A significant improvement in molecular hydrogen uptake properties is revealed by our ab initio calculations for Li-decorated metal-organic framework 5. We have found that two Li atoms are strongly adsorbed on the surfaces of the six-carbon rings, one on each side, carrying a charge of ؉0.9e per Li atom. Each Li can cluster three H 2 molecules around itself with a binding energy of 12 kJ (mol H2) ؊1 . Furthermore, we show from ab initio molecular dynamics simulations with a hydrogen loading of 18 H2 per formula unit that a hydrogen uptake of 2.9 wt % at 200 K and 2.0 wt % at 300 K is achievable. To our knowledge, this is the highest hydrogen storage capacity reported for metal-organic framework 5 under such thermodynamic conditions. first-principles calculations ͉ molecular adsorption ͉ molecular dynamics ͉ porous materials M etal-organic frameworks (MOFs) form a class of nanoporous materials with high surface area that are capable of binding gas molecules in a nondissociative manner (1-5). Consequently, MOFs are promising candidates for use in hydrogen storage media. In MOF systems, the hydrogen sorption processes display good reversibility and fast kinetics; however, the weak dispersive interactions that hold H 2 molecules require low operation temperatures and/or high pressures to guarantee a significant storage capacity, e.g., MOF-5 reaches an H 2 uptake of only 1.3 wt % at 78 K and 1 bar (6). Neither the thermodynamics nor the storage capacity meets the requirements established for onboard applications (7). Therefore, a great deal of effort is being focused on devising ways to strengthen hydrogen adsorption interactions and maximize volumetric and gravimetric surface area densities. The achievement of the latter is being pursued through the following approaches: (i) topological engineering of pore shape (8), (ii) insertion of other adsorbate surfaces inside the pores (9), (iii) synthesis of light-metal MOFs (10, 11), and (iv) entanglement of frameworks (framework catenation) (12, 13). To achieve stronger H 2 -surface interactions, which is the more challenging problem, most studies have turned to investigating the introduction of electron-donating ligands to the organic linkers and also to the synthesis of so-called ''open metal sites'' (14). In fact, it is well accepted that to significantly enhance the H 2 affinity in these frameworks, the binding mechanism should include other contributions, such as electrostatic and/or orbital interactions, rather than being purely dispersive, as described in ref.15.An alternative approach for strong, nondissociative H 2 binding comes from the possibility of adsorbing hydrogen molecules on light, nontransition metal ions such as Li ϩ , Na ϩ , Mg 2ϩ , and Al 3ϩ (15,16). These bare metal ions are capable of clustering several H 2 molecules, with binding energies in the range of 12-340 kJ (mol H 2 ) Ϫ1 . For the alkali metals (Li ϩ and Na ϩ ), H 2 is bound through electrostatic charge-quadrupole and chargeinduced dipole interactions. For Al 3ϩ and Mg 2ϩ , which display hig...

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The giant dielectric behavior of CaCu3Ti4O12 (CCTO) has been widely investigated owing to its potential applications in electronics; however, the loss tangent (tanδ) of this material is too large for many applications. A partial substitution of CCTO ceramics with either Al3+ or Ta5+ ions generally results in poorer nonlinear properties and an associated increase in tanδ (to ∼0.29–1.15). However, first-principles calculations showed that self-charge compensation occurs between these two dopant ions when co-doped into Ti4+ sites, which can improve the electrical properties of the grain boundary (GB). Surprisingly, in this study, a greatly enhanced breakdown electric field (∼200–6588 V/cm) and nonlinear coefficient (∼4.8–15.2) with a significantly reduced tanδ (∼0.010–0.036) were obtained by simultaneous partial substitution of CCTO with acceptor-donor (Al3+, Ta5+) dopants to produce (Al3+, Ta5+)-CCTO ceramics. The reduced tanδ and improved nonlinear properties were attributed to the synergistic effects of the co-dopants in the doped CCTO structure. The significant reduction in the mean grain size of the (Al3+, Ta5+)-CCTO ceramics compared to pure CCTO was mainly because of the Ta5+ ions. Accordingly, the increased GB density due to the reduced grain size and the larger Schottky barrier height (Φb) at the GBs of the co-doped CCTO ceramics were the main reasons for the greatly increased GB resistance, improved nonlinear properties, and reduced tanδ values compared to pure and single-doped CCTO. In addition, high dielectric constant values (ε′ ≈ (0.52–2.7) × 104) were obtained. A fine-grained microstructure with highly insulating GBs was obtained by Ta5+ doping, while co-doping with Ta5+ and Al3+ resulted in a high Φb. The obtained results are expected to provide useful guidelines for developing new giant dielectric ceramics with excellent dielectric properties.

