Enhanced properties for ultrasonic transduction, phase transitions and thermal depoling in 0.96(Bi0.5Na0.5)TiO3-0.04BaTiO3 submicrometre-structured ceramics

Submicrometre structured (grain size approximate to 500 nm), dense (Bi0.5Na0.5)(0.96)Ba0.04TiO3 ceramics were obtained from sol-gel auto-combustion nanopowders by hot-pressing (700-950 degrees C) and subsequent recrystallization (1000-1050 degrees C). Electromechanical coefficients were obtained by analysis of the resonance spectra of thin discs using the Alemany et al software. The real part of the room temperature set of coefficients of the best performing materials (epsilon(sigma)(33) = (416 - 16i), d(31) = (-22.68 + 0.55i)pCN(-1), k(t) = 44.5%, k(p) = 21.1%) can be compared with those of coarse-grained ceramics and d(33) (95 pCN(-1)) is higher. Shear-related coefficients were obtained from thickness poled and length excited plates (epsilon(sigma)(11) = (402 - 89i), d(15) = (108.3 - 21.4i) pCN(-1) and k(15) = 39.2%). At the depolarization temperature, T-d = 153 degrees C, the dielectric loss, tan delta(T), of poled samples shows a maximum and the planar resonance virtually vanishes. Shear resonance of thickness-poled plates and weak planar electromechanical resonance are observed above T-d. The relaxor behaviour extends up to the isotropization point, T-i = 238 degrees C. This can be understood as due to the coexistence of the room temperature ferroelectric phase in the stability range of the low-temperature non-polar phase at zero field, between T-d and T-i.