Nonequilibrium Molecular Dynamics Research Articles

Effect of Mg and Si contents on hot-dip 55Al-Zn plating: Experimental and molecular dynamics simulation

This paper combines experiments and molecular dynamics simulations to investigate the effects and mechanisms of different Mg and Si contents in hot-dip 55Al-Zn plating on the microstructure. Non-equilibrium molecular dynamics was used to simulate the diffusion process between plating and substrate. The structure and composition of the plated layers were analyzed by electron probe and transmission. Both experimental and simulation results show that Mg and Si elements tend to be enriched at the substrate interface and form the Mg2Si phase. The Fe-Al intermetallic compound layer can be divided into two parts, Fe4Al13(Si, Zn), which is far from the substrate, and Fe4Al13, which is close to the substrate, and grows along the direction perpendicular to the substrate interface. When the Mg content in the plating is 0, Si is enriched near the substrate and exists in the form of Si particles. When the Mg content in the plating is high, Si is present mainly in two forms: diffused in the Fe-Al intermetallic compound layer and formation of Mg2Si. The increase of Mg content in the plating promotes the breaking of Fe atomic bonds, but the aggregation of Mg elements hinders the diffusion of Fe. The Si dispersed in the intermetallic compound layer hinders the diffusion of Fe atoms and makes the intermetallic compound layer thinner. The above results explore the microscopic mechanism of the effect of Mg and Si elements on the plating layer at the atomic level and provide new ideas for the study of conventional hot-dip galvanized steel.

Non-Phenomenological Description of the Time-Resolved Emission in Solution with Quantum-Classical Vibronic Approaches-Application to Coumarin C153 in Methanol.

We report a joint experimental and theoretical work on the steady-state spectroscopy and time-resolved emission of the coumarin C153 dye in methanol. The lowest energy excited state of this molecule is characterized by an intramolecular charge transfer thus leading to remarkable shifts of the time-resolved emission spectra, dictated by the methanol reorganization dynamics. We selected this system as a prototypical test case for the first application of a novel computational protocol aimed at the prediction of transient emission spectral shapes, including both vibronic and solvent effects, without applying any phenomenological broadening. It combines a recently developed quantum-classical approach, the adiabatic molecular dynamics generalized vertical Hessian method (Ad-MD|gVH), with nonequilibrium molecular dynamics simulations. For the steady-state spectra we show that the Ad-MD|gVH approach is able to reproduce quite accurately the spectral shapes and the Stokes shift, while a ∼0.15 eV error is found on the prediction of the solvent shift going from gas phase to methanol. The spectral shape of the time-resolved emission signals is, overall, well reproduced, although the simulated spectra are slightly too broad and asymmetric at low energies with respect to experiments. As far as the spectral shift is concerned, the calculated spectra from 4 ps to 100 ps are in excellent agreement with experiments, correctly predicting the end of the solvent reorganization after about 20 ps. On the other hand, before 4 ps solvent dynamics is predicted to be too fast in the simulations and, in the sub-ps timescale, the uncertainty due to the experimental time resolution (300 fs) makes the comparison less straightforward. Finally, analysis of the reorganization of the first solvation shell surrounding the excited solute, based on atomic radial distribution functions and orientational correlations, indicates a fast solvent response (≈100 fs) characterized by the strengthening of the carbonyl-methanol hydrogen bond interactions, followed by the solvent reorientation, occurring on the ps timescale, to maximize local dipolar interactions.

Rheological Properties of Small-Molecular Liquids at High Shear Strain Rates.

