Vapor–Liquid Equilibria of Alternative Refrigerants by Molecular Dynamics Simulations

Alternative refrigerants HFC-152a (CHF₂CH₃), HFC-143a (CF₃CH₃), HFC-134a (CF₃CH₂F), and HCFC-142b (CF₂ClCH₃) are modeled as a dipolar two-center Lennard–Jones fluid. Potential parameters of the model are fitted to the critical temperature and vapor–liquid equilibrium data. The required vapor–liquid equilibrium data of the model fluid are computed by the Gibbs–Duhem integration for molecular elongations L=0.505 and 0.67, and dipole moments *²=0, 2, 4, 5, 6, 7, and 8. Critical properties of the model fluid are estimated from the law of rectilinear diameter and critical scaling relation. The vapor–liquid equilibrium data are represented by Wagner equations. Comparison of the vapor–liquid equilibrium data based on the dipolar two-center Lennard–Jones fluid with data from the REFPROP database shows good-to-excellent agreement for coexisting densities and vapor pressure.     

Dislocations in the Crystallites of Nanocrystalline α-Fe —A Molecular Dynamics Study

Molecular dynamics simulations are carried out in order to Study the atomic structure of crystalline component of nanocrystalline α-Fe when it is consolidated from small grains. A two-dimensional computational block is used to simulate the consolidation process. All the preset dislocations in the original grains glide out of them in the consolidation process, but new dislocations can generate when the grain size is large enough. It shows that dislocations exist in the consolidated material rather than in the original grains. Whether dislocations exist in the crystalline component of the resultant model nano-material depends upon grain size. The critical value of grain size for dislocation generation appears to be about 9 nm. This result agrees with experiments qualitatively.     

Molecular Dynamics Simulations of Li x Mn2O4 Spinel Solid Solutions with Simple Potential Models

Molecular dynamics simulations of Li _x Mn₂O₄ (0 < x 1) spinel solid solutions were carried out with the use of simple pair potentials available in the literature. The results demonstrate that computer simulations using the existing potentials with an exponential repulsion term fail to adequately reproduce appreciable Li⁺ mobility in a stable (near-zero mobility of the manganese and oxygen ions) crystalline phase. Lennard-Jones potentials make it possible to simulate such a phase at high temperatures (on the order of 1000 K).     

Formation Process of Empty and Metal-Containing Fullerenes—Molecular Dynamics and FT-ICR Studies

Abstract

The formation mechanism of empty and metal-containing fullerene was studied through MD (molecular dynamics) simulations and FT-ICR (Fourier transform ion cyclotron resonance) mass spectroscopy of laser vaporized carbon cluster. Multi-body classical potential functions for metal-carbon and metal-metal interactions were constructed based on DFT (density functional theory) calculations of various forms of small clusters MC_n and M_n (M = La, Sc, Ni). Using the modified Brenner potential for carbon-carbon interaction, the clustering process starting from 500 isolated carbon atoms and 5 metal atoms in gas phase was simulated under the controlled temperature condition at 3000K. The difference of clustering process of La@C_n, Sc@C_n and NiC_n were compared with empty fullerene formation simulation.

FT-ICR mass spectrometer directly connected to the laser vaporization cluster beam source was implemented in order to experimentally study the clustering process. The increase of cluster nozzle pressure roughly corresponded to the later stage of the molecular dynamics simulation. The FT-ICR mass spectra of metal-carbon composite clusters were compared for various sample materials used for arc-discharge generation of metal-containing fullerene and SWNT (single-wall carbon nanotube); La, Y., Sc, Gd, Ce, Ca, and Ni-Y. Positive La-C, Y-C, Sc-C, Gd-C, Ce-C binary clusters commonly showed strong MC_2n+ signal in the range of 36 < 2n with intense magic numbers at MC₄₄ ⁺, MC₅₀ ⁺ and MC₆₀ ⁺. It was speculated that the even-numbered clusters corresponded to the annealed random caged clusters observed in the MD simulation.     

