Heterogeneous vapor bubble nucleation was numerically simulated on surfaces containing nanometer-sized rectangular cavities of various sizes (ranging from 2.72–9.36 nm width & 4.09–8.34 nm depth). Non-equilibrium molecular dynamics simulations at constant wall temperature were carried out for the two-dimensional (2D) Lennard-Jones (LJ) fluid which was in thermal contact with LJ solid wall. Nucleation was induced by expanding the system volume to a metastable state while keeping the temperature of the solid wall constant. When the aspect ratio (depth-to-width) of the cavity was large (∼3-to-1), the nucleation rate was increased nearly three-fold compared to the flat surface for relatively large wettability; however, the nucleation rate was nearly the same for lower wettability surfaces. Nucleation was generally favored within the cavities due to the stabilization of the cavity side-walls. The value of cutoff radius, wall temperature, and solid-liquid wettability was varied to determine their effect on nucleation rate. The 2D model was validated by comparing thermodynamic averages of pressure and potential energy to predicted values from a 2D LJ equation of state.

This study reports non-equilibrium molecular dynamics (MD) simulations predicting the thermal conductivity of amorphous mesoporous silica. The heat flux was imposed using the Muller-Plathe method and interatomic interactions were modeled using the van Beest, Kramer and van Santen (BKS) potential. First, simulations were validated against results reported in the literature for dense quartz and amorphous silica. The BKS potential was found to significantly overestimate the thermal conductivity of dense amorphous silica and results depended on the length of the simulation cell. Then, highly ordered pores were introduced in an amorphous silica matrix by removing atoms within selected areas of the simulation cell. Effects of the simulation cell length, pore size, and porosity on the thermal conductivity were investigated at room temperature. Results were compared with predictions from commonly used effective medium approximations as well as with previously reported experimental data for films with porosity and pore diameter ranging from 20% to 48% and 30 to 180 Å, respectively. Predictions of MD simulations overestimated the experimental data and agreed with predictions from the coherent potential model. However, MD simulations confirmed that thermal conductivity in sol-gel amorphous mesoporous materials was independent of pore size and depended only on porosity.

A two dimensional toy model is developed to study thermal transport in cage like structures such a skutterudites and clathrates. The model consists of host atoms on a rectangular lattice with fillers in the center of each rectangle. The thermal conductivity is calculated by using Green-Kubo equilibrium molecular dynamics simulations. It is generally believed that the smaller and the heavier the filler, the lower is the thermal conductivity. We show that the thermal conductivity decreases with atomic displacement parameter while it has local minima versus filler mass. Our study shows that it is very important to include the correct band dispersion to get the right features of the thermal conductivity. We show that by having a double well potential one can further reduce the thermal conductivity.

An ice nucleation protein induces a phase transition from liquid water to ice in air. A specific hydrophilic surface of the protein may have an influence on the network of hydrogen bonds touching on the protein. However, microscopic characteristics of the ice nucleation protein and behavior of water molecules on it have not been clarified. So we carried out molecular dynamics simulations in various quasi-two-dimensional densities of water molecules on the ice nucleation protein. The percolation threshold of water clusters was confirmed. Comparing another hydrophilic protein, the threshold density in both cases had nearly the same value. But percolation probabilities and mean cluster sizes near the threshold were different between both cases. Those results implied that the threshold density was consistent with the conventional theory, but the forming of water clusters near the threshold was influenced by the hydrophilicity on the ice nucleation protein.

Molecular dynamics simulation has been applied for water to compare the Wolf method to the IPS method and the Ewald sum by evaluating the diffusion coefficient and liquid structure. In our previous study, we applied the IPS method for bulk water and found notable deviation of the radial distribution function g(r). The Wolf method gives a good estimation for the potential energy and the self-diffusion coefficient at a cutoff radius, r c , greater than 2.2 nm while avoiding the notable deviation of g(r) which appeared in the case of IPS. The distance dependent Kirkwood factor G k (r) was also calculated, and the truncation of a long-range interaction of the cutofflike method (such as cutoff with or without the switch function and the reaction field) show serious shortcomings for dipole-dipole correlations in bulk water systems. This was observed by comparing the shape to that of the Ewald sum. G k (r) of the cutofflike method greatly deviates from that of the Ewald sum. However, the discrepancy of G k (r) for the Wolf method was found to be much less than that of other typical cutoff-like methods. We conclude that the Wolf method is an adequately accurate technique for estimating transport coefficients and the liquid structure of water in a homogeneous system at long cutoff distances.

