Nonequilibrium Molecular Dynamics Computer Research Articles

Phonon transport across GaAs/Ge heterojunctions by nonequilibrium molecular dynamics

Interfaces are ubiquitous in modern electronics and assessing their properties is crucial when it comes to device reliability. Here, we present nonequilibrium molecular dynamics calculations of heat transport across GaAs/Ge heterojunctions. We compute the thermal boundary resistance, considering different interface morphologies, ranging from atomically flat to gradual or rough interfaces. We also discuss the implications for thermal rectification and predict a rectification coefficient as large as 30%.

Microscopic thermal characteristics of parallelly arranged nanowires in a liquid: the role of interface thermal resistance, solid-like liquid layer, and the restricted phonon transport

Nanofluids based on extended nanostructures, such as nanowires, have been demonstrated improved thermal conductivities (κ). However, the lack of a complete understanding at the microscopic level hinders the development of such nanofluids towards practical applications. We aim to provide it by investigating how the interface thermal resistance (R b ), ballistic phonon transport, and the solid-like liquid layer affect the heat conduction in nanowire-based nanofluids. By employing Non-Equilibrium Molecular Dynamics (NEMD), it is found that the heat conduction in the parallelly arranged liquid and the nanowires exhibit a coupled thermal behavior owing to the R b . This contradicts the predictions of the classical parallel heat conduction model, therefore, a novel model is proposed taking this coupled behavior into account. Using this model, it is shown that the high κ of the solid phase has a limited contribution to the effective κ of nanofluids having short nanowires due to the dominant R b effect. For the case of long nanowires, however, the individual nanowire κ becomes a vital parameter defining the effective κ. Further, NEMD calculations reveal that the κ of suspended nanowires in a liquid is markedly reduced, questioning the validity of classical effective medium theories which use the bulk parameters. This reduction is attributed to surface atoms’ restricted vibrational freedom and the nanowire’s phonon-boundary scattering. By substituting this reduced κ of the solid phase into the new mathematical model, the theoretical predictions align closely with the NEMD calculations, exhibiting deviations below 10%. The sole contribution from the solid-like liquid layer to the κ enhancement lies between 20%–30% in the nanofluids presently considered. Therefore, the findings of this study highlight the important roles play by the identified microscopic thermal characteristics in defining the effective κ of nanofluids based on nanowires.

Surface phonon localization and heat flux regulation in nanophononic metamaterials

Nanophononic metamaterials (NPM) have received surging research attention for thermal conductivity modification. The local resonance hybridization mechanism of NPM is of great significance to the performance optimization of heat flux regulation devices. In this Letter, using silicon nanofilm with a periodic array of nanopillars as example, we systemically studied the phonon localization effect and heat flux regulation. The results demonstrate that the phonon localization effect produced by the nanopillars is mainly concentrated in the region close to surface layer. Moreover, the effects of nanopillar height, spacing, and atomic mass of atoms in the nanopillars on the localization are also explored. Finally, the relationship between phonon localization and heat flux density is demonstrated through non-equilibrium molecular dynamics calculations. This study provides not only the better understanding of the phonon resonance hybridization mechanism in nanophononic metamaterials but also useful guidelines for the design of nanoscale heat flux regulation devices.

Enhanced mechanical and thermal properties of two-dimensional SiC and GeC with temperature and size dependence

Two-dimensional materials with novel mechanical and thermal properties are available for sensors, photodetectors, thermoelectric, crystal diode and flexible nanodevices. In this investigation, the mechanical and thermal properties of pristine SiC and GeC are explored by molecular dynamics simulations. First, the fracture strength and fracture strain behaviors are addressed in the zigzag and armchair directions at 300 K. The excellent toughness of SiC and GeC is demonstrated by the maximal fracture strain of 0.43 and 0.47 in the zigzag direction, respectively. The temperature-tunable tensile strength of SiC and GeC is also investigated. Then, using non-equilibrium molecular dynamics (NEMD) calculations, the thermal performances of SiC and GeC are explored. In particular, the thermal conductivity of SiC and GeC shows a pronounced size dependence and reaches up to 85.67 W⋅m−1⋅K−1 and 34.37 W⋅m−1⋅K−1, respectively. The goal of our work is to provide a theoretical framework that can be used in the near future. This will enable us to design an efficient thermal management scheme for two-dimensional materials in electronics and optoelectronics.

