Relaxor-based single-crystal materials for ultrasonic transducer applications

Relaxor ferroelectric single crystals of Pb(Zn_1/3Nb _2/3)O₃ (PZN), Pb(Mg_1/3Nb_2/3)O₃ (PMN) and their solid solutions with normal ferroelectric PbTiO₃ (PT) were investigated for ultrasonic transducer applications. Crystals offer adjustable properties not only by compositional tailoring but also by domain state engineering associated with different crystallographic orientation, which is not achievable in polycrystalline materials. Longitudinal coupling coefficients (k₃₃) as high as 94% and dielectric constants (K₃^T) in the range of 3500 - 6000 were achieved with low dielectric loss (less than 1%) using <001> oriented rhombohedral crystals of (1-x)PZN-xPT and (1-y)PMN-yPT, where x less than 0.09 and y less than 0.35. Dicing direction as well as poling direction were critical for high coupling under laterally clamped condition. Dicing parallel to the (001) yields 90% of laterally clamped coupling (k_bar) out of 94% longitudinal coupling (k₃₃) for PZN-8%PT. On the other hand, samples diced parallel to (110) exhibited no dominant mode present. Thickness coupling (k_T) as high as 64% and low dielectric constant (K₃^T) less than 600 with low loss (less than 1%) could be achieved using tetragonal crystals of (1-x)PZN-xPT and (1-y)PMN-yPT, where x greater than 0.1 and y greater than 0.4. The performance gains associated with these ultra-high coupling coefficients and range of dielectric constants are evident in relation to broader bandwidth and electrical impedance matching. Specifically, rhombohedral crystals offer the possibility of extremely broad bandwidth devices for transducer arrays and tetragonal crystals for single element transducers. Transducer simulation was performed using the KLM model. The pulse/echo response simulated a 124% bandwidth subdiced array element with a center frequency of 10 MHz. An optimized array design of the same geometry constructed of PZT 5H displays an 87% bandwidth.