Structure stability and high-temperature distortion resistance of trilayer complexes formed from graphenes and boron nitride nanosheets.

The molecular dynamics was employed to study the structure stability and high-temperature distortion resistance of a trilayer complex formed by a monolayer graphene sandwiched in bilayer boron nitride nanosheets (BN-G-BN) and graphenes (G-G-G). The investigation shows that the optimal interlayer distances are about 0.347 nm for BN-G-BN and 0.341 nm for G-G-G. Analysis and comparison of the binding energy, van der Waals interactions between layers and radial distribution function (RDF) revealed that the BN-G-BN achieves a more stable combined structure than G-G-G. The interlayer graphene in the trilayer complex nanosheets, especially the graphene in BN-G-BN, is more integrated than monolayer graphenes in a crystal structure. The structures at high temperature of 1500 K show that the BN-G-BN exhibits less distortion than G-G-G; especially, fixing the atomic positions on up-down layers can obviously further reduce structural deformation of interlayer graphene. The result further indicates that the high-temperature distortion resistance of interlayer graphene in the trilayer complex is related to both material type and conditions of constraints at the up-down layers.

Ultra-sensitive analysis of a cantilevered single-walled carbon nanocone-based mass detector.

The ultra-sensitivity of mass detectors using individual cantilevered single-walled carbon nanocone (SWCNC) resonators is first investigated. A higher-order gradient theory, derived at the atomic level, is applied for modeling SWCNC resonators. Numerical simulations using a mesh-free computational framework based on moving Kriging interpolation are conducted to investigate the mass sensitivity of cantilevered SWCNC resonators with extra mass loading as well as with equivalent single-walled carbon nanotube (SWCNT) resonators. Comparison of the magnitude of resonant frequency shifts, the key criterion for mass sensitivity, of these two kinds of resonators demonstrates a far higher mass sensitivity for SWCNC resonators than for SWCNT resonators, thus suggesting a new method for ultra-sensitive mass detection via SWCNC resonators. The dependence of the mass sensitivity of SWCNC resonators on height and top radii has been examined. A reduction in the height of SWCNC resonators gives rise to a considerable increase in mass sensitivity. The mass sensitivity of a 6 nm high SWCNC resonator can even reach a level of 10(-22) g. It is noteworthy that the top radii of SWCNC resonators have a slight effect on frequency shifts. Another interesting observed phenomenon is that a deviation in the height of 19.2° SWCNC resonators leads to little loss in precision of mass detection when the attached mass is smaller than 10(-20) g. This superior characteristic indicates that SWCNC-based mass detectors have great potential in practical applications.

Influence of hydroxyl groups on the adsorption of HCHO on TiO2-B(100) surface by first-principles study.

The adsorption of formaldehyde (HCHO) on both clean and hydroxylated TiO(2)-B(100) surfaces with terminal and bridging hydroxyl groups is systematically investigated by using first principles density functional theory calculations. The discussion is mainly focused on the two different chemical adsorption configurations of HCHO in periodicity (2 × 2), in which the C atom of HCHO is bonded with two coordinated O atoms on a step (Structure I) or on a terrace (Structure II). The study indicates that bridging hydroxyl groups on most of the adsorption sites near to HCHO will weaken the adsorption of HCHO, while terminal hydroxyl groups on most of adsorption sites will facilitate it. The investigation of the effects of hydroxyl groups and H(2)O molecule on HCHO in different periodicities shows that the terminal hydroxyl groups or H(2)O molecules have significantly facilitated the adsorption of H(2)O at larger periodicities, while bridging hydroxyl groups do not have this trend. The analysis of the adsorption mechanisms of HCHO molecules on both clean and hydroxylated surfaces indicate that the terminal hydroxyl groups can extract electrons from the surface and facilitate adsorption of HCHO due to the adsorption energy being higher than that on the clean surface, while the bridging hydroxyl groups donate electrons to the surface and weaken the adsorption. In all chemical adsorption configurations, HCHO acts as an electron acceptor. Interestingly, though the adsorptions are weaker, HCHO in Structure II gains more electrons on both the clean and hydroxylated surfaces than in Structure I. This unique mechanism provides a novel angle to understand the interaction of HCHO with the hydroxylated TiO(2) surface.

Position effects of single vacancy on transport properties of single layer armchair h-BNC heterostructure.

Based on certain single layer armchair h-BNC heterostructures, six molecular devices with different positions of single vacancy atoms are investigated to explain the modulating process of negative differential resistance (NDR) behaviors and rectifying performance. The results show that NDR behaviors can be observed clearly with vacancy atoms near the interface of graphene nano-ribbon and BN nano-ribbon, and rectifying performance can be enhanced obviously when there are vacancy atoms in the moiety of the BN nano-ribbon. The first-principles analysis of the microscopic nature reveals that strength of electronic transmission, evolutions of molecular orbitals and distributions of molecular states are the intrinsic responses to these transport properties.

