Kedong Bi

Mode dependent lattice thermal conductivity of single layer graphene

Molecular dynamics simulation is performed to extract the phonon dispersion and phonon lifetime of single layer graphene. The mode dependent thermal conductivity is calculated from the phonon kinetic theory. The predicted thermal conductivity at room temperature exhibits important quantum effects due to the high Debye temperature of graphene. But the quantum effects are reduced significantly when the simulated temperature is as high as 1000 K. Our calculations show that out-of-plane modes contribute about 41.1% to the total thermal conductivity at room temperature. The relative contribution of out-of-plane modes has a little decrease with the increase of temperature. Contact with substrate can reduce both the total thermal conductivity of graphene and the relative contribution of out-of-plane modes, in agreement with previous experiments and theories. Increasing the coupling strength between graphene and substrate can further reduce the relative contribution of out-of-plane modes. The present investigations also show that the relative contribution of different mode phonons is not sensitive to the grain size of graphene. The obtained phonon relaxation time provides useful insight for understanding the phonon mean free path and the size effects in graphene.

Phonon mean free path of graphite along the c-axis

Phonon transport in the c-axis direction of graphite thin films has been studied using non-equilibrium molecular dynamics (MD) simulation. The simulation results show that the c-axis thermal conductivities for films of thickness ranging from 20 to 500 atomic layers are significantly lower than the bulk value. Based on the MD data, a method is developed to construct the c-axis thermal conductivity as an accumulation function of phonon mean free path (MFP), from which we show that phonons with MFPs from 2 to 2000 nm contribute ∼80% of the graphite c-axis thermal conductivity at room temperature, and phonons with MFPs larger than 100 nm contribute over 40% to the c-axis thermal conductivity. These findings indicate that the commonly believed value of just a few nanometers from the simple kinetic theory drastically underestimates the c-axis phonon MFP of graphite.

Phonon transport properties in pillared silicon film

The phonon transport property of pillared silicon film is systematically investigated by molecular dynamics simulation and lattice dynamics calculation. It is found that the thermal conductivity can be reduced to as low as 28.6% of the conductivity of plain ones. Although the reduced thermal conductivity can be explained qualitatively by increased surface roughness, our calculations show that the pillars modify the phonon dispersion relation and reduce the phonon group velocity due to the local resonance effects. Furthermore, by analyzing the participation ratio spectra, it is shown that the pillars reduce the mode participation ratio over the whole range of frequency. We found that the mode localization around the pillars is another important factor to reduce the thermal conductivity of pillared film. The present investigations indicate that the pillared film may have potential application in thermoelectric energy conversion.

Thermal Conductivity of Single-wall Carbon Nanotubes Filled with Argon

Thermal conductivities of single-wall carbon nanotubes (SWNTs) filled with argon (Ar) with types (10, 10) and (15, 15) respectively, are calculated over a temperature range of 500-1200 K using equilibrium molecular dynamics (EMD) simulation method. The Tersoff potential for C-C bonding interactions and the Lennard-Jones (LJ) potential for Ar-C nonbonding interactions are employed. The effects of filled argon on the thermal conductivity of SWNTs as well as the temperature dependence of their thermal conductivities are investigated. It is found that the thermal conductivities of filled (10, 10) and (15, 15) SWNTs, showing qualitatively similar behavior to that of a bare unfilled nanotube, decrease as the temperature increases. Moreover, it is demonstrated that the thermal conductivity of nanotubes filled with argon is much higher than that of an empty nanotube at all temperatures. This increase is perhaps attributed to the heat transfer derived from the Ar-nanotube interaction, and the mass transport due to the active movement of the Ar atom in SWNTs

Axial tensile strain effects on the contact thermal conductance between cross contacted single-walled carbon nanotubes

The axial strain effects on the contact thermal conductance between two cross contacted single walled carbon nanotubes (SWCNTs) are assessed using nonequilibrium molecular dynamics simulation. The results show that the contact thermal conductance can be decreased by ∼44% as the axial strain increases from 0 to 10%. The calculated vibrational density of state reveals that the enhanced phonon scattering resulting from the blue shift of the low frequency phonon is the main factor leading to the reduction of the contact thermal conductance. We also studied the effect of the defects caused by hydrogenation and vacancy in SWCNTs on the contact thermal conductance and found that this effect can be neglected.

