Microstructure and piezoelectricity of Bi3TiNbO9 ceramics from mechanochemically activated precursors

Bi3TiNbO9 is one of the Aurivilius-type structure compounds, of general formula [Bi2O2][A(n-1)B(n)O(3n+1)], made from pseudoperovskite (with n=2) layers alternating with Bi2O2 layers. It belongs to an ensemble of high transition temperature ferroelectrics that attracts nowadays a great deal of interest as piezoelectric materials for high temperature use (p.e. in non-destructive testing of materials by ultrasounds for siderurgy, nuclear plants or petrochemical industry). When prepared as thin films, they have interest as non-volatile computer memories (FERAM), due to their reduced fatigue, large polarization retention and low leakage currents. In solid state reaction and sintering, the lamellae-like growing habit, that resembles the layered crystalline structure, gives place to porous ceramics. Previous works used hot uniaxial pressing in order to obtain dense ceramics at low temperatures, avoiding changes in stoichiometry and exagerated grain growth. But, such ceramics have anisotropic properties. In order to overcome these problems, Bi3TiNbO9 ceramics were obtained by cold-pressing of amorphous powder and sintering. The amorphous precursor was obtained by mechanochemical activation of a stoichiometric oxides mixture. The quantitative microstructural, ferro and piezoelectric characterization of the ceramics reveals that these ceramics have lower porosity, higher hardness, are better polarizable and show higher piezoelectric response that those prepared from solid state reaction powder.