Heat Conduction In Nanostructures Research Articles

Nanoscale limit of the thermal conductivity in crystalline silicon carbide membranes, nanowires, and phononic crystals

Silicon carbide (SiC) aims to be the number one material for power microelectronics due to its remarkable thermal properties. Recent progress in SiC technology finally enabled the fabrication of crystalline SiC nanostructures. Yet, the thermal properties of SiC at the nanoscale remain overlooked. Here, we systematically study heat conduction in SiC nanostructures, including nanomembranes, nanowires, and phononic crystals. Our measurements show that the thermal conductivity of nanostructures is several times lower than that in bulk and that the values scale proportionally to the narrowest dimension of the structures. In the smallest nanostructures, the thermal conductivity reached 10% of that in bulk. To better understand nanoscale thermal transport in SiC, we also probed phonon mean free path and coherent heat conduction in the nanostructures. Our theoretical model links the observed suppression of heat conduction with the surface phonon scattering, which limits the phonon mean free path and thus reduces the thermal conductivity. This work uncovers thermal characteristics of SiC nanostructures and explains their origin, thus enabling realistic thermal engineering in SiC microelectronics.

Thermal Rectification in Asymmetric Graphene/Hexagonal Boron Nitride van der Waals Heterostructures.

Graphene/hexagonal boron nitride (h-BN) heterostructures assembled by van der Waals (vdW) interactions show numerous unique physical properties such as quantum Hall effects and exotic correlated states, which have promising potential applications in the design of novel electronic devices. Understanding thermal transport in such junctions is critical to control the performance and stability of prospective nanodevices. In this work, using nonequilibrium molecular dynamics simulations, we systematically investigate the thermal transport in asymmetric graphene/h-BN vdW heterostructures. It is found that the heat prefers to flow from the monolayer to the multilayer regions, resulting in a significant thermal rectification (TR) effect. To determine the optimum conditions for TR, the influences of sample length, defect density, asymmetric degree, ambient temperature, and vdW interaction strength are studied. Particularly, we found that the TR ratio could be improved by about 1 order of magnitude via increasing the coupling strength from 1 to 10, which clearly distinguishes from the commonly held notion that the TR ratio is practically insensitive or even decreasing with the interaction strength. Detailed spectral analysis reveals that this unexpected increase of the TR ratio can be attributed to heavily modified phonon properties of encased graphene due to enhanced interlayer coupling. Our results elucidate the importance of vdW interactions to heat conduction in nanostructures.

Unexpected thermal conductivity enhancement in pillared graphene nanoribbon with isotopic resonance

Thermal transport in nanoribbon based nanostructures is critical to advancing its applications. Wave effects of phonons can give rise to controllability of heat conduction in nanostructures beyond that by particle scattering. In this paper, by introducing pillars to form structural resonance, we systematically studied the thermal conductivity of graphene nanoribbon based phononic metamaterials (GNPM) through non-equilibrium molecular dynamical simulation. Interestingly, it is found that the thermal conductivity of GNPM is counter intuitively enhanced by isotope engineering, which is strong contrast to the common notion that isotope engineering reduces thermal conductivity.Further mode analysis and atomic Green function calculation reveal that the unexpected increasing in thermal conductivity originates from the breaking of the resonant hybridization wave effect between the resonant modes and the propagating modes induced by isotope engineering. Besides, factors including the system width and pillar height can also efficiently tune the thermal conductivity of GNPM. This abnormal mechanism provides a new dimension to manipulate phonon transport in nanoribbon based nanostructures through wave effect.

Heat conduction in nanostructures

Specific features of the heat transfer in nanostructures and methods of their investigation are discussed. Phonon heat conduction that is characteristic of semiconductors and insulators is considered. In nanostructures, the Fourier law is violated and the methods of classical theory of heat conduction are inapplicable. Analysis of the physics of heat-transfer processes has shown that the heat conduction of nanostructures depends on the shape and size of the sample, properties of its surface, and direction of the heat flow with respect to the nanostructure geometry. This fact has led to the development of radically new approaches to the determination of thermal conductivity of solids on the nano- and micrometer scales. Therefore, main attention is paid to the review of existing methods for finding thermal conductivity of nanostructures and the results of its theoretical and experimental determination. Various models of heat transfer in nanostructures are presented. The knowledge of statistical thermodynamics, kinetic theory, and solid-state physics is fundamental for this field of thermal physics.

An efficient two-step Monte Carlo method for heat conduction in nanostructures

Research on the heat conduction in nanostructures has drawn much attention due to their potential applications in thermoelectric devices. Although the phonon tracing Monte Carlo (MC) technique, where the trajectories of individual phonons are simulated independently, has been extensively used for simulating the heat conduction in nanomaterials, it cannot efficiently simulate the phonon transport in the large area periodic nanostructures yet, due to the demand of absorbing boundaries. In the present work, we develop a two-step phonon tracing MC method to solve this problem. At the first step, the initial phonon transmittance and the phonon emission distributions at the internal virtual boundary are obtained by simulating phonon transport in the initial simulation unit that is directly in contact with the phonon bath. At the second step, the internal phonon transmittance is calculated for the internal simulation units according to the internal boundary phonon emission distributions. Since the whole structure can be simplified as a one-dimensional phonon transport system, the total phonon transmittance can be readily calculated via the combination of initial and internal phonon transmittances, and the effective thermal conductivity is then derived. Furthermore, for verification, we calculate the effective thermal conductivities of three typical nanostructures, that is, the cross-plane and in-plane nanofilms and the periodic nanoporous structures, by using the theoretical models, the standard and the two-step MC simulations, respectively. The two-step MC method well predicts the results calculated by the standard MC simulations and the theoretical models. More importantly, the computation time of the two-step MC simulation is at least one order of magnitude less than that of the standard MC simulation, while its under-prediction can be less than 10% even 5%.

