Effects of Charge Compensation on Colossal Permittivity and Electrical Properties of Grain Boundary of CaCu3Ti4O12 Ceramics Substituted by Al3+ and Ta5+/Nb5+

The effects of charge compensation on dielectric and electrical properties of CaCu₃Ti_4-x(Al_1/2Ta_1/4Nb_1/4)_xO₁₂ ceramics (x = 0−0.05) prepared by a solid-state reaction method were studied based on the configuration of defect dipoles. A single phase of CaCu₃Ti₄O₁₂ was observed in all ceramics with a slight change in lattice parameters. The mean grain size of CaCu₃Ti_4-x(Al_1/2Ta_1/4Nb_1/4)_xO₁₂ ceramics was slightly smaller than that of the undoped ceramic. The dielectric loss tangent can be reduced by a factor of 13 (tanδ ~0.017), while the dielectric permittivity was higher than 10⁴ over a wide frequency range. Impedance spectroscopy showed that the significant decrease in tanδ was attributed to the highly increased resistance of the grain boundary by two orders of magnitude. The DFT calculation showed that the preferential sites of Al and Nb/Ta were closed together in the Ti sites, forming self-charge compensation, and resulting in the enhanced potential barrier height at the grain boundary. Therefore, the improved dielectric properties of CaCu₃Ti_4-x(Al_1/2Ta_1/4Nb_1/4)_xO₁₂ ceramics associated with the enhanced electrical properties of grain boundaries. In addition, the non-Ohmic properties were also improved. Characterization of the grain boundaries under a DC bias showed the reduction of potential barrier height at the grain boundary. The overall results indicated that the origin of the colossal dielectric properties was caused by the internal barrier layer capacitor structure, in which the Schottky barriers at the grain boundaries were formed.     

Significantly Improved Colossal Dielectric Properties and Maxwell—Wagner Relaxation of TiO2—Rich Na1/2Y1/2Cu3Ti4+xO12 Ceramics

In this work, the colossal dielectric properties and Maxwell—Wagner relaxation of TiO₂–rich Na_1/2Y_1/2Cu₃Ti_4+xO₁₂ (x = 0–0.2) ceramics prepared by a solid-state reaction method are investigated. A single phase of Na_1/2Y_1/2Cu₃Ti₄O₁₂ is achieved without the detection of any impurity phase. The highly dense microstructure is obtained, and the mean grain size is significantly reduced by a factor of 10 by increasing Ti molar ratio, resulting in an increased grain boundary density and hence grain boundary resistance (R_gb). The colossal permittivities of ε′ ~ 0.7–1.4 × 10⁴ with slightly dependent on frequency in the frequency range of 10²–10⁶ Hz are obtained in the TiO₂–rich Na_1/2Y_1/2Cu₃Ti_4+xO₁₂ ceramics, while the dielectric loss tangent is reduced to tanδ ~ 0.016–0.020 at 1 kHz due to the increased R_gb. The semiconducting grain resistance (R_g) of the Na_1/2Y_1/2Cu₃Ti_4+xO₁₂ ceramics increases with increasing x, corresponding to the decrease in Cu⁺/Cu²⁺ ratio. The nonlinear electrical properties of the TiO₂–rich Na_1/2Y_1/2Cu₃Ti_4+xO₁₂ ceramics can also be improved. The colossal dielectric and nonlinear electrical properties of the TiO₂–rich Na_1/2Y_1/2Cu₃Ti_4+xO₁₂ ceramics are explained by the Maxwell–Wagner relaxation model based on the formation of the Schottky barrier at the grain boundary.     

Effects of surface concentration on the porphine monolayers: Molecular simulations at the nanoscale water-gas interface

The effect of surface concentration on the structure and stability of porphine (PH₂) monolayers at the water-gas interface was studied by using molecular dynamics simulation. Five monolayer systems having different surface concentrations were investigated in order to cover a full range of the experimental π-A isotherm. The simulation results show that increment of a number of the PH₂ molecules not only affects the significantly decreasing water density at the interface but also the monolayer surface tensions. The calculated surface tensions of the five systems indicate that the monolayer phase transfer corresponding to gaseous, expanded, condensed, and collapsed phases are observed. The hydrogen bonding between water and the PH₂ molecules at the interface plays an important role on the monolayer film formation, especially at the lower surface concentrations. The PH₂ orientations for all surface concentrations, except the highest one, are favored to be the β-structure as observed in the copper porphyrazine (CuPz) monolayer.