| The compound CaCu3Ti4O12(CCTO) with cubic perovskitelike structure has a giant relative dielectric constant (?r>104) with a good temperature stability in a wide temperature range (100K-600K), which makes it have good application foreground in the field of microelectronic device. However, unlike BaTiO3, the phase transition between ferroelectrocity and paraelectrocity has not been found in CCTO yet, thus, the intrinsic mechanism for the giant dielectric constant in CCTO is still on debate.Till now, based on many experimental results, it seems that the giant dielectric constant in CCTO ceramic, which has application merit, can be well explained by the internal barrier layer capacitor (IBLC) model. According to this model, CCTO ceramic is composed by the semiconducting grains and insulating grain boundary, as there is giant difference of electrical properties between them, grain-boundary dipole can be formed due to the separation and accumulation of positive and negative charges near grain boundary and the giant dielectric constant in CCTO ceramic is contributed from the grain-boundary dipole. Therefore, the dielectric properties of CCTO ceramic are strictly related with the properties of grain and grain boundary in ceramic. Till now, many research groups have reported their works concerning the manipulation of dielectric properties in CCTO ceramic by changing the composition, structure and electrical properties in grain and grain boundary via many methods. These methods mainly include changing sintering technology and element doping. However, many problems have not been resolved yet, for example, the intrinsic mechanism for the impact of sintering technology on the properties of grain and grain boundary is not clear; the electrical properties for the CCTO ceramic sintered by some special technologies have not been widely reported; doping some ions with changed valance or big radius may make a big impact on the electrical properties of the CCTO ceramic, but the related works have not been reported yet; in addition, the large low-frequency dielectric loss is a critical shortcoming for application. Therefore, in order to deeply understanding the mechanism for the special dielectric properties in the CCTO ceramic and exploring the effective methods for lowering its dielectric loss, we have carried out the research works concerning the above problems from two aspects:sintering technology and element doping. These works are all listed below:(1) CCTO ceramics have been sintered by the conventional technology at different temperatures (1035?and1080?) for different time (3h and48h). The results of composition and morphologies characterization show the formation of Cu-rich and Ti-poor grain boundary in the ceramics sintered at the higher temperature. The results of room-temperature dielectric and dc electrical conduction measurements show that with respect to prolonging sintering time, raising sintering temperature does more favor to the increase of electrical current density and dielectric constant. By using the complex impedance measured at different temperatures, we found that the ceramics sintered at the higher temperature has smaller grain resistivity, but the activation energies for grain conduction do not change with sintering conditions; While prolonging sintering time does favor to the reducing of grain-boundary resistivity and the enhancement of the activation energies for grain-boundary conduction, which may be related to the increase in the amount of Ti3+near grain boundary with increasing sintering time. But with respect to grain boundary, the electrical properties in grain are more important for the dielectric properties of CCTO ceramics.(2) CCTO ceramics were sintered in1atm air,1atm O2and3atm O2, respectively. The data of dielectric properties, dc electrical conduction and complex impedance were collected at room temperature, and the results show that the sintering in oxygen-rich atmosphere can lead to the increase of grain resistivity by4magnitudes and the drastic decrease of electrical current density and low-frequency dielectric constants, which is accompanied by the appearance of strong low-frequency dielectric relaxation. Based on the data of dielectric properties and complex impedance measured at different temperatures, we found that for the ceramic sintered in air, the activation energy for the conduction in grain is0.09eV, close to the value for the first-ionization for oxygen vacancies, but for the ceramics sintered in oxygen-rich atmosphere, the grain activation energies are clearly enhanced to be close to the activation energy for electrical conduction at grain boundary. These results indicate that the dielectric properties of the CCTO ceramics are strictly related with the electronic structure and the amount of oxygen vacancies in grain. Furthermore, the heights of the energy barrier for the low-frequency dielectric relaxation in the ceramics sintered in oxygen-rich atmosphere are larger, and this low-frequency dielectric relaxation may be attributed to the polarization of space charge at the interfaces of oxygen-rich surface layer and electrode.(3) A CCTO ceramic with depth of-2mm was sintered via spark plasma sintering technology at900?for5min, and graphite foil was used to prevent possible adhesion. The ceramic was polished to different depths, and the electrical conduction, complex impedance and dielectric properties were measured at room temperature. The results show that current density, grain-boundary resistivity and dielectric constant all decrease with depth decreasing, which may be attributed to the decreasing amount of oxygen vacancies those were formed due to the reducing reaction between carbon and CCTO. Furthermore, we found that as the depth of the ceramic is larger than1.4mm, the nonlinear coefficients for current-voltage curves are between1and2, and the low-frequency dielectric relaxation appears, but as the depth is smaller than1.4mm, the nonlinear coefficients are close to2, satisfying the space charge limited current (SCLC) mechanism, and the low-frequency relaxation disappears. These results indicate that the oxygen vacancies formed from the reducing reaction between carbon and CCTO are mainly concentrated in the surface layers with depth of0.3and0.4mm, and at the interface between this surface layer and inner part, there exist space charge to form dipoles.(4) The Rb+doped CCTO ceramics were prepared by replacing Ca2+by Rb+, the doping concentration is not larger than10%. Based on the characterization of phase, composition and surface morphologies, we found that doping Rb+can lead to the formation of a small amount of CuO and TiO2and the Cu-rich and Ti-poor grain boundary, and the depth of the boundary layer increases with increasing doping concentration. The results of measurement for dc electrical conduction, complex impedance and dielectric properties show that with increasing doping concentrations the change trend of electrical properties is not monotonic:As doping concentration is 30%, grain-boundary resistivity, the height of grain-boundary Schottky potential barrier increase; dielectric constants do not show clear change, while grain resistivity and low-frequency dielectric loss decrease. But as doping concentration is larger than30%, grain-boundary resistivity, the height of grain-boundary Schottky potential barrier and dielectric constants clearly decrease, but grain resistivity and low-frequency dielectric loss clearly increase. We think that these results should be attributed to the competitive impacts from the changed properties of grain and grain boundary due to the doping of Rb+.As the doping concentration is small, the increase in the amount of oxygen vacancies in grain plays a fundamental role, but as the doping concentration is larger, the changed composition of grain and grain boundary and the increase in the depth of grain boundary becomes dominant.(5) The Fe3+doped CCTO ceramics were prepared by replacing Cu2+ions by trace amount (not larger than2%) of Fe3+. Based on the results of measurement for electrical conduction, complex impedance and dielectric properties, we found that as the doping concentration is not larger than1%, the electrical properties can be greatly changed, and the results are very similar with those observed for the ceramics sintered in the oxygen-rich atmosphere:The grain resistivity increases by4and5magnitudes, the activation energy for conduction in grain increases greatly, while current density and low-frequency dielectric constants are greatly reduced, which are accompanied by the appearance of the strong low-frequency dielectric relaxation with larger energy barrier. These results indicate that besides sintering in oxygen-rich atmosphere, the replacement of Cu2+ions by trace amount of Fe3+can also reduce the amount of oxygen vacancies, leading to the giant change of electrical behaviors. |
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