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Abstract
Barium titanate is synthesized under hydrothermal conditions by the reaction of a variety of titania precursors with aqueous solutions of Ba(OH)$\sb2$ at 80$\sp\circ$C. Particles processed at relatively low concentrations of Ba(OH)$\sb2$ are micro-sized and highly aggregated, but increasing concentrations cause the particle size to decrease, resulting in nanometer-sized and fairly monodispersed particles. The change in particle size and morphology at various Ba(OH)$\sb2$ concentrations is controlled by the dissolution of titania and precipitation of BaTiO$\sb3$. In order to explain the origin of "raspberry-like" BaTiO$\sb3$ particles and the generation of hierarchically ordered BaTiO$\sb3$ aggregate comprised of primary, crystalline particles, which exhibit an unusually high degree of crystallographic alignment, the role of colloidal stability and therefore controlled aggregation of precipitated primary particles is taken into account. Formation of SrTiO$\sb3$ on BaTiO$\sb3$ particles reveal that two different morphologies for the growing SrTiO$\sb3$ exists and that the form taken by SrTiO$\sb3$ depends on the degree of supersaturation. In concentrated solutions, homogeneous nucleation and aggregation growth occur. In dilute solutions, heterogeneous nucleation and continuous growth of SrTiO$\sb3$ promote epitaxial growth. BaTiO$\sb3$ particles prepared by the alkoxide (Ti(OC$\sb3$H$\sb7)\sb4$) -hydroxide (Ba(OH)$\sb2$) route under hydrothermal conditions show that secondary processed such as aggregation and recrystallization are important to control the particle size and morphology. Particle clustering, and rearrangement of nanometer-sized BaTiO$\sb3$ particles, and particulate uniformity can then be explained in terms of solution reactions and colloidal behavior.
