Enhanced the thermal conductivity of flexible copper foil by introducing graphene

Journal Pre-proof Enhanced the thermal conductivity of flexible copper foil by introducing graphene

Jiao Li, Ping Zhang, Hong He, Bo Shi PII:

S0264-1275(19)30811-1

DOI:

https://doi.org/10.1016/j.matdes.2019.108373

Reference:

JMADE 108373

To appear in:

Materials & Design

Received date:

24 September 2019

Revised date:

12 November 2019

Accepted date:

18 November 2019

Please cite this article as: J. Li, P. Zhang, H. He, et al., Enhanced the thermal conductivity of flexible copper foil by introducing graphene, Materials & Design(2019), https://doi.org/ 10.1016/j.matdes.2019.108373

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© 2019 Published by Elsevier.

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Enhanced the thermal conductivity of flexible copper foil by introducing graphene Jiao Li1, Ping Zhang1*, Hong He1, Bo Shi2* 1

School of Mechanical and Electrical Engineering, Guilin University of Electronic

Technology, No. 1 Jinji Road, Guilin, Guangxi 541004, China 2

College of Energy and Power Engineering, Nanjing University of Aeronautics and

Astronautics, Jiangsu 210016, China

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*corresponding author, E-mail: [email protected]; [email protected]

Copper-graphene (Cu-rGO) film was prepared by combination of electrophoretic deposition and vacuum hot pressing.

The introduction of graphene makes the Cu-rGO film have outstanding thermal

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Highlights

Cu-rGO film exhibits excellent thermal management capabilities, especially in

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conductivity and a low coefficient of thermal expansion.

high temperature environments.

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Graphical abstract

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Abstract: Owing to the miniaturization, portability and wearable of electronic devices, the demand for advanced thermal management materials has increased significantly, especially for highly thermally conductive materials. In this paper, a simple and effective strategy for synthesizing high thermal conductivity copper-graphene (Cu-rGO) was proposed. The microstructure, thermal properties and thermal management capabilities of the composite film were studied. It was found that

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the obtained Cu-rGO film exhibits excellent flexibility and high thermal conductivity

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(637.7 W m-1 K-1). It indicates that the introduction of graphene effectively enhances

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the thermal conductivity of Cu matrix. Besides, the experimental results show that the Cu-rGO film has a lower coefficient of thermal expansion (CTE) and excellent heat

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transfer properties. This work provides important guidance for the preparation of high

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in the future.

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thermal conductivity composites that meet the growing heat dissipation requirements

Keywords: Graphene/copper composite film; Thermal conductivity; Electrophoretic

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deposition; vacuum hot pressing

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1 Introduction Nowadays, with the rapid development of microelectronics technology, the computing speed of chips is getting faster and faster, and the packaging density of integrated circuits is getting larger and larger [1-4]. Extreme heat generated by electronic components during high-frequency operation may cause a sharp rise in operating temperature, which puts higher demands on the heat dissipation

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performance of the system. Excessive operating temperatures have a negative impact on the reliability, stability and longevity of electronic equipment [3, 5]. Therefore, the

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heat dissipation problem of electronic devices, especially in high-power density

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instruments, has become one of the technical bottlenecks of the electronic information

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industry. As the earliest discovered and most widely used material by humans, copper

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(Cu) has attracted much attention since its discovery. Due to the excellent advantages of Cu and its alloys, such as good thermal conductivity and electrical conductivity,

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low price and easy processing, they are widely used in electrical, aerospace,

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machinery manufacturing [6, 7]. However, Cu has a relatively large density (8.9 g/cm3) and coefficient of thermal expansion (CTE) (0.17 ppm/K) as well as imperfect thermal conductivity, which limits its application in certain fields [8, 9]. On the other hand, the pure Cu material is relatively soft and the surface is easily scratched and oxidized, which greatly reduces the heat dissipation effect of the Cu. Thus, traditional metal materials cannot meet the heat dissipation requirements of modern thermal management. The development of advanced thermal management materials will be a new direction. Two-dimensional nanomaterials are considered to be promising materials for thermal management due to their outstanding thermal conductivity [10-12]. For example, graphene and its derivatives (such as graphene oxide (GO) and reduced

