Characteristics of relaxor-based piezoelectric single crystals for ultrasonic transducers

For ultrasonic transducers, piezoelectric ceramics offer a range of dielectric constants (K approx.1000-5000), large piezoelectric coefficients (d_ijapprox.200-700pC/N), and high electromechanical coupling (k_T≅50%, k₃₃≅75%). For several decades, the material of choice has been polycrystalline ceramics based on the solid solution Pb(Zr_1-x, Ti_x)O₃ (PZT), compositionally engineered near the morphotropic phase boundary (MPB). The search for alternative MPB systems has led researchers to revisit relaxor-based materials with the general formula, Pb(B₁,B₂)O₃ (B₁:Zn²⁺, Mg²⁺, Sc³⁺, Ni²⁺ ..., B₂:Nb⁵⁺, Ta⁵⁺...). There are some claims of superior dielectric and piezoelectric performance compared to that of PZT materials. However, when the properties are examined relative to transition temperature (Tc), these differences are not significant. In the single crystal form, however, Relaxor-PT materials, represented by Pb(Zn_1/3Nb_2/3)O₃ - PbTiO₃ (PZN-PT), Pb(Mg_1/3Nb_2/3)O₃ - PbTiO₃ (PMN-PT) have been found to exhibit longitudinal coupling coefficients (k₃₃)>90%, thickness coupling (k_T)>63%, dielectric constants ranging from 1000 to 5000 with low dielectric loss <1%, and exceptional piezoelectric coefficients d₃₃>2000pC/N, the later promising for high energy density actuators. For single crystal piezoelectrics to become the next generation material of ultrasonic transducers, further investigation in crystal growth, device fabrication and testing are required.