| In this thesis we have studied the perovskite-like CaCu3Ti4O12(CCTO)material with a giant dielectric constant.CCTO can maintain a relatively high dielectric constant at a broad temperature range from 100 K to 600 K,at the same time,CCTO exhibits an excellent nonlinear current-voltage property,with its nonlinear coefficient approaching 900.Thus CCTO shows great potential in applications,such as miniaturization of integrated circuit,energy storage and sensors.However,the giant dielectric constant of CCTO is sensitive to its preparing processes and the relatively large dielectric loss hinders its practical applications.Therefore,exploring and optimizing the preparing processes and lowering down the dielectric loss of CCTO are of great importance to the practical applications of CCTO.Until now,the internal barrier layer capacitor model(IBLC)is a widely accepted theory to give a reasonable explanation to the origin of the giant dielectric constant in CCTO.According to this model,the CCTO is composed of semiconducting grains and insulating grain boundaries and the electric properties of grain and grain boundary determine the dielectric properties of CCTO material.Therefore,we have started our research based on the electric properties of grain and grain boundary.We studied the influences of preparing process,non-stoichiometry and Ru doping on the electric and dielectric properties of CCTO.We hope that from our studies,we can deep our understandings of the origin of giant dielectric constant in CCTO and find out the optimized method to prepare CCTO material and lower down its dielectric loss.The main contents of our studies are as follows:We have studies the influences of sintering temperature(1000 ℃ 1025 ℃,1050 ℃,1075 ℃,and 1100 ℃)and sintering time(3 h,6 h,12 h,18 h,and 36 h)on the properties of CCTO with the traditional solid state reaction method.EDX characterization shows that high temperature and long time sintering will make the phenomenon of Cu rich and Ti deficiency at the grain boundaries become more and more serious.Sintering at 1100 ℃for a long time will produce TiO2 impurity phase.Room temperature current-voltage measurements show that properly improving the sintering temperature and time will increase the current density of CCTO ceramics.Dielectric response and impedance spectroscopy show that high temperature sintering and prolonged sintering time will increase the dielectric constant of CCTO.Our study demonstrates that dielectric constant of CCTO is closely related to the semiconducting properties of the CCTO grains,and low resistance of the grains will result in larger dielectric constant.After comparing the dielectric properties of all the samples,we find that CCTO sample sintering at 1100 ℃for 18 h exhibits best performance.CCTO samples with Cu-deficiency,Cu-rich,and standard stoichiometry are prepared.The conditions of Cu-deficiency and Cu-rich do not change the crystal structures of the resulting CCTO samples,but will result in a contraction and expansion of the lattice,respectively.Room temperature current-voltage measurement shows that both Cu-deficiency and Cu-rich will decrease the current density of the corresponding samples.The concentrations of Cu+ and Ti3+ are increased in the grain boundaries of the Cu-deficient and Cu-rich samples,therefore,changing the mixed covalent structure of Cu+/Cu2+ and Ti3+/Ti4+ in the grains.Impedance measurement at varying temperatures shows that Cu-rich will result in an increase in the activation energy of grain and grain boundary;while Cu-deficiency exhibit negligible influence on the activation energy.Cu-rich and Cu-deficiency will increase and decrease the resistance of the grains,leading to a decrease and increase in the corresponding dielectric constant,respectively.Therefore,the dielectric constant of CCTO is mainly determined by the semiconducting properties of the grains.Due to the increased resistance of the grain boundaries,the dielectric losses in the low frequency range is suppressed in the Cu-rich and Cu-deficient samples.In total,low resistance in the grain and high resistance in the grain boundary will result in high dielectric constant and low dielectric loss in CCTO ceramics.Ru is successfully doped into the Cu site of CCTO using a complex solid state reaction method.XRD and EDX measurements demonstrate that Ru doping does not change the crystal structure of CCTO and can increase the Cu content in the grains,thus easing the Ti-rich Cu-deficiency condition in the grains.Ru doping can shift the Debye-like relaxation process to a lower frequency range.A low dielectric loss of 0.04 is obtained in the Ru doping CCTO samples,and in the frequency range from 3 MHz～10 MHz,dielectric loss less than 0.05(tan8<0.05)is also demonstrated.A simplified solid state reaction method is used to prepare Ru doped CCTO samples with Ru doping in the Cu site and Ti site,respectively.Ru doping in the Cu site or Ti site will not change the crystal structure of the CCTO samples.Room temperature current-voltage measurement shows that with increasing Ru doping concentration,current densities of the samples decrease first and then increase.Impedance measurements at varying temperature indicate that Ru doping in the Cu site or Ti site will increase the resistance and activation energy of the grains,but has little effect on the resistance and activation energy of the grain boundaries.Dielectric measurement shows that a new dielectric relaxation process is introduced when Ru is doped in the Ti site.The corresponding activation energy is about 0.5 eV,which is equal to the activation energy of the space charge polarization process in the interface between the electrode and CCTO surface. |
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