Improving the electrical conductivity of copper/graphene composites by reducing the interfacial impurities using spark plasma sintering diffusion bonding

The copper/graphene (Cu/Gr) composites made of Gr/Cu/Gr foils with a thickness of 25 μm were prepared using spark plasma sintering diffusion bonding (SPS) and hot press diffusion bonding (HP) in an Ar atmosphere at 900 °C for 20 min under a pressure of 50 MPa. The electrical conductivity of the prepared Cu/Gr composites was measured by the van der Pauw method, the microstructure was characterized using Raman spectroscopy, scanning electron microscope and transmission electron microscope, and the corresponding mechanism was analyzed by employing the first-principles calculations. The result indicates that, SPS process reduces the interfacial impurities of Cu/Gr composites, rendering a much higher electrical conductivity (108.6% IACS) than that prepared by using HP (98.8% IACS). This finding is correlated to the reduced oxygen (O) impurities on Gr/Gr interface, which is attributed to the high temperature plasma sputtering effect of SPS. In the prepared Cu/Gr composites, the impurity-free Gr/Gr interface has an ideal spacing of ∼3.3 Å wherein carbon (C) across the interface do not bond. Despite the generated C–C covalent bonding within the same Gr layer bounds some of the electrons, the remaining electrons of the p orbital on each C atom are free electrons, leading to the excellent electrical conductivity. In contrast, absorption of O impurities on the Gr/Gr interface increases the spacing between the Gr layers to ∼3.8 Å, and generates covalent C–O–C bonding across the interface due to p orbital hybridization. This reduces the free electron concentration and the corresponding electrical conductivity.