Preparation, Structure And Dielectric Properties Of CaCu₃Ti₄O₁₂-based Ceramics

CaCu₃Ti₄O₁₂(CCTO)has attracted a lot of researches for its unusual dielectric property,giant dielectric constant(＞10⁴)at radio frequency and at room temperature. However,it is a matter of disputation of what is the origin of giant dielectric response. Moreover,the large dielectric loss(＞0.10)of CCTO hinders its application in electronic fields.In this work,we tried to explore the physical mechanism by the means of composition design,structural control and the research on structure-property relationship.Meanwhile,we also tried to find out the new CCTO-based dielectric system with high dielectric constant and low dielectric loss to facilitate its practical application.For the preparation of ceramics with optimum properties,it is necessary to obtain highly pure CCTO powder first.To aim at synthesizing single phase CCTO, which is a technological difficulty,the detailed procedure of microwave synthesis was investigated.By the microwave synthesis method,the formation of single phase CCTO powder can be obtained at a relatively low calcining temperature and in short timeThe ferroelectricity of CCTO ceramic depends on the microstructures closely.It is found that CCTO exhibits relaxor behavior.As grain size decreases,the frequency dispersion and the relaxation strength increases,which is due to the higher stress from grain boundary in smaller grains.The structures and the dielectric properties of CCTO ceramics with different Ti stoichiometry,Zr⁴⁺,Ta⁵⁺,and Sc³⁺substitution for Ti⁴⁺were investigated.That is, with B-site modification,the effect of B-site ions on structures and dielectric properties were used to explore the mechanics of giant dielectric response in CCTO. The results show that the origins of formation of insulating barrier layer and the discontinuous growth of grains partially arise from Ti presenting at grain boundary. The nature of giant dielectric response in CCTO can be explained well by internal barrier layer capacitors model(IBLC)with Maxwell-Wagner relaxation behavior. And in the temperature ranges of 300-400K and 450-550K,two relaxation behaviors have been observed which are attributed to grain boundary and domain boundary, respectively.In addition,the effects of the different valence ions substituted for Ti illustrate that under an optimum sintering conditions,the CCTO-based materials have high dielectric constant and relatively low dielectric loss:at the radio frequency,(1)At the level of 0.5%mol Zr substitution,the ceramics exhibit good dielectric properties,with a high dielectric constant(＞5000)and a low dielectric loss(＜0.8)in the temperature range from -50 to 70℃.(2)At the level of 1.0mol%Ta substitution,the ceramic exhibit a high dielectric constant(＞20000)and dielectric loss of 0.09.(3)When Sc concentration is 1.0%mol of Ti,the samples exhibit a high dielectric constant(＞6000), and a low dielectric loss(＜0.10)in the temperature range of 20-100℃.Considering the low dielectric loss of SrTiO₃,the structure and the dielectric properties of(1-x)CCTO-xSrTiO₃ ceramics have been studied,where x is volume ratio.The system is typical two phases composite.The changes in dielectric constant can be explained well by Lichtenecker's logarithmic law of composite.According to IBLC,the dielectric loss drops largely due to the high resistance of SrTiO₃.At the level of x=0.4,after sintered at 1060℃for 3 hours,an optimum dielectric ceramic has been obtained,with a high temperature stable dielectric constant of 2000 that is better than ones of XTR,and a low dielectric loss(＜0.10)in the temperature range from room temperature to 150℃.