Investigation of nanoparticles shape that influence the thermal conductivity and viscosity in argon-based nanofluids: A molecular dynamics simulation

The shape of nanoparticles plays an essential role in the thermophysical properties of nanofluids, yet their mechanisms and characteristics remain lacking in comprehensive studies. Molecular dynamics simulations of non-equilibrium molecular dynamics (NEMD) and reversing perturbation non-equilibrium molecular dynamics (RNEMD) calculation methods were used to study thermal conductivity and viscosity of Cu/Au-Argon based nanofluids, taking into account a variety of influencing factors, as well as nanoparticles shape and volume fraction. Through the analysis of the number density distribution, radial distribution function (RDF) and mean square displacement (MSD), the influences of nanoparticles shape (represented by the surface-to-volume ratio) were described and investigated: the highest value was 0.182 WK−1m−1 containing columnar particles of Au at 2% volume fraction, while the thermal conductivity of Cu nanofluids was 0.162 WK−1m−1 under the same conditions; the average values of viscosity had very close values of 2.82 × 10−4 Pa·s (Cu-Ar) and 2.86 × 10−4 Pa·s (Au-Ar) at 1.5% particle volume fraction for nanofluids containing spherical particles, respectively. Simulations of argon-based nanofluids containing five different shapes of Au nanoparticles indicate that the thermal conductivity improves with the growth of the surface-to-volume ratio. And the enhancement of the interfacial nanolayer density turns out to be the main factor that nanoparticles with higher S/V value facilitated the enhancement of the thermal conductivity of argon-based nanofluids. This study enriches the molecular dynamics studies of nanofluids and provides important insights for understanding the effect of nanoparticle's shape on the thermal properties of nanofluids.

Wide-Temperature Tunable Phonon Thermal Switch Based on Ferroelectric Domain Walls of Tetragonal KTN Single Crystal.

Ferroelectric domain walls (DWs) of perovskite oxide materials, which can be written and erased by an external electric field, offer the possibility to dynamically manipulate phonon scattering and thermal flux behavior. Different from previous ferroelectric materials, such as BaTiO3, PbTiO3, etc., with an immutable and low Curie temperature. The Curie temperature of perovskite oxide KTa1-xNbxO3 (KTN) crystal can be tuned by altering the Ta/Nb ratio. In this work, the ferroelectric KTa0.6Nb0.4O3 (KTN) single crystal is obtained by the Czochralski method. To understand the role of ferroelectric domains in thermal transport behavior, we perform a nonequilibrium molecular dynamics (NEMD) calculation on monodomain and 90° DWs of KTN at room temperature. The calculated thermal conductivity of monodomain KTN is 9.84 W/(m·k), consistent with experimental results of 8.96 W/(m·k), and distinctly decreased with the number of DWs indicating the outstanding performance of the thermal switch. We further evaluate the thermal boundary resistance (TBR) of KTN DWs. An interfacial thermal resistance value of 2.29 × 10-9 K·m2/W and a large thermal switch ratio of 4.76 was obtained for a single DW of KTN. Our study shows that the ferroelectric KTN can provide great potential for the application of thermal switch at room temperature and over a broad temperature range.

Tunable thermal properties of the biphenylene and the lateral heterostructure formed with graphene: A molecular dynamics investigation

Recently, biphenylene was successfully synthesized as a novel allotrope of carbon. In this investigation, non-equilibrium molecular dynamics calculations are conducted to explore the intrinsic thermal properties of biphenylene. The isotropic thermal conductivity of biphenylene is obtained, which is also sensitive to size and temperature. Furthermore, the graphene/biphenylene lateral heterostructure is constructed to possess an interfacial thermal conductance of about 2.84 × 109 W K−1 m−2. The external tensile strain can induce a redshift of the vibrational density of states of pristine graphene and biphenylene, and the improved overlap also results in an enhanced heat flux in the biphenylene/graphene heterostructure. Our approach can provide theoretical guidance to design a thermal management device based on graphene and biphenylene.

Phonon Transport in GaAs and InAs Twinning Superlattices.

Crystal phase engineering gives access to new types of periodic nanostructures, such as the so-called twinning superlattices, where the motif of the superlattice is determined by a periodic rotation of the crystal. Here, by means of atomistic nonequilibrium molecular dynamics calculations, we study to what extent these periodic systems can be used to alter phonon transport in a controlled way, similar to what has been predicted and observed in conventional superlattices based on heterointerfaces. We focus on twinning superlattices in GaAs and InAs and highlight the existence of two different transport regimes: in one, each interface behaves like an independent scatterer; in the other, a segment with a sufficiently large number of closely spaced interfaces is seen by propagating phonons as a metamaterial with its own thermal properties. In this second scenario, we distinguish the case where the phonon mean free path is smaller or larger than the superlattice segment, pointing out a different dependence of the thermal resistance with the number of interfaces.

