Elsevier

Carbon

Volume 171, January 2021, Pages 639-645
Carbon

Research Article
Efficient and inexpensive preparation of graphene laminated film with ultrahigh thermal conductivity

https://doi.org/10.1016/j.carbon.2020.09.039Get rights and content

Abstract

With the rapid development of electronic technology, the demand for thermal management materials is increasing continuously. Recent years graphene paper is considered as a potential material to replace the traditional graphitized polyimide film. Although the current route to graphene paper can achieve a high thermal conductivity, the extremely high energy consumption in graphitization procedure and severe heavy metal contamination limit the application of graphene paper. Here we develop a high-efficiency, low-energy cost, and inexpensive technology to produce graphene paper in large quantities. Without graphitization, the thermal conductivity of this graphene laminated film can reach 975 W m−1 K−1 when the thickness of film is 65  μm. Based on this easy-operated technology, we have already completed the pilot plant test, attained capacity of 3 kg graphene per reactor in 8 h and 800 mm × 100 m of graphene film each roll which was continuously and macroscopically assembled on our self-made equipment.

Introduction

The electronic components of integrated circuits are becoming smaller and more complex, which has become an inevitable trend in the development of electronic technology. High-power electronics and portable devices, such as smartphones and tablets, inevitably generate a large amount of heat during operation, thereby reducing performance and the life of the original electronic components and even affecting other components nearby [1,2]. Moreover, the upcoming 5G communication era puts higher demands on thermal management. This also poses new challenges for inexpensive and efficient high thermal conductivity materials.

Traditional thermal management materials are mainly made of metallic materials such as copper and aluminum. And silver is known as the metal material with the highest thermal conductivity (K), which is K ≈ 429 W m−1 K−1 at room temperature mainly attributed to electrons transport in high concentration. Although the strength of the metal is high and easy for machining, due to its high density and relatively low K, non-metallic materials are preferred in some applications where volume and weight are demanded. After decades of development, non-metallic thermal materials have been greatly enriched, for example diamond film (K = 900–2320 W m−1 K−1) [3], highly oriented pyrolytic graphite (HOPG) (K = 1950 W m−1 K−1) [4], c-BN (K = 740 W m−1 K−1) [5], graphite paper (K = 200–500 W m−1 K−1 in commercial), SiC (K = 490 W m−1 K−1) [6], carbon nanotubes (CNT) (K = 3000–3500 W m−1 K−1) [7,8], CNT film (K = 800 W m−1 K−1) [9], graphitized polyimide (PGS 25) (K = 1750 W m−1 K−1) [10]. Graphene, the material with the highest thermal conductivity at room temperature (K ≈ 5300 W m−1 K−1 in suspended single-layer) [11,12], was the most studied material due to its special two-dimensional structure in a honeycomb lattice arranged by one layer of conjugated atoms [[13], [14], [15]]. Beside the ultrahigh thermal conductivity, the other unique properties such as low atomic mass, flexibility, simple crystal structure, and strong bonding make graphene promising as a new generation of materials to meet the thermal management requirements in ultrahigh K value.

A simple summary of the existing work shows that there are three ways to use graphene for thermal management. First, gas-phase precursor-based methods such as epitaxial growth and chemical vapor deposition (CVD) can construct graphene films with excellent electrical and thermal conduction properties, but the high cost and low production yield limit these approaches for large-scale applications [[16], [17], [18], [19]]. Second, the most studied and widely reported method, using graphene oxide (GO) as the main initial raw material to fabricate graphene or hybrid-graphene paper through chemical reduction or high-temperature annealing. Preparation of GO film is a very common and simple process, can be realized by vacuum filtration or evaporation treatment [[20], [21], [22], [23]]. Due to hydrogen bonding between the layers, the GO film has an outstanding flexibility and tensile strength, interlayer structure denser. However, K of GO film is mainly limited by the defects concentration (D, f(D)), K∝ϕ(L)/f(D), such as holes, impurities, and boundaries. Chemical reduction and thermal annealing (about 1000 °C) can partially promote the K of GO film, yet the structural defects in graphene sheets cannot be fully repaired [[24], [25], [26], [27], [28]]. Graphitization, an effective means of repairing the defects of carbon materials to increase the crystallinity, can greatly improve the K value. Different annealing temperatures and conditions directly determine the thermal conductivity of the final product [10,[29], [30], [31], [32], [33], [34]]. In general, higher annealing temperatures connote better thermal conductivity, so much so that the K of graphene paper can reach as high as 1940 W m−1 K−1 when graphitization at 3000 °C [29]. The advantage of this approach is ensuring the graphene paper possess extremely high thermal conductivity and high tensile strength, but the disadvantages are also obvious. Manganese ions are not only difficult to remove in the GO gel, but also bring more serious pollution in the wastewater during the preparation of GO. And the high energy consumption caused by the high temperature graphitization process will greatly increase the cost of the product. The third method is fabricating graphene paper by mechanically exfoliated graphene [35,36]. Although mechanical exfoliation is a very inexpensive and environmentally friendly method for preparing graphene, the sheets length of the obtained graphene is generally less than 5 μm. As well known, K is mainly scaled with the size of crystal domain (L, ϕ(L)), thus this small-scale graphene sheet is not conducive to thermal conduction [11,37,38]. Moreover, more edge defects caused by smaller scale lead to a further decrease in K value, which is only 259 W m−1 K−1 before graphitization [35].