Molecular-scale understanding of rheological properties of small-molecular liquids and polymers is critical to optimizing their performance in practical applications such as lubrication and hydraulic fracking. We combine nonequilibrium molecular dynamics simulations with two unsupervised machine learning methods: principal component analysis (PCA) and t-distributed stochastic neighbor embedding (t-SNE), to extract the correlation between the rheological properties and molecular structure of squalane sheared at high strain rates (106-1010s-1) for which substantial shear thinning is observed under pressures P∈0.1-955 MPa at 293 K. Intramolecular atom pair orientation tensors of 435×6 dimensions and the intermolecular atom pair orientation tensors of 61×6 dimensions are reduced and visualized using PCA and t-SNE to assess the changes in the orientation order during the shear thinning of squalane. Dimension reduction of intramolecular orientation tensors at low pressures P=0.1,100 MPa reveals a strong correlation between changes in strain rate and the orientation of the side-backbone atom pairs, end-backbone atom pairs, short backbone-backbone atom pairs, and long backbone-backbone atom pairs associated with a squalane molecule. At high pressures P≥400 MPa, the orientation tensors are better classified by these different pair types rather than strain rate, signaling an overall limited evolution of intramolecular orientation with changes in strain rate. Dimension reduction also finds no clear evidence of the link between shear thinning at high pressures and changes in the intermolecular orientation. The alignment of squalane molecules is found to be saturated over the entire range of rates during which squalane exhibits substantial shear thinning at high pressures.

In silico Study on the Binding Interactions of SSTA and 18F-SSTA Towards Somatostatin Receptor Subtype 2.

Somatostatin analogs (SSTAs) are versatile drugs that target a group of proteins known as somatostatin receptors. SSTAs are used for the treatment and PET-molecular imaging of Neuro Endocrine Tumors (NET), for they are labeled with the radionuclide 18F, a positron emitter radionuclide. The aim of this work was to theoretically study the binding interactions of SSTA labeled with 18F (half-life of 109.7 min) and somatostatin receptor subtype 2. As the labeling of SSTA with 18F required the use of a prosthetic group, a hydrophilicity enhancer, and a linker, the influence of these traits on the interactions of 18F-SSTA with the SSTR-2 binding site was studied. The binding modes of 18F-labeled analogues with SSTR-2 were studied by using protein homology modelling, non-equilibrium molecular dynamics, and molecular docking calculations, by means of three docking software: MVD, MOE, and VINA. The results showed the main role of Asp122, Asn276, Phe272 and Phe294 from the SSTR-2 binding site, which form interactions with residues Lys, Trp, Tyr, and Thr from 18F-labeled somatostatin analogues. The interaction between Lys (from 18F-SSTA) and Asp122 (from SSTR-2) was identified as the most energetic and considered the one that drives the binding between 18F-SSTA and SSTR-2 (the anchor interaction). Despite the presence of prosthetic groups, linkers, and hydrophilicity enhancers, all the studied 18F-SSTA formed the anchor interaction. The trend in the results agreed with the experimental reports, identifying the main role of Asp122 in the binding of somatostatin-14 to SSTR-2.

Cationic stabilized layered graphene oxide (GO) membrane for shale gas wastewater treatment: An atomistic insight

In recent years graphene oxide (GO) membranes have emerged as one of the most promising candidates for wastewater treatment. However, one major issue with layered GO membranes is their tendency to swell in an aqueous environment. Cation intercalation inside the interlayer gallery of layered GO membrane is one of the promising solutions to mitigate this problem. However, the effect of cationic concentration inside membrane channels on the mechanism of water permeance and salt rejection is yet to be investigated in detail. This study investigates shale gas wastewater (NaCl, CaCl2, and crude oil) treatment via nonequilibrium molecular dynamics (MD) simulations with cation (K+, Mg2+) intercalated GO membranes. The effect of cationic concentration on water permeance and salt rejection has been presented in detail. This study reveals that the presence of a higher concentration of cations inside the GO membrane induces higher water resistance in the membrane. In addition to the concentration of intercalated ions, interlayer distance also plays a significant role in determining water permeance. We observed that Mg2+ ion intercalated and K+ ion intercalated GO membranes maintain an interlayer distance of around 13.6 Å and 11.4 Å, respectively. As a result, Mg2+ ion intercalated GO membranes exhibit superior water permeance than K+ ion intercalated GO membranes. We also observed a difference in the crude oil aggregate formation (a composition of alkanes, cycloalkanes, and aromatics) in the two systems (more stable aggregate in K+ ion intercalated GO membrane system as compared to Mg2+ ion intercalated GO membrane system) during the forward osmosis (FO) process. This can be attributed to weaker interactions of K+ ions with the crude oil and lower water permeance of K+ ion intercalated GO membranes than Mg2+ ion intercalated GO membranes. This aggregate formation is an important factor for crude oil rejection. Moreover, aromatics are more concentrated at the crude oil-water interface than other hydrocarbons. On the other hand, more cations inside the GO membrane layers results in better Na+ rejection but decreased Cl− rejection.