Yttrium Aluminum Garnet (YAG, Y 3 Al 5 O 12 ) and its varieties have applications in thermographic phosphors, lasing mediums, and thermal barriers. In this work, thermal properties of crystalline YAG where aluminum atoms are substituted with gallium atoms (Y 3 (Al 1−x Ga x ) 5 O 12 ) are explored with molecular dynamics simulations. For YAG at 300K, the simulations gave values close to experimental values for constant-pressure specific heat, thermal expansion, and bulk thermal conductivity. For various values of x, the simulations predicted no change in thermal expansion, an increase in specific heat, and a decrease in thermal conductivity for x = 50%. Furthermore, the simulations predicted a decrease in thermal conductivity with decreasing system size.

Many random substitutional solid solutions (alloys) will display a tendency to chemically order given the appropriate kinetic and thermodynamic conditions. Such order-disorder transitions will result in major crystallographic reconfigurations, where the atomic basis, symmetry, and periodicity of the alloy change dramatically. Consequently, phonon behavior in these alloys will vary greatly depending on the type and degree of ordering achieved. To investigate these phenomena, the role of the order-disorder transition on phononic transport properties of Lennard-Jones type binary alloys is explored via non-equilibrium molecular dynamics simulations. Particular attention is paid to regimes in which the alloy is only partially ordered. It is shown that, through exploitation of the long-range order parameter, thermal conductivity of binary alloys can be effectively tuned across half an order of magnitude at low-to-moderate temperatures.

We investigated the initial stage of nucleate boiling on ideally smooth surface with a molecular dynamics simulation technique. Lennard-Jones model liquid was confined in a rectangular simulation cell, contacting with a flat smooth heating wall. Extra kinetic energy was given to particles in vicinity of the wall. As the temperature of liquid on the wall increases, the liquid thermally expands, which causes the pressure decrease, leading to formation of bubble nuclei, or cavitation, of atomic scale. We found that the nucleation is affected by surface wettability (hydrophobic or hydrophilic) as well as the magnitude of heat flux. When the surface is hydrophobic and the heating area is small, a size oscillation of generated bubbles was observed, which is determined by the balance among the heating flux from the wall, thermal diffusion into the surrounding liquid, and latent heat consumption during the phase change.

Nonequilibrium molecular dynamics simulations are carried out on single component lipid bilayers with ambient water in order to investigate the effect of acyl chain length on heat transport characteristics along and across the membranes. In this study, dipalmitoyl-phosphatidyl-choline (DPPC), dilauroyl-phosphatidyl-choline (DLPC), and stearoyl-myristoyl-phosphatidyl-choline (SMPC) which has two acyl chains of both sixteen C atoms, both twelve C atoms, and eighteen and fourteen C atoms, respectively, were used as lipid molecules. In the direction along the membranes, thermal conductivity corresponds with that of each membrane. On the other hand, in the direction across membrane, the highest thermal resistance exists at the center of lipid bilayer where lipid acyl chains face each other. However, asymmetric chain length reduces thermal resistance at the interface between lipid monolayers. Therefore, thermal conductivity across the membrane which consists of asymmetric chain length is higher than those which consist of symmetric chain length.

We performed molecular dynamics simulations of the interface which is comprised of self-assembled monolayer (SAM) and water solvent to investigate heat transfer characteristics. In particular, local thermal boundary conductance (TBC), which is an inverse of so-called Kapitza resistance, at the SAM–solvent interface was evaluated by using the nonequilibrium MD (NEMD) technique in which the one-dimensional thermal energy flux was imposed across the interface. By using two kinds of SAM terminal with hydrophobic and hydrophilic properties, the local TBCs of these interfaces with water solvent were evaluated, and the result showed a critical difference due to an affinity between SAM and solvent. In order to elucidate the reason for this difference, microscopic components contributing to thermal energy flux across the interface were evaluated in detail, i.e., the total thermal energy flux is decomposed into the contribution of molecular transport and that of energy exchange by molecular interactions.

