## 3 Results and Discussion
### Effect of Holding Time under Compression Pressure  
To analyze the effect of holding time under compression pressure, green bodies were fabricated at a compression pressure of 130 MPa for varying holding times of 1, 2, and 3 min. The green bodies then underwent carbonization and the bulk density, porosity, flexural strength, Shore hardness, electrical resistivity, and microstructure of the obtained carbon blocks were analyzed. Figure 6a shows changes in the bulk density and porosity of the carbon blocks with respect to holding time under compression pressure. As the holding time increased, the  
bulk density tended to increase as follows: 1.391 g/cm\({}^{3}\) at 1 min, 1.403 g/cm\({}^{3}\) at 2 min, and 1.401 g/cm\({}^{3}\) at 3 min. At a compression pressure of 130 MPa, the volume reduction after carbonization (i.e., the volume difference between the green body and the corresponding carbon block) was  
Figure 6: Changes in mechanical properties as a function of holding time under compression pressure (a) bulk density and porosity, (b) flexural strength and Shore hardness, and (c) electrical resistivity. 3.83%, 5.08% and 4.40% for holding times of 1, 2, and 3 min, respectively. The reduction was the smallest at a holding time of 1 min. This trend is consistent with that observed in the bulk density measurements, as shown in Figure 6a. Figure 6b shows changes in the flexural strength and Shore hardness of the carbon blocks as a function of holding time under compression pressure. As the holding time increased, the flexural strength decreased, while the Shore hardness tended to increase slightly. At a holding time of 1 min, the largest flexural strength was achieved at 27.4 MPa, and it was lowest at 24.7 MPa when the holding time was 3 min. Figure 6c includes a curve that represents changes in the electrical resistivity of the carbon blocks with respect to holding time under compression pressure. The electrical resistivity was the lowest at 61.2 \(\upmu\Omega\)m at a holding time of 1 min. It increased to 62.8 \(\upmu\Omega\)m and 63.6 \(\upmu\Omega\)m when the holding times were 2 and 3 min, respectively. This trend of increasing electrical resistivity with a longer holding time is not consistent with the patterns observed in the porosity and electrical resistivity measurements, as shown in Figure 3a. As previously shown in Figure 3a, while the porosity decreased by 8.2% from 30.4% to 27.9%, the electrical resistivity decreased by 9.2% from 67.3 \(\upmu\Omega\)m to 61.2 \(\upmu\Omega\)m. Here, an arithmetical relationship between the porosity reduction and the electrical resistivity reduction was estimated to be 0.89. In Figure 6a, however, while the porosity increased by 1.1% from 27.9% to 28.2%, the electrical resistivity increased by 3.9% from 61.2 \(\upmu\Omega\)m to 63.6 \(\upmu\Omega\)m. This can be translated into an arithmetical relationship of 0.28. The degree of correlation is 3.2 times smaller. Consequently, it can be reasoned that the observed increase in porosity, accompanied by an increase in electrical resistivity, with a longer holding time can be attributed to a factor other than their mutual relationship. The following microstructural analysis provides insights into this interpretation. Figure 7 shows OM images of the carbon blocks fabricated with different holding times under compression pressure. In the images, the size of granules varied, depending on the holding time under compression pressure. Granules were larger at a holding time of 2 or 3 min compared with 1 min. At holding times of 2 and 3 min, granules larger than 200 \(\upmu\)m were observed, and smaller granules with a size of about 100 \(\upmu\)m surrounded the larger ones. When the holding time was 1 min, pores were the smallest; at holding times of 2 and 3 min, those larger than 100 \(\upmu\)m appeared. These pores were primarily located around large granules in the form of ellipses or elongated shapes. The trend of increasing bulk density and porosity with increasing holding time in Figure 6a can be explained by the microstructure.