Characterization of Nanostructured Phases and Peculiar Phase Transitions in BNBT Lead-Free Piezoceramics

Submicron-structured (Bi0.5Na0.5)0.94Ba0.06TiO3 (BNBT6) dense ceramics, from nanometric powder synthesized by sol gel auto-combustion at 500ºC and obtained by hot-pressing (800ºC-2h) and subsequent recrystallization at moderate temperature (1000-1050ºC-1h), have been studied. In-situ measurements at the shear mode of electromechanical resonance of non-standard thickness-poled shear plates as a function of the temperature show higher depolarization temperature than measurements at the radial mode of thin disks. Shear mode related material coefficients are measurable up to 160ºC, being k15?30% and d15?250 pC.N-1 at 130ºC. Depolarization is a complex phenomena caused by a ferroelectric (FE) macrodomains thermal randomization and a phase transition from the field-induced FE phase to a relaxor phase. The early stage of such a transition involves a non-negligible piezoelectricity arising most probably by the percolative coexistence of ferroelectric macrodomains in the resonator under the given stress field for each resonance mode.