Microstructure and permittivity of sintered BaTiO3: influence of particle surface chemistry in an aqueous medium

The influence of changes in the surface chemistry and surface composition of colloidal BaTiO3, due to its dissolution and adsorption/precipitation of Ba2+ in an aqueous medium, on the microstructure and permittivity of sintered powder compacts was investigated. For BaTiO3 powder with Ba-deficient (Ti-excess) surface prepared at pH 3, grain growth was enhanced at 1350 °C (above the eutectic) and permittivity was reduced (relative to stoichiometric BaTiO3 prepared at pH 9) with increasing sintering temperature due to the liquid phase formed at grain boundaries. This same sample showed minimal grain growth and moderate enhancement of sinterability at 1300 °C (below the eutectic) attributed to sliding of the Ti-excess surface phase. BaTiO3 powder treated at pH 3 and subsequently adjusted to pH 10 results in a core-shell structure with a varying near-surface stoichiometry, and produced abnormal grain growth for the compact sintered at 1350 °C. Permittivity of this sample was significantly reduced at 1350 °C due to the formation of the liquid phase, while exhibiting a similar permittivity to that of the stoichiometric sample when sintered at 1300 °C, despite significant microstructural coarsening. We conclude that changes in the surface-phase Ba/Ti ratio of particulate precursors, due to dissolution, adsorption and precipitation reactions in aqueous media, are as significant in determining the mechanical and electronic properties of the sintered material as are variations in the bulk stoichiometry of BaTiO3.