Thermal depoling process and piezoelectric properties of bismuth sodium titanate ceramics

Stoichiometric and nonstoichiometric (Bi0.5Na0.5)TiO3 (BNT) ceramics were prepared by a conventional ceramic fabrication process. This study revealed that the high conductivity of BNT ceramics is associated with Bi vaporization during sintering. An x-ray study revealed that a tetragonal phase exists in the temperature range between 330 and 480 degrees C in BNT ceramic as well as BNT single crystals. In addition, the depolarization temperature T-d, rhombohedral-tetragonal phase transition temperature TR-T, and the temperature T-m of the maximum dielectric constant were determined to be 187, approximately 300, and 325 degrees C, respectively, from the temperature dependences of dielectric properties using unpoled and poled specimens. The piezoelectric properties of all vibration modes and the temperature dependences of the piezoelectric properties were measured using fully poled BNT ceramics. It was also revealed that BNT ceramics exhibit three thermal depoling processes at T-d, between T-d and TR-T, and between TR-T and T-m from the effects of annealing on the field-induced strain, x-ray diffraction patterns, and dielectric constant of poled specimens.