Multishell structure and size effect of barium titanate nanoceramics induced by grain surface effects

Abstract

A quantitative multishell structure model of the grain size effect of nano-BaTiO3 ceramics is proposed on the basis of Ginsburg-Landau-Devonshire thermodynamic theory. The existence of surface defects is considered by assuming that surface energy varies inversely with the distance to the grain surface. The surface effects lead to the multishell structure in BaTiO3 nanoparticles consisting of a surface paraelectric layer and inner ferroelectric core. It is found that the ferroelectric-paraelectric phase boundary moves to the center of the grain with decreasing grain size. When the grain size is less than 5 nm, our model predicts that the ferroelectric phase disappears, leaving only one stable cubic structure. The dielectric constant of nano-BaTiO3 ceramics with multishell structure was then calculated using an iterative method. The results show that the dielectric constant of BaTiO3 ceramics is reduced at room temperature with decreasing grain size. The three temperature-dependent dielectric peaks are depressed and diffuse in the nano-BaTiO3 ceramics. The two low dielectric peaks move to higher temperatures, while the higher dielectric peak moves to lower temperature with decreasing grain size. The results of the calculation are also compared with experimental data and discussed in detail. © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.