Conductive enhancement of copper/graphene composites based on high-quality graphene

Abstract

Copper is a well-known traditional metal and has been widely used for thousands years due to its combination of properties, especially its electrical conductivity. Any efforts to increase copper’s electrical conductivity, by even a small percentage, will make a great contribution to the economic effectiveness of society. In this paper, we report an electrical conductivity enhanced copper/graphene composite based on high-quality graphene (HQG) via processes involving graphene-coated copper powders through ball milling, and subsequent spark plasma sintering (SPS). The HQG is converted from regular reduced graphene oxide (RGO) by using a hot-pressing treatment. The experimental results reveal that: (1) on comparing with the copper/RGO composite, the electrical conductivity of the copper/HQG composites is significantly increased; (2) the highest electrical conductivity of the copper/HQG composite was obtained at the optimal mass percentage, 1 wt%, of HQG, at which an 8% increase was achieved when compared with pure copper. We believe that the electrical conductivity enhancement is related to the high electron mobility of HQG, and the formation of a graphene conductive network in the copper/HQG composites. In addition, the hardness of both the copper/RGO and copper/HQG composites is much higher than that of pure copper, while the copper/HQG composite shows the highest value when the amount of HQG is 0.5 wt%. It is expected that the copper/HQG composites have broad prospects of application in the electrical and electronics industry, light industry, machinery manufacturing, architecture construction, national defense, etc.

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1 Introduction

As a new type of two dimensional carbon material, graphene has extremely excellent mechanical, physical and chemical properties, such as high values of specific surface area (2630 m² g⁻¹),¹ high thermal conductivity (about 5000 W m⁻¹ K⁻¹ (ref. 2)), high electron mobility (up to 2 × 10⁵ cm² V⁻¹ s⁻¹ at room temperature³), excellent mechanical properties, etc. At present, the applications of graphene have drawn extensive attention throughout the world,^4,5 and are expected to be widely applied in composites,⁶ biology,⁷ hydrogen storage,⁸ nano-electron devices,⁹ transistors,¹⁰ field-emission cathodes,¹¹ energy storage materials,¹² organic photovoltaics,^13–16 batteries¹⁷ and catalysis on industrial scales.¹⁸ As an area for the first breakthrough and wide applications, graphene-involved composites, such as polymer/graphene composites and metal/graphene composites, have achieved multi-functional development, based upon graphene’s excellent strength, great toughness and high electrical conductivity. Especially, the applications of graphene in reinforcing polymer composites for enhancing mechanical properties and electrical conductivity have made a great success.^19–21

However, there are still challenges to face for graphene applications in metal composites, due to reasons including an agglomeration tendency in liquids, and poor wettability between graphene and metal, which cause difficulties in the preparation of high performance metal/graphene composites. Therefore, up to now, there are few works on metal/graphene composites and complex preparation methods with low costs.

Electrochemical deposition²² and laser physical vapor deposition²³ have been proven capable of realizing uniform dispersion of reduced graphene oxide (RGO) in a metal matrix for preparation of metal/graphene composite films. Z. Li et al.²⁴ successfully prepared aluminum/RGO composites and found that a composite reinforced with only 0.3 wt% of RGO showed an 18 and 17% increase in the elastic modulus and hardness, respectively. They firstly achieved uniform distribution of RGO in an Al matrix via simple electrostatic interaction between graphene oxide (GO) and Al flakes, and a densified RGO/Al composite was obtained by hot pressing the RGO/Al composite powders.

It is well known that structural integrity and lack of defects are two preconditions for achieving excellent properties of graphene. That is to say, there is a close relationship between graphene’s properties and quality involving species and quantity of defects, and different qualities of graphene exhibit enormous variation in performance. For instance, the graphene prepared by micromechanical cleavage has a high electron mobility up to 2 × 10⁵ cm² V⁻¹ s⁻¹ (ref. 3) and a larger specific surface area (calculated value ∼2630 m² g⁻¹).¹ However, the RGO synthesized by chemical exfoliation with a number of oxygenic functional groups and defects shows a low electron mobility (100 cm² V⁻¹ s⁻¹) and smaller specific surface area (700 m² g⁻¹).²⁵ Therefore, the controllable and large scale preparation of high-quality graphene (HQG) plays a key role in its further application and guarantees the desired properties.

Recently, chemical exfoliation has been considered as a process with advantages including simplicity, low cost and mass production.^26,27 But its disadvantages are also detrimental, these include poor quality, a large amount of defects, and high degree of disorder of the carbon atoms, which lead to poor properties, and then limits the applications.

