Peculiar thermodynamic properties of LJ<Subscript> N</Subscript> (N = 39–55) clusters

The solid-liquid phase transitions of Lennard-Jones clusters LJ_N (N=39–55) were simulated by a microcanonical molecular dynamics method using Lennard-Jones potential, and their thermodynamic quantities were calculated. The caloric curves of clusters (except N=42) have S-bend. To understand this behaviour, configurational and total entropies were evaluated, and dents on the entropy curves were taken as a sign of negative heat capacity. The heat capacities were evaluated for N=39–55 clusters using configurational entropy data. The potential energy distributions have bimodal behaviour for all clusters in the given range at the melting temperature. The distinct melting behaviour of LJ₄₂ was explained by the topology of the potential energy surface by examining the isomer distributions at phase transitions for LJ₃₉-LJ₅₅. The isomer distributions were found to be a useful way to interpret this behaviour and melting dynamics in general. Melting temperature, latent heat and entropy change upon melting values were reported and are consistent with literature values and values calculated from bulk thermodynamic properties. The dependence of these quantities on the size of the clusters was examined and it is found that latent heat is the key quantity to determine the magic numbers.     

Phase Transition and Melting Curves of Calcium Fluoride via Molecular Dynamics Simulations

The phase transition and melting curves of CaF2 are investigated by using the general utility lattice programme （CULP） via the shell model with molecular dynamics method. By calculating the entropy H （at OK） and Cibbs free energy G^＊ （at 30OK）, we find that the phase transition pressure from the face-centred cubic （fee） structure to the orthorhombic structure is 11.40 CPa and 9.33 CPa at OK and 300K, respectively. The modified melting point of the fee CaF2 is in the range of 1650-1733K at OCPa. All these results are well consistent with the available experimental data and other theoretical results. We also obtain that the melting temperature of high pressure phase is 990-1073 K at 10 CPa. Moreover, the temperature dependences of the elastic constants Cij, bulk module B and shear module G are also predicted.     

Connection between nanostructured materials’ size-dependent melting and thermodynamic properties of bulk materials

Based on the thermodynamic and thermophysical properties of bulk materials, Gibbs free energy for nanostructured materials is obtained and used to study the size-dependent melting point depression phenomenon. The effects of volume change due to fusion, the thermal expansion and the temperature dependency of surface free energy of bulk materials on the melting point depression are investigated. Conversely, the solid surface free energy of bulk materials is also researched by means of the size-dependent melting temperature of nanostructured materials.     

Molecular dynamics study of the micro-spallation

We present a complete molecular dynamics analysis of the micro-spallation, which corresponds to the various events following the reflexion on a free surface of a unsustained shock wave causing the melting of the material. In a first stage the various curves required for a purely thermodynamic analysis (Hugoniot, melting curve and isentropes) are calculated by the mean of both equilibrium and non equilibrium molecular dynamics. Next we define two cases. In the first one the shock pressure is above the melting pressure (SM case). In the second one the melting occurs during the release wave following the shock (RM case). These two configurations provide quite similar results if the melting kinetic of the SM case is slow enough and we observe the formation of a solid micro-spall. In the other SM cases a direct transition from a liquid to a gas is obtained. As other results we found that (i) the melting under shock is a stationary process in the front shock referential and so is supported by the Rayleigh line and (ii) the spall strength of the micro-spallation is negligible.     

Adsorbate dynamics on a substrate: III. Surface structural disordering

Surface structural behavior of an fcc Lennard-Jones system is investigated using the molecular dynamics simulation by studying simultaneously the surface order and the surface mobility. Below the bulk melting temperature, the (111) Lennard-Jones surface is found to progressively disorder as the temperature is raised. Concomitant with this disordering, surface atomic mobility is found to be significantly enhanced. Results are discussed in terms of the surface premelting phenomenon.     

The effect of molecular shape anisotropy on surface melting

We report molecular dynamics (MD) simulations of the behaviour of the (101̄0) and (0001) crystal-vapour interfaces of Lennard-Jones dimers near the triple point. For slightly anisometric molecules, where the bulk melts from an orientationally disordered state, both interfaces were found to surface melt, with behaviour resembling that of the atomic Lennard-Jones system. In contrast, at larger anisometries, where the molecules are orientationally ordered close to the triple point, the (101̄0) face exhibits a first-order surface melting transition, while the (0001) face does not surface melt at all.     

