# Synthesis of Silicon Carbide Powders from Methyl-Modified Silica Aerogels
## 3 Results and Discussion
As described above, the powder sample synthesized from the hydrophobic silica aerogel containing surface methyl groups was single-phase \(\beta\)-SiC, although the C/SiO\({}_{2}\) ratio was 0.5 and the reduction temperature was 1525 \({}^{\circ}\)C. By contrast, the XRD analysis indicates that the powder sample synthesized from the hydrophilic silica aerogel is \(\alpha\)-cristobalite. Characteristic peaks corresponding to the crystalline \(\beta\)-SiC phase could not be observed. This phenomenon suggests that the surface methyl groups play important roles in the enhancement of the carbothermal reduction reaction of silica. With respect to quartz, \(\alpha\)-cristobalite remained in the synthesized powder, although the reduction temperature and C/SiO\({}_{2}\) ratio were higher than those of the hydrophobic silica aerogel. A schematic illustration of the proposed carbothermal reduction reaction mechanism for silica aerogels is shown in Figure 7. The surface of the hydrophobic silica aerogel was modified to contain methyl groups. During the carbothermal reduction of the aerogel, the C-H bonds were broken, whereas the tetrahedral environments of silicon and carbon were maintained. It can be inferred that amorphous hydrogenated SiC or hydrogenated Si-C-O (silicon oxycarbide) has a cristobalite form at temperatures between 1000 and 1400 \({}^{\circ}\)C [38]. During this process, condensation reactions between -Si(CH\({}_{3}\))\({}_{3}\) and neighboring -Si(CH\({}_{3}\))\({}_{3}\) groups occur, which lead to the formation of CH\({}_{4}\) and H\({}_{2}\) from the consumed CH\({}_{3}\) groups [39]. Above 1400 \({}^{\circ}\)C, SiC particles nucleate and grow toward the inside of the silica aerogel particles via a gas-phase reaction between SiO and CO, as has typically been observed for the carbothermal reduction of silica. It appears that the surface methyl groups not only act as the carbon source but also serve as a template for the formation of the SiC particles [40; 41; 42]. As observed in Figures 6 and 7, the SiC particles maintained the spherical morphology of the silica aerogel powder. Figure 8 shows the SEM images of the silica aerogel and \(\beta\)-SiC powders synthesized from the SiO\({}_{2}\)-C mixtures at various reduction temperatures. The C/SiO\({}_{2}\) ratio was 0.5. Based on Figure 8a, the starting silica aerogel had spherical morphology and significant aggregation cannot be observed. The particle size was estimated to be 5 to 10 \(\upmu\)m. An interesting feature can be observed in Figure 8b-e, that is, the synthesized \(\beta\)-SiC powders exhibit spherical morphology, regardless of the reduction temperature. Figure 5: XRD patterns of the powder samples synthesized from the SiO\({}_{2}\)–C mixtures with the C/SiO\({}_{2}\) ratio of 0.5 at various reduction temperatures. The particle size and morphology of the SiC powder samples obtained at different C/SiO\({}_{2}\) ratios were analyzed using SEM. Figure 9 shows the SEM images of the \(\beta\)-SiC powders synthesized from SiO\({}_{2}\)-C mixtures with C/SiO\({}_{2}\) ratios of 0.5 and 3. The carbothermal reduction was performed at 1525 \({}^{\circ}\)C for 1 h. The comparison of Figure 9a,b shows that the morphology of the SiC particles was completely different. At a C/SiO\({}_{2}\) ratio of 0.5, the SiC particles were spherical. By contrast, a high C/SiO\({}_{2}\) ratio of 3 yielded an agglomerated structure consisting of fine SiC particles. In addition, nanofiber-like SiC was observed between the fine SiC particles. The BET specific surface areas of the SiC powders prepared using C/SiO\({}_{2}\) ratios of 1 and 3 were estimated to be 6.4 and 12.7 m\({}^{2}\)/g, respectively. This shows that the specific surface areas increase with the C/SiO\({}_{2}\) ratio. These results suggest that the C/SiO\({}_{2}\) ratio affects the nucleation and growth mechanisms during the synthesis of SiC powder. To elucidate the effect of the porous structure of the SiC on oxidation, the oxygen content in the SiC powder samples was determined using an oxygen-nitrogen analyzer.