Thermally Conductive Graphene-Polymer Composites: Size

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Article pubs.acs.org/cm

Thermally Conductive Graphene-Polymer Composites: Size, Percolation, and Synergy Effects Michael Shtein,*,† Roey Nadiv,‡ Matat Buzaglo,‡ Keren Kahil,‡ and Oren Regev*,†,‡ †

Ilse Katz Institute for Nanoscale Science and Technology and ‡Department of Chemical Engineering, Ben-Gurion University of the Negev, Beer-Sheva 8410501, Israel S Supporting Information *

ABSTRACT: 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.

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shear mixing,10−13 or NF surface treatment (physical14 or chemical,15,16 respectively). The latter improves the compatibility between the filler and the matrix by minimizing the contact resistance and interfacial phonon scattering.16−20 So far, the best reported isotropic TC enhancements (e.g., 5.5 W/m K13) has been achieved by dispersing laboratory-prepared graphene nanoplatelets (GnPs, few layers of graphene with micron-sized lateral dimension) in a polymer using long sonication (few hours to days). These results demonstrate the current limit of the state of the art, the lack of an efficient dispersion technique of commercially available material at high concentration, which precludes the large-scale production of GnP-based composites. In addition to the aggregation problems of NF, most GnPbased composites are also electrically conductive,12,13,21 which further restricts their application to electromagnetic and radio frequency interference shielding,22 as replacements for solder interconnects23 or as thermal interface materials13,24,25 (e.g., between heat sources and heat sinks). It has been reported12,15,16,18,20,24,25 that the loading of more than one filler (termed hybrid filler loading) could produce composites whose heat removal properties are enhanced relative to that of a

INTRODUCTION Efficient thermal management has become a necessity in nextgeneration miniaturized electronics, optoelectronics, and medical devices. As the electronic components of these devices continuously become denser and faster,1−3 the heat they generate could be effectively dissipated by a polymer-based composite with enhanced thermal conductivity (TC) as an alternative to conventional heat removal solutions (e.g., metal heat sink). Traditionally, target TC values (>1 W/m K) were met by dispersing high loadings4,5 (50−80 vol %) of thermally conductive micron-size fillers in thermally isolating polymers (TC < 0.2 W/m K6). Such high loading, however, resulted in heavyweight and expensive composites with inferior mechanical properties (e.g., susceptible to thermal cracking and challenging to process), all of which combined to limit their application. In recent years, the extraordinary thermal properties and growing availability of nanosized fillers (NFs, e.g., graphene with TC = ∼5000 W/m K7 or boron nitride nanoparticles (BNs) with TC = 360 W/m K8) have driven research of NFbased composites with enhanced TC at low NF loading. The use of NF, however, must be carefully considered: NF easily aggregate due to van der Waals forces and their large surface area,9 which reduces dramatically the properties of the composite. Therefore, efficient dispersion methods are required, among which are sonication, roll milling, and high © XXXX American Chemical Society

Received: December 11, 2014 Revised: February 24, 2015

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DOI: 10.1021/cm504550e Chem. Mater. XXXX, XXX, XXX−XXX

Article

Chemistry of Materials

Table 1. GnP Lateral Dimensions (Determined by EM, Figure 1a and Figure S4 in the Supporting Information), Thickness (AFM, Figure 1c), Aspect Ratio (Calculated), Defect Density (XPS, Figure 1d), and GnP Combustion Temperature (TGA, Figure 1e)

a

GnP grade

sizea [μm]

thickness [nm]

aspect ratio

defect densityb [% ]

CT [°C]

TCEFc

M5 M15 H15 M25 C2

10 ± 7 19 ± 9 20 ± 7 34 ± 20 2±1

5−20 9−20 15−100 >100 5−20

725 ± 125 1255 ± 145 568 ± 298 340 ± 200 175 ± 25

17 17 23 28 35

740 757 748 741 689

64.0 107 112 48.5 5.50

n > 100 particles. bQualitative defect density =100-sp2%. cThermal conductivity enhancement factors at 0.15 volume fraction.

composite with a single filler (synergistic effect). We postulate that the loading of a secondary, electrically insulating filler (e.g, boron nitride15,18,24,26,27) may not only synergistically increase the TC of the composite, but more importantly, it will also decrease its electrical conductivity (EC), thereby facilitating its application in bonding, potting, encapsulation, and packaging of electronic devices,15,27 processes that require a low EC. In this study, we chose an epoxy (TC = 0.19 W/m K6), a material commonly used in microelectronic applications, as a polymer matrix. We explored the structural parameters of commercially available GnPs and their effects on the TC of the composite. We found that loading the epoxy with a GnP of large lateral dimensions and low defect density produced a ultrahigh isotropic enhancement of TC (12.4W/m K). Finally, we demonstrated that GnP-BN-epoxy hybrid composites had a synergistic effect on TC. Because this hybrid may also be EC tunable, it is therefore suitable for a wide spectrum of thermal management applications.

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deaerated the materials in the container. The combination of rotation and revolution generated a spiral flow along with rising and falling convection currents.30 Air bubbles within the material were efficiently removed. Zirconia balls measuring 5 mm in diameter were added to the mixing container to enhance the compression forces during the mixing process. The planetary motion threw the balls strongly against each other, generating high impact energy.31 The materials were mixed (10 min at 2000 rpm), deaerated (5 min at 2200 rpm). Then, the zirconia balls were removed, and the composites were cast into silicone molds (5.9 mm diameter and 1−2.3 mm thickness) and cured for 12 h at 80 °C. Thermal Conductivity (TC). TC was measured using a special differential scanning calorimetry32 (DSC) (Mettler Toledo Star System operated at 80 mL/min N2 and equipped with 70 μL alumina crucibles) procedure that allows the TC measurement of low volume samples (