Nonequilibrium 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.

Elastic and inelastic phonon scattering effects on thermal conductance across Au/graphene/Au interface

Heat dissipation from graphene devices is predominantly limited by heat conduction across the metal contacts with complex phonon scattering. In this work, the effects of elastic and inelastic phonon scattering on the interfacial thermal conductance (ITC) across the Au/graphene/Au interface are studied using both atomistic Green's function (AGF) and reverse non-equilibrium molecular dynamics methods. The results show that the contribution of inelastic phonon scattering to the ITC increases with the enhancement of interfacial bonding strength. Moreover, the overlap of the vibrational density of states across the interface shows that the coupling between the Au layer (adjacent to the Au/graphene interface) and graphene's out-of-plane modes plays the dominant role in ITC across the Au/graphene interface. By comparing the transmission functions calculated with AGF and spectral heat current decomposition methods, the inelastic phonon scattering process facilitates phonon transmission in the lower and higher frequency range but hinders phonon transmission in the intermediate frequency range. It is expected that this study can contribute to a better understanding of the thermal conduction mechanism across the metal/graphene interface, providing guidance for thermal management and heat conduction optimization of graphene in microelectronic devices.

Molecular dynamics study of the rheology of benzene-based nanofluids with metal particles

The purpose of this paper is to study the rheology of the nanofluids with ordinary spherical particles. The rheology of benzene and benzene-based nanofluids with copper and aluminium particles of 3 and 6 nm diameter been studied by the nonequilibrium molecular dynamics method. The maximum volume concentration of the nanoparticles was 6.11 %. It has been shown that all fluids considered, including benzene and benzene-based nanofluids, exhibit Newtonian behavior at low shear rates. However, with increasing shear rate, both benzene and nanofluids become pseudoplastic. The mechanisms of the rheology change of simple liquid and nanofluids have studied. This change in their rheology is a threshold phenomenon, in which the critical shear rate depends on the concentration, size, and material of the nanoparticles. The critical shear rate increases with increasing particle size and decreases with increasing particle concentration. Correlations describing the critical shear rates at which the rheology of nanofluids changes are obtained. It is shown that the rheology change is due to a change in the structure of both the base fluid and the nanofluid. In all cases, the change in rheology results from the weakening of the short-range order in these fluids. As the shear rate increases, the difference between viscosity of the base fluid and all studied nanofluids decreases monotonically.

Effect of the electric field orientation on the thermal resistance of the solid–liquid interface

ABSTRACT With the rapid development of nanotechnology, using the applied electric field to regulate interfacial heat transfer has become increasingly important. In the present work, the relationship between the thermal resistance of Kapitza and the direction of the applied electric field is explored with nonequilibrium molecular dynamics method at a solid–liquid interface consisting of CU (0, 0, 1) and liquid water. It found that the electric field orientation induces the ordering of water molecules near the solid, which affects the magnitude of the Kapitza thermal resistance. In addition, the electric field orientation affects the degree of mismatch between solid and liquid vibrational dynamical density (VDOS), which affects the phonon transport at the solid–liquid interface, and ultimately affects the process of interfacial heat transfer. Furthermore, it’s found that there is a weak correlation between the interfacial thermal resistance and the dimension of the copper-water model.

Thermal transports of 2D phosphorous carbides by machine learning molecular dynamics simulations

Carbon phosphide is a newly discovered two-dimensional semiconductor material which wrinkles and has a significant carrier mobility. Due to lack an accurate force field, the use of molecular dynamics to study its phonon-dominated thermal conductivity which lead to inaccurate results. At present, the use of machine learning to construct a high-precision force field has become the mainstream research method to solve this problem. The main work of this study is to construct a comprehensive training sets for Phosphorus-Doped Graphene (PCn) (n = 3, 5, 6) and to use the fitted potential to calculate the related thermal properties. The research found that (PC5) exhibited anisotropic behavior, with a thermal conductivity of 106.6 Wm−1K−1 in the y-direction and 63.6 Wm−1K−1 in the x-direction. In comparison, (PC6) and (PC3) showed isotropic behavior, with thermal conductivity of approximately 104 Wm−1K−1 and 76.83 Wm−1K−1, respectively. Compared to monolayer graphene, the lower thermal conductivity of PCn is mainly attributed to phonon-phonon scattering effects, which are limited by the regular wrinkled structure. Additionally, low-frequency phonon have been found to have a significant impact on the thermal performance of PCn. Furthermore, we investigated the influence of uniaxial strain on the PC6 and observed an increase in the thermal conductivity with increasing strain. This study used key computational and analytical techniques, including phonon dispersion relations, homogeneous nonequilibrium molecular dynamics method, spectral thermal conductivity analysis. These findings provide a theoretical basis for understanding the thermal transport properties of PCn and will guide its potential applications value.

