## 3 Results and Discussion  
### In-Plane Electrical Conductivity Results  
The values of the in-plane electrical conductivity of the epoxy/G composites with different G filler loadings at different types of G particle sizes are shown in Figure 2. The composites in Figure 2 were prepared at curing conditions of 110 \({}^{\circ}\)C and 60 min. As shown in Figure 2a, the in-plane conductivity increases with the addition of 65 to 80 wt.% G filler (G150) to the epoxy/G composites. This phenomenon indicated that the in-plane electrical conductivity value obtained is in accordance with the percolation theory, i.e., the electrical conductivity increases with increasing conductive filler load in the polymer matrix [49; 50]. An increase in the in-plane electrical conductivity with filler loading was expected and reported by many researchers [26; 27; 28; 29], and the higher the filler loading was, the more network paths there were for electrons to pass through. A maximum value of about 23 S/cm was achieved for epoxy/G150 composites at 80 wt.% G150. The comparison of these results with those obtained in previous studies is dependent on the type and size of G and matrix used as well as the processing techniques. A study conducted by [PERSON] et al. [13] on the epoxy/G composites reported a value of only about 7 S/cm in-plane conductivity at 80 wt.% of 44 \(\upmu\)m G particle size. In a similar work by [PERSON] et al. [23], by optimizing the curing conditions, the conductivity value reached about 28 S/cm for epoxy/G composites using 150 \(\upmu\)m G particle size. [PERSON] and [PERSON] [18], who prepared PP/G composite materials by melt compounding of the components and then compression molding the compounds, reported a 7 S/cm in-plane electrical conductivity for composites containing 80 wt.% G of 10 \(\upmu\)m particle size, while the value reached up to 23 S/cm for the composites prepared by using a solution blending technique due to the better dispersion of the fillers. Another significant factor that affects the value of electrical conductivity, as mentioned earlier, is dispersion of the conductive filler in the polymer matrix [15; 51]. The agglomeration and uneven distribution of the conductive fillers results in a low electrical conductivity of the composite material, which was clearly observed from the SEM images of the surface and fracture surface of the epoxy/G150 composites in Figure 3. The SEM surface images showed that the large G particles with non-regular plate-like shapes distributed on the matrix sometimes oriented and folded over each other while the smaller particles agglomerated on the in-plane surface of the large particles, leaving unfilled voids between the large particles in the through-plane direction. The lack of a binder in some areas and the less particle-to-particle contacts in the in-plane surface of the composite material resulted in less continuity in the phase of the composite material. The continuity in the phase appeared much better when the plate-like particles oriented, which positively reflected on the value of the in-plane electrical conductivity, especially at 80 wt.% G. The smooth surfaces of the particles and the interfacial cracks indicated poor adhesion between the matrix and the particles, as well as brittle-like behavior, as shown from the fracture surfaces of the epoxy/G150 composites. Even though the increase in the value was approximately 34% at 80 wt.% G150 compared to that at 65 wt.%, the value of 23 S/cm is still not high enough and needs to be improved to meet the requirements for engineering applications. Epoxy/G150 with 80 wt.% G150 was selected as the master batch formula based on its high in-plane electrical conductivity value. In an attempt to further improve the in-plane electrical conductivity and to determine the effect of the incorporation of different sizes of G particles into the matrix, up to 10 wt.% of G150 was replaced by smaller G particle size, G75 or G44, and the results are shown in Figure 2b.