•Ballistic thermal wave propagation along nanowires is investigated using a phonon-traced Monte Carlo method.•The effects of boundary scattering on thermal wave propagation differ for ballistic-diffusive and diffusive phonon transport.•Different phonon transport regimes will be measured using different temporal resolution, and their dependencies on phonon scattering regimes are different.The propagation of ballistic thermal waves when the phonon transport is in the ballistic-diffusive regime is markedly affected by the boundary. This work simulates ballistic thermal wave propagation in nanowires with a phonon-traced Monte Carlo method to investigate the effects of the nanowire characteristics including the radial Knudsen number and the specularity parameter, and the effects of the temporal resolution of the measurements. The phonon boundary scattering accelerates the evolution of the phonon transport from ballistic to ballistic-diffusive and finally to diffusive transport and increases the thermal conduction resistance by reducing the effective thermal conductivity. High heat pulse frequencies lead to thermal wave propagation in ballistic regime, moderate heat pulse frequencies lead to thermal wave propagation in ballistic-diffusive regime and very low heat pulse frequencies lead to purely diffusive thermal wave propagation, i.e. the Fourier thermal conduction. The ballistic-diffusive thermal wave propagation relies heavily on the specific type of the dominated phonon scattering mechanism while the purely ballistic and diffusive propagations do not. Thus, ballistic-diffusive thermal wave propagation should be modeled by a new constitutive equation with new characteristic parameters.

•The ultimate strength, Young's Modulus and maximum strain of carbyne are sensitive to temperature.•Opposite trends of length dependence of the overall ultimate strength at room temperature and very low temperature.•Ends constraints and ends effect of the carbyne on the overall ultimate strength are investigated.Carbyne is an ideal one-dimensional conductor and the thinnest interconnection in an ultimate nano-device and it requires an understanding of the mechanical properties that affect device performance and reliability. Here, we report the mechanical properties of finite-size carbyne, obtained by a molecular dynamics simulation study based on the adaptive intermolecular reactive empirical bond order potential. To avoid confusion in assigning the effective cross-sectional area of carbyne, the value of the effective cross-sectional area of carbyne (4.148 Å2) was deduced via experiment and adopted in our study. Ends-constraints effects on the ultimate stress (maximum force) of the carbyne chains are investigated, revealing that the molecular dynamics simulation results agree very well with the experimental results. The ultimate strength, Young's Modulus and maximum strain of carbyne are rather sensitive to the temperature and all decrease with the temperature. Opposite tendencies of the length dependence of the overall ultimate strength and maximum strain of carbyne at room temperature and very low temperature have been found, and analyses show that this originates in the ends effect of carbyne.Download high-res image (191KB)Download full-size image

•Applicability of potentials is examined for model thermal transport of graphene.•Tersoff, REBO, and AIREBO significantly underestimate thermal conductivities.•Opt-Tersoff with LJ potential accurately predicts the interlayer phonon scattering.•Opt-Tersoff is the most suitable potential for single- and few-layer graphene.This work employs non-equilibrium molecular dynamics (NEMD) simulations to examine the applicability of four kinds of interatomic potential models: the Tersoff, the REBO, the opt-Tersoff and the AIREBO, which are widely used to model the thermal transport in single- and multi-layer graphene, as well as graphite crystallites. Thermal conductivities of ∼17 × 5 nm2 and ∼50 × 5 nm2 graphene are calculated in the temperature range of 200∼500 K with the four potentials and quantum correction is applied due to an extremely high Debye temperature of about 2100 K for graphene. The predicted thermal conductivities are compared with experimental data and phonon spectrum functions are calculated to quantify the degree of phonon scattering. The results show that two original potentials, the Tersoff and the REBO, as well as the AIREBO significantly underestimate thermal conductivities of single-layer graphene but they can qualitatively describe the trend of thermal conductivities with temperature. The opt-Tersoff is found to be the most suitable potential for modeling the thermal conductivity of both single- and multi-layer graphene because it predicts a larger frequency range and a larger frequency value for the high frequency peak, while appropriately capturing phonon scattering in thicker multi-layer graphene when Lennard-Jones term is added into the opt-Tersoff to describe interlayer atomic interactions.

