The addition of nitrophenyl groups to the surface of few-layer epitaxial graphene (EG) by the formation of covalent carbon-carbon bonds changed the electronic structure and transport properties of the EG from near-metallic to semiconducting.

Covalent derivatization of the acidic functional groups in oxidized graphite with octadecylamine renders graphite soluble in common organic solvents. Atomic force microscopic characterization of the soluble species supports the idea that the solutions consist of single and few layer graphene sheets, and we report the first solution properties of graphite.

Natural graphite was intercalated, thermally exfoliated, and dispersed in acetone to prepare graphite nanoplatelets (GNPs, G n ) of controlled aspect ratio. Thermal conductivity measurements indicate that few graphene layer G n , where n ∼ 4, with a thickness of ∼2 nm function as a very efficient filler for epoxy composites. When embedded in an epoxy matrix, the G4 GNPs provide a thermal conductivity enhancement of more than 3000% (loading of ∼25 vol %), and a thermal conductivity κ = 6.44 W/mK, which surpasses the performance of conventional fillers that require a loading of ∼70 vol % to achieve these values. We attribute the outstanding thermal properties of this material to a favorable combination of the high aspect ratio, two-dimensional geometry, stiffness, and low thermal interface resistance of the GNPs.

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][2][3][4] Single-walled carbon nanotubes (SWNTs) considerably improve the heat transport in polymer composites as a result of their one-dimensional (1D) structure, high thermal conductivity and high aspect ratio. [5][6][7][8][9][10][11][12] Recently, two-dimensional (2D) nanostructures such as graphite nanoplatelets (GNPs), have emerged as a promising filler in polymer matrices [13][14][15][16][17][18][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 nanostructured 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][25][26][27][28][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.[...