Molecular dynamics simulations of thermal transport in porous nanotube network structures

Lateral and flexural thermal transport in stanene/2D-SiC van der Waals heterostructure

Thermal management is one of the key challenges in nanoelectronic and optoelectronic devices. The development of a van der Waals heterostructure (vdWH) using the vertical positioning of different two-dimensional (2D) materials has recently appeared as a promising way of attaining desirable electrical, optical, and thermal properties. Here, we explore the lateral and flexural thermal conductivity of stanene/2D-SiC vdWH utilizing the reverse non-equilibrium molecular dynamics simulation and transient pump-probe technique. The effects of length, area, coupling strength and temperature on the thermal transport are studied systematically. The projected lateral thermal conductivity of a stanene/2D-SiC hetero-bilayer is found to be 66.67 [Formula: see text], which is greater than stanene, silicene, germanene, MoSe₂ and even higher than some hetero-bilayers, including MoS₂/MoSe₂ and stanene/silicene. The lateral thermal conductivity increases as the length increases, while it tends to decrease with increasing temperature. The computed flexural interfacial thermal resistance between stanene and 2D-SiC is 3.0622 [Formula: see text] [Formula: see text] K.m² W^-1, which is close to other 2D hetero-bilayers. The interfacial resistance between stanene and 2D-SiC is reduced by 70.49% and 50.118% as the temperature increases from 100 K to 600 K and the coupling factor increases from [Formula: see text] to [Formula: see text], respectively. In addition, various phonon modes are evaluated to disclose the fundamental mechanisms of thermal transport in the lateral and flexural direction of the hetero-bilayer. These results are important in order to understand the heat transport phenomena of stanene/2D-SiC vdWH, which could be useful for enhancing their promising applications.

The impact of tilt grain boundaries on the thermal transport in perovskite SrTiO3 layered nanostructures. A computational study

Thermal management at solid interfaces presents a technological challenge for modern thermoelectric power generation. Here, we define a computational protocol to identify nanoscale structural features that can facilitate thermal transport in technologically important nanostructured materials. We consider the highly promising thermoelectric material, SrTiO3, where tilt grain boundaries lower thermal conductivity. The magnitude of the reduction is shown to depend on compositional and structural arrangements at the solid interface. Quantitative analysis indicates that layered nanostructures less than 10 nm will be required to significantly reduce the thermal conductivity below the bulk value, and it will be virtually independent of temperature for films less than 2 nm depending on the orientation with a reduction of thermal transport up to 75%. At the nanoscale, the vibrational response of nanostructures shows concerted vibrations between the grain boundary and inter-boundary regions. As the grain boundary acts markedly as a phonon quencher, we predict that any manipulation of nanostructures to further reduce thermal conductivity will be more beneficial if applied to the inter-boundary region. Our findings may be applied more widely to benefit other technological applications where efficient thermal transport is important.

Developing nanotube junctions with arbitrary specifications

Experimentally synthesized carbon nanotube (CNTs) junctions (either single or with 2D/3D CNT network topology) are expected to have random orientation of defect sites (non-hexagonal rings) around the junction. This random and irregular nature of the junction topology and defect characteristics is expected to affect their strength and durability as well as impact the associated mesoscopic and macroscopic properties. On the contrary, theoretical and computational studies often investigate structure-property relationships of pristine and regular junctions of carbon nanostructures. In this study, we developed a computational framework to model a variety of junction structures between CNTs with arbitrary spatial (orientation and degree of overlap) and intrinsic (chirality) specifications. The developed computational model also has the ability to tune the degree of topological defects around the junction via a variety of defect annihilation approaches. Our method makes use of the primal/dual meshing concept, where the development and manipulation of the junction nodes occur using triangular meshes (primal mesh), which is eventually converted to its dual mesh (honeycomb mesh) to render a fully covalently bonded CNT junction. Here each carbon atom has 3 bonded neighbors (mimicking sp² hybridization). Under a given set of CNT orientation, overlap and chirality specifications, the approach creates a number of CNT junction configurations with varying degrees of energetic stability, offering an opportunity to investigate the effect of topological arrangement of defects around the junction on mechanical, electrical and thermal properties. In addition, it is shown via few examples that the discussed methodology can easily be extended to create multi-junction nanotube clusters, multi-wall nanotube junctions, as well as true 3D random network structures.