Determining surface free energies of crystals with highly disordered surfaces from simulation

A method for calculating surface free energies by gradually creating slabs from a simulation of bulk crystals is tested for three low index faces of a Lennard-Jones crystal at temperatures up to the melting point. The path by which the interactions between atoms in different slabs are turned off must be chosen with care; here it is done in three stages, first the interaction energy is reduced until the well in the pair potential is considerably less than kT, then the effective particle sizes are reduced and finally the interaction energy is reduced to zero. The results show a slow and steady decrease in the surface free energy with temperature, while both the surface energy and the surface entropy increase rapidly when the upper layers of the crystal disorder. There is no evidence for a first order surface roughening transition although the (111) surface shows a sharper onset of disorder than do the (110) and (100) surfaces.     

Molecular dynamics simulation of surface melting behaviours of the V（110） plane

The modified analytic embedded-atom method and molecular dynamics simulations are applied to the investigation of the surface premelting and melting behaviours of the V（110） plane by calculating the interlayer relaxation, the layer structure factor and atomic snapshots in this paper. The results obtained indicate that the premelting phenomenon occurs on the V（110） surface at about 1800K and then a liquid-like layer, which approximately keeps the same thickness up to 2020K, emerges on it. We discover that the temperature 2020K the V（110） surface starts to melt and is in a completely disordered state at the temperature of 2140K under the melting point for the bulk vanadium.     

Structure and dynamics of graphite-supported bimetallic nanoclusters

Molecular dynamics simulations are used to analyze the structure and dynamics of isolated bimetallic nanoclusters of 343 (Cu-Ni) and 1000 atoms (Cu-Ni and Pt-Au) deposited on a graphite substrate. The metal-metal interactions are modeled with the many-body Sutton-Chen potential, and a Lennard-Jones potential is used to describe the metal-carbon interactions. The nanocluster melting temperature is determined from caloric and heat capacity curves, and the atomic distribution is studied layer-by-layer as a function of temperature in a direction perpendicular to the substrate plane. Changes in the nanocluster shape as temperature increases are monitored through deformation parameters that show clear evidence of structural and melting transitions as well as of atomic surface diffusion in the cluster. Dynamic properties such as atomic and whole-cluster diffusion, and the motion of the metal atoms at the interface metal/graphite are characterized as a function of temperature.     

Dislocation lines as the precursor of the melting of crystalline solids observed in Monte Carlo simulations

The microscopic mechanism of the melting of a crystal is analyzed by the constant-pressure Monte Carlo simulation of a Lennard-Jones fcc system. Beyond a temperature of the order of 0.8 of the melting temperature, we found that the relevant excitations are lines of defects. Each of these lines has the structure of a random walk of various lengths on an fcc defect lattice. We identify these lines with the dislocation ones proposed in recent phenomenological theories of melting. Near melting we find the appearance of long lines that cross the whole system. We suggest that these long lines are the precursor of the melting process.     

Free energy landscape from path-sampling: application to the structural transition in LJ38

We introduce a path-sampling scheme that allows equilibrium state-ensemble averages to be computed by means of a biased distribution of non-equilibrium paths. This non-equilibrium method is applied to the case of the 38-atom Lennard-Jones atomic cluster, which has a double-funnel energy landscape. We calculate the free energy profile along the Q₄ bond orientational order parameter. At high or moderate temperature the results obtained using the non-equilibrium approach are consistent with those obtained using conventional equilibrium methods, including parallel tempering and Wang-Landau Monte Carlo simulations. At lower temperatures, the non-equilibrium approach becomes more efficient in exploring the relevant inherent structures. In particular, the free energy agrees with the predictions of the harmonic superposition approximation.     