Thermal rectification can help develop modern thermal manipulation devices but has been rarely engineered. Here, we validated the nanoscale bimaterial interface-induced thermal rectification experimentally for the first time and investigated its underlying mechanism via molecular dynamics simulations. The thermal diode consists of polyamide (PA) and silicon (Si) nanowires in contact with each other. The thermal rectification ratio measured by a high-precision nanoscale experiment reached 4% with an uncertainty of <1%. The temperature has little influence on the ratio, while the decrease in contact length or increase in temperature differences can increase the ratio. The molecular dynamics simulations further confirmed the thermal rectification in the PA/Si nanowires. We found that the localized modes generally gather on the edge, and the higher extent of phonon localization is responsible for the lower thermal conductance in the thermal rectification. Our findings not only have guiding significance, but can also promote the development of interface-based solid-state thermal diodes.

In this letter, graded pillared graphene structures with carbon nanotube–graphene intramolecular junctions are demonstrated to exhibit ultrahigh thermal rectification. The designed graded two-stage pillared graphene structures are shown to have rectification values of 790.8 and 1173.0% at average temperatures 300 and 200 K, respectively. The ultrahigh thermal rectification is found to be a result of the obvious phonon spectra mismatch before and after reversing the applied thermal bias. This outcome is attributed to both the device shape asymmetry and the size asymmetric boundary thermal contacts. We also find that the significant and stable standing waves that exist in graded two-stage pillared graphene structures play an important role in this kind of thermal rectifier, and are responsible for the ultrahigh thermal rectification of the two-stage ones as well. Our work demonstrates that pillared graphene structure with SWCNT–graphene intramolecular junctions is an excellent and promising phononic device.Keywords: carbon nanotube-graphene intramolecular junctions; molecular dynamics; phonon density of states; pillared graphene; standing wave; thermal rectification;

Research on heat conduction in periodic nanoporous silicon films has drawn much attention due to its importance for developing highly efficient thermoelectric devices. Here, the thermal transport in two-dimensional (2D) periodic silicon nanoporous films is studied by a phonon Monte Carlo (MC) method and the theoretical analyses based on the Boltzmann transport equation (BTE). It is found that both the cross-plane and the in-plane effective thermal conductivities are significantly reduced when compared to those in the diffusive limit, and decrease with the increasing porosity or the decreasing period length. Importantly, our work reveals a strong anisotropy of the effective thermal conductivity of the 2D periodic nanoporous films; that is, the effective thermal conductivity in the in-plane direction is significantly less than that in the cross-plane direction, due to the anisotropic effects of material removal and pore boundary scattering. Interestingly, even the influence of the specular parameter that depends on the pore boundary roughness is anisotropic in this case, which could provide an anisotropic tuning method on the effective thermal conductivities of the periodic nanoporous films. In addition, the effective thermal conductivity models, which well concern the anisotropy and the specular parameter dependence, are derived on the basis of Matthiessen’s rule by introducing the geometrical factors that can be obtained from the MC simulations. The good agreements have been achieved between the present models and the MC simulations, verifying the validity of the models.