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graphene oxide (rGO)) have attracted great research interest due to their unique structure and excellent electrical, mechanical and thermal properties [13-15]. Graphene has a high intrinsic thermal conductivity, especially for single-layer graphene, which has a thermal conductivity of up to 5000 W m-1 K-1 [16]. Although single-layer graphene has excellent thermal conductivity, it is difficult to be directly used as a heat-dissipating material. Therefore, graphene is usually presented in the

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form of a composite in practical applications. Chu et al. obtained graphene nanosheets

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composites with in-plane thermal conductivity up to 458 W m-1 K-1 by vacuum filtration and spark plasma sintering (SPS) [17]. Goli et al. used chemical vapor

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deposition (CVD) to grow a single atomic plane of graphene on both sides of 9 μm

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thick Cu films with a thermal conductivity of 369.5 W m-1 K-1 at room temperature

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[18]. Although the SPS method has the advantages of fast heating rate and short

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sintering time, but it is excessively dependent on the graphite mold, so that the actual heating temperature cannot be controlled [19]. In addition, the CVD method has the

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disadvantages of complicated process and high cost [20, 21]. In recent decades, with the development of composite materials and nanomaterials, electrophoretic deposition (EPD) method have drawn great attentions [22-25]. Research indicate that EPD is a versatile technology technique that can be applied to any stable suspension [26] The EPD process is based on the migration of charged particles and subsequent deposition in a stable suspension, which is induced by the application of an electric field between the two electrodes [27, 28]. Compared with other advanced forming technologies, EPD has the advantages of good sample uniformity, thickness control, simple operation process, easy to use, and high cost-effectiveness. It is well known that GO is the most commonly used precursor for the

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preparation of graphene materials. GO is obtained by using the modified Hummer's method to treat graphite powder, followed by dispersion and flaking in water or a suitable organic solvent. Briefly, GO can be considered to be composed of a single layer of graphene, and the base and edges of the graphene are decorated with oxygen functional groups. Oxygen-containing functional groups in GO cause significant structural defects compared to the original graphene, resulting in the deterioration of

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performance. However, these functional groups can be used as chemical modification

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or functionalization sites for GO, and various active substances can be immobilized

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by covalent or non-covalent bonds, which provides a new idea for material design [29]. Furthermore, recent studies have shown that the presence of Cu substrate is

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beneficial for the reduction of GO [30, 31]. In this work, we report an effective

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strategy for preparing high thermal conductivity Cu-rGO films through a two-step

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process involving EPD and subsequent vacuum hot pressing. The microstructure of the composite film was analyzed by SEM, XRD and XPS. The thermal property

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measurement results indicate that the Cu-rGO film exhibits excellent thermal conductivity (637.7 W m-1 K-1) and relatively low CET. More importantly, the excellent thermal management capability of the Cu-rGO film was further confirmed by infrared thermal imaging.

2 Experimental 2.1 Preparation of suspension Graphite oxide was prepared from graphite through the modified Hummers method, as previously reported by our group [32]. Then the graphite oxide was dispersed in deionized water via an ultrasonic to obtain GO well-distributed

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suspension, followed by centrifugation at 1000 rpm for 10 min to remove some unexfoliated materials.

2.2 Synthesis of Cu-rGO film The composite film was prepared by a combination of EPD and vacuum hot pressing. First, Cu foil (99.99%) was cut to the desired size and subjected to high roughness pretreatment. Then, two Cu foils of the same size were immersed in a GO

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suspension (3 mg ml-1), one as a positive electrode and as a negative electrode. Adjust

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the distance (2 cm) between the electrodes and apply a DC voltage of 30 V to operate.

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Different composite materials can be obtained by changing the deposition parameters

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[23]. After EPD, the EPD-GO film was obtained at the anode, which was dried in a vacuum oven at 50 °C for 12 hours. Subsequently, the EPD-GO film was subjected to

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vacuum hot pressing to obtain a Cu-rGO film. The detailed preparation process of the

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composite material is shown in Fig. 1.