Cross-plane phonon transport properties of molybdenum disulphide

The cross-plane thermal conductivity of a molybdenum disulphide (MoS2) film is calculated from the nonequilibrium molecular dynamics simulation. The results show that, unlike graphite which has a slow convergent speed, the thermal conductivity of MoS2 tends to a convergent value when the film thickness is beyond about 40 nm. We also construct the cross-plane thermal conductivity of bulk MoS2 as an accumulation function of the phonon mean free path (MFP). It is found that phonons with MFPs below 40 nm contribute ~90% of the MoS2 cross-plane thermal conductivity at room temperature. This critical size of the phonon MFP is about two orders of magnitude smaller than that of graphite. Further calculations show that the shorter cross-plane phonon MFPs in bulk MoS2 may result from the lower phonon cut-off frequency and the mismatch of phonon density of state between Mo and S due to the mass difference. The phonon transport properties obtained would be helpful in the design and optimization of MoS2-based devices.

The contact area dependent interfacial thermal conductance

The effects of the contact area on the interfacial thermal conductance σ are investigated using the atomic Green’s function method. Different from the prediction of the heat diffusion transport model, we obtain an interesting result that the interfacial thermal conductance per unit area Λ is positively dependent on the contact area as the area varies from a few atoms to several square nanometers. Through calculating the phonon transmission function, it is uncovered that the phonon transmission per unit area increases with the increased contact area. This is attributed to that each atom has more neighboring atoms in the counterpart of the interface with the increased contact area, which provides more channels for phonon transport.

Molecular Dynamics Simulation of Thermal Conductivity of Single-Walled Carbon Nanotubes Filled With Water

Thermal conductivities for single-walled carbon nanotubes (SWNTs) filled with water are calculated with non-equilibrium molecular dynamics (NEMD) simulation method. Simulation results demonstrate the thermal conduction for the tube filed with water is better than the pure nanotube at the same conditions. It is believed the translational movement of the water molecules along the tube axis helps carry energy from the hot bath to the heat sinks, which results in the increase of the thermal conductivities. In addition, with the introduction of the water molecules into the nanotube, the additional interaction between the carbon atoms and the water molecules provide extra channels for phonon transport, which further intensifies the energy transport along the nanotubes. The effects of the temperature variation and the tube length on the thermal conductivities are also analyzed in this paper.© 2008 ASME

Phonon filtering for reduced thermal conductance in unconventional superlattices

The thermal transport of an unconventional superlattice is investigated by nonequilibrium molecular dynamics simulation. It is shown that the thermal conductance of a two-order superlattice decreases by 15–29% as compared with that of a conventional superlattice. This result is unambiguously explained by the phonon transmission functions of similar one-dimensional superlattice atomic chains calculated by Greens function method. It is demonstrated that the multiscale structure introduces additional phonon bandgaps, leading to the reduction in thermal conductance due to phonon filtering effects. The proposed unconventional superlattice may find potential applications in phonon engineering, such as thermoelectrics and thermal isolation.

First Principles Study of Thermal Conductance Across Cu/Graphene/Cu Nanocomposition and the Effect of Hydrogenation

In this work, the interfacial thermal conductance across Cu/graphene/Cu interfaces is investigated using the density functional theory (DFT) and the nonequilibrium Green’s function (NEGF) method. In order to study how hydrogenation of graphene affects thermal transport behaviors at the interfaces of Cu/graphene/Cu, we also analyze the interfacial thermal conductance across Cu/hydrogenated-graphene/Cu (Cu/H-graphene/Cu) with both double-sided and single-sided hydrogenated graphene. Our results show that, the interfacial thermal conductance across Cu/H-graphene/Cu interfaces is almost twice of the value across Cu/graphene/Cu interfaces. For Cu/H-graphene/Cu with double-sided hydrogenated graphene (Cu/DH-graphene/Cu), the hydrogen atoms between graphene and Cu layers provide additional thermal transport channels. While for Cu/H-graphene/Cu with single-sided hydrogenated graphene (Cu/SH-graphene/Cu), the hydrogen atoms not only provide additional thermal transport channels at the hydrogenated side of graphene, but also reduce the equilibrium separation between graphene and Cu layers at the non-hydrogenated side of graphene due to the transfer of massive electrons, which enhances the interface coupling between graphene and Cu layers. The phonon transmission shows that both double-sided and single-sided hydrogenation of graphene can increase the heat transport across the interface. Our calculation indicates that the interfacial thermal conductance of Cu/graphene/Cu nanocomposition can be improved by hydrogenation.Copyright