Thermal depolarization of lead-free (Bi0.5Na0.5)0.94Ba0.06TiO3 piezoceramics monitored by shear resonance of thickness-poled plates

Submicron-structured (Bi0.5Na0.5)0.94Ba0.06TiO3 ceramics, at the Morphotropic Phase Boundary of the solid solution, were prepared from nanometric powder synthesized by sol gel auto-combustion at 500ºC. Samples were obtained by hot-pressing (800ºC-2h) and subsequent recrystallization at moderate temperature (1050ºC-1h) [1]. Accurate resonance measurements at the uncoupled shear mode of thickness-poled plates and their analysis by Alemany software [2] were carried out to obtain electromechanical coupling factors (k15) and piezoelectric (e15, d15) and elastic (sE55)coefficients. Evolution of these as a function of the temperature was determined to study the depolarization process. Results show that irreversible depoling starts from a temperature higher than 120ºC. Shear resonance modes are measurable well above the reported depolarization temperature (100ºC). A value of d15=105 pC.N-1 was measured at 160ºC. This is most probably due to a difuse phase transition from a field-induced ferroelectric (FE) phase to the low temperature non-polar phase at zero field (LTNPZF) phase. The macroscopic ferro-piezoelectricity observed may arise from the coexistence of the FE phase at the nano-scale in the LTNPZF phase