Facile fabrication of layer-structured graphene/copper composite with enhanced electrical and mechanical properties

Copper (Cu) and its related composites are widely used and hence extremely important in industrial applications. Much efforts have been made to improve the physical properties of Cu, but usually lead to degradation of electrical performance. Herein, this work develops a facile way to fabricate layer-structured graphene/copper composite using graphene powder sprayed on Cu foil as building blocks. Compared to pure Cu bulk, significant improvement of both the electrical conductivity of 105.12% IACS and also ∼17% enhanced tensile strength is achieved. This strategy provides a versatile way to produce high-performance Cu composite in large with low cost for practical applications.


Introduction
Copper (Cu) is one of the most important metals widely used in various industrial fields because it is of significant electrical conductivity, thermal conductivity, mechanical strength and also high annual yield.The rapid development of aerospace, machinery industry and electronics industry in recent years the requirement of materials with higher physical properties, especially the high electrical conductivity, for improving the energy efficiency [1][2][3].Lots of research work have been made to study and improve Cu and its alloy which is the metal with second highest electrical conductivity in conventional operating environment [4][5][6][7][8] .Further improvement of the electrical conductivity can be achieved by improving the purity or getting single crystallization, which is quite limited for practical use due to its high cost and low yield.In addition, metals with higher purity and fewer boundaries are usually of lower strength and are difficult to meet the requirements of many practical applications [9].
Many works have been tried to improve the electrical conductivity of Cu matrix composites by adding reinforcing materials with excellent electrical conductivity.In the meantime, the second phase strengthening can also improve the strength of the material.Graphene (Gr) is a new carbon material with superior physical properties, especially its remarkable inherent carrier mobility and unique carrier transport properties [10][11][12][13][14][15].Gr has been considered to be the excellent candidate as a reinforcing phase to improve the electrical conductivity of Cu matrix composites.Until now, lots of studies on Cu/Gr composites had achieved the improvement of its mechanical properties, and few works reported the enhanced electrical conductivity [16][17][18].The Cu/Gr composites have been prepared in various methods, including electrochemical deposition, molecular mixing, in-situ synthesis and powder metallurgy etc [19][20][21][22][23]. Typically, Cu/Gr composite materials obtained by sintering and other processes usually exhibits the obvious increase of mechanical properties accompanying with the significant reduce of electrical conductivity.Hwang et al [24] reported that the tensile strength of Gr/Cu composite prepared by molecular level mixing method can be increased to 305 MPa, but the corresponding electrical conductivity can reach only 51% IACS.Wang et al [25]realized the integrity and uniform dispersion of graphene in the composite based on in-situ grown method.The tensile strength of 0.7% Gr (volume fraction)/ Cu composite was 60% higher than that of pure copper, but the electrical conductivity was still lower than that of pure copper.It can be observed in these works that the graphene is randomly distributed in the Cu/Gr composite matrix, which usually leads to the increase of electrical contact interface and hence the higher electrical resistance.
Recent works pave the new way to improve the electrical conductivity of Cu/Gr composite by configuring the structure in order.For example, Zhang et al [26] established a continuous three-dimensional network in Cu matrix by in-situ reaction combined with hot pressing welding process, and the corresponding electrical conductivity reached 103% IACS.In addition, Cao et al [27]fabricated the layered Cu/Gr composite material with the nacre structure, which exhibited an electrical conductivity of 97.1% IACS due to the construction of oriented Gr channels and the good interface bonding between in-situ grown Gr and Cu matrix.Furthermore, Cao et al [9]deposited Gr on surface of Cu foil by CVD method and then prepared Cu/Cu/Gr sandwich composite material by hot pressing Cu/Gr foil in stack.The reported Cu/Gr composite with sandwich structure presents the electrical conductivity of 117.4% IACS, which is still the highest record up to now.The above researches point out a feasible research direction for preparing Gr reinforced Cu matrix composites with high electrical conductivity.However, the high cost and difficult fabrication process hinder it from the practical application.For this purpose, Cu/Gr lamellar composites with Gr powder as reinforcement were prepared in a facile way in this study.Ordered layer structure is constructed by hot pressing graphene powder coated Cu foil, which leads to both improvement of electrical and mechanical performance.This work emphasizes the importance of configuration design in the development of Cu matrix composites with high electrical conductivity, and provides an easy and low-cost way for preparing high-performance Cu/Gr composite for practical applications.

Materials
Graphene slurry (5 wt%), which has a mean thickness of 2.4 nm (approximately 7 layers), is supplied by Ningbo Morsh Technology Co., Ltd, China.Rolled copper foil (C10200) with a purity of above 99.99% and thickness of 25um is commercially purchased.

