Improved thermal conductivity of epoxy resin by graphene–nickel three-dimensional filler

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Improved thermal conductivity of epoxy resin by graphenenickel three-dimensional filler Yanjie Liu1,2, Jiangyin Lu3, Yanbin Cui 1, 4* 1State

Key Laboratory of Multiphase Complex System, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China 2Zhengzhou

3Key

Institute of Emerging Industrial Technology, Zhengzhou, Henan 450000, China

Laboratory of Oil & Gas Fine Chemicals, Ministry of Education, College of Chemistry and Chemical Engineering, Xinjiang University, Urumqi 830046, China 4Dalian

National Laboratory for Clean Energy, Dalian 116023, China

Improved thermal conductivity of epoxy resin by graphenenickel three-dimensional filler Yanjie Liu1,2, Jiangyin Lu3, Yanbin Cui 1, 4* 1State

Key Laboratory of Multiphase Complex System, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China 2Zhengzhou

3Key

Institute of Emerging Industrial Technology, Zhengzhou, Henan 450000, China

Laboratory of Oil & Gas Fine Chemicals, Ministry of Education, College of Chemistry and Chemical Engineering, Xinjiang University, Urumqi 830046, China 4Dalian

Abstract:

National Laboratory for Clean Energy, Dalian 116023, China

Owing to high thermal conductivity, carbon nanotube and

graphene have been used as nanofillers to improve the thermal conductivity of polymer. However, the agglomeration of nanofillers in polymer inhibits their applications in improving the thermal conductivity of composite. To overcome this problem, graphene was grown on Ni foam by chemical vapor deposition in this work. And graphene-nickel three-dimensional filler was

added into epoxy resin to improve the thermal conductivity of epoxy resin. Ni foam can prevent the agglomeration of graphene in epoxy resin and a thermally conductive network by graphene and Ni foam was formed in epoxy resin. By adding graphenenickel three-dimensional filler into epoxy resin, the thermal conductivity of graphene-nickel/epoxy composite can reach up to 2.6549 W·m−1·k−1, which was 9 times higher than that of raw epoxy resin. Keyword: epoxy resin, graphene,

1.

Ni foam, thermal conductivity, composite

Introduction

Ultrathin, lightweight and multiple functions smart electronics have attracted widespread interest in recent years. Due to the continued miniaturization of electronic devices, the heat dissipation of electronic devices is crucial to maintain the devise worked with high performance and long lifespan

[1, 2].

Smart

electronic devices will generate excess heat when they work. If the generated heat could not be dissipated effectively and quickly, the performance and reliability of electronic devices will be reduced

[3, 4].

For example, a small increase of the operating

temperature (10-15 oC) can result in a two-fold reduction in the lifespan and reliability of an electronic devices [5, 6]. High thermal conductive material could reduce the local hot spot temperature

in smart electronics quickly and the operating temperature of electronics can be maintained at a desired level. Because of their corrosion resistance, lightweight, ease of processing and low cost, polymers are wildly used in electronics. Epoxy resins are considered as one of the most important classes of thermosetting polymers and extensive use in various fields of electronic packaging and substrate materials. However, low thermal conductivity of polymer limits its potential used in electronic. The thermal conductivity of polymer is very low and thermal conductivity of most polymer are lower than 0.5 W·m−1·k−1 at 25 °C

[7-10].

Due to low thermal conductivity,

polymers cannot dissipate the generated heat of electronics effectively. Therefore, it is very important to improve the thermal conductivity of polymers. Various strategies were used to enhance the thermal conductivity of polymers. Adding high thermal conductive fillers into polymers is one of the most efficient ways to improve the thermal conductivity of polymer. Many kinds of high thermal conductive fillers have been added into polymer to improve the thermal conductivity of polymers [1113].

The high thermal conductive fillers can be classified into

metallic, ceramic and carbonous, etc

[14-17].

