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.     

Size effect in the melting and freezing behaviors of Al/Ti core-shell nanoparticles using molecular dynamics simulations

The thermal stability of Ti@Al core/shell nanoparticles with different sizes and components during continuous heating and cooling processes is examined by a molecular dynamics simulation with embedded atom method. The thermodynamic properties and structure evolution during continuous heating and cooling processes are investigated through the characterization of the potential energy, specific heat distribution, and radial distribution function(RDF). Our study shows that, for fixed Ti core size, the melting temperature decreases with Al shell thickness, while the crystallizing temperature and glass formation temperature increase with Al shell thickness. Diverse melting mechanisms have been discovered for different Ti core sized with fixed Al shell thickness nanoparticles. The melting temperature increases with the Ti core radius. The trend agrees well with the theoretical phase diagram of bimetallic nanoparticles. In addition, the glass phase formation of Al–Ti nanoparticles for the fast cooling rate of 12 K/ps, and the crystal phase formation for the low cooling rate of 0.15 K/ps. The icosahedron structure is formed in the frozen 4366 Al–Ti atoms for the low cooling rate.     

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.     

Energetic and structural evolution of gold nanowire under heating process: A molecular dynamics study

The energetic and structural evolution of a squared gold nanowire under heating process is investigated via molecular dynamics with many-body potential. The simulations reveal that the nanowire undergoes distinct energetic and structural developments during the following four heating processes: low temperature, melting, breaking and high temperature. The cross-section of nanowire is found to change from a square to a circle shape with rising temperature at first. A neck is then found to be initiated above the overall melting point, followed by the formation of a two- to five-atom-thick chain structure before the breaking of neck. The nanowire transforms to a spherical cluster after the final breaking.     

Influence of Size on Melting Thermodynamics of Nanoparticles: Mechanism,Factors, Range,and Degree

Particle size plays a crucial role in melting process of nanoparticles, but the mechanism, factors, range, and degree of the size effect are still unclear. Here, the precise equations of the integral melting enthalpy and entropy with radius of nanoparticles are deduced, without any adjustable parameters, and the influencing mechanism and the factors are discussed. Experimentally, the melting of spherical nano‐Au with different radii (0.9–37.4 nm) is taken as a system to research the melting behavior of nanoparticles. Combining the results of theory and experiments, the influencing regularities, range, and degree are discussed. The results indicate that there are significant effects of particle size on the temperature, integral enthalpy, and integral entropy of melting, which decrease with the radius decreasing. These effects can be attributed to specific surface area, surface tension, and its temperature coefficient. When the radius exceeds 10 nm, specific surface area is the decisive factor, there exists the linear relationships of temperature, integral enthalpy, and integral entropy of melting with the reciprocal of radius. However, when the radius is less than 10 nm, the effects of surface tension and its temperature coefficient gradually hold the main position, the linear relations do not exist.     

Heat-transfer processes upon laser heating of inert-matrix-hosted inclusions

A model to describe the heating of metal inclusions in inert media by a laser radiation pulse with allowance for the heat-transfer and melting processes in the matrix and inclusion materials is proposed. The time regularities of the heating of the matrix and inclusions were examined, and the dependences of the maximum temperature on the particle surface on the laser pulse energy density and on the particle radius were obtained. Approximate formulae for the maximum heating temperature and for the radius of most heated particles are proposed. We show that melting processes result in a reduction of the maximum heating temperature and in an insignificant variation of the radius of most heated particles.     

Melting Behaviour of Shell-symmetric Aluminum Nanoparticles: Molecular Dynamics Simulation

用基于镶嵌原子方法势能的分子动力学模拟研究了含有561个原子的铝纳米粒子. 利用总势能和比热来计算铝纳米粒子的熔点：二十面体、十面体、切去顶端的八面体铝纳米粒子的熔点分别是540±10、500±10和520±10 K. 均方位移、键参数和回转半径的变化趋势与势能和比热的变化一致. 通过拟合均方位移得到了Kohlraush-William-Watts弛豫法则中的弛豫时间和伸缩参数,计算表明在高温区域弛豫时间和温度之间遵循标准阿伦尼乌斯关系.     

Decline of nucleation in the heating process with a high heating rate

The effect of the heating rate on the nucleation of metallic glass in a rapid heating process starting from the glass transition temperature is investigated. The critical nucleus radius increases with the increase of the temperature of the undercooling liquid. If the increment rate of the critical nucleus radius, owing to the heating process, is higher than the growth rate of the nuclei, the nuclei generated at the low temperature will become the embryos at the high temperature. This means that the high heating rate can make no nucleation happen in the heating process. In consideration of the interfacial energy, the growth rate of the nuclei increases with the increase of their size and the growth rate of the critical nucleus is zero. Thus, the lower heating rate can also make the nuclei decline partially. Finally, this theory is used to analyze the nucleation process during laser remelting metallic glass.     

