## 3 Results and Discussion
### Effect of Post-Processing on Properties  
#### 3.2.1 Vacuum Pressure Impregnation of Phenolic Resin Solution and Carbonization  
Figure 5 shows the comprehensive properties of carbonized graphite preforms after vacuum pressure infiltration in phenolic resin solution (select 3# mixture powder). The porosity and open porosity of the samples without vacuum pressure infiltration are 76.51% and 65.70%, respectively, and these are loose and high porosity characteristics, which may account for the lower thermal conductivity (1.40 W\(\cdot\)m\({}^{-1}\)K\({}^{-1}\)) and compressive strength (1.25 MPa). However, the thermal conductivity and compressive strength of the carbonized samples after vacuum pressure infiltration are improved to a certain extent. After three times the vacuum infiltration in phenolic resin solution, the porosity and the open porosity of the samples were decreased 20.39% (from 76.51% to 56.12%) and 27.35% (from 65.70% to 38.35%), respectively compared with those without infiltration (Figure 5a). The porosity decreased by vacuum pressure infiltration in phenolic resin solution, which was also confirmed by the SEM images shown in Figure 6.  
Figure 5: Effect of phenolic resin solution infiltration times on properties: (**a**) porosity and open porosity; (**b**) thermal conductivity and compressive strength. The graphite preforms were formed with graphite flakes that bonded with each other by resin during the process of SLS. Although the resin content of the mixture powder used for 3D printing is high (30 wt %), only a small amount of molten resin plays a role in bonding during the laser rapid scanning process, so the strength of SLS preforms is low (1.25 MPa). In addition, during the SLS process, the mixture powder spread freely under the action of gravity, and the loose accumulation structure between powder remains after SLS, so the preform has a high porosity (76.51%). The porous microstructure was also observed by SEM image, as shown in Figure 6a. By vacuum pressure infiltration, the internal porous structure is filled, and the graphite flakes are coated by liquid phenolic resin. The filling and binding of phenolic resin account for the increase in density and compressive strength of the samples. In the process of carbonization, the phenolic resin (approximately 70-80 wt %) is heated at high-temperature and transformed from organic binder to inorganic binder (glass carbon) [8], which has higher strength than a phenolic resin binder and consequently results in the higher compressive strength of carbonized samples. Figure 7 shows the glass carbon binder bridge formed by the phenolic resin binder during the process of carbonization in samples without infiltration. As a result of vacuum pressure infiltration, the more resin infiltrated into the pores, the more glass carbon converted, the lower the porosity, and the greater the strength and density of the samples [24]. Figure 6: SEM image of different infiltration times of phenolic resin solution. (**a**) 0 times, (**b**) 1 time, (**c**) 2 times, (**d**) 3 times. Figure 7: SEM image of glassy carbon. After the first vacuum pressure infiltration, the strength and density increased significantly by 304.8% (from 1.25 MPa to 5.06 MPa) and 73.58% (from 0.53 g\(\cdot\)cm\({}^{-3}\) to 0.92 g\(\cdot\)cm\({}^{-3}\)), respectively, while the thermal conductivity only increased by 13.57%. The second vacuum pressure infiltration caused an increase in compressive strength, density, and thermal conductivity of about 22.53%, 5.43%, and 3.14%, respectively, and the third infiltration resulted in the increment of no more than approximately 2% in density and thermal conductivity, while the strength increment is only about 8.23% and not significant compared with the first immersion (Figure 6b). The results show that after infiltration twice, there is little potential to adjust the strength, porosity, and thermal conductivity of the samples by continuous infiltration.