High-temperature thermal stability and in-plane compressive properties of a graphene and a boron-nitride nanosheet.

The structural performance of graphene and boron-nitride nanosheet (BNNS) with zigzag and armchair types, when subjected to high temperatures, is investigated through molecular dynamics simulations. It is found that the degree of structure distortion is related to chirality; materials at high temperature of 3500 K, the zigzag nanosheet always exhibits less distortion than the armchair for the same material, and the BNNS exhibits less distortion than graphene for the same chirality. Graphene and BNNS with different in-plane compressive strains are optimized by using the Universal Force Field (UFF) method. It is found that there are two entirely different buckling modes, i.e., the lateral buckling of graphene begins to occur at the middle part, whereas buckling of BNNS begins to occur at near both ends and shows lateral deformation in two opposite directions. The coefficient of elasticity of graphene is slightly smaller than that of BNNS for the same chirality, the coefficient of elasticity of zigzag is slightly bigger than that of armchair for the same material, buckling strain of zigzag nanosheet is larger than that of armchair for the same material, and buckling strains of graphene are always larger than those of BNNS. These phenomena are also analyzed on the basis of radial distribution function (RDF) and system energy. The results indicate that there are thermal expansion anisotropy and planar stress anisotropy in a graphene and a BNNS. Among these materials, zigzag graphene has the highest resistance to compressive buckling but zigzag BNNS can have the highest resistance to distortion at high-temperature distortion and have high compression elasticity.

A collaborative emergency decision making approach based on BWM and TODIM under interval 2-tuple linguistic environment.

Emergencies require various emergency departments to collaborate to achieve timely and effective emergency responses. Thus, the overall performance of emergency response is influenced not only by the efficiency of each department alternative but also by the coordination effect among different department alternatives. This paper proposes a collaborative emergency decision making (CEDM) approach considering the synergy among different department alternatives based on the best-worst method (BWM) and TODIM (an acronym in Portuguese of interactive and multiple attribute decision making) method within an interval 2-tuple linguistic environment. First, the evaluation information provided by decision makers (DMs) is represented by interval 2-tuple linguistic variables to reflect and model the underlying diversity and uncertainty. On the basis of the DMs' evaluations, the individual and collaborative performance evaluations of multi-alternative combinations composed of different department alternatives are constructed. Then, the BWM is extended into interval 2-tuple linguistic environment to obtain the weights of evaluation criteria, where the group decision making is taken into account in an interval fuzzy mathematical programming model. Furthermore, to derive more practical and accurate decision results, an interval 2-tuple linguistic TODIM (ITL-TODIM) method is proposed by considering the DMs' psychological behaviours. In the developed ITL-TODIM method, both the gain and loss degrees of one alternative relative to another are simultaneously computed. Finally, a numerical example is presented to illustrate the applicability of the proposed method. Sensitivity and comparative analyses are also provided to demonstrate the effectiveness and advantages of the proposed approach.

High-temperature thermal stability and axial compressive properties of a coaxial carbon nanotube inside a boron nitride nanotube.

The structural performance of double-walled C(5, 5)@BN(10, 10) and C(5, 5)@C(10, 10) nanotubes subject to high temperatures is investigated through molecular dynamics simulations. It is found that the inner tube C(5, 5) in the C(5, 5)@BN(10, 10) exhibits less distortion than that in the C(5, 5)@C(10, 10) at annealing temperatures of 3500 and 4000 K. The C(5, 5)@BN(10, 10) and C(5, 5)@C(10, 10) models with different axial compressive strains are optimized using the universal force field (UFF) method. It is found that the critical buckling strains of the inner tubes in the C(5, 5)@BN(10, 10) and C(5, 5)@C(10, 10) are 12.74% and 9.1%, respectively. The critical buckling strain of the former is larger than that of the latter; although the former exhibits greater deformation and energy loss after buckling than does the latter. These phenomena are also analyzed on the basis of the radial distribution function (RDF) and system energy. The results of this study indicate that the outer tube boron nitride nanotube (BNNT) has a better protective effect on the inner tube than does the outer tube carbon nanotube (CNT) under both high-temperature and lower compressive strain conditions. In these cases, the thermal stability and compressive resistance properties of the C(5, 5)@BN(10, 10) are superior to those of the C(5, 5)@C(10, 10).

Nonlocal continuum model and molecular dynamics for free vibration of single-walled carbon nanotubes.

