Molecular dynamics simulation of iron nanoparticle sintering during flame synthesis

The sintering process of iron nanoparticles produced in a flame environment is investigated by molecular dynamic (MD) simulations. The thermodynamic characteristics and restructuring pathways are studied for two-body and three-body sintering processes. The melting point, energy, and structures are computed for nanoparticles before and after sintering. A simplified model is proposed to predict the equilibrium temperature of nanoparticles upon sintering. The MD results are used to explain the formation mechanisms of two size ranges of nanoparticles during the flame synthesis. The role of sintering during nanoparticle growth is analyzed.     

Molecular dynamics investigation of shape effects on thermal characteristics of platinum nanoparticles

Using molecular dynamics simulations with the quantum corrected Sutton-Chen type many-body potential, we investigate the thermal characteristics and structural evolution of Pt nanoparticles with spherical and polyhedral shapes under the heating process. The main focus of this work is the shape effects on the thermal characteristics of Pt nanoparticles. The simulation results show that all types of nanoparticles present the same overall melting temperature in spite of their different shapes. These nanoparticles can hold their initial shapes and structures at low temperature. However, polyhedral nanoparticles undergo a remarkable shape transformation before their overall melting. The critical temperature of shape transformation depends on their shapes and associated Miller index of the surface. Our study indicates that octahedron-truncated nanoparticle displays a better thermal stability than other polyhedral nanoparticles.     

Computer study of the temperature dependence of physical properties of noncrystalline silicon nanoparticles

Variations in the structure and kinetic properties of vitreous and amorphous Si₄₀₀ nanoparticles upon heating from 300 to 1700 K are studied by molecular dynamics. The nanoparticle density increases with temperature and approaches the density of bulk solid silicon. A transition from a unimodal to a bimodal distribution of bond lengths is observed upon heating. This transition is more pronounced in the case of the vitreous nanoparticle. The average bond length in the amorphous nanoparticle is, as a rule, larger than that in the vitreous one, and the average number of bonds per atom is lower than that in the vitreous nanoparticle for nearly all studied temperatures. Negative values of the excess potential energy correspond to middle concentric layers of nanoparticles. Liquid layers form in the surface region of nanoparticles in the vicinity of the melting transition. A kinetic test indicating the beginning of nanoparticle melting is formulated.     

Hetero-phase fluctuations in the pre-melting region in ionic crystals

The theory of the pre-melting phenomena in ionic crystals on the basis of the concept of the hetero phase fluctuation has been applied to KCl and AgCl crystal. The large scale molecular dynamics simulations (MD) in KCl and AgCl crystals are also performed to examine the ionic configuration in premelting region in the vicinity of their melting points. The size of the liquid like clusters are estimated by the theory and MD. The structural features of liquid like clusters are discussed by MD results using the Lindemann instability condition. The ionic conductivities in the pre-melting region are also discussed on the same theoretical basis.     

Atomic mechanism of the heating-induced phase transitions of the simple monatomic glasses

Atomic mechanism of the heating-induced phase transitions of the monatomic Lennard-Jones (LJ) glass has been studied via molecular dynamics (MD) simulations. Monatomic LJ glass was heated up at two different heating rates, crystallization occurs at the lowest one and further heating leads to the melting of LJ crystal. Thermodynamics of the phase transitions and corresponding evolution of structural properties upon heating have been analyzed in details. Atomic mechanism of a crystallization of the glassy state was monitored via spatio-temporal arrangements of the atoms involved in the 1421 bond-pair of the fcc crystalline structure. The 1421 bond-pair was detected via the Honeycutt-Andersen analysis [J.D. Honeycutt, H.C. Andersen, J. Phys. Chem. 91 (1987) 4950]. We found that crystallization of the monatomic LJ glass occurs via homogeneous local rearrangements of atoms in the glassy matrix and we found an important role of the liquid-like atoms (existed in the glassy state) in crystallization of the system. In addition, spatio-temporal arrangements of the liquid-like atoms in the system upon further heating were shown in order to clarify the atomic mechanism of a melting of the obtained LJ crystal. Liquid-like atoms were defined by the Lindemann melting criterion. Our results provide previously un-reported data and give deeper understanding of the heating-induced phase transitions in the less stable metallic glasses, which have been observed in practice.     

Atomic mechanism of homogeneous melting of bcc Fe at the limit of superheating

Atomic mechanism of homogeneous melting of bcc Fe is studied via monitoring spatiotemporal arrangements of the liquid-like atoms, which are detected by the Lindemann criterion of melting, during the heating process. Calculations are performed by molecular dynamics (MD) simulations. Calculations show that liquid-like atoms occur randomly in the crystalline matrix at temperature far below the melting point due to local instability of the crystalline lattice. Number of liquid-like atoms increases with increasing temperature and they have a tendency to form clusters. Subsequently, a single percolated liquid-like cluster is formed in the crystalline model and at the melting point 99% atoms in the model become liquid-like to form a liquid phase. Melting is also accompanied by the sudden changes in various static and thermodynamic quantities. However, total melting is reached just at the point above the melting one. Three characteristic temperatures of the homogeneous melting of bcc Fe are determined.     

