CaCu <sub> 3 </ Sub> Ti <sub> 4 </ Sub> O <sub> 12 </ Sub> Relaxation Characteristics Of The Giant Dielectric Material And Modified

With the development of electronic technology industry, microelectronic devices demand more compact size, quicker speed, higher frequency, lower power to realize miniaturization and integration. Recently, the compound CaCu₃Ti₄O₁₂ (CCTO) has attracted much interest because of its extraordinarily high permittivity (ε_r-10⁴) at room temperature and very small temperature dependence. However, there are still some problems that need to be resolved. For example, there is no consensus on the origin of giant dielectric constant and the dielectric loss of CCTO is relatively too high to be applied commercially.In this thesis, high permittivity material CaCu₃Ti₄O₁₂ ceramic samples were prepared by using the conventional ceramic solid state reaction. CaCu₃Ti_4-xFe_xO₁₂ (x=0, 0.01, 0.04,0.12, 0.2) (CCTFO) ceramics and CaCu_3-xSr_xTi₄O₁₂(x=0, 0.05, 0.1, 0.2, 0.4) (CCSTO) ceramics were synthesized respectively. The microstructures, dielectric properties and the mechanism giant dielectric permittivity of the materials were investigated systematically.The origin of giant dielectric constant under different frequency range had been investigated through analyzing the effect of the substitution of Fe³⁺ for Ti⁴⁺ on the dielectric relaxation process of CCTFO ceramics. The main results and the conclusions are as follows: with an increase of Fe content, the semiconductivity of grain vanishes gradually and dielectric constant decreases. For the specimens with x≤0.04, the two dielectric relaxation processⅠandⅡappeared in the frequency range of 10⁶-10⁸ Hz and 10³-10⁴ Hz respectively. These two dielectric relaxation processes were considered to be associated with grain boundaries and interfacial polarization between the electrode and ceramic surface respectively. In addition, the third dielectric relaxationⅢwas detected in the high-temperature dielectric spectroscopy of CaCu₃Ti_3.99Fe_0.01O₁₂ ceramic, which was caused by a hopping process of localized charge carriers. The activation energy of this thermally excited relaxation is 0.78 eV obtained by using an Arrhenius formula.Sr doping in Cu-deficiency CaCu_3-xSr_xTi₄O₁₂ (x=0, 0.05, 0.1,0.2, 0.4) (CCSTO) ceramics were carried out in this work. It has been found that this design can decrease dielectric loss and remain higher dielectric constant (ε_r>3000) by decreasing grain sizes and increasing the impedance of grain boundary. So, we can conclude that Sr-doped CCTO ceramics have outstanding electrical properties for practical applications. As compared with Ca²⁺ (1.34 (?)), Sr²⁺ (1.44 (?)) is too large to substitute Cu²⁺ (0.57 (?)), so the substituting Ca²⁺ with Sr²⁺ made the surplus of Ca, Sr and Ti. The analysis of X-ray diffraction patterns and SEM images show that the white particles in grain boundaries are mainly composed of the solutions of CaTiO₃ and SrTiO₃, exactly whose presence restrained the egregious growth of grains. CCTO ceramics shows the microstructure distributed uniformly with the grain size of 100-200μm and the clear grain boundaries. In contrast, the abnormally large grains gradually disappear by increasing the content of Sr. When x=0.4, the evenly distributed grains of CCSTO ceramics have an average size smaller than 10μm. So we can conclude that Sr-doping can control the microstructure and then modify the grain boundaries of CCTO ceramics to optimal performance. It is necessary to know the origin of dielectric loss in CCTO to find an effective method to lower it. The Maxwell-Wagner (M-W) relaxation is widely accepted as the primary mechanism for the giant dielectric constant in CCTO ceramics. Accompanied by a strong M-W relaxation, the dielectric loss mainly originates from the conductivity of insulating barriers, which is called leakage loss. The analysis results of complex impedance spectroscopy and C-V characteristics show that the impedance increase is the direct cause of decreasing the dielectric loss of CCTO ceramics.