Molecular Dynamics Simulation of Tensile Deformation and Fracture of γ-TiAl with and without Surface Defects

Molecular dynamics simulation of uniaxial tension along [001] has been performed to study the influence of various surface defects on the initiation of plastic deformation and fracture of γ-TiAl single crystals.The results indicate that brittle fracture occurs in perfect bulk; surfaces and edges will be detrimental to the strength of materials and provide dislocation nucleation site. The defects on surfaces and edges cause further weakening with various effects depending on defect type, size, position and orientation,while the edge dimples are the most influential. For γ-TiAl rods with surface dimples, dislocations nucleate from an edge of the rod when dimples are small, dimple dislocation nucleation occurs only when the dimples are larger than a strain rate dependent critical size. The dislocations nucleated upon [001]tension are super dislocations with Burger vectors 011] or 1/2 112] containing four 1/6 112 partials. The effects of surface scratches are orientation and shape sensitive. Scratches parallel to the loading direction have little influence, while sharp ones perpendicular to the loading direction may cause crack and thus should be avoided. This simulation also shows that, any type of surface defect would lower strength,and cause crack in some cases. But some may facilitate dislocation nucleation and improve ductility of TiAl if well controlled.     

Metastable States in Liquid–Gas Phase Transition. Simulation by the Method of Molecular Dynamics

The method of molecular dynamics is used to investigate the p, , T-properties and the structural characteristics of the Lennard–Jones fluid in the stable and metastable states in the liquid–gas phase transition. The calculation results demonstrate the presence of phase separation in molecular models at close-to-spinodal supersaturation. The effect of the cut-off radius of the interparticle potential on the process of phase transformation is analyzed, and the position of spinodal of superheated liquid and supersaturated vapor is estimated.     

A Molecular Dynamics Simulation Study of the Self-Diffusion Coefficient and Viscosity of the Lennard–Jones Fluid

Self-diffusion coefficients and viscosities for the Lennard–Jones fluid were obtained from extensive equilibrium molecular dynamics simulations using the Einstein plot method. Over 300 simulated state points cover the entire fluid region from the low-density gas to the compressed liquid close to the melting line in the temperature range T*=Tk/=0.7 to 6.0. The translational–translational, translational–configurational, and configurational–configurational contributions to the viscosity are resolved over this broad range of fluid states, thus providing coherent insight into the nature of this transport property. The uncertainties of the simulation data are conservatively estimated to be 0.5% for self-diffusion coefficients and 2% for viscosities in the liquid region, increasing to 15% at low-density gaseous states.     

Synthesis and alteration of α-LiFeO2 by mechanochemical processes

The effects of grinding on a stoichiometric mixture of LiOH · H₂O and -FeOOH were studied. It was found that, in the course of grinding, losses of structural water occurred and a phase structurally related to disordered -LiFeO₂ was formed. X-ray diffraction data suggest the occurrence of an ordered phase as intermediate and both -Fe₂O₃ and -Fe₂O₃ were undetected during the comminution process. A prolonged mechanical treatment of this mixture originated an elimination of Li⁺ from the -LiFeO₂ structure and the appearance of the spinel phase, -LiFe₅O₈. Additionally, the mechanical activation of a sample of -LiFeO₂ prepared at high temperatures also leads to a similar rearrangement of cations. The structural transformation is explained with the help of a model in which the vacancies of Li⁺ created during grinding promote the migration of the Fe³⁺ ions from octahedral to tetrahedral sites.     

Molecular Resolution Nanostructure and Dynamics of the Deep Eutectic Solvent—Graphite Interface as a Function of Potential

Interest in deep eutectic solvents (DESs), particularly for electrochemical applications, has boomed in the past decade because they are more versatile than conventional electrolyte solutions and are low cost, renewable, and non-toxic. The molecular scale lateral nanostructures as a function of potential at the solid–liquid interface—critical design parameters for the use of DESs as electrochemical solvents—are yet to be revealed. In this work, in situ amplitude modulated atomic force microscopy complemented by molecular dynamics simulations is used to probe the Stern and near-surface layers of the archetypal and by far most studied DES, 1:2 choline chloride:urea (reline), at the highly orientated pyrolytic graphite surface as a function of potential, to reveal highly ordered lateral nanostructures with unprecedented molecular resolution. This detail allows identification of choline, chloride, and urea in the Stern layer on graphite, and in some cases their orientations. Images obtained after the potential is switched from negative to positive show the dynamics of the Stern layer response, revealing that several minutes are required to reach equilibrium. These results provide valuable insight into the nanostructure and dynamics of DESs at the solid–liquid interface, with implications for the rational design of DESs for interfacial applications.     

