# Synthesis of Silicon Carbide Powders from Methyl-Modified Silica Aerogels
## 3 Results and Discussion
On the other hand, in the powder sample synthesized from the SiO\({}_{2}\)-C mixture with a C/SiO\({}_{2}\) ratio of 0.3, the \(\alpha\)-cristobalite peak intensity was decreased and peaks corresponding to \(\beta\)-SiC appeared. At a C/SiO\({}_{2}\) ratio of 0.5, all peaks due to \(\alpha\)-cristobalite disappeared and single-phase \(\beta\)-SiC powder was obtained. Three peaks (35.5\({}^{\circ}\), 41.5\({}^{\circ}\), and 59.9\({}^{\circ}\)) are attributable to various \(\beta\)-SiC facets, and the small peak observed at 34.0\({}^{\circ}\) is due to stacking faults in \(\beta\)-SiC. This result is unexpected because the C/SiO\({}_{2}\) ratio of 0.5 is also much lower than the stoichiometric ratio required for the carbothermal reduction of silica. Generally, a C/SiO\({}_{2}\) ratio above the stoichiometric value is required for the carbothermal reduction of silica [14; 23; 26]. Thus, the result observed in Figure 4 strongly suggests that the surface methyl groups, which modify the silica aerogel particles, represent an efficient carbon source for the carbothermal reduction. Figure 5 illustrates the XRD patterns of the powder samples synthesized from the SiO\({}_{2}\)-C mixture with a C/SiO\({}_{2}\) ratio of 0.5 at various reduction temperatures for 1 h. At 1450 \({}^{\circ}\)C, the powder sample consists of \(\beta\)-SiC and \(\alpha\)-cristobalite. As the reduction temperature increases, the peak intensity of \(\alpha\)-cristobalite gradually decreases, while that of \(\beta\)-SiC increases. Based on Figure 5, it can be inferred that a reduction at 1525 \({}^{\circ}\)C is required to synthesize a single-phase \(\beta\)-SiC at a C/SiO\({}_{2}\) ratio of 0.5. We calculated the equilibrium compositions for the carbothermal reduction reaction as a function of temperature in a previous study. The results indicated that \(\beta\)-SiC forms at temperatures above 1516 \({}^{\circ}\)C [35]. Based on the XRD analysis (Figure 5), \(\beta\)-SiC formed at 1450 \({}^{\circ}\)C, which is \(\sim\)70 \({}^{\circ}\)C lower than the temperature obtained from the thermodynamic calculation (1516 \({}^{\circ}\)C). This discrepancy can be attributed to the enhanced reactivity between silica and carbon or the increased surface energy (nano-size effect) [36; 37]. In this study, surface methyl groups are the carbon source, which are bonded to the silica aerogels at the atomistic level. Therefore, it can be expected that the carbothermal reaction proceeds in a highly reactive state. In addition, the high mesopore volume and high specific surface area of the silica aerogels allow the nano-size effect on the conversion of silica to SiC [25]. The thermodynamic calculation of the carbothermal reduction of silica considering the surface energy contribution indicates that the formation temperature for SiC can be reduced to 1410 \({}^{\circ}\)C [35]. To confirm that the surface methyl groups of the silica aerogel were the carbon source for the carbothermal reduction of silica, the surface methyl groups were removed by calcining the silica aerogel at 600 \({}^{\circ}\)C for 3 h in air. The resulting hydrophilic silica aerogel powder was mixed with carbon black, and the carbothermal reduction was performed as described in Section 2. For comparison,  
Figure 4: XRD patterns of the powder samples synthesized under various C/SiO\({}_{2}\) ratios at 1525 \({}^{\circ}\)C.  
a commercial quartz powder was purchased from Junsei (extra pure, Japan) and used for an additional carbothermal reduction experiment. Figure 6 illustrates the XRD patterns of the powder samples synthesized from the hydrophobic silica aerogel, aerogel without surface methyl groups (hydrophilic silica aerogel), and quartz powders. The carbothermal reductions of the hydrophobic silica aerogel (Figure 6a,b) and other samples (Figure 6c,d) were performed at 1525 and 1550 \({}^{\circ}\)C, respectively.