Thermal Transport Properties Of Graphene Research Articles

Effect of out-of-plane acoustic phonons on the thermal transport properties of graphene

The lattice thermal conductivity of graphene is evaluated using a microscopic model that takes into account the lattice's discrete nature and the phonon dispersion relation within the Brillouin zone. The Boltzmann transport equation is solved iteratively within the framework of three-phonon interactions without taking into account the four-phonon scattering process. The Umklapp and normal collisions are treated rigorously, thereby avoiding relaxation-time and long-wavelength approximations. The mechanisms of the failures of these approximations in predicting the thermal transport properties are discussed. Evaluation of the thermal conductivity is performed at different temperatures and frequencies and in different crystallite sizes. Reasonably good agreement with the experimental data is obtained. The calculation reveals a critical role of out-of-plane acoustic phonons in determining the thermal conductivity. The out-of-plane acoustic phonons contribute greatly and the longitudinal and transverse acoustic phonons make small contributions over a wide range of temperatures and frequencies. The out-of-plane acoustic phonons dominate the thermal conductivity due to their high density of states and restrictions governing the anharmonic phonon scattering. The selection rule severely restricts the phase space for out-of-plane phonon scattering due to reflection symmetry. The optical phonon contribution cannot be neglected at higher temperatures. Both Umklapp and normal processes must be taken into account in order to predict the phonon transport properties accurately.

Softened sp2−sp3 bonding network leads to strong anharmonicity and weak hydrodynamics in graphene+

Graphene+, a novel carbon monolayer with $s{p}^{2}\text{\ensuremath{-}}s{p}^{3}$ hybridization, was recently reported to exhibit unprecedented out-of-plane half-auxetic behavior and graphenelike Dirac properties [Yu et al., Cell Rep. Phys. Sci. 3, 100790 (2022)]. Herein, based on comprehensive state-of-the-art first-principles calculations, we reveal the significant effect of softened $s{p}^{2}\text{\ensuremath{-}}s{p}^{3}$ bonding on the lattice thermal transport in graphene+. At room temperature, the thermal conductivity (\ensuremath{\kappa}) of graphene+ is obtained as \ensuremath{\sim}170 W/mK, which is much lower than that of graphene (\ensuremath{\sim}3170 W/mK). It is found that the softened $s{p}^{2}\text{\ensuremath{-}}s{p}^{3}$ bonding significantly suppresses the vibrations of acoustic phonons in graphene+, which leads to strong anharmonicity and weak phonon hydrodynamics. Thus, the large reduction in \ensuremath{\kappa} stems from the softened $s{p}^{2}\text{\ensuremath{-}}s{p}^{3}$ bonding network. Our study provides fundamental physical insights into the thermal transport properties of graphene+, which would provide prospective guidance for the promising application in the field of thermal management.

Effects of Different Phonon Scattering Factors on the Heat Transport Properties of Graphene Ribbons.

Understanding the effect of phonon scattering is of primary significance in the study of the thermal transport properties of graphene. While phonon scattering negatively affects the thermal conductivity, the exact effect of microscopic phonon scattering is still poorly understood when full phonon dispersions are taken into account. The heat transport properties of graphene ribbons were investigated theoretically by taking into account different polarization branches with different frequencies in order to understand the physical mechanism of the thermal transport phenomenon at the nanoscale. The effects of grain size, chiral angle, Grüneisen anharmonicity parameter, specularity parameter, and mass-fluctuation-scattering parameter were evaluated, taking into account of the restrictions imposed by boundary, Umklapp, and isotope scattering mechanisms. The contribution from each phonon branch was estimated, and the anisotropic coefficients were determined accordingly. The results indicated that the graphene ribbons are very efficient at conducting heat in all the cases studied. All the acoustical branches contribute significantly to the heat transport properties, and the temperature strongly affects the relative contribution of the phonon branches. The lattice thermal conductivity varies periodically with the chiral angle. The maximum thermal conductivity is achieved in the zigzag direction, and the minimum thermal conductivity is obtained in the armchair direction. The lattice thermal conductivity and anisotropic coefficient depend heavily upon the roughness of the edges and the width of the ribbons. The specularity parameter and mass-fluctuation-scattering parameter significantly affect the lattice thermal conductivity, and the effect arising from isotope scattering is significant in the context of natural isotopic abundance. The dependence of the Grüneisen anharmonicity parameter on phonon branches must be taken into account when making predictions. The results have significant implications for the understanding of the relations between phonon scattering and thermal properties.