Molecular dynamics simulation of a graphite-supported copper nanocluster: thermodynamic properties and gas adsorption

Molecular dynamics simulations were conducted for a cubic Cu cluster supported on a graphite bilayer. The Sutten–Chen and Lennard–Jones potentials were used for metal–metal and metal–graphite interactions, respectively. Heating and cooling processes were performed by NVT simulations at different temperatures in the range 200 to 1800?K. The melting point was identified on the basis of caloric and heat capacity curves. The calculated melting point was 770?K, far below the bulk melting point of crystalline copper. Several phenomena such as the appearance of a hysteresis (irreversibility) in caloric curves, surface melting, and cluster-induced surface wetting were justified from the results. The simulation of cluster in the presence of gas atmosphere showed that the CO gas is adsorbed more than H₂ and it has a greater impact on the cluster's structure.     

LN晶体的激光损伤研究

测量了ＬＮ晶体的表面和体损伤阈值，以及重复频率脉冲的积累效应，研究了晶体中的非线性吸收过程。分析了损伤机理，发现在表面和体内都会发生多光子吸收，并且是引起晶体破坏的根源，造成宏观破坏的原因在体内是应力炸裂，在表面是热烧熔化和等离子体抛射。     

Modeling the thermodynamic properties of bimetallic nanosolids

A model has been developed to account for size, shape, surface segregation, composition and dimension dependent cohesive energy of bimetallic nanosolids, and further been extended to predict the size dependent thermodynamic properties, such as melting temperature, Curie temperatures, ordering temperature and phase diagram. The cohesive energy, melting temperature, Curie temperatures and ordering temperature of bimetallic nanosolids decrease with decreasing the particle size. The depression is dramatic in the lower range of size, while it becomes smoothly in large size. For nano phase diagram, the solidus and liquidus curves drop and the two-phase zones become small, as the size of the nanosolids decreases. The two-phase zones of the nano phase are always lower than the regions indicated in the bulk Ag-Pd alloy phase diagram, and they may deteriorate into a curve at a critical size. It is also found that the thermodynamic properties of nanosolids not only depend on the compositions, the atomic diameter and the cohesive energy of each component, but also depend on the size and the shape. The model predictions are consistent with the corresponding simulation, semi-empirical model and experimental data.     

Evolution of topological order in Xe films on a quasicrystal surface

We report results of the first computer simulation studies of a physically adsorbed gas on a quasicrystalline surface Xe on decagonal Al-Ni-Co. The grand canonical Monte Carlo method is employed, using a semiempirical gas-surface interaction, based on conventional combining rules, and the usual Lennard-Jones Xe-Xe interaction. The resulting adsorption isotherms and calculated structures are consistent with the results of LEED experimental data. The evolution of the bulk film begins in the second layer, while the low coverage behavior is epitaxial. This transition from epitaxial fivefold to bulklike sixfold ordering is temperature dependent, occurring earlier (at lower coverage) for the higher temperatures.     

Thermal instability of decahedral structures in platinum nanoparticles

We conduct molecular dynamics simulations of 887 and 1389-atom decahedral platinum nanoparticles using an embedded atom potential. By constructing microcanonical caloric curves, we identify structural transitions from decahedral to fcc in the particles prior to melting. The transitions take place during phase coexistence and appear to occur via melting of the decahedral structure and subsequent recrystallisation into the fcc structure.     

A Lennard—Jones embedded—atom potential and its application to the study of melting

The nearest-neighbour Lennard-Jones potential from the embedded-atom method is extended to a form that includes more than nearest neighbours.The model has been applied to study melting with molecular dynamics.The calculated melting point,fractional volume change on melting,heat of fusion and linear coefficients of thermal expansion are in good agreement with experimental data.We have found that the second and third neighbours influence the melting point distinctly.     

On the size dependence of surface tension in the temperature range from melting point to critical point

The problem of size dependence of surface tension was investigated in view of a more general problem of the applicability of Gibbs’ thermodynamics to nanosized objects. For the first time, the effective surface tension (coinciding with the specific excess free energy for an equimolecular dividing surface) was calculated within a wide temperature range, from the melting temperature to the critical point, using the thermodynamic perturbation theory. Calculations were carried out for Lennard-Jones and metallic nanosized droplets. It was found that the effective surface tension decreases both, with temperature and particle size.