Synthesis of Silicon Carbide Powders from Methyl-Modified Silica Aerogels

applied sciences

Article Synthesis of Silicon Carbide Powders from Methyl-Modiﬁed Silica Aerogels

Kyoung-Jin Lee 1, Yanggu Kang 1,2, Young Hun Kim 3, Se Won Baek 3 and Haejin Hwang 1,*

1 Department of Materials Science and Engineering, Inha University, 100 Inha-ro, Incheon 22212, Korea; [email protected] (K.-J.L.); [email protected] (Y.K.) 2 KOMEX, Ansung, Gyeonggi 17604, Korea 3 Basic Materials & Chemicals R&D, LG Chem, 188 Munji-ro, Yuseong-gu, Daejeon 34122, Korea; [email protected] (Y.H.K.); [email protected] (S.W.B.) * Correspondence: [email protected]; Tel.: +82-32-860-7521

Received: 3 August 2020; Accepted: 3 September 2020; Published: 4 September 2020

Abstract: β-silicon carbide (SiC) powders were synthesized by the carbothermal reduction of methyl-modiﬁed silica aerogel/carbon mixtures. The correlations between the phase evolution and morphologies of the SiC powders and the C/SiO2 ratio were investigated. At a C/SiO2 ratio of 3, β-SiC formed at 1425 ◦C and single-phase SiC powders were obtained at 1525 ◦C. The methyl groups (-CH3) on the silica aerogel surfaces played important roles in the formation of SiC during the carbothermal reduction. SiC could be synthesized from the silica aerogel/carbon mixtures under lower temperature and C/SiO2 ratios than those needed for quartz or hydrophilic silica. The morphology of the SiC powder depended on the C/SiO2 ratio. A low C/SiO2 ratio resulted in β-SiC powder with spherical morphology, while agglomerates consisting of ﬁne SiC particles were obtained at the C/SiO2 ratio of 3. High-purity SiC powder (99.95%) could be obtained with C/SiO2 = 0.5 and 3 at 1525 ◦C for 5 h.

Keywords: silicon carbide (SiC); silica (SiO2); aerogel; surface methyl (-CH3) group; carbothermal reduction

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 ◦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 [3–5] 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 [9–12]. Various techniques have been developed for the synthesis of high-quality SiC powders such as the carbothermal reduction of silica (SiO2)[13–15], direct carbonization [16,17], the thermal decomposition of preceramic polymers [18], and chemical vapor deposition (CVD) [19,20]. The synthesis of α-SiC using the carbothermal reduction of SiO2 is the most common technique, which is characterized by

Appl. Sci. 2020, 10, 6161; doi:10.3390/app10186161 www.mdpi.com/journal/applsci Appl. Sci. 2020, 10, 6161 2 of 11 relatively low-cost processing (inexpensive starting materials) and can be easily scaled up [21,22]. The carbothermal reaction can be expressed as follows:

SiO (s) + 3C(s) SiC(s) + 2CO(g) (1) 2 →

AC/SiO2 powder mixture with a stoichiometric C/SiO2 ratio above 3 is required for the reaction to obtain SiO2-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, SiO2 and C, as well as on the reaction conditions. Parmentier 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]. Meng et al. proposed a novel sol-gel process to obtain SiO2 xerogels containing carbon nanoparticles [26]. They fabricated nanostructured α-SiC powder at 1650 ◦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 (~95% porosity), and their surfaces were modified to contain methyl groups (-CH3). 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 β-SiC powders through the carbothermal reduction of surface-modified SiO2 aerogels. The effects of the surface methyl groups (-CH3) and mesoporous structure of the silica aerogel on the carbothermal reduction reaction were investigated for different SiO2/C ratios. The synthesized SiC powders were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM).

