Thermal Transport Properties in Graphene

Experimental and theoretical studies show that graphene has unusually high thermal conductivity in the range from 3000 to 5500 W/mK at room temperature, making it a promising material for application in electronic devices. It has been demonstrated that defects or external strains may lead to significant effects on graphene thermal transport properties. Therefore, in this work, we systematically investigated the thermal transport properties of graphene sheets from both atomistic simulation and continuum level modeling perspectives. We investigated the effects of external shear strains on both pristine (zigzag- and armchair-edge) and defective (with GBs) graphene sheets, see Fig. a. From Fig. a, it is clear that introducing GBs and shear strains can effectively reduce the graphene thermal conductivities. This work has been published in the journal Computational Materials Science (DOI:10.1016/j.commatsci.2012.12.037).

We also developed a simple yet general semi-classical model for calculating the lattice specific heat, thermal conductance, and thermal conductivity of 2D nanomaterials. In this model, we took both the long-wavelength 1D and 2D guided waves, and the short-wavelength 3D-like bulk waves into accounts, thereby evaluating contributions from three normal modes of guided and bulk waves over a wide temperature range. As an illustration, we computed the specific heat (left panel of Fig. b), thermal conductance (middle panel of Fig. b), and thermal conductivity (right panel of Fig. b) of graphene sheets. The graphene thermal conductivities from our models across both nanoribbon lengths and temperatures are in good agreements with both experimental and large-scale MD simulation results (see the right panel of Fig. b), and our model revealed the transition temperature of thermal transport from ballistic to diffusive regimes. Note that the parameters required can be readily extracted from molecular dynamics (MD) simulations or from experimental measurements, and therefore, can be easily extended to other 2D materials such as boron nitride (BN) and molybedum disulfide ([IMAGE png]). This work has been published in Applied Physics Letters (DOI:10.1063/1.4826693).

Figure: (a) the thermal conductivities of pristine graphene sheets, and graphene sheets with GBs under shear strains; note that the thermal conductivities of graphene GBs were computed by considering heat fluxes across the GBs; all thermal conductivities are normalized by the thermal conductivity of the pristine zigzag graphene under zero strain (123.5 W/mK). (b) left panel: theoretical prediction of specific heat for graphene sheet, as a function of temperature; the red and green lines are the specific heat contributions from guided and bulk waves, respectively. The blue line represents the total specific heat, which is calculated by [IMAGE png]. Middle panel: thermal conductance of graphene sheet as a function of temperature; the blue line represents the total thermal conductance [IMAGE png] with contributions from the guided waves [IMAGE png] (red line) and bulk waves [IMAGE png] (green line); contributions from each polarization are displayed in dashed lines. Right panel: thermal conductivity of graphene with different temperatures and sheet sizes; the colored lines represent the thermal conductivities of graphene of sizes from 1 to 10 [IMAGE png]. The experimental data are labeled as rectangular symbols with different temperatures, and the thermal conductivity of a [IMAGE png] [IMAGE png] [IMAGE png] graphene sheet from 300 to 500 K labeled by red circles using the non-equilibrium molecular dynamics (NEMD) simulation.