In our previous work,²⁸ we developed a simple and effective route to convert RGO to high quality graphene (HQG) by using a hot-pressing treatment, at a high temperature (1500 °C) and moderate pressure (40 MPa). The experimental results revealed that the HQG was free of defects and oxygen-containing functional groups on the surface, and had a much higher electron mobility (1000 cm² V^–1 s⁻¹). Preliminarily, we prepared HQG/poly vinylidene fluoride (PVDF) composite films by spin coating. It was found that the storage modulus of the HQG/PVDF composite was nearly twice that of the RGO/PVDF composite. On comparing with pure PVDF, the storage modulus of the HQG/PVDF composite was eight times higher than that of pure PVDF, and the optimum additive amount of the HQG to PVDF was between 3 and 5 wt%.¹⁹

Copper is a well-known traditional metal and has been widely used for thousands years due to its combination of properties, especially its electrical conductivity. Any efforts to increase copper’s electrical conductivity, by even a small percent, will make a great contribution to the economic effectiveness of society. Moreover, SPS is a newly developed rapid sintering technique with great potential for achieving fast densification results with minimal grain growth in a short sintering time.²⁹ Because SPS reduces impurity segregation at grain boundaries,³⁰ it can greatly improve the graphene/copper composite during sintering.³¹

In this paper, we present a process for producing a copper/graphene composite. Firstly, preparing the graphene-coated copper powders by ball milling; and then, preparing the densified composites via high temperature and high pressure sintering. The electrical performance tests revealed that the electrical conductivity was further enhanced by 8% compared to that of pure copper when 1 wt% high quality graphene (HQG) was added to the copper matrix, and it is much better than the regular copper/RGO composite. In addition, the mechanical properties were also improved. We expect that the present result will open a new era on copper’s industrial applications in the future.

2 Methods

Preparation of graphite oxide (GO)

GO was synthesized from natural graphite (∼325 mesh, 99.95%) by a modified Hummers method:³² (1) the mixture of graphite powders (1.0 g), K₂S₂O₄ (0.5 g) and P₂O₅ (0.5 g) were put into an 80 °C solution of concentrated H₂SO₄ (10 mL) for 4 h. Then, the dark mixture was filtrated by deionized water several times and dried in a stove; (2) the pre-oxidized graphite was put into 15 mL 98% H₂SO₄, and 4 g KMnO₄ was gradually added with stirring and cooling with an ice-water bath; (3) 30 mL deionized water was added to the solution after stirring for 2 hours at 35 °C; (4) the solution was kept at 85 °C for 30 minutes. Then, 30% H₂O₂ was added to the solution until the color of the mixture turned to bright yellow. The GO was obtained after filtration, pickling, washing, and drying; (5) finally, the GO was kept in a tube furnace (OTF-1200X, HEFEI KE JING, China) under Ar atmosphere at 700 °C (heating rate ∼10 °C min⁻¹) for 1 h to obtain the RGO.³³

Preparation of the high quality graphene (HQG)

The RGO was post-treated at 1500 °C and 40 MPa for 5 min in a Spark Plasma Sintering (SPS) system (SPS-3.20MK-II, Sumitomo Heavy Industries). The HQG was scraped by a knife, and the HQG was obtained after ultrasonication in deionized water for 24 hours.

Preparation of the copper/graphene composites

Different amounts of RGO or HQG were mixed with copper powders (∼5 μm, 99.7% produced by Sinopharm Chemical Reagent Co., Ltd), and the mixture was treated by ball milling (QM-3SP4J, Nanjing NanDa Instrument Plant, China) at 1700 rpm for 4 hours by using different diameter balls for the purpose of producing the graphene-coated copper powders. The schematic illustration is shown in Fig. 1. Subsequently, the powders were processed at 650 °C and 60 MPa for 5 minutes by SPS to obtain the densified copper composites.³⁴

Fig. 1 Schematic illustration of fabrication process for the copper/HQG composite by ball milling.