Recently, carbon

nanotube (CNT) and graphene have gained considerable interest

to improve thermal conductivity of polymer due to their superior thermal conductivity [18-20]. The thermal conductivity of CNT can reach up to ~3000 W·m−1·k−1 and it is ~5000 W·m−1·k−1 for graphene

[21].

However, high thermal conductive fillers have

limited impact on the improvement of thermal conductivity of polymer composites. The type of fillers is not the only factor which influence the thermal conductivity of polymer composites [22, 23].

Other issues can also play an important role in the thermal

conductivity of polymer composites, such as the dispersion of fillers, the interfacial thermal resistance between polymer and fillers, the structural defects of fillers, etc [24, 25]. In consequence, the thermal conductivity of polymer composites is lower than expected. If homogeneous dispersion of fillers in polymer can be achieved, the thermal conductivity of polymer composite can be improved significantly. Numerous attempts have been made to improve filler dispersion in polymer matrices to fully utilize the potentials of high thermal conductive filler

[26, 27].

On the other

hand, improving the interfacial interaction between polymer and fillers and constructing a three-dimensional network of fillers in polymeric are also favorable to improve the thermal conductivity of polymer composites [28]. In fact, no graphene network formed in polymer is one critical issue that has prevented graphene filler

to reach their full potential to improve the thermal conductivity of polymer. In this paper, graphene was deposited on Ni foam by chemical vapor deposition (CVD) method. Then, Ni foam with graphene deposition was embedded into epoxy resin. The Ni foam promotes the formation of three-dimensional graphene network fillers in epoxy resin, which also provides a skeleton to support graphene. Besides, Ni foam is in favor for inhibition of the agglomeration of graphene in epoxy as graphene is deposited on Ni foam. By adding graphene-nickel three-dimensional (3D) filler, the thermal conductivity of graphene-nickel/epoxy composite is 2.6549 W·m−1·k−1, which is 9 times higher than that of raw epoxy resin G/Ni/epoxy film has great application prospects in heat dissipation of electronic devices (such as, mobile phone, computer and chips), power batteries, LED, etc. 2. Experimental Firstly, commercial Ni foam (K. J. GROUP, purity >99.9%, surface density: 350 g/m2) was cleaned with alcohol, hydrochloric acid and deionized water, respectively. The clean Ni foam was dried at 100 °C and cut into 4×4 cm2 to deposit graphene. Then, graphene was grown on Ni foam as following procedure: (1) Graphene was grown in a one-zone quartz tube (2-inch diameter) furnace (Hefei kejing materials technology Co., Ltd). Ni foam was placed in the middle of quartz tube and the quartz tube was pumped down to 5 mTorr to remove any ambient gas. (2) The furnace was heated up to 1000 °C at 10 oC/min under 500 mL·min-1 Ar flow. (3) The Ni foam was kept at 1000 °C for 30 min in mixed gas (H2/Ar, 100/500 mL·min-1) to reduce the oxide layer of Ni foam.

(4) The Ni foam was exposed to CH4 (100 mL·min-1 in H2/Ar, 100/500 mL·min-1) at 1000 °C for a certain time. (5) CH4 gas flow was turned off and the furnace was cooled down to room temperature with the H2/Ar (100/500 mL·min-1) continuing. The obtained graphene/nickel filler was transferred to a polyethylene mold and 25 grams of epoxy was added into the mold. After curing at 40 °C for 6 h, the sample was cooled down to room temperature. Then, the mold was peeled off and the graphene-Ni/epoxy composite was polished for use. Fig. 1 shows the schematic illustrates for the prepare process of graphene-Ni/composite.

Fig. 1. Schematic illustration for the preparation of graphene-Ni/epoxy resin composite.

