The Self-Association of Graphane Is Driven by London Dispersion and Enhanced Orbital Interactions

We investigated the nature of the cohesive energy between graphane sheets via multiple CH center dot center dot center dot HC interactions, using density functional theory (DFT) including dispersion correction (Grimmes D3 approach) computations of n]graphane sigma dimers (n = 6-73). For comparison, we also evaluated the binding between graphene sheets that display prototypical pi/pi interactions. The results were analyzed using the block-localized wave function (BLW) method, which is a variant of ab initio valence bond (VB) theory. BLW interprets the intermolecular interactions in terms of frozen interaction energy (Delta E-F) composed of electrostatic and Pauli repulsion interactions, polarization (Delta E-pol), charge-transfer interaction (Delta E-CT), and dispersion effects (Delta E-disp). The BLW analysis reveals that the cohesive energy between graphane sheets is dominated by two stabilizing effects, namely intermolecular London dispersion and two-way charge transfer energy due to the sigma CH -> sigma*(HC) interactions. The shift of the electron density around the nonpolar covalent C-H bonds involved in the intermolecular interaction decreases the C-H bond lengths uniformly by 0.001 angstrom. The Delta E-CT term, which accounts for similar to 15% of the total binding energy, results in the accumulation of electron density in the interface area between two layers. This accumulated electron density thus acts as an electronic glue for the graphane layers and constitutes an important driving force in the self-association and stability of graphane under ambient conditions. Similarly, the double faced adhesive tape style of charge transfer interactions was also observed among graphene sheets in which it accounts for similar to 18% of the total binding energy. The binding energy between graphane sheets is additive and can be expressed as a sum of CH center dot center dot center dot HC interactions, or as a function of the number of C-H bonds.

Effect of Particle Concentration on Shape Deformation and Secondary Atomization Characteristics of a Burning Nanotitania Dispersion Droplet

Secondary atomization characteristics of burning bicomponent (ethanol-water) droplets containing titania nanoparticles (NPs) in dilute (0.5% and 1 wt.%) and dense concentrations (5% and 7.5 wt.%) are studied experimentally at atmospheric pressure under normal gravity. It is observed that both types of nanofuel droplets undergo distinct modes of secondary breakup, which are primarily responsible for transporting particles from the droplet domain to the flame zone. For dilute nanosuspensions, disruptive response is characterized by low intensity atomization modes that cause small-scale localized flame distortion. In contrast, the disruption behavior at dense concentrations is governed by high intensity bubble ejections, which result in severe disruption of the flame envelope.

A systematic asymptotic approach to determine the dispersion characteristics of structural-acoustic waveguides with arbitrary fluid loading

Structural-acoustic waveguides of two different geometries are considered: a 2-D rectangular and a circular cylindrical geometry. The objective is to obtain asymptotic expansions of the fluid-structure coupled wavenumbers. The required asymptotic parameters are derived in a systematic way, in contrast to the usual intuitive methods used in such problems. The systematic way involves analyzing the phase change of a wave incident on a single boundary of the waveguide. Then, the coupled wavenumber expansions are derived using these asymptotic parameters. The phase change is also used to qualitatively demarcate the dispersion diagram as dominantly structure-originated, fluid originated or fully coupled. In contrast to intuitively obtained asymptotic parameters, this approach does not involve any restriction on the material and geometry of the structure. The derived closed-form solutions are compared with the numerical solutions and a good match is obtained. (C) 2016 Elsevier Ltd. All rights reserved.

A Continuation Method of Parameter Inversion for Non-Equilibrium Convection-Dispersion Equation

Based on the homotopy mapping, a globally convergent method of parameter inversion for non-equilibrium convection-dispersion equations (CDEs) is developed. Moreover, in order to further improve the computational efficiency of the algorithm, a properly smooth function, which is derived from the sigmoid function, is employed to update the homotopy parameter during iteration. Numerical results show the feature of global convergence and high performance of this method. In addition, even the measurement quantities are heavily contaminated by noises, and a good solution can be found.

Polysiloxane surfactants for the dispersion of carbon nanotubes in nonpolar organic solvents.

We develop two new amphiphilic molecules that are shown to act as efficient surfactants for carbon nanotubes in nonpolar organic solvents. The active conjugated groups, which are highly attracted to the graphene nanotube surface, are based on pyrene and porphyrin. We show that relatively short (C18) carbon tails are insufficient to provide stabilization. As our ultimate aim is to disperse and stabilize nanotubes in siloxane matrix (polymer and cross-linked elastomer), both surfactant molecules were made with long siloxane tails to facilitate solubility and steric stabilization. We show that the pyrene-siloxane surfactant is very effective in dispersing multiwall nanotubes, while the porphyrin-siloxane makes single-wall nanotubes soluble, both in petroleum ether and in siloxane matrix.

