© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 6724 www.advmat.de www.MaterialsViews.com wileyonlinelibrary.com COMMUNICATION Enhanced Mechanical Properties of Graphene/Copper Nanocomposites Using a Molecular-Level Mixing Process Jaewon Hwang, Taeshik Yoon, Sung Hwan Jin, Jinsup Lee, Taek-Soo Kim, Soon Hyung Hong,* and Seokwoo Jeon* J. Hwang, S. H. Jin, J. Lee, Prof. S. H. Hong, Prof. S. Jeon Department of Materials Science and Engineering KAIST, Graphene Research Center(GRC) KAIST Institute for the NanoCentury 291 Daehak-ro, Yuseong-gu Daejeon, 305–701, Republic of Korea E-mail: shhong@kaist.ac.kr; jeon39@kaist.ac.kr T. Yoon, Prof. T. Kim Department of Mechanical Engineering KAIST, Graphene Research Center(GRC) KAIST Institute for the NanoCentury 291 Daehak-ro, Yuseong-gu Daejeon, 305–701, Republic of Korea DOI: 10.1002/adma.201302495 Graphene, which has a 2D layered structure of carbon atoms, is of great interest because of its excellent mechanical [1,2] and electrical properties. [3,4] Monolayer graphene has a Young’s modulus of 1 TPa and a tensile strength of 130 GPa. The elec- tron mobility of suspended graphene is 200 000 cm 2 V -1 s -1 . [4] Graphene also has a large surface area (≈2600 m 2 g -1 ) and a low density (2.2 g cm -3 ), which makes it suitable as a reinforce- ment for nanocomposites. Numerous graphene/polymer com- posites have been studied for that reason. Stankovich et. al. first reported possibilities of graphene-based polymer composites with homogeneous dispersion of graphene at relatively low percolation threshold of ≈0.1 vol% in polystyrene matrix. Since then, graphene/polymer nanocomposites have been reported based on epoxy, [5,6] poly(methyl methacrylate) (PMMA), [7] poly- styrene (PS), [8] polyurethane (PU), [9] and polypropylene (PP) [10] polymers. Recently, our research group also introduced non- covalent, [11–13] PBA functionalization to graphene flakes and synthesized graphene/epoxy composite resulting in enhanced thermal conductivities and mechanical properties. [12] Graphene could be an ideal 2D reinforcement nanomaterial not only for polymer matrix but also to metal matrix. However, only few work on graphene/metal nanocomposites have been reported [14–17] and, sometimes, exhibited even lower mechanical properties with the addition of graphene flakes. [14] Two major reasons for this behavior are: i) poor bondings between gra- phene flakes and metals and ii) relatively high processing temp- erature (over 1000 °C in case of copper) at which the graphene is easily decomposed or damaged. Previous work of graphene/ metal composites was mostly based on the traditional process of powder metallurgy, which cannot effectively prevent agglom- eration of the graphene in the metal matrix because graphene is prone to segregate from the metal particles due to its poor affinity to metal in the absence of any binding sites and the for- mation of agglomerates of graphene by van der Waals forces. Furthermore, general sintering and melting process are not easily applicable to graphene/metal nanocomposites because the process temperatures of most metals are beyond the decomposition temperature of reduced graphene oxide (RGO) (≈600 °C) found from TGA (Figure S1, Supporting Informa- tion). Also, the large density difference between the metal and graphene causes the graphene to float on top of the melt. To achieve the best mechanical properties, graphene flakes must be homogeneously dispersed in metal matrix without signifi- cant thermal damage or conversion into metal carbides during densification and sintering. We propose here a molecular-level mixing process and spark plasma sintering (SPS) process to investigate the strengthening effects of graphene in a metal matrix. The suggested method avoids the issues of dispersion and thermal damage of gra- phene flakes during the synthesis of graphene/metal nano- composite. The first key process is a molecular-level mixing process that consists of attaching functional groups onto gra- phene flakes and making chemical bondings between graphene and composite matrix. The second key process is SPS that con- solidates metal powders through local joule heating and spark plasma generated between individual powders. Fast heating and cooling rate of SPS process not only limits grain growth and diffusion but also lowers average sintering temperature due to localized heating at the contact point of the powders. The strategies was proven successfully for carbon nanotube (CNT)/metal nanocomposites by our previous work [18] and for graphene/ceramic nanocomposites. [19] Because graphene has the same surface characteristics with CNTs except curvature, the application of molecular-level mixing process to graphene/ metal nanocomposites is very promising. To the best of our knowledge, our result is the first report proving that graphene flakes can be effective nanofillers to sig- nificantly enhance mechanical properties of metal composites. Graphene/copper (Cu) nanocomposites containing 2.5 vol% RGO had an elastic modulus of 131 GPa and a yield strength of 284 MPa, which are 1.3 and 1.8 times higher, respectively, than those of pure Cu. The strengthening mechanism was studied by measuring the adhesion energy between gra- phene and Cu using a double cantilever beam (DCB) test. [20] The adhesion energy between sintered graphene and Cu was 164 J m -2 , which is much stronger than the adhesion energy of 0.72 J m -2 for as-grown graphene on a Cu substrate. We believe the successful application of molecular level mixing process Adv. Mater. 2013, 25, 6724–6729