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2.3 Characterization

Optical microscope (OLS4100 3D) was used to observe the original Cu foil surface. The microstructures of the original Cu foil and film composites were examined by a field emission scanning microscopy (FE-SEM, FEI Quanta 450FEG). The structure was characterized by X-ray diffraction (XRD, Bruker D8 Advance). X-ray photoelectron spectroscopy (XPS) measurements were carried out using a Thermo ESCALAB 250Xi spectrometer. The CTE of the samples in the temperature range from room temperature to 300 °C was measured by using a TMA Q400 thermomechanical analyzer under a preloaded force of 0.05 N with a 10 ℃ min-1 heating rate. The sample was cut to the appropriate size (45 mm in diameter) for the

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analysis of heat transfer characteristics. As presented in Fig. 6a, the sample was placed on the same heating pad and heated at a power of 0.08 W. At the same time, we used a glass cover to protect the experimental device in order to avoid the influence of other factors. The time-dependent temperature of the samples was recorded by an infrared thermograph (FLIR E50). The thermal conductivity of the sample at room

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temperature was measured using Hot Disk (2500S, Sweden).

Fig. 1. Schematic depicts the preparation process of composite film.

3 Results and discussion 3.1 Structural characterization

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Fig. 2. Optical and scanning electron microscopy of Cu and EPD-GO composites. Optical image of the surface of Cu film (a) and pretreated Cu film (b). SEM image of the surface of Cu film (c) and EPD-GO (d). EDS scanning of the surface of EPD-GO composites, and spectral results of elemental Cu (e) and oxygen (f).

The morphology and microstructure of the original Cu foil and EPD-GO composite film were characterized. Fig. 2a and Fig. 2b are optical micrographs of original and pretreated Cu foil, respectively, indicating that the pretreatment can effectively remove oxides on the surface of the Cu foil. The surface of the pretreated Cu foil has a certain roughness (Fig. 2c), which is beneficial to increase the contact surface between Cu foil and deposited layer, thereby enhancing the interfacial

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bonding ability between them [33, 34]. After the EPD process, the EPD-GO film was synthesized. As shown in Fig. 2d, it can be seen clear that the deposited layer has obvious irregular free-distributed wrinkles. The distribution of elements in the EPD-GO deposit was further confirmed via energy-dispersive spectrometry (EDS) (Fig. 2e, f), and the presence of Cu atoms indicates that GO is successfully reduced [31, 35]. This phenomenon is explained by the principle that EPD method removes

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oxygen-containing groups and forms reactive groups, while Cu anodic electrolysis of

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Cu ions contributes to the reduction of GO. However, the presence of oxygen atoms

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(Fig. 2f) suggests that the reduction of GO is incomplete, that is, some oxygen-containing functional groups are still present in the EPD-GO film.

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The Cu-rGO film obtained after vacuum hot pressing at 900 ℃ is take as an

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example, as shown in Fig. 3a. It can be clearly seen that Cu-rGO film has a silver-gray metallic luster and a very considerable flexibility. A large number of regular Cu

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particles are distributed in graphene layer, and their average diameter is about 1 μm

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(Fig. 3b, Fig. 2S). This indicates that the EPD process successfully embeds some Cu particles into the graphene layer and accumulates and reduces Cu ions with the aid of vacuum hot pressing [36]. It can be seen that the oxygen atom concentration is reduced by 85% after vacuum hot pressing, indicating that the oxygen-containing functional groups in the graphite oxide have been substantially removed (Fig. S1). At the same time, the concentration of Cu element distributed between the surface layer of graphene or between the layers is as high as 44.5% (Table 1), which strongly promotes the bonding of graphene layer and the Cu matrix and provides an effective thermal conduction pathway. As shown in Fig. 3c, the experimentally obtained deposited layer has a thickness of 2 μm. The composition of was characterized by

Journal Pre-proof XRD and the results are depicted in Fig. 3d. The plane peak appears at 2θ=26.2, which can be indexed into C(002) of graphene [37, 38]. It can be calculated that the interlayer space layer spacing of graphene families are 0.3393 nm (Table S1). Three high intensity diffraction peaks appear at 43.5, 50.7, and 74.7, which correspond to the (111), (200), and (220) planes of the face centered cubic Cu. The results demonstrate that Cu particles are not oxidized and exist in the form of Cu0 in the

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Cu-rGO film.

Cu 44.94 44.53

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EPD-GO Cu-rGO

C 40.56 53.34

Atomic concentration (wt %) O 14.51 2.13

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Sample

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Table 1: Atomic concentration of C, O, Cu atoms in the surface of EPD-GO, Cu-rGO film.