2. Materials and Methods

2.1. Preparation of Silica Aerogel Silica aerogel and carbon black (Raven425, Birla Carbon, Seoul, Korea) were used as starting materials for the carbothermal reduction. Silica aerogel powders were prepared from water glass (silica content, 28–30 wt.%; SiO2/Na2O = 3.52:1; Young IL Chemical, Incheon, Korea) and n-hexane (95%, Samchun Pure Chemicals, Pyeongtaek, Korea) using an emulsion polymerization technique. The detailed procedure is described elsewhere [27]. Silica aerogel and carbon black were mixed in n-hexane using an ultrasonic bath. Homogeneous slurry was obtained after 10 min and dried at 100 ◦C in an electric oven. The C/SiO2 molar ratios were 0, 0.5, 1, 2, and 3. The SiO2/C powder mixture was poured into an alumina (Al2O3) boat, which was placed inside a tube furnace under a constant argon ﬂow (100 sccm). The carbothermal reduction was performed at 1450–1525 ◦C for 1 and 5 h. The synthesized powders were further heat-treated at 800 ◦C for 1 h in air to remove residual carbon black.

2.2. Evaluation To measure tap density, 0.2 g of the silica aerogel powder was placed in a 5 mL cylinder (9 mm in diameter) and tapped 1000 times using a tapping density tester (TAP-2S, Logan Instruments Co., Somerset, NJ, USA). Fourier transform infrared (FT-IR) spectroscopy (FTS-165, Bio-Rad, Hercules CA, USA) was used to confirm the surface chemical structure of the aerogels in the wave number range of 1 400–4000 cm− . The powder was mixed with potassium bromide (KBr) and pressed to form a sample Appl. Sci. 2020, 10, x FOR PEER REVIEW 3 of 11 aAppl. sample Sci. 2020 disk, 10 ,for 6161 FT-IR measurements. The phases of the obtained powders were identified using3 of 11 XRD (D/MAX 2200V/PC, RIGAKU Co., Ltd., Tokyo, Japan) with Ni-filtered CuKα radiation. The thermogravimetry (TG) and differential thermal analysis (DTA) (Diamond TG/DTA Lab System, Perkindisk for Elmer) FT-IR measurements.was performed The in air phases up to of 800 the °C obtained at a heating powders rate were of 5 identified °C · min− using1 to investigate XRD (D/MAX the α thermal2200V/PC, behavior RIGAKU of Co.,the silica Ltd., Tokyo,aerogel Japan) powder. with The Ni-filtered carbon, CuKhydrogen,radiation. and oxygen The thermogravimetry contents in the starting(TG) and silica diff erentialaerogel thermaland synthesized analysis SiC (DTA) pow (Diamondders were TGdetermined/DTA Lab using System, an elemental Perkin Elmer) analyzer was performed in air up to 800 C at a heating rate of 5 C min 1 to investigate the thermal behavior of (EA, Thermo EA1112, Thermo◦ Fisher Scientific, Waltham,◦ · − MA, USA) and an oxygen–nitrogen analyzerthe silica (EMGA-920, aerogel powder. Horiba, The Japan), carbon, respectively. hydrogen, and The oxygen surface contents area, pore in thevolume, starting and silica mean aerogel pore sizeand synthesizedof the starting SiC silica powders aerogel were powder determined were usingmeasured an elemental using Brunauer–Emmett–Teller analyzer (EA, Thermo EA1112, (BET) equipmentThermo Fisher (0.01 Scientific, < p/p0 < 1; Waltham, ASAP 2010; MA, Micrometri USA) andcs, an oxygen–nitrogenNorcross, GA, USA). analyzer The equipment (EMGA-920, is used Horiba, to Japan), respectively. The surface area, pore volume, and mean pore size of the starting silica aerogel measure the amount of nitrogen that is adsorbed as the pressure changes. Before N2 adsorption, the powder sample were measured was degassed using at Brunauer–Emmett–Teller 200 °C. The microstructures (BET) of equipmentthe obtained (0.01 powder< p/ p0samples< 