Synthesis and sintering improvement of Aurivillius type structure ferroelectric ceramics by mechanochemical activation

Abstract

An oxide mixture of composition 3Bi ₂ O ₃ :Nb ₂ O ₅ :2TiO ₂ has been mechanochemically activated in a laboratory mill for different times from 3 to 336 h. The as-milled powder and an unmilled mixture of identical composition were annealed at different temperatures up to the formation of the Aurivillius-type oxide Bi ₃ NbTiO ₉ , and examined by X-ray diffraction (XRD) and differential thermal analysis (DTA). The sample milled for 336 h shows complete amorphization. Its DTA curve exhibits two exothermic process at 370 and 560 °C, corresponding to the formation of a metastable fluorite phase and the Aurivillius-type oxide, respectively. The crystallization temperature of this Aurivillius phase from the unmilled oxide mixture is reported to be 1050 °C, whereas from the amorphous powder obtained by 336 h of mechanochemical activation this temperature becomes as low as 600 °C. Ceramics of this composition must be prepared by hot-pressing in order to obtain low porosities, owing to the lamellar morphology of the conventionally crystallized powder, which gives rise to textured anisotropic materials. Non-textured ceramics were obtained both from the conventionally crystallized and the amorphous powder by natural sintering at 1100 °C. Ceramics obtained from amorphous powder show lower porosity and higher electromechanical coupling factors.