Using lattice dynamics theory, we derive the spectral energy density and the relation between the spectral energy density and the phonon frequencies and relaxation times. We then calculate the spectral energy density and phonon frequencies and relaxation times for a test system of Lennard-Jones argon using velocities obtained from molecular dynamics simulations. The phonon properties, which can be used to calculate thermal conductivity, are compared to predictions made using (i) anharmonic lattice dynamics calculations and (ii) a technique that performs normal mode analysis on the positions and velocities obtained from molecular dynamics simulations.

Superlattices made by superposing dielectric and metal nanolayers are of great interest as their small size may increase the thermoelectric figure of merit. Thus it is essential to predict their thermal conductivity. Potentials for Au and Si are discussed and the potential of 2NN MEAM is chosen as being the best for simulating heat transfer in Si/Au systems. Full 2NN MEAM Si/Au cross-potential parametrization is developed and the results are compared with ab initio calculus to test its ability to reproduce local density approximation (LDA) calculations. NVT molecular dynamics simulations are performed to deposit Au atoms on an Si substrate by physical vapor deposition and the results of the intermixing zone are in good agreement with the Cahn and Hilliard theory. Non-equilibrium molecular dynamics simulations are performed for an average temperature of 300 K to determine the Kaptiza conductance of Si/Au systems, and the value of 188 MW/m 2 K obtained is in good agreement with the results of Komarov et al. for Au deposited on isotopically-pure Si-28 and natural Si, with values ranging between 133 and 182 MW/m 2 K.

The high thermal conductivities of carbon nanotubes (CNTs) measured experimentally and predicted from theory suggest that they are good candidates for next-generation thermal management materials. The quantities of CNTs needed in applications preclude the use of pristine products. Limited work, however, has been done to study thermal transport in CNTs with defects. In this paper, the thermal conductivities of pristine CNTs and CNTs with various defect types (adatoms, single vacancies, double vacancies, and Stone-Wales) are systematically predicted using molecular dynamics simulation and a direct application of the Fourier law. We investigate the correlation between the thermal conductivity and defect energy.

Based on first-principles density-functional calculations, we have developed and tested a force-field for silicon, which can be used for molecular dynamics simulations and the calculation of its thermal properties. This force field uses the exact Taylor expansion of the total energy about the equilibrium positions up to 4th order. In this sense, it becomes systematically exact for small enough displacements, and can reproduce the thermodynamic properties of Si with high fidelity. Having the harmonic force constants, one can easily calculate the phonon spectrum of this system. The cubic force constants, on the other hand, will allow us to compute phonon lifetimes and scattering rates. Results on equilibrium Green-Kubo molecular dynamics simulations of thermal conductivity as well as an alternative calculation of the latter based on the relaxation-time approximation will be reported. The accuracy and ease of computation of the lattice thermal conductivity using these methods will be compared. This approach paves the way for the construction of accurate bulk interatomic potentials database, from which lattice dynamics and thermal properties can be calculated and used in larger scale simulation methods such as Monte Carlo.

The aim of this study is to estimate the interfacial thermal resistance between a carbon nano-particle and alkali molten salt eutectics using molecular dynamics simulations. Additionally the effect of particle shapes and sizes on the interfacial thermal resistance was investigated using three different shapes of the carbon nanoparticles. Transient heat transfer simulation between a carbon particle and molecules of a molten salt was performed with the lumped capacitance method. A carbonate salt eutectic which consists of lithium carbonate (Li 2 CO 3 ) and potassium carbonate (K 2 CO 3 ) in 62:38 molar ratio was used as a solvent medium for the nanoparticles. Three carbon particles of a single walled carbon nanotube (SWNT), a fullerene (C 60 ), and a graphite sheet were used to represent different shapes of cylinders, a spheres, and disks, respectively. The interfacial thermal resistance was determined by a correlation with a specific heat of the carbon particle, their surface area, and the time constant of decaying particle temperature. The results show the interfacial thermal resistance values are independent of the particle size for SWNT and graphite particles. For three carbon particles with a similar particle size, similar resistances were obtained in our simulations. The purpose of this study is to design and develop novel high-temperature Thermal Energy Storage (TES) materials in order to improve the operational efficiencies for harnessing solar thermal power at cheaper costs for Concentrated Solar Power (CSP) systems.