Free transverse, longitudinal and torsional vibrations of single-walled carbon nanotubes (SWCNTs) are investigated through nonlocal beam model, nonlocal rod model and verified by molecular dynamics (MD) simulations. The nonlocal Timoshenko beam model offers a better prediction of the fundamental frequencies of shorter SWCNTs, such as a (5, 5) SWCNT shorter than 3.5 nm, than local beam models. The nonlocal rod model is employed to study the longitudinal and torsional vibrations of SWCNT and found to enable a good prediction of the MD results for shorter SWCNTs. Nonlocal and local continuum models provide a good agreement with MD results for relatively longer SWCNTs, such as (5, 5) SWCNTs longer than 3.5 nm. The scale parameter in nonlocal beam and rod models is estimated by calibrations from MD results.

Atomistic insights into structure evolution and mechanical property of calcium silicate hydrates influenced by nuclear waste caesium.

The fundamental mechanisms underlying the influence of nuclear wastes on concrete properties remain poorly understood, especially at the molecular level. Herein, caesium ions (Cs+) are introduced into calcium silicate hydrates (CSH) to investigate its effect using molecular dynamics simulation. Structurally, a swelling phenomenon is observed, attributed to the CSH interlayer expansion as Cs+ occupies larger space than Ca2+. The diffusion of interlayer water, Ca2+ and Cs+, following an order of water > Cs+ > Ca2+, is accelerated with increasing Cs+ content, owing to three mechanisms: expanded interlayer space, weakened interfacial interaction, and loss of chemical bond stability. Mechanically, the Young's modulus and strength of CSH are degraded by Cs+ due to two mechanisms: (1) the load transfer ability of interlayer water and Ca2+ is weakened; (2) the load transfer provided by Cs+ is very weak. Additionally, a "hydrolytic weakening" mechanism is proposed to explain the mechanical degradation with increasing water content. This study also provides guidance for studying the influence of other wastes (like heavy metal ions) in concrete.

Interface enhancement of glass fiber/vinyl ester composites with carbon nanotubes synthesized from ethanol flames.

Multi-walled carbon nanotubes synthesized from ethanol flames (F-MWCNTs) and nanotubes functionalized by n-hexadecylamine (H-MWCNTs) were applied to the preparation of glass fiber/vinyl ester composites by overcoating the original glass fiber. Scanning electron microscopy of the composites containing multi-walled carbon nanotubes (MWCNTs) showed better bonding between the glass fiber and the resin matrix which may be attributed to the existence of a flexible interphase introduced by the nanotubes. It was found that the bonding in the composites treated with H-MWCNTs was much stronger. Moreover, the dynamic mechanical properties and impact strengths of the resulting composites were investigated. The results revealed that treating glass fiber with MWCNTs effectively improved the mechanical properties of the composite materials. Furthermore, the dynamic properties showed that H-MWCNTs have become integral parts of the crosslinked polymer structure, rather than acting as separate fillers.

Electron-conduction properties of Fe-Al alloy nanowires.

Helical and nonhelical shell structures of Fe-Al alloy nanowires are obtained using molecular dynamics (MD) and density functional theory (DFT) calculations. The electrical transport properties of alloy nanowires are investigated and compared with those of pure metallic aluminum and iron nanowires. The calculations indicate that the conductance of the Fe-Al alloy nanowire is less than that of the pure Al or Fe nanowires. The results show that the conductance of a carbon-coated Fe-Al alloy nanowire (28,7) is significantly larger than that of Fe-Al alloy nanowire. The difference in the electrical behavior of the Fe-Al alloy nanowire and the carbon-coated structure can be attributed to the two interfering pathways between the CNT and the alloy nanowire. The nonlinear feature of the current-voltage (I-V) for all alloy nanowires suggests that it does not follow the Ohmic pattern.

Experimental study on a comparison of typical premixed combustible gas-air flame propagation in a horizontal rectangular closed duct.

Research surrounding premixed flame propagation in ducts has a history of more than one hundred years. Most previous studies focus on the tulip flame formation and flame acceleration in pure gas fuel-air flame. However, the premixed natural gas-air flame may show different behaviors and pressure dynamics due to its unique composition. Natural gas, methane and acetylene are chosen here to conduct a comparison study on different flame behaviors and pressure dynamics, and to explore the influence of different compositions on premixed flame dynamics. The characteristics of flame front and pressure dynamics are recorded using high-speed schlieren photography and a pressure transducer, respectively. The results indicate that the compositions of the gas mixture greatly influence flame behaviors and pressure. Acetylene has the fastest flame tip speed and the highest pressure, while natural gas has a faster flame tip speed and higher pressure than methane. The Bychkov theory for predicting the flame skirt motion is verified, and the results indicate that the experimental data coincide well with theory in the case of equivalence ratios close to 1.00. Moreover, the Bychkov theory is able to predict flame skirt motion for acetylene, even outside of the best suitable expansion ratio range of 6

Methane molecules drive water molecules along diameter-gradient SWCNTs with junctions.

