Alicia Castro, Pilar Millán, Lorena Pardo and Basilio Jiménez
An oxide mixture of composition 3Bi2O3:Nb2O5:2TiO2 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 Bi3NbTiO9, 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.