1; ASAP were examined2010; Micrometrics, using field-emission Norcross, GA,SEM USA). (FESEM, The S-4300, equipment Hitachi, is used Japan). to measure the amount of nitrogen that is adsorbed as the pressure changes. Before N2 adsorption, the powder sample was degassed at 3.200 Results◦C. The and microstructures Discussion of the obtained powder samples were examined using field-emission SEM (FESEM, S-4300, Hitachi, Japan). Some of the physical properties of the starting silica aerogel powder are listed in Table 1. The silica3. Results aerogel and has Discussion a highly porous structure with mesopores, and its surface was modified by methyl groups (-CH3). The Fourier transform infrared spectra of the starting silica aerogel powder are shown Some of the physical properties of the starting silica aerogel powder are listed in Table1. The silica in Figure 1. The absorption peaks near 1100, 800, and 460 cm−1 were assigned to the asymmetry, aerogel has a highly porous structure with mesopores, and its surface was modified by methyl groups symmetry, and bending modes of Si-O-Si, respectively [28,29]. These peaks are characteristic peaks (-CH ). The Fourier transform infrared spectra of the starting silica aerogel powder are shown in showing3 a typical silica aerogel network structure. By contrast, the peaks at 1260 and 850 cm−1 indicate Figure1. The absorption peaks near 1100, 800, and 460 cm 1 were assigned to the asymmetry, symmetry, the presence of a Si-C bond, while the peaks at 2900 and− 1450 cm−1 are due to C-H stretching [30,31]. and bending modes of Si-O-Si, respectively [28,29]. These peaks are characteristic peaks showing Thus, it can be inferred that the silica aerogel was modified into a hydrophobic form by1 the surface a typical silica aerogel network structure. By contrast, the peaks at 1260 and 850 cm− indicate the methyl groups (-CH3). A N2 adsorption–desorption isotherm of th1 e starting silica aerogel powder is presence of a Si-C bond, while the peaks at 2900 and 1450 cm− are due to C-H stretching [30,31]. shown in Figure 2. N2 absorption sharply increases near the high relative pressure (Type IV Thus, it can be inferred that the silica aerogel was modified into a hydrophobic form by the surface adsorption–desorption isotherm), which indicates that the silica aerogel is mesoporous [32,33]. methyl groups (-CH3). A N2 adsorption–desorption isotherm of the starting silica aerogel powder is shown in FigureTable2.N 1. Some2 absorption physical properties sharply increases of the starting near silica the aerogel high relative powder. pressure (Type IV adsorption–desorption isotherm), which indicates that the silica aerogel is mesoporous [32,33]. Tap Density, Porosity, BET Surface Area, Pore Volume, Pore Diameter, g·cmTable−3 1. Some% physical § propertiesm2 of·g− the1 starting silicacm aerogel3·g−1 powder. nm Silica 0.12 Tap Density,94.5 Porosity, 730BET Surface Area, Pore 2.35 Volume, Pore Diameter, 12.89 aerogel g cm 3 % § m2 g 1 cm3 g 1 nm · − · − · − Silica aerogel§ porosity 0.12 = ((1-tap 94.5 density)/ρSiO2) × 730100, ρSiO2 = 2.19 g·cm 2.35−3. 12.89 § porosity = ((1-tap density)/ρ ) 100, ρ = 2.19 g cm 3. SiO2 × SiO2 · −

Figure 1. FT-IR spectra of the starting silica aerogel powder. The inset is an SEM image of the starting Figuresilica aerogel 1. FT-IR powder. spectra of the starting silica aerogel powder. The inset is an SEM image of the starting silica aerogel powder. Appl. Sci. 2020, 10, x FOR PEER REVIEW 4 of 11 Appl. Sci. 2020, 10, 6161 4 of 11 Appl. Sci. 2020, 10, x FOR PEER REVIEW 4 of 11

