## 3 Results
### Ceramic SiC and SiC/\(\mathrm{C_{f}}\) Microspheres  
The precursor polysiloxane microspheres were subjected to ceramization in the atmosphere of argon at 1600 \({}^{\circ}\)C for 5 h. Micrographs of representative ceramic microspheres are displayed in Figure 3. The particles preserved spherical shape although their surfaces indicated the formation of macroopores. The presence of macroopores was confirmed by SEM, see Figure 3b,d,h. The ceramic yield for DVB cross-linked microspheres ranged from about 30 to 46.3%, while for the DVTMDS microspheres the yield was much lower, due to the small content of carbon, Table 2. A sharp increase in the yield of ceramics was observed when the weight ratio of DVB to PHMS was above 0.48, Figure 4. This increase in ceramic yield was accompanied by a change in color of the microspheres from beige-green to black, Table 2. The beige-green color is consistent with the presence of SiC [6], while the black color may indicate the presence of free carbon. These observations are consistent with the changes in the composition of the preceramic microspheres discussed earlier. Oxygen-rich siloxane particles are converted to amorphous SiC\({}_{\mathrm{n}}\)O\({}_{\mathrm{m}}\) ceramics with various content of carbon when heated above 900 \({}^{\circ}\)C. Subsequent heating of SiC\({}_{\mathrm{n}}\)O\({}_{\mathrm{m}}\) material to 1600 \({}^{\circ}\)C causes separation of SiO\({}_{\mathrm{2}}\), SiC and \(\mathrm{C_{f}}\) phases, Equation (1), followed by a carbothermal reduction of SiO\({}_{\mathrm{2}}\) by the free carbon, Equation (2). However, when the amount of free carbon is low, it is completely consumed before all SiO\({}_{\mathrm{2}}\) is converted to SiC. Under these conditions, SiO\({}_{\mathrm{2}}\) reacts with SiC to form volatile SiO and CO according to Equation (3) [3; 44; 45]. The removal of oxygen from microspheres in the form of SiO significantly reduces the final ceramic yield. A particularly low yield is observed for the carbon-poor microspheres obtained from PHMS/DVTMDS, Table 3, sample B-1. The large amounts of gases released during the pyrolysis of these microspheres generate their high porosity, as described in paragraph 3.4 and crevices on their surfaces seen in Figure 3h. \[2\;\mathrm{SiO_{2(s)}}+\mathrm{SiC_{(s)}}\to 3\;\mathrm{SiO_{(g)}}+ \mathrm{CO_{(g)}} \tag{3}\]  
Figure 2: Effect of DVB proportion on the ratio of ([SiOH] + [SiOSil)/[SiC]—\(\blacktriangle\) and the carbon content in the precursor siloxane microspheres—\(\blacklozenge\). Figure 3: SEM images of SiC and SiC/C\({}_{\rm f}\) microspheres representative for different DVB/PHMS \(w/w\) ratio: (**a,b**) (A-1) = 0.17, (**c,d**) (A-3) = 0.48, (**e,f**) (A-6) = 0.97, (**g,h**) (B-1) DYTMDS/PHMS = 0.22. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Ceramized** & **Color** & **Ceramic Yield** & **Average** & **Weight Loss** & **w\%** & **w\%** & **Calculated** \\
**Sample** & **Color** & **w\%** & **Diameter \(\mu\)m** & **\% TGA \({}^{*}\)** & **C\({}_{\rm total}\)** **\({}^{**}\)** & **O \({}^{**}\)** & **w\% C\({}_{\rm f}\)** \\ \hline A-1 & Beige/Green & 30.4 & 7.1 & 0.18 & 28 & 1.12  
Obviously, the carbon-rich siloxane particles, which are formed when the \(w/w\) ratio of DVB to PHMS was above 0.48, are converted to SiC mainly by carbothermal reduction, Equation (2), leading to a higher yield of ceramic. These microspheres also contain residual free carbon. The content of C\({}_{\rm f}\) in the SiC microspheres was determined by combustion at 800 \({}^{\circ}\)C in air by a thermogravimetric analyzer, TGA.