| (CCTO) ceramic has drawn much interest recently due to its extraordinary high dielectric constant which is of good temperature stability over a large range. The high dielectric constant makes CCTO have a potential application in miniaturizing electronic devices. However, its relatively high dielectric loss and sensitive dependence on the fabrication process become a bottleneck in practical applications. Therefore, for practical applications, it is necessary to investigate the mechanism for improving the dielectric properties of CCTO ceramics and develop optimal processes for the CCTO ceramics.In this work, CCTO ceramics were prepared by using solid-state reaction method and effects of doping process, such as SiO2, V, B, Mg, Sr, Y, La and Y-Al elements, on electrical properties, low-temperature synthesis and sintering performance were investigated experimentally. Moreover, the cooling process was also exploited for improving dielectric properties of CCTO. All experiment designs were based on the main route of modifying grain and grain boundary characteristics. Some new results were obtained as following:SiO2is one suitable additive to modify grain boundary characteristics because the solid-state reaction between SiO2and CCTO phase didn’t occur at the sintering temperature. The secondary phase of SiO2is believed to be present among the CCTO grains, which had enhanced the grain size as the additive amount beyond2wt%. The dielectric constant and dielectric loss decreased as the content of SiO2increasing. Impedance spectroscopy showed that the grain boundary resistance increased due to the addition of SiO2to CCTO, which should be main reason for the decrease in dielectric loss.The CCTO can be synthesized at a lower temperature about870～920℃by doping with V and B for B-site Ti. The densifcation of doped CCTO ceramics could be achieved at920℃. The V doped CCTO showed dielectric constant value as1.4-2.3X105at low frequency, but it decreased quickly to50as frequency increasing to105-106Hz. It had high dielectric loss. The B doped CCTO ceramics sintered at920℃showed high dielectric constant and low dielectric loss. When the B doping amount was4to6mol%, the dielectric constant could reach to5×103and its loss maintained at0.1～0.8within the frequency range from300to106Hz.Mg doped Ca1-xMgxCu3Ti4O12ceramics were prepared by the solid state reaction method. The substitution of Mg for Ca resulted in decrease of the lattice constant of Ca1-xMgxCu3Ti4O12with x when x≤0.03. Grain growth was promoted by Mg doping, and reached a maximum when x=0.05. Mg doping was found to increase the grain boundary resistance in the Ca1-xMgxCu3Ti4O12ceramics. An increase in dielectric constants and large decrease in dielectric losses were simultaneously achieved in the Ca1-xMgxCu3Ti4O12ceramics with x=0.05within the frequency range between1kHz and20kHz. For other doping concentrations, the dielectric losses were generally lowered, but their dielectric constants were markedly enhanced. Ca1-xSrxCu3Ti4O12ceramics were also investigated. The sinterability of the Ca1-xSrxCu3Ti4O12ceramics was largely enhanced, with the reduced sintering temperature from1100℃to1010℃and promotion of densification and grain growth during the sintering stage. Sr doping was found to increase grain boundary resistance in the Ca1-xSrxCu3Ti4O12ceramics. Large increase in dielectric constants and decrease in dielectric losses were simultaneously achieved in the Ca1-xSrxCu3Ti4O12ceramics with x=0.2within the frequency range between0.2kHz and20kHz.For substitution of rare earth Y for Ca, the single CCTO phase couldn’t be affected when Y doping amount was less than7mol%. However, the Cu4O3(or CuTiO3) phase would be present when La doping was about lmol%. The grain size of CCTO ceramis doped with Y and La was decreased and the threshold of Y doping amount was3mol%, which was less than the La doping of7mol%. Y doping was found to increase dielectric constant and decrease dielectric loss simultaneously when doping at1-3mol%. But for La doping, there was a great increase in dielectric loss, though a large enhancement in dielectric constant was achived.Y-Al co-doped Ca(1-x)YxCu3Ti(4-y)AlyO12ceramics showed that lattice constant was reduced with doping amount increasing due to Al3+size is smaller than Ti4+size. And the threshold to suppressing grain size raised to7mol%, which was two times higher than Y doping. Y-Al co-doping would weaken the improvement in dielectric constant. However it could reduce the defects in grain boundary introduced by high doping only withY elemrnt, which would increase the grain boundary resistance and obtain lower dielectric loss of CCTO ceramics with an increase in dielectric constants simultaneously.The CCTO ceramics cooled in air showed higher dielectric constant and lower dielectric loss than the sample cooled in furnace, which mainly originated from the enhancement in grain conductance and increase in grain boundary resistance due to the cooling atmosphere in air. Moreover, the dielectric constant showed better frequency stability, which would be owing to the change in chemical co-vanlence of Cu+/Cu2+and Ti3+/Ti4+composition. Additionally, the initial cooling temperature had great effect on the optimization of dielectric properties for the ceramics cooled in air atmosphere and the initial temperature of980℃could rechieve the optimal improvement in dielectric properties with dielectric constant increased by1.5times and dielectric loss lowered by50%. It is concluded that the effectiveness on optimizing properties of CCTO ceramics is determined by the proper combination of oxygen concentration in cooling atmosphere, initial tempreture and cooling rate, i.e. these factors should make sure that grain boundary could be oxdizied enough, but no oxidization on the grains. And high grian boundary resistance and low grain resistance can be controlled by this method, which could provide a new and effective way to optimize the dielectric properties of CCTO ceramics.In summary, large increase in dielectric constants and decrease in dielectric losses were simultaneously achieved in CCTO ceramics by doping and proper cooling process, which is rarely reported in the literature. Although there will be more room for further improvements in the dielectric properties of CCTO based ceramics, the results in this work should provide a basis for future work towards obtaining more desirable dielectric properties in this material and promoting its commercial application. |
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