Figure 2. N2 adsorption–desorption isotherm (a) and pore size distribution of the starting silica Figure 2. N2 adsorption–desorption isotherm (a) and pore size distribution of the starting silica aerogel aerogelFigure powder2. N2 adsorption–desorption (b). isotherm (a) and pore size distribution of the starting silica powder (b). aerogel powder (b). TheThe TG TG was was carried carried out out to to quantitatively quantitatively analyze analyze the the presence presence and and content content of of surface surface carbon carbon in in thethe silica silicaThe aerogel TG aerogel was powder. powder.carried outThe The toTG TG quantitatively curves curves are are illustra illustrated analyzeted thein in Figure Figurepresence 3.3. Weight Weightand content loss loss was was of surfaceobserved observed carbon in in two two in the silica aerogel powder. The TG curves are illustrated in Figure 3. Weight loss was observed in two temperaturetemperature regions. regions. The The weight weight loss loss of of ~0.5% ~0.5% observed observed from from room room temperature temperature to to 200 200 °C◦C is is due due to to thetemperaturethe release release of of physicallyregions. physically The adsorbed adsorbed weight losswater water of or ~0.5% or the the evaporation observed evaporation from of of residual room residual temperature solvents solvents on on to the the 200 surface surface °C is dueof of the the to silicathe release aerogel. of Onphysically the other adsorbed hand, the water weight or the loss evaporation of 11.8% in of the residual temperature solvents range on the of 300surface to 800 of °Cthe silica aerogel. On the other hand, the weight loss of 11.8% in the temperature range of 300 to 800 ◦C issilica due aerogel.to the combustion On the other of hand,the surface the weight methyl loss group of 11.8% (-CH in3). theIt is temperature assumed that range the ofDTG 300 peak to 800 and °C is due to the combustion of the surface methyl group (-CH3). It is assumed that the DTG peak and sharpis due weight to the losscombustion at ~475 °C of can the besurface attributed methyl to thegroup oxidation (-CH3). of It the is assumedsurface methyl that the groups DTG (-CHpeak3 )and of sharp weight loss at ~475 ◦C can be attributed to the oxidation of the surface methyl groups (-CH3) of hydrophobicsharphydrophobic weight silica loss silica ataerogel aerogel ~475 °C particles particles can be attributed[34]. [34]. The The silica silicato the aerogel, aerogel,oxidation which which of the is is chemicallysurface chemically methyl modified modiﬁed groups by by (-CH surface surface3) of methylhydrophobicmethyl groups, groups, silica exhibits exhibits aerogel a ahydrophobic–hydrophilic hydrophobic–hydrophilicparticles [34]. The silica aerogel, tr transitionansition which upon upon is heatingchemically heating in in air,modified air, which which byis is duesurface due to to themethyl oxidation groups, of -CHexhibits3. The a EAhydrophobic–hydrophilic analysis revealed that thetransition carbon uponand hydrogen heating in contents air, which of the is due silica to the oxidation of-CH3. The EA analysis revealed that the carbon and hydrogen contents of the silica aerogeltheaerogel oxidation powder powder of were were-CH 11.783. 11.78 The and EA and 3.04analysis 3.04 wt.%, wt.%, revealed respectively. respectively. that the This This carbon value value and(11.78 (11.78 hydrogen wt.% wt.% of of carbon)contents carbon) corresponds correspondsof the silica toaerogel ~0.70 powdermol% of were SiO2. 11.78 Therefore, and 3.04 the wt.%, actual respectively. C/SiO2 ratios This are value 0.70, (11.78 1.30, wt.% 1.88, of 3.06, carbon) and corresponds4.23 for the to ~0.70 mol% of SiO2. Therefore, the actual C/SiO2 ratios are 0.70, 1.30, 1.88, 3.06, and 4.23 for the nominalto ~0.70 C/SiOmol%2 ofratios SiO2. of Therefore,0, 0.5, 1, 2, andthe actual3, respectively. C/SiO2 ratios are 0.70, 1.30, 1.88, 3.06, and 4.23 for the nominal C/SiO2 ratios of 0, 0.5, 1, 2, and 3, respectively. nominal C/SiO2 ratios of 0, 0.5, 1, 2, and 3, respectively.

