Study On Size-Controlled Synthesis, Properties And Composites Of Pre-Perovskite And Perovskite PbTiO₃ Nanofibers

Due to their ferroelectricity, piezoelectricity, high-temperature superconductivity, giant magneto resistance effect and other abundant physical properties, perovskite-structure oxides have promising potential applications in non-volatile ferroelectric memory (FeRAM), infrared detector, thin-film capacitor, piezoelectric nanogenerator and so on. With the minimization and multi-functionalization of electron devices, the controllable size, performance optimization and new material exploration of perovskite-structure oxides become increasingly important.In this dissertation, the crystal structure and the current status of size effect for perovskite oxides have been reviewed firstly. Then, the process and main problems on preparation and properties of low-dimension perovskite ferroelectric oxides have been summarized and analyzed. At last, brief introductions to dielectric, ferroelectric and anti-ferroelectric properties of perovskite-structure oxides have been provided. Based on current status and development tendency, size-controlled pre-perovskite and perovskite structure PbTiO3 (PT) nanofibers have been synthesized by combining modified sol-gel method with polymer-assisted hydrothermal method. Besides, after preparing PT nanofibers with high aspect ratio and good dispersity, PT nanofibers were mixed with PVDF to manufacture PVDF/PP-PT and PVDF/PT nanocomposite thin films. Moreover, the PT/PZ composite fibers have been synthesized successfully for the first time by a hydrothermal method. The crystal structure, microstructure, element distribution and growth mechanism have been investigated by employing several measurement methods, such as high-resolution transmission electron microscopy (HRTEM), scanning electron microscopy (SEM), X-ray diffraction (XRD) and energy dispersive spectrometer (EDS). Then their dielectricity, photoluminescence properties have been tested and analyzed. The main contents and results are summarized as follows:(1) By adjusting the ratio of TEA and Ti(C4H9O)4 in the modified sol-gel process, size-controlled pre-perovskite PT nanofibers were synthesized after subsequent polymer-assisted hydrothermal method. It has been found that TEA solution can effectively modify the Ti precursor, leading to the good dispersity and uniformity of the initial reactants. Therefore, the number of crystal nucleation sites for the pre-perovskite PT nanofibers (the molecular ratio of Pb/Ti= 1.1) increases during the hydrothermal reaction, which suppresses the growth rate of the nanofibers. As a result, the length of the nanofibers decreased and the dispersity was enhanced, as well as the diameter uniformity of a single nanofiber. The length of pre-perovskite PT nanofibers decreased from 35-90μm to 5-12μm and the diameter remained in the range of 100-300 nm, accompanying with a reduction of aspect ratio from 225-330 to 42-60. At the same time, the dispersity and uniformity of as-prepared nanofibers have been improved significantly. On the other hand, the pre-perovskite PT nanofiber is metastable phase and grows along [001] with the outer surface enclosed by{110}. It is concluded that the ratio of Pb/Ti has a decisive impact on the diameter of the nanofibers, where the enclosed surface{110} is supposed to be Pb-O termination. Therefore, if the amount of Pb remains unchanged, the diameter of the nanofiber would be kept in a certain range. The result of photoluminescence measurements revealed that there was a broad green emission band at the wavelength of 530-550 nm in each curve, without an obvious size effect.(2) PVA/PVP-assisted hydrothermal method was used to prepare single-crystal pre-perovskite PbTiO3 (PP-PT) nanofibers, in which PVA and PVP acted as surface modification agents. The as-prepared nanofibers had high aspect ratio (100-200) and good dispersity, which was beneficial to improve the dielectric behavior of nanocomposites. Subsequently, poly(vinylidene fluoride)/pre-perovskite PbTiO3 nanofibers (PVDF/PP-PT) nanocomposite thin films were successfully fabricated. The obtained results indicated that an increase of PP-PT nanofibers resulted in an increase of the dielectric constants of nanocomposite thin films in the low frequency range. As a comparison, the dielectric constant of PVDF/PP-PT nanocomposite thin film that contained 20% PP-PT nanofibers was 43.7% larger than that of pure PVDF thin film (εr= 6.77) at 5 kHz, whereas the loss tangent kept very low. The Cole-Cole diagrams showed that t increased with increasing the quality fraction of PP-PT nanofibers, which was probably due to the effect of interface between the nanofibers and polymer.(3) Ferroelectric/anti-ferroelectric heterostructured PbTiO3/PbZrO3 composite fibers were fabricated via hydrothermal method for the first time. Single-crystal perovskite tetragonal PbTiO3 (PT) nanofibers were employed as growing core to react with PbZrO3 (PZ) precursors under hydrothermal conditions. The length of composite fibers was in the range of 15-65 μm, and the diameter was 0.70-2.05 μm. Structural characterization indicated that the composite fiber consisted of single-crystal PT and PZ phases. Moreover, an epitaxial growth between the PT nanofiber and PZ crystal was revealed to occur at (011)PT/(221)PZ. At the interface, Zr4+ions have been determined to diffuse into the nanofiber within～20nm and possibly substitute partial Ti4+ positions in perovskite PT. The growth mechanism including diffusion and epitaxial growth was proposed to explain the growth of PT/PZ heterostructure.(4) To increase the diffusion power of Zr4+and Ti4+ ions at the interface of heterostructure, PT/PZ composite fibers were performed by annealing treatment and then one-dimensional Pb(Zr0.44Tio.56)03 (PZT) crystals have been synthesized.