Synthesis And Characterization Of One-dimensional Ferroelectric Nanomaterials In BaO-TiO₂ System

As the ferroelectric devices are taken development on minimization, integration and functionalization, there is growing interest in preparing low-dimensional ferroelectric nanostructure materials. The preparing of new one-dimensional ferroelectric nanostructures and exploring the relationship between their ferroelectric property and the scale and dimension are of great importance for both of fundamental research and technical applications.In this dissertation, the BaO-TiO2 system materials were chosen as the object of study. One-dimensional nanomaterials such as BaTiO3 nanowires and BaTi2O5 nanowires were synthesized by molten salt synthesis method or templating method. The synthesis process and the structure of the materials were studied intensively. Moden testing methods were used to explore the properties of the individual ferroelectric one-dimensional materials, such as piezoelectric property, ferroelectric property and leakage current. The main contents of the dissertation are described as follows:(1) Single-crystalline BaTi2O5 nanowires have been successfully prepared by a molten salt synthesis rout in high yield. The as-synthesized BaTi2O5 nanowires have a uniform structure with rectangular cross section (about 80-200 nm in width,70-150 nm in thickness) and lengths reaching up to tens of micrometers. The effects of raw material, salt type, the ratio of mixed salts, annealing temperature and annealing time on the phase composition and morphology of the products have been discussed. The optimum process condition has been obtained. The growth of BaTi2O5 nanowires is resulted from the anisotropy of the crystal structure. In the molten salt environment, BaTi2O5 crystal was more easily to form a wire-like structure. BaTiO3, Ba4Ti13O30 were formed together with BaTi2O5 phases at the beginning of the synthesis. And Ba4Ti13O30 was in a one-dimensional structure. With the annealing time prolong, pure phase BaTi2O5 nanowires are obtained after series of reactions under the synthesis temperature.(2) The molten salt synthesis route, previously reported to yield BaTiO3 nanowires was full of disputes and has been re-examined. The effects of surfactant, salt type, the ratio of mixed salts, annealing temperature and annealing time on the products'phase composition and morphology have been discussed. The results show that it is difficult to get BaTiO3 nanowires by molten salt synthesis method. The small amount of 1-D nanotructures in the sample can be indexed as BaTi2O5, BaTi5O11, BaTi3O7, Ba6Ti17O40 phases. None of the examined 1-D nanostructures in the sample could be indexed as BaTiO3. BaTiO3 is hard to grow anisotropically in the molten salt enviroment due to its high symmetry crystal structure. However, the BaTiO5, BaTi5O11, BaTi3O7,Ba6Ti17O40 phases with high anisometric crystal structures are more easily to form anisometric shapes. Based on above discussion, by adjusting the Ba/Ti ratio of the raw material and the process condition, pure phase one-dimensional Ba4Ti13O30 and BaTi5O11 nanomaterials were also obtained by molten salt synthesis method. This paper provides a new methodology for the synthesis of one-dimensional nanomaterial in BaO-TiO2 system.(3) One-dimensional BaTiO3 materials have been synthesized by template method. A new method based on templating and molten salt treatment is introduced to synthesize single-crystalline BaTiO3 nanowires. Pure phase single-crystalline BaTi2O5 nanowires were coated with BaC2O4·0.5H2O shell by the precipitation process. And then, the precursor was annealed in molten salt. Single-crystalline BaTiO3 nanowires were obtained finally. Compared with the BaTiO3 nanowires obtained by just annealing the precursor in air, the molten salt environment benefit the synthesis of BaTiO3 nanowires with perfect crystallinity. Besides, using K2Ti4O9 fibers as template, BaTiO3 fibers were obtained via the K2Ti4O9 fibers ion-exchanged with barium ions by hydrothermal process. Using H2Ti8O17 nanowires coated with BaC2O4·0.5H2O by a precipitation process as template, BaTiO3 nanowires were also obtained after annealed in air for a period of time. This kind of method is a self-sacrificial guide in-situ reaction, provides an efficient way to synthesis complex oxides with one-dimensional nanostructure. The coating process is a key factor in template synthesis of BaTiO3 nanowires.(4) The investigation of individual property of ferroelectric BaTiO3 and BaTi2O5 nanowires is still a challenge. The properties of individual BaTiO3 and BaTi2O5 nanowires were explored by SPM, Radiant Precision Workstation and nanomanipulate system. Piezoresponce force microscope measurements indicated that the d33* of BaTiO3 and BaTi2O5 were 15pm/V and 300 pm/V, respectively. BaTi2O5 has the better piezoresponce. The ferroelectric test along the nanowire's diameter direction revealed that the Pr of BaTi2O5 is about 10 times bigger than BaTiO3 under the same voltage. And the test along the nanowire's growth direction revealed that Pr of BaTi2O5 and BaTiO3 are similar. The Leakage current test of individual BaTi2O5 nanowires revealed that the leakage current obeyed the ohmic law under low voltage, and obeyed the space-charge limited current (SCLC) law under higher voltage. The conductivity value of BaTi2O5 nanowires is about 2×10-6 S/cm.