Figure 3. TG and its derivative curves for the silica aerogel powder. Figure 3. TG and its derivative curves for the silica aerogel powder. Figure4 illustratesFigure the 3. XRD TG and patterns its derivative of the curves powder for samples the silica synthesizedaerogel powder. from SiO 2-C mixtures withFigure various 4 illustrates C/SiO2 ratios the XRD at 1525 patterns◦C for of 1 the h. Thepowder powder samples samples synthesized obtained from at CSiO/SiO2-C2 ratiosmixtures of 0 withand various 0.1Figure are 4 αC/SiO -cristobaliteillustrates2 ratios the at that XRD1525 formed patterns°C for from 1 h.of The amorphousthe powder silica.samples This obtainedsynthesized indicates at C/SiO thatfrom the 2SiO ratios carbothermal2-C ofmixtures 0 and with various C/SiO2 ratios at 1525 °C for 1 h. The powder samples obtained at C/SiO2 ratios of 0 and Appl. Sci. 2020, 10, x FOR PEER REVIEW 5 of 11 Appl. Sci. 2020, 10, 6161 5 of 11 0.1 are α-cristobalite that formed from amorphous silica. This indicates that the carbothermal reduction did not proceed under these two conditions. This is reasonable because the C/SiO2 ratios reduction did not proceed under these two conditions. This is reasonable because the C/SiO2 ratios of 0 andof 0 0.1 and are 0.1 much are lowermuch thanlower the than stoichiometric the stoichiometric value (3). value On the(3). other On the hand, other in thehand, powder in the sample powder sample synthesized from the SiO2-C mixture with a C/SiO2 ratio of 0.3, the α-cristobalite peak synthesized from the SiO2-C mixture with a C/SiO2 ratio of 0.3, the α-cristobalite peak intensity was intensity was decreased and peaks corresponding to β-SiC appeared. At a C/SiO2 ratio of 0.5, all peaks decreased and peaks corresponding to β-SiC appeared. At a C/SiO2 ratio of 0.5, all peaks due to due to α-cristobalite disappeared and single-phase β-SiC powder was obtained. Three peaks (35.5°, α-cristobalite disappeared and single-phase β-SiC powder was obtained. Three peaks (35.5◦, 41.5◦, and 41.5°, and 59.9°) are attributable to various β-SiC facets, and the small peak observed at 34.0° is due 59.9◦) are attributable to various β-SiC facets, and the small peak observed at 34.0◦ is due to stacking to stacking faults in β-SiC. This result is unexpected because the C/SiO2 ratio of 0.5 is also much lower faults in β-SiC. This result is unexpected because the C/SiO2 ratio of 0.5 is also much lower than the than the stoichiometric ratio required for the carbothermal reduction of silica. Generally, a C/SiO2 stoichiometric ratio required for the carbothermal reduction of silica. Generally, a C/SiO2 ratio above theratio stoichiometric above the valuestoichiometric is required value for the is carbothermalrequired for the reduction carbothermal of silica reduction [14,23,26]. of Thus, silica the [14,23,26]. result observedThus, the in Figureresult 4observed strongly in suggests Figure 4 that strongly the surface suggests methyl that groups, the surface which methyl modify groups, the silica which aerogel modify particles,the silica represent aerogel anparticles, eﬃcient represent carbon source an efficient for the carbon carbothermal source for reduction. the carbothermal reduction.

Figure 4. XRD patterns of the powder samples synthesized under various C/SiO2 ratios at 1525 ◦C. Figure 4. XRD patterns of the powder samples synthesized under various C/SiO2 ratios at 1525 °C.

