Frequency dependent properties of high permittivity PMNT piezoelectrics for ultrasonic transducer applications

Pb(Zr,Ti)O₃-5H piezoelectric ceramics are widely used in medical ultrasonic applications due to their high electromechanical coupling (k ₃₃ ≤ 75%). However, their relatively low clamped dielectric permittivity (< 1500) limits their usefulness in high frequency medical transducers with small aperture and array elements. Thus, high permittivity piezoelectric materials are desired. Relaxor based Pb(Mg_1/3Nb _2/3)O₃-PbTiO₃ (PMNT) piezoelectrics with high permittivity and coupling have attracted much attention in medical ultrasonic applications in the form of both polycrystalline and single crystals. In this work, the electromechanical coupling of monolithic PMNT and 1-3 composites were investigated as a function of frequency. Meanwhile, fine grain size (< 1 micron) PMNT polycrystalline ceramics (dielectric permittivity >5000, coupling k₃₃ ̃78%) were developed from the point of view of manufacturability and improved mechanical and dielectric strength. High frequency PMNT single crystal/epoxy 1-3 composites were prepared with electromechanical coupling factors in the range of 72-85%, using various fabrication methods, including dice/fill and deep reactive ion etching.