Fabrication of graphene/copper nanocomposites via in-situ delamination of graphite in copper by accumulative roll-compositing

Abstract

The application and performance of graphene-reinforced composites can be benefited from the easy fabrication of graphene and good dispersion of them in the matrix. Here we reported the fabrication of in-situ graphene/copper (Gr/Cu) nanocomposites by an accumulative roll-compositing (ARC) process featured by ultrahigh rolling cycles/strains. By sandwiching in copper sheets, commercial graphite foils can be effectively delaminated into few-layer graphene sheets by ARC, without the need of any additives or chemical treatments. The ARC-induced delamination of graphite was mediated by the shear-induced slip and debonding of graphitic planes, for which the critical stress/strain conditions were analyzed. The plastic-deformation-based method simultaneously enabled the uniform incorporation of the delaminated graphene sheets into Cu matrix. The as-produced bulk Gr/Cu nanocomposites exhibit an excellent combination of strength, ductility and electrical conductivity, with the maximum tensile strength of ~686 MPa and electrical conductivity above 70% IACS. Our study showed the high effectiveness of ARC processing for exfoliating graphite into graphene nanosheets, and the great potential for enhancing the contributions of graphene by improving their dispersion and bonding with the matrix.

Introduction

Graphene has been extensively applied in various fields owing to its unique properties, such as superior strength (~130 GPa) and Young's modulus (~1.0 TPa) [1], high electrical and thermal conductivities [2,3]. The superb properties of graphene make it a promising reinforcement for producing nanocomposites with unique structural-functional properties [[4], [5], [6], [7], [8], [9], [10], [11], [12]]. The application and performances of graphene-reinforced composites can be benefited from the easy fabrication of graphene sheets and uniform incorporation of them in the matrix. For fabrication of graphene as raw materials, various methods have been developed, such as mechanical exfoliation of graphite [[13], [14], [15]], liquid exfoliation of graphite [[16], [17], [18]], epitaxial growth [19,20], chemical vapor deposition [[21], [22], [23]] and chemical oxidation and reduction [24,25]. Production of graphene by non-complicated processes is highly desired for versatile applications. Since the famous trial to exfoliate graphene from graphite mechanically [13], applicable methods based on mechanical milling have been reported to have the potential to directly exfoliate graphite into graphene [14,15]. Nevertheless, more effective methods for the mechanical exfoliation of graphite into graphene are of great interests.

On the other hand, a great challenge for developing graphene-reinforced composites has been the easy-stacking and agglomeration of graphene nanosheets in the matrix, which can limit utilizing the extraordinary properties of 2D graphene [26]. Using graphene as reinforcement has been expected to overcome the contradictory between mechanical and physical properties as found in Cu alloys and Cu matrix composites reinforced by ceramic particles [[27], [28], [29], [30]]. Various efforts have been applied to fabricate high performance graphene-reinforced copper matrix (Gr/Cu) composites by facilitating the incorporation of graphene into Cu matrix. By using molecular level mixing and self-assembly, Gr/Cu composites with nacre-like structure were fabricated, and the composites have a high tensile strength of 748 MPa, an electrical conductivity of 62.86% IACS (International Annealed Copper Standard) and a limited tensile elongation of 1.45% [31]. Multi-layer Gr/Cu composites fabricated by ball milling and high-ratio differential speed rolling exhibit a tensile strength of 425.5 MPa and a total elongation of 16.4% [32]. Yang et al. [33] reported a novel configuration design of nanocarbon-reinforced Cu matrix via unzipping carbon nanotubes into graphene nanoribbons. The electrical conductivity was retained at 92.6% IACS but the tensile strength was only 295 MPa. In most of the studies, graphene was ex-situ introduced into Cu matrix. Gr/Cu composites produced with in-situ grown graphene exhibit a tensile strength, total elongation and electrical conductivity of 378 MPa, 32.3% and 93.8% IACS, respectively [34]. To summarize, most of the Gr/Cu composites produced up to now exhibit only moderate enhancement efficiency in the mechanical and electrical properties as compared to the expected contributions of graphene. Meanwhile, literature data for the strength and electrical conductivity of Gr/copper composites are quite diverse and lower than the theoretical ones. A recent study [26] demonstrated that the three-dimensional continuous graphene network architecture has the advantage in achieving high performance Gr/Cu composites over the homogeneous structure with random distribution of graphene in the matrix.

Fabrication of Gr/Cu composites with excellent comprehensive properties such as high strength, good ductility and good electrical conductivity is still a great challenge. It is interesting to figure out the potential to achieve high-performance Gr/Cu composites by improving the dispersion of graphene in matrix. Moreover, a facile method for the fabrication of Gr/Cu composites by using cheap raw materials such as commercial graphite is of great interest to industrial applications. In this study, we presented a plastic-deformation-based method that simultaneously enables the in-situ formation of graphene nanosheets by exfoliating graphite and the uniform dispersion of them in Cu matrix. Our results showed that the shear-induced delamination of graphite can be functioned by applying accumulative roll-compositing (ARC) to the initial Cu/graphite/Cu sandwiches. The roll-bonding of copper layers accompanied with the in-situ delamination of graphite facilitates the uniform dispersion of graphene nanosheets in the continuous Cu matrix. The as-produced Gr/Cu composites exhibit an excellent combination of high tensile strength, ductility and electrical conductivity. We showed therefore the high effectiveness of exfoliating graphite into graphene nanosheets and the great benefit of achieving homogenous dispersion of graphene in Cu matrix by the simple plastic-deformation-based method.