Graphene Nanoplatelet (xGnP™) additives for multifunctional composite materials (original) (raw)
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Thermal management has become a critical aspect in next-generation miniaturized electronic devices. Efficient heat dissipation reduces their operating temperatures and insures optimal performance, service life, and efficacy. Shielding against shocks, vibrations, and moisture is also imperative when the electronic circuits are located outdoors. Potting (or encapsulating) them in polymer-based composites with enhanced thermal conductivity (TC) may provide a solution for both thermal management and shielding challenges. In the current study, graphene is employed as a filler to fabricate composites with isotropic ultrahigh TC (>12 W m −1 K −1) and good mechanical properties (>30 MPa flexural and compressive strength). To avoid short-circuiting the electronic assemblies, a dispersion of secondary ceramic-based filler reduces the electrical conductivity and synergistically enhances the TC of composites. When utilized as potting materials, these novel hybrid composites effectively dissipate the heat from electronic devices; their operating temperatures decrease from 110 to 37 °C, and their effective thermal resistances are drastically reduced, by up to 90%. The simple filler dispersion method and the precise manipulation of the composite transport properties via hybrid filling offer a universal approach to the large-scale production of novel materials for thermal management and other applications.
Graphene Reinforced Composites as Efficient Thermal Interface Materials
The power dissipated in computer chips has been growing with each new technology node reaching unsustainable levels. In such a situation, the search for materials that conducts heat well and fast became essential for design of the next generations of integrated circuits (ICs) and three-dimensional (3D) electronics [1]. Efficient thermal management of electronics, optoelectronics and photonic devices require better thermal interface materials (TIMs). Current TIMs are based on polymers or greases filled with thermally conductive particles such as silver or silica, which require high volume fractions of filler (up to 70%) to achieve K of ~1-5 W/mK of the composite. Carbon materials such as carbon nanotubes (CNTs) have been studied as possible fillers in TIMs. Theoretical and cost considerations suggest that chemically derived graphene and few-layer graphene (FLG) flakes can perform better than other carbon materials in TIMs. It was discovered that the intrinsic thermal conductivity of graphene is extremely high [2]. At the same time, thermal properties of graphene flakes in the composites will be determined by the flake size, thickness, and coupling to the matrix material. We report the results of the experimental investigation of thermal properties of the graphene reinforced composite materials. The TIM samples were produced using the chemically derived graphene and FLG flakes. The number of atomic planes in FLG flakes was determined with the micro-Raman spectroscopy [3]. Thermal properties of the resulting graphene-epoxy composites were measured with the “laser flash” and “hot disk” thermal conductivity techniques. The thermal conductivity enhancement factor exceeded ~ 2300% at 10% of the volume loading fraction. To achieve such strong enhancement with the conventional filler materials one would need a loading fraction of ~70%. The computer simulations of thermal properties of TIM composites carried out using the modified effective medium approximation, which included the thermal boundary resistance effects, were in agreement with our experimental data. Our results suggest that graphene and FLG flakes can become excellent filler materials in the next generation of TIMs. The work at UCR was supported, in part, by the Office of Naval Research (ONR) award on Graphene Quilts for Thermal Management of HighPower Density Electronics and DARPA – SRC through the FCRP Center on Functional Engineered Nano Architectonics (FENA).
2006
Since the late 1990’s, research has been underway in our group at Michigan State University to investigate the fabrication of new nano-size carbon material, exfoliate graphite nanoplatelets [xGnP]. The xGnP is fabricated from natural graphite and can be used as a nanoreinforcement for polymers as an alternative to expensive carbon-based nanomaterials. The thickness of the xGnP became around 5-10nm. The diameter can be controlled from sub-micron level to few hundred um, indication the aspect ratio can be controlled. The surface area of the material reached more than 100m/g. Since graphite is the stiffest material found in nature (Young’s Modulus = 1060 MPa), having a modulus several times that of clay, accompanied with excellent strength, electrical and thermal conductivity, the xGnP should have similar properties to carbon-based nanomaterials, including carbon nanotubes, nanofibers, and fullerenes, yet the estimated cost is fur less than these materials. The cost of the graphite nan...
Thermally Conductive Graphene-Polymer Composites: Size, Percolation, and Synergy Effects
The rapidly increasing device densities in electronics dictate the need for efficient thermal management. If successfully exploited, graphene, which possesses extraordinary thermal properties, can be commercially utilized in polymer composites with ultrahigh thermal conductivity (TC). The total potential of graphene to enhance TC, however, is restricted by the large interfacial thermal resistance between the polymer mediated graphene boundaries. We report a facile and scalable dispersion of commercially available graphene nanoplatelets (GnPs) in a polymer matrix, which formed composite with an ultrahigh TC of 12.4 W/m K (vs 0.2 W/m K for neat polymer). This ultrahigh TC was achieved by applying high compression forces during the dispersion that resulted in the closure of gaps between adjacent GnPs with large lateral dimensions and low defect densities. We also found strong evidence for the existence of a thermal percolation threshold. Finally, the addition of electrically insulating boron-nitride nanoparticles to the thermally conductive GnP-polymer composite significantly reduces its electrical conductivity (to avoid short circuit) and synergistically increases the TC. The efficient dispersion of commercially available GnPs in polymer matrix provides the ideal framework for substantial progress toward the large-scale production and commercialization of GnP-based thermally conductive composites.
Many research efforts have been focusing on exfoliated clay systems, the same nanoreinforcement concept can be applied to another layered material, graphite. The key to utilizing graphite as a platelet nanoreinforcement is in the ability to exfoliate this material. If the appropriate surface treatment can be found for graphite, its exfoliation and dispersion in a polymer matrix will result in a composite with not only excellent mechanical properties but electrical properties as well, opening up many new structural applications as well as non - structural ones where electromagnetic shielding and high thermal conductivity are required. In this research, a special thermal treatment was applied to the graphite flakes to produce exfoliated graphite reinforcements. Intercalated natural crystalline graphite compounds (GICs) were formed followed by exfoliation and milling to produce sub-micron graphite flakes. SEM and TEM images showed that the average size of graphite became 0.86 um with a...
Enhanced thermal conductivity of graphene nanoplatelets epoxy composites
Materials Science-Poland
Efficient heat dissipation from modern electronic devices is a key issue for their proper performance. An important role in the assembly of electronic devices is played by polymers, due to their simple application and easiness of processing. The thermal conductivity of pure polymers is relatively low and addition of thermally conductive particles into polymer matrix is the method to enhance the overall thermal conductivity of the composite. The aim of the presented work is to examine a possibility of increasing the thermal conductivity of the filled epoxy resin systems, applicable for electrical insulation, by the use of composites filled with graphene nanoplatelets. It is remarkable that the addition of only 4 wt.% of graphene could lead to 132 % increase in thermal conductivity. In this study, several new aspects of graphene composites such as sedimentation effects or temperature dependence of thermal conductivity have been presented. The thermal conductivity results were also com...