Mechanical properties of connected carbon nanorings via molecular dynamics simulation

Stable, carbon nanotori can be constructed from nanotubes. In theory, such rings could be used to fabricate networks that are extremely flexible and offer a high strength-to-density ratio. As a first step towards realizing such nanochains and nanomaile, the mechanical properties of connected carbon nanorings were investigated via molecular dynamics simulation. The Young's modulus, extensibility and tensile stength of nanorings were estimated under conditions that idealize the constraints of nanochains and nanomaile. The results indicate nanorings are stable under large tensile deformation. The calculated Young's modulus of nanorings was found increase with deformation from 19.43 GPa to 121.94 GPa (without any side constraints) and from 124.98 GPa to 1.56 TPa (with side constraints). The tensile strength of unconstrained and constrained nanorings is estimated to be 5.72 and 8.522 GPa, respectively. The maximum strain is approximately 39% (nanochains) and 25.2% (nanomaile), and these deformations are completely reversible

New concepts of metallic bonding based on valence-bond ideas

From generalized-valence-bond calculations on numerous Li-atom clusters (Li_n and Li_n^+, n <- 13), we conclude that the optimum bonding involves singly occupied orbitals localized interstitially (in tetrahedra). Rules based on the calculations are used to predict low-energy isomers (leading for Li^+_(13) to low-symmetry structures that are significantly more stable than the icosahedron but retain local fivefold-symmetry axes) and are applied to infinite metallic systems

Mechanically bonded macromolecules

Mechanically bonded macromolecules constitute a class of challenging synthetic targets in polymer science. The controllable intramolecular motions of mechanical bonds, in combination with the processability and useful physical and mechanical properties of macromolecules, ultimately ensure their potential for applications in materials science, nanotechnology and medicine. This tutorial review describes the syntheses and properties of a library of diverse mechanically bonded macromolecules, which covers (i) main-chain, side-chain, bridged, and pendant oligo/polycatenanes, (ii) main-chain oligo/polyrotaxanes, (iii) poly[c2]daisy chains, and finally (iv) mechanically interlocked dendrimers. A variety of highly efficient synthetic protocols—including template-directed assembly, step-growth polymerisation, quantitative conjugation, etc.—were employed in the construction of these mechanically interlocked architectures. Some of these structures, i.e., side-chain polycatenanes and poly[c2]daisy chains, undergo controllable molecular switching in a manner similar to their small molecular counterparts. The challenges posed by the syntheses of polycatenanes and polyrotaxanes with high molecular weights are contemplated

Projective covers and minimal free resolutions

Using a generalization of the definition of the projective cover of a module, a special type of surjective free resolution, known as the projective cover of a complex, may be defined. The projective cover is shown to be a direct summand of every surjective free resolution and to be the direct sum of the minimal free resolution and an exact complex. Necessary and sufficient conditions for the projective cover and minimal free resolution to be identical are discussed

Temperature Dependence of Blue Phosphorescent Cyclometalated Ir(III) Complexes

The photophysical properties for a series of facial (fac) cyclometalated Ir(III) complexes (fac-Ir(C^N)_3 (C^N = 2-phenylpyridyl (ppy), 2-(4,6-difluorophenyl)pyridyl (F2ppy), 1-phenylpyrazolyl (ppz), 1-(2,4-difluorophenyl)pyrazolyl) (F2ppz), and 1-(2-(9,9′-dimethylfluorenyl))pyrazolyl (flz)), fac-Ir(C^N)_2(C^N′) (C^N = ppz or F2ppz and C^N′ = ppy or F2ppy), and fac-Ir(CC′)_3 (C^C′ = 1-phenyl-3-methylbenzimidazolyl (pmb)) have been studied in dilute 2-methyltetrahydrofuran (2-MeTHF) solution in a temperature range of 77−378 K. Photoluminescent quantum yields (Φ) for the 10 compounds at room temperature vary between near zero and unity, whereas all emit with high efficiency at low temperature (77 K). The quantum yield for fac-Ir(ppy)_3 (Φ = 0.97) is temperature-independent. For the other complexes, the temperature-dependent data indicates that the luminescent efficiency is primarily determined by thermal deactivation to a nonradiative state. Activation energies and rate constants for both radiative and nonradiative processes were obtained using a Boltzmann analysis of the temperature-dependent luminescent decay data. Activation energies to the nonradiative state are found to range between 1600 and 4800 cm^−1. The pre-exponential factors for deactivation are large for complexes with C^N ligands (1011−1013 s^−1) and significantly smaller for fac-Ir(pmb)_3 (109 s^−1). The kinetic parameters for decay and results from density functional theory (DFT) calculations of the triplet state are consistent with a nonradiative process involving Ir−N (Ir−C for fac-Ir(pmb)_3) bond rupture leading to a five-coordinate species that has triplet metal-centered (^3MC) character. Linear correlations are observed between the activation energy and the energy difference calculated for the emissive and ^3MC states. The energy level for the ^3MC state is estimated to lie between 21700 and 24000 cm^−1 for the fac-Ir(C^N)_3 complexes and at 28000 cm^−1 for fac-Ir(pmb)_3