Effects of Topological Parameters on Thermal Properties of Carbon Nanotubes via Molecular Dynamics Simulation

Due to their unique properties, carbon nanotubes (CNTs) are finding a growing number of applications across multiple industrial sectors. These properties of CNTs are subject to influence by numerous factors, including the specific chiral structure, length, type of CNTs used, diameter, and temperature. In this topic, the effects of chirality, diameter, and length of single-walled carbon nanotubes (SWNTs) on the thermal properties were studied using the reverse non-equilibrium molecular dynamics (RNEMD) method and the Tersoff interatomic potential of carbon–carbon based on the Large-scale Atomic/Molecular Massively Parallel Simulator (LAMMPS). For the shorter SWNTs, the effect of chirality on the thermal conductivity is more obvious than for longer SWNTs. Thermal conductivity increases with increasing chiral angle, and armchair SWNTs have higher thermal conductivity than that of zigzag SWNTs. As the tube length becomes longer, the thermal conductivity increases while the effect of chirality on the thermal conductivity decreases. Furthermore, for SWNTs with longer lengths, the thermal conductivity of zigzag SWNTs is higher than that of the armchair SWNTs. Thermal resistance at the nanotube–nanotube interfaces, particularly the effect of CNT overlap length on thermal resistance, was studied. The simulation results were compared with and in agreement with the experimental and simulation results from the literature. The presented approach could be applied to investigate the properties of other advanced materials.

The function of nano layer in enhancing the thermal conductivity of TiO2/water nanofluids

Nanoparticles have the capability to effectively improve the thermal conductivity of base fluids, thus improving the heat transfer coefficient of heat transfer systems. In this study, a non-equilibrium molecular dynamics (NEMD) method based on the Fourier law is employed to study the thermal conductivity of TiO2 (r-TiO2)/water nanofluids with temperatures ranging between303K and 333K and volume fractions in the range of1-2%. The ordered layer structure as a shell is analyzed and its influence is surveyed by calculating the number density and radial distribution function (RDF).The results revealed that a clear, solid-like nanolayer of about 0.5 nm can be observed around the nanoparticle. In this regard, the thickness of the nanolayer is less affected by variations in volume fraction and temperature. The g(r) values and the number density decreased with the increase in temperature. Additionally, the g(r) values and the number density at the level of the nanolayer were much higher compared to those at other parts. This indicates the existence of more water molecules in the nanolayer, thereby reducing contact thermal resistance and improving thermal conductivity. Macroscopically, the thermal conductivity increases with the increase in volume fraction. It was found that the increase in the volume fraction from 1%to 2%at303Kresulted in an increase in the effective thermal conductivity from1.027 and 1.042, respectively. In other words, the thermal conductivity of the nanofluid was 2.7% and 4.2% higher than that of the base liquid under the same conditions.

Interface Thermal Conductance and Phonon Thermal Transport Characteristics of Diamond/Carbon Nanotube Interface

