Study On Fabrication And Modification Of CaCu₃Ti₄O₁₂ Ceramics By Spark Plasma Sintering

The cubic perovskite-like CaCu₃Ti₄O₁₂（CCTO）has drawn great attention in recent years because of its giant dielectric constant（～10⁴-10⁵）at room temperature and excellent temperature stability（100-600 K）.Additionally,the non-ohmic characteristics of CCTO have also been found,which make CCTO possible candidate for energy storage components and overvoltage protection devices.Unfortunately,this material exhibits a high dielectric loss（tanδ）and low breakdown field despite of its high dielectric constant,which is undesirable for most industrial and electronic applications.Firstly,in this paper,CCTO ceramics were synthesized by spark plasma sintering（SPS）method at different temperature（750°C,800°C,and 850°C）,and the effect of SPS process on microstructure,current-voltage characteristics and dielectric properties of CCTO ceramics was investigated.It can be seen that SPS process is beneficial to improve the microstructure of CCTO ceramics.The pore size and porosity of the samples were reduced,and the grain size was smaller and more uniform and density was calculated to be over 98%of theoretical density.It was found that through SPS process,the dielectric loss can be reduced significantly and the current-voltage properties can also be enhanced,which are ascribed to the increase of the grain boundary resistivity in the sintering and annealing treatment.Among all the samples,sample SPS-750 has much better nonlinear J-E characteristics and lower dielectric loss,therefore,the sintering temperature for preparing CCTO ceramics by SPS at 750°C was determined in this work.Secondly,the effects of heat treatment in different atmospheres on the electrical properties and the defect structure relaxation process of CCTO ceramics were studied.It was found that the breakdown field and nonlinear coefficient of the sample after being heat-treated in oxygen atmosphere were increased to 9.57 k V/cm and 6.01,respectively,and the dielectric loss at 1 k Hz was greatly reduced to 0.036.By analyzing the loss spectrum of the samples,it was found that the grain boundary activation energy of the sample increased after being heat-treated in oxygen atmosphere,thereby improving the nonlinear characteristics.In addition,it was found that the activation energy of dc conductance of the grain boundary was significantly increased to 1.31 e V after being heat-treated in oxygen atmosphere.Finally,the effect of co-doping of ZnO and Al₂O₃ on the microstructure,electrical properties and defect structure of CCTO ceramics was investigated.The grains of the obtained samples were uniform and the grain size was suppressed to around 2μm.Dramatically enhanced electrical properties of CCTO ceramics were obtained when 1mol%Zn O and Al₂O₃ were co-doped.The breakdown field was improved to 12.13k V/cm and the nonlinear coefficient was increased to 6.59,accompanying with a low dielectric loss of 0.02 at around 1 k Hz,which were ascribed to the elevation of Schottky barrier height from 0.54 e V to 0.80 e V.Comparing with the pure CCTO sample,the activation energy of grain boundary obtained from the impedance spectra increases from 0.64 e V to 0.92 e V.Additionally,the grain and grain boundary resistance were also discussed based on the Cole-Cole plot,from which the grain boundary resistance was found to be increased significantly from 3.8×10⁵Ωto 4.0×10⁶Ωafter 1 mol%Zn and Al co-doping.