Molecular-dynamics study of lattice-defect-nucleated melting in metals using an embedded-atom-method potential

Applying Classical, Ab Initio, and Machine-Learning Molecular Dynamics Simulations to the Liquid Electrolyte for Rechargeable Batteries

Rechargeable batteries have become indispensable implements in our daily life and are considered a promising technology to construct sustainable energy systems in the future. The liquid electrolyte is one of the most important parts of a battery and is extremely critical in stabilizing the electrode-electrolyte interfaces and constructing safe and long-life-span batteries. Tremendous efforts have been devoted to developing new electrolyte solvents, salts, additives, and recipes, where molecular dynamics (MD) simulations play an increasingly important role in exploring electrolyte structures, physicochemical properties such as ionic conductivity, and interfacial reaction mechanisms. This review affords an overview of applying MD simulations in the study of liquid electrolytes for rechargeable batteries. First, the fundamentals and recent theoretical progress in three-class MD simulations are summarized, including classical, ab initio, and machine-learning MD simulations (section 2). Next, the application of MD simulations to the exploration of liquid electrolytes, including probing bulk and interfacial structures (section 3), deriving macroscopic properties such as ionic conductivity and dielectric constant of electrolytes (section 4), and revealing the electrode-electrolyte interfacial reaction mechanisms (section 5), are sequentially presented. Finally, a general conclusion and an insightful perspective on current challenges and future directions in applying MD simulations to liquid electrolytes are provided. Machine-learning technologies are highlighted to figure out these challenging issues facing MD simulations and electrolyte research and promote the rational design of advanced electrolytes for next-generation rechargeable batteries.

Compression Induced Deformation Twinning Evolution in Liquid-Like Cu2Se

For practical applications of copper selenide (Cu₂Se) thermoelectric (TE) materials with liquid-like behavior, it is essential to determine the structure-property relations as a function of temperature. Here, we investigate β-Cu₂Se structure evolution during uniaxial compression over the temperature range of 400-1000 K using molecular dynamics simulations. We find that at temperatures above 800 K, Cu₂Se exhibits poor stability with breaking order that is described as a liquid-like or hybrid structure comprising a rigid Se sublattice and mobile Cu ions. A uniaxial load causes accumulated structural heterogeneity that is alleviated by diffusion-induced accommodation of local deformations. With increasing strain, the deformation mode changes into a combination of compression and shear, accompanied by restructuring in terms of twinning. Interestingly, in addition to a plastic behavior rarely found in inorganic semiconductors, we find that higher temperature promotes deformation twinning in liquid-like Cu₂Se, showing the role of thermal instability, including Cu diffusion, in structural adaptation and mechanical modulation. These findings reveal the micromechanism of hybrid structural evolution as well as performance tuning through twinning, which provides a theoretical guide toward advanced Cu₂Se TE materials design.

Superheating of grain boundaries within bulk colloidal crystals

Whether grain boundaries (GBs) premelt is a longstanding question, because of the difficulty of direct experimental tests. Here, we focused an optical beam to locally heat single GBs within bulk hard-sphere colloidal crystals, observing the melting dynamics at single-particle resolution by video microscopy. The melting point is determined by analysing both the Lindemann parameter and the critical nucleus size for homogeneous nucleation. We found that all the GBs, including the high-energy GBs, can be superheated and melt via a heterogeneous nucleation mechanism. Based on the classical nucleation theory of GBs, we measured the incubation time and contact angle of the critical nucleus to compute all relevant kinetic factors, as well as the energy barrier, nucleation rate and the diffusion coefficient at the solid–liquid interface under weak superheating. The superheat limits of GBs with various misorientations have also been measured to further explore the instability mechanism. Under traditional uniform heating, premelting occurs only at triple junctions, whereas GBs retain their original structures up to the melting point. The premelted regions at triple junctions further interrupt high-energy GBs from superheating, through intrusion by uniform liquid layers. Overall, our experiments confirm the existence of superheating of GBs.

Understanding the dynamics of grain boundaries and their melting behaviour is important for controlling the mechanical properties of materials. Now, experiments show that grain boundaries can be superheated, and that they melt via a nucleation mechanism.