Fabrication of Cu/Gr composite material
First, starting from Gr slurry (5 wt%), a dispersed slurry of Gr (2 wt% viscous aqueous slurry) with excellent dispersibility was obtained by adding an appropriate amount of deionized water in a sand mill disperser.Then, 0.25 mg ml −1 Gr −1 dispersion was prepared just by diluting Gr (2 wt% viscous aqueous slurry) with deionized water and sonicating for 60 min to get a gray smooth dispersion.Subsequently, the Gr was sprayed onto Cu substrate using a UC360C ultrasonic precision spray coater.The Gr film thickness could be controlled by adjusting spray times and liquid feeding speed.After spraying process, the as-obtained Gr powder coated Cu foils were cut into square-shaped samples using laser cutting machine.The Gr coated Cu foils were stacked and placed in a hot press furnace which was then raised to the specified temperature at a ramping rate of 10 °C/min under vacuum.Then, a hot press with different designed pressures is applied and maintained at this temperature for 1 h.The Cu/Gr bulk composites were finally obtained after cooling at 10 °C/min to 200 °C and then to room temperature in the furnace.

Characterization
Raman spectroscopy (Renishaw in Via Reflex) was performed by using 532 nm laser as excitation source to characterize the graphene film in Cu/Gr composite foils.The surface morphology was characterized by scanning electron microscope (SEM, S-4800) and optical microscope (OM, L-3800).x-ray diffraction (XRD) analysis for the composite was performed on a Bruker D8 ADVANCE x-ray diffractometer using Cu Kα radiation.The diffraction range was 20°−100°, with a scanning rate of 4°•min −1 .Electron backscatter diffraction (EBSD) analysis for crystallographic orientations of the composite cross-section was carried out on universal testing machine (Zwick/Roell Z030).The electrical conductivity of the prepared Cu/Gr composite was determined using the van der Pauw method, and all the samples were wire-cut into a dimension of 20 mm × 20 mm × ∼0.6 mm.The thickness of Cu/Gr bulk composite is calculated based on measured density, weight and dimension.