Modeling the cohesive energy and melting point of nanoparticles by their average coordination number

We present a relation between the average coordination number and the cohesive energy for nanoparticles that shows that the ratio of nanoparticles cohesive energy to the bulk value is equal to the ratio of the nanoparticles average coordination number to that of the bulk. We consider the effect of lattice and surface packing factors on the average coordination numbers of the atoms in the nanoparticle. The melting temperature of nanoparticles has been calculated from the obtained relation for cohesive energy, and predictions for the cohesive energy and melting temperature of the nanoparticles have been compared with other theoretical models and available experimental data and the results of molecular dynamics simulations.     

The melting mechanism in binary Pd0.25Ni0.75 nanoparticles: molecular dynamics simulations

The melting mechanism for Pd_0.25Ni_0.75 alloy nanoparticles (NPs) was investigated using molecular dynamics (MD) simulations with quantum Sutton-Chen many-body potentials. NPs of six different sizes ranging from 682 to 22,242 atoms were studied to observe the effect of size on the melting point. The melting temperatures of the NPs were estimated by following the changes in both the thermodynamic and structural quantities such as the total energy, heat capacity and Lindemann index. We also used a thermodynamics model to better estimate the melting point and to check the accuracy of MD simulations. We observed that the melting points of the NPs decreased as their sizes decreased. Although the MD simulations for the bulk system yielded higher melting temperatures because of the lack of a seed for the liquid phase, the melting temperatures determined for both the bulk material and the NPs are in good agreement with those predicted from the thermodynamics model. The melting mechanism proceeds in two steps: firstly, a liquid-like shell is formed in the outer regions of the NP with increasing temperature. The thickness of the liquid-like shell increases with increasing temperature until the shell reaches a critical thickness. Then, the entire Pd–Ni NP including core-related solid-like regions melts at once.     

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     

铝纳米颗粒的热物性及相变行为的分子动力学模拟

采用分子动力学方法模拟了纳米金属铝在粒径为0.8-3.2 nm 时的熔点、密度和声子热导率的变化, 研究了粒径为1.6 nm的铝纳米颗粒的密度、比热和声子热导率随温度的变化. 采用原子嵌入势较好地模拟了纳米金属铝的热物性及相变行为, 根据能量-温度曲线和比热容-温度曲线对铝纳米颗粒的相变温度进行了研究, 并利用表面能理论、尺寸效应理论对铝纳米颗粒熔点的变化进行了分析. 随着纳米粒径的不断增大, 铝纳米颗粒的熔点呈递增状态, 当粒径在2.2-3.2 nm时, 熔点的增幅减缓, 但仍处于递增趋势. 随着纳米粒径的增大, 铝纳米颗粒的密度呈单调递减, 热导率则呈线性单调递增, 且热导率的变化情况符合声子理论. 随着温度的升高, 粒径为1.6 nm的铝纳米颗粒的密度、热导率均减小. 该模拟从微观原子角度对纳米材料的热物性进行了研究, 对设计基于铝纳米颗粒的相变材料具有指导意义.     

Superheating and solid-liquid phase coexistence in nanoparticles with nonmelting surfaces

We present a phenomenological model of melting in nanoparticles with facets that are only partially wet by their liquid phase. We show that in this model, as the solid nanoparticle seeks to avoid coexistence with the liquid, the microcanonical melting temperature can exceed the bulk melting point and that the onset of coexistence is a first-order transition. We show that these results are consistent with molecular dynamics simulations of aluminum nanoparticles which remain solid above the bulk melting temperature.     

Effects of iron oxide nanoparticles on polyvinyl alcohol: interfacial layer and bulk nanocomposites thin film

Iron oxide (α-phase) nanoparticles with coercivity larger than 300 Oe have been fabricated at a mild temperature by an environmentally benign method. The economic sodium chloride has been found to effectively serve as a solid spacer to disperse the iron precursor and to prevent the nanoparticles from agglomeration. Higher ratios of sodium chloride to iron nitrate result in smaller nanoparticles (19 nm for 20:1 and 14 nm for 50:1). The presence of polyvinyl alcohol (PVA) limits the particle growth (15 nm for 20:1 and 13 nm for 50:1) and favors nanoparticle dispersion in polymer matrices. Obvious physicochemical property changes have been observed with PVA attached to the nanoparticle surface. With PVA attached to the nanoparticle surface, the nanoparticles are found not only to increase the PVA cross-linking with an increase in melting temperature but also to enhance the thermal stability of the PVA. The nanoparticles are observed to be uniformly dispersed in the polymer matrix. Scanning electron microscopy (SEM) microstructure also shows an intermediate phase with a strong interaction between the nanoparticles and the polymer matrices, arising from the hydrogen bonding between the PVA and hydroxyl groups on the nanoparticle surface. The addition of nanoparticles favors the cross-linkage of the bulk PVA matrices, resulting in a higher melting temperature, and an enhanced thermal stability of the polymer matrix.     