Analytical Formulation of (0°, ± α°) Braided Composites and Its Application In Crashworthiness Simulations

New types of tridimensional composite materials have been developed during the last few years. These new materials consist of braided, weft or warp knitted, and 3D woven composites preforms, and they present excellent out-of-plane mechanical properties. In this article, tubular energy absorbers, constituted of carbon-glass hybrid braided composites, are analyzed by means of the finite-element method, with explicit time integration of the equations of motion. A new material model for (0°, - f °) braided composites is introduced in a commercial finite-element code by means of user-developed subroutines. Braided materials with carbon fiber in the 0° direction and glass fibers in the - f ° directions are studied. Moreover, different fiber orientations and geometry of the triggering devices are analyzed in order to obtain the optimum configuration in terms of specific energy absorption.     

Investigation of α-phase and liquid uranium by the method of quantum molecular dynamics

The thermodynamic properties of α-phase and liquid uranium have been investigated by the method of quantum molecular dynamics. The isotherms and equilibrium properties (pressure, temperature, and internal energy) along isochores and isotherms have been calculated for densities of 16.5–41.1 g/cm³ and temperatures up to 60000 K. The dependence of the thermal pressure on temperature and internal energy is analyzed. The melting boundary of α-phase uranium is estimated. The shock adiabat is calculated for pressures up to 1.5 TPa based on the equation of state derived here. The results obtained are in good agreement with the published experimental data and the results of calculations according to different theoretical models.     

Dynamics of Polymeric Fluids：Part Ⅱ The Molecular Theory of Die Swell： Correlation of Ultimate Die Swelling Effect to the Molecular Parameters and the Operational Variables

A general expression for the correlation of the simple shear (tan ) to the molecular parameters and the shear rate ( ) was deduced. It shows that the simple shear (tan ) may be resolved into free recoil (recoverable strain) and viscous heating (unrecoverable strain). The magnitudes of the simple shears for recoil (tan E) and (tan V) for viscous heating not only depended on the molecular parameters and the operational variables, but also on the exponential fractions of the recoverable (1- ) and unrecoverable ( ) conformations for recoil and viscous heating. Therefore the magnitudes of the simple shears (tan E) for recoil and (tan V) for viscous heating are, respectively, expressed as the partition function to the (1- )th power and the partition function to the ( )th power. Thus correlations of the total recoil and the ultimately recoverable strains to the molecular parameters [ , a, η0, GN0 and (1- )] and the operational variables ( ), (L=D) and tr) were deduced respectively, which show that at very different shear rates ( ) the polymeric liquids may exhibit a very different viscoelastic behaviors. After introducing the uniform two-dimensional extension, the definition of swelling ratio and the ratio of L to D [De=(L/D)], two expressions for the ultimate die swelling effect and the ultimate extrudate swelling ratio BEVT5 to the molecular parameters [ , a, η0, GN0 and (1- )] and the operational variables ( , (L/D) and tr) were obtained. The two correlation expressions were verified by the experimental data of high-density polyethylene (HDPE) which shows that the two correlation expressions can be used to predict the correlations of the ultimate extrudate swelling behaviors of polymeric liquids to the molecular parameters and the operational variables.     

Dynamics of Polymeric Fluids: Part II The Molecular Theory of Die Swell: Correlation of Ultimate Die Swelling Effect to the Molecular Parameters and the Operational Variables

