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The intermolecular potential between a 18-crown-6/K+ complex and a water molecule is derived from 1200 energy points obtained from quantum chemical calculations using the 6-31G** basis set. The ab initio fitted potential was then applied to study the structural properties of the complex in an aqueous solution using the Monte Carlo simulation method. The radial distribution function (RDF) centered at K+ to the oxygen atom of water shows a sharp first peak at 2.88 A. The corresponding coordination number, integrated up to the first minimum at 3.76 A, is 2 water molecules. The results indicate clearly that the 18-crown-6/K+ complex was solvated by the two nearest neighbors, one above and other below the ligand's plane. Evaluation was focused on the precise position and orientation of the two water molecules. It was found that the oxygen atoms of the two nearest neighbors bind to the K+ while their hydrogen atoms rotate freely around the vector perpendicular to the ligand's molecular plane.  相似文献   
2.
CaCu3-xZnxTi4O12 ceramics (x = 0, 0.05, 0.10) were successfully prepared by a conventional solid-state reaction method. Their structural and dielectric properties, and nonlinear electrical response were systematically inspected. The X-ray diffraction results indicated that single-phase CaCu3Ti4O12 (JCPDS no. 75–2188) was obtained in all sintered ceramics. Changes in the lattice parameter are well-matched with the computational result, indicating an occupation of Zn2+ doping ions at Cu2+ sites. The overall tendency shows that the average grain size decreases when x increases. Due to a decrease in overall grain size, the dielectric permittivity of CaCu3-xZnxTi4O12 decreases expressively. Despite a decrease in the dielectric permittivity, it remains at a high level in the doped ceramics (~3,406–11,441). Besides retention in high dielectric permittivity, the dielectric loss tangent of x = 0.05 and 0.10 (~0.074–0.076) is lower than that of x = 0 (~0.227). A reduction in the dielectric loss tangent in the CaCu3-xZnxTi4O12 ceramics is closely associated with the enhanced grain boundary response. Increases in grain boundary resistance, breakdown electric field, and conduction activation energy of grain boundary as a result of Zn2+ substitution are shown to play a crucial role in improved grain boundary response. Furthermore, the XPS analysis shows the existence of Cu+/Cu2+ and Ti3+/Ti4+, indicating charge compensation due to the loss of oxygen lattice. Based on all results of this work, enhanced dielectric properties of the Zn-doped CCTO can be explained using the internal barrier layer capacitor model.  相似文献   
3.
CaCu3Ti4O12 ceramics have been extensively studied for their potential applications as capacitors in recent years; however, these materials exhibit very large dielectric losses. A novel approach to reducing the dielectric loss tangent in two steps, while increasing the dielectric permittivity, is presented herein. Doping CaCu3Ti4O12 with a Zn dopant reduces the loss tangent of the ceramic material from 0.227 to 0.074, which is due to the increase in grain boundary (GB) resistance by an order of magnitude (from 6.3× 103 to 3.93 × 104 Ω cm). Zn-doping slightly changes the microstructure and dielectric permittivity of the CaCu3Ti4O12 ceramic, which reveals that the primary role of the Zn dopant is to tune the intrinsic properties of the GBs. Surprisingly, the addition of the Ge4+ dopant into the Zn2+-doped CaCu3Ti4O12 ceramic sample led to a further decrease in the loss tangent from 0.074 to 0.014, due to enhanced GB resistance (3.1 × 105 Ω cm). The grain size increased remarkably from 2–3 μm to 85–90 μm, corresponding to a significant increase in the dielectric permittivity (~1–4 × 104). The large increase in GB resistance is due to the intrinsic potential barrier height at the GBs and the segregation of the Cu-rich phase in the GB region. First-principles calculations revealed that Zn and Ge are preferentially located at the Cu sites in the CaCu3Ti4O12 structure. The substitution of the Ge dopant does not hinder the role of the Zn dopant in terms of improving the electrical properties at the GBs. These phenomena are effectively explained by the internal barrier layer capacitor model. This study provides a way of improving the dielectric properties of ceramics for their practical use as capacitors.  相似文献   
4.
High dielectric permittivity (ε′ ≈ 2000―6900) was accomplished in Sn4+-doped CaCu2.95Mg0.05Ti4O12 ceramics while retaining a low loss tangent (tanδ ≈ 0.027―0.075). Further, significant improvements in the nonlinear electrical properties, such as high values of the breakdown electric field (Eb ≈ 1.2―1.3 × 104 V cm?1) and nonlinear coefficient (α ≈ 31), were achieved. In addition, the nonlinear electrical parameters significantly improved, which is consistent with the increase in the electrical resistivities of the grains and grain boundaries due to the decrease in the Cu+/Cu2+ ratio. According to our first-principles calculations, the Sn atom at the Ti site prefers to be close to the Mg atom at the Cu site, while the oxygen vacancy prefers to be located at large distances from the Sn and Mg co-dopants. This confirms that the dielectric behavior and the nonlinear electrical properties originate from the interface between the grain and grain boundary.  相似文献   
5.
The giant dielectric behavior of CaCu3Ti4O12 (CCTO) has been widely investigated owing to its potential applications in electronics; however, the loss tangent (...  相似文献   
6.
In this study we employed the B3LYP/6-311++G(d,p) method combined with the CIS/6-311++G(d,p) calculation to investigate the effects of the type and the number of alkali metal atoms(Li, Na, K) on the geometric, electronic, and optical properties of alkali metals substituted into adamantanes. Substituting alkali metal(Li, Na, K)atoms caused significant changes in the electronic and optical properties of adamantane. The Ad-1Li, Ad-1Na,and Ad-1K structures showed a dramatically decreased energy gap and ionization potential, while adding more alkali metal atoms slightly decreased these properties. Substituting more alkali metals led to a shift in the maximum absorption wavelength from the visible to the infrared region, depending on the type of alkali metal atom substituted. The magnitude of shift occurred in the following order: Li b Na b K. These characteristics suggest the possibility of tunable electronic structures of this material for optoelectronic device applications.  相似文献   
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