Equilibrium Molecular Dynamics Method Research Articles

Thermal conductivity of irregularly shaped nanoparticles from equilibrium molecular dynamics.

The computation of thermal conductivity for finite nanoparticulate systems, particularly those of irregular shapes, poses significant challenges. Much of the previous work on the thermal conductivity of nanoparticles has been carried out by applying traditional nonequilibrium molecular dynamics (NEMD) methods. One of our recent works [Physical Review B 103, 035417 (2021)] proposed that equilibrium molecular dynamics (EMD) methods can be used for the simulation of thermal conductivity of finite-scale systems and demonstrated their equivalence to NEMD methods. In this work, we explored the application of the EMD approach to calculating the thermal conductivity of zero-dimensional nanoparticles. In our initial step, we merged both methodologies to substantiate the equivalence in thermal conductivity calculation for cube and cylinder nanoparticles. Post data filtration, the usefulness of EMD for thermal conductivity evaluation of zero-dimensional materials was confirmed. The NEMD method struggles with accuracy in nanoparticle systems characterized by a cross-sectional area change in the direction of transport, whereas in the case where the volume can be determined, EMD methods can be used to calculate the thermal conductivity. In a subsequent study, we used the latest machine learning potential (NEP) to calculate the thermal conductivity of spherical nanoparticles and evaluated the results against the classical Tersoff potential.Ultimately, predictions were made for the thermal conductivity of nanoparticles with various unique shapes in all directions. Collectively, our findings underscore the simplicity and effectiveness of using EMD methods for thermal conductivity calculations in the context of nanoparticles. This paves the way for novel strategies in studying the thermal transport properties of finite nanoparticle systems as well as nanopowders.

Thermal conductivity of GeTe crystals based on machine learning potentials

GeTe has attracted extensive research interest for thermoelectric applications. In this paper, we first train a neuro-evolution potential (NEP) based on a dataset constructed by ab initio molecular dynamics, with the Gaussian approximation potential (GAP) as a reference. The phonon density of states is then calculated by two machine learning potentials and compared with density functional theory results, with the GAP potential having higher accuracy. Next, the thermal conductivity of a GeTe crystal at 300 K is calculated by the equilibrium molecular dynamics method using both machine learning potentials, and both of them are in good agreement with the experimental results; however, the calculation speed when using the NEP potential is about 500 times faster than when using the GAP potential. Finally, the lattice thermal conductivity in the range of 300 K–600 K is calculated using the NEP potential. The lattice thermal conductivity decreases as the temperature increases due to the phonon anharmonic effect. This study provides a theoretical tool for the study of the thermal conductivity of GeTe.

Theoretical insights into the lattice thermal conductivity and thermal expansion of CoNiFe medium-entropy alloys

In this paper, we investigate the impacts of elemental concentration, tensile strain and temperature on the lattice thermal conductivity of CoNiFe medium-entropy alloys using the equilibrium molecular dynamics method.

Molecular dynamics simulation study to evaluate mechanical properties of plumbene using bending, oscillation and equilibrium MD approaches

Due to their exclusive structure and amazing properties, prodigious precedence is given to two-dimensional (2D) nanomaterials, in current years. Plumbene, the newly reported 2D allotrope of lead is cognate to stanene, silicene, graphene and germanene in atomic structure (hexagonally arranged and single-layered). Using multi-scale modeling a plumbene sheet is designed for the purpose of evaluating the mechanical properties of plumbene under different approaches. Earlier mechanical properties of plumbene were investigated using tensile modeling. To schlenter an immeasurable view of mechanical properties of the material for efficacious application in diverse engineering fields, investigation of plumbene sheets under different loading approaches are studied using molecular dynamics (MD) simulation. Young’s modulus of plumbene is estimated using bending, oscillation and equilibrium MD methods and compared with previously reported value from tensile modeling of plumbene. Effect of varying load on plumbene sample under similar conditions are also evaluated. Bulk’s modulus and Poisson’s ratio of plumbene are also estimated using equilibrium MD method. Present solicited techniques will mentor experimental creep up on plumbene sheets designing with precise mechanical properties for desired applications.

