Molecular dynamics simulation of the sintering of metallic nanoparticles

The sintering of two different-sized nickel nanoparticles is simulated by a molecular dynamics method in this work. The particles are partitioned into different regimes where tracing atoms are arranged to investigate the sintering kinetics. The detailed sintering process of two nanoparticles, 3.52 and 1.76 nm in diameter, respectively, is subsequently examined by the shrinkage ratio, gyration radius, mean square displacement, sintering diffusivity, and activation energy. A three-stage sintering scenario is established, and the layered structure shows a regime dependent behavior of diffusivity during the sintering process. Besides the surface diffusion, sintering of different-sized nanoparticles is found to be affected by a few other mechanisms.     

Modeling and simulation of titanium dioxide nanoparticle synthesis with finite-rate sintering in planar jets

Numerical simulations of titanium dioxide nanoparticle synthesis in planar, non-premixed diffusion flames are performed. Titania is produced by the oxidation of titanium tetrachloride using a methane–air flame. The flow field is obtained using the two-dimensional Navier–Stokes equations. The methane–air flame and oxidation of titanium tetrachloride are modeled via one-step reactions. Evolution of the particle field is obtained via a nodal method which accounts for nucleation, condensation, coagulation, and coalescence with finite-rate sintering. The modeling of finite-rate sintering is accomplished via the use of uniform primary-particle size distribution. Simulations are performed at two different jet-to-co-flow velocity ratios as well as with finite-rate and instantaneous sintering models. In doing so we elucidate the effect of fluid mixing and finite-rate sintering on the particle field. Results show that highly agglomerated particles are found on the periphery of the eddies, where the collisions leading to nanoparticle coagulation occur faster than nanoparticle coalescence.     

Structural transformations in single-crystal iron during shock-wave compression and tension: Molecular dynamics simulation

The molecular dynamics method is used to simulate shock-wave propagation in the [100] direction of a single-crystal bcc iron target in order to study structural transformations in compression and rarefaction waves and the mechanisms of spall fracture. The specific features of structural transformations near the lateral target surface have been analyzed.     

Control of nanoparticle size and agglomeration through electric-field-enhanced flame synthesis

The isolated study of electrophoretic transport of nanoparticles (that are innately charged through thermionic emission), with no ionic wind, has been conducted under uniform electric fields. Titania nanoparticles are produced using a burner-supported low-pressure premixed flame in a stagnation-point geometry from corresponding organometallic vapor precursor. The material processing flow field is probed in-situ using laser-induced fluorescence (LIF) to map OH-radical concentrations and gas-phase temperatures. The experimental results of particle growth under different applied electric fields are compared with computations using monodisperse and sectional models. The results show that such electric field application can decrease aggregate particle size (e.g. from 40 to 18 nm), maintain metastable phases and particle crystallinity, and non-monotonically affect primary particle size (e.g. from 6 to 5 nm) and powder surface area. A specific surface area (SSA) for anatase titania nanopowder of 310 m²/g, when synthesized under an applied electric field of 125 V/cm, is reported. Results are also given for the synthesis of alumina nanoparticles.     

Molecular dynamics simulation study of deep hole drilling in iron by ultrashort laser pulses

Classical molecular dynamics simulation technique is applied for investigation of the iron ablation by ultrashort laser pulses at conditions of deep hole for the first time. Laser pulse duration of 0.1 ps at wavelength of 800 nm is considered. The evolution of the ablated material in deep hole geometry differs completely from the free expansion regime as two major mechanisms are important for the final hole shape. The first one is the deposition of the ablated material on the walls, which narrows the hole at a certain height above its bottom. The second mechanism is related to ablation of the material from the walls (secondary ablation) caused by its interaction with the primary ablated particles. Properties of the secondary ablated particles in terms of the velocity and the angular distribution are obtained. The material removal efficiency is estimated for vacuum or in Ar environment conditions. In the latter case, the existence of well-defined vapor cloud having low center of the mass velocity is found. The processes observed affect significantly the material expulsion and can explain the decrease of the drilling rate with the hole depth increase, an effect observed experimentally.     