Grain boundary-induced premelting and solid ↔ melt phase transformations: effect of interfacial widths and energies and triple junctions at the nanoscale

The strong effect of dry grain boundary (GB) and isolated solid-melt interface widths and energies and the triple junction energy on GB-induced premelting and the transformations between solid, premelt, and melt at the nanoscale are studied using a thermodynamically consistent original phase field approach of Ginzburg-Landau type. One order parameter is considered for describing solid ↔ melt transformation and another N- 1 independent order parameters are used to describe N grains in the samples. A variable energy of the GB regions is assumed which depends on the order parameter related to solid ↔ melt transformation, and the energy of the GBs reduces from their dry state energy γ_gb in solid to zero at melt. The complex 'phase diagrams', showing the variation of the transformation temperatures with the non-dimensional width ratio (ratio of dry GB width and isolated solid-melt interface width), are plotted for 2D bicrystals with a planar GB and a circular GB for the cases when (i) solid-melt interface width is fixed and GB width varies, and (ii) GB width is fixed and solid-melt interface width varies. A bistable region, where the premelt spontaneously appears or transforms back to the solid, is shown indicating that these transformations accompany hysteresis. The effect of the width ratio on the effective melt layer thickness and the disjoining potential between the solid-melt interfaces on premelting/melting is also studied. The effect of triple junctions on the transformations is studied in tricrystals having three planar GBs meeting at the interior of the sample with equal dihedral angles. A premelt/melt pocket appears at the junction in a jump-like manner at temperatures significantly lower than the temperatures at which the GBs premelt. The premelt/melt that appeared at the junction eventually wets the adjacent GBs, and finally, the entire sample melts at the melting point. The results show a plausible reason for differences in the premelting and melting temperature ranges of the GBs observed in various experimental and atomistic studies.

Atomic Simulations of Packing Structures, Local Stress and Mechanical Properties for One Silicon Lattice with Single Vacancy on Heating

The effect of vacancy defects on the structure and mechanical properties of semiconductor silicon materials is of great significance to the development of novel microelectronic materials and the processes of semiconductor sensors. In this paper, molecular dynamics is used to simulate the atomic packing structure, local stress evolution and mechanical properties of a perfect lattice and silicon crystal with a single vacancy defect on heating. In addition, their influences on the change in Young’s modulus are also analyzed. The atomic simulations show that in the lower temperature range, the existence of vacancy defects reduces the Young’s modulus of the silicon lattice. With the increase in temperature, the local stress distribution of the atoms in the lattice changes due to the migration of the vacancy. At high temperatures, the Young’s modulus of the silicon lattice changes in anisotropic patterns. For the lattice with the vacancy, when the temperature is higher than 1500 K, the number and degree of distortion in the lattice increase significantly, the obvious single vacancy and its adjacent atoms contracting inward structure disappears and the defects in the lattice present complex patterns. By applying uniaxial tensile force, it can be found that the temperature has a significant effect on the elasticity–plasticity behaviors of the Si lattice with the vacancy.

Ab initiobased interionic interactions in calcium aluminotitanate oxide melts: structure and diffusion

Calcium aluminotitanate (CaO-Al₂O₃-TiO₂) ternary oxides are of fundamental interest in Materials as well as Earth and environmental science, and a key system for several industrial applications. As their properties at the atomic scale are scarcely known, interionic interactions for the melts are built from a bottom up strategy consisting in fitting first only Al₂O₃, CaO and TiO₂single oxide compounds separately with a unified description of the oxygen charge and O-O interaction term. For this purpose, a mean-square difference minimization of the partial pair-correlation functions with respect to theab initioreference was performed. The potentials for the ternary oxide are finally built straightforwardly by adding purely Coulomb terms for dissimilar cation-cation interactions without further fit. This general and unified approach is transferable and successfully describes the structural and diffusion properties of the three single oxides as well as the ternary melts simultaneously. A possible underlying structural mechanism at the origin of the diffusion evolution with TiO₂content is proposed based on the formation of Ti induced triply bonded oxygen.

