Elsevier

Carbon

Volume 49, Issue 3, March 2011, Pages 773-778
Carbon

Thermal conductivity of exfoliated graphite nanoplatelet paper

https://doi.org/10.1016/j.carbon.2010.10.003Get rights and content

Abstract

Exfoliated graphite nanoplatelets (xGnP) were produced by acid intercalation followed by thermal exfoliation and a controlled size reduction to produce graphite nanoplatelets of 1–15 μm in lateral dimension and approximately 10 nm in thickness. These highly hydrophobic nanoparticles were dispersed and stabilized in a DI water/polyethyleneimine (PEI, a cationic polyelectrolyte) solution. A free standing, mechanically robust paper of xGnP was prepared by vacuum filtration. The effect of xGnP size, polyelectrolyte coating and paper porosity on thermal transport properties was investigated. It was found that the annealing process improves the thermal conductivity by decomposing the PEI molecule that is adsorbed on the xGnP particles while still maintaining the porosity of the paper. Mechanically compressing the sample effectively reduces the pore volumes within the paper and increases the contact area among individual platelet. The strong alignment effect and larger contact area was evidenced by a 80% increase in in-plane thermal conductivity (178 ± 12 W/mK) and a 10% reduction in through-plane conductivity (1.28 ± 0.12 W/mK). This flexible, lightweight, low-cost, paper material made of xGnP particles is a promising candidate for applications requiring 2D heat conduction.

Graphical abstract

Exfoliated graphite nanoplatelet paper prepared by vacuum assisted self-assembly exhibits very high anisotropy in heat conduction with in-plane thermal conductivity as high as 190 W/mK for nanoplatelets of 15 μm in size.

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Research highlights

Polyethyleneimine (PEI) functionalized xGnP form good suspension in water. ► The as-made paper prepared by vacuum filtration is porous. ► Decomposition of PEI improves thermal conductivity of the paper. ► Cold pressing reduces paper porosity and improves thermal contact. ► Paper exhibits strong anisotropy in heat conduction after annealing and pressing.

Introduction

Free-standing paper-like graphitic materials have great technological importance to modern society because of their chemical inertness, good mechanical properties, and impermeability to gases and fluids. Early work has shown that these exfoliated graphite flakes can be rolled into flexible thin sheets and used as gaskets [1], [2], [3], [4], flow field plates in fuel cells,1 and electrodes for Li ion batteries [5]. The discovery of carbon nanotubes leads to densely packed CNT mats prepared by vacuum filtration and their superior strength and electrical conductivity are useful in many applications [6], [7], [8]. More recently, it is shown that graphite oxide can be exfoliated in water to yield hydrophilic, oxygenated graphene oxide nanosheet that can be assembled macroscopically to a paper-like structure through flow directed assembly [9]. Ruoff et al. also shows that these graphite oxide nanosheets can be chemically reduced to graphene with hydrazine in the presence of a strong base such as KOH to restore the outstanding mechanical and electrical properties of graphene [10].

However, there are very few reports on thermal transport properties of those papers. Hone reported the thermal conductivity of densely packed CNT paper to be a few tens W/mK [11], much lower than conventional inorganic materials for heat dissipation although individual carbon nanotube is believed to have very high thermal conductivity [12]. Balandin et al. recently used micro-Raman spectroscopy, a non-contact, optical method to determine the thermal conductivity of single layer graphene, they concluded κ = 4400–5300 W/mK, among the highest in nature [13]. Yet, it remains elusive how well a paper-like material composed of individual graphite nanosheet conducts heat and how to tailor the thermal conductivity of these papers for various purposes.

In this paper, the thermal transport property of a free-standing paper consisting of graphite nanoplatelets was explored. Graphite nanoplatelets, which are a stack of single layer graphenes was produced by exfoliating acid intercalated natural graphite with thermal methods. Research led by Drzal has developed a process that can produce exfoliated graphite nanoplatelet (xGnP) of 4–10 nm in thickness and from 1 to 15 μm in diameter. The large aspect ratio, sp2 type carbon bonding in the basal plane of individual platelets, contributes to the very high electrical conductivity approaching 1000 S/cm reported by Biswas and Drzal [14]. Also, these xGnP particles, capable of being produced at an expected cost of around $15/lb would be a very promising replacement for carbon nanotubes and carbon black in various applications due to their good mechanical and electrical properties [15].

To make a paper with xGnP particles, these highly hydrophobic nanoparticles were dispersed in a polyethyleneimine/water solution. The cationic polyelectrolyte induces steric and electrostatic stabilization [16] between the nanoplatelets [17] and a paper was prepared by subsequent vacuum filtration and drying. The thermal conductivity of the as-made xGnP paper consisting of individual platelets of different sizes and made with various processing steps (e.g. thermal annealing, cold pressing) was measured and related to the heat conduction in this paper-like structure.

Section snippets

Exfoliated graphite nanoplatelet preparation (xGnP)

Exfoliated graphite nanoplatelets (xGnP) were prepared by exfoliating the H2SO4/HNO3 intercalated natural graphite (GIC, A3772) purchased from Asbury Graphite Mills, Inc., NJ. Microwave exfoliation proves superior to conventional thermal treatments due to the fast heating process and high energy density, which vaporizes the acids within the layers of graphite causing a significant and rapid expansion of the graphite gallery [18]. The volume of exfoliated graphite (referred to as “worm”) is over

Results and discussions

To create a more ordered layer structure, a multi-filtration process (5 ml at a time) is necessary. During each filtration, water easily flows through the pores within the paper and the highly hydrophobic nanoplatelet floats at the water–air interface forming a very densely packed structure to minimize the interfacial energy. The first few layers were stacked more orderly because of the uniformity of water flow rate through the tubular shaped pores in the filter paper. However, as the thickness

Conclusions

Exfoliated graphite nanoplatelets stabilized by PEI were successfully dispersed in water and a paper-like structure was prepared by flow directed self assembly. Thermal diffusivity and conductivity of both xGnP-15 and xGnP-1 papers were measured at three different conditions. Annealing that helps decompose the polymer enhances thermal conductivity possibly due to the reduction of thermal interface resistance. However, the dominant factor in nanoscale heat transfer is the density of

Acknowledgements

The authors wish to acknowledge the support of the State of Michigan 21st Century Jobs Fund for partial support of this research.

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