Results and discussion
The fabrication process of Cu/Gr composite is illustrated in figure 1(a).With the graphene powder coated Cu foil as building block, layer-structured Cu/Gr composites are produced by stacking and hot-press many layers of building blocks.In order to ensure the accuracy and reproducibility of the data, the laser cutting technique was used to cut all Cu/Gr foils to the precisely same rectangular shape in size of 20 × 20 mm 2 , and each composite was made with 40 stacked layers.Figure 1 presents the detailed structure information of a typical Cu/Gr composite using 50 times sprayed graphene coated Cu foil as marked as Cu@Gr50.As shown in figure 1(b), clear layer interface can be observed in the cross-sectional morphology of the as obtained Cu/Gr composite.The distance between each layer is about 25 μm which is similar to the original thickness of Cu foil.Since the Cu/Gr composite is very dense to be disassembled, the surface morphology of Gr coated Cu foil is studied instead in detail, as show in figure 1(b).Islands in dark color suggests the existence of graphene powder, which can be confirmed by the Raman test, as shown in figure 1(c).More information about the Cu/Gr composite foils were characterized by SEM, optical microscope (OM) and Raman spectroscopy.(see figures S1-S3 in supporting information).
Since the coverage of graphene on copper foil is not completely full, a strong mechanical bond can be realized by the thermal diffusion between the uncovered Cu parts.The copper contact at the upper and lower interfaces is fused into a large grain in this process, and hence it forms the Cu/Gr composite with a dense and ordered layered structure, as shown in figure 1(d).It should be emphasized that the enlarged picture in figure 1(d) shows a good interface bond between the copper substrate and the graphene layer.Since the wettability between graphene and copper is poor, as a bridge between the reinforcement phase and the matrix, this void-free interface is of good electrical conducitivy to reach the excellent comprehensive performance of the composite [28].Furthermore, figure 1(e) shows the XRD patterns of the pure Cu foil, the Gr coated Cu foil (Cu@Gr), and the Cu/Gr composite.It can be found that the hot-press process helps the Cu grains tend to preferentially oriented on (111), which is similar to the result of CVD grown Cu foil.
The electrical performance of various Cu/Gr composites has been evaluated using a standard van der Pauw method (figure S4, supporting information).Figure 2(a) compares the electrical conductivities of Cu/Gr composite material built using different blocks with different spraying times.The surface morphology of these samples is presented in figure S1 in supporting information, which clearly shows the increase of coverage as increasing the spraying time.For the pure Cu without any Gr powder, the electrical conductivity of the pure Cu bulk using same hot-press process is about 101% IACS which is due to the high purity of original Cu foil (C10200).The electrical conductivity can be further improved as increasing the introduction of Gr powder.For the Cu/Gr50 (50 spraying times) sample, the electrical conductivity reaches the highest value of 105.12%IACS.However, the electrical conductivity of Cu/Gr composites turns to be lower as further increasing the spraying time from 50 to 100 and 150.But it is still higher than that of the pure Cu bulk.The result might be due to the quite high coverage of Gr powder and hence the quite low contact between the exposed area on Cu foil, as shown in figures 2(b) and S1 in supporting information.The higher quality and coverage of CVD-grown Cu foil result in the formation of pure (111) Cu/Gr composite which also presents higher electrical conductivity [9].
To understand the influence of the microstructure on the electrical conductivity of the Cu/Gr composite, samples with different fabrication parameters are fabricated and analyzed which reveals quite different microstructures.Considering the high electrical conductivity happens on the Cu/Gr composite with suitable Gr coverage, Gr coating Cu foil with spraying 50 times (Cu@Gr50) is used as the source materials for building different Cu/Gr composites.Figure 3(a) presents the electrical conductivities of Cu/Gr composites fabricated using same source material and same hot-press pressure at different temperatures.It clearly shows that the electrical conductivity of Cu/Gr composite increase as increasing the hot-press temperature.Since it is limited by the temperature range of the experimental apparatus, the highest electrical conductivity is limited to 105% IACS.It should be also mentioned that the Cu/Gr composite would not combine together tightly if the hot-press temperature is lower than 650 °C (see figure S5 in supporting information).On the other hand, figure 3(b) compares the electrical conductivities of Cu/Gr composites fabricated using same source material and temperature but at different hot-press pressures.The electrical conductivity of Cu/Gr composite increases as increasing the hot-press pressure.Be similar with the samples fabricated at low temperature, the sample fabricated at low hot-press pressure would be easily separated.
The detailed studies on the microstructure formed at different conditions could disclose more information through the XRD analysis and EBSD results of the cross-sectional structure.As shown in figure 3(c), the grain orientation of the Cu/Gr composite prepared at 750 °C/50 MPa and 850 °C/30 MPa is mainly (200), compared to the main peak of (111) in the Cu/Gr composite prepared at 850 °C/50 MPa.The two-dimensional graphene sheet in the layered composite is regarded as the brick, and the copper matrix in the middle is regarded as the slurry, as shown in the cross-section microstructure of the Cu/Gr composite material shown by EBSD in figure 3(d).The 2D graphene sheet and the Cu grain layer are alternately stacked as a ' brick-and-slurry' structure.This phenomenon is quite different form the sample fabricated using 850 °C/50 MPa hot-press process, which shows a strong Cu (111) orientation accompanied with significant grain coarsening.Different from the 'brick-and-slurry' structure, the Cu/Gr composite fabricated at 850 °C/50 MPa hot-pressing condition has a large grain crossing the Gr interface between different layers of Cu grain to form a dense 'slurry-filled-brick' structure (graphene sheets are encased in copper grains).On the other hand, the formation of integrated Cu grain with embedded Gr layer interface could be benefit for enhancing the mechanical and electrical performance.The above result suggests that the microstructure within the Cu matrix play a crucial role on the physical properties of as-obtained Cu/Gr bulk composites.The integrity of graphene, coupled with the construction of orientation channels within its layered structure, enables the simultaneous achievement of high electron density and exceptional electron mobility.This breakthrough has significantly enhanced the material's electrical conductivity [9,27,29,30].In this work, using spraying Gr layers instead of the CVD-grown graphene on Cu foil, it still improves the electrical conductivity of layer-structured Cu/Gr composite with a lower improvement but also significant reduction of fabrication cost.Furthermore, due to the limitations of current hot-pressing equipment, further advance can be expected by fabricating at higher temperature and pressure.
Mechanical properties are also very important for the composite in practical applications.Figure 4(a) shows the representative engineering stress-strain curves of the Cu/Gr bulk composites using Gr powder coated Cu foils with different spraying times (the hot-pressing process is based on 850 °C/50 MPa conditions).The corresponding mechanical data is also summarized in table 1.Compared to the tensile strength of 202 MPa in the pure Cu bulk, the highest one of 237 ± 5 MPa is realized in the Cu/Gr composite using Cu@Gr50 building block.It is noted that the change rule is quite similar to that of the electrical conductivity.The value increases as increasing the content of Gr powder until Cu@Gr50 is used, and then it decreases as further increasing the Gr  content.This phenomenon is associated with the microstructure because too much Gr powder would block the interconnection of Cu layers through the Gr interface [31][32][33][34].On the other hand, the elongation rate decreases as increasing the Gr content, because excessive agglomeration of graphene hinders the effective bridging and expansion path of cracks.It is not conducive to the dissipation of energy, so that the composite material prematurely breaks after necking [35,36].Furthermore, figures 4(b)-(e) present the detailed morphologies of the fracture surface of the Cu/Gr composites (Cu@Gr50) and the hot-pressed pure Cu bulk as comparison.It can be seen that well-developed dimples distribute over the entire fractured surface for pure Cu bulk, indicating a typical ductile fracture with high plastic deformation as shown in figures 4(b) and (c).After introducing graphene in the Cu matrix as layer-structure, the ' slurry-filled-brick' is formed.As shown in figure 4(e), the exposed Gr wafer presents a jagged edge fracture phenomenon in the matrix, indicating that strong Cu-Cu bonding between Cu-Gr-Cu structures can significantly improve the load transfer capacity of Cu/Gr composites.Effective load transfer is the main strengthening mechanism for current composites, which is consistent with other reported strengthening mechanisms for graphene/metal composites [37].On the other hand, the failure behavior (either pull-out or fracture of graphene) is determined by its relative size compared to the ratio of tensile strength of graphene to the yield shear strength of Cu matrix.In the tensile test, stress is predominantly localized on the graphene sheet, whereas the Cu-Gr interface positioned at a greater distance from the graphene exhibits relatively low stress levels.The existence of micron-scale graphene leads to that the shear stress on the Cu matrix at the interface is always below its yield shear strength, which is the main reason for the failure behavior resulting in graphene fracture rather than pull-out.This is consistent with previous reports [26,38].
The simultaneous attainment of both high strength and high electrical conductivity in metals is significantly important for many piratical applications.Figure 5 and table 2 summarizes the data of both electrical conductivity and tensile strength in various works and current work.It can be found that there are a few reports on Cu/Gr composites in the first quadrant where strength and electrical conductivity are increased in the same time.Most studies report the results in the fourth quadrant where strength is increased but electrical conductivity is decreased.Herein, this work exhibits the electrical conductivity of 105.12%IACS and also ∼17% enhanced tensile strength compared to pure Cu bulk.Although the data is slightly lower than that of the  composite based on CVD-grown Cu foils, the fabrication method is much easier to be performed in large quantity with low cost, which is very useful for production and applications.