Molecular Combustion Properties of Nanoscale Aluminum and Its Energetic Composites: A Short Review

Remarkable progress has been established in the field of nanoenergetic materials (mixture of nanoscale fuel and oxidizer) since the advent of nanotechnology. Combustion of nanoenergetic materials depends on many key factors like synthesis route, equivalence ratio, morphology of constituents, and arrangements and handling of materials. For tailoring and tuning of the combustion properties of nanoenergetics, sound knowledge of the reaction mechanism is needed; in this review article a schematic study on the reaction mechanism is presented. By employing various routes and strategies in synthesizing and nanoengineering of the fuel or/and oxidizer to realize a significant evolution from normal physical mixing of nanopowders to the formulation of core/shell nanostructures, the nanoenergetic materials achieved the best ever combustion properties in terms of combustion reactivity, ignition sensitivity, energy density, etc. Overall, in this article, a critical state-of-the-art review of the existing literatures has been conducted to feature the main developments in the molecular combustion modeling of melting, oxidation, and core-shell reaction/diffusion of nanoaluminum and the molecular modeling of combustion reactivity and ignition sensitivity of nanoenergetic materials.

Nonequilibrium Molecular Dynamics Simulations of Coal Ash

Both molecular dynamics (MD) and nonequilibrium molecular dynamics (NEMD) simulations were performed to simulate coal ashes using the Guillot-Sator model in this work. The structural and transport properties of coal ashes at high temperatures have been obtained. Superheating of coal ash system with anorthite crystal structure initial configuration has been observed for MD simulation which explains the discrepancy between previous MD simulation results and FactSage thermochemical calculations. The fluxing effects of both calcium oxide and sodium oxide have been investigated systematically through MD and NEMD simulations. Moreover, the viscosities of coal ash systems have been computed by two methods: (1) Stokes-Einstein equation; (2) NEMD simulations. Estimations of viscosities for various coal ash systems based on Stokes-Einstein equation exhibit a strong temperature dependence of viscosity, which agrees with previous experimental results. On the other hand, NEMD simulation results that showed a strong shear-thinning feature, failed to reproduce this strong temperature dependence of viscosity, possibly due to the short simulation time. Nevertheless, NEMD simulations not only provide us detailed information about atoms dynamics under shear, but also allow us to model the coal ash system far from equilibrium which cannot be accessed by thermodynamics calculation using software like FactSage.

Ab initioinvestigation of the temperature-dependent elastic properties of Bi, Te and Cu

Using density functional theory andab initiomolecular dynamics, we have investigated the elastic properties of Bi, Te and Cu as a function of temperature. We compare calculated quantities which can be used to determine the effectiveness of our proposed method, such as the bulk (K), shear (G), and Young's (E) moduli. We also computed Poisson's ratio (ν) and the Pugh ratio (γ) for each of these materials at different temperatures to investigate changes in ductility. We have used the elastic moduli to calculate the Debye temperatureθDand minimum thermal conductivitykminof these materials as a function of temperature. We found that the elastic properties calculated in this work are in good agreement with experimental work. The inclusion of temperature effects has allowed for the proper prediction of ductility for each of these materials, a feat that standard density functional theory calculations has previously been unable to accomplish for Bi and Te.

Thermodynamics and structural properties of CaO: A molecular dynamics simulation study

A detailed theoretical study of CaO in the solid and liquid states by means of combined classical and ab initio molecular dynamics simulations is presented. Evolution of the specific heat capacity at constant pressure as a function of temperature is studied, and the melting temperature and enthalpy of fusion are determined. It is shown that an empirical Born-Mayer-Huggins potential gives a good representation of pure CaO in the liquid and solid states as compared to available experimental data and density functional theory calculations. Consistency of the predicted results obtained for CaO with the data available in commercial thermodynamic databases and experimental values in the literature is discussed. The present methodology and theoretical results provide a new accurate basis for calculations of thermodynamic properties in a temperature range that is hardly accessible by experiments.

Predicting Melting Points of Biofriendly Choline-Based Ionic Liquids with Molecular Dynamics

In this work, we introduce a simulation-based method for predicting the melting point of ionic liquids without prior knowledge of their crystal structure. We run molecular dynamics simulations of biofriendly, choline cation-based ionic liquids and apply the method to predict their melting point. The root-mean-square error of the predicted values is below 24 K. We advocate that such precision is sufficient for designing ionic liquids with relatively low melting points. The workflow for simulations is available for everyone and can be adopted for any species from the wide chemical space of ionic liquids.

