Forward and Backward Acoustic Waves in Crystals with High Piezoactivity and Dielectric Permittivity

Results are presented from studying the characteristics of acoustic waves of different types in plates of ceramic materials with high piezoactivity and dielectric permittivity (e.g., 0.95(Na0.5Bi0.5)TiO3–0.05BaTiO3, (K,Na)(Nb,Ta)O3, and Ba(Zr0.2Ti0.8)O3–50(Ba0.7Ca0.3)TiO3). The dependences of the phase velocities and electromechanical coupling coefficients of these waves on parameter hf (where h is the thickness of a plate, and f is the frequency of a wave) are calculated for directions XY and YX of propagation. It is found that the investigated materials are characterized by frequency ranges of the existence of backward acoustic waves whose phase and group velocities are oriented in different directions. The existence of extremely broad frequency ranges that depend very weakly on a change in the electrical boundary conditions on the surface of a plate is established for waves with negative group velocity in the case of a 0.95(Na0.5Bi0.5)TiO3–0.05BaTiO3 single crystal. It is shown that the electromechanical coupling coefficient of acoustic waves in the investigated materials is greatest for an SH1-wave in YX Ba(Zr0.2Ti0.8)O3–50(Ba0.7Ca0.3)TiO3, and can be as high as 7‒30% in the range hf = 1330–2000 m/s.