Published: February 21, 2011 r2011 American Chemical Society 3872 dx.doi.org/10.1021/jp109978x | J. Phys. Chem. C 2011, 115, 3872–3880 ARTICLE pubs.acs.org/JPCC Effective Heat Transfer Properties of Graphene Sheet Nanocomposites and Comparison to Carbon Nanotube Nanocomposites Khoa Bui, †,‡ Hai M. Duong 4 , § Alberto Striolo, †,‡ and Dimitrios V. Papavassiliou †,‡, * † School of Chemical, Biological, and Materials Engineering, University of Oklahoma, Norman, Oklahoma, United States ‡ Carbon Nanotube Technology Center (CANTEC), University of Oklahoma, Norman, Oklahoma, United States § Department of Mechanical Engineering, National University of Singapore, Singapore 1. INTRODUCTION Carbon nanotubes (CNTs) with their outstanding electrical, thermal, and mechanical properties have been suggested as reinforcement fillers in a variety of composite materials. By incorporating CNTs into a polymer matrix, or by dispersing CNTs into a solution, the effective thermal conductivity of the resulting composite can be increased. For example, this enhance- ment has been found to be from 80 to 125% at 1 wt % over pure polymer for the case of epoxy composites 1 or by a factor of almost 4 in the case of high volume fraction single-walled carbon nanotubes (SWNTs) in polystyrene. 2 However, on the basis of the properties of pure CNTs, one would expect a much higher increase of the effective thermal conductivity of such composite materials, up to an order of magnitude according to the classical theory of Maxwell. The presence of resistance to heat transfer at the CNT-polymer interface, known as the Kapitza interfacial thermal resistance, is the reason for this difference. The value of the Kapitza resistance can be roughly estimated by the acoustic mismatch theory, 3 which attributed this resis- tance to phonon transport across the interface of dissimilar materials. The overall effective thermal conductivity of a system with nanoinclusions depends on the volume fraction of the nanoinclusions and on the interfacial thermal resistance. It has been suggested that the effective medium theory can provide insights about the thermal behavior of such systems 4,5 by taking into account the Kapitza resistance and different geometries of nanoinclusions. The effective thermal conductivity, K eff , of CNT nanocomposites can also be calculated numerically with Monte Carlo (MC)-based methods, following the approach developed by Duong et al. 6 This method offers the advantage of explicitly accounting for the random or controlled placement of the CNTs, the Kapitza resistance between the CNTs and the matrix, and even the presence of a thermal boundary resistance between neighboring CNTs in contact with each other. 7,8 This method has been validated by comparisons to experimental data, 9 and it has also been used to estimate the Kapitza resistance effects for suspension systems. 10,11 The nature of interfacial thermal resistance at the atomic scale can be explored with molecular dynamics (MD) simulations. 12-14 It has been reported that the overlap of the thermal vibration spectra between two materials is the key point to control the Kapitza resistance at the interface. 15,16 Multiscale modeling, in which the Kapitza resistance is examined by atomic-scale simula- tions and a meso/macroscopic approach is employed to study the thermal properties of bulk materials, can be seen as a suitable approach to this problem. Clancy et al. 13 employed MD simula- tions together with effective medium theory to study the effect of functionalization on Kapitza resistance and thermal conductivity Received: October 18, 2010 Revised: January 17, 2011 ABSTRACT: By incorporating nanoinclusions (carbon nano- tubes, graphene sheets) with exceptional thermal conductivity into a polymer matrix, one would expect to improve the heat performance of resulting nanocomposites. However, the effec- tive thermal conductivity of carbon-based nanocomposites is strongly influenced by the Kapitza interfacial resistance. In this study, a comparison between carbon nanotubes and graphene sheet nanocomposites that takes into account dispersion pat- terns of the nanoinclusions and the Kapitza resistance is performed by means of a Monte Carlo simulation. It is found that graphene-based nanocomposites can be more efficient thermal conductors than carbon nanotube ones not only because of smaller Kapitza resistance but also because of the geometry of the graphene sheet. When the Kapitza resistance is reduced by appropriate functionalization of the graphene sheets and when the graphene sheet inclusions yield nematic patterns, our calculations suggest the possibility of obtaining composite materials with effective thermal conductivity up to 350 times larger in the direction parallel to the graphene sheets than in the direction perpendicular to them.