Thermal conductivity of defective graphene: an efficient molecular dynamics study based on graphics processing units

The exceptional thermal transport properties of graphene are affected due to the presence of various topological defects, which include single vacancy, double vacancies and Stone–Wales defects. The present article is intended to study on thermal transport properties of defective graphene by comparing the effects of topological defects on the thermal conductivity of graphene. This study developed a program for constructing defective graphene models with customizable defect concentrations and distribution types. The efficient molecular dynamics method based on graphics processing units is applied, which can achieve efficient and accurate calculation of material thermal conductivity. It is revealed that the existence of topological defects has a considerable reduce on the thermal conductivity of graphene, and the declining rate of the value get less with increasing defects concentration. At the same concentration, the weakening effect of SW defects on the thermal conductivity of graphene is evidently less than the other two defects. We also explored the effect of temperature on the thermal conductivity of graphene with different defects. These findings were discussed from the phonon perspective that elucidate the atomic level mechanisms, which provide guidance for thermal management of graphene devices.

Thermal Transport of Graphene Sheets with Fractal Defects.

Graphene combined with fractal structures would probably be a promising candidate design of an antenna for a wireless communication system. However, the thermal transport properties of fractal graphene, which would influence the properties of wireless communication systems, are unclear. In this paper, the thermal transport properties of graphene with a Sierpinski fractal structure were investigated via the reverse non-equilibrium molecular dynamics simulation method. Simulation results indicated that the thermal conductivity of graphene with fractal defects decreased from 157.62 to 19.60 (W m−1 K−1) as the fractal level increased. Furthermore, visual display and statistical results of fractal graphene atomic heat flux revealed that with fractal levels increasing, the real heat flux paths twisted, and the angle distributions of atomic heat flux vectors enlarged from about (−30°, 30°) to about (−45°, 45°). In fact, the fractal structures decreased the real heat flow areas and extended the real heat flux paths, and enhanced the phonon scattering in the defect edges of the fractal graphene. Analyses of fractal graphene thermal transport characters in our work indicated that the heat transfer properties of fractal graphene dropped greatly as fractal levels increased, which would provide effective guidance to the design of antennae based on fractal graphene.

Thermoelectric and thermal transport properties of graphene under strong magnetic field

We investigate theoretically the thermoelectric and thermal transport properties of graphene under strong magnetic field in the presence of short-range scatterers. The numerical results are in good agreement with available experimental data for all thermoelectric quantities except for the Seebeck coefficient near zero chemical potential. This anomaly is attributed to the overestimation of the longitudinal resistivity. Furthermore, we find an anomalous oscillation in the transverse thermal conductivity at the lowest Landau level and double peaks in the longitudinal thermal conductivity at higher Landau levels, which are expected to be observed in future experiments on high mobility graphene samples. An important finding of our work is that the thermoelectric figure of merit ZT can be as high as 2.4 for a magnetic field under which the chemical potential is pinned to the n=1 Landau level. This finding can greatly advance the thermoelectric application of graphene.

On the diversity in the thermal transport properties of graphene: A first-principles-benchmark study testing different exchange-correlation functionals

Due to the extraordinary mechanical, electronic, optical, and thermal transport properties, graphene had been one of the most fascinating and extensively studied two-dimensional materials in recent years. Benefiting from the extremely high thermal conductivity, the thermal transport in graphene has received exceptional attention in both experimental and theoretical studies. In this paper, based on first-principles calculations, we performed systematic study on the thermal transport properties of graphene using 10 different exchange-correlation (XC) functionals combined with ultrasoft or projector augmented wave (PAW) pseudopotentials. The thermal transport properties of graphene show distinct diversity for different XC functionals despite their overall good agreement with experimental measurements. It is found that the thermal conductivity of graphene could range from 1936 to 4376 W m−1 K−1 with different XC functionals employed. The reason for the diversity in the thermal transport properties of graphene is analyzed based on the insight into the mode level phonon behaviors. Our study executed a comprehensive investigation of the thermal transport properties of graphene with different XC functionals employed, which would shed light on future researches related to graphene and other novel materials.

Phonon Thermal Transport Properties of Graphene Periodically Embedded with Four- and Eight-membered Rings: a Molecular Dynamics Study

Phonon Thermal Transport Properties of Graphene Periodically Embedded with Four- and Eight-membered Rings: a Molecular Dynamics Study

Experimental investigation of the thermal transport properties of graphene oxide/Co3O4 hybrid nanofluids

