RESUMO
The effects of sintering conditions on the microstructure, giant dielectric response, and electrical properties of Na1/2Y1/2Cu3Ti3.975Ta0.025O12 (NYCTTaO) were studied. A single phase of Na1/2Y1/2Cu3Ti4O12 and a high density (>98.5%) were obtained in the sintered NYCTTaO ceramics. First-principles calculations were used to study the structure of the NYCTTaO. Insulating grain boundaries (i-GBs) and semiconducting grains (semi-Gs) were studied at different temperatures using impedance and admittance spectroscopies. The conduction activation energies of the semi-Gs and i-GBs were Eg ≈ 0.1 and Egb ≈ 0.6 eV, respectively. A large dielectric constant (ε' ≈ 2.43-3.89 × 104) and low loss tangent (tanδ ≈ 0.046-0.021) were achieved. When the sintering temperature was increased from 1070 to 1090 °C, the mean grain size slightly increased, while ε' showed the opposite tendency. Furthermore, the breakdown electric field (Eb) increases significantly. As the sintering time increased from 5 to 10 h, the mean grain size did not change, whereas ε' and Eb increased. Variations in the dielectric response and non-linear electrical properties were primarily described by the intrinsic (Egb) and extrinsic (segregation of Na-, Cu-, Ta-, and O-rich phases) properties of the i-GBs based on the internal barrier layer capacitor effect.
RESUMO
The microstructure, dielectric response, and nonlinear current-voltage properties of Sr2+-doped CaCu3Ti4O12/CaTiO3 (CCTO/CTO) ceramic composites, which were prepared by a solid-state reaction method using a single step from the starting nominal composition of CCTO/CTO/xSrO, were investigated. The CCTO and CTO phases were detected in the X-ray diffraction patterns. The lattice parameter increased with increasing Sr2+ doping concentration. The phase compositions of CCTO and CTO were confirmed by energy-dispersive X-ray spectroscopy with elemental mapping in the sintered ceramics. It can be confirmed that most of the Sr2+ ions substituted into the CTO phase, while some minor portion substituted into the CCTO phase. Furthermore, small segregation of Cu-rich was observed along the grain boundaries. The dielectric permittivity of the CCTO/CTO composite slightly decreased by doping with Sr2+, while the loss tangent was greatly reduced. Furthermore, the dielectric properties in a high-temperature range of the Sr2+-doped CCTO/CTO ceramic composites can be improved. Interestingly, the nonlinear electrical properties of the Sr2+-doped CCTO/CTO ceramic composites were significantly enhanced. The improved dielectric and nonlinear electrical properties of the Sr2+-doped CCTO/CTO ceramic composites were explained by the enhancement of the electrical properties of the internal interfaces.