Thermal Conductivity of Water/carbon Nanotube Composite Systems: Insights from Molecular Dynamics Simulations

The effective thermal conductivity of water/carbon nanotube (CNT) composite systems is predicted using molecular dynamics simulation. Both empty and water-filled CNTs with diameters ranging from 0.83 nm to 1.26 nm are considered. Using a direct application of the Fourier law, we explore the transition to diffusive phonon transport with increasing CNT length and identify the correlation between CNT diameter and fully-diffusive thermal conductivity. Using Green-Kubo linear response theory, we explore how the thermal conductivity of water inside CNT varies with tube diameter. We predict the effective thermal conductivity of the composite systems and examine how the phonon modes in the CNT are affected by interactions with the water molecules. INTRODUCTION Due to their high thermal conductivities, graphene sheets and carbon nanotubes (CNT) are ideal candidates for nextgeneration thermal management devices [1]. Most experimental and theoretical investigations of thermal transport in these materials, however, have focused on their behavior in a vacuum. As components of a thermal management device, the atoms in CNTs and graphene sheets will exchange energy with atoms and molecules in an adjacent solid, liquid, or gas. Understanding how such interactions with non-bonded atoms modify thermal transport is an important next step in predicting how these materials will behave in heat transfer applications. ∗Address all correspondence to this author. The high thermal conductivity of graphitic structures is attributed to their rigid crystalline structure and resulting long phonon mean free paths [2]. In CNTs and graphene sheets shorter than about 100 nm, thermal energy is transported diffusely by phonons that scatter inside the system and ballistically by phonons that scatter with the system boundaries. With increasing system size, longer wavelength vibrational modes that can travel ballistically through the system are accessed and the thermal conductivity off the system increases [2, 3]. In systems longer than about 1 μm, however, the contribution to the thermal conductivity from these long-wavelength ballistic modes becomes negligible, most modes become diffusive, and the thermal conductivity becomes independent of the CNT length [4]. Information concerning how this transition length varies with CNT diameter and interactions with an adjacent material is not yet available. In this work we use molecular dynamics (MD) simulation to investigate the thermal conductivity of CNTs filled with water, a common coolant and well-characterized fluid. We begin by describing our simulation procedure. Next, we investigate the variation in CNT thermal conductivity with diameter and length for hollow CNTs and explore the transition to fully diffusive phonon transport. We then predict the thermal conductivity of water confined inside CNTs and examine how changes in the water structure with CNT diameter modify the CNT thermal conductivity. We finish by examining the thermal conductivity of water/CNT composite systems. 1 Copyright c © 2009 by ASME