Molecular dynamics simulation of neck growth in laser sintering of different-sized gold nanoparticles under different heating rates

The neck growth in the laser sintering of different-sized gold (100) nanoparticles under different heating rates is investigated using a molecular dynamics method. The numerical simulations are carried out for four pairs of two spherical nanoparticles under three different heating rates. For each pair, one nanoparticle has the same diameter of 4 nm and the other nanoparticle’s diameter is varied, ranging from 4 nm to 20 nm. The results show that the solid state sintering automatically takes place by local potential at room temperature. The stable neck width increases as the size of the other nanoparticle increases. Once the limit stable neck width is reached, it no longer is affected by the nanoparticle size. For the subsequent laser heating to the same final temperature, a lower heating rate results in a larger stable neck width due to the longer sintering process. The neck growth mechanisms and rate under various sintering conditions are discussed.     

气体分子与纳米粒子碰撞的分子动力学仿真分析

当气体分子与纳米粒子碰撞的时候,纳米粒子传输理论预测到当纳米粒子的直径由小变大时,碰撞会由镜面反射转化为漫反射.文章利用分子动力学仿真研究了气体分子与纳米粒子碰撞的过程.在验证了这种转化存在同时,又探讨了碰撞转化的机理,即漫反射的起因.仿真结果揭示了漫反射的起因是由于纳米粒子对气体分子的吸附作用.这种吸附作用是由于纳米粒子对能量的容纳特性而产生的.     

气体分子与纳米粒子碰撞的分子动力学仿真分析

当气体分子与纳米粒子碰撞的时候,纳米粒子传输理论预测到当纳米粒子的直径由小变大时,碰撞会由镜面反射转化为漫反射.文章利用分子动力学仿真研究了气体分子与纳米粒子碰撞的过程.在验证了这种转化存在同时,又探讨了碰撞转化的机理,即漫反射的起因.仿真结果揭示了漫反射的起因是由于纳米粒子对气体分子的吸附作用.这种吸附作用是由于纳米粒子对能量的容纳特性而产生的.     

Molecular dynamics simulation of zirconia melting

The melting point for the tetragonal and cubic phases of zirconia (ZrO₂) was computed using Z-method microcanonical molecular dynamics simulations for two different interaction models: the empirical Lewis-Catlow potential versus the relatively new reactive force field (ReaxFF) model. While both models reproduce the stability of the cubic phase over the tetragonal phase at high temperatures, ReaxFF also gives approximately the correct melting point, around 2900 K, whereas the Lewis-Catlow estimate is above 6000 K.     

Molecular dynamics simulation of effect of liquid layering around the nanoparticle on the enhanced thermal conductivity of nanofluids

The effect of the molecular layering at liquid–solid interface on the thermal conductivity of the nanofluid is investigated by an equilibrium molecular dynamics simulation. By tracking the position of the nanoparticle and the liquid atoms around the spherical nanoparticle, it was found that a thin layer of liquid is formed at the interface between the nanoparticle and liquid; this thin layer will move with the Brownian motion of the nanoparticle. Through the analysis of the density distribution of the liquid near the nanoparticle, it is found that more argon atoms are attracted to form the layer around the nanoparticle when the diameter of the nanoparticle is larger, and therefore lead to the more significant enhancement of the thermal conductivity of the nanofluid.     

