Coating of BaCO₃Crystals with TiO₂: Versatile Approach to the Synthesis of BaTiO₃Tetragonal Nanoparticles

The fabrication of the very thin (<1 μm) dielectric layers for the next generation of miniaturized multilayer ceramic capacitors will require barium titanate, BaTiO 3, particles of 100−200 nm with narrow particle size distributions, high density, and high tetragonality. Composite particles, consisting of a BaCO 3 core and a TiO 2 shell, can be efficiently used for the solid-state synthesis of nanocrystalline tetragonal BaTiO 3 powders at moderate temperatures. A uniform layer of amorphous titania has been formed on the surface of fine barium carbonate crystals suspended in a peroxotitanium(IV) solution by a precipitation process. Formation of single phase BaTiO 3 from the BaCO 3 @TiO 2 particles occurs by a single step reaction at 600−650 °C, whereas the conversion of mechanical mixtures of nanocrystalline raw materials BaCO 3 and TiO 2 requires temperatures ≥800 °C and produces much coarser powders. The lowering of the reaction temperature can be attributed to the maximization of the contact surface between the reactants and to the minimization of diffusion distances. The BaCO 3 @TiO 2 particles undergo spontaneous fragmentation and spheroidization during the reaction, and, consequently, the resulting BaTiO 3 nanoparticles have no memory of the initial shape of the barium carbonate crystals. The final powders have high density (94−98%), high tetragonality ( c / a ratio: 1.006), average particle size of 140 nm, and no agglomerates bigger than 300 nm. The solid-state reaction of BaCO 3 @TiO 2 particles is a simple process that could be competitive with the oxalate and the hydrothermal routes for the mass production of high-quality BaTiO 3 nanopowders.