The morphology of graphene/Ni foam were characterized by a field-emission scanning electron microscope (FE-SEM, JEOL JSM-7610F, Japan) at the accelerating voltage of 10 kV. Raman spectra of the graphene growth on Ni foam were implemented by a LabRAM HR800 at 514.5 nm (Horiba Jobin Yvon, France). Xray diffraction (XRD) was carried out using a X'Pert PRO MPD (PANalytical, the Netherland) with Cu Kα radiation (λ=1.54059° A) from 22° to 35° with a scanning speed of 5°/min. The thermal conductive properties of the sample were measured with a transient hot disk thermal analyzer (HESON, HS-DR-5, China). The accuracy of thermal conductivity instrument is 0.005 W·m−1·k−1.To

eliminate the

effect of surface roughness and dust, the sample surface was carefully polished and then cleaned with ethanol. The measurements were taken by putting the sensor (3 mm diameter)

between two samples (40×40×2 mm3). The sensor supplied a heat pulse of 0.03 W for 20 s to specimen and the temperature change of the sample at five different position was measured. All the thermos-conductivity date of the sample was the average value of three measurements. Then, the thermal conductivity was obtained by fitting the data based on Gustavsson equation

[10].

To

demonstrate the heat-transfer rate of graphene-nickel/epoxy composite in real electronics, graphene-nickel/epoxy was used as a heat-dissipation film in a smartphone and the infrared thermal imaging was performed with XIN TEST (HT-02, China).

3. Results and discussion Fig. 2 are SEM images of graphene/Ni foam which graphene was grown on Ni foam at various growth time. As shown in Fig. 2, Ni foam is wrapped by graphene after CVD process. After 5 min of graphene growth, only partial struts of Ni foam were coated by graphene (Fig. 2a). As the graphene growth time increased to 45 min, all the skeleton of Ni foam was wrapped by graphene (Fig. 2c). Fig. 2b shows that the graphene films are ultrathin and transparent. The graphene films in Fig. 2d possess straight ridges and complicated folding structures due to thicker graphene film grown on Ni foam.

Fig. 2. SEM images of graphene growth on Ni foam with 5 min (a-b) and 45 min (c-d) growth time.

XRD patterns of graphene growth on Ni foam with different reaction time are shown in Fig. 3. The main XRD pattern of graphene are similar to those of graphite due to their intrinsic nature

[29].

As shown in Fig. 3, the peak of graphene growth at

different reaction times was close to that of graphite (26.62°). In addition, the [002] peaks of graphene are up-shifted gradually with the increasing reaction time. When the growth time increased from 5 to 45 min, the [002] peaks of graphene shifted from 26.33° to 26.60°. According to Bragg’s law (nλ= 2dsinθ),

we calculated the graphene interlayer spacing and it was decreased with the increasing of reaction time. For example, the d002 spacing of graphene growth on Ni foam with 5 min (2θ=26.33°) is 3.382 Å and the d002 spacing of graphene growth on Ni foam with 45 min (2θ=26.60°) is 3.348 Å. Soin et al. also found that graphene interlayer spacing decreased with the increasing of reaction time

[30].

The reason for the decrease of

interlayer spacing is due to the increasing of graphene layers.

Fig. 3. XRD patterns of graphene growth on Ni foam at different reaction times.

Raman spectra is the most useful tool to characterize carbon materials. Typical Raman spectra of graphene is shown in Fig. 4.

Two obvious peaks can be observed in the Raman spectra (1580 cm−1 and 2670 cm−1), which typically correspond to the characteristic G and 2D modes of graphene

[31].

The G peak

corresponds to the optical mode vibration of two neighboring carbon atoms in graphene layer and the 2D peak is due to a double-resonance process of two phonons with opposite wavevectors[32]. It has been reported that the intensity ratio of 2D and G peaks (I2D/IG) decreases as the graphene layer number increases[32, 33]. In Fig. 4, the I2D/IG ratio for the samples decrease gradually from 0.49 to 0.37 as the growth time increases from 5 min to 45 min, indicating that the layer number of graphene increases with the growth time increases [34].

Fig. 4. Raman spectra of the samples grown at different reaction times.