As an ultra-wide band gap semiconductor material, diamond is an ideal material for high-power, high-frequency, high-temperature, and low-power loss electronic devices. However, high-frequency and high-power working environment leads to ultra-high local hot spots. Thermal interface material (TIM) is urgently needed to improve interface heat dissipation. Carbon nanotube (CNT), a brand-new generation of TIM, has ultra-high thermal conductivity (6000 W/(m·K)) and is expected to solve the heat dissipation problem of diamond semiconductor. Based on this, we first propose to combine diamond and CNT together so as to improve the performance and stability of semiconductor device and reduce packaging size to achieve miniaturized device design. Here we use reverse non-equilibrium molecular dynamics (RNEMD) method to study the thermal transport characteristics and interface thermal conductance (ITC) at the diamond/CNT interface. Results reveal that increasing CNT layers enhances the overall vibration density of states (VDOS) of CNT and shifts the peak value to the low frequency band, which is more conducive to interface heat transfer. Alternatively, the enhancement of the phonon overlap energy strengthens the coupling vibration of phonon and thus improves the efficiency of the interfacial heat transfer. Moreover, in a certain range, the increase of system temperature and CNT length-to-diameter ratio can raise the cutoff frequency of the VDOS of diamond and CNT near the interface and the peak value of the low frequency band. This further improves the coupling vibration of phonon on both sides. Finally, by orthogonal test simulation, the optimal value of ITC was determined, which is 2.65 GW/(m<sup>2</sup>·K) when the temperature, chirality, layers and length are 900 K, (6, 6), 6 layers and 5 nm respectively. This result significantly surpasses the current ITC of general semiconductors/metal. Compared to general composite materials, diamond/CNT composite material has great potential to enhance heat dissipation. Furthermore, according to P value test, the influence of the number of layers on interfacial thermal transport is extremely significant, while the influence of length, temperature and diameter decreases in turn.

Effective thermal contact conductance of encased silicon ball from molecular dynamics simulations

The heat dissipation performance of point-contact structures in many micro/nano-devices is critical. In this study, the effects of the normal load and diameter of the silicon ball on the effective thermal contact conductance (Geff) of the encased silicon ball between two silicon plates were studied by using the non-equilibrium molecular dynamics method. The results show that increasing the normal load increases the real contact area between the silicon ball and silicon plate, and thus improves the transmission channel and the density of states of low-frequency phonons, resulting in the observation that the Geff increases with increasing normal load. It is also found that the Geff is not sensitive to the diameter of the silicon ball from 2.66 to 4.16 nm at low normal load due to the competition between the enhanced interfacial thermal conductance across the contact region and the reduced internal thermal conductance of the silicon ball. The variation of Geff in the encased silicon ball structure with the external load and diameter has important guidance for the packaging and thermal design of micro/nano-devices.

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.

A sensitivity analysis on thermal conductivity of Al2O3-H2O nanofluid: A case based on molecular dynamics and support vector regression method

The current study examines the sensitivity study of the thermal conductivity of water-based alumina nanofluids with variations in concentration, sphericity, and temperature. Thermal conductivity data of alumina nanofluids were obtained by simulation using the non-equilibrium molecular dynamics method with the parameters of nanoparticle volume fraction (1.24–6.2%), particle sphericity (0.6975–1.0), and temperature (290–360 K). Volume fraction and temperature have been discovered to have a major effect on the thermal conductivity of nanofluids, but sphericity should also be taken into consideration. Further, for parameter sensitivity analysis, a support vector machine regression model of thermal conductivity versus parameters was created with a coefficient of determination up to 0.98. The findings show that an Al2O3-H2O nanofluid's thermal conductivity increases with temperature and volume fraction while decreasing with sphericity. Temperature, sphericity, and volume fraction are the variables that are most sensitive to modification, according to a sensitivity analysis. A change in thermal conductivity of up to 4 % can occasionally be brought on by temperature differences of merely 1 %. The use of spherical particles makes it possible for the thermal conductivity of the nanofluid to be more stable, and the sensitive volume fraction region that can cause thermal conductivity to fluctuate across a large range is between 2.5 % and 5.5 %.

Thermo-mechanical characteristics of oxide-coated aluminum nano-powder

This paper presents a comprehensive study on the thermo-mechanical characteristics of oxide-coated aluminum nano-powder, which is a crucial property for their application in powder metallurgy, particularly in warm compaction process. A representative volume element (RVE) of oxide-coated aluminum nano-powder is analyzed under the hydrostatic compression test at different temperatures using the Reactive Molecular Dynamics (RMD) method that can predict the thermal conductivity of nano-powders by incorporating the interaction effects of density and temperature. The thermal conductivity is evaluated based on the Müller-Plathe's reverse non-equilibrium molecular dynamics (rNEMD) method, and the size dependence of the method is investigated. The computational model is validated with the experimental data. The results illustrate that the thermal conductivity of oxide-coated aluminum nano-powder is significantly lower than that of the bulk aluminum due to restricted surface contact between individual aluminum particles. Moreover, it is observed that increasing the density and temperature leads to an increase in the thermal conductivity of oxide-coated aluminum nano-powder.