Mechanical Spectroscopy Investigation of Point Defect-Driven Phenomena in a Cr Martensitic Steel

The paper presents and discusses results of mechanical spectroscopy (MS) tests carried out on a Cr martensitic steel. The study regards the following topics: (i) embrittlement induced by Cr segregation; (ii) interaction of hydrogen with C–Cr associates; (iii) nucleation of Cr carbides. The MS technique permitted characterising of the specific role played by point defects in the investigated phenomena: (i) Cr segregation depends on C–Cr associates distribution in as-quenched material, in particular, a slow cooling rate (~150 K/min) from austenitic field involves an unstable distribution, which leads to Cr concentration fluctuations after tempering at 973 K; (ii) hydrogen interacts with C–Cr associates, and the phenomenon hinders hydrogen attack (HA) because hydrogen atoms bound by C–Cr associates are not able to diffuse towards grain boundaries and dislocation where CH4 bubbles may nucleate, grow, and merge to form the typical HA cracks; (iii) C–Cr associates take part in the nucleation mechanism of Cr7C3 carbides, and specifically these carbides form by the aggregation of C–Cr associates with 1 Cr atom.

Melting of superheated molecular crystals

Melting dynamics of micrometer scale, polycrystalline samples of isobutane, dimethyl ether, methyl benzene, and 2-propanol were investigated by fast scanning calorimetry. When films are superheated with rates in excess of 10⁵ K s^-1, the melting process follows zero-order, Arrhenius-like kinetics until approximately half of the sample has transformed. Such kinetics strongly imply that melting progresses into the bulk via a rapidly moving solid-liquid interface that is likely to originate at the sample's surface. Remarkably, the apparent activation energies for the phase transformation are large; all exceed the enthalpy of vaporization of each compound and some exceed it by an order of magnitude. In fact, we find that the crystalline melting kinetics are comparable to the kinetics of dielectric α-relaxation in deeply supercooled liquids. Based on these observations, we conclude that the rate of non-isothermal melting for superheated, low-molecular-weight crystals is limited by constituent diffusion into an abnormally dense, glass-like, non-crystalline phase.

Inflating hollow nanocrystals through a repeated Kirkendall cavitation process

The Kirkendall effect has been recently used to produce hollow nanostructures by taking advantage of the different diffusion rates of species involved in the chemical transformations of nanoscale objects. Here we demonstrate a nanoscale Kirkendall cavitation process that can transform solid palladium nanocrystals into hollow palladium nanocrystals through insertion and extraction of phosphorus. The key to success in producing monometallic hollow nanocrystals is the effective extraction of phosphorus through an oxidation reaction, which promotes the outward diffusion of phosphorus from the compound nanocrystals of palladium phosphide and consequently the inward diffusion of vacancies and their coalescence into larger voids. We further demonstrate that this Kirkendall cavitation process can be repeated a number of times to gradually inflate the hollow metal nanocrystals, producing nanoshells of increased diameters and decreased thicknesses. The resulting thin palladium nanoshells exhibit enhanced catalytic activity and high durability toward formic acid oxidation.

Owing to their unique properties, hollow metal nanocrystals demonstrate greater catalytic promise than their solid counterparts. Here the authors produce hollow and inflated palladium nanocrystals with thin shells via a repeated Kirkendall cavitation process, and demonstrate their activity for formic acid oxidation.

Addressing the discrepancy of finding the equilibrium melting point of silicon using molecular dynamics simulations

We performed molecular dynamics simulations to study the equilibrium melting point of silicon using (i) the solid-liquid coexistence method and (ii) the Gibbs free energy technique, and compared our novel results with the previously published results obtained from the Monte Carlo (MC) void-nucleated melting method based on the Tersoff-ARK interatomic potential (Agrawal et al. Phys. Rev. B72, 125206. (doi:10.1103/PhysRevB.72.125206)). Considerable discrepancy was observed (approx. 20%) between the former two methods and the MC void-nucleated melting result, leading us to question the applicability of the empirical MC void-nucleated melting method to study a wide range of atomic and molecular systems. A wider impact of the study is that it highlights the bottleneck of the Tersoff-ARK potential in correctly estimating the melting point of silicon.

