William Evans

Effect of aggregation on thermal conduction in colloidal nanofluids

Using effective medium theory the authors demonstrate that the thermal conductivity of nanofluids can be significantly enhanced by the aggregation of nanoparticles into clusters. Predictions of the effective medium theory are in excellent agreement with detailed numerical calculation on model nanofluids involving fractal clusters and show the importance of cluster morphology on thermal conductivity enhancements.

Role of Brownian motion hydrodynamics on nanofluid thermal conductivity

We use a simple kinetic theory based analysis of heat flow in fluid suspensions of solid nanoparticles (nanofluids) to demonstrate that the hydrodynamics effects associated with Brownian motion have a minor effect on the thermal conductivity of the nanofluid. Our conjecture is supported by the results of molecular dynamics simulations of heat flow in a model nanofluid with well-dispersed particles. Our findings are consistent with the predictions of the effective medium theory as well as with recent experimental results on well dispersed metal nanoparticle suspensions.

Thermal conductivity of graphene ribbons from equilibrium molecular dynamics: Effect of ribbon width, edge roughness, and hydrogen termination

We use equilibrium molecular dynamic simulations to compute thermal conductivity of graphene nanoribbons with smooth and rough edges. We also study effects of hydrogen termination. We find that conductivity is the highest for smooth edges and is essentially the same for zigzag and armchair edges. In the case of rough edges, the thermal conductivity is a strong function of the ribbon width indicating the important effect of phonon scattering from the edge. Hydrogen termination also reduces conductivity by a significant amount.

Inter-tube thermal conductance in carbon nanotubes arrays and bundles: Effects of contact area and pressure

We use molecular dynamics simulations to compute junction thermal conductance of carbon nanotubes as a function of crossing angle and pressure, and conductivity of arrays and bundles consisting of multiple junctions as a function of pressure. Two types of arrays are investigated: crossbar structures consisting of alternating orthogonal layers of nanotubes and close-packed bundles of parallel oriented tubes. Conductance of 90° junction increases with pressure 4 fold before saturation; cross-plane thermal conductivity of crossbar structures increases by a factor of 2. For parallel junctions pressure doubles the conductance while thermal conductivity of nanotubes bundles is more or less pressure independent.

Thermal Conductivity of Ordered Molecular Water

The authors use molecular dynamics simulation to investigate the thermal transport characteristics of water with various degree of orientational and translational orders induced by the application of an electric field. The authors observe that the orientational ordering of the water dipole moments has a minor effect on the thermal conductivity. However, electric-field-induced crystallization and associated translational order result in approximately a three fold increase of thermal conductivity with respect to the base water, i.e., to values comparable with those characterizing ice crystal structures.

Improvement of heat transfer efficiency at solid-gas interfaces by self-assembled monolayers

Using molecular dynamics simulations, we demonstrate that the efficiency of heat exchange between a solid and a gas can be maximized by functionalizing solid surface with organic self-assembled monolayers (SAMs). We observe that for bare metal surfaces, the thermal accommodation coefficient (TAC) strongly depends on the solid-gas interaction strength. For metal surfaces modified with organic SAMs, the TAC is close to its theoretical maximum and is essentially independent from the SAM-gas interaction strength. The analysis of the simulation results indicates that softer and lighter SAMs, compared to the bare metal surfaces, are responsible for the greatly enhanced TAC.

Thermal conductivity of carbon nanotube cross-bar structures

We use non-equilibrium molecular dynamics (NEMD) to compute the thermal conductivity (κ) of orthogonally ordered cross-bar structures of single-walled carbon nanotubes. Such structures exhibit extremely low thermal conductivity in the range of 0.02-0.07 W m(-1) K(-1). These values are five orders of magnitude smaller than the axial thermal conductivity of individual carbon nanotubes, and are comparable to the thermal conductivity of still air.

Thermal Transport in Self-Assembled Conductive Networks for Thermal Interface Materials

We present a concept for development of high thermal conductivity thermal interface materials (TIMs) via a rapid formation of conductive network. In particular we use molecular dynamics simulations to demonstrate the possibility of a formation of a network of solid nanoparticles in liquid solution and establish wetting and volume fraction conditions required for a rapid formation of such network. Then, we use Monte-Carlo simulations to determine effective thermal conductivity of the solid/liquid composite material. The presence of a percolating network dramatically increases the effective thermal conductivity, as compared to values characterizing dispersed particle structures.