Nanoparticle growth based on three-dimensional heterogeneous nucleation was simulated by classical molecular dynamics. To collectively observe the effects of the dimension of seeds and thermodynamic conditions, seed size and system supersaturation ratio were the factors that were examined to see if they influenced the nucleation rates. Two stages were found to exist within the system, where the first stage is from the seed growth and the second from homogeneous nucleation. The Yasuoka-Matsumoto method was used to calculate the rates. The homogeneous nucleation characteristics coincided with the classical nucleation theory, but heterogeneous nucleation showed an irregular form, which at the current state cannot not be fully understood. Kinetic analysis was also performed to calculate the critical nucleus size and better understand the seed growth characteristics. All in all, the seed effects were insignificant to the overall nucleation characteristics for this system.

Thermal bubble nucleation was studied using molecular dynamics for both homogeneous and heterogeneous systems using isothermal-isobaric (NPT) and isothermal-isostress (NP zz T) ensembles. Simulation results indicate that homogeneous thermal bubble nucleation is induced from cavities occurring spontaneously in the liquid when the temperature exceeds the superheat limit. In contrast to published results using NVE and NVT ensembles, no stable nanoscale bubble exists in NPT ensembles, but instead, the whole system changes into vapor phase. For a heterogeneous system composed of a nanochannel with an initial distance of 3.49 nm between the two solid plates, it is found that if the liquid-solid interaction is equal to or stronger than that between liquid argon atoms, the bubble nucleation temperature of the confined liquid argon can be higher than the corresponding homogeneous nucleation temperature, because of the more ordered arrangement of atoms within two solid walls nanometers apart. This observation is in contradiction to the common understanding that homogeneous bubble nucleation temperature sets an upper limit for thermal phase change under a given pressure. Compared to the system where the liquid-solid interaction is the same as that between liquid argon atoms, the system with reduced liquid-solid interaction possesses a significantly reduced bubble nucleation temperature, while the system with enhanced liquid-solid interaction only has a marginally increased bubble nucleation temperature.

Multicanonical molecular dynamics simulations are applied, for the first time, to study the liquid-solid and solid-solid transitions in Lennard-Jones (LJ) clusters. The transition temperatures are estimated based on the peak position in the heat capacity versus temperature curve. For LJ 31 , LJ 58 and LJ 98 , our results on the solid-solid transition temperature are in good agreement with previous ones. For LJ 309 , the predicted liquid-solid transition temperature is also in agreement with previous result.

Molecular dynamics simulations of single water or water-IPA (Isopropyl-alcohol) mixture droplets on a solid surface were performed. The interaction for solid-water molecules was modeled as the combination of L-J and Coulomb potentials, and the effects of Coulomb interaction were independently investigated by appending arbitrary electric charge as a parameter on the solid surface. The water droplet became more wettable both with positive and negative surface charges as the absolute value of the electric charge increased, and the cosine of the contact angle was roughly a linear function of the absolute value of the electric charge although the correlation was obviously different between positive and negative charge values. Multiple molecular orientations seemed possible as local equilibrium states near the adsorption layer on negatively charged surface, and the mean rotational diffusion was higher there. On the other hand, mean rotational diffusion was reduced in the vicinity of the positively charged surface.

The thermal properties of a half-defective single-walled carbon nanotube, and one-dimensional (1D) nonlinear lattices were investigated to unveil the mechanism of defect-induced thermal rectification. We propose a model of an asymmetrically defective low-dimensional material to explain the underlying mechanism of thermal rectification obtained in a past experiment in which C 9 H 16 Pt was asymmetrically deposited on a nanotube. These numerical approaches show the applicability of asymmetrically defective low-dimensional material to solid-state thermal rectification.