT. This paper reviews the strengths and limitations of single crystals for medical ultrasound applications. It is likely impractical to use this material for devices with center frequencies higher than 6 MM, because of the exceptional fragility, the low sound velocity and the low coercive field of single crystal material. For very low frequency arrays with small elements, electrical impedance becomes very high, which causes severe mismatch to the cable and system. An example of a 2.5MHz fine-pitch array with PMN-PT and a triple-layer PZT-5H with simulated and experimental data shows that the single layer PMN-PT array underperforms the multi-layer PZT transducer. Our experience suggests that optimal applications for single crystals in medical ultrasound applications are transducers with 3-5MHz center frequencies and preferably large elements. The results for a 300 mu m pitch, 3MHz PMN-PT array with 90% fractional bandwidth compared to 75% for the same PZT array are presented.