## 3 Results and Discussion  
The graphite fibers were coated with Cu through an electroless plating process, thus forming the CGC powders with a volume ratio of 30 wt.% graphite and 70 wt.% Cu as shown in Figure 1a,b. Thickness of the deposited Cu was observed to be within the range of 2 to 3 \(\upmu\)m. Subsequently, these composite powders were consolidated during the spark plasma sintering (SPS) process as displayed in Figure 1c,d. Cross-sectional images of the sintered CGCs uncovered that the graphite powders were layered in the Cu matrix, representing anisotropy in the mixed phase. The graphite layers (black colored region) were spatially distributed in the Cu matrix (white colored region), thus aligning in the plane direction perpendicular to the pressing direction during the sintering process. To clearly identify interfacial reconstruction in between the Cu matrix and the graphite oxide layers and, more importantly, to rationalize the structure formation driven during the SPS process, cross-sectional TEM observations were performed. Such these significant structural changes were made from pristine graphite to graphite oxide (GO), and to the reduced graphite oxide (rGO), which took place during the SPS process. Previous studies of rGO phases formed in the Cu matrix have focused on discussing mechanical properties dominated by the interface strength between Cu and graphene and the graphene dispersion in the composite [28; 29; 30]. It is worth noting that the copper-graphite interface with a well-defined thickness of the graphite oxide layers ranging from 8 to 10 nm were formed as obviously shown in Figure 2. Figure 3 shows the XPS results for the chemical composition and ionization etch depth at the copper-graphite composite interface. The Cu and Cu\({}_{2}\)O phases, and the composition of carbon, oxygen, and copper compounds was confirmed to be 10 nm from the depth profile of 40 nm on the surface. It could be seen that the composition and quantitative changes in carbon, oxygen, and copper compounds were found to be 20 nm in the depth direction from the surface of the composite material. The amount of carbon and oxygen atoms decreased with distance from the surface, while the pure copper composition increased in weight. This strongly reflected that the rGO and GO interfacial layers formed the anisotropic composite structure [31; 32; 33]. Structural changes of the CGCs during the SPS process were further investigated using Raman spectroscopy. The Raman spectrum of the CGC powder showed a \(G\) band shift at 1581 cm\({}^{-1}\) corresponding to the primary scattering of graphite, where the intensity was higher than that of the \(D\) band as displayed in Figure 4. The Raman spectrum of the interfacial graphite and Cu after the SPS process showed a weak intensity peak at 1594 cm\({}^{-1}\), as well as the \(G\) band at 1581 cm\({}^{-1}\). In addition, the intensity of the \(D\) band at 1363 cm\({}^{-1}\) increased remarkably. These observations were due to the co-existence of the graphite oxide and graphite phases; hence, we reached the conclusion that the graphite surface was highly expected to be oxidized during the electroless plating and the SPS processes. Furthermore, the increased intensities in \(G\) and \(D\) bands (1594 and 1352 cm\({}^{-1}\), respectively) originated from the formation of GO [34; 35]. Since GO and pristine graphite are the different allotropes of carbon, the graphite oxide surface can also be readily bonded to an oxygen-containing group, thereby resulting in ascending distance between the carbon atoms. Therefore, we concluded that the weak peak at 1594 cm\({}^{-1}\) observed in the Raman spectrum of the CGC material interface turned up after the SPS process [8; 36]. Figure 1: Field emission electron microscopy (FE-SEM) images of Cu-coated graphite fibers (**a**,**b**) and Cu-graphite composites after the spark plasma sintering process (**c**,**d**). Figure 2: High-resolution transmission electron microscopy image of the interface between the copper and graphite composite.