Molecular dynamics simulation of Sc2O3-Gd2O3-Y2O3 dopant effect on the crystal structure and thermophysical properties of zirconia-based ceramic materials

This paper aims to provide an insight into the effects of different doping ratios on the crystal structure, thermal conductivity, ion diffusion, and high-temperature thermal expansion coefficient of Sc2O3-Gd2O3-Y2O3 co-doped ZrO2 (ScGdYSZ) materials. They were investigated by a non-equilibrium molecular dynamics method. The results show that with Gd2O3-Sc2O3-Y2O3 co-doping, the thermal expansion of the resulting materials is all relatively close to that of 8YSZ materials and linear with temperature. At 1473 K, 1 mol%Sc2O3–3 mol%Gd2O3–4 mol%Y2O3 stabilized ZrO2 (1Sc3Gd4YSZ) coating material has the smallest thermal conductivity, with a thermal conductivity of 5.66 W·(m·K)−1. The mean square displacement (MSD) of O2− was linear with the time when the sum of the doping ratio of Sc2O3 and Gd2O3 was 4 mol%, and the MSD of O2− in the material increased with the doping ratio of Sc2O3. increases, the MSD of O2− in the material increases, and the migration diffusion rate is accelerated.

Effects of lattice orientation and defect degree on Si/Al solid interfacial structure and thermal resistance

ABSTRACT The effect of variation in the Si/Al interface structure on the thermal properties is explored with non-equilibrium molecular dynamics method in present work, and two distinct approaches are employed to manipulate the interface structure. The first is to change the arrangement of the atoms in the Al region by changing the orientation of the axis. The second one is to dig out several layers of atoms near the Al interface, so that the interface appears as a groove-like defect with different depths. Both methods exhibit a direct impact on atomic surface density. The high atomic surface density indicates a more compact atomic arrangement inside the structure, which improves the heat transport. In addition, the internal mechanism of thermal resistance change is probed from the aspects of Grain Boundary (GB) energy, vibrational density of states (VDOS) and the overlap of phonon spectrums parameter. It is found that structures with high atomic surface density correspond to smaller GB energies and larger overlap of phonon spectral parameters. Furthermore, by comparing the two ways of changing the interface structure, it is found that changing the degree of interface defects has a larger effect on the interfacial thermal resistance.

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.

Non-equilibrium molecular dynamics studies of thermal diffusion of hydrogen isotopes in low concentration zirconium hydrides

Tritium permeability in zirconium-based tritium getter critically impacts tritium storage and environmental safety during operation of tritium-producing burnable absorber rods (TPBARs). Previous experiments indicated that during irradiation operation, the hydrogen equilibrium pressured is increased. Further experimental and modeling studies suggested that the enhanced tritium release observed for reactor scale assemblies might be related to a thermal diffusion known as the Soret effect. A direct measurement of the Soret factor, however, has not been performed. To improve TPBAR and other nuclear applications, here we have applied two non-equilibrium molecular dynamics methods to study thermal diffusion of hydrogen isotopes in low-concentration zirconium hydrides. One of the methods produces sufficiently converged results to distinguish crystal orientation, isotope type, and concentration effects. With this method, crystal orientation, isotope type, and concentration effects are discussed.

Improving inherent thermal conductivity of epoxy resins based on contribution components of thermal conductivity: A molecular dynamics study

