Jingchao Zhang

Thermal transport across graphene and single layer hexagonal boron nitride

As the dimensions of nanocircuits and nanoelectronics shrink, thermal energies are being generated in more confined spaces, making it extremely important and urgent to explore for efficient heat dissipation pathways. In this work, the phonon energy transport across graphene and hexagonal boron-nitride (h-BN) interface is studied using classic molecular dynamics simulations. Effects of temperature, interatomic bond strength, heat flux direction, and functionalization on interfacial thermal transport are investigated. It is found out that by hydrogenating graphene in the hybrid structure, the interfacial thermal resistance (R) between graphene and h-BN can be reduced by 76.3%, indicating an effective approach to manipulate the interfacial thermal transport. Improved in-plane/out-of-plane phonon couplings and broadened phonon channels are observed in the hydrogenated graphene system by analyzing its phonon power spectra. The reported R results monotonically decrease with temperature and interatomic bond strengths. No thermal rectification phenomenon is observed in this interfacial thermal transport. Results reported in this work give the fundamental knowledge on graphene and h-BN thermal transport and provide rational guidelines for next generation thermal interface material designs.

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 (MD) simulation of the novel thermal properties of graphene. The novel thermal properties of graphene, which include anisotropic thermal conductivity, decoupled phonon thermal transport, thermal rectification and tunable interfacial thermal conductance, have attracted enormous interest in the development of next-generation nano-devices. Molecular dynamics simulation is one of the main approaches in numerical simulation of the novel thermal properties of graphene. In this paper, the widely used potentials of MD for modeling the novel thermal properties of graphene are described first. Then MD simulations of anisotropic thermal conductivity, decoupled phonon thermal transport, thermal rectification and tunable interfacial thermal conductance are discussed. Finally, the paper concludes with highlights on both the current status and future directions of the MD simulation of the novel thermal properties of graphene.

Monolayer and bilayer polyaniline C3N: two-dimensional semiconductors with high thermal conductivity

Polyaniline (PANI) has been extensively studied in the past few decades owing to its broad applications in electronic devices. However, two dimensional PANI was not realized until very recently. In this work, the thermal transport properties of one of the newly synthesized 2D PANI structures, C3N, are systematically investigated using classical molecular dynamics simulations. The in-plane thermal conductivity (κ) of monolayer and bilayer C3N structures is computed, and the κ values for infinite-length systems are found to be as high as 820 and 805 W m-1 K-1, respectively. Both the values are markedly higher than those of many prevailing 2D semiconducting materials such as phosphorene, hexagonal boron nitride, MoS2 and MoSe2. The effects of different modulators, such as system dimension, temperature, interlayer coupling strength and tensile strain, on the calculated thermal conductivity are evaluated. Monotonic decreasing trends of thermal conductivity with temperature and tensile strain are found, while a positive correlation between the thermal conductivity and system dimension is revealed. Interlayer coupling strength is found to have negligible effects on the in-plane thermal conductivity of bilayer C3N. The cross-plane interfacial thermal resistance (R) between two adjacent C3N layers is evaluated in the temperature range from 100 to 500 K and at different coupling strengths. The predicted R at temperature 300 K equals 3.4 × 10-8 K m-2 W-1. The maximum reductions of R can amount to 59% and 68% with respect to temperature and coupling strength, respectively. Our results provide theoretical guidance to future applications of C3N-based low-dimensional materials in electronic devices.

Understanding thermal transport in asymmetric layer hexagonal boron nitride heterostructure

In this work, thermal transport at the junction of an asymmetric layer hexagonal boron-nitride (h-BN) heterostructure is explored using a non-equilibrium molecular dynamics method. A thermal contact resistance of 3.6xa0×xa010-11 Kxa0·xa0m2 W-1 is characterized at a temperature of 300 K with heat flux from the trilayer to monolayer regions. The mismatch in the flexural phonon modes revealed by power spectra analysis provides the driving force for the calculated thermal resistance. A high thermal rectification efficiency of 360% is calculated at the layer junction surpassing that of graphene. Several modulators, i.e. the system temperature, contact pressure and lateral dimensions, are applied to manipulate the thermal conductance and rectification across the interfaces. The predicted thermal rectification sustains positive correlations with temperature and phonon propagation lengths with little change to the coupling strength.

