Research Of Giant Dielectric Response And Doping On CaCu₃Ti₄O₁₂ Ceramics

CaCu₃Ti₄O₁₂（CCTO） ceramics possess a relative high dielectric constant of 104¹05 at room temperature and show good temperature stability in the range of 100K⁶00K. These properties make CCTO potential dielectric materials for the large-capacity capacitors and the field-effect transistors gate oxides. But higher dielectric loss and the sensitivity to raw materials and preparation process have limited the application of CCTO materials. The internal barrier layer capacitance（IBLC） effect is generally considered as the origin of giant dielectric constant, however there are still controversies on it. In this paper, CCTO ceramics have been prepared via the solid-state reaction. By changing the Cu content and TiO₂/CaTiO₃ addition, the author has studied the dielectric responses of the nonstoichiometric CCTO ceramics. The effects of Co, Ni and Mn doping on the microstructure and dielectric properties of CCTO ceramics are also studied.The dielectric frequency spectrum of nonstoichiometric CCTO ceramics with different Cu content was studied. The comparative results show that there exist three dielectric response behaviors at room temperature, i.e. the surface barrier layer capacitance（SBLC） effect, the internal barrier layer capacitor（IBLC） effect and the intrinsic displacement polarization, the former two lead to the giant dielectric constant together. When the Cu content decreases and deviates from the stoichiometry, a uniform fine grain microstructure is obtained and the SBLC effect enhances, while the IBLC effect decreases relatively.The correlation of current-voltage characteristics with Cu content also confirms this variation. In addition, it is found that surface polishing can enhance the SBLC effect of the Cu-deficient samples.When temperature is elevated or DC bias is applied, local carriers transition will happen and the low-frequency dielectric constant is further enhanced.The effect of TiO₂ additive on the microstructure and electrical properties of CCTO ceramics was studied. The low-frequency dielectric constant increases with TiO₂ additive amount, especially when the amount reached 0.08 mol%, the dielectric constant is significantly elevated. TiO₂ is believed to reduce the local carrier activation energy. With the elevated dielectric constant, the loss peak resulting from high-frequency relaxation also increased.The effect of CaTiO₃ additive on the microstructure and electrical properties of CCTO ceramics was studied and it is found that CaTiO₃ additive has significant influence on the microstructure of CCTO ceramics, that is, grains become fine and uniform, opening pores increased and bulk density decreased, which is similar to Cu-deficient samples. Dielectric response behaviors are also consistent with Cu-deficient samples. Therefore, the reason why the microstructure and dielectric response behaviors of Cu-deficient samples differs from the standard stoichiometric samples is essentially due to the CaTiO₃. CaTiO₃ can suppress the precipitation of Cu-rich phase and the abnormal grain growth.The Co-doping experiments were done on CCTO ceramics. Co substituting Cu can enhance the low-frequency dielectric constant and reduce the dielectric loss at the same time. Ca(Cu_2.95Co_0.05)Ti₄O₁₂ exhibits the lowest loss tanδ=0.03 at 500 Hz. However, Co substituting Ti and Co₃O₄ doping will produce the impurity phase CoO and inhibits the abnormal large grains growth. The low-frequency dielectric constant declines by an order when Cu substitutes Ti. These results show that the Co atoms tend to substitute the Cu atom sites, not the Ti atom sites, in CCTO crystalsThe Ni-doping experiments were done on CCTO ceramics. The XRD results show Ni can substitute Cu at a large amount without generating any impurity phases, and it can promote the abnormal grains growth. The low frequency dielectric constant gradually increased with Ni substitution. Ca(Cu_2.6Ni_0.4)Ti₄O₁₂ exhibits extremely high permittivity with εr²Ã—105 at 100 kHz. Both Ni substituting Ti and NiO doping can enhance the low-frequency dielectric constant. The above results indicate that Ni element can either substitute the elements Cu or the Ti element in CCTO, the two substituents can improve the low-frequency dielectric constant.The Results of MnCO₃ doping experiments show a substantial decline in the low-frequency dielectric constant, but the volume resistivity raises by an order in reverse.