# Synthesis of Silicon Carbide Powders from Methyl-Modified Silica Aerogels
## 3 Results and Discussion  
Some of the physical properties of the starting silica aerogel powder are listed in Table 1. The silica aerogel has a highly porous structure with mesopores, and its surface was modified by methyl groups (-CH\({}_{3}\)). The Fourier transform infrared spectra of the starting silica aerogel powder are shown in Figure 1. The absorption peaks near 1100, 800, and 460 cm\({}^{-1}\) were assigned to the asymmetry, symmetry, and bending modes of Si-O-Si, respectively [28,29]. These peaks are characteristic peaks showing a typical silica aerogel network structure. By contrast, the peaks at 1260 and 850 cm\({}^{-1}\) indicate the presence of a Si-C bond, while the peaks at 2900 and 1450 cm\({}^{-1}\) are due to C-H stretching [30,31]. Thus, it can be inferred that the silica aerogel was modified into a hydrophobic form by the surface methyl groups (-CH\({}_{3}\)). A N\({}_{2}\) adsorption-desorption isotherm of the starting silica aerogel powder is shown in Figure 2. N\({}_{2}\) absorption sharply increases near the high relative pressure (Type IV adsorption-desorption isotherm), which indicates that the silica aerogel is mesoporous [32,33]. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline  & **Tap Density,** & **Porosity,** & **BET Surface Area,** & **Pore Volume,** & **Pore Diameter,** \\  & g-cm\({}^{-3}\) & **\%** \%** & **m\({}^{2}\)g-1** & **cm\({}^{2}\)g-1** & **nm** \\ \hline Silica aerogel & 0.12 & 94.5 & 730 & 2.35 & 12.89 \\ \hline \multicolumn{6}{c}{\({}^{\lx@sectionsign}\) porosity = ((1-tap density)/\(\rho_{\text{SiCO2}})\times 100\), \(\rho_{\text{SiCO2}}\) = 2.19 g-cm\({}^{-3}\).} \\ \end{tabular}
\end{table} 
Table 1: Some physical properties of the starting silica aerogel powder. Figure 1: FT-IR spectra of the starting silica aerogel powder. The inset is an SEM image of the starting silica aerogel powder. The TG was carried out to quantitatively analyze the presence and content of surface carbon in the silica aerogel powder. The TG curves are illustrated in Figure 3. Weight loss was observed in two temperature regions. The weight loss of \(\sim\)0.5% observed from room temperature to 200 \({}^{\circ}\)C is due to the release of physically adsorbed water or the evaporation of residual solvents on the surface of the silica aerogel. On the other hand, the weight loss of 11.8% in the temperature range of 300 to 800 \({}^{\circ}\)C is due to the combustion of the surface methyl group (-CH\({}_{3}\)). It is assumed that the DTG peak and sharp weight loss at \(\sim\)475 \({}^{\circ}\)C can be attributed to the oxidation of the surface methyl groups (-CH\({}_{3}\)) of hydrophobic silica aerogel particles [34]. The silica aerogel, which is chemically modified by surface methyl groups, exhibits a hydrophobic-hydrophilic transition upon heating in air, which is due to the oxidation of-CH\({}_{3}\). The EA analysis revealed that the carbon and hydrogen contents of the silica aerogel powder were 11.78 and 3.04 wt.%, respectively. This value (11.78 wt.% of carbon) corresponds to \(\sim\)0.70 mol% of SiO2. Therefore, the actual C/SiO\({}_{2}\) ratios are 0.70, 1.30, 1.88, 3.06, and 4.23 for the nominal C/SiO\({}_{2}\) ratios of 0, 0.5, 1, 2, and 3, respectively. Figure 4 illustrates the XRD patterns of the powder samples synthesized from SiO\({}_{2}\)-C mixtures with various C/SiO\({}_{2}\) ratios at 1525 \({}^{\circ}\)C for 1 h. The powder samples obtained at C/SiO\({}_{2}\) ratios of 0 and 0.1 are \(\alpha\)-cristobalite that formed from amorphous silica. This indicates that the carbothermal  
Figure 3: TG and its derivative curves for the silica aerogel powder. Figure 2: N\({}_{2}\) adsorption–desorption isotherm (**a**) and pore size distribution of the starting silica aerogel powder (**b**). reduction did not proceed under these two conditions. This is reasonable because the C/SiO\({}_{2}\) ratios of 0 and 0.1 are much lower than the stoichiometric value (3).