Significantly reduced c-axis thermal diffusivity of graphene-based papers

Owing to their very high thermal conductivity as well as large surface-to-volume ratio, graphene-based films/papers have been proposed as promising candidates of lightweight thermal interface materials and lateral heat spreaders. In this work, we study the cross-plane (c-axis) thermal conductivity (k c ) and diffusivity (α c ) of two typical graphene-based papers, which are partially reduced graphene paper (PRGP) and graphene oxide paper (GOP), and compare their thermal properties with highly-reduced graphene paper and graphite. The determined α c of PRGP varies from (1.02xa0±xa00.09)xa0×xa010-7 m2 s-1 at 295 K to (2.31xa0±xa00.18)xa0×xa010-7 m2 s-1 at 12 K. This low α c is mainly attributed to the strong phonon scattering at the grain boundaries and defect centers due to the small grain sizes and high-level defects. For GOP, α c varies from (1.52xa0±xa00.05)xa0×xa010-7 m2 s-1 at 295 K to (2.28xa0±xa00.08)xa0×xa010-7 m2 s-1 at 12.5 K. The cross-plane thermal transport of GOP is attributed to the high density of functional groups between carbon layers which provide weak thermal transport tunnels across the layers in the absence of direct energy coupling among layers. This work sheds light on the understanding and optimizing of nanostructure of graphene-based paper-like materials for desired thermal performance.

Special issue on ?Heat Transfer Analysis in Processes of Developing and Applying Renewable Energies and Novel Materials?

Developing and applying renewable energies have moved to the forefront of scientific studies and industrial applications in recent years due to the rapid depletion of fossil fuel reserve. Likewise, in the last two decades, developing and applying novel materials have been pronounced as a core national strategy by many countries for both civil and military purposes. Heat transfer, a fundamental transport phenomenon, can be found in all processes of developing and applying renewable energies and novel materials. Though heat transfer problems in processes of developing and applying renewable energies and novel materials share some commons to those related to traditional energies and materials, the particularities of renewable energies and novel materials can bring several unique issues on heat transfer. For example, the heat transfer mechanisms of solar energy conversion through photovoltaic materials are very different from those in processes of developing and applying traditional energies and materials. To highlight the up-to-date progress on heat transfer studies related to processes of developing and applying renewable energies and novel materials, a special issue is launched to serve as a platform for researchers in these fields to report their recent activities. After the strict peer-review process, 15 research articles, ranging from nanoscale to macroscale and from theoretical to numerical, have been collected to discuss the fundamental mechanisms and practical applications of heat transfer in processes of developing and applying renewable energies and novel materials. The topics of the published papers include computational fluid dynamics simulation of heat transfer in micromixer and solar cavity receiver, experimental design and measurement of thermal properties for photovoltaic systems and two-dimensional materials, and theoretical analysis of heat transfer in novel nanofluids, etc. These topics, to some extent, represent the frontiers of heat transfer analysis in processes of developing and applying renewable energies and novel materials. From these papers, we can gain an in-depth insight of the state-of-the-art status on heat transfer analysis in processes of developing and applying renewable energies and novel materials. Finally, we would like to use this opportunity to sincerely appreciate the editorial effort of Editor-in-Chief, Professor S. A. Sherif who made this special issue seamlessly published. Professional comments and suggestions from reviewers to significantly improve the quality of each published manuscript are also acknowledged, which are indispensable to the success of this special issue. Although this special issue is unable to cover all the aspects on heat transfer studies in processes of developing and applying renewable energies and novel materials, we hope that it can provide some inspirations to readers for their future research efforts.