The effects of grain size and fractal porosity on thermal conductivity of nano-grained graphite: A molecular dynamics study

The thermal conductivity of isotropic graphite is a crucial parameter in mechanical and nuclear engineering. However, obtaining its value accurately is still difficult, especially under irradiation and high ambient temperature conditions. In this study, the nano-grained graphite models with different grain sizes and arrangements are designed, and corresponding thermal conductivity is calculated with equilibrium and non-equilibrium molecular dynamics methods. The results show that the thermal conductivity of nano-grained graphite varies between 3 to 5 W/(m·K) and decreases exponentially with the increase of the number of nanograins, while the arrangement of nanograins has almost no effect on the thermal conductivity. In addition, the effect of porosity of nano-polycrystalline graphite is analyzed by constructing fractal porous structures with different fractal dimensions and stages, of which the pore characteristics are determined with the initial zero-stage structure. The thermal conductivity of porous graphite increases with increasing fractal dimension and decreasing fractal porosity and the outcomes of molecular dynamics simulations are verified with the theoretical model results.

Molecular dynamics simulation of interfacial heat transfer characteristics of CO2, R32 and CO2/R32 binary zeotropic mixture on a smooth substrate

Mixture fluids have the advantage of a flexible adjustment of the physicochemical properties by actively tuning the composition and concentrations, showing promising potential in thermal and power-related applications. However, mixtures are mainly zeotropic with heat transfer deterioration during the boiling process, resulting in a challenge to gain a comprehensive understanding of their microscopic phase change. In this study, molecular dynamics simulations were employed to investigate the nanoscale behavior of CO2, R32 and CO2/R32 mixtures. The equilibrium molecular dynamics (MD) method and the non-equilibrium MD method was utilized to study the vapor-liquid interface characteristics and the boiling heat transfer characteristics of zeotropic mixture, respectively. The results revealed that the thickness of the vapor-liquid interface was linearly related to the temperature, and the surface tension displayed significant fluctuations in both solid-liquid and vapor-liquid interfaces, which indicated a potential barrier for molecular transport. Compared to the pure components, the inception of nucleation boiling and vapor film formation of the mixture fluid was significantly delayed. The heating and evaporation rate, heat flux of mixture were lower, while the thermal resistance was significantly higher. Moreover, the time required for bubble nucleation and the formation of vapor film decreased notably with the enhanced wetting characteristics. The findings in this study provide a molecular-scale insight into zeotropic mixture boiling heat transfer and the possible wettability regulation to improve the heat transfer performance of zeotropic mixtures and promote their wider applications.

Lattice thermal conductivity of defected tungsten evaluated by equilibrium molecular dynamics simulation

Exposed to high fluxes of helium particles and heat, tungsten divertor plates will suffer various forms of damage thus degrading its performance such as its thermal conductivity. In this paper, the equilibrium molecular dynamics method is employed to evaluate the lattice thermal conductivity (LTC) changes of the tungsten lattice containing one type of defects (dislocation loops, helium bubbles and impurity helium atoms). It is found that the LTC of tungsten is slightly reduced in the presence of dilute dislocation loops and vacuum bubbles, while a remarked reduction of the thermal conductivity is engendered by dilute helium particles. A tiny number of helium atoms as randomly-distributed tetrahedral interstitials (accounting for 0.2 % of the total number of atoms in the simulation cell), result in a drastic drop of the LTC to 15 % of that of the perfect tungsten lattice. The phonon density of states of the perfect and defected tungsten lattices are compared to identify the causes of the LTC reduction at the atomic level.

Interfacial thermal resistance calculations for weak solid–liquid atom interactions using equilibrium molecular dynamics

ABSTRACT This study investigates the use of equilibrium molecular dynamics (EMD) simulations for determining interfacial thermal resistance (ITR) at solid-liquid interfaces. The Green-Kubo theorem is typically used for this purpose, but it may not be suitable for hydrophobic conditions where the interaction between liquid and solid atoms is weak. Two EMD simulation methods were used to calculate ITR at interfaces under different wetting conditions: one using instantaneous temperature differences and the other using instantaneous heat flux. The data derived from the EMD simulation using instantaneous temperature difference could not converge due to weak intermolecular interactions, leading to inaccurate ITR calculations. To address this issue, a method for calculating ITR in the frequency domain is proposed. Results showed that both EMD methods produced different results from non-equilibrium molecular dynamics (NEMD), with the ITR of EMD using instantaneous heat flux being higher than that of NEMD, while the ITR of EMD using instantaneous temperature was lower, and in good agreement with NEMD. Overall, this study highlights the limitations of using EMD simulations for calculating ITR at solid-liquid interfaces in hydrophobic conditions and proposes a new method for improving accuracy in these situations.

