Sol-gel Synthesis And Properties Of BaTiO₃Dielectric Ceramics Doped With Rare Earth Cations

Barium titanate (BaTiO3) is the earliest discovered perovskite-type functional material with an ABO3structure. The high permittivity and excellent dielectric and ferroelectric properties of BaTiO3make it a significant base material for the application of multilayer ceramic capacitors (MLCC). However, a sudden change of permittivity would occur for pure BaTiO3near the Curie temperature (Tc), leading to the unstability of dielectric-temperature properties. Therefore, doping routes should be adopted to modify the temperature dependence of permittivity. For the traditional solid-state method of preparing electronic powders and ceramics, however, there are many disadvantages, e.g. the large powder size and inhomogenous distribution of dopants. In addition, the high sintering temperature of solid-state method makes it difficult to co-fire BaTiO3dielectric layers with electrodes. And the abnormal growth of grains due to high sintering temperature reduces the reliability of MLCC devices.Sol-gel method, as one of the wet chemical synthesis methods, has been widely used in the fields of inorganic nonmetallic materials, especially for the preparation of high-quality electronic ceramic powders. Via sol-gel method, nanopowders can be achieved with a small particle size, homogenous distribution of dopants, pure phase and high sintering activity, which are in agreement with the requirements of small size and high iniegration in electronic devices. In this thesis, sol-gel method was used to prepare Sc3+singly doped and Sc3+、Dy3+(DyScO3) co-doped BaTiO3nanopowders and ceramics for the first time. The effect of Sc3+and Dy3+on the phase transition, microstructure and dielectric properties was investigated systematically. The main achievements are as follows:1. Sc-doped BaTiO3nanopowders and ceramics were prepared by sol-gel method and the optimum process conditions were investigated. Via sol-gel method, single-phase Sc-doped BaTiO3nanopowders with a narrow particle size distribution between20～30nm have been achieved at low temperatures of750～800℃for2h. Based on that, BaTi1-xSCxO3-δ(0≤x≤0.17) ceramics with different Sc3+content was synihesized at low temperatures. From the XRD results, all of the nanopowders showed a pseudo-cubic phase and the diffraction peaks shifted to the low angles, which indicates that Sc3+has been well doped into the lattice of BaTiO3. The powders were then pressed into pellets and sinterted at1000～1400℃for6h. Their phase compositions were analysized to construct the phase diagram of BaTi1-xScxO3-δ(x= 0～0.17) solid solutions. Results showed that single-phase BaTi1-xScxO3δ ceramics could be densified at1200℃. In addition, doping with Sc3+can dramaticly refine the grain size of BaTiO3ceramics, leading to the change of dielectric properties. For different Sc3+content in BaTiO3, the resistivity of ceramics was dominated by bulk or grain boundary resistance, resulting in two types of impedance response. The Curie temperature and permittivity also decreased with increasing Sc3+content. When x increases to0.10, the Curie point and ferroelectric effect disappears. And relaxor behaviour occurs below room temperatures.2. Based on the research of Sc-doped BaTiO3, a novel perovskite-based (1-x)BaTiO3-xDyScO3(0≤x≤0.06) nanopowders were synthesized via sol-gel method. Pellets pressed from the nanopowders can be densified at a lower sintering temperature of1150℃compared with the solid-state method, due to the high sintering activity of particles and the increasing concentration of chemical defects. This ensures that the (1-x)BaTiO3-xDyScO3ceramics can be co-fired with70Ag-30Pd electrodes at a low temperature. The grain growth of BaTiO3ceramics was inhibited by doping with DyScO3, with a decreased grain size of200nm when x=0.06. The dielectric peaks broadened gradually and the Curie temperature droped from120℃at x=0to5℃at x=0.06, accompanied by an increased room-temperature permittivity. Especially, when x=0.05and0.06,(1-x)BaTiO3-xDyScO3ceramics conformed to the Y5V and Z5U specification by EIA, which made the ceramics a high potential for technological application of MLCC. Furthermore, a typical relaxor behavior was observed when x≥0.05.