Elsevier

Carbon

Volume 101, May 2016, Pages 120-128
Carbon

Ultrahigh electrically and thermally conductive self-aligned graphene/polymer composites using large-area reduced graphene oxides

https://doi.org/10.1016/j.carbon.2016.01.088Get rights and content

Abstract

In this work, for the first time we fabricated highly self-aligned large-area reduced graphene oxide/poly (vinylidene fluoride-co-hexafluoropropylene) (rLGO/PVDF-HFP) composite films through simple solution casting followed by low temperature chemical reduction process. The resulting free-standing rLGO/PVDF-HFP composite thin film revealed excellent electrical conductivity of ∼3000 S/m and ultrahigh in-plane thermal conductivity of ∼19.5 W/mK at rLGO content of 27.2 wt %. This ultrahigh electrical and thermal conductivity were attributed to the good interfacial interaction, effective chemical reduction, high aspect ratio, and preferential orientation of graphene sheets along the film direction. We believe that our new fabrication procedure can be effectively used for large-scale production and commercialization of conductive composite materials for many thermal and electrical conduction applications.

Introduction

Heat management is a key issue for safety, performance and reliability in many applications, such as electronic devices, solar energy devices, LED, and vehicles [1], [2], [3], [4]. Traditionally, metals (like copper) were used as thermally conductive materials (TCMs) for heat pipes, heat sinks, and heat spreaders because of their excellent electrical and thermal conduction [5]. Unfortunately, metals are heavy, bear low flexibility, and their relatively large coefficient of thermal expansion become problematic for cooling silicon and other semiconductor devices. Polymer composites with enhanced electrical and thermal conductivity can be used as an alternative to conventional TCMs because of their lightweight, resistance to corrosion, flexibility, good processability, and low cost compared with the metal-based materials [6], [7], [8], [9], [10], [11], [12]. Graphene-based polymer composites have drawn significant interest for the fabrication of highly electrically and thermally conductive composite materials due to super electrical conductivity (∼6000 S/m) and thermal conductivity (∼5000 W/mK) of pure graphene [13], [14]. Graphene-based polymer composites with enhanced electrical and thermal conductivity have been achieved through dispersion of multilayer graphene, graphite nanoplatelet, and graphite into various polymer matrices [15], [16], [17], [18], [19]. So far, however, the best reported isotropic thermal conductivity of ∼6.44 W/mK has been reported for graphene nanoplatelet/epoxy composite at 25 vol % filler loading [19]. These improvements are limited due to poor dispersion, limited direct contact and insufficient alignment of graphene fillers and weak thermal coupling at graphene/matrix interface.

Presently, graphene/polymer researches are focused on alignment of graphene to improve electrical and thermal properties of the resulting composites because of the fact that in-plane conductivity of graphene sheet is much larger than out-of-plane conductivity. Thus, high in-plane conduction properties of graphene can impart very large electrical and thermal conductivity along in-plane direction of aligned graphene to the resulting composite [20]. Therefore, controlled orientation of graphene in polymer matrix is critically important to achieve the lightweight, flexible, and high thermally conductive composite materials for efficient thermal management in present electronics sector. However, only limited successes have been achieved for graphene alignment in polymer matrix due to lack of efficient fabrication procedure [7], [20], [21], [22], [23], [24]. For example, Yousefi et al. reported the maximum electrical conductivity of ∼1 S/m of aligned rGO/epoxy composite at graphene content of 3 wt % [22]. Moreover, Jung and co-workers reported the through-plane thermal conductivity of 10 W/mK at 25 vol % graphene content from PVDF/graphene nanoflakes composite fabricated through melt processing method [24].

In addition to alignment, graphene sheet size also plays a significant role in improvement of conductivity [4], [25]. In our previous work we demonstrated that low temperature chemically reduced highly-aligned large-area graphene thin film exhibit greater electrical conductivity and larger in-plane thermal conductivity compared with that of small-area GO counterpart [4]. This is probably due to the fact that large-area GO sheets have fewer defects in their sp2 structure, mostly caused by edge boundaries, more compact, aligned and lower intersheet contact resistance than conventional small area. However, aligned graphene polymer composites using large-area graphene still need to be further explored.

In this work, for the first time we demonstrate a robust and scalable route for the fabrication of highly-aligned new composite thin films through self-assembly of large-area GO dispersed poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) solution, followed by low temperature hydriodic (HI) acid reduction at 80 °C. Resulting large-area rLGO/PVDF-HFP composite film revealed the greatest electrical conductivity (∼3000 S/m) and in-plane thermal conductivity (∼19.5 W/mK) at 27.2 wt % of rGO than that of all previously reported results in literature.

Section snippets

Materials

Graphite flakes, potassium permagnate (KMnO4), hydriodic acid (HI, 57%), and PVDF-HFP (density 1.78 g/cm3) were purchased from Sigma Aldrich. Sulphuric acid (H2SO4), hydrogen peroxide (H2O2), hydrochloric acid (HCl), and N,N-dimethylformamide (DMF) were obtained from Daejung Chemicals, Republic of Korea.

Preparation of large-area graphene oxide

First, the aqueous GO dispersion was prepared using modified Hummer's method from oxidation of natural graphite flakes [4], [26], [27]. Preparation method for large-area GO sheets has been

Results and discussion

Fig. S1 shows typical SEM micrographs of both large area LGO and small area SGO sheets, including their size distributions. GO sheets size distributions were obtained using Image J software. The average area of GO sheets was measured as ∼25 μm2 with the largest GO sheet area of 1350 μm2. Interestingly, the number fraction was dominated by smaller sheet size (area ≤5 μm2), whereas, area fraction was dominated by larger sheet size (<25 μm2, ∼70%). In contrast, in SGO sample (Fig. S1b,d), both

Conclusions

In conclusion, we report a simple, economical and scalable approach for preparation of highly-aligned and ultrahigh electrically and thermally conductive rGO/PVDF-HFP composites through simple casting of PVDF-HFP solution of dispersed large-area graphene oxide, followed by hydriodic acid reduction. GO sheets in matrix were preferably orientated during self-assembly process and aligned along the film surface direction. The aligned rGO/PVDF-HFP composite films exhibited excellent electrical

Acknowledgments

This work was supported by Graphene Part & Material Development Program, Fundamental R&D Program for Core Technology of Materials, and Industrial Strategic Technology Development Program funded by the Ministry of Trade, Industry and Energy, Republic of Korea and partially by Korea Institute of Science and Technology. Synchrotron X-ray scattering tests were carried out at Pohang Light Source, Republic of Korea.

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