Conclusion
In summary, we have successfully designed and fabricated Cu/Gr composites with a ' slurry brick' layered configuration using graphene powder coated Cu foil as building blocks.The layer-structure allows the good connection between Cu layers with sintered grain and also unblocked electrical transfer in the matrix.Compared to pure Cu bulk, significant improvement of both the electrical conductivity of 105.12%IACS and also ∼17% enhanced tensile strength are achieved through a facile fabrication route.The strategy paves a new way for the development of Cu-based composites with good combined structure and comprehensive properties, which is very important and useful for practical applications.

Figure 1 .
Figure 1.(a) Scheme of fabrication process of Cu/Gr composites with graphene powder.(b) SEM image and (c) Raman spectroscopy of Gr distributed on a Cu matrix.(d) cross-sectional SEM image of the Cu/Cu/Gr interface structure.(e) XRD patterns of pure Cu foil, Cu@Gr50 foil and Cu/Gr50 composite.

Figure 2 .
Figure 2. (a) The electrical conductivities of Cu/Gr composites with different graphene spraying times.(b) graphene coverage with different graphene spraying times.

Figure 4 .
Figure 4. (a) Engineering stress-strain curves for Cu/Gr composites and the unreinforced Cu matrix fabricated by hot-press at 50 MPa and 850 °C.SEM morphology of the fracture surface of (b) and (c) pure Cu and (d) and (e) Cu/Gr50.

Figure 5 .
Figure 5.Comparison of tensile strength and electrical conductivity of the Cu/Gr composites with other reported Cu/Gr composites.The theoretical electrical conductivity and tensile strength of pure copper are taken as the coordinate origin.(The detailed information of all Cu/Gr composites in the figure is listed in table 2).

Table 1 .
). Tensile properties of the Gr/Cu bulk composites and the unreinforced Cu matrix.