Characterizations of the samples

The morphologies and microstructures of the graphene sheets and composites were characterized by scanning electron microscopy (SEM, FEI, Netherlands, with the energy spectrum of EDS) operated at 20 kV, high-resolution transmission electron microscopy (HRTEM, JEM-2010FEF, JEOL, Japan) operated at 200 kV, and Raman spectroscopy (LabRAM HR, HORIBA JobinYvon, France). The power of the Raman laser was 15 mW, and the laser excitation was 488 nm. Scans were taken on an extended range (1000–3000 cm⁻¹) and the exposure time was 5 seconds. The samples were sonicated in ethanol and drop-casted onto a SiO₂ substrate for optical observation. The electrical conductivity of the samples was measured using a precise power source/measure unit (B2902A, Agilent, USA), and the test voltage was continuously changed from −20 mV to 20 mV in same distance. The current–voltage curve was measured at 20 °C. The relative conductivity was calculated from the slope of the current–voltage curve.

3 Results and discussion

In general, Raman spectroscopy is a convenient and effective method for characterizing carbon materials. The G peak (1580 cm⁻¹) and 2D peak (2700 cm⁻¹) represent the structure and order-degree of carbon materials, and the D peak (1350 cm⁻¹) represents the concentration of defects or disorder-degree in graphite. Fig. 2 illustrates Raman spectra of samples including graphite, GO, RGO and HQG, respectively. The 2D peak in graphene is the second order of the D peak and caused by double resonant Raman scattering with two-phonon emissions. The 2D peak is sensitive to the number of graphene layers, in which the graphene should be free of defects and have no D peak. However, the defects and functional groups will inevitably be introduced on the RGO sheets, which decreases the intensity of the 2D peak. Obviously, only the G peak and 2D peak remained, and the D peak completely disappeared for the HQG, which meant that the oxygenic functional groups and defects on the surface of the RGO have been removed after the SPS treatment at high temperature and moderate pressure.

Fig. 2 Raman spectra of graphite, graphite oxide (GO), RGO and HQG.

Fig. 3 shows the SEM morphology of the HQG. Clearly, the graphene still kept the integrated layers, which indicated that the graphene sheets did not transform into graphite after the hot-pressing treatment. Fig. 4 shows the HRTEM micrographs of RGO and HQG. These show the integrated structure and that no defects are observed within the HQG directly, which is consistent with the Raman spectra. In our previous work,²⁸ the experimental results revealed that the average electron mobility was about 1000 cm² V^–1 s⁻¹ for the HQG sheets (about 8 times higher than the RGO precursor sheets, where the RGO sheets show a mobility of about 130 cm² V⁻¹ s⁻¹). This high electron mobility of the HQG is of significance to improve the conductivity of the composites.

Fig. 3 SEM morphology of the HQG.

Fig. 4 HRTEM micrographs of RGO (a) and HQG (b).

Fig. 5 shows the SEM morphologies of the HQG-coated copper powder after ball milling. It can be observed that when the copper powder is coated by graphene sheets, the surface becomes rough with folds. Actually, the graphene coating structure can be clearly observed from the edge of the copper powder due to the transparency of graphene, as shown in Fig. 5(d).

Fig. 5 SEM morphologies of the HQG-coated copper powder during ball milling. (a) Un-coated copper powder; (b and c) surfaces of the HQG-coated copper powder; (d) edge of the HQG-coated copper powder.

The graphene-coated copper powder was further sintered by SPS to get the densified copper/graphene composite for measuring electrical conductivity, as shown in Fig. 6. The composite bulks are 10 mm in diameter and 2 mm in thickness after sintering at 650 °C and 60 MPa for 5 minutes. The gold poles were plated onto the composite surface after being polished. When measuring the electrical conductivity of the composites, the testing voltage was set continuously from −20 mV to 20 mV, and the current–voltage curves were obtained with a variation of the graphene content, as shown in Fig. 6(b) and (c). On comparing to pure copper, the electrical conductivity of the copper/HQG composites changed obviously. That is, when the HQG content was less than 1 wt%, the electrical conductivity increased gradually with increasing the HQG content. However, while the content was higher than 1 wt%, the electrical conductivity began to decrease obviously. When the HQG content reached 5 wt%, the electrical conductivity almost equalled that of pure copper. The highest electrical conductivity of the copper/HQG composite was obtained at 1 wt%, the optimal mass percentage of the HQG; an 8% increment was achieved when comparing with pure copper. However, for the regular copper/RGO composites, the electrical conductivity only increased at 0.3% content, as shown in Fig. 6(c). Therefore, we believe that the integrated structure and high electron mobility of HQG play important roles in improving the conductivity of the copper/HQG composites.