Graphene/biphenylene heterostructure: Interfacial thermal conduction and thermal rectification

The allotrope of carbon, biphenylene, was prepared experimentally recently [Fan et al., Science 372, 852–856 (2021)]. In this Letter, we perform first-principles simulation to understand the bonding nature and structure stability of the possible in-plane heterostructure built by graphene and biphenylene. We found that the graphene–biphenylene in-plane heterostructure only exhibits along the armchair direction, which is connected together by strong covalent bonds and energetically stable. Then, the non-equilibrium molecular dynamics calculations are used to explore the interfacial thermal properties of the graphene/biphenylene heterostructure. It is found that the graphene/biphenylene in-plane heterostructure possesses an excellent interfacial thermal conductance of 2.84 × 109 W·K−1·m−2 at room temperature. Importantly, the interfacial thermal conductance presents different temperature dependence under opposite heat flux direction. This anomalous temperature dependence results in increased thermal rectification ratio with temperature about 40% at 350 K. This work provides comprehensive insight into the graphene–biphenylene heterostructure and suggests a route for designing a thermal rectifier with high efficiency.

The lower limit of thermal conductivity in multicomponent solutions of rare gas solids

The isochoric thermal conductivity of Kr0.63Xe0.37 solid solution has been investigated for three samples with different molar volumes in the temperature range from 80 K to the onset of melting, as well as the thermal conductivity of the ternary (Kr0.63Ar0.185Xe0.185) and quaternary (Kr0.63Ar0.12Xe0.12(CH4)0.13) solid solutions. It is found that for multi-component alloys, the thermal conductivity reaches a minimum, whereby adding more impurity atoms in the solid solution does not affect the thermal conductivity in agreement with the recent nonequilibrium molecular dynamics calculations for argon. The phonon contribution to the total thermal conductivity depends significantly on molar volume of sample and impurity concentration, in contrast to the contribution of diffusive modes, which is practically independent of these factors.

Phonon transport across crystal-phase interfaces and twin boundaries in semiconducting nanowires.

We combine state-of-the-art Green's-function methods and nonequilibrium molecular dynamics calculations to study phonon transport across the unconventional interfaces that make up crystal-phase and twinning superlattices in nanowires. We focus on two of their most paradigmatic building blocks: cubic (diamond/zinc blende) and hexagonal (lonsdaleite/wurtzite) polytypes of the same group-IV or III-V material. Specifically, we consider InP, GaP and Si, and both the twin boundaries between rotated cubic segments and the crystal-phase boundaries between different phases. We reveal the atomic-scale mechanisms that give rise to phonon scattering in these interfaces, quantify their thermal boundary resistance and illustrate the failure of common phenomenological models in predicting those features. In particular, we show that twin boundaries have a small but finite interface thermal resistance that can only be understood in terms of a fully atomistic picture.

Volume-based mixing rules for viscosities of methane + n-butane liquid mixtures

We present a study of viscosities of methane, n-butane and their mixtures by the non-equilibrium molecular dynamics simulations and derivation of semiempirical volume-based mixing rules. The Batchinski equation η=C/(V−b) is used to describe the viscosities of pure components, with parameters fitted to reproduce molecular dynamics results. Cubic root, Arrhenius and Batchinski mixing rules are tested for mixtures. The viscosities of pure components used in mixing equations are expressed as functions of volume of component rather than pressure. This allows to apply the mixing rules to metastable and stable liquids, dense supercritical fluids and solutions of gas in liquid. To obtain volumes of components in mixture, molecular dynamics method is used. The mixing rules predictions are compared against direct non-equilibrium molecular dynamics calculations of mixture viscosities. The best agreement with the molecular dynamics data is found when Batchinski mixing is used. The proposed viscosity model predictions are in agreement with the experimental data on viscosities of methane-butane mixtures. The model can be used for the interpretation and interpolation of the experimental data on viscosities of liquids, which is demonstrated on the example of methane + propane system.