Here, we abandoned the above methods and used graphene prepared by non-oxidizing chemical intercalation method as precursor material. As reported in our previous work, such as-prepared graphene has very low oxygen content, very few plane defects, and crystallization in high degree, avoiding the problem caused by GO as mentioned above [39,40]. In addition, without damage of shearing force, our as-prepared graphene sheets maintains a relatively large size compared to the mechanically exfoliated graphene, which greatly reduces boundary scattering. Finally, without graphitization, the thermal conductivity of this graphene laminated film can reach 975 W m−1 K−1 when the thickness of film is 65  μm, and the conductivity can reach 5267 S cm−1. And the tensile strength can reach 20.6Mpa through adjusting the interlaminar force by several reagents to improve the layer compactness. The preparation process is simple and easy to operate, on this account we have already amplified this process and completed the pilot plant test, attained capacity of 3 kg graphene per reactor in 8 h and 800 mm × 100 m of graphene film each roll which was continuously and macroscopically assembled on self-made equipment. Therefore, cheap and common chemical raw materials and low-energy processes ensure low cost of the product. This technology can be used not only for production of graphene paper in thermal management, but also for graphene lithium battery anodes and current collectors, as we have previously reported [40,41].

Section snippets

Preparation of graphene

In a typical synthesis of graphene, as reported in our previous work [39,40], the flake graphite was dispersed in chlorosulfonic acid (CSA) and ultrasonicated for 10 min or stirred for 20 min, forming a stable dispersion which attributed to the protonation of the graphitic material. The dispersion was allowed to settle for 2 h, allowing better intercalation of CSA. Then, hydrogen peroxide was slowly added into the dispersion to react with the intercalated CSA. The generated O2, HCl, and SO3

Results and discussion

It is well known that lattice defects and crystal size directly affect the thermal conductivity of materials, in this case graphene must have fewer defects and larger in-plane sizes. For this purpose, we use flake graphite of 50–100 mesh as starting materials for exfoliation. The as-prepared GS were performed a sequence of characterizations to inspect the quality (Fig. 1). The exfoliation state of GS can be roughly assessed through Electron scanning microscopy (SEM) measurements (Fig. 1a–d),

Conclusions

There are several ways to improve thermal conductivity, such as increasing crystallinity, reducing defects, reducing voids, removing impurities and increasing density. Therefore, most of the literatures choose the method of ultra-high temperature graphitization with high energy consumption. Conversely, we prepared a kind of GS with low defect, high graphitization and large scale, to solve the above problems from the source of materials and avoid heat treatment process with thousands of degrees.

CRediT authorship contribution statement

Tongshun Wu: Material preparation, experimental design, pilot experiment, equipment design, Data curation, Writing - original draft, writing. Youliang Xu: equipment design, equipment production. Haoyu Wang: pilot experiment. Zhonghui Sun: sample testing. Luyi Zou: Writing - review & editing.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work is supported by the Guangdong Province Key Area R&D Program (2019B010933001), National Natural Science Foundation of China ( NSFC), China (No. 21627809, No. 21501169). the National Natural Science Foundation of China (51725204, 51572179, 21771132, 21471106, 21501126).

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