Study On The Dielectric Properties Of CaCu₃Ti₄O₁₂-Based Ceramics

Recently, the CaCu₃Ti₄O₁₂ compound, CCTO, has drawn a lot of attention due to its properties. In general, it possesses a very high dielectric constant of about 10⁴ at radio frequency and below room temperature, and it also shows good temperature stability over a wide temperature range from 100 to 600K. So it probably will be used widely in high indensity energy memory and thin-film devices or dielectric capacitor. On the other hand, it has high dielectric loss which limits its applications. Moreover, electronics devices are prepared smaller and smaller, and their function becomes stronger and stronger. So it is a crucial problem to decrease dielectric loss. The research shows that properties of samples will be even influnced by different prepration method and different technologies. The experiments results showed that it is a good choice to decrease the dielectric loss of CCTO ceramics by doping.In this work, at first, CCTO powders were prepared by sol-gel method and microstructure of samples were studied to clarify the influence from different sintering temperature. XRD results showed that perovskite-like phase is obtained when the sintering temperature reached 750℃. The result of XRD analysis is agreed with those of Raman and FT-IR one.By solid state method, CaCu₃Ti₄O₁₂-based ceramics were prepared. Microstructure of a part of samples were measured by XRD, and the results showed that CCTO ceramics sintered at 1100℃for 5h presented perovskite-like phase with good crystalline, and its dielectric constant is almost temperature independent over a wide temperature range which is between 5000 and 10000. Still, CCTO ceramics had nonlinear I-V properties. By doping dielectric loss of samples could be decreased, and at the same time, their dielectric constants be decreased. On the other hand, different dopant has different effect on dielectric properties of samples. The dielectric loss of La-doped CCTO ceramics decreases apparently, and the results of Sr-doped CCTO ceramics is not as good as those of La-doped. For Sr and Mg doped samples, the dielectric loss also decrease, on the other hand, their dielectric constant is decreased too which would limit their application.The impedance spectroscopy analysis showed that dielectric constant and dielectric loss are influence greatly by frequency and temperature, which implied the existence of frequency dispersion and relaxation. The peak of dielectric loss will shift to higher temperature with the frequency increasing, which is a thermally activated relaxation process. According to the equivalent circuit of the samples, we may conclude that the grain boundary resistance and capacitance are main factors affecting the properties of CaCu₃Ti₄O₁₂-based ceramics in the frequency range from 10kHz to 1MHz. Small amount impurities of second phase are regarded as layers in the grain boundary which affect the sample's dielectric properties. Cole-Cole plot analysis of the samples also demonstrated the grain boundary, the domain boundary and the grain of crystal structure are three main factors which influenced the CCTO-based ceramics's electric properties.