| Recently,a body-centered cubic perovskite-related CaCu3Ti4O12(CCTO)compound has attracted much interest and become a hot research topic in recent years due to its unusual dielectric response and attractive application in electronic devices.In this thesis,we studied CCTO in the following four aspects.1.A sol-gel process has been used to synthesize CCTO ceramics and the effect of synthesizing temperatures on the microstructure,dielectric properties and I-Vnonlinear behavior of CCTO ceramics have been studied.XRD diffraction,and SEM images show that the calcining temperature for gel precursors and sintering temperatures for pellets have significant effects on the microstructure of CCTO ceramics.In the sintering process of CCTO at high temperature,some Cu2+ moved to grain boundaries and formed melting Cu-rich phase which helped melt and promoted grain growth.Positron annihilation lifetime and Doppler broadening results indicate thatthe concentration of Cu2+ vacancies increases with the increase of sintering temperature.Therefore,the sample with larger grain-size also has higher Cu2+vacancy concentration.Dielectric properties and I-V nonlinear behavior measurement results show that the samples with larger grain-size present very high dielectric constant at low frequencies,but poor stability of permittivity,large dielectric loss values at low frequencies and poor nonlinear behavior.We think that this is mainly attributed to higher concentration of Cu2+ in them.Therefore,synthesizing temperature has significant effect on the microstructures and electric properties of CCTO ceramics,and appropriate synthesizing temperatures for CCTOceramics prepared by Sol-Gel process is very necessary.2.Sol-gel process has been used to synthesize Ca1-xSrxCu3Ti4O12((x=0,0.05,0.1,0.15,0.2,0.3,0.4)and CaCu3-xZnxTi4O12(x=0,0.001,0.005,0.01,0.03,0.05)ceramics,and the effects of Sr and Zn doping on the microstructure,dielectric properties and I-Vnonlinear behavior of CCTO based ceramics have been studied.(1)XRD diffraction result shows that(220)peak shifts towards small angle and the lattice parameter increases with increasing Sr dopant.In addition,weak peaks of Sr4Ti3O10,TiO2 and Ca1.7Sr0.3CuO3 begin to appear as x>0.2.The calculating and measuring results of density,SEM morphologyand positron annihilation lifetime results show that the appropriate amountof Sr doping(x≤0.15)is favorable forthe increase of density,grain growth and reduction of the defect concentration,which is helpful to improve the dielectric properties and I-Vnonlinear behavior of CCTO based ceramics.With increasing Sr-dopant content(0.2 ≤ x≤ 0.4),Sr-doping results in the appearance of second phases at grain boundaries,reduction of grain-size and increase of the concentration of vacancies,which deteriorate the dielectric properties and I-Vnonlinear behaviors of Ca1-xSrxCu3Ti4O12 ceramics.(2)XRD diffraction of CaCu3-xZnxTi4O12 ceramics shows no second phases.X Ray Fluorescence(XRF)results reveal that the deviation of Zn content between the theoretical and experimental values increases with increasing Zn dopant because of volatilization of Zn in sintering process at high temperature.Positron annihilation lifetime and Doppler broadening results indicate that,with increasing Zn dopant,the concentration and size of Cu vacancy defects increases.The Cu deficiency and high concentration of Cu vacancy defects results in a rapid reduction of dielectric constant and I-Vnonlinear behavior of CaCu3-xZnxTi4O12 ceramics.3.CCTO-xNiO(x=0,0.003,0.006,0.01,0.015 and 0.02)ceramicshave been prepared by the solid-state reaction method and the effects of NiO doping on the microstructures and dielectric properties were studied.XRD diffraction,Raman spectroscopy and SEM images results show that the appropriate amount NiO doping can promote grain growth,but excess NiO doping can hold up grain growth.Positron annihilation lifetime results show that the averagelifetime of positron has increased after small amount of NiO doping,and raised to maximum value at x=0.3,and then decreased gradually with increasing amount of NiO doping,but also longer than that of CCTO ceramics.The condition of defects in CCTO ceramics is related to grain-size,bigger-sized grains have higher defect concentration and larger defect volume.The dielectric property measurement results show that appropriate amount NiO doping is helpful to increase the dielectric constant and decrease the dielectric loss of CCTO ceramics.The results of impedance spectroscopy results indicate that NiO doping can increase theresistance of grains boundary,which is helpful to reduce the dielectric loss of CCTO ceramics.4.CCTO-silicone rubber composites with different CCTO mass fractions(The mass fraction of CCTO is 10%,20%,30%,40%and 50%respectively.)have been prepared.The experimental results reveal that the composites possess excellent mechanical and dielectric properties.The calculated and experimental results of the dielectric constant of CCTO-silicone rubber composites show that experimental values are lower than calculated values,and the deviation increases gradually with increasing CCTO fraction.This indicates that the dielectric properties of composites are affected not only by the dielectric constant of composition phases,but also by the dispersion degree of CCTO particles in the silicone rubber matrixandthe particle size of CCTO.The composite with CCTO mass fraction of 50%has not only high dielectric constant,but also excellent mechanical property. |
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