Hydrothermal synthesis of barium strontium titanate (BST) powders, and continuous and patterned thin films

Barium strontium titanate (BST) solid-solution oxides represent an important class of ferroelectric materials due to their high dielectric constants, composition dependent Curie temperature, and potential applications in dynamic random access memory (DRAM). Hydrothermal technique is a new low-temperature (\u3c250°C) method for the synthesis of advanced ceramic materials. The objective of this research is to use the hydrothermal method to synthesize BST nano powders, continuous and patterned thin films and to gain a better understanding of the ceramic growth mechanism under hydrothermal conditions. ^ Homogeneous crystalline BST nano powders have been hydrothermally synthesized at 200°C. The mean particle sizes were found to be between 80 nm to 150 nm. All powders were found to be single-phase solid solutions, and were determined to be cubic using XRD. However, Raman spectroscopy has revealed, for the first time, the presence of tetragonal phase in Ba-rich hydrothermal BST powders. ^ BST thin films with thickness from 0.1 μm to 1 μm have been synthesized on as-polished and pre-treated Ti foils at 200°C by the hydrothermal method. All the BST thin films are crack-free, polycrystalline, and adhered strongly to the substrates. The dielectric constants of BST hydrothermal thin films were found to be around 150. Study on film morphology, and film thickness showed that the nucleation rate of hydrothermal thin film is mainly determined by the temperature; and growth rate is determined by the pH value in the hydrothermal solutions. ^ Based on the understanding of the hydrothermal thin film growth mechanism, patterned BST thin films were synthesized using a two-step method. In this two-step method, first, Ti substrate is anodized to produce a surface layer of amorphous titanium oxide nano-tubes arrays (“honeycomb”). In the second step, the anodized substrate is subjected to hydrothermal treatment in aqueous (Ba,Sr)(OH)2 solutions, where the nano-tube arrays serve as templates for the hydrothermal growth of polycrystalline BST nano-tubes. The optimum hydrothermal conditions for the synthesis of BST nano-tube thin films have been presented in a phase stability diagram. A growth model has been proposed for the synthesis of hydrothermal BST nano-tube thin films.