| Many reports exist in the literature which indicate that BaTiO{dollar}sb3,{dollar} when prepared as a powder with small particle size {dollar}({lcub}<{rcub}1mu{dollar}m) or as a polycrystalline ceramic with small grain size, displays dramatic deviations in crystal structure and electrical properties, compared with forms involving larger crystallite sizes. Specifically, ceramists have been concerned for decades primarily with three characteristics for BaTiO{dollar}sb3{dollar} which have been reported with decreasing crystallite size to the nano scale: (i) a progression at room temperature from the normally tetragonal structure to one of apparent cubic symmetry, when examined by conventional x-ray diffraction (XRD), (ii) substantially decreasing values of the dielectric constant (K{dollar}spprime{dollar}) for grain sizes below 0.5{dollar}mu{dollar}m, and (iii) the concept that ferroelectricity should become unstable below a limiting critical crystallite size. This thesis reports an experimental investigation which substantially clarifies and leads to a plausible explanation of these phenomena for chemically prepared BaTiO{dollar}sb3.{dollar}; Research in this thesis determined that the evolution of structure from tetragonal to apparent cubic crystal symmetry for polycrystals does not result from a shift of the normal cubic-tetragonal transformation {dollar}rm (Tsb{lcub}c{rcub} = 130spcirc C){dollar} below room temperature. A more complete characterization of the size effect, including XRD, Raman and infrared spectroscopy, differential scanning calorimetry, and hot-stage transmission electron microscopy, revealed that {dollar}rm Tsb{lcub}c{rcub}{dollar} shifts only by {dollar}{lcub}approx{rcub}5spcirc C,{dollar} while the orthorhombic-tetragonal transformation shifts above room temperature, before ferroelectric transformation characteristics (spontaneous strain, enthalpy) become suppressed. Also, it was found that ultrafine-grain materials, which appear cubic when examined by XRD, exhibit light scattering characteristics of lower symmetry acentric phases. With regard to the second issue above, this thesis reports a new explanation for original electrical data measured for BaTiO{dollar}sb3{dollar} ceramics with grain sizes as small as 40nm, prepared by a special high pressure (8GPa) forming method. A microstructure model is developed which considers the dielectric behavior of bulk ceramics in terms of grain interiors and series limiting boundary regions. Curie-Weiss characteristics, in addition to dielectric properties measured in the temperature range of stability for the tetragonal phase, are fully described by dielectric mixing of the transforming primary phase and grain boundary regions with a value of {dollar}rm Ksp{lcub}prime{rcub}=130{dollar} and a thickness of 8 A. These findings underscore the importance of interfaces in determining the properties for materials of ultrafine microstructure. Finally, with regard to a critical size for ferroelectricity for BaTiO{dollar}sb3,{dollar} it is reported in this thesis that ceramic specimens with grain size of 40nm exhibit Raman scattering characteristics of the acentric orthorhombic phase, thermal depolarization consistent with Curie-Weiss behavior, and polarization-reversal characteristics indicative of ferroelectricity. Thus, the conclusion is made that such a critical size, if it exists, must be less than 40nm. |