The in-situ growth and chemical co-precipitation method was used for the synthesis of uniform dispersion of Co3O4 nanoparticles on the graphene oxide (GO) nanosheet. The reductions of aqueous cobalt chloride in the presence of GO with sodium borohydrate result in the formation of hybrid GO/Co3O4 nanoparticles. The synthesized GO/Co3O4 nanoparticles were characterized using X-ray power diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, transmission electron microscopy (TEM) and vibrating sample magnetometer (VSM). The hybrid nanofluids were prepared by dispersing synthesized GO/Co3O4 nanoparticles in water, ethylene glycol, and ethylene glycol/water mixtures. The properties such as thermal conductivity and viscosity were estimated experimentally at different volume concentrations and temperatures. The thermal conductivity enhancement of water-based nanofluid is 19.14% and ethylene glycol-based nanofluid is 11.85% at 0.2% volume concentration and at a temperature of 60°C respectively compared to their respective base fluids. Similarly, the viscosity enhancement of water-based nanofluid is 1.70-times and ethylene glycol-based nanofluid is 1.42-times at 0.2% volume concentration and at a temperature of 60°C respectively. The obtained thermal conductivity and viscosity data is compared with the literature values.

Tailoring thermal transport properties of graphene by nitrogen doping

The influence of two different nitrogen doping configurations, graphite-like and pyridinic-like nitrogen doping (denoted as graphite-N and pyridinic-N hereafter, respectively), on the thermal conduction of graphene is carefully studied via non-equilibrium molecular dynamic (NEMD) simulations. The thermal conductivity is more strongly suppressed in the pyridinic-N-doped graphene than that in the graphite-N-doped sample, which can be well understood from the changes in bond strength between nitrogen and carbon atoms, phonon group velocities, phonon density of states, participation ratio, and phonon transmission. Our study indicates that the pyridinic-N doping is an efficient method to tune the thermal conduction in graphene, especially for the situation where low thermal conductivity is requested, e.g., thermoelectric applications and thermal shielding.

Annealing and polycrystallinity effects on the thermal conductivity of supported CVD graphene monolayers.

The thermal transport properties of graphene are strongly influenced by its contact environment and the strength of such interactions can be used to tailor these properties. Here we find that annealing suppresses the basal plane thermal conductivity (κ) of graphene supported on silicon dioxide, due to the increased conformity of graphene to the nanoscale asperities of the substrate after annealing. Intriguingly, increasing the polycrystallinity of graphene, grown by chemical vapor deposition on copper, increases the severity of this suppression after annealing, revealing the role of grain boundaries and associated defects in aiding phonon scattering by the substrate. In highly polycrystalline graphene, the value of κ after annealing is comparable to that after significant fluorination of an identical unannealed sample. Our experiments employ the suspended micro-bridge platform for basal plane thermal conductivity measurements. Using xenon difluoride gas for the final release also enables the investigation of thermal transport in graphene in contact with polymers. We find evidence for weaker phonon scattering in graphene, due to a 10 nm thick polymer layer on top compared to the pre-existing silicon dioxide substrate, which is a promising result for flexible electronics applications of graphene.

Tunable thermal transport properties of graphene by single-vacancy point defect

Graphene, a representative two-dimensional nanomaterial, possess great potential in the field of nanoelectronics. However, the defects which are usually unavoidable in the fabrication of graphene may cause significant thermal effects in real applications. Thus, the effective and precise manipulation of thermal transport is critical for the practical applications of graphene-based electronic devices. In this paper, the effect of single-vacancy point defect on thermal transport properties of graphene was analyzed with non-equilibrium molecular dynamics (NEMD) method. We found that the existence of single-vacancy point defect can reduce the thermal conductivities of graphene. However, different from previous conclusions, the thermal conductivity of graphene depends not only on the defect density, but also on the topological configuration of defect. Our present results revealed that thermal conductivity of graphene with randomly distributed defects decreases monotonously with the increase of defect density. On the contrary, the thermal conductivity of graphene with regularly distributed defects expresses obvious non-monotonicity for the same case. This phenomenon was furthermore explained by performing the analysis of phonon properties. These results indicate that the precise manipulation on the thermal conductivity of nanodevices can be realized by regulating the topological configuration of defect on graphene. These understandings will provide important references for the development of nanoelectronic devices.

Comparison on thermal transport properties of graphene and phosphorene nanoribbons.

We investigate ballistic thermal transport at low temperatures in graphene and phosphorene nanoribbons (PNRS) modulated with a double-cavity quantum structure. A comparative analysis for thermal transport in these two kinds of nanomaterials is made. The results show that the thermal conductance in PNRS is greater than that in graphene nanoribbons (GNRS). The ratio kG/kP (kG is the thermal conductivity in GNRS and kP is the thermal conductivity in PNRS) decreases with lower temperature or for narrower nanoribbons, and increases with higher temperature or for wider nanoribbons. The greater thermal conductance and thermal conductivity in PNRS originate from the lower cutoff frequencies of the acoustic modes.

Networked nanoconstrictions: An effective route to tuning the thermal transport properties of graphene

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.