In this paper, we investigate the fast flow in nanochannels, which is induced by the travelling surface waves. The nanoscale fluid mechanism in nanochannels has been influenced by both amplitude and frequency of travelling surface waves, and the hydrodynamic characteristics have been obtained by molecular dynamics simulations. It has been found that the flow rate is an increasing function of the amplitude of travelling surface waves and can be up to a sevenfold increase. However, the flow rate is only enhanced in the limited range of frequency of travelling surface waves such as low frequencies, and a maximum fivefold increase in flow rate is pronounced. In addition, the fluid–wall interaction (surface wettability) plays an important role in the nanoscale transport phenomena, and the flow rate is significantly increased under a strong fluid–wall interaction (hydrophilicity) in the presence of travelling surface waves. Moreover, the friction coefficient on the wall of nanochannels is decreased obviously due to the large slip length, and the shear viscosity of fluid on the hydrophobic surface is increased by travelling surface waves. It can be concluded that the travelling surface wave has a potential function to facilitate the flow in nanochannels with respect to the decrease in surface friction on the walls. Our results allow to define better strategies for the fast nanofluidics by travelling surface waves.

•Size effects are found in non-linear heat conduction with flux-limited behaviors.•The heat flux will not exist in problems with sufficiently small scale.•The existence needs the sizes larger than certain critical sizes.•The non-linear heat conduction models predict the possibility of multiplicity.•The possibility can be avoided by sizes larger than other critical sizes.Size effects are discussed for several non-linear heat conduction models with flux-limited behaviors, including the phonon hydrodynamic, Lagrange multiplier, hierarchy moment, nonlinear phonon hydrodynamic, tempered diffusion, thermon gas and generalized nonlinear models. For the phonon hydrodynamic, Lagrange multiplier and tempered diffusion models, heat flux will not exist in problems with sufficiently small scale. The existence of heat flux needs the sizes of heat conduction larger than their corresponding critical sizes, which are determined by the physical properties and boundary temperatures. The critical sizes can be regarded as the theoretical limits of the applicable ranges for these non-linear heat conduction models with flux-limited behaviors. For sufficiently small scale heat conduction, the phonon hydrodynamic and Lagrange multiplier models can also predict the theoretical possibility of violating the second law and multiplicity. Comparisons are also made between these non-Fourier models and non-linear Fourier heat conduction in the type of fast diffusion, which can also predict flux-limited behaviors.

Tuning the thermal transport properties of graphene is under intense investigation to achieve novel material functionalities. Here we propose a strategy of networked nanoconstrictions to maintain the ultrahigh thermal conductivity like in design of graphene-based integrated circuits, or to reduce to the minimum for thermoelectrics of energy conversion. By using molecular dynamics simulations, we study the thermal transport behavior in the 18.2-nm-long graphene sheet and firstly report the characteristics of the thermal resistance arising from single-nanoconstriction, inversely proportional to the constriction width and independent of geometry shapes, which agrees well with the derived two-dimensional ballistic resistance model. After the nanoconstrictions are networked, the results elucidate a parallel relationship between ballistic resistances in parallel systems, and especially, a complicated superimposed effect of arrangement mode on ballistic resistances in series systems governed by the phonon localization and corresponding change of phonon transmission angle. Such anomalous phenomenon causes a decrease or further increase in the total ballistic resistance, e.g., tuning the thermal transport property of graphene as much as or more than 96% with specific nanoconstriction networks. We believe this feasible and versatile route will effectively expand potential applications of two-dimensional graphene and also pave the way for three-dimensional materials in the future.

Understanding capillary filling dynamics in nanoconfined geometries is crucial in the nanotechnology field, such as nanofluidic devices, lab-on-a-chip, 3D printers, porous nanomaterials. The spontaneous capillarity-driven flow behaviors of polyethylene (PE) melts through anodized aluminum oxide (AAO) nanopores are investigated experimentally by using the nanoporous template wetting technique first in our study. The diameter of the pores is 100 nm and 200 nm. The displacement of the polymer melts is measured by the thickness of the fabricated nanowire array as a function of time. Besides, considering the effect of the shear rate on the polymer viscosity in the capillary nanoflow, a theoretical model, i.e. a modified Lucas–Washburn law that combines the Lucas–Washburn law with the polymer rheological model, is established. Based on the experimental results it is speculated that the rise of the meniscus agrees with the modified Lucas–Washburn law. It also suggests that the zero-shear-rate viscosity of the PE melts decreases in their flows through the nanopores and the induced unconventional rheological behavior may be caused by nanoconfinement of the nanopores.

