Study On The Preparation And Characterization Of Nanosized Barium Titanate Powders

The dielectric and ceramic microstructural properties of sintered ceramics using nanosized BaTiO3powders prepared by high gravity reactive precipitation method (HGRP) were systematically studied. The critical size and lattice expansions of nanosized BaTiO3particles in the size ranging from30nm to250nm, as well as the influencing factors of ferroelectric phase transformation were investigated. In addition, a novel high gravity reactive precipitation plus hydrothermal recrystallization method was developed to synthesize nanosized BaTiO3powders with characteristics of low concentrations structural defects, stoichimetric composition, tailored particle size in the range of45-125nm, and pseudo-cubic structure. At last, the particle growth mechanism during the hydrothermal recrystallization process was also investigated. As a result, the following results had been obtained.Although nanosized HGRP BaTiO3powders with particle size of35nm had high sinterability and the grain size of sintered ceramics was about0.5μm, the dielectric constants of nanosized BaTiO3powders were very low (about2600) and the dielectric dissipations were about20%. If the powders calcined at500℃and900℃to increase the particle size (below100nm) and promote the crystal structure changing from cubic to tetragonal, the dielectric constant increased to3600and4500and the dielectric dissipation downed to2%-3%, respectively. These results were in accordance with the conclusion that better dielectric properties were obtained by tetragonal powders, as opposed to cubic ones.Critical size and anomalous lattice expansion in nanosized BaTiO3powders were investigated. The lattice constants of BaTiO3powders were obtained from the XRD patterns of the particles in the size range of30-250nm prepared by calcinations method at the temperature range of400-1200℃. The present results indicated that the structural change from a tetragonal (ferroelectric) phase to a cubic (paraelectric) one around70nm in diameter, which was in good agreement with a critical diameter recently reported. Large lattice expansions of more than1.7%were detected in the particles down to30nm in diameter. The origin of the expansion was discussed on the basis of elastic constraints. The crystal structure of samples with higher concentrations of structural defects displayed to cubic paraelectric phase, while the ones with smaller particle size and lower concentrations of structural defects changed to tetragonal. These results suggested that the widely cited’size effect’ model was inappropriate and the distortion of Ti-O bonds within the BaTiO3lattice was possible, even for particles as small as40nm and having less concentrations of hydroxyl lattice defects and barium and titanium vacancies.Regarding the structural defects existed in the nanosized BaTiO3powders, the author had attempted to develop a novel synthesis method, high gravity reactive precipitation plus hydrothermal recrystallization method, to synthesize BaTiO3powders characterized by less structural defects, precise stoichiometric composition, and particle size below100nm. Based on LaMer model for monodispersed particle formation, we realized that one of the extreme measures to separate growth from nucleation may be the’seeding’method and a quick change of the temperature immediately after a limited nucleation, which leaded to the higher supersaturation within the nucleation range to plunge into the lower supersaturation below maximum concentration for nucleation (’supersaturation quenching’). The experiment was first conducted at beaker trials, and the experimental results of beaker trials can achieve those goals. The experimental parameters obtained from beaker trials were applied to the HGRP platform. Nanosized BaTiO3powders with tailored particle size in the range of45-125nm, spherical morphology, and fewer concentrations of structural defects were synthesized by controlling synthesis parameters during the hydrothermal recrystallization process. The hydrothermal recrystallization growth kinetics was determined by monitoring the particle size as a function of seeding time and at different seeding temperatures. The results showed that the growth kinetics of the BaTiO3particles follows the Lifshitz-Slyozov-Wagner theory for Ostwald ripening. In this model, the higher curvature and hence chemical potential of smaller particles provides a driving force for dissolution, and the larger particles continue to grow by diffusion-limited transport of species dissolved in solution.