Effect Of Microstructure On The Dielectric And Non-ohmic Properties Of CaCu₃Ti₄O₁₂ System

Ca Cu3Ti4O12(CCTO), a typical giant dielectric material with nonlinear current-voltage characteristics, is an important potential candidate for capacitor applications. To modify its electrical properties and explore the origin of the giant dielectric response, in this work, the effects of various doping and preparation conditions on the microstructure and electrical performance of CCTO-based ceramics are investigated.1. CCTO-x Sn O2(x=0-2.0 wt%) ceramics were prepared by a solid state reaction process. The doping modification of CCTO was studied by incorporation of Sn O2 as the second phase. The results indicate that there is no obvious influence of Sn O2 on the crystal structure of CCTO. While the grain growth is promoted by the small amount of dopant(x=0.3-0.6 wt%), which is beneficial to the increase of dielectric constant. The variation of grain boundary conductivity as a result of doing is also the influential factor on the dielectric performance. The increase in dielectric loss of low-doped samples is caused by the higher leakage current due to the decease of grain boundary resistance.2. The Gd3+/Y3+ doped CCTO-based ceramics were prepared by a solid state reaction process respectively, and the influence of different valence ionic doping on the microstructure and performance of CCTO were investigated systematically. The results suggest that both the two series of samples keep the single phase of CCTO as doping content x<0.05, and the ion-doping results in the variation of crystal lattice parameters and the various degree of restraint for grain growth. The positron annihilation lifetime experiment results show the influence of doping content on the defect size and concentration. In Gd-doped samples, both the dielectric performance and non-Ohmic properties of CCTO are enhanced. The non-Ohmic behaviors of CCTO are improved by doping Y3+ ions. And especially for x=0.01 sample, the nonlinear coefficient and breakdown electric field reach the maximum among all samples. It can be inferred that both grain morphology and defect characteristics affect the polarization mechanism and barrier height at grain boundary of CCTO-based ceramics, and then result in the various dielectric and non-Ohmic properties.3. CCTO ceramics were synthesized with different forming pressure(11-800 MPa). The effects of forming pressure on the microstructure and electrical performances were studied. There are slight variations in lattice parameters and polarizability concerned with the vibration of Ti-O caused by different forming pressure. The large grain growth in the sintering process is promoted by an appropriate increase of forming pressure, inducing the enhancement of dielectric constant for samples. For samples formed at P=600-800 MPa, the dielectric constant of CCTO increase by an order of magnitude compared with the sample formed at 11 MPa. The nonlinear coefficient and breakdown electric field reach the maximum at P=200 MPa( a =5.32, bE =1620 V/cm), which suggests that an appropriate forming pressure is beneficial to the enhancement of grain boundary barrier height and the improvement of non-Ohmic properties.