Studies Of The Synthesis, Characterization And Formation Mechanism Of PbZr_0.52Ti_0.48O₃ (PZT) And Bi_3.25La_0.75Ti₃O₁₂ (BLT) Nanosized Powders And One-Dimensional Nanostruct

In the thesis, the synthesis, structure and properties of Pb(Zr_xTi_1-x)O₃ (PZT) and Bi_4-xLa_xTi₃O₁₂ (BLT) and the synthesis strategies and growth mechanisms of 1 dimensional nanostructure were reviewed in detail, especially, the development on the researching of the 1 dimensional nanostructure of ferroelectric perovskite oxides such as BaTiO₃ and PZT was discussed. Combining the advantages of partial oxalate solid state reaction method and coprecipitation method, the pure perovskite PZT powders were directly synthesized by a two-stages prcipitation route at low temperature that is a new method and developed in these work. Nanosized PbZr_0.52Ti_0.48O₃ (PZT) powder with well disperse was prepared by poly(vinyl alcohol) (PVA) assisted two-stages precipitation route. The PZT precursor cluster was separated alone well by PVA gel. The synthesis of nanosized Bi_3.25La_0.75Ti₃O₁₂ (BLT) powder with Bi-layer perovskite structure was realized by coprecipitation method. And innovatedly, in this work, single-crystalline tetragonal perovskite PbZr_0.52Ti_0.48O₃ (PZT), which is a quarternary oxide, nanorods and nanowires as well as poly-crystalline perovskite PZT nanotubes were synthesized by PVA or poly(acrylic acid) (PAA) assisted in the hydrothermal reaction method.Differential thermal analysis (DTA) and thermal gravitative analysis (TG) were employed to analyze the thermal behavior of the PZT and BLT precursor and X-ray diffraction (XRD) was used to follow the evaluation of PZT and BLT phase with the calcinations temperature. The morphology of the powder obtained was observed by field emission scanning electron microscopy (FESEM). In order to evaluate the chemical composition and thermal stability, the composition of the PZT and BLT powders was measured with ICP and EDX.Despite of the formation of the main phase that is the perovskite PZT phase, minor PbO appears in the PZT powder synthesized by coprecipitation method. The minor PbO phase will evaporate at high temperature and makes the PZT powder calcined at high temperature lack of Pb and the PZT powder calcined at low temperature with poor thermal stability. By enhancing the combination of the Zr⁴⁺ and Ti⁴⁺ ions, the evaporation of PbO is effectively avoided in the two-stage precipitation route. Not only is the stoichiometric and homogeneous PZT powder synthesized but also the powder has high thermal stability.It is revealed by studying on the activation energies of the precursor for the formation of PZT phase that the phase transformation from precursor to perovskite PZT follows a consecutive mechanism, which can described as:Amorphous Precursor Powder------>Pyrochlore------> Perovskite PZT,which is not only suitable for the coprecipitation method but also for the partial oxalate coprecipitation and the two-stage precipitation route.A model for phase formation of PZT by the two-stage precipitation method is proposed based on the existence of two interfaces (PbO-PZT and PZT-ZT) of reaction and diffusion of cations. The Pb2+ ion diffuses to the interface of PZT-ZT and reacts with ZT to form PZT phase, which controlled by the Pb2+ ion diffusion through the PZT layer. Because of the resolution of TiC>2 in PbO phase, the rhombohedral PZT phase is preferential formed for low calcinations temperatures.The powders synthesized by the three precipitation methods mentioned above does not have very clean surface or boundary in distinct particles, in which the particles combine to chain because of sintering and the agglomeration is very heavy. However, when the two-stage precipitation is assisted with PVA, the powder obtained not only has clean surface or boundary in distinct particles but also is of nanosized particle and narrow distribution. The effect of PVA and quantity of PVA on the particle morphology was studied.For the synthesis of pure BLT phase, the critic factor is the preparation of transparent precursor solution. It is revealed that the bismuth oxynitrate derived from bismuth nitrate hydrolysis are not favorable for the formation of BLT phase with Bi-layer perovskite structure. The bismuth nitrate hydrolysis can be avoided by using dense concentration nitric acid solution as solvent. A single phase of BLT powder can be synthesized by directly coprecipitating the transparent BLT precursor solution in ammonia solution under stirring and no pyrochlore Bia^O? phase was found. The particle size of the BLT powder synthesized by calcinations at 700°C for 2 h is not bigger than lOOnm and the interaction of particles is loose with good dispersion.The single crystalline tetragonal perovskite PZT nanorods and nanowires as well as poly-crystalline PZT nanotube were firstly synthesized by polymer assisted hydrothermal reaction method. X-ray diffraction (XRD), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM) and selected area electron diffraction (SAED) were employed to characterize the 1 dimensional nanostructure synthesized. The effect of polymer on the synthesis of PZTnanostructure was studied and for the growth of single crystalline tetragonal perovskite PZT nanorods and nanowire or of polycrystalline PZT nanotube a model was proposed, respectively.The straight molecular chain of PVA will stretch under strong basicity by alcoholysis. Because of the hydrogen band and molecular effect, the stretched molecular chain assembles to a column polymolecular chain. The PZT crystallites formed in the hydrothermal reaction initially are absorbed on the surface of the column polymolecular chain and grow to a polycrystalline PZT nanotube.The adsorption of polymer on the surface of PZT crystallite, which forms a thin film, will result in the surface energy decreased. Because the tetragonal perovskite PZT is of poor anisotropic structure, the growth cell may deposit on the facet with high surface energy, which can be named as growing facet. The deposition of growth cell will breaks the polymer films and will enhance the deferential of surface energy between the facets with films and the growing facet, resulting in the [001] orientation growth of single crystalline tetragonal perovskite PZT. Because of the adsorption of PVA or PAA on the PZT crystal surface by hydrogen band or by chemical band, respectively, the single crystalline PZT nanorods are synthesized by only introducing PVA and the single crystalline PZT nanowires are synthesized by introducing both PVA and PAA.It is a feasible route for the synthesis of the single crystalline oxides, which are of poor anisotropic crystal structure, nanowires by introducing the polymer surface modified agent in the thermal reaction system to adsorb on the crystal facet for enhancing the deferential between the facets. The innovative work carried out in our researching not only is important for the synthesis of tetragonal oxides nanosturcture but also facilitates the researches on the properties of relative oxides nanostructure.