Fig. 5 shows the thermal conductivity of G/Ni/epoxy with different growth time of graphene on Ni foam.

The pure epoxy’s thermal

conductivity was 0.2617 W·m−1·k−1. The thermal conductivity of

G/Ni/epoxy resin was increased with the increasing of reaction time. In the initial growth stage, only a little graphene was formed on Ni foam and thermal conductivity network was not formed. As graphene growth time prolonged, a layer of graphene was deposited on Ni foam and a thermal conductive network of graphene was formed. As a result, the thermal conductivity of composite G/Ni/epoxy is increased rapidly. For instance, the thermal conductivity of G/Ni/epoxy resin was increased to 1.6012 W·m−1·k−1 when the graphene growth time was 5 min. The thermal conductivity of G/Ni/epoxy was further increased when the deposition time increased from 5 min to 45 min. When the graphene deposition time was 45 min, the thermal conductivity of G/Ni/epoxy resin was 2.6549 W·m−1·k−1, which was 914.5% higher than that of pure epoxy. With prolonged time, more graphene was deposited on Ni foam and graphene 3D network was formed on Ni foam. It was proved that 3D network of filler was beneficial to improve thermal conductivity of polymer [35-37]. The thermal conductivity of epoxy resin, Ni foam, Ni foam/epoxy resin and G/Ni/epoxy was compared in Fig. 6 are significantly improved with increased graphene. The thermal conductivity of epoxy resin is 0.2617 W·m−1·k−1 and the thermal conductivity of Ni foam is 1.1315 W·m−1·k−1. By adding Ni foam

into epoxy, the thermal conductivity of Ni foam/epoxy resin is 0.6135 W·m−1·k−1. But the thermal conductivity of epoxy resin is much lower than that of Ni foam, the thermal conductivity of Ni foam/epoxy resin composite is lower than that of Ni foam.

As Ni foam is served as thermal conduction network in epoxy resin, the thermal conductivity of Ni foam/epoxy resin composite increases from 0.2617 W·m−1·k−1 to 0.6135 W·m−1·k−1. By further deposition graphene on Ni foam and adding graphene-Ni foam filler into epoxy, the thermal conductivity of G/Ni/epoxy improved significantly, which is due to the high thermal conductivity of graphene (5000 W·m−1·k−1). The thermal conductivity of G/Ni/epoxy prepared in our work was also compared with other carbon materials/polymer composites reported in literatures (Table 2). Singh et. al. prepared multiwalled carbon nanotube (MWCNT)-epoxy by mixing method

[38].

The thermal conductivity of epoxy with 0.5 wt%

MWCNTs is 72.5% higher than that of pure epoxy. Chang et. al. added MWCNTs/graphene nanoplatelets (GNPs) hybrid fillers into epoxy and the thermal conductivity of epoxy is enhanced by 287% at 1.525 wt% MWCNTs and 4.575 wt% GNPs [39]. Tien et al. used graphene flakes to improve the thermal conductivity of epoxy. The thermal conductivity of epoxy was enhanced by 350% at 12 wt% graphene flakes

[40].

The thermal conductivity of

G/Ni/epoxy composite in our work was improved by 914.5% at 10.1 wt% graphene. It is clearly seen that G/Ni/epoxy composites prepared by adding graphene-nickel three-dimensional filler into epoxy resin have much higher thermal conductivity than other polymer composites at similar filler concentration.

Fig. 5. Thermal conductivity of G/Ni/epoxy resin with different growth time of graphene on Ni foam.

Fig. 6. Thermal conductivity of epoxy resin, Ni foam, Ni foam/epoxy resin and G/Ni/epoxy resin.

Table 2. Thermal conductivity comparison of different composites.

Polymer

Filler

Filler

Thermal

Thermal

addition

conductivity

conductivity

(wt%)

(W·m−1·k−1)

enhancement (%)

Ref.

