Status of piezoelectric single crystal growth for medical transducer applications

Single crystal piezoelectrics such as PMN-PT are leading to dramatic improvements in transducer, sensor and actuator technology. The potential benefit of implementing these materials is clearly promising in terms of increased bandwidth, sensitivity and source levels due to ultrahigh electromechanical coupling factors (k(33)>94%), high piezoelectric coefficients (d(33)>2500pC/N), high strain levels (>0.6%), and low hysteresis. TRS is investigating a variety of uses for piezoelectric single crystals in medical transducer applications such as ultrasound harmonic imaging and high frequency transducers. Harmonic imaging requires very broad bandwidth ultrasonic transducers. By using single crystals, which inherently have more bandwidth, the complex engineering normally associated with achieving increased bandwidth will be reduced. Simple ultrasound transducers that have been constructed from crystals exhibiting bandwidths of 100% (if high impedance, backing layers are used, 140% can be achieved) and insertion losses less than similarly constructed PZT transducers with only a 70% bandwidth. To date, commercialization of these materials for ultrasound has been limited by low yields, small size, and high cost. The primary cost driver for single crystals is the amount of useable material yielded per growth run. Currently these crystals are grown by the Bridgman method, which involves moving a crucible containing a molten charge relative to a stationary temperature gradient resulting in unidirectional solidification. One limiting factor of the Bridgman growth technique for PMN-PT is that useful compositions of this system have a segregation coefficient less than 1, so there is a significant compositional gradient along the growth axis, which severely limits the amount of crystal yielded per growth run. The current state of the art is Bridgman-grown 1.5-2" diameter boules. Scaling up and the use of zone melting in the Bridgman technique to increase useable length are expected to yield price reductions for large volume production that are 2 to 3 times that of high quality PZT ceramic (similar to$20 to 30/cc). The current status of the growth development including current issues as well as preliminary results of utilizing crystals in medical transducer applications will be discussed.