## 3 Results and Discussion  
Figure 1 shows the SEM images of cured samples prepared with different acetone content which were obtained to analyze the porosity and compactness of all samples. The surface of the pure L-PCS was much more porous after curing (Figure 1a), which was mainly attributed to the thermal evaporation of micro-molecules in the PCS during the curing process [26]. With that porosity, inner cracks would be easily generated when external forces are applied, which would further induce the destruction of the whole structure. Besides this, the Energy-dispersive X-ray spectroscopy (EDS) spectrum (Figure 1a) shows that the main elements in pure PCS after curing were C and Si, and no O element was detected, which was consistent with the results obtained from Figure 1h. With the increasing of acetone content, less pores and higher compactness of cured samples were detected (Figure 1b,g), which demonstrated that acetone can contribute to the improvement of compactness and carrying capacity. Besides, Figure 2h was the EDS spectrum after L-PCS-0 curing, which showed that no other extraneous elements appeared during the curing process (the nitrogen was from the Karstedt's Catalyst). Figure 2 shows the SEM images of each sample after pyrolyzing. Many through-holes were detected in the fracture surface of the pure PCS sample, and the surface images of pure L-PCS also indicated many pores (Figure 2a). By comparing the fracture morphologies of PCS prepared with different acetone content, which were shown in Figure 2b-g, it can be concluded that the pure PCS presented plenty of pores after pyrolyzing, because those pores had already formed during curing and the PCS was already curing during high temperature (>100 \({}^{\circ}\)C), which made L-PCS have poor fluidity and filling pores impossible. But, with the increase of acetone content, the fracture porosity of PCS tended to decrease gradually, and the pore diameter and the proportion of through-holes were also reduced at the same time. In addition, about 30% of the porosities presented in Figures 1 and 2 were open porosities, and the two figures both presented the perfect compactness of samples produced by the optimized impregnation method, which showed its superiority at the same time. What's more, combined with analysis of the EDS spectrum, the main elements in L-PCS-0 after pyrolyzing were C and Si, and no O element was detected, as shown in Figure 2h, which was consistent with the reference [27]. Figure 1: Scanning electron microscopy (SEM) images of all samples prepared with different acetone ratios after curing at 200 \({}^{\circ}\)C, and Energy-dispersive X-ray spectroscopy (EDS) energy spectrum after pure polycarbosilane (PCS) curing. (**a**) without acetone, (**b**) 1 wt.%, (**c**) 2 wt.%, (**d**) 5 wt.%, (**e**) 10 wt.%, (**f**) 30 wt.%, (**g**) 40 wt.% and (**h**) EDS energy spectrum of (**a**). Figure 2: SEM images of all samples prepared with different acetone content after pyrolyzing and EDS energy spectrum after pure PCS pyrolyzing. (**a**) without acetone, (**b**) 1 wt.%, (**c**) 2 wt.%, (**d**) 5 wt.%, (**e**) 10 wt.%, (**f**) 30 wt.%, (**g**) 40 wt.%, and (**h**) EDS energy spectrum of (**a**). Figure 3 illustrates the influences of acetone content on pore diameter and porosity (which were obtained both by using python to process SEM images). We could see that when the acetone content was relatively high, the pore diameter (shown in the histograms) dropped apparently. However, there were still some exceptions, such as the cured samples after adding 1 wt.% and 10 wt.% acetone and the pyrolyzed samples after adding 2 wt.% and 30 wt.% acetone, as shown in Figure 3a,b. It was easy to understand that there were two major factors that affect pore diameters, one was the heating temperature and the other was the number of small molecules.