Numerical simulation of flame dynamics associated with negative velocity induced by deformed flame shape

In this study, the influence of the negative velocity field formed ahead of an abruptly deformed flame tip on the propagation behaviour of a laminar premixed flame is numerically investigated. A strong deformation in the flame front is induced by imposing a very narrow, in-line pre-heating zone in the unburned region. The simulation is performed under low Mach number approximation by using a multi-scale multi-physics Computational Fluid Dynamics (CFD) solver FrontFlow/Red with one-step finite rate chemistry in order to track the time-dependent flame dynamics. The computed results unveil that the flame front is deformed significantly within a short time due to the narrow in-line pre-heating effect. The flame deformation induces a strong negative velocity field ahead of the deformed flame tip, acting in the direction of propagation, which gives rise to a strong pair vortex. This strong pair vortex interacts with the flame tip and then slides down along the flame surface in the upstream direction during propagation. This flame-vortex interaction causes further deformation in the flame surface in the upstream direction, and consequently, the flame exhibits a wave-like surface, which enhances the flame propagation speed. The auto-generation of a strong pair vortex ahead of the flame front due to the localised thermal input could be applied as one of the methods to control the combustion externally. It is also expected that the results obtained in this study could have a significant impact on the detailed understanding of the local thermo-fluid dynamical interaction process of turbulent combustion in practical combustors.     

Modelling nanoparticle dynamics: coagulation,sintering, particle inception and surface growth

In this paper we investigated a stochastic particle method (SPM) for solving an extension to the sintering–coagulation equation and modelled two particle systems: the production of SiO₂ and TiO₂. A new mass-flow stochastic algorithm to find numerical solutions to the particle model is stated. The stochastic method calculates fully the evolution of the bivariate particle size distribution (PSD) and is computationally very efficient in comparison to traditional finite element methods. The SPM was compared to a bivariate sectional method for a system with coagulation and sintering as the only mechanisms. The results obtained agree closely to those in the literature and were obtained in a small fraction of the time. An extended model with particle inception and surface growth was then used to model the TiCl₄ → TiO₂ system under various conditions. At low precursor concentration the effect of varying temperature was investigated, whilst at high precursor concentration the effect of surface growth on the system was explored. The results agree well with the conclusions reached previously in the literature.     

Molecular dynamics simulation of the formation of twin boundaries during agglomeration of nanoparticles

The processes controlling early stages of agglomeration of nanoparticles have been investigated by the molecular dynamics method. It has been established that the formation of boundaries with twin misorientation is the main mechanism of structural relaxation during primary agglomeration of nanoparticles. It has been shown that an increase in the temperature leads to an increase in the number of twin boundaries and that their mutual arrangement depends on the misorientation of the nanoparticles. In the case where twin boundaries are noncoplanar, structure relaxation results in the formation of pentagonal twin boundaries. The role of twinning in the formation of interfaces upon compaction of nanoparticles has been discussed.     

Molecular dynamics simulation of cylindrical Richtmyer-Meshkov instability

The microscopic-scale Richtmyer-Meshkov(RM) instability of a single-mode Cu-He interface subjected to a cylindrically converging shock is studied through the classical molecular dynamics simulation. An unperturbed interface is first considered to examine the flow features in the convergent geometry, and notable distortions at the circular inhomogeneity are observed due to the atomic fluctuation. Detailed processes of the shock propagation and interface deformation for the single-mode interface impacted by a converging shock are clearly captured. Different from the macroscopic-scale situation, the intense molecular thermal motions in the present microscale flow introduce massive small wavelength perturbations at the single-mode interface, which later significantly impede the formation of the roll-up structure. Influences of the initial conditions including the initial amplitude,wave number and density ratio on the instability growth are carefully analyzed. It is found that the late-stage instability development for interfaces with a large perturbation does not depend on its initial amplitude any more. Surprisingly, as the wave number increases from 8 to 12, the growth rate after the reshock drops gradually. The distinct behaviors induced by the amplitude and wave number increments indicate that the present microscopic RM instability cannot be simply characterized by the amplitude over wavelength ratio(η). The pressure history at the convergence center shows that the first pressure peak caused by the shock focusing is insensitive to η, while the second one depends heavily on it.     