This work provides a comprehensive investigation on the spectral phonon properties in graphene nanoribbons (GNRs) by the normal mode decomposition (NMD) method, considering the effects of edge chirality, width, and temperature. We find that the edge chirality has no significant effect on the phonon relaxation time but has a large influence to the phonon group velocity. As a result, the thermal conductivity of around 707 W/(m K) in the 4.26 nm-wide zigzag GNR at room temperature is higher than that of 467 W/(m K) in the armchair GNR with the same width. As the width decreases or the temperature increases, the thermal conductivity reduces significantly due to the decreasing relaxation times. Good agreement is achieved between the thermal conductivities predicted from the Green–Kubo method and the NMD method. We find that optical phonons dominate the thermal transport in the GNRs while the relative contribution of acoustic phonons to the thermal conductivity is only 10.1% and 13% in the zigzag GNR and the armchair GNR, respectively. Interestingly, the ZA mode is found to contribute only 1–5% to the total thermal transport in GNRs, being much lower than that of 30–70% in single layer graphene.

The superhigh-speed unidirectional rotation of a carbon nanotube (CNT) induced by a linear shear flow is investigated by molecular dynamics simulations. We have identified three rotational types: “continuous rotation”, “interrupted rotation” and “simple oscillation”, corresponding to a decreased number of unidirectional rotation circles over the same time duration. It was found that the unidirectional motion and oscillation respectively originate from the applied shear and rotary Brownian motion by a decoupled analysis of the rotational features. The angular velocity of the unidirectional motion is over one order of magnitude larger than the Jeffery's theory. To construct a CNT-based rotary motor with good performance, the high-speed unidirectional angular velocity can be achieved by carefully selecting the shear rate (e.g. ∼2 × 108 rad s−1 at 35 GHz) and the continuous rotating state can be approached by using a low aspect ratio carbon nanotube.

Thermophoresis can be used for particle manipulation and separation in microfluidics. This work reports an experimental investigation on thermophoresis and the associated rotational patterns of dilute peanut-like colloids in DI water and SDS surfactant solutions. A microfluidic device is utilized for generating a linear temperature gradient and for directly visualizing the thermophoretic motion and rotation of peanut-like particles. Thermophilic behavior was observed for the peanut-like particles, and their thermophoretic mobilities were found smaller than those of the spherical particles of similar sizes. The peanut-like particles’ rotation characterized by the rotational diffusion coefficient is found to be free diffusive dominated. A very small orientation order toward a preferred direction is distinguished along with the random Brownian rotation.

The thermal properties of body-centered tetragonal C4 (bct-C4), a new allotrope of carbon, were investigated using molecular dynamics (MD) simulations. The calculations gave a high and anisotropic thermal conductivity that is the first of its kind. The cross-plane thermal conductivity is 1209 W/(m K) at room temperature, which is even higher than that of diamond. The thermal conductivity decreases as the temperature increases from 80 to 400 K. The density of states of bct-C4 was analyzed, which has a prominent peak at 36 THz. The relaxation times were calculated by fitting a heat flux autocorrelation function. The results showed that the acoustic phonons play the dominant role in the heat conduction, with a contribution of more than 99%. The relaxation times decrease with increasing temperature, as does the contribution of the acoustic phonons. Finally, the thermal conductivity based on lattice dynamics agreed well with that from the MD method, with which the group velocity and mean free path were deduced. This outstanding thermal property makes bct-C4 a promising substitute for diamond, especially as thermal interface materials in microelectronic packaging.