Effect of phonon scattering by substitutional and structural defects on thermal conductivity of 2D graphene

The ability to tailor the thermal conductivity of graphene by introducing crystalline defects has attracted considerable research attention. In this study, nonequilibrium molecular dynamics calculation is used to investigate the effect of crystalline defects on the thermal conductivity of 2D graphene. The defects considered include substitutional nitrogen and silicon, pure structural single vacancy and Stone–Wales defects, and structurally different pyridinic nitrogen. In particular, this study focuses on the unique phonon scattering behaviors arising from the low dimensionality of graphene. The results reveal that the low dimensionality of graphene has a negligible effect on phonon scattering in substitutionally defected graphene, for which the Klemens scattering model is accurate without the need for any corrections. The substitutional silicon defect leads to more effective reduction of the thermal conductivity than the structural defects because of the effect of change in the hybridization and the mass on the scattering. Almost equal reductions are observed for the two structural defects, the scattering strengths of which are significantly weakened by the two dimensionality of graphene. Callaway analysis of the vacancy scattering reveals that even with the perturbation of the vacancy, the 2D honeycomb structure preserves considerable phonon stability compared with a 3D material. In addition, the absence of mass deficiency for the Stone–Wales defect suggests that the contribution of mass deficiency is minimized for structural defects of graphene. Finally, opposite to the findings for the substitutional nitrogen defect, the introduction of pyridinic nitrogen leads to further reduction of the thermal conductivity compared with that for a single vacancy defect.

Orientation effect of alpha-to-epsilon phase transformation in single-crystal iron

The dynamic response of iron, especially the phase transformation from the ambient body-centered-cubic (bcc) up-phase to the hexagonal-closed packed (hcp) -phase, has been studied extensively in the last 60 years due to its importance in industry and its role as a main constituent of Earth. Recently, this topic has attracted a lot of attention in the aspects of the kinetic characteristics and mechanism of the shock-induced phase transition, including orientation-, temperature-, time- and strain rate-dependences. But only a few data have been published on the crystal orientation effect. The systematic experimental results to identify the predictions of the non-equilibrium molecular dynamics (NEMD) simulation are still lacking. For this reason, we study the shock responses of the [100], [110] and [111] orientated iron single crystals by using a three-independent-sample method in one shot. Unlike previously reported [001] single-crystal iron, a clear three-wave structure consisting of a PEL wave (elastic wave), a P1 wave (plastic wave) and a P2 wave (phase transition wave) is observed in the measured wave profiles for all single-crystal iron samples. The elastic-plastic transition process is in accordance with the numerical simulation of dislocation-based constitutive model for visco-plastic deformation. It is found that the values of Hugoniot elastic limit HEL ((111)/(HEL) (110)/(HEL) (100)/(HEL)) are greater than 6 GPa and dependent on the initial crystal orientation. Such a high yield strength is consistent with the nanosecond X-ray diffraction of [001] single-crystal iron where the uniaxial compression of the lattice has been observed at a shock pressure of about 5.4 GPa. Moreover, the onset pressures PPT for the phase transition are obtained to be 13.890.57 GPa, 14.530.53 GPa and 16.050.67 GPa along the [100], [110], and [111] directions, respectively. Based on these results, it is concluded that the crystal orientation effect of PPT is consistent with the reported NEMD calculations. However, the measured values are lower. In addition, the transition strain-ratio of singlecrystal iron is found to be higher than that of polycrystalline iron, reflecting the influence of the transformation kinetics (i.e., transformation kinetics coefficient) on the wave profile evolution. Our observations indicate that the strong coupling between plasticity and phase transition in single crystal iron might be a key point for understanding the origin of the phase transition and also for ending the controversy of metastable -phase. The fine multi-wave profiles also provide an important experimental reference for improving the phase field modeling of shock-induced phase transition.

Ionic conductivity in Sm-doped ceria from first-principles non-equilibrium molecular dynamics

Sm-doped ceria is a prospective electrolyte material for intermediate-temperature solid-oxide fuel cells (IT-SOFC). Equilibrium ab initio molecular dynamics (AIMD) studies of oxygen ion diffusion in this material are currently impractical due to the rareness of diffusive events on the accessible timescale. To overcome this issue we have performed ab initio non-equilibrium molecular dynamics calculations of Sm-doped ceria using the color-diffusion algorithm. Applying an external force field we have been able to increase the frequency of diffusive events over the simulation time, while keeping the physical mechanism of diffusion intact. We have investigated the temperature dependence of the maximum strength of the applied external field that could be used while maintaining the response of the system in a linear regime. This allows one to obtain the diffusivity at zero field. The bulk ionic conductivity has been calculated and found to match the experimental data well. We have also compared the description of the diffusion process by our method to previous findings and show that the migration mechanism and site preference of oxygen vacancies with respect to the Sm dopants is well reproduced.