Asymmetric Junctions Boost in-Plane Thermal Transport in Pillared Graphene

Hybrid 3D nanoarchitectures by covalent connection of 1D and 2D nanomaterials are currently in high demands to overcome the intrinsic anisotropy of the parent materials. This letter reports the junction configuration-mediated thermal transport properties of Pillared Graphene (PGN) using reverse nonequilibrium molecular dynamics simulations. The asymmetric junctions can offer ∼20% improved in-plane thermal transport in PGN, unlike the intuition that their wrinkled graphene sheets cause phonon scattering. This asymmetric trait, which entails lower phonon scattering provides a new degree of freedom to boost thermal properties of PGN and potentially other hybrid nanostructures.

Lateral and flexural phonon thermal transport in graphene and stanene bilayers

The predicted in-plane thermal conductivity of the graphene/stanene hetero-bilayer is 311.1 W m⁻¹ K⁻¹, higher than most 2D materials such as phosphorene, hexagonal boron nitride, MoS₂ and MoSe₂.

Modeling the Role of Bulk and Surface Characteristics of Carbon Fiber on Thermal Conductance across the Carbon-Fiber/Matrix Interface

The rapid heating of carbon-fiber-reinforced polymer matrix composites leads to complex thermophysical interactions which not only are dependent on the thermal properties of the constituents and microstructure but are also dependent on the thermal transport between the fiber and resin interfaces. Using atomistic molecular dynamics simulations, the thermal conductance across the interface between a carbon-fiber near-surface region and bismaleimide monomer matrix is calculated as a function of the interface and bulk features of the carbon fiber. The surface of the carbon fiber is modeled as sheets of graphitic carbon with (a) varying degrees of surface functionality, (b) varying defect concentrations in the surface-carbon model (pure graphitic vs partially graphitic), (c) varying orientation of graphitic carbon at the interface, (d) varying interface saturation (dangling vs saturated bonds), (e) varying degrees of surface roughness, and (f) incorporating high conductive fillers (carbon nanotubes) at the interface. After combining separately equilibrated matrix system and different surface-carbon models, thermal energy exchange is investigated in terms of interface thermal conductance across the carbon fiber and the matrix. It is observed that modifications in the studied parameters (a-f) often lead to significant modulation of thermal conductance across the interface and, thus, showcases the role of interface tailoring and surface-carbon morphology toward thermal energy exchange. More importantly, the results provide key bounds and a realistic degree of variation to the interface thermal conductance values at fiber/matrix interfaces as a function of different surface-carbon features.

Dimensional Crossover of Thermal Transport in Hybrid Boron Nitride Nanostructures

Although boron nitride nanotubes (BNNT) and hexagonal-BN (hBN) are superb one-dimensional (1D) and 2D thermal conductors respectively, bringing this quality into 3D remains elusive. Here, we focus on pillared boron nitride (PBN) as a class of 3D BN allotropes and demonstrate how the junctions, pillar length and pillar distance control phonon scattering in PBN and impart tailorable thermal conductivity in 3D. Using reverse nonequilibrium molecular dynamics simulations, our results indicate that although a clear phonon scattering at the junctions accounts for the lower thermal conductivity of PBN compared to its parent BNNT and hBN allotropes, it acts as an effective design tool and provides 3D thermo-mutable features that are absent in the parent structures. Propelled by the junction spacing, while one geometrical parameter, e.g., pillar length, controls the thermal transport along the out-of-plane direction of PBN, the other parameter, e.g., pillar distance, dictates the gross cross-sectional area, which is key for design of 3D thermal management systems. Furthermore, the junctions have a more pronounced effect in creating a Kapitza effect in the out-of-plane direction, due to the change in dimensionality of the phonon transport. This work is the first report on thermo-mutable properties of hybrid BN allotropes and can potentially impact thermal management of other hybrid 3D BN architectures.

A comprehensive review on the molecular dynamics simulation of the novel thermal properties of graphene

This review summarizes state-of-the-art progress in the molecular dynamics simulation of the novel thermal properties of graphene.

