Modelling transient heat conduction in solids at multiple length and time scales: A coupled non-equilibrium molecular dynamics/continuum approach

A method for controlling the thermal boundary conditions of non-equilibrium molecular dynamics simulations is presented. The method is simple to implement into a conventional molecular dynamics code and independent of the atomistic model employed. It works by regulating the temperature in a thermostatted boundary region by feedback control to achieve the desired temperature at the edge of an inner region where the true atomistic dynamics are retained. This is necessary to avoid intrinsic boundary effects in non-equilibrium molecular dynamics simulations. Three thermostats are investigated: the global deterministic Nosé–Hoover thermostat and two local stochastic thermostats, Langevin and stadium damping. The latter thermostat is introduced to avoid the adverse reflection of phonons that occurs at an abrupt interface. The method is then extended to allow atomistic/continuum models to be thermally coupled concurrently for the analysis of large steady state and transient heat conduction problems. The effectiveness of the algorithm is demonstrated for the example of heat flow down a three-dimensional atomistic rod of uniform cross-section subjected to a variety of boundary conditions.     

Carbon-Based Materials: Growth,Properties, MEMS/NEMS Technologies,and MEM/NEM Switches

Micro- or nanoelectromechanical system (MEMS/NEMS) is a multidisciplinary field, which has witnessed explosive growth during the past decades. The current materials for MEMS are dominated by Si thanks to the fully based technology for CMOS. However, Si has its own intrinsic limitations such as poor mechanical or tribological properties and poor thermal stability. Carbon-based materials such as diamond, carbon nanotube, and graphene possess excellent properties such as low mass, high Young's modulus, high thermal conductivity, hydrophobic surface, and tailorable electronic configuration, which make these materials promising for MEMS/NEMS applications with diverse and much better performance than Si. In this review, we describe the recent progress of carbon-based materials for MEMS/NEMS with focus on diamond, carbon nanotube, and graphene. The growth of these carbon materials is briefly described and only selected properties are discussed with respect to MEMS/NEMS applications. The fabrication process for suspended structures of these materials toward MEMS/NEMS is comprehensively reviewed from the recent literatures. Special attention is devoted to the state-of-the-art of MEM/NEM switches based on these carbon materials, one of the most important fields in MEMS/NEMS. Finally, the application of carbon-based MEM/NEM switches to microwave field is critically analyzed.     

尺寸效应对微通道内固液界面温度边界的影响

采用非平衡分子动力学方法模拟不同浸润性微通道内液体的传热过程,分析了尺寸效应对固液界面热阻及温度阶跃的影响.研究结果表明,界面热阻随微通道尺寸的变化可分为两个阶段,即小尺寸微通道的单调递增阶段和大尺寸微通道的恒定值阶段.随着微通道尺寸的增加,近壁区液体原子受对侧固体原子的约束程度降低,微通道中央的液体原子自由移动,固液原子振动态密度近似不变,使得尺寸效应的影响忽略不计.上述两种阶段的微通道尺寸过渡阈值受固液作用强度与壁面温度的共同作用:减弱壁面浸润性,过渡阈值向大尺寸区域迁移;相较于低温壁面,高温壁面处的过渡阈值更大.增加微通道尺寸,固液界面温度阶跃呈单调递减趋势,致使壁面温度边界和宏观尺度下逐渐符合.探讨尺寸效应有助于深刻理解固液界面能量输运及传递机制.     