Passive CO₂ removal in urban soils:evidence from brownfield sites

Management of urban brownfield land can contribute to significant removal of atmospheric CO2 through the development of soil carbonate minerals. However, the potential magnitude and stability of this carbon sink is poorly quantified as previous studies address a limited range of conditions and short durations. Furthermore, the suitability of carbonate-sequestering soils for construction has not been investigated. To address these issues we measured total inorganic carbon, permeability and ground strength in the top 20 cm of soil at 20 brownfield sites in northern England, between 2015 and 2017. Across all sites accumulation occurred at a rate of 1–16 t C ha−1 yr−1, as calcite (CaCO3), corresponding to removal of approximately 4–59 t CO2 ha−1 yr−1, with the highest rate in the first 15 years after demolition. C and O stable isotope analysis of calcite confirms the atmospheric origin of the measured inorganic carbon. Statistical modelling found that pH and the content of fine materials (combined silt and clay content) were the best predictors of the total inorganic carbon content of the samples. Measurement of permeability shows that sites with carbonated soils possess a similar risk of run-off or flooding to sandy soils. Soil strength, measured as in-situ bearing capacity, increased with carbonation. These results demonstrate that the management of urban brownfield land to retain fine material derived from concrete crushing on site following demolition will promote calcite precipitation in soils, and so offers an additional CO2 removal mechanism, with no detrimental effect on drainage and possible improvements in strength. Given the large area of brownfield land that is available for development, the contribution of this process to CO2 removal by urban soils needs to be recognised in CO2 mitigation policies

Sodium Diffusion through Aluminum-Doped Zeolite BEA System: Effect of Water Solvation

To investigate the effect of hydration on the diffusion of sodium ions through the aluminum-doped zeolite BEA system (Si/Al = 30), we used the grand canonical Monte Carlo (GCMC) method to predict the water absorption into aluminosilicate zeolite structure under various conditions of vapor pressure and temperature, followed by molecular dynamics (MD) simulations to investigate how the sodium diffusion depends on the concentration of water molecules. The predicted absorption isotherm shows first-order-like transition, which is commonly observed in hydrophobic porous systems. The MD trajectories indicate that the sodium ions diffuse through zeolite porous structures via hopping mechanism, as previously discussed for similar solid electrolyte systems. These results show that above 15 wt % hydration (good solvation regime) the formation of the solvation cage dramatically increases sodium diffusion by reducing the hopping energy barrier by 25% from the value of 3.8 kcal/mol observed in the poor solvation regime

Understanding and Applying Ecological Principles in Cities

Renaturing cities requires a thorough understanding of how plants and animals interact with the urban environment and humans. But cities are a challenging environment for ecologists to work in, with high levels of heterogeneity and rapid rates of change. In addition, the hostile conditions often found in cities mean that each city, and region of a city, can have their own unique geographical context. In this chapter, we contrast urban ecological research in the UK and Brazil, to demonstrate the challenges and approaches needed to renature cities. In so doing, we provide a platform for global transferability of these locally contextualised approaches. The UK has a long history of urbanisation and, as a result of increasing extinction debts over 200 years, well-established urban ecological research. Research is generally focused on encouraging species back into the city. In contrast, Brazil is a biodiversity hotspot with relatively rich urban flora and fauna. This rich ecosystem is imperilled by current rapid urbanisation and lack of support for urban nature by city-dwellers. By working together and transferring expertise, UK and Brazilian researchers stand a better chance of understanding urban ecological processes and unlocking renaturing processes in each location. We present one such method for applying ecological knowledge to cities, so-called Ecological Engineering, in particular by discussing ecomimicry—the adaptive approach needed to apply global ecological principles to local urban challenges. By reading the ecological landscape in which urban developments sit and applying tailored green infrastructure solutions to new developments and greenspaces, cities may be able to reduce the rate at which extinction debt is accumulated

VETA-1 x ray detection system

The alignment and X-ray imaging performance of the Advanced X-ray Astrophysics Facility (AXAF) Verification Engineering Test Article-I (VETA-I) was measured by the VETA-I X-Ray Detection System (VXDS). The VXDS was based on the X-ray detection system utilized in the AXAF Technology Mirror Assembly (TMA) program, upgraded to meet the more stringent requirements of the VETA-I test program. The VXDS includes two types of X-ray detectors: (1) a High Resolution Imager (HRI) which provides X-ray imaging capabilities, and (2) sealed and flow proportional counters which, in conjunction with apertures of various types and precision translation stages, provide the most accurate measurement of VETA-I performance. Herein we give an overview of the VXDS hardware including X-ray detectors, translation stages, apertures, proportional counters and flow counter gas supply system and associated electronics. We also describe the installation of the VXDS into the Marshall Space Flight Center (MSFC) X-Ray Calibration Facility (XRCF). We discuss in detail the design and performance of those elements of the VXDS which have not been discussed elsewhere; translation systems, flow counter gas supply system, apertures and thermal monitoring system

A humanness dimension to visual object coding in the brain

Neuroimaging studies investigating human object recognition have primarily focused on a relatively small number of object categories, in particular, faces, bodies, scenes, and vehicles. More recent studies have taken a broader focus, investigating hypothesized dichotomies, for example, animate versus inanimate, and continuous feature dimensions, such as biologically similarity. These studies typically have used stimuli that are identified as animate or inanimate, neglecting objects that may not fit into this dichotomy. We generated a novel stimulus set including standard objects and objects that blur the animate-inanimate dichotomy, for example, robots and toy animals. We used MEG time-series decoding to study the brain's emerging representation of these objects. Our analysis examined contemporary models of object coding such as dichotomous animacy, as well as several new higher order models that take into account an object's capacity for agency (i.e. its ability to move voluntarily) and capacity to experience the world. We show that early (0–200 ​ms) responses are predicted by the stimulus shape, assessed using a retinotopic model and shape similarity computed from human judgments. Thereafter, higher order models of agency/experience provided a better explanation of the brain's representation of the stimuli. Strikingly, a model of human similarity provided the best account for the brain's representation after an initial perceptual processing phase. Our findings provide evidence for a new dimension of object coding in the human brain – one that has a “human-centric” focus