Heat conduction in nanostructured materials

An overview of the direct and ab initio simulation as well as experimental tools addressing heat conduction in dielectric nanostructures and nanostructured materials is proposed. Recent advancements based on density functional theory to compute phonon properties and thermal conductivity are exposed and the state of the art in metrology is detailed. Finally, new progresses in the experimental demonstration of coherence effects in heat conduction are illustrated, emphasizing the open questions in the still poorly understood field of phononic heat transport.

The effective thermal conductivity of ballistic–diffusive heat conduction in nanostructures with internal heat source

In nanostructures whose characteristic lengths are comparable to the phonon mean free path, the ballistic–diffusive heat conduction leads to the size effect, geometry dependence and anisotropy of the effective thermal conductivity. In the present work, we have studied the effective thermal conductivity of the ballistic–diffusive heat conduction in nanostructures (including nanofilms and nanowires) with internal heat source using Monte Carlo simulation and Boltzmann transport equation. It is found that the effective thermal conductivity of nanostructures with internal heat source is significantly lower than that with temperature difference, though it still increases with the increasing characteristic length. The models for the effective thermal conductivity and the temperature distribution of the cross-plane heat conduction in the nanofilms with internal heat source are directly derived from the phonon Boltzmann transport equation, and the comparisons with the Monte Carlo simulations well confirm their validities. As for the effective thermal conductivity of the in-plane nanofilms and nanowires with internal heat source, referring to the Matthiessen’s rule, the models are in the form of keff/kbulk=1/(1+αKn), with the parameter α obtained by the best fitting with the Monte Carlo simulations. Moreover, the diffusive heat conduction equation with the effective thermal conductivity can well characterize the temperature distributions in the in-plane nanofilms and long nanowires, while it fails in the short nanowires due to the influence of the axial constraints.

Molecular Dynamics Simulations of Heat Conduction in Nanostructures: Effect of Heat Bath

We investigate systematically the impacts of heat bath used in molecular dynamics simulations on heat conduction in nanostructures exemplified by Silicon Nanowires (SiNWs) and Silicon/Germanium nano junction. It is found that multiple layers of Nos\'e-Hoover heat bath are required to reduce the temperature jump at the boundary, while only a single layer of Langevin heat bath is sufficient to generate a linear temperature profile with small boundary temperature jump. Moreover, an intermediate value of heat bath parameter is recommended for both Nos\'e-Hoover and Langevin heat bath in order to achieve correct temperature profile and thermal conductivity in homogeneous materials. Furthermore, the thermal rectification ratio in Si/Ge thermal diode depends on the choice of Nos\'e-Hoover heat bath parameter remarkably, which may lead to non-physical results. In contrast, Langevin heat bath is recommended because it can produce consistent results with experiment in large heat bath parameter range.

Nanoscale heat transfer and nanostructured thermoelectrics

Heat transfer at nanoscales differs significantly from that in macroscales because of size effects on the phonon and electron transport. Nanoscale heat transfer effects have significant implications for the microelectronic and microphotonic industries, from the thermal management, the device design and reliability, and the active cooling considerations. Past studies have shown that heat conduction in nanostructures can be significantly impeded below that of the predictions of the Fourier theory. Such size effects imply higher device temperatures than anticipated and demands more stringent thermal management measures. On the other hand, same size effects can be exploited for developing highly efficient thermoelectric (TE) materials for direct cooling. This paper starts with a discussion on some nanoscale heat transfer effects and their impacts on the device performance, particularly using thermal conductivity reduction in superlattices as an example, followed by a review of recent developments in nanostructured TE materials

Phonon heat conduction in nanostructures

Phonon heat conduction mechanisms in nanostructures may differ significantly from those in macrostructures. In this paper, we illustrate the fundamental differences between heat conduction in nanostructures and macrostructures based on examples we have been working on, including particularly heat conduction in superlattices, nanowires, and nanoparticles. The prospects of control heat conduction in nanostructures through phonon engineering are discussed. Examples are given to demonstrate the importance of nanoscale heat transfer in microelectronics and energy technology.

Particularities of Heat Conduction in Nanostructures

Heat conduction in nanostructures differs significantly from that in macrostructures because the characteristic length scales associated with heat carriers, i.e., the mean free path and the wavelength, are comparable to the characteristic length of nanostructures. In this communication, particularities associated with phonon heat conduction in nanostructures, the applicability of the Fourier law, and the implications of nanoscale heat transfer effects on nanotechnology are discussed.