Fig. 3. (a) Optical images of Cu-rGO film. SEM images of the surface (b) and side (c) of the Cu-rGO film. (d) XRD patterns of Cu-rGO film.

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Fig. 4 (a) XPS spectrum of the GO, EPD-GO, and Cu-rGO films. (b-g) High-resolution XPS spectra of C1s, O1s and Cu2p.

XPS was used to analyze the chemical composition and bonding morphology of the sample. As shown in XPS survey spectra in Fig. 4a, all samples have two remarkable peaks, corresponding to the spectra of C 1s and O 1s, while EPD-GO and Cu-rGO films show Cu 2p peaks. From GO to EPD-GO to Cu-rGO film, the C atomic concentration increased from 61.09% to 94.67%, while the O atomic concentration ranged from 38.91% to 4.65% (Table S2). Fig. 4b-g show the evolution of the

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high-resolution XPS spectra of C 1s, O1s, and Cu 2p, respectively. The evolution of C 1 on the surface of GO, EPD-GO and Cu-rGO is shown in Table S3. Fig. 4b displays the broad C 1s spectrum of GO, which can be resolved into four peaks. one at binding energy 284.8 eV which features the C-C or C-H bonds in the graphene skeleton, other three at 286.7 eV, 287.1 eV, and 288.4 eV which are attributed to C-O, C=O and O-C=O bonds at the defect sites of graphene [38, 39]. Comparing GO and EPD

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(Fig.4c), it is not difficult to find that the integrated relative peak area of C1s of the

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oxygen-containing groups decreased from 55.4% to 47.7%, which clarified indicates

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that EPD technology has the effect of removing some oxygen -containing groups. After vacuum hot pressing, the relative peak area of the C-C or C-H bonds increased

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to 81.7%, and peak area of C1s of the C-O, C=O and O-C=O groups further decreased

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to 18.26%, indicating that almost all of the oxygen -containing groups were removed.

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As shown in Fig.4d, there is a slight change in chemical shift, and the binding energies of C-O, C=O and O-C=O are 285.8 eV,287.0 eV, and 288.9 eV, respectively

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[40]. Fig. 4e shows a high-resolution O 1s XPS spectrum with a binding energy of 531.2 eV. It is clear that the relative peak area is significantly attenuated from GO to EPD-GO to Cu-rGO. These results mighty suggest that the graphene defects of the Cu-rGO film are effectively repaired [41]. The XPS spectrum of GO-EPD film shows Cu 2p3/2 and Cu 2p1/2 signals of Cu2+ appear at 931.7 and 951.6 eV. From the XPS broad spectrum of Cu-rGO film, it is known that a peak at 932.7 is assigned to 2p3/2 signal, and a peak at 952.6 is assigned to 2p1/2 signal, all of which are derived from Cu0. This is consistent with the XRD results in the previous section.

3.2 Thermal performance

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Fig. 5. Thermal properties of Cu, EPD-GO and Cu-rGO film materials at room temperature. (a)

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Times-dependent thermal conductivity of Cu-rGO film composites. (b) Thermal conductivity

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enhancement ratio of Cu-rGO film material. (c) The thermal conductivity of the Cu foil, the EPD-GO and Cu-rGO film, respectively. (d) Temperature-dependent linear change (dL/L0) of pure Cu and Cu-rGO composite.

Fig. 5a shows the thermal conductivity of the Cu-rGO as a function of deposition time. The thermal conductivity of the Cu-rGO film increases linearly with the increase of deposition time, indicating that most of the introduced graphene hybrids contribute to the formation of thermal conduction pathways in the Cu-rGO composite. It is not difficult to find that the thermal conductivity has a large growth rate at the deposition time of 30 s. The increased layer-by-layer Cu-rGO with the deposition time produces an outstanding heat conduction pathways. However, when deposition time is 40 s, the

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thermal conductivity of the Cu-rGO film is only slightly increased than before. Because a certain concentration of GO suspension has been largely completely reduced to graphene as the deposition time increases. A parameter η was proposed to further illustrate the extent of the improvement, which is defined as η=

𝐾 − 𝐾0 𝐾0

(1)

where 𝐾 and 𝐾0 represent the thermal conductivity of the composites and matrix

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material, respectively. Fig. 5b shows the thermal conductivity enhancement ratio of

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Cu-rGO film. To better explore the thermal conductivity of the composite film, we

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studied the composite film that was synthesized in 40 s of deposition time (Fig. 5c).

