Sergei Shenogin

Role of thermal boundary resistance on the heat flow in carbon-nanotube composites

We use classical molecular dynamics simulations to study the interfacial resistance for heat flow between a carbon nanotube and octane liquid. We find a large value of the interfacial resistance associated with weak coupling between the rigid tube structure and the soft organic liquid. Our simulation demonstrates the key role played by the soft vibration modes in the mechanism of the heat flow. These results imply that the thermal conductivity of carbon-nanotube polymer composites and organic suspensions will be limited by the interface thermal resistance and are consistent with recent experiments.

Effect of chemical functionalization on thermal transport of carbon nanotube composites

We use molecular dynamics simulations to analyze the role of chemical bonding between the matrix and the fiber on thermal transport in carbon nanotube organic matrix composites. We find that chemical bonding significantly reduces tube-matrix thermal boundary resistance, but at the same time decreases intrinsic tube conductivity. Estimates based on the effective medium theory predict increase, by about a factor of two, of the composite conductivity due to functionalization of single-walled nanotubes with aspect ratios within 100–1000 range. Interestingly, at high degree of chemical functionalization, intrinsic tube conductivity becomes independent of the bond density.

On the lack of thermal percolation in carbon nanotube composites

Recent experiments demonstrated very low percolation thresholds for carbon nanotube composites signified by steep increases in electrical conductivity at very low nanotube loadings. By contrast, thermal transport measurements, even on the same samples, showed no signature of the percolation threshold. These contrasting behaviors are particularly intriguing considering that both transport processes are described by the same continuum equation. In this letter we present a theoretical analysis based on finite element calculations that expose the underlying reasons for markedly different behaviors of electrical and thermal transport in high aspect ratio fiber composites.

Bonding-induced thermal conductance enhancement at inorganic heterointerfaces using nanomolecular monolayers

Manipulating interfacial thermal transport is important for many technologies including nanoelectronics, solid-state lighting, energy generation and nanocomposites. Here, we demonstrate the use of a strongly bonding organic nanomolecular monolayer (NML) at model metal/dielectric interfaces to obtain up to a fourfold increase in the interfacial thermal conductance, to values as high as 430 MW m(-2) K(-1) in the copper-silica system. We also show that the approach of using an NML can be implemented to tune the interfacial thermal conductance in other materials systems. Molecular dynamics simulations indicate that the remarkable enhancement we observe is due to strong NML-dielectric and NML-metal bonds that facilitate efficient heat transfer through the NML. Our results underscore the importance of interfacial bond strength as a means to describe and control interfacial thermal transport in a variety of materials systems.

Molecular dynamics simulation of interfacial thermal conductance between silicon and amorphous polyethylene

Using nonequilibrium molecular dynamics simulations, we study the interfacial thermal conductance between single crystal silicon and amorphous polyethylene (PE). We estimate that the silicon-PE interfacial thermal conductance is about 20MW∕m2K at room temperature, which is equivalent to the resistance of 16nm thick layer of bulk amorphous polyethylene. We also study the role of solid stiffness and the bonding strength across the interface on the interfacial thermal resistance. With strong interfacial bonding, our results are in agreement with the diffuse mismatch model and phonon radiation limit predictions, suggesting that in this case, heat carrying acoustic phonons in solids have transmission coefficients to polymer equal almost to unity.

Predicting the thermal conductivity of inorganic and polymeric glasses: The role of anharmonicity

The thermal conductivity of several amorphous solids is numerically evaluated within the harmonic approximation from Kubo linear-response theory following the formalism developed by Allen and Feldman [Phys. Rev. B 48, 12581 (1993)]. The predictions are compared to the results of molecular dynamics (MD) simulations with realistic anharmonic potentials and to experimental measurements. The harmonic theory accurately predicts the thermal conductivity of amorphous silicon, a model Lennard-Jones glass, and a bead-spring Lennard-Jones glass. For polystyrene and amorphous silica at room temperature, however, the harmonic theory underestimates the thermal conductivity by a factor of about 2. This result can be explained by the existence of additional thermal transport via anharmonic energy transfer. More surprisingly, the thermal conductivity of polystyrene and amorphous silica at low temperature (MD and experimental) are significantly below the predictions of the harmonic theory. Potential reasons for the failur...

Thermal energy exchange between carbon nanotube and air

Using molecular dynamics simulations the authors impose a heat flux between single-walled carbon nanotubes and air to study thermal interfacial conductance. They estimate that the nanotube-air interfacial thermal conductance is about 0.1MW∕m2K at room temperature and atmospheric pressure. The associated interfacial thermal resistance is equivalent to the resistance of 250nm thick layer of air. They also show that the interfacial resistance is a strong function of the interaction parameters between air atoms and carbon nanotubes.

Limited thermal conductance of metal-carbon interfaces

The thermal conductance for a series of metal-graphite interfaces has been experimentally measured with time-domain thermoreflectance (TDTR). For metals with Debye temperatures up to ∼400 K, a linear relationship exists with the thermal conductance values. For metals with Debye temperatures in excess of ∼400 K, the measured metal-graphite thermal conductance values remain constant near 60 MW m−2 K−1. Titanium showed slightly higher conductance than aluminum, despite the closeness of atomic mass and Debye temperature for the two metals. Surface analysis was used to identify the presence of titanium carbide at the interface in contrast to the aluminum and gold-carbon interfaces (with no detectable carbide phases). It was also observed that air-cleaved graphite surfaces in contact with metals yielded slightly higher thermal conductance than graphite surfaces cleaved in vacuo. Examination of samples with scanning electron microscopy revealed that the lack of absorbed molecules on the graphite surface resulted...

Thermal relaxation mechanism and role of chemical functionalization in fullerene solutions

Using molecular-dynamics simulations we investigate thermal relaxation of C60 and C84 molecules suspended in octane liquid. Pristine fullerenes exhibit relatively slow relaxation due to weak thermal coupling with the liquid. A comparison of the interfacial transport characteristics obtained from relaxation simulations with those obtained from equilibrium simulations and fluctuation-dissipation theorem analysis demonstrates that the relaxation process involves two main steps: (i) energy flow from high- to low-frequency modes within the fullerene, and (ii) energy flow from low-frequency fullerene modes to the liquid. Functionalization of fullerenes with alkene chains leads to significant reduction of the thermal relaxation time. The relaxation time of functionalized fullerenes becomes independent from the functionalizing chain length beyond approximately 10 carbon segments; this can be understood in terms of thermal conductivity along the chain and heat transfer between the chain and the solvent.

Air flow through carbon nanotube arrays

Using molecular dynamics simulations, we studied the air flow through carbon nanotube arrays. We found that for 1.4nm diameter tubes separated by 25nm, the air flow can be well described by the free molecular flow theory. We estimate that for such array, the pressure gradient is about 0.1atm∕μm at 1atm air pressure and 5m∕s flow velocity, indicating that the flowing air can only pass through an array of no more than about 400 carbon nanotubes in series. Our findings provide design rules for arrays of nanotubes for thermal energy exchange with air.