Study On Preparation And Properties Of (Na_0.5Bi_0.5) TiO₃ Low-dimension Nanomaterials

Sodium bismuth titanate, (Na_0.5Bi_0.5)TiO₃ (NBT), is a kind of perovskite-type ferroelectric with a relatively large remanent polarization (Pr =38pC/cm2) at room temperature and a relatively high Curie temperature (Tc=320oC). For its strong ferroelectrics at room temperature, NBT has been considered to be a promising candidate material for lead-free piezoelectric ceramics. Nanoscale one and two dimensional materials such as nanotubes, nanowires, nanorods and thin films have attracted great attention due to their unique properties and potentials applications. To develop practical nanoscale devices, such as signal storage, switch and transducer, etc, one important challenge is the ability to reveal the formation mechanisms of the nanostructures and to rationally exploit their size-dependent properties. In this dissertation, a sol-gel ceramic fabrication method was employed to prepare the NBT-based thin films and prepared Na_0.5Bi_0.5TiO₃ nanowires by a sol–gel–hydrothermal method at a low-temperature, the synthesis, structure characteristics and optical properties were investigated. The relation between optical properties and the composition and structure was also studied. The main work and results are as follows:(1) LaNiO₃ (LNO) thin films were prepared on Si (100) substrates with various baking temperatures from 150 to 300oC by sol-gel route. Their crystal structure, surface morphologies, and conductive property were experimentally investigated by using X-ray diffraction, atomic force microscopy, scanning electron microscopy, and 4-point probe method. All the LNO films crystallized into a pseudo cubic perovskite phase with the dense, uniform and crack-free surfaces. We also found that the baking temperature has obvious influences on the grain orientations and resistivities of the LNO films. The LNO film showed more preferential <110>-orientation at the baking temperature of 210oC than at the other temperature. The LNO film obtained at the baking temperature of 210oC exhibited the smallest resistivity as low as 8.98×10^-4Ω·cm.(2) This study fabricated (Na_0.5Bi_0.5)TiO₃ thin films with pervoskite structure on α-Al₂O₃ substrate using a sol-gel method. The effect of precursor concentration on the structure and the optical properties were investigated. The optical constant (refractive index, absorption coefficient and extinction coefficient) in the wavelength range of 200²500 nm and the thickness of the films were obtained through optical transmittance measurements. The optical band gaps of the samples were estimated to be 3.32³.53 eV. The dispersion of the refractive index in the interband transition region was analyzed by using the single electronic oscillator model.(3) We described a sensitive single beam Z-scan technique for measurement the optical nonlinear refractive index for a wide variety of materials. By means of the technique, the transmittance of (Na_0.5Bi_0.5)TiO₃ thin films was measured through a finite aperture in the far field as the sample was moved along the propagation path (Z) of a focused Gaussian beam. The n2 of the sample was achieved.(4) Perovskite ferroelectric (Na_0.5(Bi1-xTbx)_0.5TiO₃ thin films were grown onα-Al2O3 substrate using a sol-gel method. For the first time, their optical nonlinearites and dispersion properties of the basic optical constants were studied by means of single beam Z-scan and optical transmittance methods, respectively. The films exhibited pronounced self-defocusing effects under 1064 nm laser irradiation, and no nonlinear optical absorption at 1064 nm, the nonlinear indices (n2) were determined to be -( 2. 56～3.29 )×10^-8 esu.(5) Ferroelectric (Na_1-xK_x)_0.5Bi_0.5TiO₃ (NKBT) (x=0.1, 0.2, and 0.3) thin films with perovskite crystal structure were fabricated on quartz substrates by a sol-gel process. The nonlinear optical properties were investigated by Z-scan technique performed at 532nm with a picosecond laser. A variational two-photon absorption effects were obtained. The nonlinear refractive index n2, nonlinear absorption coefficientβ, and the real part and imaginary part of third-order optical nonlinear susceptibilityχ(3) were calculated corresponding to x=0.1, 0.2 and 0.3, respectively. All the results suggested that the nonlinearity is dominated by the refractive for the film, and the BNKT thin film is promising material for applications in nonlinear optical devices. (6) Na_0.5Bi_0.5TiO₃(BNT) nanowires with diameters of 10nm and lengths of 300 nm have been prepared at low temperature by a sol-gel-hydrothermal method. The effect of processing parameters on the growth and morphology of derived BNT powers were investigated. It is cleared that alkali concentration and reaction temperature play an important role in obtaining purity BNT. It is suggested that the gel precursor and the hydrothermal environment were the key to format nanowires.(7) Na_0.5Bi_0.5TiO₃ nanowires with perovskite structure are synthesized by sol-gel- hydrothermal method, using sodium acetic, bismuth nitrate and butyl titanate as the starting materials and NaOH served as a mineralizer. The structure and components of Na_0.5Bi_0.5TiO₃ nanowires were studied using X ray diffraction, transmission electron microscope, X ray photoelectron spectroscopy. Investigations exhibit that the as-prepared samples are made up of large quantity nanowires with diameters about 10¹5nm and lengths reaching up to 1μm and single crystalline in nature. The authors have also investigated the photoluminescence properties of Na_0.5Bi_0.5TiO₃ nanowires. The Na_0.5Bi_0.5TiO₃ nanowires exhibited a broad and structureless band appearing around 520 nm.