Propagation characteristics of longitudinal modes in dusty plasmas

The space-time correlation function has been obtained in strongly coupled dusty plasmas (SCDPs) using equilibrium molecular dynamics (EMD) simulations. The simulated results for three-dimensional (3D) SCDPs with suitable normalization are computed over a wide domain of plasma parameters (Γ, κ) in a microcanonical ensemble. The EMD simulations indicate that different modes of propagated wave in SCDPs are analyzed for four different values of wave number (k). New investigations of normalized longitudinal current correlation function CL(k, t) show that the amplitude of oscillation and frequency of propagated modes increase with an increase in k. The obtained results for longitudinal modes of oscillation indicate that the dust particles remain in damping behavior at the low Γ, damped oscillation with decreasing amplitude inside decaying exponential envelope at intermediate Г, and sinusoidal oscillation at high Г, depending on κ. The system size (N) does not significantly affect the propagated modes of oscillation, while the periodic oscillation shifts toward higher Γ with increasing N and κ. The computations show that normalized longitudinal CL(k, t) current correlation particularly depend on Coulomb coupling (Γ), Debye screening (κ), and wave number (k). In our simulations, the frequency and the amplitude of oscillation of the dust particles decrease with an increment of κ and system size (N), but the frequency increases and the amplitude decreases with increasing Γ, as expected. It has been demonstrated that the EMD method is used to study the different propagated modes in dusty plasma systems and can be used to predict the damping behavior, damped oscillation, and periodic phenomena in 3D strongly coupled SCDPs.

In Silico Screening of Metal-Organic Frameworks and Zeolites for He/N2 Separation.

In silico screening of 10,143 metal-organic frameworks (MOFs) and 218 all-silica zeolites for adsorption-based and membrane-based He and N2 separation was performed. As a result of geometry-based prescreening, structures having zero accessible surface area (ASA) and pore limiting diameter (PLD) less than 3.75 Å were eliminated. So, both gases can be adsorbed and pass-through MOF and zeolite pores. The Grand canonical Monte Carlo (GCMC) and equilibrium molecular dynamics (EMD) methods were used to estimate the Henry's constants and self-diffusion coefficients at infinite dilution conditions, as well as the adsorption capacity of an equimolar mixture of helium and nitrogen at various pressures. Based on the obtained results, adsorption, diffusion and membrane selectivities as well as membrane permeabilities were calculated. The separation potential of zeolites and MOFs was evaluated in the vacuum and pressure swing adsorption processes. In the case of membrane-based separation, we focused on the screening of nitrogen-selective membranes. MOFs were demonstrated to be more efficient than zeolites for both adsorption-based and membrane-based separation. The analysis of structure-performance relationships for using these materials for adsorption-based and membrane-based separation of He and N2 made it possible to determine the ranges of structural parameters, such as pore-limiting diameter, largest cavity diameter, surface area, porosity, accessible surface area and pore volume corresponding to the most promising MOFs for each separation model discussed in this study. The top 10 most promising MOFs were determined for membrane-based, vacuum swing adsorption and pressure swing adsorption separation methods. The effect of the electrostatic interaction between the quadrupole moment of nitrogen molecules and MOF atoms on the main adsorption and diffusion characteristics was studied. The obtained results can be used as a guide for selection of frameworks for He/N2 separation.