A solid–state reaction method was used to prepare (Sr2+, Ge4+) co-doped CaCu3Ti4O12 ceramics. A single-phase of CaCu3Ti4O12 was detected in all the ceramics. An enormous evolution of grain growth in (Sr2+, Ge4+) co-doped CaCu3Ti4O12 ceramics was observed, which was due to a liquid phase sintering mechanism. Theoretical calculations showed that Ge dopant ions are more likely substituted in Cu sites rather than Ti sites. High dielectric permittivity, ∼69,889, with a low dielectric loss tangent, ∼0.038, was achieved in a Ca0.95Sr0.05Cu3Ti3.95Ge0.05O12 ceramic. Furthermore, dielectric permittivity at 1 kHz of this ceramic is more temperature-stable than that of the CaCu3Ti4O12 and Ca0.95Sr0.05Cu3Ti4O12 ceramics. The enhanced dielectric permittivity with reduced loss tangent in the co-doped ceramics originated from a metastable insulating phase created by a liquid phase sintering mechanism. The local insulating phase along the grain boundary layers can increase the grain boundary resistance as well as the conduction activation energy of the grain boundaries, resulting in a decreased dielectric loss tangent. An internal barrier layer capacitor model supports the origin of the giant dielectric properties in CaCu3Ti4O12-based ceramics by all results in this work.

The effects of DC bias on the dielectric and electrical properties of co-doped (In1/2Nb1/2)xTi1−xO2 (IN-T), where x = 0.05 and 0.1, and single-doped Ti0.975Nb0.025O2 ceramics are investigated.

The effects of the sintering temperature on microstructures, electrical properties, and dielectric response of 1%Cr3+/Ta5+ co-doped TiO2 (CrTTO) ceramics prepared using a solid-state reaction method were studied. The mean grain size increased with an increasing sintering temperature range of 1300–1500 °C. The dielectric permittivity of CrTTO ceramics sintered at 1300 °C was very low (ε′ ∼198). Interestingly, a low loss tangent (tanδ ∼0.03–0.06) and high ε′ (∼1.61–1.9 × 104) with a temperature coefficient less than ≤ ±15% in a temperature range of −60 to 150 °C were obtained. The results demonstrated a higher performance property of the acceptor Cr3+/donor Ta5+ co-doped TiO2 ceramics compared to the Ta5+-doped TiO2 and Cr3+-doped TiO2 ceramics. According to a first-principles study, high-performance giant dielectric properties (HPDPs) did not originate from electron-pinned defect dipoles. By impedance spectroscopy (IS), it was suggested that the giant dielectric response was induced by interfacial polarization at the internal interfaces rather than by the formation of complex defect dipoles. X-ray photoelectron spectroscopy (XPS) results confirmed the existence of Ti3+, resulting in the formation of semiconducting parts in the bulk ceramics. Low tanδ and excellent temperature stability were due to the high resistance of the insulating layers with a very high potential barrier of ∼2.0 eV.

Hydrogen binding energies for the primary and secondary adsorption sites in the Cd- and Zn-based metal organic framework-5 (MOF-5) were studied using density functional theory. Out of the three exchange-correlation functionals employed in our study, we find that the local density approximation yields a qualitatively correct description of the interaction strengths of H(2) in MOF-5 systems. The H(2) adsorption energies for all trapping sites in Zn- and Cd-based MOF-5 are seen to be of the same order of magnitude but with a generally stronger binding in Cd-based MOF-5 as compared to Zn-based MOF-5. In particular, the H(2) binding energy at the secondary adsorption sites in Cd-based MOF-5 is increased by around 25% compared to Zn-based MOF-5. This result suggests that Cd-based MOF-5 would be better suited to store hydrogen at higher temperatures than Zn-based MOF-5.