Giant Dielectric Response Of CaCu₃Ti₄O₁₂ Ceramics And Its Modification

CaCu₃Ti₄O₁₂ giant dielectric constant material has attracted much attention because of its great importance in device miniaturization and the potential application for high-capacity capacitors.In the present thesis,the origin of dielectric relaxation and the giant dielectric response for CaCu₃Ti₄O₁₂ ceramics were systematically investigated,and the modification of dielectric properties was also carried out.In order to explore the physical mechanism and structural origin for the unusual dielectric responses in CaCu₃Ti₄O₁₂ ceramics,the dielectric relaxations at low and high temperatures as well as the giant dielectric constant step were investigated by using broadband dielectric spectrometer,X-ray photoelectron spectroscopy and ultrasonic attenuation measurements.The Debye-like dielectric relaxations in low and high temperature ranges were both thermal activated process.The high-temperatrue dielectric relaxation could be suppressed by O₂-annealing which was suggested to the results of defects,while the low-temperature dielectric relaxation was not affected by O₂-annealing which was induced by mixed-valent structure of Cu⁺/Cu²⁺ and Ti³⁺/Ti⁴⁺. Moreover,the giant dielectric constant step was resulted from the competing balance of the low and high temperature relaxations and the giant dielectric constant step could be modified by optimizing the competing balance between the two dielectric relaxations.Determining the effect of invariable-valent element substitution for the variable-valent element(Cu or Ti) on the dielectric properties should be helpful in deepening the understanding of the giant dielectric response in CaCu₃Ti₄O₁₂ ceramics. The effects of Zn-substitution on the giant dielectric response in CaCu₃Ti₄O₁₂ ceramics were investigated,and Ca(Cu_1-xZn_x)₃Ti₄O₁₂(x=0,0.05,0.1) ceramics were obtained by solid-state reaction.As the content of Zn-substitution increased,the dielectric constant increased together with the obvious increase of Ti³⁺ content.The enhanced giant dielectric response should be primarily caused by the increase of Ti³⁺ ions and the modification of such mixed-valent structure.Investigation of Mg-substitution for Cu was also performed to confirm the results in the Zn-substituted system.As supposed,the giant dielectric response was significantly enhanced by Mg-substitution.The results of X-ray photoelectron spectroscopy analysis confirmed again that the enhanced giant dielectric response primarily originated from the corresponding modification of mixed-valent structure caused by the obvious increase of Ti³⁺ content.The enhanced dielectric response caused by the increase of Ti³⁺ content in the Mg or Zn substituted systems indicated that the variable-valent Ti element should be the primary factor affecting the giant dielectric constant in CaCu₃Ti₄O₁₂ ceramics. Therefore,the effects of invariable-valent Sn substitution for Ti on the dielectric properties were investigated.The dielectric constant of CaCu₃(Ti_1-xSn_x)₄O₁₂(x=0, 0.05,0.1) ceramics decreased with increasing Sn substitution content.The decrease of Ti³⁺ content should be the dominating factor for the depressed dielectric properties.Modification of dielectric properties for CaCu₃Ti₄O₁₂ ceramics by introducing low loss SrTiO₃ phase was investigated.The dielectric constant of (1-x)CaCu₃Ti₄O₁₂-xSrTiO₃(x=0.2,0.4,0.6,0.8) composite ceramics decreased with increasing SrTiO₃ content,which fitted well with Lichtenecker's logarithmic law.As x increasing,the dielectric loss increased first,and then rapidly decreased.The dielectric loss was 0.06 for x=0.8 which was much smaller than that for x=0 (tanδ=0.16).The temperature dependence of dielectric constant enhanced with increasing content of SrTiO₃.