Exactly equivalent thermal conductivity in finite systems from equilibrium and nonequilibrium molecular dynamics simulations

In a previous paper (Dong et al., 2021), we showed that the equilibrium molecular dynamics (EMD) method can be used to compute the apparent thermal conductivity of finite systems. It has been shown that the apparent thermal conductivity from EMD for a system with domain length 2 L is equal to that from nonequilibrium molecular dynamics (NEMD) for a system with domain length L . Taking monolayer silicene with an accurate machine learning potential as an example, here we show that the thermal conductivity values from EMD and NEMD agree for the same domain length if the NEMD is applied with periodic boundary conditions in the transport direction. Our results thus establish an exact equivalence between EMD and NEMD for thermal conductivity calculations. • Large-scale MD simulations are performed with an accurate machine learned potential. • Exactly equivalent thermal conductivity in EMD and NEMD has been established. • The exact equivalence is explained in terms of phonon-boundary scattering. • We suggest that EMD serves as a feasible alternative method to NEMD in finite systems.

Molecular dynamics study of sucrose aqueous solutions: From solution structure to transport coefficients

In this work, we study the applicability of the equilibrium molecular dynamics methods for the prediction of solution structure, equation of state and transport properties of sucrose aqueous solution for various concentrations and temperatures. The OPLS-AA force field with 1.14*CM1A charge corrections is used to describe the interactions in the model. We analyze the prediction power of the model against the experimental data and simulation results, obtained previously in CHARMM, OPLS-AA, GLYCAM06 and their modifications. The glycosidic linkage conformations of sucrose in solution are found. Their independence from the sugar concentration and the appearance of the conformation region at temperatures from 283 K to 308 K are observed. Sucrose solution densities are calculated for different sugar concentrations and temperatures. The obtained viscosities and sucrose diffusion coefficients match the experimental data at low sucrose concentrations, and diverge within 41% from the experiment as the concentration increases. The change of boundary condition in Stokes–Einstein formula is observed with an increase of the sucrose concentration above 40%.

Strong reduction of thermal conductivity of WSe2 with introduction of atomic defects

The thermal conductivities of pristine and defective single-layer tungsten diselenide (WSe2) are investigated by using equilibrium molecular dynamics method. The thermal conductivity of WSe2 increases dramatically with size below a characteristic of ~5 nm and levels off for broader samples and reaches a constant value of ~2 W/mK. By introducing atomic vacancies, we discovered that the thermal conductivity of WSe2 is significantly reduced. In particular, the W vacancy has a greater impact on thermal conductivity reduction than Se vacancies: the thermal conductivity of pristine WSe2 is reduced by ~60% and ~70% with the adding of ~1% of Se and W vacancies, respectively. The reduction of thermal conductivity is found to be related to the decrease of mean free path (MFP) of phonons in the defective WSe2. The MFP of WSe2 decreases from ~4.2 nm for perfect WSe2 to ~2.2 nm with the addition of 0.9% Se vacancies. More sophisticated types of point defects, such as vacancy clusters and anti-site defects, are explored in addition to single vacancies and are found to dramatically renormalize the phonons. The reconstruction of the bonds leads to localized phonons in the forbidden gap in the phonon density of states which leads to a drop in thermal conduction. This work demonstrates the influence of different defects on the thermal conductivity of single-layer WSe2, providing insight into the process of defect-induced phonon transport as well as ways to improve heat dissipation in WSe2-based electronic devices.

Size and stoichiometric dependence of thermal conductivities of InxGa1-xN: A molecular dynamics study

The thermal conductivities κ of wurtzite InxGa1-xN are investigated using equilibrium molecular dynamics (MD) method. The κ of InxGa1-xN rapidly declines from InN (κInN = 141 W/mK) or GaN (κGaN = 500 W/mK) to InxGa1-xN (x≠ or 1), and reaches a minimum (κmin = 19 W/mK) when x is around 0.5 at 300 K. The mean free path (MFP) of InxGa1-xN, ranging from 2 to 5 nm and following the same trend with the κ, is extrapolated in our simulation and a parabolic relationship between x and MFP is established. We find that the κ of InxGa1-xN decreases with increasing temperatures. The evolution of κ of InxGa1-xN is also examined by projecting the momentum-energy relationship of phonons from MD trajectories. The phonon dispersion and phonon density of states for InxGa1-xN reflect a slightly more flattened dispersive phononic curve of the alloying system. Despite an overestimated κ than experimental values, our calculated κ at 300 K agrees well with the results obtained by solving Boltzmann transport equation and also has the same stoichiometric trend with the experimental data. Our study provides the coherent analysis of the effect of thickness, temperature and stoichiometric content on the thermal transport of InxGa1-xN which is helpful for the thermal management of InxGa1-xN based devices.