自由分子区内纳米颗粒的热泳力计算

基于非平衡态分子动力学模拟方法,研究了自由分子区内纳米颗粒的热泳特性.理论研究表明,纳米颗粒与周围气体分子之间的非刚体碰撞效应会明显地改变其热泳特性,经典的Waldmann热泳理论并不适用,但尚未有定量的直接验证.模拟计算结果表明:对于纳米颗粒而言,当气-固相互作用势能较弱或气体温度较高时,气体分子与纳米颗粒之间的非刚体碰撞效应可以忽略,Waldmann热泳理论与分子动力学模拟结果吻合较好;当气-固相互作用势能较强或气体温度较低时,非刚体碰撞效应较为明显,Waldmann热泳理论与模拟结果存在较大误差.基于分子动力学模拟结果,对纳米颗粒的等效粒径进行了修正,并考虑了气体分子与纳米颗粒之间的非刚体碰撞效应,理论计算结果与分子动力学模拟结果吻合较好.     

Melting evolution and diffusion behavior of vanadium nanoparticles

Molecular dynamics calculations have been performed to study the melting evolution, atomic diffusion and vibrational behavior of bcc metal vanadium nanoparticles with the number of atoms ranging from 537 to 28475 (diameters around 2–9 nm). The interactions between atoms are described using an analytic embedded-atom method. The obtained results reveal that the melting temperatures of nanoparticles are inversely proportional to the reciprocal of the nanoparticle size, and are in good agreement with the predictions of the thermodynamic liquid-drop model. The melting process can be described as occurring in two stages, firstly the stepwise premelting of the surface layer with a thickness of 2–3 times the perfect lattice constant, and then the abrupt overall melting of the whole cluster. The heats of fusion of nanoparticles are also inversely proportional to the reciprocal of the nanoparticle size. The diffusion is mainly localized to the surface layer at low temperatures and increases with the reduction of nanoparticle size, with the temperature being held constant. The radial mean square vibration amplitude (RMSVA) is developed to study the anharmonic effect on surface shells.     

面心立方铁纳米粒子的相变与并合行为的分子动力学研究

本文采用分子动力学模拟结合Finnis-Sinclair多体势研究了面心立方铁纳米粒子在加温过程中的相变与并合行为. 模拟结果表明: 纳米粒子在熔化之前均发生了由面心立方至体心立方的马氏体相变; 大小相等的两纳米粒子在并合之前发生了相对转动; 而大小不等的两纳米粒子在并合过程中并未出现转动, 小纳米粒子倾向于吸附在大纳米粒子上, 并随着温度的升高而熔化, 最终形成更大的纳米粒子. 关键词： 纳米粒子 相变 并合 分子动力学     

Molecular dynamics study of nanoparticle evolution in a background gas under laser ablation conditions

Long-time evolution of nanoparticles produced by short laser interactions is investigated for different materials. To better understand the mechanisms of the nanoparticle formation at a microscopic level, we use molecular dynamics (MD) simulations to analyse the evolution of a cluster in the presence of a background gas with different parameters (density and temperature). In particular, we compare the simulation results obtained for materials with different interaction potentials (Morse, Lennard-Jones, and Embedded Atom Model). Attention is focused on the evaporation and condensation processes of a cluster with different size and initial temperature. As a result of the MD calculations, we determinate the influence of both cluster properties and background gas parameters on the nanoparticle evolution. The role of the interaction potential is discussed based on the results of the simulations.     

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.     

Changes in the structure of nanoclusters of nickel and copper under the action of temperature

Molecular dynamics with tight-binding potential was used to study the processes of thermal impact on Ni nanoclusters with radii of up to 0.8 nm. The performed simulation demonstrated that under the action of heating, there is a structural transition from the initial face-centered cubic phase to a icosahedral modification. It was observed that upon an increase of cluster size, the transition temperature shifted to the melting point of the cluster. The obtained relationships were tested on the copper nanoparticles of similar size. It was concluded that the size of a metal cluster of 200–250 atoms is probably the limit below which it is impossible to maintain the initial face-centered cubic modification under actual conditions of exploitation.     

On structural transitions in nanoparticles

Using two independent computer programs based on isothermal molecular dynamics method, we studied size effects during the melting and crystallization of Lennard-Jones nanoparticles and metallic clusters, the interaction in which was described by the cooperative potentials. We found that at nanoparticle radii exceeding a certain characteristic value, the melting point of nanoparticles is higher than their crystallization temperature.     

Anisotropic Thermal Processing of Polymer Nanocomposites via the Photothermal Effect of Gold Nanorods

By embedding metal nanoparticles within polymeric materials, selective thermal polymer processing can be accomplished via irradiation with light resonant with the nanoparticle surface plasmon resonance due to the photothermal effect of the nanoparticles which efficiently transforms light into heat. The wavelength and polarization sensitivity of photothermal heating from embedded gold nanorods is used to selectively process a collection of polymeric nanofibers, completely melting those fibers lying along a chosen direction while leaving the remaining material largely unheated and unaffected. Fluorescence‐based temperature and viscosity sensing was employed to confirm the presence of heating and melting in selected fibers and its absence in counter‐aligned fibers. Such tunable specificity in processing a subset of a sample, while the remainder is unchanged, cannot easily be achieved through conventional heating techniques.