Fig. 6 Electrical conductivity measurements of the composites: (a) densified composites and testing configuration (inset shows the gold electrode on the surface); (b) current–voltage curve of the copper/HQG composites with variant HQG content (inset shows the intercept of each line); (c) the relative conductivity relationship of copper/HQG and copper/RGO composites with variant content.

In order to explore the mechanism of the variation of electrical properties, we further characterized the microstructures and composition distributions of the copper/HQG composites, as shown in Fig. 7 and 8. It was found that with increasing the HQG content, the cavities in the composite gradually increased in quantity and size because of the poor wettability between HQG and copper. That is to say, when a large amount of HQG was added, it would agglomerate and be difficult to disperse homogeneously via ball milling. Therefore, during the subsequent SPS treatment, the HQG became the impurity phase in the copper matrix, and formed the cavities. From Fig. 8, we can clearly find the carbon distribution within the cavities. This is the reason that the electrical conductivity of the composites was reduced when the HQG content was over the optimum. However, on the other hand, the electrical conductivity could be improved by adding an appropriate amount of HQG.

Fig. 7 SEM surface morphologies of the copper/HQG composites with variant HQG content: (a) 0 wt%; (b) 0.2 wt%; (c) 0.5 wt%; (d) 1 wt%; (e) 3 wt%; (f) 5 wt%.

Fig. 8 EDS mapping around the cavity of the copper/HQG composite with 5 wt% HQG content: (a) SEM morphology; (b) carbon distribution.

In fact, according to Ohm’s law, the electrical conductivity of the material can be expressed as:

σ = neμ (1)

where n is the carrier concentration in material, e is the electronic charge, and μ is the carrier mobility. According to the quantum statistical theory and periodic boundary conditions, the carrier mobility μ can be expressed as:where r is the scattering factor, τ₀ is the relaxation time, m* is the carrier effective mass, s is the spin quantum number, k_B is the Boltzmann constant, and F_1/2(ξ) is the Fermi integral function.

According to formula (2), the main factors affecting the electrical conductivity of materials include the scattering factor, effective mass, relaxation time, Fermi level, etc. For a given material system, such as the present copper/graphene composite, all the above factors are constant. Thus, regulating the microstructures and reducing the scattering of carriers in the interface and defects as much as possible are the prominent ways to improve the carrier mobility.

Fig. 9 illustrates the FT-IR spectra of the samples. Obviously, almost all of the O-containing functional groups had been removed in the copper/HQG composite, which is similar to our previous work.²⁸ Therefore, we believe that there are two factors related to the enhancement of the conductivity of the copper/HQG composite, i.e., (1) the much higher electron mobility of HQG than that of regular RGO; and (2) an efficient and integrated conductive network of HQG in the copper matrix. In other words, when the composite has a low HQG content, the contribution to increasing the electrical conductivity is limited because the system cannot construct a whole network structure. However, if the HQG content is too much, it is easy to form a large amount of cavities and the scattering of carriers is increased, which makes the electrical conductivity decrease. Therefore, there is an optimum content of HQG which is 1 wt%. As for regular RGO, there is little contribution to increasing the conductivity of copper due to the presence of functional groups and defects in the surface.

Fig. 9 FT-IR spectra of pure copper, copper/RGO and copper/HQG.

In addition, we also measured the Vickers hardness of samples, as shown in Fig. 10. The measurement conditions were a 2.942 N load force for 20 seconds. The hardness of the copper/HQG composite was increased by 13% compared to that of pure copper, when 0.5 wt% HQG was added, and was obviously higher than that of regular copper/RGO composites.

Fig. 10 Vickers hardness of the copper/HQG and copper/RGO composites.

4 Conclusions

The graphene-coated copper powder was prepared effectively and facilely by ball milling. Compared to pure copper and regular copper/RGO composites, the copper/HQG composite exhibits an improved electrical conductivity. This is because the HQG is of an integrated structure without defects due to the high temperature and high pressure treatment. It is found that the highest electrical conductivity is achieved at the optimal amount of 1 wt% of the HQG, which makes an 8% increase when compared to pure copper. Based on the present experimental results, the copper/HQG composite has great application prospects in a wide range of areas, such as in the electrical and electronics industry, machinery manufacturing, architecture construction, national defense, etc.

Acknowledgements

This work was supported by the National Nature Science Foundation of China (No. 11174227, 51209023), the Special Fund for the Development of Strategic Emerging Industries of Shenzhen City of China (No. JCYJ20140419141154246), and the National Key Technology R&D Program of the Hubei province (No. 2013BHE012).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra15189a

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