A molecular dynamics study of thermal conductivity and viscosity in colloidal suspensions: From well-dispersed nanoparticles to nanoparticle aggregates

This study investigates the microscopic transport behavior of nanofluids addressing some debatable points including the anomalous thermal conductivity (κ), against the predictions of classical effective medium theories. International Nanofluid Property Benchmark Exercise (INPBE) [J. Buongiorno et al., J. Appl. Phys. 106 (2009) 094312] has shown that no such anomaly found in well-dispersed nanofluids after conducting experiments for range of different types of nanofluids. However, a number of molecular dynamics based studies reported otherwise making inconsistent conclusion with INPBE. In this work, it is argued that the over predicted κ values reported in previous computational studies can be attributed to the ill-defined partial enthalpy formulation of the Green-Kubo method for multicomponent systems. Present study begins by addressing this issue via non-equilibrium molecular dynamics and shows that the results are in agreement with the conclusions of INPBE. Further, it is shown that the contribution of potential micro-mechanisms such as micro-convection due to Brownian motion, and the solid-like liquid layering are either absent or suppressed by the interface thermal resistance. The observed decreasing trend of viscosity enhancement to thermal conductivity enhancement ratio (Cη/Cκ) with increasing particle size indicates the improved heat transfer performance in nanofluids with larger nanoparticles. The effect of nanoparticle aggregation, the proposed originative mechanism of anomalous κ, is evaluated arranging nanoparticles as chain-like structures. A 67% improvement in κ is achieved with negligible viscosity variation. This rapidly reduces Cη/Cκ indicating better heat transfer performance with the presence of conductive paths due to the aggregation or in general extended nanostructures.

Prediction and Control of Thermal Transport at Defective State Gr/h-BN Heterojunction Interfaces.

The control of interfacial thermal conductivity is the key to two-dimensional heterojunction in semiconductor devices. In this paper, by using non-equilibrium molecular dynamics (NEMD) simulations, we analyze the regulation of interfacial thermal energy transport in graphene (Gr)/hexagonal boron nitride (h-BN) heterojunctions and reveal the variation mechanism of interfacial thermal energy transport. The calculated results show that 2.16% atomic doping can effectively improve interfacial heat transport by more than 15.6%, which is attributed to the enhanced phonon coupling in the mid-frequency region (15-25 THz). The single vacancy in both N and B atoms can significantly reduce the interfacial thermal conductivity (ITC), and the ITC decreases linearly with the increase in vacancy defect concentration, mainly due to the single vacancy defects leading to an increased phonon participation rate (PPR) below 0.4 in the low-frequency region (0-13 THz), which shows the phonon the localization feature, which hinders the interfacial heat transport. Finally, a BP neural network algorithm is constructed using machine learning to achieve fast prediction of the ITC of Gr/h-BN two-dimensional heterogeneous structures, and the results show that the prediction error of the model is less than 2%, and the method will provide guidance and reference for the design and optimization of the ITC of more complex defect-state heterogeneous structures.