接触角与液固界面热阻关系的分子动力学模拟

本文利用分子动力学方法模拟了液体在固体表面的 接触角及液固界面热阻, 并探讨了二者之间的关系. 通过分别改变液固结合强度和固体的原子性质来分析接触角和界面热阻的关系及变化趋势. 模拟结果显示增强液固间相互作用时, 接触角减小的同时界面热阻也随之单调减小; 而改变固体原子间结合强度和原子质量时, 接触角几乎保持不变, 但界面热阻显著改变. 固体原子间结合强度和原子质量影响界面热阻的原因是其改变了固体的振动频率分布, 导致液固原子间的振动耦合程度发生变化. 本文的结果表明界面热阻不仅与由接触角所表征的液固结合强度有关, 还与液固原子间的振动耦合程度有关. 接触角与界面热阻间不存在单值的对应关系, 不能单一地将接触角作为液固界面热阻的评价标准. 关键词： 液固界面 接触角 界面热阻 分子动力学模拟     

包含倾斜晶界的双晶ZnO的热输运行为

用嵌入位错线法和重合位置点阵法构建含有小角度和大角度倾斜角的双晶氧化锌纳米结构.用非平衡分子动力学方法模拟双晶氧化锌在不同倾斜角度下的晶界能、卡皮查热阻,并研究了样本长度和温度对卡皮查热阻和热导率的影响.模拟结果表明,晶界能在小角度区域随倾斜角线性增加,而在大角度区域达到稳定,与卡皮查热阻的变化趋势一致.热导率随样本长度的增加而增加,卡皮查热阻表现出相反的趋势.然而随着温度的增加,热导率和卡皮查热阻都减小.通过比较含5.45°和38.94°晶界样本的声子态密度,发现声子光学支对热传导的影响不大,主要由声子声学支贡献,大角度晶界对声子散射作用更强,声学支波峰向低频率移动.     

The scattering of phonons of arbitrary wavelength at a solid-solid interface: Model calculation and applications

A one-dimensional lattice model of a solid-solid interface is presented within which it is possible to characterize the scattering of phonons at the interface as a function of wavelength. The probability for a phonon to be transmitted across the interface is found generally to decrease with decreasing wavelength, although phenomena such as total reflexion and resonant transmission may occur. Conditions for the existence of a localized interface mode are given. The thermal boundary resistance for heat flow across the interface is expressed in terms of an average temperature-dependent phonon transmission coefficient which generally increases with decreasing temperature and approaches the continuum value at very low temperature. Applications of these results to three-dimensional interfaces in general, and particularly to heat dissipation in catalysts, high-frequency phonon radiators, and Kapitza resistance, are discussed.     

Slip, immiscibility, and boundary conditions at the liquid-liquid interface

The conventional boundary conditions at the interface between two flowing liquids include continuity of the tangential velocity. We have tested this assumption with molecular dynamics simulations of Couette and Poiseuille flows of two-layered liquid systems, with various molecular structures and interactions. When the total liquid density near the interface drops significantly compared to the bulk values, the tangential velocity varies very rapidly there, and would appear discontinuous at continuum resolution. The value of this apparent slip is given by a Navier boundary condition.     

聚乙烯/银纳米颗粒复合物的分子动力学模拟研究

通过分子动力学模拟对聚乙烯/银纳米颗粒复合物的结构、极化率和红外光谱、热力学性质、力学特性进行计算, 分析其随模拟温度和银颗粒尺寸的变化规律. 模拟结果表明: 聚乙烯/银纳米颗粒复合物为各向同性的无定形结构, 温度升高可提高银纳米颗粒的分散均匀性; 银纳米颗粒表面多个原子层呈现无定形状态, 并在银颗粒和聚乙烯基体的界面形成电极化层, 界面区域随颗粒尺寸和温度的增加分别减小和增加; 与聚乙烯体系相比, 聚乙烯/银纳米颗粒复合物的极化率高很多, 且随温度的升高和银颗粒尺寸的减小而增大; 银颗粒尺寸直接影响界面电偶极矩的强度和振动频率, 红外光谱峰强度和峰位随颗粒尺寸发生变化; 聚乙烯/银纳米颗粒复合物具有比聚乙烯体系更高的等容热容和与聚乙烯体系相反的负值热压力系数, 热容随颗粒尺寸的变化较小, 但随温度的升高而明显减小, 具有显著的温度效应; 热压力系数随温度的变化较小, 但随颗粒尺寸的增加而减小, 具有明显的尺度效应, 温度稳定性更好; 聚乙烯/银纳米颗粒复合物的力学特性表现出各向同性材料的弹性常数张量, 具有比聚乙烯体系更高的杨氏模量和泊松比, 并且都随温度的升高和银颗粒尺寸的增大而减小, 加入银纳米颗粒可有效改善聚乙烯的力学性质. 关键词： 分子动力学模拟 聚合物纳米复合物 纳米颗粒     

