## 3 Results and Discussion
Adding acetone could reduce the number of small molecules effectively (and the mechanism would be explained later), but high temperature during curing and pyrolyzing process generated bubbles in different sizes, and they burst to form pores with different diameters. In addition, the porosities of samples all showed the downward trend (which could be seen from the curves). The results confirm that acetone content does reduce the porosities and improve the compactness to a large extent during both curing and the pyrolyzing process. In order to discover the effect of acetone on the impregnation process, viscosities and wetting angles of all samples were tested. The effects of the amount of acetone on the viscosities of L-PCS solutions were presented in Figure 4. The viscosity of L-PCS was greatly reduced with the increase of acetone content under the constant shear rate 80 s\({}^{-1}\), and the viscosity of L-PCS with only 1 wt.% acetone approached 195.5 mPa\(\cdot\)s, lower than that of L-PCS solution without acetone (243.0 mPa\(\cdot\)s, consistent with published articles [20,28]). Moreover, the viscosity of L-PCS with 40 wt.% acetone decreased to 6.0 mpa-s, a factor of about 30 lower than that of L-PCS solution without any acetone. In addition, the viscosities of all samples increased slightly with the increasing shear rate, which could be caused by the \"Newtonian behavior\" [29,30], as shown in the insert image of Figure 4.  
To verify the acetone introduction's role in the improvement in impregnation efficiency of L-PCS solution, the heating treatment was exploited to decrease the viscosity of L-PCS as a comparison. We heated seven L-PCS samples to specific temperatures and stopped heating once those temperatures were achieved. The heated samples were set to cool naturally at room temperature for 3 h. Another seven samples of L-PCS were added with different acetone content and were placed at room temperature without further processing. The viscosity of heated L-PCS was approximated to 21.6 mPa\(\cdot\)s at 100 \({}^{\circ}\)C, which was consistent with the results of published articles [21,22], and the value of viscosity was only equivalent to that with 13 wt.% acetone addition. However, there was an increase in viscosity of L-PCS with the continuous heat treatment, resulting in an adverse effect in impregnation of L-PCS. Figure 5 shows the viscosity variation of L-PCS under heat treatment and acetone addition after 3 h. The viscosity approached to the minimum value of 220.5 mPa\(\cdot\)s at 30 \({}^{\circ}\)C (with no acetone addition) (Figure 5a), and the viscosity increased sharply with increasing in heat treatment temperature. However, viscosity of L-PCS with only 1 wt.% acetone introduced was 205.5 mPa\(\cdot\)s (Figure 5b),  
Figure 3: The variation of pore diameter (histograms) and porosities (curves) with different acetone content of all samples after curing and pyrolyzing. (**a**) Samples after curing and (**b**) samples after pyrolyzing. even lower than that of heating treatment at 30 \({}^{\circ}\)C, and the viscosity decreased significantly with the increase in acetone content. Therefore, the acetone addition played a significant role in decreasing the viscosity and enhancing the impregnation efficiency of L-PCS precursor and furthermore to decrease both time and economic costs to a great extent. The wetting angles and surface tension of L-PCS with different acetone content were measured and calculated by wetting angle measuring instrument and capillary lifting [31; 32; 33]. The formulas of surface tension (\(\sigma\)) and dipping height (\(l\)) were as follows [34]:  
\[\sigma=\frac{\rho glr}{2\cos\theta} \tag{1}\]  
\[l=\sqrt{\frac{tod\cos\theta}{4\mu}} \tag{2}\]  
where \(\rho\) is the density of the mixed solution, \(\theta\) is the wetting angle, \(\mu\) is the viscosity of the mixed solution, \(r\) is the capillary radius, \(t\) is impregnation time, \(h\) is the height of the object, and \(g\) is the acceleration of gravity, which is equal to 9.8 m/s\({}^{2}\).