Characterizing Particle Dispersion by Image Analysis in ICFB

A new particle image technique was developed to analyze the dispersion of tracer particles in an internally circulating fluidized bed (ICFB). The movement course and the concentration distribution of tracer particles in the bed were imaged and the degree of inhomogeneity of tracer particles was analyzed. The lateral and axial dispersion coefficients of particles were calculated for various zones in ICFB. Results indicate that the lateral diffusion coefficient in the fluidized bed with uneven air distribution is significantly higher than that in uniform bubbling beds with even air distribution. The dispersion coefficients are different along bed length and height.

Stability and Bifurcation Bahaviour of Electrostatic Torsional NEMS Varactor Influenced by Dispersion Forces

The influences of Casimir and van der Waals forces on the nano-electromechanical systems (NEMS) electrostatic torsional varactor are studied. A one degree of freedom, the torsional angle, is adopted, and the bifurcation behaviour of the NEMS torsional varactor is investigated. There are two bifurcation points, one of which is a Hopf bifurcation point and the other is an unstable saddle point. The phase portraits are also drawn, in which periodic orbits are around the Hopf bifurcation point, but the periodic orbit will break into a homoclinic orbit when meeting the unstable saddle point.

Dispersion rheology of carbon nanotubes in a polymer matrix

We report on rheological properties of a dispersion of multiwalled carbon nanotubes in a viscous polymer matrix. Particular attention is paid to the process of nanotubes mixing and dispersion, which we monitor by the rheological signature of the composite. The response of the composite as a function of the dispersion mixing time and conditions indicates that a critical mixing time t* needs to be exceeded to achieve satisfactory dispersion of aggregates, this time being a function of nanotube concentration and the mixing shear stress. At shorter times of shear mixing t< t*, we find a number of nonequilibrium features characteristic of colloidal glass and jamming of clusters. A thoroughly dispersed nanocomposite, at t> t*, has several universal rheological features; at nanotube concentration above a characteristic value nc ∼2-3 wt. % the effective elastic gel network is formed, while the low-concentration composite remains a viscous liquid. We use this rheological approach to determine the effects of aging and reaggregation. © 2006 The American Physical Society.

Characteristics of multiwalled carbon nanotube nanobridges fabricated by poly(methylmethacrylate) suspended dispersion

Transport measurements were performed on individual PECVD grown MWCNT nanobridge structures. Temperature dependent conductance measurements show that as the temperature is decreased, the conductance also decreases. The nanotubes were able to carry high current densities with the observed maximum at ∼108 A/cm2. High volatile measurements reveal that the PECVD grown MWCNTs break down in segments of nanotube shells.

Fabrication of multiwalled carbon nanotube bridges by poly-methylmethacrylate suspended dispersion

Poly-methylmethacrylate suspended dispersion was used to fabricate multiwalled carbon nanotube (MWCNT) bridges. Using this technique, nanotubes could be suspended between metal electrodes without any chemical etching of the substrate. The electrical measurement on suspended MWCNT bridges shows that the room temperature resistance ranges from under a kω to a few Mω.

Study of multi-walled carbon nanotube structures fabricated by PMMA suspended dispersion

We report on the electrical characteristics of plasma enhanced chemical vapour deposition (PECVD)-grown, multi-walled carbon nanotube (MWCNT) devices made by a new fabrication method, PMMA suspended dispersion. This method makes it possible to suspend nanotubes between metal electrodes and to remove unwanted nanotubes from the substrate. The measurements show that the MWCNTs are metallic and able to maintain a current density ∼2×106 A/cm2 for more than 15 days with a maximum current density of ∼1.8×107 A/cm2. This high current density and reliability will make PECVD-grown MWCNTs applicable to field emission cathodes. © 2002 Elsevier Science B.V. All rights reserved.

Effects of Particle Size on Dilute Particle Dispersion in a Karman Vortex Street Flow

It is shown that in a Karman vortex street flow, particle size influences the dilute particle dispersion. Together with an increase of the particle size, there is an emergence of a period-doubling bifurcation to a chaotic orbit, as well as a decrease of the corresponding basins of attraction. A crisis leads the attractor to escape from the central region of flow. In the motion of dilute particles, a drag term and gravity term dominate and result in a bifurcation phenomenon.