Bi _2/3 Cu ₃ 4 O ₁₂ - Based Ceramics And Their Dielectric Properties

In recent years, with the development of miniaturization and integration of electronic devices, materials with high dielectric permittivity, low dielectric loss and good thermal stability becomed more and more necessary. CaCu3Ti4O12(CCTO), possesses a perovskite-related structure, was found by Subramanian in2000. It exhibited the colossal dielectric permittivity (104) and weak temperature dependence between100and400K. Therefore, it attracted considerable attention. However, the dielectric loss of CCTO was relatively high for application. In order to make dielectric loss decrease, ACu3Ti4O12, has similar structure with CCTO, was obtained by replacing Ca2+in CCTO with different ions. So far, the research on ACu3Ti4O12has getted much significant results, but result on Bi2/3Cu3Ti4O12(BCTO) was so little. In this work, the microstructure, phase structure and dielectric properties of Bi2/3Cu3Ti4O12ceramics prepared in the different conditions were investigated. The Y3+and Na+substitution for Bi3+in Bi2/3Cu3Ti4O12ceramics was also investigated to improve dielectric properties.Firstly, effect of calcining temperature and sintering temperature on the microstructure and dielectric properties of BCTO ceramics were systematically investigated. All the samples were body-centered cubic perovskite-related structure. The grain size was independent of calcining temperature and sintering temperature, it still remains2-3μm. When the samples were calcined at800℃and sintered at1000℃for20h, nice dielectric properties were obtained. The dielectric constant was2873and the dielectric loss was0.039. The analysis of Complex impedance revealed the grain of all BCTO ceramics was semiconducting and the grain boundaries was insulating. The dielectric relaxation presented clearly in the temperature dependence of dielectric constant can be explained by Maxwell-Wagner effect.Then effect of Y3+substitution for Bi3+on the phase structure, microstructure and dielectric properties of Bi2/3Cu3Ti4O12ceramics was studied, respectively. The results showed that the pure phase could be more easily obtained when substituting Y3+for Bi3+. The average grain size increased with increasing Y3+substitution.(YxBi1-x)2/3Cu3Ti4O12(YBCTO) ceramics with x=0.20possessed the highest permittivity (7321) and relatively higher dielectric loss (0.058). Complex impedance presented that all samples were electrically heterogenous. The study on electric modulus demonstrated that the dielectric properties of YBCTO ceramics have a close relationship to Maxwell-Wagner relaxation occurring at grain boundaries. The dielectric relaxation between250℃and300℃clearly existed in the temperature dependence of dielectric constant was related to Maxwell-Wagner relaxation.Lastly, effect of Na+substitution for Bi3+on phase structure, microstructure, and dielectric properties were discussed. NaxBi(2-x)/3Cu3Ti4O12ceramics from x=0.00to x=0.50were systematically studied. With the increase of Na+concentration, both the average grain size and the low-frequency dielectric constant increased. The dielectric constant of NaxBi(2-x)/3Cu3Ti4O12ceramics with x=0.00,0.05,0.20,0.35, and0.50were found to be2873,3655,6471,7665, and14279at1kHz, respectively. NaxBi(2-x)/3Cu3Ti4O12ceramics with x=0.35showed the lowest dielectric loss (0.015) at1kHz with a relatively highdielectric permittivity (-7,600). The complex impedance spectroscopy at room temperature suggested that all the NaxBi(2-x)/3Cu3Ti4O12ceramics were electrically heterogeneous. The calculated grain boundary resistance and grain resistance decreased with increasing Na+concentration. The investigation of temperature dependence of complex impedance displayed that the resistence behavior at the grain boundaries of NaxBi(2-X)/3Cu3Ti4O12ceramics was related to second ionization of oxygen vacancies. In view of the analyses of electric modulus, it was confirmed that the giant low-frequency dielectric constant observed in NaxBi(2-x)/3Cu3Ti4O12ceramics was attributed to Maxwell-Wagner polarization at the grain boundaries. Furthermore, dielectric relaxations (named as Al, A2and A3, respectively), which occur at-140℃,240℃, and-350℃, were clearly observed in the temperature dependence of dielectric constant of NaxBi(2-x)/3Cu3Ti4O12ceramics. A1was a dipolar relaxation which was induced by the hopping motions of confined carriers related to single-ionized oxygen vacancies; A2was attributed to Maxwell-Wagner relaxation associated with grain boundaries; A3was likely to be resulted from the hopping motions of carriers related to the second ionization of oxygen vacancies. In addition, the dielectric constant of NaxBi(2-x)/3Cu3Ti4O12ceramics with x=0.65reached11800, but it has relative high dielectric loss (0.055). Compared with NaxBi(2-x)/3Cu3Ti4O12(0.00≤x≤0.50), there were two kinds of dielectric response in complex impedance and electric modulus, respectively. The one in low frequency region was resulted from Maxwell-Wagner effect, the other in high frequency region from grain effect.