Microstructure Tailoring And Property Study Of CaCu₃Ti₄O₁₂ Ceramics With Colossal Dielectric Constant

CaCu₃Ti₄O₁₂（CCTO）ceramics with high dielectric constant have great potential for the miniaturization of capacitive components.However,there are still some problems in the research of CCTO,such as its high dielectric loss and the mechanism of the giant dielectric constant effect.Therefore,in this paper rare earth ions with diferent content,radius and valence were used to substitute the ions at A and B sites of CCTO.The defects associated with doping and their effect on the dielectric properties of CCTO ceramics were investigated by positron annihilation technique（PAT）.Firstly,Lu³⁺ was substituted into Ca²⁺ at A sites of CCTO.PAT results revealed that the defects can be controlled by adjusting the doping content.It was found that the cationic defect concentration increased with the increase of doping content.The internal barrier layer capacitor（IBLC）model can not be used to explain the variation of dielectic constant with increasing doping content when the defect concentration is more than 7.9×1022m-3.It is necessary to use defect-dipoles polarization effect to explain the high dielectric behavior of the doped CCTO samples,whose dielectric properties can be modified by adjusting the defect concentration.Secondly,in order to study the influence of doping ionic radius on the defect,microstructure and electrical properties of CCTO,the substitution of Ca²⁺ by rare earth ions in CCTO was performed.The doping content was controlled below the level at which IBLC model is adapted.It was found that the doping ionic radius can affect the precipaitation of Cu-rich phase at the grain boundary of CCTO,and the change of defect concentration decreased with the decrease of the doping ionic radius,revealing that Cu-rich phase precipitated at the grain boundary decreased in turn.The corresponding mean grain size and the dielectric constant also decreased monotonously.The dielectric loss of the doping samples decreased significantly while the dielectric constant was still above 103.Thirdly,Y³⁺,Zr⁴⁺ and Ta⁵⁺ ions were also employed to partially replace Ti⁴⁺of CCTO.The effect of the valence state of the doped ions on the properties of CCTO was investigated.The variation of the defects of the samples was identified by PAT.The donor Ta⁵⁺ doping can increase the titanium cation vacancy,inhibit the grain growth and decrease the dielectric constant and loss,while the acceptor Y³⁺ doping increase the copper vacancy and promote the precipitation of Cu-rich phase at the grain boundary,which is beneficial to the growth of the grain and the enhancement of the dielectric constant.The substitution of Zr⁴⁺ has little influence on the defect types and microstructure of CCTO,whose dielectric properties are not changed obviously.Finally,in order to further reduce the dielectric loss,SrTiO₃ with low dielectric loss was added into CCTO ceramics.It was found that Sr²⁺ entered the lattice when the addition of SrTiO₃ is appropriate.Low melting point liquid formed by TiO₂ and Cu-rich phase promoted the grain growth,which enhanced the dielectric constant and maintained the dielectric loss at low frequency and decreased the dielectric loss at high frequency.The grain boundary density and thickness increased with the increase of the addition content of SrTiO₃ content,resulting in the increase of the grain boundary resistence and the significant decrease of the dielectric loss.The dielectric loss of the sample was reduced to 0.03 when the addition content of SrTiO₃ is up to 2%,while the dielectric constant is still above 103.In addition,the effects of preparation process on the microstructure and electrical properties of CCTO ceramics were also investigated.