Separation of SO2 and NO2 with the Zeolite Membrane: Molecular Simulation Insights into the Advantageous NO2 Dimerization Effect.

NO2 and SO2, as valuable chemical feedstock, are worth being recycled from flue gases. The separation of NO2 and SO2 is a key process step to enable practical deployment. This work proposes SO2 separation from NO2 using chabazite zeolite (SSZ-13) membranes and provides insights into the feasibility and advantages of this process using molecular simulation. Grand canonical ensemble Monte Carlo and equilibrium molecular dynamics methods were respectively adopted to simulate the adsorption equilibria and diffusion of SO2, NO2, and N2O4 on SSZ-13 at varying Si/Al (1, 5, 11, 71, +∞), temperatures (248-348 K), and pressures (0-100 kPa). The adsorption capacity and affinity (SO2 > N2O4 > NO2) demonstrated strong competitive adsorption of SO2 based on dual-site interactions and significant reduction in NO2 adsorption due to dimerization in the ternary gas mixture. The simulated order of diffusivity (NO2 > SO2 > N2O4) on SSZ-13 demonstrated rapid transport of NO2, strong temperature dependence of SO2 diffusion, and the impermeability of SSZ-13 to N2O4. The membrane permeability of each component was simulated, rendering a SO2/NO2 membrane separation factor of 26.34 which is much higher than adsorption equilibrium (6.9) and kinetic (2.2) counterparts. The key role of NO2-N2O4 dimerization in molecular sieving of SO2 from NO2 was addressed, providing a facile membrane separation strategy at room temperature.

Molecular dynamics study of effects of point defects on thermal conductivity in cubic silicon carbide

Silicon carbide (SiC) has been widely used in nuclear technology due to its excellent properties. In the irradiation environment, the energetic incident particles can cause the atoms in the material to deviate from the position of the crystal lattice, thereby producing the vacancies, interstitial atoms, anti-site atoms and other point defects. These defects will change the thermal properties of the material and degrade the service performance of the material. Therefore, in this work the equilibrium molecular dynamics method (Green-Kubo method) is used to study the effect of point defects on the heat transfer properties of cubic SiC (<i>β</i>-SiC or 3<i>C</i>-SiC) with the help of the Tersoff-type potential. The point defects considered include Si interstitial atoms (Si<sub>I</sub>), Si vacancies (Si<sub>V</sub>), Si anti-site atoms (Si<sub>C</sub>), C interstitial atoms (C<sub>I</sub>), C vacancies (C<sub>V</sub>) and C anti-site atoms (C<sub>Si</sub>). It is found that the thermal conductivity (<i>λ</i>) decreases with the increase of the point defect concentration (<i>c</i>). The excessive thermal resistance (Δ<i>R</i> = <i>R</i><sub>defect </sub>– <i>R</i><sub>perfect</sub>, <i>R</i> = 1/<i>λ</i>, <i>R</i><sub>defect</sub> is the thermal resistance of the defective material, and <i>R</i><sub>perfect</sub> is the thermal resistivity of the material without defects) has a linear relation with the concentration of point defects in the considered range (0.2%–1.6%), and its slope is the thermal resistivity coefficient. It can be found that the thermal resistivity coefficient of vacancy and interstitial atoms are higher than that of anti-site atoms; the thermal resistivity coefficient of point defects at high temperature is higher than at low temperature; the thermal resistivity coefficient of Si vacancies and Si interstitial atoms are higher than that of C vacancies and C interstitial atoms. These results are helpful in predicting the thermal conductivity of silicon carbide under irradiation and controlling the thermal conductivity of silicon carbide.

Research on sintering process and thermal conductivity of hybrid nanosilver solder paste based on molecular dynamics simulation

Nanosilver solder paste has become one of the widely materials in the field of electronic packaging due to its excellent thermal conductivity and mechanical properties. This paper considers the rapid sintering technology characteristics, the sintering processes of the same diameters and hybrid diameters nanosilver solder paste are simulated from the microscope perspective based on the molecular dynamics simulation theory. By tracing the atoms' trajectories on the surface of the nanosilver particles, the dominant sintering mechanism is obtained. Then, the thermal conductivity of the sintered silver nanoparticles is calculated by the equilibrium molecular dynamics method. The simulation results show that the densest sintered silver layer with the highest thermal conductivity is obtained when the diameter ratio of hybrid nanosilver balls is around 1:1.4, which provides a technical reference for the future proportioning of nanosilver solder paste.