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The thermal conductivity of the EPD-GO and Cu-rGO films were 449.2 W m-1 K-1 and 637.7 W m-1 K-1, respectively. Obviously, the thermal conductivity of the Cu-rGO

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film was increased by 79.6%, which is higher than the other reported Cu-based

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composites [18, 42]. These results indicate that graphene was successfully introduced and improved the thermal conductivity of Cu.

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Dimensional stability plays a very important role in the thermal management design of materials [43]. To examine the dimensional reliability of the Cu-rGO, the CTE of the sample is determined in the temperature range from room temperature to 300 °C. Fig. 5d shows the temperature-dependent linear change of pure Cu and Cu-rGO composites along the through-plane directions.It can be seen that all the curves present an increased linear change with increasing temperature, and the CTE values of Cu-rGO is always lower than that of Cu, which is decreased by 28.1%. The results show that the presence of graphene can effectively retard the thermal expansion of Cu matrix upon heating, so that the linear CTE of the Cu-rGO (0.1464 ppm/K) film is lower than that of Cu matrix.

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3.3 Thermal management capability

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Fig. 6. (a) Experimental setup for thermal infrared imaging. (b) Optical images of Cu, EPD-GO, and Cu-rGO film. (c) Surface temperature variation with heating time of Cu, EPD-GO, and Cu-rGO film from the room temperature. (d) Corresponding infrared thermal images of heating process.

The surface temperature of the composite during heating was recorded by infrared thermography to demonstrate the thermal management properties of the composite. As shown in Fig. 6a, Cu, EPD-GO and Cu-rGO films of the same size (Fig. 6b) were arranged on identical equipment It can be seen that the surface temperature of Cu-rGO film increases with time at a higher rate (Fig 6c). It is worth noting that the surface temperature of the Cu-rGO film reached 58.5 ℃ after 10 s, which is 91.8%

Journal Pre-proof higher than Cu (30.5 ℃). Meanwhile, a satisfactory result is that the temperature gradient (∆T) between EPD-GO and Cu-rGO increases with time (∆T=10, 11.4, 13.7, 14.2, 18.9 ℃). This indicates that the heat transfer of the Cu-rGO film is much faster than that of the EPD-GO film and the Cu foil, especially in an increasingly high temperature environment. Therefore, it means that Cu-rGO film has great potential in thermal management applications.

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4 Conclusions

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In summary, Cu-rGO film was successfully fabricated by combining EPD and

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vacuum hot pressing. Thermal performance measurements demonstrate that the

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composite has excellent thermal conductivity. It is impressive that the experimentally

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obtained Cu-rGOfilm has a thermal conductivity of up to 637.7 W m-1 K-1and a η value of 79.6% compared to pure Cu. In addition, the thermal conductivity of the

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prepared Cu-rGO film is 40.9% higher than that of the EPD-GO film. The significant

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increase in thermal conductivity is attributed to the introduction of graphene, while the vacuum hot pressing further repairs the defects of graphene. In addition, it is known by experimental measurement that the Cu-rGO film has a lower CTE value, which is 28.1% lower than that of pure Cu. Meanwhile, the Cu-rGO film exhibits excellent heat transfer capability, especially in high temperature environments. This further clarifies that Cu-rGO films have great potential in thermal management applications.

Acknowledgments The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: The authors acknowledge the financial

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support provided by National Natural Science Foundation of China (Project No. 51506033), Guangxi Natural Science Foundation (Grant No. 2017JJA160108), and Guangxi Colleges and Universities Program of Innovative Research Team and Outstanding Talent.

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Credit Author Statement Jiao Li: Investigation, Writing-Original Draft, Formal analysis, Ping Zhang: Conceptualization, Writing-Review and Editing, Data Curation Visualization, Visualization, Funding acquisition, Hong He: Resources, Software

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Bo Shi: Supervision, Project administration, Methodology

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Declaration of Interest Statement The authors declared that they have no conflicts of interest to this work. We declare that we do not have any commercial or associative interest that

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represents a conflict of interest in connection with the work submitted.

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