Polymer chain swelling induced by dispersed nanoparticles

The dimensions of individual deuterated polystyrene (d-PS) chains in a well-dispersed mixture of protonated polystyrene and chemically identical nanoparticles was determined by neutron scattering. A 10%-20% increase in the radius of gyration of d-PS was found when the nanoparticles are homogeneously dispersed in the polymer, an effect that occurs only when the radius of gyration of the polymer is larger than the nanoparticle radius. These results are reconciled with the existing literature.     

高温电离气体中微细颗粒的加热

本文分析表明，自由分子流区高温电离气体中颗粒的团相加热、熔化、液相加热与强烈蒸发等加热阶段所需的时间，均正比于颗粒半径，其中蒸发阶段需时最长．颗粒表面温度高于约４０００Ｋ时，热电子发射开始显著影响颗粒的加热速率，辐射则在低得多的温度下（如２０００Ｋ）开始影响颗粒的加热．     

金属Cu熔化结晶过程的分子动力学模拟

采用常温、常压分子动力学模拟技术,研究了在周期性边界条件下,由８６４个Ｃｕ原子构成的模型系统的熔化、结晶过程。原子间相互作用势采用ＥＡＭ势。模拟结果表明：在连续升温过程中,金属Ｃｕ在１５２０Ｋ熔化;以不同的冷速进行冷却,在较慢冷却条件下,液态Ｃｕ在１０１０Ｋ结晶;当冷速较快时,液态Ｃｕ形成非晶态。分析了升降温过程中熔体偶分布函数、原子体积、能量、ＭＳＤ随温度的变化特征。     

Instantaneous in-flame measurement of soot volume fraction, primary particle diameter, and aggregate radius of gyration via auto-compensating laser-induced incandescence and two-angle elastic light scattering

A new combination of soot diagnostics employing two-angle elastic light scattering and laser-induced incandescence is described that is capable of producing non-intrusive, instantaneous, and simultaneous, in situ measurements of soot volume fraction, primary particle size, and aggregate radius of gyration within flames. Controlled tests of the new apparatus on a well-characterized laminar flame show good agreement with existing measurements in the literature. From a detailed and comprehensive Monte Carlo uncertainty analysis of the results, it was found that the uncertainty in all three measured parameters is dominated by knowledge of soot properties and aggregation behavior. The soot volume fraction uncertainty is dominated by uncertainty in the soot refractive index light absorption function; the primary particle diameter uncertainty is dominated by uncertainty in the fractal prefactor; while the uncertainty in the aggregate radius of gyration is dominated by the uncertainty in the width of the distribution of aggregate sizes.     

Pt-Au核-壳结构纳米粒子热稳定性的分子动力学研究

采用分子动力学方法结合嵌入原子势, 对Pt-Au核-壳纳米粒子的热稳定性进行了研究. 计算结果表明: Pt-Au纳米粒子的熔点明显高于Au纳米粒子而低于Pt纳米粒子. 通过计算Lindemann指数发现: 壳层中的Au首先熔化, 然后逐渐向内部扩展, 最终导致核中的Pt完全熔化; 熔化所经历的温度区间明显宽于单质纳米粒子, 而且该熔化过程呈现典型的两阶段熔化特征; 在两次熔化之间, 存在着固(核)液(壳)共存的结构. 关键词： 纳米粒子 熔化 分子动力学     

Molecular dynamics simulations on the melting of gold nanoparticles

Molecular dynamics is employed to study the melting of bulk gold and gold nanoparticles. PCFF, Sutton-Chen and COMPASS force fields are adopted to study the melting point of bulk gold and we find out that the Sutton-Chen force field is the most accurate model in predicting the melting point of bulk gold. Consequently, the Sutton-Chen force field is applied to study the melting points of spherical gold nanoparticles with different diameters. Variations of diffusion coefficient, potential energy and translational order parameter with temperature are analyzed. The simulated melting points of gold nanoparticles are between 615～1115 K, which are much lower than that of bulk gold (1336 K). As the diameter of gold nanoparticle drops, the melting point also descends. The melting mechanism is also analyzed for gold nanoparticles.     

Sintering of titanium dioxide nanoparticles: a comparison between molecular dynamics and phenomenological modeling

Molecular dynamics simulations were used to determine the melting points of anatase and rutile nanoparticles. The melting points decrease with decrease in particle diameter and are in reasonable agreement with the empirical formula derived by Buffat and Borel. The phenomenological model of Koch and Friedlander is unable to predict the temperature rise during initial stages of sintering with acceptable accuracy. It is argued that the Koch and Friedlander assumption of linear surface reduction rate upon sintering may be inadequate for the time scales under consideration. A theoretical model using direct area measurement from molecular dynamics simulations and a single adjustable parameter is able to predict temperature rise during initial stages of sintering within acceptable error limits.     