Kapitza resistance at the liquid—solid interface

Using equilibrium and non-equilibrium molecular dynamics simulations, we determine the Kapitza resistance (or thermal contact resistance) at a model liquid-solid interface. The Kapitza resistance (or the associated Kapitza length) can reach appreciable values when the liquid does not wet the solid. The analogy with the hydrodynamic slip length is discussed.     

The effect of interfacial energy anisotropy on planar interface instability in a succinonitrile alloy under a small temperature gradient

The morphological stability of a planar interface with different crystallographic orientations is studied under a small positive temperature gradient using a transparent model alloy of succinonitrile.Novel experimental apparatus is constructed to provide a temperature gradient of about 0.37 K/mm.Under this small temperature gradient,the planar interface instability depends largely on the crystallographic orientation.It is shown experimentally that the effect of interfacial energy anisotropy on planar interface stability cannot be neglected even in a small temperature gradient system.Higher interfacial energy anisotropy leads the planar interface to become more unstable,which is different from the stabilizing effect of the interfacial energy on the planar interface.The experimental results are in agreement with previous theoretical calculations and phase field simulations.     

Theory on Morphological Instability in Driven Systems

Motivated by recent findings from simulation of a driven lattice gas under shifted periodic boundary conditions, we study within the context of a continuum model the interfacial stability of driven diffusive systems. In this model, an external driving field maintains the system away from equilibrium. Well below criticality, steady-state solutions of the associated bulk kinetic equation are obtained. Our results successfully account for the novel features found in simulation. In particular, the solution describing a pair of interfaces tilted with respect to the driving field under periodic boundary conditions shows a tilt-dependent bulk density (and internal energy), and boundary layers near one of the interfaces. Focusing on the interface dynamics, one finds that such an interface exhibits a characteristic Mullins-Sekerka instability. This is argued to be responsible for the onset of the single- to multistrip transformation observed in simulation.     

Negative differential thermal resistance in a double-chain system

Using nonequilibrium molecular dynamics simulations, we study the phenomenon of negative differential thermal resistance (NDTR) in a double-chain system. We investigate the dependence of NDTR on the external potential, inter- and intra-chain interaction and the system size. It is reported that the NDTR can occur in a small double-chain system with weak external potential and weak inter- and intra-chain interaction. We also present the influence of the external potential, inter- and intra-chain interaction and the system size on the heat current of the system through the phonon spectral analysis.     

General theories and features of interfacial thermal transport

A clear understanding and proper control of interfacial thermal transport is important in nanoscale devices. In this review, we first discuss the theoretical methods to handle the interfacial thermal transport problem, such as the macroscopic model, molecular dynamics, lattice dynamics, and quantum transport theories. Then we discuss various effects that can significantly affect the interfacial thermal transport, such as the formation of chemical bonds at interface, defects, interface roughness, strain, substrates, atomic species, mass ratios, and structural orientations. Then importantly, we analyze the role of inelastic scattering at the interface, and discuss its application in thermal rectifications. Finally, the challenges and promising directions are discussed.     

A multiscale coupling method for the modeling of dynamics of solids with application to brittle cracks

We present a multiscale model for numerical simulations of dynamics of crystalline solids. The method combines the continuum nonlinear elasto-dynamics model, which models the stress waves and physical loading conditions, and molecular dynamics model, which provides the nonlinear constitutive relation and resolves the atomic structures near local defects. The coupling of the two models is achieved based on a general framework for multiscale modeling – the heterogeneous multiscale method (HMM). We derive an explicit coupling condition at the atomistic/continuum interface. Application to the dynamics of brittle cracks under various loading conditions is presented as test examples.     

