Research Of In-situ Synthesis Of Three-dimensional Graphene And The Fabrication And Property Of Its Copper Matrix Composites

The realization of integration of structural and functional properties is an important branch of high-performance metal matrix composites(MMCs).Copper matrix composites(CMCs),as a kind of important conductive materials used in lead frames of integrated circuit and overhead lines in high-speed railways,are required to have the performance of not only high electrical and thermal conductivity,but also high strength and balanced plasticity and toughness.Owing to its excellent comprehensive properties such as high modulus,strength,conductivity and low density,graphene is regarded as an ideal reinforcement for CMCs.However,graphene could not make full use of these advantages due to the limitations of the common preparation methods and the traditional homogenous architecture.Herein,the three-dimensional graphene network(3D-GN)/copper composites were achieved through the architecture design and novel fabrication methods,and the simultaneous improvement of the mechanical and physical properties were realized.In this thesis,a discontinuous 3D-GN and a continuous 3D-GN reinforced structures were separately constructed in the Cu matrix by combining the methods of chemical vapor deposition(CVD)and powder metallurgy.The formation mechanism of 3D-GN and the relationship between the microstructure and mechanical properties of the composites were studied.The research purpose is to reveal the strengthening and fracture mechanisms of composites with the network architecture,and provide the theory foundation and realization ways for obtaining 3D-GN reinforced MMCs with integrated high strength and conductivity.The main research contents and results of this thesis are as follows:1)A new method for preparing a discontinuous 3D-GN/Cu(3D-DGN/Cu)was presented.First,a mixture consisted of sodium chloride as the template,glucose as the carbon source,and copper salt as the catalyst precursor was freeze-dried and calcinated to in-situ synthesize 3D-GN powders decorated with Cu nanoparticles(NPs);used as the reinforcement,the hybrid powder was then fabricated into 3D-DGN/Cu bulk material by impregnating with Cu(NO₃)₂ followed by calcinating-reducting and hot pressing.The in-situ synthesis mechanism of Cu NPs decorated 3D-GN structure and the evolution of bulk microstructure were studied.The results reveals that glucose is firstly pyrolyzed into amorphous carbon on the flat surface of the sodium chloride assemblies,and then it is converted into high-quality 3D-GN by reduced Cu catalyst at high temperature;The distribution pattern of graphene in the composite is a discontinuous type,which is homogenous on the macroscopic view but inhomogeneous on the microscopic level.2)A new methods for the synthesizing continuous 3D-GN/Cu composites(3D-CGN)was obtained.Using Cu powder as the matrix and template,sucrose as the solid-state carbon source,graphene was grown on the surface of metal powder by a CVD method.Then,a 3D-CGN was in-situ constructed in the bulk Cu matrix by hot pressing.The effects of the proportion of the ingredients in the precursor on the morphology of the CVD-grown graphene was studied.The structure evolution of 3D-CGN/Cu and the formation mechanism of 3D-CGN were also discussed.The results demonstrates that the ratio of Cu powder to sucrose has a significant impact on the surface morphology and the thickness of graphene,but has little effect on its crystallinity;Under appropriate conditions,the activated sp³-type carbon atoms on the graphene surface transforms into sp²-type ones by the Cu catalyst,which facilitates the“welding”between graphene layers through covalent bonds and thus achieves an integrated structure of 3D-CGN.3)The microstructure and mechanical properties of 3D-DGN/Cu and 3D-CGN/Cu were studied,and the strengthening and toughening mechanisms of graphene/Cu with network architecture were also clarified.It is revealed that,both 3D-DGN/Cu and 3D-CGN/Cu exhibited improved strength and toughness compared with pure copper:the yield strength,tensile strength and toughness modulus of 3D-DGN/Cu containing 4.0 vol.%graphene increased by 126%,39%and 73%,respectively;3D-CGN/Cu with 2.24 vol.%graphene increased by 111%,44%and 9%,respectively.The main strengthening mechanisms of 3D-GN/Cu is load transfer strengthening,dislocation strengthening and grain refinement strengthening.The toughening mechanism is extrinsic toughening dominated by crack deflection and bridging;moreover,the toughness of 3D-DGN/Cu is also affected by intrinsic toughening induced by the heterogeneous grain structure.4)The interface structure of in-situ synthesized graphene/Cu composites and its influence on the mechanical properties were studied.Using in-situ synthesized 3D-GN and reduced graphene oxide(RGO)as the reinforcement separately,two types of graphene/Cu bulk composites,namely 3D-DGN/Cu and reduced graphene oxide/Cu(RGO/Cu)were prepared by impregnating,calcinating-reducting followed by spark plasma sintering.The microstructure and interface structure of the two materials were studied comparatively.Besides,the relationship between the interface structure and mechanical properties of the composites was analysed.The results confirms that 3D-DGN/Cu and RGO/Cu have notably different interface structures:The interface structure of 3D-DGN/Cu is pure graphene/Cu interface connected by Cu-O-C bonding,while that of RGO/Cu is a complex combination of RGO/transition zones made up of amorphous carbon/Cu O_x/Cu;the 3D-DGN/Cu interface is more beneficial to achieve the load transfer strengthening as well block the crack propogation compared with that of RGO/Cu.5)The study of the electrical conductivity of 3D-DGN/Cu and the electrical and thermal conductivity of 3D-CGN/Cu in comparison with Cu were carried out.It is indicated that at room temperature,the electrical conductivity of 3D-DGN/Cu containing 4.0 vol.%graphene is equivalent to that of pure copper(56.6 MS/m);both the electrical conductivity(60.0 MS/m)and the horizontal thermal conductivity(413.0W/(m·K))of 3D-CGN/Cu with 1.12 vol.%graphene were superior to those pure Cu,respectively.It was noteworthy that with the increase of the test temperature,the thermal conductivity of pure Cu decreased significantly while that of 3D-CGN/Cu could be well maintained.The result above corroborates that the network architecture was effective in improving the physical properties of CMCs by optimizing the continuity of graphene.A composite reinforced by a continuous network structure is more capable to improve the overall performance than that with a discontinuous graphene network.The results of the research can provide instructions for preparing structural metallic materials with excellent performance,and is of vital significance for developing novel MMCs with integration of structural and functional properties.