Substrate effect on thermal transport properties of graphene on SiC(0 0 0 1) surface

Both suspended graphene and graphene on Si-terminated (0001) SiC system with the lattice parameter of 4.92Å (strained) or 5.33Å (unstrained) were simulated using first-principles method to investigate the substrate effect on thermal transport properties of graphene on SiC. The thermal conductance of graphene on substrate is figured out to have a dramatical reduction, with a maximum value of 42% which can only be found on the buffer graphene layer, compared to that of suspended graphene at room temperature. The results will help to accelerate application of graphene in the situation where substrate effect cannot be ignored.

Effects of nanosized constriction on thermal transport properties of graphene.

Thermal transport properties of graphene with nanosized constrictions are investigated using nonequilibrium molecular dynamics simulations. The results show that the nanosized constrictions have a significant influence on the thermal transport properties of graphene. The thermal resistance of the nanosized constrictions is on the order of 107 to 109 K/W at 150 K, which reduces the thermal conductivity by 7.7% to 90.4%. It is also found that the constriction resistance is inversely proportional to the width of the constriction and independent of the heat current. Moreover, we developed an analytical model for the ballistic thermal resistance of the nanosized constrictions in two-dimensional nanosystems. The theoretical prediction agrees well with the simulation results in this paper, which suggests that the thermal transport across the nanosized constrictions in two-dimensional nanosystems is ballistic in nature.PACS65.80.CK; 61.48.Gh; 63.20.kp; 31.15.xv

Thermal transport properties of defective graphene: A molecular dynamics investigation

In this work the thermal transport properties of graphene nanoribbons with randomly distributed vacancy defects are investigated by the reverse non-equilibrium molecular dynamics method. We find that the thermal conductivity of the graphene nanoribbons decreases as the defect coverage increases and is saturated in a high defect ratio range. Further analysis reveals a strong mismatch in the phonon spectrum between the unsaturated carbon atoms in 2-fold coordination around the defects and the saturated carbon atoms in 3-fold coordination, which induces high interfacial thermal resistance in defective graphene and suppresses the thermal conductivity. The defects induce a complicated bonding transform from sp2 to hybrid sp—sp2 network and trigger vibration mode density redistribution, by which the phonon spectrum conversion and strong phonon scattering at defect sites are explained. These results shed new light on the understanding of the thermal transport behavior of graphene-based nanomaterials with new structural configurations and pave the way for future designs of thermal management phononic devices.

Characterizing phonon thermal conduction in polycrystalline graphene

Abstract

Thermal Conductivity Measurement of Graphene Composite

ABSTRACTGraphene composites (GCs) have attracted much attention recently. It is interesting to explore thermal properties of GCs in which graphene filler concentrations are tunable. Here, we use 3ω method to measure the thermal conductivity of GCs synthesized from reduced graphene oxide (RGO) dispersed in polystyrene. To avoid the detrimental effect of lithography processes to GCs, we have developed a novel method employing polyvinyl alcohol and poly(methyl methacrylate) (PMMA) as a holder film to transfer micrometer-sized metal heaters/sensors onto GC surface. Room temperature measurements of the thermal conductivity of GCs are performed. The thermal conductivity is enhanced by ∼ 35 % when adding 5 vol.% of RGO filler concentration. Our measurements will be helpful to probe and understand the thermal transport properties of graphene based composites.

Impact ignition of aluminum-teflon based energetic materials impregnated with nano-structured carbon additives

The inclusion of graphene into composite energetic materials to enhance their performance is a new area of interest. Studies have shown that the addition of graphene significantly enhances the thermal transport properties of an energetic composite, but how graphene influences the composite’s ignition sensitivity has not been studied. The objective of this study is to examine the influence of carbon additives in composite energetic material composed of aluminum and polytetrafluoroethylene (Teflon™) on ignition sensitivity due to low velocity, drop weight impact. Specifically, three forms of carbon additives were investigated and selected based on different physical and structural properties: spherically shaped amorphous nano particles of carbon, cylindrically shaped multi walled carbon nanotubes, and sheet like graphene flakes. Results show an interesting trend: composites consisting of carbon nanotubes are significantly more sensitive to impact ignition and require the lowest ignition energy. In contrast, graphene is least sensitive to ignition exhibiting negligible reduction in ignition energy with low concentrations of graphene additive. While graphene does not significantly sensitize the energetic composite to ignition, graphene does, however, result in greater overall reactivity as observed through images of the reaction. The enhanced thermal transport properties of graphene containing composites may promote greater energy transport once ignited, but those properties do not also increase ignition sensitivity. These results and the understanding of the structural arrangement of particles within a composite as a key parameter affecting impact ignition sensitivity will have an impact on the safe handling and use of composite energetic materials.