Anisotropy in surface-initiated melting of the triclinic molecular crystal 1,3,5-triamino-2,4,6-trinitrobenzene: A molecular dynamics study

Surface-initiated melting of 1,3,5-triamino-2,4,6-trinitrobenzene (TATB), a triclinic molecular crystal, was investigated using molecular dynamics simulations. Simulations were performed for the three principal crystallographic planes exposed to vacuum, with the normal vectors to the planes given by b × c, c × a, and a × b (where a, b, and c define the edge vectors of the unit cell), denoted as (100), (010), and (001), respectively. The best estimate of the normal melting temperature for TATB is 851 ± 5 K. The nature and extent of disordering of the crystal-vacuum interface depend on the exposed crystallographic face, with the (001) face exhibiting incomplete melting and superheating. This is attributed to the anisotropy of the inter-molecular hydrogen bonding and the propensity of the crystal to form stacking faults in directions approximately perpendicular to the (100) and (010) faces. For all three crystal orientations, formation of molecular vacancies in the lattice at the crystal-vacuum (or crystal-quasi-liquid layer) interface precedes the complete loss of order at the interface.

A comparison of methods for melting point calculation using molecular dynamics simulations

Accurate and efficient prediction of melting points for complex molecules is still a challenging task for molecular simulation, although many methods have been developed. Four melting point computational methods, including one free energy-based method (the pseudo-supercritical path (PSCP) method) and three direct methods (two interface-based methods and the voids method) were applied to argon and a widely studied ionic liquid 1-n-butyl-3-methylimidazolium chloride ([BMIM][Cl]). The performance of each method was compared systematically. All the methods under study reproduce the argon experimental melting point with reasonable accuracy. For [BMIM][Cl], the melting point was computed to be 320 K using a revised PSCP procedure, which agrees with the experimental value 337-339 K very well. However, large errors were observed in the computed results using the direct methods, suggesting that these methods are inappropriate for large molecules with sluggish dynamics. The strengths and weaknesses of each method are discussed.

Thermodynamic stability of nanometre-sized Cu₃Au systems

Classical molecular dynamics simulations have been used to investigate the order-to-disorder transition in bulk and nanometre-sized Cu₃Au systems. The Helmholtz free energy difference between ordered and disordered phases was evaluated at different temperatures through the Bennett's method. The change of free energy differences with temperature was employed to identify the transition temperatures. The obtained information was used to study the dynamics of a nanometre-sized Cu₃Au particle in a He gas thermostat at the transition temperature. It is shown that the system underwent rapid fluctuation in the chemical order degree related to the formation of vacancies and of atoms with defective coordination. Additional information on surface energies was also gained through a thermodynamic description of the transition process.

Structural disjoining potential for grain-boundary premelting and grain coalescence from molecular-dynamics simulations

We describe a molecular-dynamics framework for the direct calculation of the short-ranged structural forces underlying grain-boundary premelting and grain coalescence in solidification. The method is applied in a comparative study of (i) a Sigma9115120 degrees twist and (ii) a Sigma9110{411} symmetric tilt boundary in a classical embedded-atom model of elemental Ni. Although both boundaries feature highly disordered structures near the melting point, the nature of the temperature dependence of the width of the disordered regions in these boundaries is qualitatively different. The former boundary displays behavior consistent with a logarithmically diverging premelted layer thickness as the melting temperature is approached from below, while the latter displays behavior featuring a finite grain-boundary width at the melting point. It is demonstrated that both types of behavior can be quantitatively described within a sharp-interface thermodynamic formalism involving a width-dependent interfacial free energy, referred to as the disjoining potential. The disjoining potential for boundary (i) is calculated to display a monotonic exponential dependence on width, while that of boundary (ii) features a weak attractive minimum. The results of this work are discussed in relation to recent simulation and theoretical studies of the thermodynamic forces underlying grain-boundary premelting.