A general expression for the correlation of the simple shear (tan(?)) to the molecular parameters and the shear rate (γ) was deduced. It shows that the simple shear (tan(?)) may be resolved into free recoil (recoverable strain) and viscous heating (unrecoverable strain). The magnitudes of the simple shears for recoil (tan(?)E) and (tan(?)V) for viscous heating not only depended on the molecular parameters and the operational variables, but also on the exponential fractions of the recoverable (1-Wγ) and unrecoverable (W-γ) conformations for recoil and viscous heating. Therefore the magnitudes of the simple shears (tan(?)E) for recoil and (tan(?)V) for viscous heating are, respectively, expressed as the partition function to the (1-Wγ)th power and the partition function to the (Wγ)th power. Thus correlations of the total recoil and the ultimately recoverable strains to the molecular parameters [n', a,η0, GN0 and (1-Wγ)] and the operational variables (γ, (L/D) and tr) were deduced respectively, which show that at very different shear rates (0≤γ≤∞) the polymeric liquids may exhibit a very different viscoelastic behaviors. After introducing the uniform two-dimensional extension, the definition of swelling ratio and the ratio of L to D [De=(L/D)], two expressions for the ultimate die swelling effect and the ultimate extrudate swelling ratio BEVT5 to the molecular parameters [n', a,η0, GN0 and (1-Wγ)] and the operational variables (γ, (L/D) and tr) were obtained. The two correlation expressions were verified by the experimental data of high-density polyethylene (HDPE) which shows that the two correlation expressions can be used to predict the correlations of the ultimate extrudate swelling behaviors of polymeric liquids to the molecular parameters and the operational variables.     

Dynamics of Polymeric Fluids: Part Ⅱ The Molecular Theory of Die Swell: Correlation of Ultimate Die Swelling Effect to the Molecular Parameters and the Operational Variables

A general expression for the correlation of the simple shear(tanφ)to the molecular parameters and the shear rate(γ)was deduced. It shows that the simple shear(tanφ)may be resolved into free recoil(recoverable strain)and viscous heating(unrecoverable strain). The magnitudes of the simple shears for recoil(tanφE)and(tanφv)for viscous heating not only depended on the molecular parameters and the operational variables,but also on the exponential fractions of the recoverable(1-(W)γ)and unrecoverable((W)γ)conformations for recoil and viscous heating. Therefore the magnitudes of the simple shears(tanφE)for recoil and(tanφv)for viscous heating are, respectively, expressed as the partition function to the(1-(W)γ)th power and the partition function to the(-(W)γ)th power. Thus correlations of the total recoil and the ultimately recoverable strains to the molecular parameters [n', a, η0, G0NN and(1--(W)γ)] and the operational variables(·γ, (L/D)and tr)were deduced respectively, which show that at very different shear rates(0≤·γ≤∞)the polymeric liquids may exhibit a very different viscoelastic behaviors. After introducing the uniform two-dimensional extension, the definition of swelling ratio and the ratio of L to D [De=(L/D)], two expressions for the ultimate die swelling effect and the ultimate extrudate swelling ratio BEVT5 to the molecular parameters [n', a, η0, G0N and(1--(W)·γ)] and the operational variables(·γ,(L/D)and tr)were obtained. The two correlation expressions were verified by the experimental data of high-density polyethylene (HDPE) which shows that the two correlation expressions can be used to predict the correlations of the ultimate extrudate swelling behaviors of polymeric liquids to the molecular parameters and the operational variables.     

Synthesis of monodisperse and high-purity α-Si3N4 powder by carbothermal reduction and nitridation

Carbothermal reduction-nitridation method is an effective means for synthesizing Si₃N₄ powder. Herein, spherical monodisperse silica was used as silicon source. The effects of reaction temperature, nitrogen flow rate and Si₃N₄ seeds content on the products were studied. It was found that high-purity α-Si₃N₄ (>99.0 wt%) was synthesized from C/SiO₂ = 3:1 at 1400 °C, reaction time of 6 h and nitrogen flow rate of 800 ml/min. The powder, with an average size of 0.5 μm, showed good dispersity and regular morphology because spherical monodisperse silica could be completely coated with carbon. The more contact sites between SiO₂ and C, the higher concentration of SiO(g) would be produced in the initial stage. It also indicated that the nucleation rate of α-Si₃N₄ increased, thereby inhibiting the formation of an agglomerate phase and suppressing the grain growth of α-Si₃N₄. Furthermore, higher nitriding temperature and Si₃N₄ seeds content both decreased the grain size and increased β-Si₃N₄ content. The forming mechanism of non-agglomerated and submicron-sized α-Si₃N₄ was clarified.