DOI: 10.1002/adma.200800401 Enhanced Thermal Conductivity in a Hybrid Graphite Nanoplatelet – Carbon Nanotube Filler for Epoxy Composites** By Aiping Yu, Palanisamy Ramesh, Xiaobo Sun, Elena Bekyarova, Mikhail E. Itkis, and Robert C Haddon* The increased heat generated in high density electronics has intensified the search for advanced thermal interface materials (TIMs) and prompted fundamental studies at the nanoscale level to develop filler materials with enhanced thermal performance. [1–4] Single-walled carbon nanotubes (SWNTs) considerably improve the heat transport in polymer compo- sites as a result of their one-dimensional (1D) structure, high thermal conductivity and high aspect ratio. [5–12] Recently, two-dimensional (2D) nanostructures such as graphite nano- platelets (GNPs), have emerged as a promising filler in polymer matrices [13–19] and it has been shown that they provide even higher thermal conductivity enhancement than SWNTs. [16] In this study we combine 1D-SWNTs and 2D-GNPs to prepare a series of hybrid graphitic nanofillers and we observe a synergistic effect between the GNPs and SWNTs in the enhancement of the thermal conductivity of epoxy composites to the point that at certain filler loadings the hybrid composition outperforms composites utilizing pure GNP or SWNT fillers. The increased thermal conductivity is ascribed to the formation of a more efficient percolating nanoparticle network with significantly reduced thermal interface resistances. The idea of using a hybrid filler comprised of two or more traditional filler materials has already been explored in the literature and it has been demonstrated that improved composite performance can be achieved by combining the advantages of each filler. [20,21] Commercially available thermal greases and adhesives often utilize several components to achieve the desired combination of thermal and electrical conductivities, viscosity and low coefficient of thermal expansion. In our study, we utilize two different nanostruc- tured graphitic fillers for incorporation into epoxy resin: purified SWNTs and graphite nanoplatelets (GNPs) comprised of few graphene layer G n , where n  4. The SWNT component of the hybrid filler is electric arc produced purified SWNTs with a typical length of 0.3–1.0 mm and an average diameter of 1.4 nm. The purification process [22] leaves the SWNTs ends and side-walls functionalized with carboxylic acid groups and this facilitates their homogeneous dispersion into the polymer matrix. In addition, the epoxy curing process is accompanied by a cross-linking reaction between the carboxylic acid groups of the SWNTs and the epoxy groups of the polymer, [23] thus improving the integration of SWNTs into the polymer matrix. GNPs are typically prepared by intercalation and exfoliation of graphite; [24–29] and by control of the exfoliation conditions we were able to obtain GNPs comprised of 2 to 8 graphene layers with a lateral dimension of 200–1000 nm and an aspect ratio in the range of 50 to 300. [16] This was achieved by thermal shock exfoliation of natural graphite flakes at 800 8C [25,26] followed by high shear mixing and sonication in order to separate the exfoliated graphite flakes into nanoplatelets. [16] A series of composites were prepared with a hybrid filler loading between 5 wt % and 40 wt % in the epoxy (EPON 682/ EPIKURE) matrix. The ratio of SWNTs and GNPs in the hybrid filler was varied in order to study their efficiency in enhancing the thermal conductivity of the composite. The thermal and electrical conductivity measurements were performed using composite disks with a diameter of 2.54 cm and thickness of 4–12 mm. A detailed composite preparation procedure was reported in our previous publications, [12,16] and it is described briefly in the experimental section. Figure 1a shows the thermal conductivity (k) of GNP-SWNT/epoxy composites as a function of the GNP fraction in the hybrid filler at a hybrid filler loading of 10 wt %. The epoxy composites prepared with a 10 wt % loading of the individual fillers gave thermal conductivities of k SWNT ¼ 0.85 W m 1 K 1 for the SWNT filled composite [12] and k GNP ¼ 1.49 W m 1 K 1 for the GNP-filled composite. [16] In the case of a hybrid filler (HYB), the thermal conductivity is expected to increase monotonically as the fraction of the more efficient GNP filler increases, in accord with the rule of mixtures. However, the experimental data show a pronounced maximum of k HYB ¼ 1.75 W m 1 K 1 at a GNP:SWNT filler ratio of 3:1 (7.5 wt % GNPs and 2.5 wt % SWNTs in epoxy). Thus the hybrid filler demonstrates a strong synergistic effect and surpasses the performance of the individual SWNT and GNP fillers. This synergistic behavior is quite remarkable COMMUNICATION [*] Prof. R. C Haddon, A. Yu, Dr. P. Ramesh, Dr. X. Sun, Dr. E. Bekyarova, Dr. M. E Itkis Center for Nanoscale Science and Engineering Departments of Chemistry and Chemical & Environmental Engineering University of California – Riverside Riverside, California 92521 (USA) E-mail: haddon@ucr.edu [**] We acknowledge the financial support from DOD/DMEA under award # H94003-06-20604 and # H94003-08-2-0803 and technical help in graphite exfoliation from Yasir Khalid Ali and Kimberly Worsley. Supporting Information is available online from Wiley InterScience or from the authors. 4740 ß 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Adv. Mater. 2008, 20, 4740–4744