Semi-Empirical Methods to Predict the Physical Properties of Ionic Liquids: An Overview of Recent Developments

The field of ionic liquids (ILs) has shown rapid growth in recent years. Much of this work has involved the synthesis of new ILs and their application in an ever-increasing number of areas. In contrast, there have been relatively few studies that investigate and attempt to predict the fundamental physical properties of ILs, which are extremely important for their applications. The quantitative prediction of the physical properties of unknown salts remains an important goal in IL research. This will allow the design of new ILs with specific properties tailored for particular applications, without the need for time-consuming trial and error syntheses. Recently, several studies have shown that it is possible to make predictions of the physical properties of ILs e.g. melting points, conductivities, viscosities, densities, surface tensions and refractive indices. This paper gives an overview of these semi-empirical methods and makes some comparisons regarding the accuracy of their predictions and their applicability to predicting the properties of unknown salts.

TWO-STATE STRUCTURE OF NANOMETER-SIZED Cu PARTICLES

Molecular dynamics simulations have been employed to investigate the static and dynamic properties of unsupported spherical Cu particles with size ranging between 1 and 10 nm. The potential energy, the structural arrangement, and the mobility of atomic species were studied for each nanometer-sized particle within the temperature range between 300 K and the melting point. Two distinct regions, namely an internal domain and a surface layer, can be identified within each nanoparticle on the basis of the radial profiles of such quantities. The atomic species belonging to the interior of the particle display a bulk-like behavior. By contrast, the surface layer is characterized by an excess potential energy associated with a disordered structure. At relatively low temperatures, the surface atoms possess structural and energetic features intermediate between the ones of a superheated bulk solid and of an undercooled bulk liquid. Pre-melting processes at the surface are also evident at temperatures close to the melting point. The nanometer-sized particles can be thus regarded as heterogeneous two-state systems consisting of roughly distinguishable bulk-like and surface regions.

Molecular dynamics simulations of the melting mechanisms of perfect and imperfect crystals of dimethylnitramine

The melting mechanisms of perfect and imperfect crystalline dimethylnitramine have been studied using molecular dynamics simulations. The imperfect crystal was created by removing approximately 10% of the molecules from the center of the simulation cell. The density, diffusion coefficient, translational and orientational order parameters, and void size were calculated as functions of temperature and simulation time. Upon melting, the volume of the imperfect crystal slowly decreases with time due to the shrinkage of the void then suddenly decreases to a minimum value due to collapse of the structure around the void with concomitant diffusion of molecules into the void. The simulation cell volume then increases as the liquid nucleus formed at the void expands. The melting of perfect crystals must occur by a different mechanism. As the temperature of the perfect crystal reaches the maximum superheating temperature, there is an increase in the thermal motions of the molecules that result in the formation of liquid centers (characterized by translational order parameter consistent with the liquid phase) at random locations. The liquid centers rapidly grow, resulting in a complete transition to the liquid phase. The increases in orientational and translational freedom occur simultaneously in the imperfect crystal, and in the perfect crystal, orientational freedom significantly precedes translational freedom.

Effect of surface wettability on liquid density, structure, and diffusion near a solid surface

Molecular dynamics and Langevin dynamics simulations are used to elucidate the behavior of liquid atoms near a solid boundary. Correlations between the surface wettability and spatial variations in liquid density and structure are identified. The self-diffusion coefficient tensor is predicted, revealing highly anisotropic and spatially varying mass transfer phenomena near the solid boundary. This behavior affects self-diffusion at a range of time scales. Near a more-wetting surface, self-diffusion is impeded by strong solid-liquid interactions that induce sharp liquid density gradients and enhanced liquid structure. Conversely, near a less-wetting surface, where solid-liquid interactions are weaker, the liquid density is low, the atoms are disordered, and diffusion is enhanced. These findings suggest that altering the wettability of a micro- or nanochannel may provide a passive means for controlling the diffusion of select targets towards a functionalized surface and controlling the reaction rate in diffusion-limited reactions.