Influence of synthesis parameters on iron nanoparticle size and zeta potential

Zero valent iron nanoparticles are of increasing interest in clean water treatment applications due to their reactivity toward organic contaminants and their potential to degrade a variety of compounds. This study focuses on the effect of organophosphate stabilizers on nanoparticle characteristics, including particle size distribution and zeta potential, when the stabilizer is present during nanoparticle synthesis. Particle size distributions from DLS were obtained as a function of stabilizer type and iron precursor (FeSO₄·7H₂O or FeCl₃), and nanoparticles from 2 to 200 nm were produced. Three different organophosphate stabilizer compounds were compared in their ability to control nanoparticle size, and the size distributions obtained for particle volume demonstrated differences caused by the three stabilizers. A range of stabilizer-to-iron (0.05–0.9) and borohydride-to-iron (0.5–8) molar ratios were tested to determine the effect of concentration on nanoparticle size distribution and zeta potential. The combination of ferrous sulfate and ATMP or DTPMP phosphonate stabilizer produced stabilized nanoparticle suspensions, and the stabilizers tested resulted in varying particle size distributions. In general, higher stabilizer concentrations resulted in smaller nanoparticles, and excess borohydride did not decrease nanoparticle size. Zeta potential measurements were largely consistent with particle size distribution data and indicated the stability of the suspensions. Probe sonication, as a nanoparticle resuspension method, was minimally successful in several different organic solvents.     

Molecular dynamics simulation of compression of single-layer graphene

The compression of a single-layer graphene sheet in the “zigzag” and “armchair” directions has been investigated using the molecular dynamics method. The distributions of the xy and yx stress components are calculated for atomic chains forming the graphene sheet. A graphene sheet stands significant compressive stresses in the “zigzag” direction and retains its integrity even at a strain of ～0.35. At the same time, the stresses which accompany the compressive deformation of single-layer graphene in the “armchair” direction are more than an order in magnitude lower than corresponding characteristics for the “zigzag” direction. A compressive strain of ～0.35 in the “armchair” direction fractures the graphene sheet into two parts.     

Molecular dynamics simulation of thermodynamic properties of YAG

In this paper we study the thermodynamic properties of Y分子动力学;热力学;扩散;热学YAG, diffusion, elastic constant, molecular dynamicsProject supported by the National Natural Science Foundation of China (Grant No~10744002).2/2/2007 12:00:00 AMIn this paper we study the thermodynamic properties of Y3Al5O12 （YAG） by using molecular dynamic method combined with two- and three-body potentials. The dependences of melting process, elastic constant and diffusion coefficient on temperature of crystal YAG are simulated and compared with the experimental results. Our results show that anion O has the biggest self-diffusivity and cation Y has the smallest self-diffusivity in a crystal YAG. The calculated diffusion activation energies of ions O, Al and Y are 282.55, 439.46, 469.71kJ/mol, respectively. Comparing with experimental creep activation energy of YAG confirms that cation Y can restrict the diffusional creep rate of crystal YAG.     

Molecular dynamics simulation of brittle fracture in silicon

Brittle fracture in silicon is simulated with molecular dynamics utilizing a modified embedded atom method potential. The simulations produce propagating crack speeds that are in agreement with previous experimental results over a large range of fracture energy. The dynamic fracture toughness is found to be equal to the energy consumed by creating surfaces and lattice defects in agreement with theoretical predictions. The dynamic fracture toughness is approximately 1/3 of the static strain energy release rate, which results in a limiting crack speed of 2/3 of the Rayleigh wave speed.     

Ni-Co合金快淬过程的分子动力学模拟

采用分子动力学模拟熔体旋淬技术的合金快淬过程,在不同的冷却速度下研究Ni-Co合金在快淬后的结构特征.模拟发现:Ni-Co二元合金的凝固过程对冷却速率的变化较为敏感,体系对形成非晶对冷却速率要求较高,约在80 K/ps以上,可以采用增加添加元素的方法来降低材料对冷却速率的要求;在75 K/ps的冷却速率下合金最终完全晶...