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 ?500 nm), dense (Bi0.5Na0.5)0.96Ba0.04TiO3 ceramics were obtained from sol-gel auto-combustion nanopowders by hot-pressing (700-950°C) and subsequent recrystallization (1000-1050°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 (??33= (416-16i), d31=(-22.68+0.55i)pCN-1, kt=44.5%, kp=21.1%) can be compared with those of coarse-grained ceramics and d33(95pCN-1) is higher. Shear-related coefficients were obtained from thickness poled and length excited plates (?? 11=(402-89i), d15=(108.3-21.4i) pCN-1 and k15=39.2%). At the depolarization temperature, Td = 153°C, the dielectric loss, tan ?(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 Td. The relaxor behaviour extends up to the isotropization point, Ti = 238°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 Td and Ti