Solid-state synthesis of nanocrystalline BaTiO3:: Reaction kinetics and powder properties

The formation of BaTiO3 nanoparticles by a solid-state reaction between nanocrystalline raw materials BaCO3 and TiO2 was studied as a function of temperature (400 degrees-800 degrees C), time (1-24 h), and titania particle size (15 and 30 nm). The reaction starts at 500 degrees C and a high reaction rate is already observed at 600 degrees C for the finest titania, with up to 90% conversion after 2 h. Two main reaction stages were observed at 600 degrees-700 degrees C. The first step is dominated by nucleation and growth of BaTiO3 at the TiO2-BaCO3 contact points and at the TiO2 surface. Surface diffusion of BaCO3 is, most likely, the prevailing mass transport mechanism responsible for the rapid formation of BaTiO3, even in the absence of a significant contribution from lattice diffusion. The second stage begins when the residual TiO2 cores are completely covered by the product phase. For longer times, the reaction can only proceed by the slower lattice diffusion, resulting in a strong decrease of the reaction rate. Single-phase BaTiO3 nanopowders with a specific surface area of 12-15 m(2)/g, an average particle size of 70-85 nm, a relative density of 96.5%-98.3%, and a tetragonality of 1.005 were obtained by calcination at 700 degrees-800 degrees C. Critical parameters in the preparation of ultrafine powders by solid-state reactions are the particle size of both raw materials, the absence of large hard agglomerates, and the homogeneity of the mixture. The use of fine raw materials and optimization of the reaction conditions make mechanical activation unnecessary.