Understanding the adsorption of air entraining agents on portlandite: A combined DFT and molecular dynamics investigation

The molecular activity of four air entraining agents (AEA) widely used in cement and concrete systems is firstly calculated by DFT method and then adsorption behaviors including adsorption conformation, adsorption interaction and adsorption stability at portlandite-water interface are investigated by the classical atomistic molecular dynamics (MD) and non-equilibrium MD (named SMD) simulations. The molecular activity decreases as Rosin>SDBS>CDEA>SDS and the negative electrostatic potential (ESP) charges of oxygen atoms of adsorption groups decrease as Rosin>CDEA>SDBS>SDS, which is agree with the trends of adsorption interaction and adsorption stability of four AEAs at portlandite-water interface. The adsorption interaction and adsorption stability are determined by the electrostatic interaction and assembly density of AEAs at portlandite-water interface has less effect on the adsorption and adsorption stability. Conformation of SDS exhibit noticeable orientation reversal and a bilayer structure tends to form. The angles of the hydrophobic tail chains of SDS with z-axis are smallest, indicating the more perpendicular orientation and a denser assembly of the hydrophobic chains. Strong interaction of water molecules with portlandite surface causes competitive adsorption between AEA and water molecules on the portlandite surface.

Low lattice thermal conductivity with two-channel thermal transport in the superatomic crystal PH4AlBr4

Designing novel crystalline materials composed of light and nontoxic elements, but with ultralow thermal conductivity insensitive to temperature, has been a long-standing challenge. One effective strategy is to utilize superatoms as building blocks to introduce hierarchical bonding and vibration for suppressing phonon velocity and enhancing high-order scattering. However, far fewer theoretical efforts have been made to understand the lattice dynamics and thermal transport in superatom-based materials. Herein, different from most of the existing works reported hitherto on atom-superatom hybrid systems, we carry out a comprehensive study on the phonon interaction and thermal transport in superatomic crystal $\mathrm{P}{\mathrm{H}}_{4}\mathrm{Al}{\mathrm{Br}}_{4}$ solely consisting of superatoms (superalkali and superhalogen), by employing homogeneous nonequilibrium molecular dynamics with active-learning potential, combined with density functional theory and unified theory of phonon thermal transport. We find that the supersalt $\mathrm{P}{\mathrm{H}}_{4}\mathrm{Al}{\mathrm{Br}}_{4}$ crystal exhibits amorphouslike ultralow lattice thermal conductivity of $0.329\ensuremath{-}0.286\phantom{\rule{0.16em}{0ex}}\mathrm{W}\phantom{\rule{0.16em}{0ex}}{\mathrm{m}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{K}}^{\ensuremath{-}1}$ from 200 to 600 K. In particular, due to the strong quartic anharmonicity, the contribution of four-phonon scattering reaches 38% of total scattering rates at 300 K, while the phonon coherence's contribution is comparable with that of population, resulting from significant phonon localization. These theoretical findings demonstrate that superatom-assembled materials exhibit features distinguished from atom-based crystals and provide a unique platform for exploring ultralow thermal conductivity with enhanced two-channel thermal transport.

Differentiating Thermal Conductances at Semiconductor Nanocrystal/Ligand and Ligand/Solvent Interfaces in Colloidal Suspensions.

Infrared-pump, electronic-probe (IPEP) spectroscopy is used to measure heat flow into and out of CdSe nanocrystals suspended in an organic solvent, where the surface ligands are initially excited with an infrared pump pulse. Subsequently, the heat is transferred from the excited ligands to the nanocrystals and in parallel to the solvent. Parallel heat transfer in opposite directions uniquely enables us to differentiate the thermal conductances at the nanocrystal/ligand and ligand/solvent interfaces. Using a novel solution to the heat diffusion equation, we fit the IPEP data to find that the nanocrystal/ligand conductances range from 88 to 135 MW m-2 K-1 and are approximately 1 order of magnitude higher than the ligand/solvent conductances, which range from 7 to 26 MW m-2 K-1. Transient nonequilibrium molecular dynamics (MD) simulations of nanocrystal suspensions agree with IPEP data and show that ligands bound to the nanocrystal by bidentate bonds have more than twice the per-ligand conductance as those bound by monodentate bonds.