Molecular dynamics studies of melting and solid-state transitions of ammonium nitrate

Molecular dynamics simulations are used to calculate the melting point and some aspects of high-temperature solid-state phase transitions of ammonium nitrate (AN). The force field used in the simulations is that developed by Sorescu and Thompson [J. Phys. Chem. A 105, 720 (2001)] to describe the solid-state properties of the low-temperature phase-V AN. Simulations at various temperatures were performed with this force field for a 4 x 4 x 5 supercell of phase-II AN. The melting point of AN was determined from calculations on this supercell with voids introduced in the solid structure to eliminate superheating effects. The melting temperature was determined by calculating the density and the nitrogen-nitrogen radial distribution functions as functions of temperature. The melting point was predicted to be in the range 445 +/- 10 K, in excellent agreement with the experimental value of 442 K. The computed temperature dependences of the density, diffusion, and viscosity coefficient for the liquid are in good agreement with experiment. Structural changes in the perfect crystal at various temperatures were also investigated. The ammonium ions in the phase-II structure are rotationally disordered at 400 K. At higher temperatures, beginning at 530 K, the nitrate ions are essentially rotationally unhindered. The density and radial distribution functions in this temperature range show that the AN solid is superheated. The rotational disorder is qualitatively similar to that observed in the experimental phase-II to phase-I solid-state transition.

Nucleation and melting from nanovoids

Melting in solids containing nanovoids is studied via molecular dynamics simulation, and the finite size effects are treated with different size systems. For the first time, we have found four typical stages in void melting that are different from the melting of bulk materials and nanoparticles. Melting in each of the stages is governed by the interplay among different thermodynamic mechanisms arising from the changes in the interfacial free energies, the curvature of the interface, and the elastic energy induced by the density change at melting. As a result, the local melting temperatures show a strong dependence on the void size, which is the root cause of the observed complex hierarchical melting sequence.

Formation and Decomposition of Ethane, Propane, and Carbon Dioxide Hydrates in Silica Gel Mesopores under High Pressure

The experimental data on decomposition temperatures for the gas hydrates of ethane, propane, and carbon dioxide dispersed in silica gel mesopores are reported. The studies were performed at pressures up to 1 GPa. It is shown that the experimental dependence of hydrate decomposition temperature on the size of pores that limit the size of hydrate particles can be described on the basis of the Gibbs-Thomson equation only if one takes into account changes in the shape coefficient that is present in the equation; in turn, the value of this coefficient depends on a method of mesopore size determination. A mechanism of hydrate formation in mesoporous medium is proposed. Experimental data providing evidence of the possibility of the formation of hydrate compounds in hydrophobic matrixes under high pressure are reported. Decomposition temperature of those hydrate compounds is higher than that for the bulk hydrates of the corresponding gases.

Comparative study of microstructural evolution during melting and crystallization

Molecular dynamics simulations, with the interaction between atoms described by a modified analytic embedded atom method, have been performed to obtain the atomic-scale details of isothermal melting in nanocrystalline Ag and crystallization from supercooled liquid. The radial distribution function and common neighbor analysis provide a visible scenario of structural evolution in the process of phase transition. The results indicate that melting at a fixed temperature in nanocrystalline materials is a continuous process, which originates from the grain boundary network. With the melting developing, the characteristic bond pairs (555), (433), and (544), existing in liquid or liquidlike phase, increase approximately linearly till completely melted. The crystallization from supercooled liquid is characterized by three characteristic stages: nucleation, rapid growth of nucleus, and slow structural relaxation. The homogeneous nucleation occurs at a larger supercooling temperature, which has an important effect on the process of crystallization and the subsequent crystalline texture. The kinetics of transition from liquid to solid is well described by the Johnson-Mehl-Avrami equation.

An effective pair potential for thermodynamics and structural properties of liquid mercury

The properties of liquid mercury are investigated by using an empirical effective pair potential. Its parameters are determined with the aid of Monte Carlo simulation along the liquid branch of the liquid-vapor coexistence curve. The complexity of the electronic structure of dense metal mercury supposes a state dependence of the interatomic interactions, while no more state dependence is found in the metal-nonmetal transition region. It is shown that the use of this effective potential leads to an accurate description of the structural and thermodynamic properties of the expanded liquid mercury. Then, the melting and freezing phenomena are investigated with that potential. Sharp melting and freezing temperatures are observed at 234 and 169 K, respectively. This large hysteresis loop between freezing and melting is consistent with the experiments for the bulk mercury.

