Thermal Conductivity Of Graphene Research Articles

Experimental Measurements of Viscosity and Thermal Conductivity of Single Layer Graphene Based DI-water Nanofluid

Experimental measurements of viscosity and thermal conductivity of single layer of graphene . based DI-water nanofluid are performed as a function of concentrations (0.1-1wt%) and temperatures between (5 to 35ºC). The result reveals that the thermal conductivity of GNPs nanofluids was increased with increasing the nanoparticle weight fraction concentration and temperature, while the maximum enhancement was about 22% for concentration of 1 wt.% at35ºC. These experimental results were compared with some theoretical models and a good agreement between Nan’s model and the experimental results was observed. The viscosity of the graphene nanofluid displays Newtonian and Non-Newtonian behaviors with respect to nanoparticles concentration and temperature, and about 111% enhancement was obtained compared to the base fluid with GNPs weight fraction concentration of 1wt.% at 35ºC. Based onthe experimental data, correlations were developed for predicting thermophysical properties of the GNPs based DI-water nanofluid.

Lattice thermal conductivity of δ-graphyne — A molecular dynamics study

Thermal conductivity of δ-graphyne was investigated using reverse non-equilibrium molecular dynamics simulations. The dependence of the thermal conductivities with the temperature, acetylenic linkages, and external strain were explained by the phonon density of states. Our simulations revealed that as the temperature increased, the thermal conductivity of graphene first increased and then decreased, whereas that of δ-graphyne monotonically decreased. Owing to the presence of the acetylenic linkages, a significant reduction was found in the thermal conductivity of δ-graphyne, which resulted in a phonon vibration mismatch or weakened coupling. Moreover, the temperature profile changed from mono linear to the ladder the number of acetylenic linkages increased. These results play a guidance role in the design and application of thermoelectrics devices using 2D carbon materials.

Ultra high stiffness and thermal conductivity of graphene like C3N

Recently, single crystalline carbon nitride 2D material with a C3N stoichiometry has been synthesized. In this investigation, we explored the mechanical response and thermal transport along pristine, free-standing and single-layer C3N. To this aim, we conducted extensive first-principles density functional theory (DFT) calculations as well as molecular dynamics (MD) simulations. DFT results reveal that C3N nanofilms can yield remarkably high elastic modulus of 341 GPa nm and tensile strength of 35 GPa nm, very close to those of defect-free graphene. Classical MD simulations performed at a low temperature, predict accurately the elastic modulus of 2D C3N with less than 3% difference with the first-principles estimation. The deformation process of C3N nanosheets was studied both by the DFT and MD simulations. Ab initio molecular dynamics simulations show that single-layer C3N can withstand high temperatures like 4000 K. Remarkably, the phononic thermal conductivity of free-standing C3N was predicted to be as high as 815 ± 20 W/mK. Our atomistic modelling results reveal ultra high stiffness and thermal conductivity of C3N nanomembranes and therefore propose them as promising candidates for new application such as the thermal management in nanoelectronics or simultaneously reinforcing the thermal and mechanical properties of polymeric materials.

In-Plane Thermal Conductivity of Polycrystalline Chemical Vapor Deposition Graphene with Controlled Grain Sizes.

Manipulation of the chemical vapor deposition graphene synthesis conditions, such as operating P, T, heating/cooling time intervals, and precursor gas concentration ratios (CH4/H2), allowed for synthesis of polycrystalline single-layered graphene with controlled grain sizes. The graphene samples were then suspended on 8 μm diameter patterned holes on a silicon-nitride (Si3N4) substrate, and the in-plane thermal conductivities k(T) for 320 K < T < 510 K were measured to be 2660-1230, 1890-1020, and 680-340 W/m·K for average grain sizes of 4.1, 2.2, and 0.5 μm, respectively, using an opto-thermal Raman technique. Fitting of these data by a simple linear chain model of polycrystalline thermal transport determined k = 5500-1980 W/m·K for single-crystal graphene for the same temperature range above; thus, significant reduction of k was achieved when the grain size was decreased from infinite down to 0.5 μm. Furthermore, detailed elaborations were performed to assess the measurement reliability of k by addressing the hole-edge boundary condition, and the air-convection/radiation losses from the graphene surface.

A molecular dynamics study of stiffness-dependent thermal conductivity of graphene

Non-equilibrium molecular dynamics (NEMD) simulations is used to investigate the effect of support stiffness on thermal conductivity property based on the ‘graphene-springs’ model. It shows that the support stiffness greatly influences the thermal conductivity. Thermal conductivity of graphene decreases nonlinearly as the support stiffness increases. The temperature stability of thermal conductivity property of graphene can be improved by the support stiffness. For patterned stiffness supported stripe, thermal conductivity is significantly dependent on the patterned area, angle and stripe distribution. The work in this paper reveals the possibility for designing and controlling of thermal conduction of graphene by using support stiffness, which is beneficial to the application of graphene in nanoscale devices.

