Weiyu Chen

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.

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.

Interfacial Thermal Conductance Between Carbon Nanotubes From Nonequilibrium Green’s Function Method

In this paper, the interfacial thermal conductance between two single-wall carbon nanotubes (SWCNTs) is evaluated using the nonequilibrium Green’s function (NEGF) method. The calculation results show that, for offset parallel contact type, interfacial thermal conductance increases almost linearly with the overlap length. This is because the coupling atom number in overlap region is the main contributor to heat flow through interface. With the same overlap length, interfacial thermal conductance of the nested contact type is much higher than that of the offset parallel contact type. By comparing the phonon transmission function between the two contact types, it is found that the nested contact type has much larger transmission function than the offset parallel contact type due to more atoms involving in the interfacial coupling in the overlap region. By adjusting the chirality of SWCNTs in the offset parallel contact type, it is found that the difference of phonon spectrum can reduce interfacial thermal transfer. We also find the transmission function profiles with only different overlap length are quite similar, that is, changing in the overlap length will not change the phonon transmission probability at the interface. Moreover, acoustic phonon is the main contributor to the interfacial thermal conductance and the radical breathing mode is the vital mode of coupling modes for CNT-CNT system. The calculated results in this paper indicate that increasing the coupling atom number between CNTs would increase the heat energy transfer in CNT-based composites.Copyright

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

The Cross-Plane Thermal Conductance of Multi-Layer Graphene Bundles

In this paper, the cross-plane thermal conductance σ of multi-layer graphene nanobundles (MLGNBs) is investigated using the non-equilibrium Green’s function method. For the normal MLGNBs, the σ has a positive dependence on the lateral area S due to more atoms involved in the heat transport in the larger S. However, the thermal conductance per unit area Λ is negative dependent on the S since high-frequency phonons contribute less to Λ with low transmission function and small number while the increased phonon branches are mainly located in the high-frequency range. Interestingly, as the S is larger than several square nanometers, the Λ converges to the macroscopic value, independently on the S. Then the staggered MLGNBs is investigated, the results show that increasing both staggering distance between neighboring graphene layers with each other and the graphene layer number in the central device can modulate the σ in a large scope due to the boundary scattering. Finally, in the MLGNBs junction, we found the variation of heat flux direction has an important effect on the σ while the layer number in the central device has weak effect on the cross-plane thermal transport. Our results help understand the cross-plane thermal transport of MLGNBs and provide a model to investigate the thermal property of layered material nanobundles.Copyright

The Effects of Van Der Waals Bonding Strength on the In-Plane Lattice Thermal Conductivities of Multilayer Thin Films

A non-equilibrium molecular dynamics (NEMD) simulation model is established to investigate the impact of vdW strength on thermal transport along the in-plane direction in argon bi-layer films (BLFs) and silicon BLFs. Simulation results indicate that higher strength leads to a higher in-plane thermal conductivity. However, interface roughness also increases with the continuous increase of the van der Waals (vdW) strength and leads to the reduction of thermal conductivities. The bonding strength does play an important role in manipulating thermal conductivity of multilayer thin films.© 2012 ASME

Intertube Thermal Resistance in Double-Wall Carbon Nanotube

This paper presents the variation of interface thermal resistance between two single-wall carbon nanotubes versus overlap length by using nonequilibrium molecular dynamics method. The simulation model is constructed through pulling the inner tube out a double wall carbon nanotube with a distance. The overlap length between the inner and outer nanotubes is proportional to the contact area, which acts as a variable in controlling the heat transport between the inner and the outer nanotubes. Simulation results show that the intertube thermal conductance increases almost linearly with increasing the overlap length. The rectification effect is undetectable due to the fixed atoms that prevent rotation of the carbon nanotubes and resulting in constant intertube coupling strength.Copyright