The inherent thermal transport properties of epoxy resin are vital factors to be studied in applications. In the present work, we constructed the amorphous crosslinking structure, the step crosslinking structure and the ordered crosslinking structure based on molecular dynamics theory, and used the reverse nonequilibrium molecular dynamics (RNEMD) method to calculate the thermal conductivity (TC) of different structures. Based on the numerical verification of the reliability of the method, the contribution of bonding to the TC was enhanced by increasing the molecular arrangement orderliness to reduce phonon scattering, taking the composition of the TC contribution as the guide. Meanwhile, the contribution of non-bonding interaction to the TC was enhanced by using compression process to achieve TC increase in the epoxy resin intrinsic structure, followed by exploring the effect of cross-linking degree on the TC of different structures. The results showed that the TC of amorphous epoxy resin decreased with the increase of crosslinking degree, and the relative high-frequency VDOS increased, but the influence of crosslinking degree on TC was minor in the range. For amorphous epoxy resin four-sided pressurization varied in the range of 0–7 Gpa, the non-bond energy percentage increased in the range of 0–60%, and the increase of non-bond energy made its contribution to TC increased significantly. The epoxy resin achieved 215% improvement in intrinsic TC by changing the molecular arrangement orderliness. The ordered structure TC varied from 0.3 to 3.5 W/(m⋅K) in the pressure variation range of 0–7 GPa. As the structure becomes more ordered, the TC increased significantly with the degree of cross-linking. The study provided the theoretical basis for the realization of resin preparation and resin thermal property evaluation with regulated intrinsic TC.

Molecular insight into the oil displacement mechanism of CO2 flooding in the nanopores of shale oil reservoir

With the increasing demand for petroleum, shale oil with considerable reserves has become an important part of global oil resources. The shale oil reservoir has a large number of nanopores and a complicated mineral composition, and the effect of nanopore confinement and pore type usually makes the effective development of shale oil challenging. For a shale oil reservoir, CO2 flooding can effectively reduce the oil viscosity and improve the reservoir properties, which can thus improve the recovery performance. In this study, the method of non-equilibrium molecular dynamics (NEMD) simulation is used to simulate the CO2 flooding process in the nanoscale pores of shale oil reservoir. The performance difference between the organic kerogen slit nanopore and four types of inorganic nanopores is discussed. Thus, the effects of nanopore type and displacement velocity on the nanoscale displacement behavior of CO2 are analyzed. Results indicate that the CO2 flooding process of different inorganic pores is different. In comparison, the displacement efficiency of light oil components is higher, and the transport distance is longer. The intermolecular interaction can significantly affect the CO2 displacement behavior in nanopores. The CO2 displacement efficiency is shown as montmorillonite, feldspar > quartz > calcite > kerogen. On the other hand, it is found that a lower displacement velocity can benefit the miscibility process between alkane and CO2, which is conducive to the overall displacement process of CO2. The displacement efficiency can significantly decrease with the increase in displacement velocity. But once the displacement velocity is very high, the strong driving force can promote the alkane to move forward, and the displacement efficiency will recover slightly. This study further reveals the microscopic oil displacement mechanism of CO2 in shale nanopores, which is of great significance for the effective development of shale oil reservoirs by using the method of CO2 injection.

Rheology of nanofluids. Experiments and molecular dynamics modeling

This article discusses the rheology of nanofluids with ordinary spherical particles. It has been shown that about a quarter of nanofluids at not too low concentrations turn out to be pseudo- or viscoplastic. Their rheology is well described by power-law fluid or Herschel–Bulkley models. To study the mechanism of rheology change, the method of nonequilibrium molecular dynamics is used. It is established that the change in rheology has a threshold character. Critical values of the shear rates of rheology change and their dependence on the concentration of nanoparticles, their size and material are found. It is shown that the change in rheology is accompanied by a change in the structure of the studied fluids, which is illustrated by the corresponding radial distribution functions.

Viscosity evolution of water glycol in deep-sea environment at high pressure and low temperature

Using water glycol as a working medium is an important development direction for deep sea hydraulic drives, its viscosity is an important parameter for hydrodynamic lubrication and seriously influenced by the deep-sea environment. In this paper, Non-Equilibrium Molecular Dynamics method was used to predict the viscosity of water glycol with 0%, 25%, 50%, 75% and 100% water content in the deep-sea environment based on the OPLS-AA force field. The simulation results have an error of less than 10% compared to available experimental data. Detailed analysis of hydrogen bonding and water molecular structure is provided to explain why the viscosity of water decreases with increasing sea depth from 2000 m to 11,000 m. A fitted equation is proposed to calculate the viscosity of water glycol by analyzing the effect of temperature, pressure, and water content on the viscosity of water glycol in deep-sea environment.