Isothermal Crystallization Kinetics and Melting Behavior of Poly(ethylene terephthalate)/Attapulgite Nanocomposites Studied by Step-scan DSC

The crystallization kinetics of poly(ethylene terephthalate)/attapulgite (AT) nanocomposites and their melting behaviors after isothermal crystallization from the melt were investigated by DSC and analyzed using the Avrami method. The isothermal crystallization kinetics showed that the addition of AT increased both the crystallization rate and the isothermal Avrami exponent of PET. Step-scan differential scanning calorimetry was used to study the influence of AT on the crystallization and subsequent melting behavior in conjunction with conventional DSC. The results revealed that PET and PET/AT nanocomposites experience multiple melting and secondary crystallization processes during heating. The melting behaviors of PET and PET/AT nanocomposites varied in accordance with the crystallization temperature and shifted to higher temperature with the increase of AT content and isothermal crystallization temperature. The main effect of AT nanoparticles on the crystallization of PET was to improve the perfection of PET crystals and weaken its recrystallization behavior.     

优化磁共振纳米医学中磁性纳米粒子的热疗效率

磁共振热疗(magnetic resonance hyperthermia)是近年来新兴的一种纳米医学治疗方法,由磁共振的硬件架构产生特定交变磁场,有效地加热磁性纳米粒子,以直接或间接地杀死癌细胞,体现诊疗一体化。提高磁性纳米粒子的加热效率是当前磁共振热疗领域亟待解决的难题之一。磁性纳米粒子的加热效率不仅与粒子本身的大小、性质以及尺寸分布有关,还和聚集状态有关。该研究利用3D Metropolis蒙特卡罗模拟方法,模拟了不同温度下磁性纳米粒子的磁共振热动力学行为及其团聚与分离现象;并通过修正过的郎之万方程,建立了相变临界温度与外加磁场频率的函数关系。模拟结果显示,磁性纳米粒子悬浮液中多聚体的相对含量随着温度的升高而降低,达到临界温度后,多聚体完全分离成单体;而提高交变磁场频率可以显著降低临界温度,且存在临界频率,高于此临界频率后临界温度不再受外加磁场频率影响,达到稳定。因而在临界频率下预热磁性纳米粒子悬浮液,使得多聚体分离成单体,可优化磁性纳米粒子的热疗效率。     

Irreversible energy flow in forced Vlasov dynamics

Molecular dynamics (MD) simulations are used to investigate the thermodynamic properties and structural changes of KCl spherical nanoparticles at various sizes (1064, 1736, 2800, 3648, 4224 and 5832 ions) upon heating. The melting temperature is dependent on both the size and shape of KCl models, and the behaviour of the first order phase transition is also found in the present work. The surface melting found here is different from the melting phenomena of KCl models or other alkali halides studied in the past. In the premelting stage, a mixed phase containing liquid and solid ions covers the surface of nanoparticles. The only peak of heat capacity spreads out a significant segment of temperature, probably exhibiting both heterogeneous melting on the surface and homogeneous melting in the core. The coexistence of two melting mechanisms, homogeneous and heterogeneous ones, in our model is unlike those considered previously. We also found that the critical Lindemann ratio of the KCl nanoparticle becomes much more stable when the size of the nanoparticle is of the order of thousands of ions. A picture of the structural evolution upon heating is studied in more detail via the radial distribution function (RDF) and coordination numbers. Our results are in a good agreement with previous MD simulations and experimental observations.     

In Situ Dimensional Characterization of Magnetic Nanoparticle Clusters during Induction Heating

The study and fundamental understanding of magnetic nanoparticle induction heating remains critical for the advancement of magnetic hyperthermia technologies. Complete characterization of not only the nanoparticles themselves but their interparticle behavior in a sample matrix is necessary to accurately predict their heating response. Herein, an in situ method for measuring the extent of nanoparticle clustering during induction heating using small-angle and ultrasmall-angle neutron scattering facilities at the National Institute of Standards and Technology Center for Neutron Research is described and implemented by comparing two sets of iron oxide nanoparticles with differing structures and magnetic properties. By fitting the scattering profiles to a piecewise model covering a wide Q-range, the magnitude of nanoparticle clustering during induction heating is quantified. Observations of the low-Q intensity before and after heating also allow for relative measurement of the cluster volume fraction during heating. The use of this method can prove to be advantageous in both developing more encompassing models to describe magnetic nanoparticle dynamics during heating as well as optimizing nanoparticle synthesis techniques to reduce aggregation during heating.     

强激光加热旋转薄柱壳的参量选择分析

为了把激光加热静止圆柱壳的实验测量结果应用到旋转圆柱壳的激光参量估计中,研究了旋转圆柱壳的激光加热效率。用积分变换法得到了旋转圆柱壳的温度分布,分析了最大温升点相对激光峰值强度点的滞后现象。基于静止圆柱壳和薄壳假设,导出了旋转圆柱壳激光加热效率及估计辐照时间的表达式。对于旋转金属圆柱壳,最大温升点相对激光峰值强度点的滞后角和激光加热效率取决于无量纲参量DR(柱壳半径R与束斑半径r0之比)、DL(横向热扩散尺度4ατL与束斑半径r0之比)及DM(加热时间τL与柱壳旋转频率fR的乘积)。达到相同的最大温升时,旋转圆柱壳的激光辐照时间和静止圆柱壳的激光辐照时间之间存在与激光功率无关的非线性关系,而激光功率决定了所需的绝对激光辐照时间。