Coating of BaCO3 crystals with TiO2:: Versatile approach to the synthesis of BaTiO3 tetragonal nanoparticles

The fabrication of the very thin (< 1 mu m) dielectric layers for the next generation of miniaturized multilayer ceramic capacitors will require barium titanate, BaTiO3, particles of 100-200 nm with narrow particle size distributions, high density, and high tetragonality. Composite particles, consisting of a BaCO3 core and a TiO2 shell, can be efficiently used for the solid-state synthesis of nanocrystalline tetragonal BaTiO3 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 BaTiO3 from the BaCO3@TiO2 particles occurs by a single step reaction at 600-650 degrees C, whereas the conversion of mechanical mixtures of nanocrystalline raw materials BaCO3 and TiO2 requires temperatures >= 800 degrees 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 BaCO3@TiO2 particles undergo spontaneous fragmentation and spheroidization during the reaction, and, consequently, the resulting BaTiO3 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 BaCO3@TiO2 particles is a simple process that could be competitive with the oxalate and the hydrothermal routes for the mass production of high-quality BaTiO3 nanopowders.