Synthesis Of Sol-gel Bi₄Ti₃O₁₂ Powder And Preparation Of Ceramics By Surperhigh Press Method

In recent years there has been much interest on ferroelectric thin films for their excellent ferroelectric, dielectric, electro-optic, piezoelectric and nonlinear optic properties. In theory, by using the spontaneous polarization in ferroelectric thin films, which are integrated with silicon semioncuctor circuit, we can fabricate ferroelcetric nonvolatile random access memories (FRAM) and ferroelectric field effect transistor. These two types of device is attractive not only for the feature of low threshold voltage, large field effect, fast swich speed, high integrated density, and tolerance against radiation, but also for the possibility to realize the nonvolatile memory and nondestructive readout. As a result, when these two types of device fully go into practice, there will be a revolution in store technology. The technological uses of ferroelectric thin films are not limited to ferroelectric memories, but extend to a wide range of applications in discrete devices, utilizing the full range of properties. Applications include multilayer capacitors for memories, piezoelectric film-based microelectro-mechanical system, boundary layer capacitor, varistors, gas sensors, radiation detector, temperature sensor, transducers switches, shutters fuel cell electrolytes, concentration cell electrolytes and electrolytes for high energy density batteries . For a long time, PZT (PbZr_xTi_1-xO₃) and La-substituted PZT serve as the most important piezoelectric and ferroelectric ceramics and have been widely used. However, Pb is harmful for human and will evaporate at relative high temperature, which make it difficult to make an accurate stoichiometry control. Recent years, researchers have made much effort to substitute PZT with lead-free ferroelcetric and pielectric ceramics, such as Na_0.5Bi_0.5TiO₃ and La-substituted bismuth titanate ceramics. Bi₄Ti₃O₁₂( BTO) is known to be one of the most important candidates for FRAM. It has high dielectric constant and good feroelectric, piezoelectric property. However, it shows degradation properties after being subjected to10⁷ read/writes witching cycles, which will decrease the stability of ferroelctric device. In 1999, PARK reported on Nature that La-substituted bismuth titanate thin films show excellent fatigue-free characteristic. These indicate that the fatigue problem during switch circles in BTO can be improved even solved by ion substitution or modulation. Amorphous Bi4Ti3Ol2 precursor was prepared by sol-gel method. The sol-gel process offers many advantages over conventional methods, such as high purity, molecular homogeneity, low-temperature processing, convenient fabrication procedures, and easy control of stoichiometry. The La substitution (Bi4-x, Lax) Ti3O12 (abbreviated as BLT) powders, ceramics and films were prepared by using Sol-Gel processing techniques with the initial materials of bismuth nitrate Bi(NO3)3·5H2O, lanthanum nitrate La(NO3)3·6H2O and tetrabutyl titanate Ti(C4H9O)4. The BLT ferroelectric thin films were prepared on quartz glass substrate by Sol-Gel method. The reaction mechanism of the Sol-Gel processing was investigated with IR spectroscopy and with this understanding, the techniques of the Sol-Gel processing was optimized. By means of modern analyses techniques, such as DTA, XRD, SEM, TEM etc., the reaction temperature, crystallized properties, particle size and distributing of BLT powders and the properties of physics/chemistry of BLT0.75 ceramics were characterized; microstructure, crystallized properties of BLT thin films were investigated. The crystal structure of Bi4Ti3O12, particularly, the structural influence of the substitution of Bi3+ with La3+ in Bi4Ti3O12 powder were studied. The main conclusions obtained from above-mentioned works with innovation significance are as follows: 1. Adding acetic acid and glycol into the sol could completely control the speed of the hydrolysis and polymerization reaction, and the BLT powders with better physical and chemical properties could be obtained by using the Sol-Gel processing put forward in this paper. 2. The BTO powder heated at 433℃were mixture Bi2O3 and TiO2, at 488℃were mixture Bi2O3 and Bi2Ti4O11, at 644℃were tetragonal Bi4Ti3O12 , and at 830℃were orthorhombic Bi4Ti3O12 3. The reaction temperature of BLT0.75 powder is at 820℃, The reaction temperature is decreasing following with decreasing the amount of La. 4. This observation indicates that the lanthanum ions in the BLT do not form minority phases or segregate from the interior grain but dissolve into the perovskite lattice. We can conclude that the BLT maintains a layered structure similar to the perovskite BTO even under extensive modifications by lanthanum.