| Dielectric materials are widely used to form capacitive devices such as capacitance, resonators and filters. Those dielectric materials with high dielectric permittivity have attracted considerable interest in recent years since they might offer the potential to enhance the performance or shrink the dimensional sizes of the microelectronic devices. Under the circumstances, this thesis work concentrates mainly on the studies of material preparations, physical properties and the related mechanisms for CaCu3Ti4O12 (CCTO)-based high-permittivity dielectric ceramics.CCTO is the oxide that has a cubic perovskite-related crystal structure and exhibits an enormously large dielectric permittivity (ε') at low frequencies in both forms of single crystals and ceramics. The dielectric permittivity is almost a constant in the low frequency range below 100 kHz at room temperature and is nearly independent of temperature over the wide temperature region. So far, several models have been proposed to explain the dielectric behavior but are quite controversial, including both the intrinsic and extrinsic mechanism explanations from the viewpoints such as crystal structure, internal barrier layer capacitance (IBLC) effect and contact-electrode depletion effect.Despite having many advantageous properties, CCTO ceramics also show a serious shortcoming of relatively high dielectric loss (tanδ), which may result in the large signal decay or the instable operation of electronic circuit in practical applications. Therefore, it is essential to find a way that could somehow reduce the dielectric loss to a sufficiently low level and maintain simultaneously the desirable characteristic of high dielectric permittivity for CCTO ceramics.On the other hand, more than twenty kinds of oxides [which can be described as ACu3Ti4O12 (A includes S,Cd,Bi2/3,Na0.5Bi0.5 or some other lanthanons respectively] with the similar crystallographic structures and chemic compositions to CCTO have been known in recent years. Therefore, further investigations on these kinds of ceramics can be helpful for people to understand high-permittivity dieletric mechanisms and the related properties for CCTO and CCTO-like oxides.Also, it is found that TiO2 raw materials with different structures and purities show some influence on the properties of CCTO ceramics. A systematic investigation on the effects of raw materials can not only help choosing the proper raw materials for the preparation of high-permittivity dieletric ceramics, but also be helpful for people to understand high-permittivity dieletric mechanisms and the related properties for CCTO.This thesis treats the CCTO-based high-dielectric ceramics prepared by conventional solid-state reaction as the study objects. The effects of CaSiO3-doping and different raw materials on the electrical properties of CCTO ceramics and the crystalline structure, microstructure, dielectric properties and complex impedance and the corresponding temperature dependences of NBCTO ceramics are investigated.1. The xCaSiO3-CaCu3Ti4O12 ceramics have been prepared with the raw materials of carbonates and oxides by the solid-state reaction technique. The dielectric response, impedance spectroscopy and the thermoelectric transport properties, crystalline structure and microstructure of CaSiO3-doped CaCu3Ti4O12 ceramics have been investigated and compared with the corresponding physical properties of CCTO. It has been revealed that the doping of CaSiO3 can help restraining the rapid decrease of the dielectric constant with the frequency in the low-frequency range, reducing the dielectric loss, increasing the resistivities of the grains (Rg) and grain boundaries (Rgb) and making the grain sizes more uniform. In particular, the enhancement of Rgb should be attributed to the huge resistivity of SiO2 that is precipated along the grain boundaries. Moreover, the method of preparing the specimens by increasing sintering times under the same sintering temperature has some positive effects on reducing the dielectric loss. After doping CaSiO3, the characteristic nequency and the strength of LR for CCTO ceramics decrease with temperatures. The Seebeck coefficients of CCTO ceramics exhibit a little decrease in the whole measuring range, but still remain negative. So the conduction is n-type. 2. A series of (Na1/2Bi1/2)Cu3Ti4O12 (NBCTO) ceramics were prepared via solid-state reaction route under different sintering temperature conditions. The crystalline structures, microstructures, dielectric properties and complex impedances and the corresponding temperature dependences were investigated. It has been revealed that:Compared with CCTO ceramics, NBCTO ceramics exhibit the lower sintering temperatures. Moreover, the microstructures of NBCTO ceramics show some changes which are less obvious than those of CCTO ceramics as the sintering temperatures increase. There are two kinds of grains with different sizes seen in the microstructures of the specimens that are sintered at low temperatures, and the grain sizes become more and more uniform with increasing the sintering temperature. NBCTO ceramics has the quite similar dielectric properties and complex impedances with CCTO ones. The NBCTO ceramics prepared under sintering temperatures between 990℃and 1060℃exhibit their low-frequency s'larger than 10000 at room temperature, and the one sintered at 1030℃has the largestε'of 33000 in particular. In contrast, only one dielectric relaxation is seen at room temperature or lower temperatures, two are seen in the dielectric spectra within the measured frequency range of 40 Hz-10 MHz at higher temperatures. We can speculate that the low-frequency dielectric relaxation should be relative to contact-electrode depletion effect from our experimental results and some correlative reports. The characteristic relaxational frequencies, which correspond to the imaginary peaks of dielectric dispersions, shift to the high-frequency direction with the enhancement of sintering temperatures. There are two or three semicirculars in the complex impedence plane within the measuring frequency range of 40 Hz-10 MHz. As a whole, the dielectric properties and complex impedances can be explained by an equivalent circuit model which contains three RC elements in series.3. A series of CaCu3Ti4O12 ceramics have been prepared with different types of TiO2 raw materials. The effects of TiO2 raw materials on the the dielectric response and impedance spectroscopy have been investigated. It has been revealed that TiO2 materials with different structures all transfered to rutile-type after thermal treatment at 1100℃. The result means that TiO2 raw materials with the anatase-type structure have the higher activity than those with the rutile-type structure. Compared with these CCTO calcined powders prepared by the anatase-type TiO2 materials, the powders synthesized by the rutile-type TiO2 materials have the larger grain size and higher density. Meanwhile, we conclude that the fact may be the reason why the CCTO ceramics prepared by the single-type TiO2 materials have the better dielectric-constant properties than the ones prepared by the mixing-type materials. Moreover, we can get the conclusion that the choice of TiO2 materials with the purity of 99.8% and rutile type should have some positive effects on maintaining the dielectric properties of CCTO ceramics after general comparison for the dielectric constants and loss between different kinds of specimens.
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