Thermal transport in graphene with defect and doping: Phonon modes analysis

The effects of defect and isotopic doping with different ratios on the thermal conductivity of graphene are investigated by using non-equilibrium molecular dynamics simulations and normal mode analysis method. In contrast to the persisted size dependent thermal conductivity in the pristine graphene, thermal conductivity of defected graphene quickly saturates when the size is greater than 100 nm. Similar to the pristine graphene, we find the thermal conductivity of defected and doped graphene follows ∼T−α behavior, and the power exponent α is sensitive to the defect and doping ratio. The spectral phonon relaxation time and normalized accumulation thermal conductivity with respect to the phonon mean free path (MFP) reveal that the long-MFP phonon modes are strongly suppressed in the defected and doped graphene, resulting in the suppressed size dependence and the weaker temperature dependence of the thermal conductivity compared to the pristine graphene. The phonon modal analysis in our work establishes a deep understanding of the defect and doping effects on the thermal transport in graphene, which would provide effective guidance to the graphene-based phonon engineering applications.

On the domain size effect of thermal conductivities from equilibrium and nonequilibrium molecular dynamics simulations

Equilibrium molecular dynamics (EMD) simulations with the Green-Kubo formula and nonequilibrium molecular dynamics (NEMD) simulations with the Fourier's Law are two widely used methods for calculating thermal conductivities of materials. It is well known that both methods suffer from domain size effects, especially for NEMD. But the underlying mechanisms and their comparison have not been much quantitatively studied before. In this paper, we investigate their domain size effects by using crystalline silicon at 1000 K, graphene at 300 K, and silicene at 300 K as model material systems. The thermal conductivity of silicon from EMD simulations increases normally with the increasing domain size and converges at a size of around 4×4×4 nm3. The converging trend agrees well with the wavelength-accumulated thermal conductivity. The thermal conductivities of graphene and silicene from EMD simulations decrease abnormally with the increasing domain size and converge at a size of around 10×10 nm2. We ascribe the anomalous size effect to the fact that as the domain size increases, the effect of more phonon scattering processes (particularly the flexural phonons) dominates over the effect of more phonon modes contributing to the thermal conductivity. The thermal conductivities of the three material systems from NEMD simulations all show normal domain size effects, although their dependences on the domain size differ. The converging trends agree with the mean free path accumulation of thermal conductivity. This study provides new insights that other than some exceptions, the domain size effects of EMD and NEMD are generally associated with wavelength and mean free path accumulations of thermal conductivity, respectively. Since phonon wavelength spans over a much narrower range than mean free path, EMD usually has less significant domain size effect than NEMD.

Substrate-induced reduction of graphene thermal conductivity

We develop the theory of heat conductivity in supported graphene, accounting for coherent phonon scattering on disorder induced by an amorphous substrate. We derive spectra for in-plane and out-of-plane phonons in the framework of Green's function approach. The energetic parameters of the theory are obtained using the molecular dynamics simulations for graphene on SiO$_2$ substrate. The heat conductivity is calculated by the Boltzmann transport equation. We find that the interaction with the substrate drastically reduces the phonon lifetime and completely suppresses the contribution of ZA phonons to the heat conductivity. As a result, the total heat conductivity is reduced by several times, which matches with the tendency observed for available experimental data. The considered effect is important for managing thermal properties of the graphene-based electronic devices.

Tailor Made Conductivities of Polymer Matrix for Thermal Management: Design and Development of Three-Dimensional Carbonaceous Nanostructures

The thermal conductivity of graphene is limited in the transverse direction due to the exfoliation of the interplanar distance. In this study, the multiwalled carbon nanotubes (MWCNTs) with high thermal conductivities were immobilized into the exfoliated few layers graphene with an objective to enhance the transverse thermal conductivity of MWCNT embedded graphene three-dimensional architectures. Reconstructing the topographically controlled synthetic hybrid nanostructure assemblies in a polymer matrix provides the opportunity to gain insight into the electrical and thermal conduction mechanism. The thickness of the resin layer between the nanohybrid fillers was found to be critical for the phonon transport through the coating whereas this epoxy insulating layer prevents tunneling of the electron. By controlled functionalization and site specific orientations of hybrid nanoscale morphologies in the fiber reinforced composite, the higher mechanical strength could be achieved with increased electrical but r...