Molecular-dynamics study of liquid nickel above and below the melting point

We have investigated the structural and dynamic properties of liquid nickel by means of large-scale molecular-dynamics simulations, using an effective-pair potential derived from the second-order pseudopotential perturbation theory. The model of interactions is assessed on the single-atom as well as collective dynamic properties. The short-range order in the stable and undercooled liquids is also examined. We show that the present model potential gives a description of the local structure in both states in close agreement with first-principles molecular-dynamics simulations.

Atomic Mechanisms of Grain Boundary Motion

We present results of atomistic computer simulations of spontaneous and stress-induced grain boundary (GB) migration in copper. Several symmetrical tilt GBs have been studied using the embedded-atom method and molecular dynamics. The GBs are observed to spontaneously migrate in a random manner. This spontaneous GB motion is always accompanied by relative translations of the grains parallel to the GB plane. Furthermore, external shear stresses applied parallel to the GB and normal to the tilt axis induce GB migration. Strong coupling is observed between the normal GB velocity vn and the grain translation rate v||. The mechanism of GB motion is established to be local lattice rotation within the GB core that does not involve any GB diffusion or sliding. The coupling constant between vn and v|| predicted within a simple geometric model accurately matches the molecular dynamics observations.

Melting Behaviors of Nanocrystalline Ag

In the atomic scale, the melting behaviors of nanocrystalline Ag with mean grain size ranging from 3.03 to 12.12 nm have been investigated with molecular dynamics simulations, and a method to determine the melting temperatures of the infinite polycrystalline nanostructured materials is presented. It is found that the melting in nanostructured polycrystals starts from their grain boundaries, and the relative numbers of the three typical bonded pairs, (1551), (1431), and (1541), existing in the liquid phase, increase rapidly with the evolvement of melting. The melting temperatures of nanocrystalline Ag decrease with decreasing mean grain size, and it can be estimated from the size-dependent melting temperature of the corresponding nanoparticles.

Simulations of the Solid, Liquid, and Melting of 1-n-Butyl-4-amino-1,2,4-triazolium Bromide

Molecular dynamics simulations are used to study the solid and liquid properties and to predict the melting point of 1-n-propyl-4-amino-1,2,4-triazolium bromide ([patr][Br]) using a force field based on the one developed by Canongia Lopes et al. (J. Phys. Chem. B 2004, 108, 2038) for dialkyl substituted imidazolium salts, which was modified by including terms from the general AMBER force field. Electrostatic charges for the intermolecular interactions were determined from gas-phase ab initio electron structure calculations of the triazolium cation. Simulations of the solid state at 100 K reproduced the experimental density to within 4%. Simulations from 100 K to the melting point and the liquid from 333 to 500 K were performed to determine the temperature dependence of the densities of the two phases. The structures of the solid and liquid phases are characterized with radial distribution functions, which show that there are strong spatial correlations among neighboring ion pairs in liquid [patr][Br]. The dynamic behavior of the ions in the liquid state is also studied by computing velocity autocorrelation functions and the mean-square displacements between the ions. The melting point is determined by simulating void-induced melting. Changes in the density, intermolecular energy, and Lindemann index are used as indicators of the melting transition. The computed melting point is 360 +/- 10 K, which is within 10% of the experimental value 333 K.

Molecular dynamics studies of melting and some liquid-state properties of 1-ethyl-3-methylimidazolium hexafluorophosphate [emim][PF6]

Molecular dynamics simulations are used to study the liquid-state properties and melting of 1-ethyl-3-methylimidazolium hexafluorosphosphate [emim][PF6] using the force field of Canongia Lopes et al. [J. Phys. Chem. B 108, 2038 (2004)] and geometric constants from crystallographic data. The structures of the solid and liquid states are characterized by carbon-carbon, carbon-phosphorous, and phosphorous-phosphorous radial distribution functions. Spatial correlations among the ions are strong in the liquid state. The cohesive energy density and the temperature dependences of the molar volume and density of the liquid have been computed. The melting point is determined by equilibrating the solid-state supercells in which void defects have been introduced to eliminate the free-energy barrier for the formation of a solid-liquid interface. The computed melting point is 375+/-10 K, which is approximately 10% higher than the experimental value of 333 K.