Enhancing the ballistic thermal transport of silicene through smooth interface coupling

We have performed nonequilibrium molecular dynamics calculations on the length () dependence of thermal conductivity () of silicene both supported on and sandwiched between the smooth surfaces, i.e. h-BN, at room temperature. We find that of silicene follows a power law , with increasing from about 0.3–0.4 under the effect of interface coupling, showing an enhancement of the ballistic thermal transport of silicene. We also find that can be further increased to about 0.6 by increasing the interface coupling strength for the silicene sandwiched between h-BN. The increase of for the supported case is found to come from the variation of the flexural acoustic (ZA) phonon mode and the first optical phonon mode induced by the substrate, whereas the unusual increase of for the sandwiched case is attributed to the increment of velocities of all three acoustic phonon modes. These findings provide an interesting route for manipulating the ballistic energy flow in nanomaterials.

Anisotropy and probe-medium interactions in the microrheology of nematic fluids

A theoretical formalism is presented to analyze and interpret microrheology experiments in anisotropic fluids with nematic order. The predictions of that approach are examined in the context of a simple coarse-grained molecular model which is simulated using nonequilibrium molecular dynamics calculations. The proposed formalism is used to study the effect of confinement, the type of anchoring at the probe-particle surface, and the strength of the nematic field on the rheological response functions obtained from probe-particle active microrheology. As expected, a stronger nematic field leads to increased anisotropy in the rheological response of the material. It is also found that the defect structures that arise around the probe particle, which are determined by the type of anchoring and the particle size, have a significant effect on the rheological response observed in microrheology simulations. Independent estimates of the bulk dynamic modulus of the model nematic fluid considered here are obtained from small-amplitude oscillatory shear simulations with Lees–Edwards boundary conditions. The results of simulations indicate that the dynamic modulus extracted from particle-probe microrheology is different from that obtained in the absence of the particle, but that the differences decrease as the size of the defect also decreases. Importantly, the results of the nematic microrheology theory proposed here are in much closer agreement with simulations than those from earlier formalisms conceived for isotropic fluids. As such, it is anticipated that the theoretical framework advanced in this study could provide a useful tool for interpretation of microrheology experiments in systems such as liquid crystals and confined macromolecular solutions or gels.

Surface morphology and strain coupling effects on phonon transport in silicon nanowires

We perform non-equilibrium molecular dynamics calculations to study the coupling effects of surface, defects, and strain on heat transport in silicon nanowires (SiNWs). It is found that, for single-crystalline SiNWs, rippled surface and vacancy defects can significantly reduce the thermal conductivity κ of SiNWs. In addition, through surface amorphization, the thermal conductivity can be further remarkably reduced, due to strong phonon scattering at the interface as well as the non-propagating diffusion of phonons in the amorphous region. However, the κ value of amorphous-surface SiNWs is not sensitive to the roughness and vacancy in amorphous surface. Besides surface and defects, it has been observed that stress-induced defects/deforms can introduce additional mechanism of phonon scattering, which decrease the thermal conductivity. The present work demonstrates that, through phonoic engineering to tune the thermal conductivity, SiNWs can be promising candidates for thermoelectric applications.

Pressure–Viscosity Coefficient of Hydrocarbon Base Oil through Molecular Dynamics Simulations

The pressure–viscosity coefficient (the α value), which represents the variation of viscosity as a function of pressure, is an important parameter for elastohydrodynamic lubrication analyses. The properties of hydrocarbons in the C20–C40 mass range are of fundamental importance as they are basic constituents of synthetic- and mineral-based lubricant stocks. The conventional acquisition of the α value requires preparation of lubricant samples and experimental testing by means of a high-pressure viscometer. In this paper, we present a method to obtain the α value of a typical base oil (1-Decene trimer) based solely on the molecular dynamics simulations. Non-equilibrium molecular dynamics (NEMD) simulations were performed to calculate the shear viscosity of the lubricant at various temperatures and pressures up to 1 GPa. Elevated temperatures and time–temperature superposition (TTS)-based extrapolations were applied to further extend the ability of the NEMD simulations, and the rotational relaxation time was calculated and used to determine the validity of the NEMD calculations. The α value at 100 °C was calculated and compared with experimental results. Effectiveness of the extrapolation was evaluated with a 95 % confidence interval.

System size effects on calculation of the viscosity of extended molecules

We consider finite size effects on calculation of the viscosity of bulk molecular fluids using molecular dynamics simulations. The results are obtained using equilibrium simulations, direct calculations from nonequilibrium molecular dynamics simulations and calculations of the transient time correlation function expression, based on the dissipation theorem for nonlinear response. As with atomic fluids, strain induced by the periodic boundary conditions on the molecular systems can become significant for small systems at high densities and low temperatures. It addition to this effect, the dissipation function has a finite size contribution below a critical size, and this becomes more important as the system size is reduced and the length of the molecule increases. In this paper we show how calculations can be carried out to obtain convergence to bulk values with limited system size simulations.