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.

Determination of thermal conductivity of eutectic Al–Cu compounds utilizing experiments, molecular dynamics simulations and machine learning

In this study, the thermal conductivity () of Al–Cu eutectics were investigated by experimental and computational methods to shed light on the role of these compounds in thermal properties of Al–Cu connections in compound casting. Specifically, the nonequilibrium molecular dynamics (MD) method was utilized to simulate the lattice thermal conductivity () of six compositions of Al–Cu with 5–30 at.% Cu. To extend the results of the MD simulations to bulk materials, instead of using conventional linear extrapolation methods, a machine learning approach was developed for the dataset acquired from the MD simulations. The bootstrapping approach was utilized to find the most suitable method among the support vector machine (SVM) with polynomial and radial basis function (RBF) kernels and the random forest method. The results showed that the SVM model with RBF kernel performed the best, and thus was used to predict the bulk . Subsequently, the chosen compositions were produced by induction casting and their electrical conductivities were measured via eddy current method for calculating the electronic contribution of using the Wiedemann–Franz law. Finally, the actual of the alloys were measured using the xenon flash method and the results were compared with the computational values. It was shown that the MD method is capable of successfully simulating the thermal conductivity of this system. In addition, the experimental results demonstrated that the of Al–Cu eutectics decreases almost linearly with formation of the Al2Cu phase due to increase in the Cu content. Overall, the current findings can be used to enhance the of cooling devices made via Al–Cu compound casting.

Water transport in reverse osmosis membranes is governed by pore flow, not a solution-diffusion mechanism.

We performed nonequilibrium molecular dynamics (NEMD) simulations and solvent permeation experiments to unravel the mechanism of water transport in reverse osmosis (RO) membranes. The NEMD simulations reveal that water transport is driven by a pressure gradient within the membranes, not by a water concentration gradient, in marked contrast to the classic solution-diffusion model. We further show that water molecules travel as clusters through a network of pores that are transiently connected. Permeation experiments with water and organic solvents using polyamide and cellulose triacetate RO membranes showed that solvent permeance depends on the membrane pore size, kinetic diameter of solvent molecules, and solvent viscosity. This observation is not consistent with the solution-diffusion model, where permeance depends on the solvent solubility. Motivated by these observations, we demonstrate that the solution-friction model, in which transport is driven by a pressure gradient, can describe water and solvent transport in RO membranes.

Evaluation of Various Shear-Thinning Models for Squalane Using Traction Measurements, TEHD and NEMD Simulations

The accurate prediction of friction in highly loaded concentrated contacts is one of the most challenging aspects of thermal elastohydrodynamic (TEHD) simulation. The correct modelling of fluid behaviour on the macroscale, in particular non-Newtonian flow behaviour, is an essential prerequisite. For many years, shear-thinning models have been developed and validated with different approaches and controversially discussed. In basic research, model fluids are often used in this context, which have a similar behaviour to practical lubricants. Accompanied by earlier research results, this paper carries out comprehensive investigations on the rheometric behaviour of the model fluid squalane. Based on traction measurements at four different tribometers, an overall parameter optimisation and performance evaluation of three different shear-thinning models is performed using numerical TEHD simulations. In order to additionally validate the theoretical viscosity behaviour, the optimised shear-thinning curves are then compared with comprehensive non-equilibrium molecular dynamics (NEMD) simulations. The key aspect of this paper is the simultaneous consideration of the shear-thinning models in terms of rheometric, experimental, and simulative investigations without changing the parameters. All investigations show that the Eyring model, despite its simplicity, provides the best agreement in both the numerical contact simulation and the NEMD simulations.