Tunable thermal conductivities of graphene and graphyne under in-plane torsion

Thermal conductivities of graphene and graphyne are revealed to be tunable under in-plane circular rotations caused by the evolution of surface morphology using non-equilibrium molecular dynamics method.

Measurement of specific heat and thermal conductivity of supported and suspended graphene by a comprehensive Raman optothermal method.

The last decade has seen the rapid growth of research on two-dimensional (2D) materials, represented by graphene, but research on their thermophysical properties is still far from sufficient owing to the experimental challenges. Herein, we report the first measurement of the specific heat of multilayer and monolayer graphene in both supported and suspended geometries. Their thermal conductivities were also simultaneously measured using a comprehensive Raman optothermal method without needing to know the laser absorption. Both continuous-wave (CW) and pulsed lasers were used to heat the samples, based on consideration of the variable laser spot radius and pulse duration as well as the heat conduction within the substrate. The error from the laser absorption was eliminated by comparing the Raman-measured temperature rises for different spot radii and pulse durations. The thermal conductivity and specific heat were extracted by analytically fitting the temperature rise ratios as a function of spot size and pulse duration, respectively. The measured specific heat was about 700 J (kg K)-1 at room temperature, which is in accordance with theoretical predictions, and the measured thermal conductivities were in the range of 0.84-1.5 × 103 W (m K)-1. The measurement method demonstrated here can be used to investigate in situ and comprehensively the thermophysical properties of many other emerging 2D materials.

Dependence of Rate of Heat Flow in Graphene on Number of Layers

Graphene attracts particular interests recently due to its extraordinary properties. Jang et al. [1] have studied thickness dependent thermal conductivity in graphene. They observed that thermal conductivity of graphene increases with increase in number of layers. We find that heat current flowing through graphene decreases with increase in number of layers.

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.

Phonon thermal properties of graphene from molecular dynamics using different potentials.

Phonon thermal transport in graphene has attracted significant interest in recent years. Phonon thermal properties of graphene are investigated by molecular dynamics simulations using the Tersoff, Tersoff-2010, REBO, and AIREBO potentials. By calculating the phonon properties and thermal conductivity of graphene, the performance of the potentials is evaluated based on comparisons with experimental data. It shows that the Tersoff-2010 and REBO display better dispersion curves for graphene than the original Tersoff and AIREBO. The Tersoff-2010 correctly provides the Γ point phonon velocities of the LA and TA branches as well as the G peak frequency with a value of 46 THz. In addition, the acoustic phonon relaxation time derived from the Tersoff-2010 satisfies the ideal relation "τ-1 ∝ ν2." It is also found that the Tersoff-2010 provides the highest graphene thermal conductivity among the used potentials, and estimates about 30.0% contribution for flexural phonons to the total thermal conductivity. By comparison, the Tersoff-2010 potential is demonstrated to be the most suitable one to describe the phonon thermal properties of graphene.

Width dependent intrinsic thermal conductivity of suspended monolayer graphene

Size dependence is one of the most important unique features of thermal conductivity in two-dimensional materials. Suspended single-layer graphene (SLG) provides a perfect platform for studying the size dependent phonon transport. Here we report measurement and theoretical analysis of heat conduction in suspended SLG as a function of width and temperature. The thermal conductivity of graphene was larger for wider SLG. This width effect was smaller at higher temperatures. In suspended SLG, the long wave-length phonons tend to be more scattered at the lateral boundaries of narrow SLG ribbon, in which the mean free path of phonons is close to the sample width. This behavior can be understood as a mode selectivity of phonon-boundary scattering for suspended SLG. The result revealed the unique width dependence of thermal conductivity in suspended SLG and provided useful guidelines for the future SLG-based thermal applications.

Analysis of the anomalies in graphene thermal properties

A comprehensive analysis of the thermal conductivity of graphene under various conditions is presented in this study. Results obtained from different experimental and theoretical methods are analyzed and discussed for numerous conditions such as preparation process, shape, sample size, wavelength, and temperature. Wide discrepancies in the measured thermal conductivity results were found in many studies in the literature. Based on the cited data for the graphene thermal conductivity, the initially measured thermal conductivities appear to be highly overestimated. Majority of the documented results reported lower values of thermal conductivity than the earlier reported results. Furthermore, large differences in the values of the thermal conductivity of graphene were noticed from the cited results using different experimental and numerical methods (0.14W/mK–20,000W/mK). This raised an important question on the accuracy of these methods when measuring thermal conductivity of graphene at nanoscale. We have established the existence of a high degree of anomalies in the value of the thermal conductivity of grapheme. Therefore, proper experimental and theoretical studies should be conducted to accurately measure the thermal conductivity of graphene.

