| This dissertation describes the synthesis, the characterization and the modeling of a triblock copolymer of polystyrene(PS)-polybutadiene(PB)-polystyrene(PS) monolayer thin film patterned with nanometer sized cubic BaTiO3 crystals, with a focus on the influence of length scales and the hierarchical structure on the ferroelectric properties of BaTiO3.; BaTiO3, a dielectric and a ferroelectric, is used extensively in multilayer capacitors, thermistors and electrooptic devices. Its ferroelectric properties are known to be size dependent in the nanometer scale. To gain further insight into the fundamental characteristics of BaTiO3 in the nanometer scale, BaTiO3 is crystallized, for the first time, in a confined environment hosted by a PS-PB-PS triblock copolymer monolayer thin film with nanodomains of tunable geometry and size.; The engineering of the PS-PB-PS triblock copolymer monolayer thin film includes fractionation, elimination of interfacial interaction, control of film thickness and minimization of pinholes. The resulting monolayer contains cylindrical PS nanodomains embedded in a PB matrix. The film thickness, the diameter of the PS domain and the domain center-to-domain center distance are 31 nm, 15 nm and 31 nm respectively. Cubic BaTiO3 nanoparticles having a narrow size distribution of 10 nm are formed and located predominantly within the PB matrices by three steps: epoxidation-hydroxylation, barium titanation and vapor-phase hydrothermal process. The volume fraction of BaTiO3 phase is 0.0113. The effective dielectric constant of the BaTiO3/PS-PB-PS composite monolayer is 5.5 ± 2.5. With the assistance of dielectric mixing rules, the dielectric constant of the cubic BaTiO3 phase is determined to be 160.; The relative low dielectric constant of the BaTiO3 phase is usually explained by the critical size above which BaTiO3 particles are tetragonal and ferroelectric and below which particles are cubic and non-ferroelectric. But, the inconsistency of the critical size ranging from 15 to 200 nm in the literature and our capability of synthesizing 250 and 500 nm sized cubic BaTiO 3 particles hydrothermally at 80°C suggest that ferroelectric properties are not a function of the particle size (tertiary structure) alone. A possible mechanism for the suppression of the ferroelectric transition induced by lattice defects (primary structure), such as hydroxyl ions and barium vacancies, whose concentrations are related to the degree of completion of hydrolysis and condensation in the hydrothermal. process is proposed. Finally, the random field theory is used to describe the suppression of the ferroelectric phase transition and the increase in lattice strain induced by lattice defects. |