Covalently interconnected carbon nanotubes network enhancing thermal conductivity of EP-based composite

The miniaturization of integrated circuits promotes the rapid rise of polymer-based carbon nanotubes (CNTs) thermal conductive composites. However, the unordered arrangement structure and interfacial thermal resistance of CNTs become important obstacles affecting their application. The directional orientation driven of CNTs by external force and the “bridge” between CNTs constructed via covalent bonding could solve these issues. Herein, the freeze-casting orientation configuration and new CNTs grew in situ by ZIF-67 constructed the hierarchical structure of CNTs in epoxy resin (EP)-based composites. The composites with directional networks of interlinked CNTs achieve an out-plane thermal conductivity value (κ⊥) of 0.98 W m−1 K−1 at filler loading of 5.8 vol%, which is 4.85 times high than that of pure EP. More importantly, non-equilibrium molecular dynamics (NEMD) simulations validated the advantage of using covalent bonding to connect CNTs. This work provides a way for the reasonable design of polymer-based CNTs thermal conductive composites.

Influence of the twin boundary and Cr segregation on the spalling of Ni-base alloys by large-scale molecular dynamic simulations

Large-scale nonequilibrium molecular dynamics simulation is used to study the effect of grain size and Cr segregation at the twin boundary (TB) on the spalling fracture mechanism of nickel based alloys. In particular, loading waves are designed so that the maximum tensile stress first appears in the grain interior for all the crystals with different grain sizes. In contrast to traditional understandings, no monotonous relationship between the spall strength and the grain size appears in our results. The spall strength is found to depend on the wave attenuation distance measured from the first maximum tensile stress position to the spalled TB as well as the accompanied microstructure evolutions. The number of spalling plane increases with the increase of TB or the decrease of the grain size. As the grain size continues to decrease, a greater impact strength is required to cause spallation fracture at multiple TBs. In this case, the spall strength becomes insensitive to the first maximum tensile stress position. With the increment of solute atom concentration, the number of the spalling plane increases when the solution is segregated. But it decreases when the solution is uniformly distributed. Such a result is explained by segregation-enhanced energy dissipation and interactions between the waves and the microstructures nearby TBs. In particular, the shock wave would induce a local lattice reorientation nearby the TB depending on its segregation degree and the lattice reorientation would modify the slip manner of stacking faults and, thus, affect void nucleation and growth. The lattice reorientation would also contribute to the generation of sub-grain boundaries inside the grains in terms of the movement of stacking faults. Void nucleation at the sub-GB is the main cause of fracture at the grain interior.

Fundamental Understanding of Ultrathin, Highly Stable Self-Assembled Liquid Crystalline Graphene Oxide Membranes Leading to Precise Molecular Sieving through Non-equilibrium Molecular Dynamics

Self-assembled graphene oxide lyotropic liquid crystal (GO LLC) structures are mostly formed in aqueous medium; however, most GO derivatives are water insoluble, so processing GO LLCs in water poses a practical limitation. The use of polar aprotic solvent (like dimethyl sulfoxide) for the formation of GO LLC structures would be interesting, because it would allow incorporating additives, like photoinitiators or cross-linkers, or blending with polymers that are insoluble in water, which hence would expand its scope. The well-balanced electrostatic interaction between DMSO and GO can promote and stabilize the GO nanosheets' alignment even at lower concentrations. With this in mind, herein we report mechanically robust, chlorine-tolerant, self-assembled nanostructured GO membranes for precise molecular sieving. Small-angle X-ray scattering and polarized optical microscopy confirmed the alignment of the modified GO nanosheets in polar aprotic solvent, and the LLC structure was effectively preserved even after cross-linking under UV light. We found that the modified GO membranes exhibited considerably improved salt rejection for monovalent ions (99%) and water flux (120 LMH) as compared to the shear-aligned GO membrane, which is well supported by forward osmosis simulation studies. Additionally, our simulation studies indicated that water molecules traveled a longer path while permeating through the GO membrane compared to the GO LLC membrane. Consequently, salt ions permeate slowly across the GO LLC membrane, yielding higher salt rejection than the GO membrane. This begins to suggest strong electrostatic repulsion with the salt ions, causing higher salt rejection in the GO LLC membrane. We foresee that the ordered cross-linked GO sheets contributed to excellent mechanical stability under a high-pressure, cross-flow, chlorine environment. Overall, these membranes are easily scalable, exhibit good mechanical stability, and represent a breakthrough for the potential use of polymerized GO LLC membranes in practical water remediation applications.

