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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. [ABSTRACT FROM AUTHOR]

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 1000K. 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. [ABSTRACT FROM AUTHOR]

The setting up process in a nanoscale bearing has been modeled by molecular dynamics simulation. Contrary to the prediction from the classical Reynolds’ theory, simulation results show that the load capacity of the nanoscale bearing does not increase monotonically with the operation speed. This is attributed to the change of the local shear rate, which will decrease with the shear velocity of the bearing as the shear velocity exceeds a critical value, i.e., the local shear rate has an upper limit. A simple nonlinear dynamic model indicates that the momentum exchange between the liquid and the solid wall is reduced with the shear velocity when the shear velocity is above a critical value. The weak momentum exchange results in a decrease of the local shear rate, which in turn causes a sharp increase of the slip length. [ABSTRACT FROM AUTHOR]

Thermal expansion and impurity effects on the lattice thermal conductivity of solid argon have been investigated with equilibrium molecular dynamics simulation. Thermal conductivity is simulated over the temperature range of 20–80 K. Thermal expansion effects, which strongly reduce thermal conductivity, are incorporated into the simulations using experimentally measured lattice constants of solid argon at different temperatures. It is found that the experimentally measured deviations from a T[sup -1] high-temperature dependence in thermal conductivity can be quantitatively attributed to thermal expansion effects. Phonon scattering on defects also contributes to the deviations. Comparison of simulation results on argon lattices with vacancy and impurity defects to those predicted from the theoretical models of Klemens and Ashegi et al. demonstrates that phonon scattering on impurities due to lattice strain is stronger than that due to differences in mass between the defect and the surrounding matrix. In addition, the results indicate the utility of molecular dynamics simulation for determining parameters in theoretical impurity scattering models under a wide range of conditions. It is also confirmed from the simulation results that thermal conductivity is not sensitive to the impurity concentration at high temperatures. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We report on a rapid and simple method to fabricate polyethylene (PE) nanofibers by one-step drawing from PE solution. The diameter of the fiber prepared with this method can be as small as 40 nm. The thermal conductivity of the drawn PE nanofiber was measured with suspended microdevices, and the highest value obtained is 8.8Wm–1 K–1, which is very close to that of electrospun PE nanofibers, and over 20 times higher than bulk value. Raman spectra of these drawn PE nanofibers indicate that molecular chains in these fibers can be as well aligned as that in electrospun fibers, which results in the enhanced thermal conductivity of the drawn PE nanofibers. [ABSTRACT FROM AUTHOR]

The effects of the contact area on the interfacial thermal conductanceσ 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 A is positively dependent on the contact area as the area varies from a fewatoms 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. [ABSTRACT FROM AUTHOR]

We report on experimental studies of the average phonon mean free path in the c-axis direction of graphite. Through systematically measuring the cross-plane thermal conductivity of thin graphite flakes with thickness ranging from 24 nm to 714 nm via a differential three omega method, we demonstrate that the average phonon mean free path in the c-axis direction of graphite is around 200 nm at room temperature, much larger than the commonly believed value of just a few nanometers. This study provides direct experimental evidence for the recently projected very long phonon mean free path along the c-axis of graphite. [ABSTRACT FROM AUTHOR]

Phonon transport in the c-axis direction of graphite thin films has been studied using nonequilibrium 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. [ABSTRACT FROM AUTHOR]

We report on experimental measurements of contact thermal resistance between individual carbon nanotubes. Results indicate that the contact thermal conductance can increase by nearly two orders of magnitude (from 10-8 to 10-6 W/K) as the contact area increases from a cross contact to an aligned contact. Normalization with respect to the contact area leads to normalized contact thermal resistance on the order of 10-9 m2 K/W at room temperature, one order of magnitude lower than that from a molecular dynamics simulation in literature. These results should have important implications in the design of carbon nanotube-polymer composites for tunable thermal properties. [ABSTRACT FROM AUTHOR]