Epoxy

MWCNTs

0.5

0.88

72.5

[38]

Epoxy

MWCNTs/GNPs

1.525/4.575

0.2–0.89

287

[39]

Epoxy

Graphene flakes

12

0.2–0.72

350

[40]

Polystyrene

Graphene

10

0.244

66

[41]

20

1.13

370

[42] [43]

Polycarbonate

Graphene platelets

Polydimethylsiloxane

Graphene foam

0.7

0.56

195

Epoxy

Graphene

10.1

2.6549

914.5

This work

The thermal conductivity of composite is determined by several factors, including filler type, filler dispersion and thermal conductivity path between fillers

[23].

In our work, an intact 3D

thermal conductivity network of graphene is formed in composite. The thermal conductivity mechanism of G/Ni/epoxy is illustrated in Fig. 7. A thermal conduction network composed by graphene and Ni foam was formed in epoxy. The heat generated by a heat source can be transmitted instantly by the heat conduction network.

Fig. 7. Illustration of heat dissipation in G/Ni/epoxy composite.

Fig 8. (a) Back of a smartphone (taken by a camera), (b) smartphone at ready mode, (c) smartphone without G/Ni/epoxy resin and (d) smartphone with G/Ni/epoxy resin heat-dissipation film.

To demonstrate the real heat dissipation performance, G/Ni/epoxy is used as the heat-dissipation film in a smartphone (Fig. 8). The smartphone was played a large mobile phone game for 167 min to generate certain energy. Then, the temperature of smartphone back was measured. The temperature of smartphone without G/Ni/epoxy heat-dissipation film was 43.4 °C. The temperature of smartphone with G/Ni/epoxy heat-dissipation film was 15.2 °C lower than that of smartphone without heatdissipation film after working for 167 min in the same condition. The average temperature of smartphone with heat-dissipation film was lower than that of smartphone without heat-dissipation film by ~6.9 °C. The temperature distribution of smartphone with heat-dissipation film is more uniform than that of smartphone without heat-dissipation film. The relatively low temperature of smartphone with heat-dissipation film indicated that G/Ni/epoxy resin has a good thermal transfer rate, thereby homogenizing the heat generated by smartphone.

Fig. 9. Mechanical properties of graphene/Ni foam.

In addition to high thermal conductivity, mechanical performance is another important property for the composite. Fig. 9 is the mechanical properties of graphene/Ni foam. As indicated in Fig. 9, the ultimate stress of Ni foams deposited with graphene increased from 5.91 MPa to 9.12 MPa with the growth time of graphene increasing from 5 min to 45 min, indicating that the mechanical strength of Ni foam can be increased by deposited graphene. As the layer number of graphene was increased with the growth time increases. The increased stress of graphene/Ni foam may due to the graphene layer number increasing on Ni foam. Besides, the increased stress of graphene/Ni foam may also due to the formation of 3D graphene network on Ni form, as the cross-linked graphene network can effectively enhance the stress of Ni foam. On the other hand, the strain of graphene/Ni foam

decreases with the growth time increased. Metal fatigue is the main reason for the strain decrease of Ni foam as Ni foam was treated at high temperatures (1000 °C) for long durations [44]. 4. Conclusion In present work, graphene was deposited on Ni foam by CVD method and graphene/Ni foam was added into epoxy as hybrid fillers to improve the thermal conductivity of epoxy resin. Ni foam was used as skeleton for the formation of thermal conductivity network and Ni foam can also prevent the agglomeration of graphene in epoxy resin. Due to the introduction of Ni foam, a complete 3D heat conduction network was constructed by graphene and Ni foam in the epoxy resin. By adding graphene-nickel 3D filler into epoxy resin, the thermal conductivity of graphene-nickel/epoxy composite can reach up to 2.6549 W·m−1·k−1 with 10.1 wt% of graphene, which was 9 times higher than that of raw epoxy resin. Compared with the data reported in literatures, the G/Ni/epoxy prepared in our work have much higher thermal conductivity than other polymer composites at similar filler concentration. Meanwhile, the G/Ni/epoxy have good real heat dissipation performance for smartphone. It implies that G/Ni/epoxy have a promising application in the field of heat