Van der Waals interaction-tuned heat transfer in nanostructures

Interfaces usually impede heat transfer in heterogeneous structures. Recent experiments show that van der Waals (vdW) interactions can significantly enhance thermal conductivity parallel to the interface of a bundle of nanoribbons compared to a single layer of freestanding nanoribbon. In this paper, by simulating heat transfer in nanostructures based on a model of nonlinear one-dimensional lattices interacting via van der Waals interactions, we show that the vdW interface interaction can adjust the thermal conductivity parallel to the interface. The efficiency of the adjustment depends on the intensity of interactions and temperature. The nonlinear dependence of the conductivity on the intensity of interactions agrees well with experimental results for carbon nanotube bundles, multi-walled carbon nanotubes, multi-layer graphene, and nanoribbons.

A novel nano-configuration for thermoelectrics: helicity induced thermal conductivity reduction in nanowires

In this article, we propose a novel helical nano-configuration towards the designing of high ZT thermoelectric materials. Non-equilibrium molecular dynamics (NEMD) simulations for 'model' bi-component nanowires indicate that a significant reduction in thermal conductivity, similar to that of flat superlattice nanostructures, can be achieved using a helical geometric configuration. The reduction is attributed to a plethora of transmissive and reflective phonon scattering events resulting from the steady alteration of phonon propagating direction that emerges from the continuous rotation of the helical interface. We also show that increasing the relative mass ratio of the two components lowers the phonon energy transmission at the interface due to differences in vibrational frequency spectra, thereby relatively 'easing' the phonon energy propagation along the helical pathway. While the proposed mechanisms result in a reduced lattice thermal conductivity, the continuous nature of the bi-component nanowire would not be expected to significantly reduce its electrical counterpart, as often occurs in superlattice/alloy nanostructures. Hence, we postulate that the helical configuration of atomic arrangement provides an attractive and general framework for improved thermoelectric material assemblies independent of the specific chemical composition.

Knitted graphene-nanoribbon sheet: a mechanically robust structure

In this paper, a new nanostructure is proposed, namely, the knitted graphene-nanoribbon sheet (KGS), which consists of zigzag and/or armchair graphene nanoribbons. The knitting technology is introduced to graphene nanotechnology to produce large area graphene sheets. Compared with pristine graphene, the chirality of a knitted graphene-nanoribbon sheet is much more flexible and can be designed on demand. The mechanical properties of KGSs are investigated by molecular dynamics simulations, including the effect of vacancies. With hydrogen atoms saturating the ribbon edges, the structure (KGS + H) is found to be of significant mechanical robustness, whose fracture does not rely on the critical bonds. The fracture strain of KGS + H remains nearly unchanged as long as there remains a single defect-free graphene nanoribbon in the tensile direction. This graphene nano knitting technique is experimentally feasible, inspired by a recent demonstration by Fournier et al. [Phys. Rev. B, 2011, 84, 035435] of lifting a single molecular wire using a combined frequency-modulated atomic force and tunnelling microscope.

Small-Angle X-ray Scattering Investigations of Biomolecular Confinement, Loading, and Release from Liquid-Crystalline Nanochannel Assemblies

This Perspective explores the recent progress made by means of small-angle scattering methods in structural studies of phase transitions in amphiphilic liquid-crystalline systems with nanochannel architectures and outlines some future directions in the area of hierarchically organized and stimuli-responsive nanochanneled assemblies involving biomolecules. Time-resolved small-angle X-ray scattering investigations using synchrotron radiation enable monitoring of the structural dynamics, the modulation of the nanochannel hydration, as well as the key changes in the soft matter liquid-crystalline organization upon stimuli-induced phase transitions. They permit establishing of the inner nanostructure transformation kinetics and determination of the precise sizes of the hydrophobic membraneous compartments and the aqueous channel diameters in self-assembled network architectures. Time-resolved structural studies accelerate novel biomedical, pharmaceutical, and nanotechnology applications of nanochannel soft materials by providing better control of DNA, peptide and protein nanoconfinement, and release from diverse stimuli-responsive nanocarrier systems.