## 3 Results and Discussion
### Effect of Post-Processing on Properties
#### 3.2.2 Vacuum Pressure Infiltration of Silica Sol Solution and High-Temperature Sintering  
Figure 8 shows the pore size distribution and micromorphology of the carbonized samples after twice vacuum pressure infiltration in phenolic resin solution. There are still many macropores with an average pore diameter of 16.24 \(\upmu\)m after carbonization. These pores provide favorable space for the filling of silica sol solution due to the suitable size of nano-silica powder (with an average size of 8-15 nm) contained in the sol. Therefore, by infiltrating silica sol solution under vacuum pressure, the pores in carbonized samples can be effectively filled with silica sol solution, so that the strength of the samples can be further improved after subsequent sintering [25]. Before vacuum pressure infiltration of silica sol solution, the sample was infiltrated twice in phenolic resin solution and carbonized. The carbonized sample was infiltrated in the silica sol solution under vacuum pressure and then sintered at high-temperature. Drying treatment was carried out before repeated infiltration in silica sol solution or high-temperature sintering. The properties and porosity of sintered samples with different times of infiltration in silica sol solution are shown in Figure 9.  
Figure 8: Pore size distribution and SEM image of 3D graphite/ceramic carbonized part  
Figure 9: Effect of silica sol solution infiltration times on properties: (**a**) porosity and open porosity; (**b**) thermal conductivity and compressive strength. In the high-temperature sintering process, when the sintering temperature is over 1440 \({}^{\circ}\)C, the molten silicon reacts with the active glass carbon carbonized by phenolic resin to form the SiC whisker reinforcement phase, which is confirmed by the XRD pattern in Figure 10 and SEM image in Figure 11. SiC has high strength, good thermal conductivity, and excellent interface bonding with graphite, and it acts as the strengthening phase to enhance the strength of the matrix. Enough SiC whiskers strongly linked by mechanical contact can be formed a three-dimensional framework, which can effectively improve the strength and thermal conductivity of the sample [26; 27]. With the increase of the number of infiltration times of silica sol solution, the intensity of the diffraction peak of silicon carbide was higher; when the silica sol solution was infiltrated three times, the reaction was incomplete due to insufficient active carbon source, leaving more silicon and silicon dioxide. After three times of vacuum pressure infiltration in silica sol solution, the strength and thermal conductivity of the sintered sample are increased by 63.56% and 21.14%, respectively. The results are consistent with the above analysis [28; 29]. The porosity of the samples can be effectively reduced by the filling of silica sol solution into open pores in vacuum pressure infiltration, which can be confirmed by the SEM microstructure in Figure 12. However, the closed pores can not be infiltrated into by silica sol solution, so the porous structure can not be eliminated completely after sintering at high temperature. Besides, the porosity can be reduced to some extent because of the volume expansion of 35% accompanied by the formation of SiC whiskers, as the new tiny pores can be caused by volume shrinkage of the silica sol solution infiltrated into the open pores due to the loss of liquid phase (such as water or organic solvent) or thermal stress during the subsequent drying and sintering process [30]. Figure 11: SEM image of SiC whisker. Figure 10: XRD pattern of different infiltration times of silica sol solution. The results of the porosity test show that after three times of sintering, the porosity and open porosity are reduced by 5.89% and 24.63%, respectively, but the closed porosity is increased by 18.74%, which shows that the new tiny pores produced in drying and sintering are mainly closed pores.