Comparison of atomic simulation methods for computing thermal conductivity of n-decane at sub/supercritical pressure

n-Decane is the most commonly used component in surrogate to study the properties of RP-3 aviation kerosene for regenerative cooling systems and the engine injection systems. In this paper, the equilibrium molecular dynamics (EMD) and the reverse non-equilibrium MD (RNEMD) methods are adopted and compared in the prediction of the thermal conductivity of the sub/supercritical n-decane. Four different united atom (UA) force field models and four all-atomic force field models are compared in the EMD simulations. It is found that the UA models predict much better than the all-atom force field models and EMD methods show better prediction accuracy than the RNEMD methods with the same UA models. The SKS model has the best prediction accuracy among all the force field models for EMD and RNEMD simulations. The MD results are compared with experimental data of RP-3 aviation kerosene; it is obtained that the overall averaged absolute relative deviation (AARD) of EMD simulations with the SKS force field model for the single component substitute of aviation kerosene by n-decane is 2.05%. Moreover, the radial distribution functions are calculated via MD simulations to make a better understanding of the temperature dependences of the thermal conductivity at sub/supercritical pressure of n-decane at a molecular level. The findings of this work could provide important guidance for the investigation of thermal conductivity of n-alkanes and n-alkane-based fuels.

Effect of Modification on the Fluid Diffusion Coefficient in Silica Nanochannels.

The diffusion behavior of fluid water in nanochannels with hydroxylation of silica gel and silanization of different modified chain lengths was simulated by the equilibrium molecular dynamics method. The diffusion coefficient of fluid water was calculated by the Einstein method and the Green–Kubo method, so as to analyze the change rule between the modification degree of nanochannels and the diffusion coefficient of fluid water. The results showed that the diffusion coefficient of fluid water increased with the length of the modified chain. The average diffusion coefficient of fluid water in the hydroxylated nanochannels was 8.01% of the bulk water diffusion coefficient, and the diffusion coefficients of fluid water in the –(CH2)3CH3, –(CH2)7CH3, and –(CH2)11CH3 nanochannels were 44.10%, 49.72%, and 53.80% of the diffusion coefficients of bulk water, respectively. In the above four wall characteristic models, the diffusion coefficients in the z direction were smaller than those in the other directions. However, with an increase in the silylation degree, the increased self-diffusion coefficient due to the surface effect could basically offset the decreased self-diffusion coefficient owing to the scale effect. In the four nanochannels, when the local diffusion coefficient of fluid water was in the range of 8 Å close to the wall, Dz was greater than Dxy, and beyond the range of 8 Å of the wall, the Dz was smaller than Dxy.

A highly accurate interatomic potential for LaMnO3 perovskites with temperature-dependence of structure and thermal properties

ABO3-type perovskites LaMnO3 is a colossal magnetoresistance material that undergoes an orthorhombic-rhombohedral phase transition as the temperature increases. Phase transition determines its structural properties and thermal conductivity that are important in industrial application. Structural transition is reflected mainly in Mn-O bond lengths and Mn-O-Mn angles between MnO6 octahedrons for Jahn-Teller effect during this progress and traditional born–mayer (BM) model has difficulty in description of structural distortion and thermal properties. In this paper, a new classic interatomic potential model for LaMnO3 was developed within the framework of bond valence (BV) theory. First-principles and intelligent optimization algorithm were used to fit the parameters, enabling the study of temperature-dependent structures by accurate large-scale molecular dynamics (MD) simulations. Calculated structural distortions by our model and thermal properties calculated with equilibrium molecular dynamics (EMD) method accorded with the experimental values within a reasonable error, and had higher accuracy than traditional born-mayer model. These results provided further insight about temperature-depended phase transition and further performance mining of LaMnO3. Most importantly, our potential model showed better accuracy than traditional potential model and is applicablefor all crystal materials with perovskite structures.