Multiscale analysis in nonlinear thermal diffusion problems in composite structures

The aim of this paper is to analyze the asymptotic behavior of the solution of a nonlinear problem arising in the modelling of thermal diffusion in a two-component composite material. We consider, at the microscale, a periodic structure formed by two materials with different thermal properties. We assume that we have nonlinear sources and that at the interface between the two materials the flux is continuous and depends in a dynamical nonlinear way on the jump of the temperature field. We shall be interested in describing the asymptotic behavior of the temperature field in the periodic composite as the small parameter which characterizes the sizes of our two regions tends to zero. We prove that the effective behavior of the solution of this system is governed by a new system, similar to Barenblatt’s model, with additional terms capturing the effect of the interfacial barrier, of the dynamical boundary condition, and of the nonlinear sources.     

SiC/C界面辐照性能的分子动力学研究

本文通过分子动力学模拟的方法,研究了5种含不同空间结构的SiC/C界面的材料受辐照后的缺陷分布随时间以及PKA位置的变化关系,并与单质SiC中缺陷分布情况进行对比.利用径向分布函数分析了辐照对界面原子排列情况的影响.研究结果表明,SiC/C界面的抗辐照能力明显低于SiC内部,不同的空间结构对界面缺陷数量存在一定影响.由径向分布函数推得界面区域石墨原子密度高则界面原子排列情况受辐照影响越大.     

Anomalous Impact of Surface Wettability on Leidenfrost Effect at Nanoscale

Leidenfrost effect is a common and important phenomenon which has many applications,however there is a limited body of knowledge about the Leidenfrost effect at the nanoscale regime.We investigate the impact of substrate wettability on Leidenfrost point temperature(LPT) of nanoscale water film via molecular dynamics simulations,and reveal a new mechanism different from that at the macroscale.In the molecular dynamics simulations,a method of monitoring density change at different heating rates is proposed to obtain accurate LPT under different surface wettability.The results show that LPT decreases firstly and then increases with the surface wettability at the nanoscale,which is different from the monotonous increasing trend at the macroscale.The mechanism is elucidated by analyzing the competitive effect of adhesion force and interfacial thermal resistance,as well as different contributions of gravity on LPT at the nanoscale and macroscale.The investigations can deepen the understanding of Leidenfrost effect at the nanoscale regime and also facilitate to guide the applications of heat transfer and flow transport.     

Molecular dynamics simulations of translational thermal accommodation coefficients for time-resolved LII

Time-resolved laser-induced incandescence demands precise knowledge of the thermal accommodation coefficient, but little is known about the gas-surface scattering physics that underlies this parameter. This paper presents a molecular dynamics simulation that shows how the thermal accommodation coefficient is influenced by the gas molecular mass and gas temperature. The molecular dynamics results also define scattering kernels that can be used as boundary conditions in Direct Simulation Monte Carlo simulations of heat and momentum transfer between soot aggregates and surrounding gas molecules.     

Boundary treatment effects on molecular dynamics simulations of interface thermal resistance

Molecular Dynamics simulations of heat conduction in liquid Argon confined in Silver nano-channels are performed subject to three different thermal conditions. Particularly, different surface temperatures are imposed on Silver domains using a thermostat in all and limited number of solid layers, resulting in heat flux in the liquid domain. Alternatively, energy is injected and extracted from solid layers to create a NVE liquid Argon system, which corresponds to heat flux specification. Imposition of a constant temperature region in the solid domain results in an unphysical temperature jump, indicating the presence of an artificial thermal resistance induced by the thermostat. Thermal resistance analyses for the components of each case are performed to distinguish the artificial and interface thermal resistance effects. Constant wall temperature simulations are shown to exhibit superposition of the artificial and interface thermal resistance values at the liquid/solid interface, while applying thermostat on wall layers sufficiently away from the liquid/solid interface results in consistent predictions of the interface thermal resistance. Injecting and extracting energy from each solid layer eliminates the artificial resistance. However, the method cannot directly specify a desired temperature difference between the two solid domains.