Covalent functionalization of carbon nanotubes and their use in dielectric epoxy composites to improve heat dissipation Charles Baudot a,b, * , Cher Ming Tan b a STMicroelectronics Asia Pacific Pte Ltd., 5A Serangoon North Avenue 5, Singapore 554574, Singapore b School of EEE, Nanyang Technological University, Block S2, Nanyang Avenue, Singapore 639798, Singapore ARTICLE INFO Article history: Received 28 October 2010 Accepted 1 February 2011 Available online 24 February 2011 ABSTRACT A method to enhance the thermal dissipation of epoxies is described. The method exploits the transport of heat by phonons so that the composite material designed remains dielec- tric. The improvement in thermal transport is guaranteed by the addition of functionalized carbon nanotube fillers which are covalently bonded to the epoxy matrix. We demonstrate that even if the covalent grafting of functional molecules affects the thermal transport within the nanotubes because of the disruption in periodicity of the structure, it improves the interfacial thermal conductance between the matrix and fillers. The trade-off has a net positive impact on the effective thermal conductivity of the composite material. Ó 2011 Elsevier Ltd. All rights reserved. 1. Introduction A key strategy to enhance the thermal properties of polymers is to add metal particles in it [1,2]. In such a case, both electri- cal and thermal transports are thus ensured by the free flow of electrons resulting from the percolated network of metal particles which is formed within the polymer matrix. How- ever, when high thermal dispersion is required from a dielec- tric material, special ceramics and glasses are still the preferred materials used [3]. Our target is to develop a high thermal conductivity composite based on a dielectric polymer with non-electrically conductive fillers and our proposed alternative is to use functionalized carbon nanotube (f-CNT) fillers in polymeric materials as thermal dissipation enhanc- ers. The individual carbon nanotube (CNT) is a well known material for its outstanding thermal conductivity of at least 3000 W/mK [4,5]. Our strategy is to properly functionalize multi wall carbon nanotubes (MWCNTs) so that their electri- cal properties are diminished while still maintaining enough structural integrity for the heat to be transported by lattice vibration. We demonstrate in this paper that, in order to en- sure a good phonon transport, molecular bridges must be set in-between the CNTs and between the CNTs and the neighboring polymer matrix. Thus, our target is to do a selec- tive covalent functionalization of the CNTs that will not only weaken the electrical properties but also enhance the phonon interfacial transport and, on top of that, also facilitate the composite fabrication process through better and faster dis- persion of the fillers in the polymer by preventing bundling and avoiding entanglement. The functionalization method described in this paper is aimed at being reproducible on a large scale by using industrially compliant processes. Besides explaining the chemical functionalization of the CNTs and the method to prepare epoxy composite samples, we also present a thorough transport characterization of the complex and an in-depth study of the transport mechanism involved in the particulate material. Moreover, we show the experi- mental evidences of the thermal enhancement while maintaining the dielectric properties of the original polymer. The study of the transport mechanisms involved in particulate materials consisting of a matrix which is more frequently amorphous and fillers which are dispersed homog- enously across the matrix had already started to raise the interest of Lord Rayleigh [6] and Maxwell [7] at the end of 0008-6223/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2011.02.003 * Corresponding author at: STMicroelectronics Asia Pacific Pte Ltd., 5A Serangoon North Avenue 5, Singapore 554574, Singapore. Fax: +65 6427 7379. E-mail address: charles.baudot@st.com (C. Baudot). CARBON 49 (2011) 2362 – 2369 available at www.sciencedirect.com journal homepage: www.elsevier.com/locate/carbon