Abstract
Traditionally, the most common method for preparing PTO and PZT powders was solid state reaction. This kind of reaction is to react a mixture of metal oxides, hydroxides or salts in the solid state. However, this method resulted in poor sintering behavior, a lack of homogeneity, large aggregates formation and poor control of cation stoichiometry; hence, this method was inadequate for various advanced applications. Chemical routes, particularly sol-gel process, offer advantages over the solid state reaction and have attracted strong interest. The most important step in sol-gel process is to obtain a solution of all target components in the form of soluble compounds, so mixing at the molecular level made possible, and if this level of mixing can be retained in the subsequent conversion to gel, and ultimately to oxides, a very homogeneous product results. In this thesis, we investigated the sol gel process to prepare the PTO and PZT nanocrystalline powders at low temperature, and also examined the crystal phase at the as-prepared state and higher temperatures. Moreover, we observed and compared the sintering and grain growth behavior upon calcination at higher temperatures for different pH values in the hydrolysis step. In the preparation of lead titanate, first we synthesized the precursor by mixing titanium isopropoxide and lead acetate in ethanol and then refluxed this mixture for 72 hours. Consequently, we could obtain nanocrystalline Pb2Ti2O6 (major) and PbTi3O7 (minor) at low temperature (150°C) by hydrolysising this precursor and dried. As a result, the pH values in the hydrolysis step did not influence the crystallinity of the powders at low temperature. The grains obtained at this low temperature had the size ranging around 2-5 nm. While further heating the as-prepared powders, the phases Pb2Ti2O6 and PbTi3O7 gradually transformed to PbTiO3 at roughly 400-450°C with some extent of PbTi3O7 remained untransformed. The temperature obtaining pure tetragonal perovskite PbTiO3 phase was lower in the powders hydrolysis in acid condition, which is around 600°C, compared with that in neutral condition (700 °C). Furthermore, we investigated the sintering and grain growth behavior of these nanocrystalline PbTiO3 powders at higher temperatures. Enhanced sinterability also could be observed at lower temperature for the powders hydrolysis in acid condition. Moreover, since the solid solution system Pb(Zr1-xTix)O3,PZT, exhibited useful ferroelectric and piezoelectric properties. In particular, compositions near the morphotropic phase boundary (MPB) around x=0.45– 0.5 have attracted considerable interest for many years due to their high piezoelectric response. Thus we further used the same reaction condition to hydrolysis PZT precursor to examine the results between the powders hydrolysising from these two precursors. Besides the investigation of crystallinity at low temperature, we also study the sintering behavior and phase transformation pathway for these two nanosized materials. In the preparation of PZT, first we synthesized the precursor by mixing titanium isopropoxide, zirconium butoxide and lead acetate in ethanol and then refluxed this mixture for 72 hours. Consequently, we could obtain white powders by hydrolysising this precursor and dried. As a result, we could obtain tetragonal perovskite PZT phase directly, without any pyrochlore phase formation, at low temperature (as-prepared state, 150°C) by sol gel process only when the hydrolysis condition was kept in acid condition (pH=3). Upon calcining the powders, the tetragonal perovskite PZT phase crystallinity was much stronger at higher temperatures. Whereas, in the neutral and basic conditions, the powders obtained at low temperature revealed the amorphous nature, and further calcining these amorphous powders, the amorphous powders were gradually transformed to pyrochlore and tetragonal perovskite PZT phase at 500-600°C, and the pyrochlore phase was eventually transformed to tetragonal perovskite PZT phase at 600-700°C. Furthermore, we investigated the sintering and grain growth behavior of these nanocrystalline lead zirconate titanate powders. Enhanced sinterability also could be observed at lower temperature for the powders hydrolysis both in acid and basic conditions.