We report the transport behavior of water molecules along a system of coaxial single-walled carbon nanotubes (SWCNTs) of different diameters with junctions under the driving force of methane molecules. The junctions are potential barriers to the transport of water molecules through SWCNTs. However, methane molecules can overcome these potential barriers and pull the water molecules across the junction region from one compartment to the next. Although a junction is an obstacle to water transport through SWCNTs, the presence of more junctions gives methane molecules a longer lasting driving force that helps them to pull the water molecules out of the SWCNTs.

Effects of boron nitride impurities on the elastic properties of carbon nanotubes.

The molecular dynamics method is used in this paper to investigate the effect of boron nitride (BN) impurities on the elastic properties of armchair (5, 5) (10, 10) and zigzag (9, 0) (18, 0) single-walled carbon nanotubes (SWCNTs). The results show the Young's moduli of armchair (5, 5) (10, 10) and zigzag (9, 0) (18, 0) SWCNTs with no impurities to be 948 GPa, 901 GPa and 804 GPa, 860 GPa, respectively. When the armchair SWCNTs are doped with BN, their Young's modulus decreases slightly. However, an increase in the doping ratio beyond a certain point does not cause any further reduction in the modulus, which continues to fluctuate at about 800 GPa and 760 GPa, respectively. The zigzag SWCNTs behave somewhat differently. When they are doped with BN, their Young's moduli drop quickly, and then rise as the doping ratio increases until it reaches 100% (i.e. boron nitride nanotubes are formed), at which point the Young's moduli of the nanotubes are 780 GPa and 835 GPa, respectively, 97% that of the corresponding pure SWCNTs. The effect of a high ratio of BN on zigzag SWCNTs is thus negligible. The reasons for this phenomenon are analyzed according to the law of electron cloud coupling between two atoms, which comes from the local density approximation (LDA) and is based on density functional theory (DFT).

Structures and electronic transport of water molecular nanotubes embedded in carbon nanotubes.

In this paper, ice nanotubes confined in carbon nanotubes are investigated by molecular dynamics. The trigonal, square, pentagonal, and hexagonal water tubes are obtained, respectively. The current-voltage (I-V) curves of water nanotubes are found to be nonlinear, and fluctuations of conductance spectra of these ice nanotubes show that the transport properties of ice nanotubes are quite different from those of bulk materials. Our studies indicate that the conductance gap of ice nanotube is related to the difference value from the Fermi energy EF to the nearest molecular energy level E0. Increasing the diameter of a water molecular nanostructure results in the increase of the conductance.

Coupling of ab initio density functional theory and molecular dynamics for the multiscale modeling of carbon nanotubes.

A multiscale technique is developed that couples empirical molecular dynamics (MD) and ab initio density functional theory (DFT). An overlap handshaking region between the empirical MD and ab initio DFT regions is formulated and the interaction forces between the carbon atoms are calculated based on the second-generation reactive empirical bond order potential, the long-range Lennard-Jones potential as well as the quantum-mechanical DFT derived forces. A density of point algorithm is also developed to track all interatomic distances in the system, and to activate and establish the DFT and handshaking regions. Through parallel computing, this multiscale method is used here to study the dynamic behavior of single-walled carbon nanotubes (SWCNTs) under asymmetrical axial compression. The detection of sideways buckling due to the asymmetrical axial compression is reported and discussed. It is noted from this study on SWCNTs that the MD results may be stiffer compared to those with electron density considerations, i.e. first-principle ab initio methods.

Resonance analysis of multi-layered graphene sheets used as nanoscale resonators.

A stacked plate model for the vibration of multi-layered graphene sheets (MLGSs), in which the van der Waals (vdW) interaction between layers is described by an explicit formula, is presented. Explicit formulae are derived for predicting the natural frequencies of double- and triple-layered graphene sheets, and they clearly indicate the effect of vdW interaction on the natural frequencies. The natural frequencies are calculated for various numbers of layered graphene sheets, and the results show that the vdW interaction has no influence on the lowest natural frequency (classical frequency) of an MLGS but plays a significant role in all higher natural frequencies (resonant frequencies) for a given combination of m and n. The vibration modes that are associated with the classical frequencies for each sheet of an MLGS are identical. In contrast, the vibration modes that are associated with the resonant frequencies are non-identical and give various vibration patterns, which indicates that MLGSs are highly suited to use as high frequency resonators.