Optimization of interfacial mixing for thermal transport along Si/Ge heterostructures: A molecular dynamics study

Interfaces can play critical roles in the dissipation of heat, generated during the electronic nanodevices operation. Therefore, engineering of the interfacial thermal resistance is a key factor in the thermal management design of nanoelectronics. In this research, the engineering of the interfacial thermal resistance of silicon/germanium heterostructure is explored, by varying the content of silicon or germanium atoms at the interphase (mixing region). For this goal, we conducted systematic non-equilibrium molecular dynamics simulations. For a specific interphase length, it is noticeably predicted that the interfacial thermal resistance versus the percentage of germanium atoms shows an optimal minimum value. By substituting germanium atoms with silicon atoms, but with preserving their atomic mass, it is interestingly shown that the interfacial thermal resistance increases significantly, although the optimal value for the interfacial thermal resistance is still observable. By altering the length of the interphase region and the samples’ temperature (300 and 600 K), the optimal interfacial thermal conductance in terms of the percentage of germanium atoms in the mixing region was also detectable. The findings of this work can be useful in the thermal management engineering of nanodevices, especially in nanoelectronics.

Nonequilibrium molecular dynamics for accelerated computation of ion–ion correlated conductivity beyond Nernst–Einstein limitation

Condensed matters with high ionic conductivities are crucial in various solid devices such as solid-state batteries. The conduction is characterized by the cooperative ionic motion associated with the high carrier density. However, the high cost of computing correlated ionic conductivities has forced almost all ab initio molecular dynamics (MD) to rely on the Nernst–Einstein dilute-solution approximation, which ignores the cross-correlation effect. Here we develop a chemical color-diffusion nonequilibrium MD (CCD-NEMD) method, which enables to calculate the correlated conductivities with fewer sampling steps than the conventional MD. This CCD-NEMD is demonstrated to well evaluate the conductivities in the representative solid electrolyte bulk Li10GeP2S12 and Li7La3Zr2O12. We also applied CCD-NEMD to the grain boundary of Li7La3Zr2O12 and demonstrated its applicability for calculating interfacial local conductivities, which is essential for investigating grain boundaries and composite electrolytes. CCD-NEMD can provide further accurate understanding of ionics with ionic correlations and promote developing solid devices.

Investigation of the heat transport in intersected graphene

Graphene networks have great potential in improving the thermal transport of epoxy matrix composites. However, low heat transfer efficiency between adjacent graphene, especially the intersected structure with a small heat transfer area, limits its application. Here, the non-equilibrium molecular dynamics is used to investigate the thermal transport of the intersected graphene. The effects of nodal gap width, number and distribution of crosslinking bonds, and intersecting angle on the heat transfer coefficient are studied. The result shows that the coupling between intersected graphene is transformed from Van der Waals interactions to covalent interactions with a critical nodal gap width of 1.8 Å. Then, crosslinking bonds are divided into independent crosslinking bonds and dependent crosslinking bonds according to whether the heat transfer of the crosslinking bond is affected by other bonds. Based on the thermoelectricity analogy theory, the nodal heat transfer coefficient is proportional to the number of independent crosslinking bonds, while the heat transfer efficiency of the dependent crosslinking bond is associated with numbers and distributions. Finally, the study on the intersecting angle shows that the heat transfer coefficient of intersected graphene increases with decreased angle under Van der Waals interactions. In contrast, the trend is opposite under covalent interactions.