ORIGIN OF HIGH DIELECTRIC PROPERTIES OF NM-SIZED BARIUM TITANATE CRYSTALLITES

摘要

Barium titanate (BaTiO_3) crystallites with various particle sizes from 17 to 1,000 nm were prepared by a modified 2-step thermal decomposition method of barium titanyl oxalate. Investigation of impurity in these particles using both TG-DTA and FT-IR measurements revealed that no impurity was detected in the BaTiO_3 lattice while hydroxyl and carbonate groups were detected only on the surface. Moreover, their relative densities were always over 99 %. The dielectric constants of these particles were measured using slurry by a powder dielectric measurement method. As a result, the dielectric constant of BaTiO_3 particles with a size of around 140 nm exhibited a maximum of around 5,000. Thus, we discussed the origin of high dielectric constant around 5,000 for BaTiO_3 particles with a size of around 140 nm. The crystal structure of the BaTiO_3 particles with sizes below 140 nm was always assigned to cubic Vm-3m by a conventional X-ray diffraction measurement because of significant line broadening. Thus, using a synchrotron radiation X-ray powder experiment, the crystal structure of the BaTiO_3 particles with sizes from 17 to 1,000 nm was investigated. As a result, in the BaTiO_3 particles with sizes over 40 nm, it was confirmed that their crystal structure was assigned to tetragonal 74mm. This means that the cla ratio continuously decreased with decreasing particle sizes from 1,000 to 40 nm. In the particle size with a maximum dielectric constant of 5,000, its cla ratio of 1.0064 was smaller than 1.011 of the BaTiO_3 single crystal. Finally, to explain the origin of high dielectric constant, the model related to "external AC field induced superparaelectric behavior" was proposed.