dissipation for electronic devices and other devices which also need to transmit the generated heat. Acknowledgements This work was supported by the Fund of State Key Laboratory of Multiphase Complex Systems (No. MPCS-2019-A-03) and Center for Mesoscience (No. COM2016A003), Institute of Process Engineering, and DNL Cooperation Fund (No. DNL180304), Chinese Academy of Sciences. References: [1]. Bai, Q., et al., Dispersion and network formation of graphene platelets in polystyrene composites and the resultant conductive properties. Composites Part A: Applied Science and Manufacturing, 2017. 96(2): p. 89-98. [2]. Pongsa, U. and A. Somwangthanaroj, Effective thermal conductivity of 3,5-diaminobenzoylfunctionalized multiwalled carbon nanotubes/epoxy composites. Journal of Applied Polymer Science, 2013. 130(5): p. 3184-3196. [3]. Tang, B., et al., Application of graphene as filler to improve thermal transport property of epoxy resin for thermal interface materials. International Journal of Heat and Mass Transfer, 2015. 85(2): p. 420-429. [4]. Zhou, Y., F. Liu and H. Wang, Novel organic-inorganic composites with high thermal conductivity for electronic packaging applications: A key issue review. Polymer Composites, 2017. 38(4): p. 803-813. [5]. Im, H. and J. Kim, Thermal conductivity of a graphene oxide–carbon nanotube hybrid/epoxy composite. Carbon, 2012. 50(15): p. 5429-5440. [6]. Sim, L.C., et al., Thermal characterization of Al2O3 and ZnO reinforced silicone rubber as thermal pads for heat dissipation purposes. Thermochimica Acta, 2005. 430(1-2): p. 155-165. [7]. Singh, A.K., et al., Study on metal decorated oxidized multiwalled carbon nanotube (MWCNT) epoxy adhesive for thermal conductivity applications. Journal of Materials Science: Materials in Electronics, 2017. 28(12): p. 8908-8920. [8]. Han, Z. and A. Fina, Thermal conductivity of carbon nanotubes and their polymer nanocomposites: A review. Progress in Polymer Science, 2011. 36(7): p. 914-944. [9]. Zhang, W., et al., Largely enhanced thermal conductivity of poly(vinylidene fluoride)/carbon nanotube composites achieved by adding graphene oxide. Carbon, 2015. 90(4): p. 242-254. [10]. Zhang, F., et al., Improved thermal conductivity of polycarbonate composites filled with hybrid exfoliated graphite/multi-walled carbon nanotube fillers. Journal of Thermal Analysis and Calorimetry, 2016. 123(1): p. 431-437. [11]. Huang, H., et al., Aligned Carbon Nanotube Composite Films for Thermal Management. Advanced Materials, 2005. 17(13): p. 1652-1656. [12]. Aravind, S.S.J. and S. Ramaprabhu, Graphene–multiwalled carbon nanotube-based nanofluids for improved heat dissipation. RSC Advances, 2013. 3(13): p. 4199. [13]. THOSTENSON, E., C. LI and T. CHOU, Nanocomposites in context. Composites Science and Technology, 2005. 65(3-4): p. 491-516.

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Highlights   

A complete 3D graphene thermal conductivity network was constructed in epoxy resin. The thermal conductivity of graphene/Ni/epoxy composite is 9 times higher than that of raw epoxy resin. The graphene/Ni/epoxy have a promising application in heat dissipation for electronic and

other devices.

Yanbin Cui: Conceptualization, Methodology, Software. Yanjie Liu: Data curation, Writing- Original draft preparation. Jiangyin Lu: Visualization, Investigation. Yanbin Cui: Supervision. Yanbin Cui: Software, Validation. Yanjie Liu: Writing- Reviewing and Editing,