# Synthesis of Silicon Carbide Powders from Methyl-Modified Silica Aerogels
## 1 Introduction  
Silicon carbide (SiC) is a typical non-oxide ceramic material that forms covalent bonds in structural units. Because of its strong covalent bonds and chemical stability, SiC exhibits excellent properties at temperatures up to 1400 \({}^{\circ}\)C such as a high hardness, wear resistance, corrosion resistance, and strength [1, 2]. Other notable features of SiC are high thermal and electrical conductivities. Based on these excellent mechanical and electronic properties, SiC has been actively studied in various fields with respect to different applications such as for gas turbine components, heat exchangers, high temperature gas filters, power devices, heat dissipation substrates, and catalyst support materials. The SiC ceramics can be fabricated by sintering SiC powders or preceramic polymers [3, 4, 5] at high temperature because of the covalent nature of the Si-C bond and low self-diffusion coefficient [6]. Single crystals of SiC for power device applications can be grown from SiC powder compacts by physical vapor transport (PVT) or sublimation epitaxial growth (SEG) [7, 8]. The defects (stacking faults and dislocations) in SiC single crystals, sinterability, microstructure, and properties of sintered SiC strongly depend on the purity, morphology, size, and distribution of the SiC starting powders [9, 10, 11, 12]. Various techniques have been developed for the synthesis of high-quality SiC powders such as the carbothermal reduction of silica (SiO\({}_{2}\)) [13, 14, 15], direct carbonization [16, 17], the thermal decomposition of preceramic polymers [18], and chemical vapor deposition (CVD) [19, 20]. The synthesis of \(\alpha\)-SiC using the carbothermal reduction of SiO\({}_{2}\) is the most common technique, which is characterized byrelatively low-cost processing (inexpensive starting materials) and can be easily scaled up [21; 22]. The carbothermal reaction can be expressed as follows:  
\[\text{SiO}_{2}(\text{s})+3\text{C}(\text{s})\rightarrow\text{SiC}(\text{s})+2 \text{CO}(\text{g}) \tag{1}\]  
A C/SiO\({}_{2}\) powder mixture with a stoichiometric C/SiO\({}_{2}\) ratio above 3 is required for the reaction to obtain SiO\({}_{2}\)-free SiC powder [23]. Because gaseous silicon monoxide (SiO) species may form and the SiC particles grow based on the reaction of SiO and C [24], the morphology and characteristics of the SiC powder obtained using carbothermal reduction strongly depend on starting the starting material, that is, SiO\({}_{2}\) and C, as well as on the reaction conditions. [PERSON] et al. synthesized SiC powders with high specific surface areas through the carbothermal reaction of mesoporous MCM-48 silica and pyrolytic carbon using the chemical vapor infiltration of propylene [25]. [PERSON] et al. proposed a novel sol-gel process to obtain SiO\({}_{2}\) xerogels containing carbon nanoparticles [26]. They fabricated nanostructured \(\alpha\)-SiC powder at 1650 \({}^{\circ}\)C using the carbothermal reduction of the precursor powder mentioned above. In our previous study [27], we developed a novel technique for the synthesis of silica aerogel powders with spherical shapes using emulsion polymerization from water glass. The synthesized silica aerogels were highly porous (\(\sim\)95% porosity), and their surfaces were modified to contain methyl groups (-CH\({}_{3}\)). When the surface-modified silica aerogel is employed as a starting material for the carbothermal reduction to synthesize SiC powder, the covalent bonds of Si-C and the high surface energy, owing to mesoporous nature of the silica aerogels, can accelerate the conversion of silica to SiC. In addition, the use of the surface-modified silica aerogel is expected to produce a spherical SiC powder. To our knowledge, there has been no attempt to use silica aerogels containing surface methyl groups to synthesize SiC powder. This study focuses on the synthesis of spherical \(\beta\)-SiC powders through the carbothermal reduction of surface-modified SiO\({}_{2}\) aerogels.