The transient heat conduction in both armchair and zigzag-edged graphene ribbons pulsed by local heating with a duration of 1 ps was studied using nonequilibrium molecular dynamics simulations. The results show that the heat pulse excites two waves which indicates non-Fourier heat conduction. One of the two waves is a sound wave (first sound), which has macroscopic momentum and propagates at the speed of sound. The other is a thermal wave (second sound), whose propagation speed is \( 1/\sqrt 3 \) of the sound velocity. The sound wave excited by the heat pulse is a longitudinal wave, whose energy is only transported in the longitudinal direction. The thermal wave excited by the heat pulse is generated by transverse lattice vibrations, with the energy only having the transverse component. The observed anisotropy of the transient heat conduction suggests that the system is in a non-equilibrium state during propagation of the heat pulse. Further statistical analyses show that the displacement of the heat pulse energy is related to the time as \( \langle \sigma^{2}\rangle \propto t^{1.80} \), which implies that heat transport is ballistic-diffusive transport in graphene. The higher proportion of the ballistic transport will lead to stronger heat waves. At the crest of the thermal wave, energy is transported ballistically, while in the diffusive region and during attenuation of the thermal wave, the energy is transported diffusively.

Molecular dynamics simulations are conducted to study the motion of carbon nanotube-based nanobearings powered by temperature difference. When a temperature difference exists between stator nanotubes, the rotor nanotubes acquire a higher temperature, which arises from the interaction between phonon currents and nanotubes. The thermal driving force increases with the increase in temperature difference between the stators, an increase that is nearly proportional to the temperature difference. Confined by the minimum energy track, the (5, 5)@(10, 10) nanotube bearings only translate along the axis direction but without successive rotation.

Generally polymer bulk structures and nanostructures are thermally insulative. In this study, we show that an improved nanoporous template wetting technique can prepare thermally conductive polymer nanowire arrays. The thermal conductivities of the fabricated high-density polyethylene (HDPE) nanowire arrays with diameters of 100 nm and 200 nm, measured by a laser flash method, are about 2 orders of magnitude higher than their bulk counterparts. The estimated thermal conductivity of a single HDPE nanowire is as high as 26.5 W/mK at room temperature. The high orientation of chains of the HDPE nanowires may arise from the integrative effects of shear rate, vibrational perturbation, translocation, nanoconfinement and crystallization. Findings in this study provide useful strategies on enhancing the intrinsic thermal properties of polymer nanostructures.

A uniform momentum source-and-sink scheme of nonequilibrium molecular dynamics (NEMD) is developed to calculate the shear viscosity of fluids in this paper. The uniform momentum source and sink are realized by momentum exchanges of individual atoms in the left and right half systems, like the reverse nonequilibrium molecular dynamics (RNEMD) method [20] [Müller-Plathe, Phys. Rev. E, 49 (359), 1999]. This method has all features of RNEMD. In addition, the present momentum swap strategy maximizes the perturbation relaxation and eliminates the boundary jumps, which often harm other NEMD methods greatly. With periodic boundary conditions quadratic velocity profiles can be constructed and from the mean velocities of the right and left half systems the shear viscosity can be easily extracted. The scheme is tested on Lennard-Jones fluids over a wide range of state points (temperature and density), momentum exchange intervals and system sizes. It is demonstrated that the present approach can give reliable results with fast convergence by properly selecting the simulation parameters, i.e. particle number and exchange interval.

Macro-thermal cloaking is typically produced by coordinate transformations, but this method is unsuitable for nanostructures. We designed a graphene-based nanoscale thermal cloak using a novel mechanism of phonon localization. The nanocloak in graphene was produced via the chemical functionalization of hydrogen, methyl and hydroxyl using molecular dynamics simulations. The cloaking performance was quantified by the ratio of thermal cloaking (RTC). We found that the RTC correlated with the functionalization fraction and it has a local maximum at a certain width, since the heat flux reduction in the exterior and the protected region reversed if the width was excessive. The atomic mass of the functional group also correlated with the RTC. Our simulations determined that phonon localization occurred due to sp2-to-sp3 bonding transitions, which caused the heat flux to avoid the transition region. Finally, the extent of phonon localization was related to the cloaking performance.