Phonons, Localization, and Thermal Conductivity of Diamond Nanothreads and Amorphous Graphene.

Recently, the domains of low-dimensional (low-D) materials and disordered materials have been brought together by the demonstration of several new low-D, disordered systems. The thermal transport properties of these systems are not well-understood. Using amorphous graphene and glassy diamond nanothreads as prototype systems, we establish how structural disorder affects vibrational energy transport in low-dimensional, but disordered, materials. Modal localization analysis, molecular dynamics simulations, and a generalized model together demonstrate that the thermal transport properties of these materials exhibit both similarities and differences from disordered 3D materials. In analogy with 3D, the low-D disordered systems exhibit both propagating and diffusive vibrational modes. In contrast to 3D, however, the diffuson contribution to thermal transport in these low-D systems is shown to be negligible, which may be a result of inherent differences in the nature of random walks in lower dimensions. Despite the lack of diffusons, the suppression of thermal conductivity due to disorder in low-D systems is shown to be mild or comparable to 3D. The mild suppression originates from the presence of low-frequency vibrational modes that maintain a well-defined polarization and help preserve the thermal conductivity in the presence of disorder.

Thermal conductivity of graphene mediated by strain and size

Based on first-principles calculations and full iterative solution of the linearized Boltzmann–Peierls transport equation for phonons, we systematically investigate effects of strain, size and temperature on the thermal conductivity k of suspended graphene. The calculated size-dependent and temperature-dependent k for finite samples agree well with experimental data. The results show that, contrast to the convergent room-temperature k=5450W/m-K of unstrained graphene at a sample size ∼8cm, k of strained graphene diverges with increasing the sample size even at high temperature. Out-of-plane acoustic phonons are responsible for the significant size effect in unstrained and strained graphene due to their ultralong mean free path and acoustic phonons with wavelength smaller than 10nm contribute 80% to the intrinsic room temperature k of unstrained graphene. Tensile strain hardens the flexural modes and increases their lifetimes, causing interesting dependence of k on sample size and strain due to the competition between boundary scattering and intrinsic phonon–phonon scattering. k of graphene can be tuned within a large range by strain for the size larger than 500μm. These findings shed light on the nature of thermal transport in two-dimensional materials and may guide predicting and engineering k of graphene by varying strain and size.

Hydrogenation of Penta-Graphene Leads to Unexpected Large Improvement in Thermal Conductivity.

Penta-graphene (PG) has been identified as a novel two-dimensional (2D) material with an intrinsic bandgap, which makes it especially promising for electronics applications. In this work, we use first-principles lattice dynamics and iterative solution of the phonon Boltzmann transport equation (BTE) to determine the thermal conductivity of PG and its more stable derivative, hydrogenated penta-graphene (HPG). As a comparison, we also studied the effect of hydrogenation on graphene thermal conductivity. In contrast to hydrogenation of graphene, which leads to a dramatic decrease in thermal conductivity, HPG shows a notable increase in thermal conductivity, which is much higher than that of PG. Considering the necessity of using the same thickness when comparing thermal conductivity values of different 2D materials, hydrogenation leads to a 63% reduction in thermal conductivity for graphene, while it results in a 76% increase for PG. The high thermal conductivity of HPG makes it more thermally conductive than most other semiconducting 2D materials, such as the transition metal chalcogenides. Our detailed analyses show that the primary reason for the counterintuitive hydrogenation-induced thermal conductivity enhancement is the weaker bond anharmonicity in HPG than PG. This leads to weaker phonon scattering after hydrogenation, despite the increase in the phonon scattering phase space. The high thermal conductivity of HPG may inspire intensive research around HPG and other derivatives of PG as potential materials for future nanoelectronic devices. The fundamental physics understood from this study may open up a new strategy to engineer thermal transport properties of other 2D materials by controlling bond anharmonicity via functionalization.

On the quantum magnetic oscillations of electrical and thermal conductivities of graphene

Abstract Oscillating thermodynamic quantities of diamagnetic materials, specially graphene, have been attracting attention of the scientific community due to the possibility to experimentally map the Fermi surface of the material. These have been the case of the de Haas–van Alphen and Shubnikov–de Haas effects, found on the magnetization and electrical conductivity, respectively. In this direction, managing the thermodynamic oscillations is of practical purpose, since from the reconstructed Fermi surface it is possible to access, for instance, the electronic density. The present work theoretically explores the quantum oscillations of electrical and thermal conductivities of a monolayer graphene under a crossed magnetic and electric fields. We found that the longitudinal electric field can increase the amplitude of the oscillations and this result is of practical and broad interest for both, experimental and device physics.