Figure5 illustrates the XRD patterns of the powder samples synthesized from the SiO 2-C mixture Figure 5 illustrates the XRD patterns of the powder samples synthesized from the SiO2-C mixture with a C/SiO2 ratio of 0.5 at various reduction temperatures for 1 h. At 1450 ◦C, the powder sample with a C/SiO2 ratio of 0.5 at various reduction temperatures for 1 h. At 1450 °C, the powder sample consists of β-SiC and α-cristobalite. As the reduction temperature increases, the peak intensity of consists of β-SiC and α-cristobalite. As the reduction temperature increases, the peak intensity of α- α-cristobalite gradually decreases, while that of β-SiC increases. Based on Figure5, it can be inferred cristobalite gradually decreases, while that of β-SiC increases. Based on Figure 5, it can be inferred that a reduction at 1525 ◦C is required to synthesize a single-phase β-SiC at a C/SiO2 ratio of 0.5. that a reduction at 1525 °C is required to synthesize a single-phase β-SiC at a C/SiO2 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 β-SiC forms at temperatures above 1516 ◦C[35]. Based on the XRD analysis (Figure5), β-SiC formed at 1450 ◦C, which is ~70 ◦C lower than the temperature obtained from the thermodynamic calculation (1516 ◦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 ◦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 ◦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 Section2. For comparison, Appl. Sci. 2020, 10, 6161 6 of 11 a commercial quartz powder was purchased from Junsei (extra pure, Japan) and used for an additional carbothermalAppl. Sci. 2020, reduction 10, x FOR PEER experiment. REVIEW 6 of 11

Figure 5. XRD patterns of the powder samples synthesized from the SiO2–C mixtures with the C/SiO2 Figure 5. XRD patterns of the powder samples synthesized from the SiO2–C mixtures with the C/SiO2 ratio of 0.5 at various reduction temperatures. ratio of 0.5 at various reduction temperatures. Figure6 illustrates the XRD patterns of the powder samples synthesized from the hydrophobic We calculated the equilibrium compositions for the carbothermal reduction reaction as a silica aerogel, aerogel without surface methyl groups (hydrophilic silica aerogel), and quartz powders. function of temperature in a previous study. The results indicated that β-SiC forms at temperatures The carbothermal reductions of the hydrophobic silica aerogel (Figure6a,b) and other samples above 1516 °C [35]. Based on the XRD analysis (Figure 5), β-SiC formed at 1450 °C, which is ~70 °C (Figure6c,d) were performed at 1525 and 1550 C, respectively. As described above, the powder sample lower than the temperature obtained from◦ the thermodynamic calculation (1516 °C). This synthesized from the hydrophobic silica aerogel containing surface methyl groups was single-phase discrepancy can be attributed to the enhanced reactivity between silica and carbon or the increased β-SiC, although the C/SiO ratio was 0.5 and the reduction temperature was 1525 C. By contrast, surface energy (nano-size2 effect) [36,37]. In this study, surface methyl groups are the◦ carbon source, the XRD analysis indicates that the powder sample synthesized from the hydrophilic silica aerogel is which are bonded to the silica aerogels at the atomistic level. Therefore, it can be expected that the α-cristobalite. Characteristic peaks corresponding to the crystalline β-SiC phase could not be observed. carbothermal reaction proceeds in a highly reactive state. In addition, the high mesopore volume and This phenomenon suggests that the surface methyl groups play important roles in the enhancement of high specific surface area of the silica aerogels allow the nano-size effect on the conversion of silica the carbothermal reduction reaction of silica. With respect to quartz, α-cristobalite remained in the to SiC [25]. The thermodynamic calculation of the carbothermal reduction of silica considering the synthesized powder, although the reduction temperature and C/SiO ratio were higher than those of surface energy contribution indicates that the formation temperature2 for SiC can be reduced to 1410 the hydrophobic silica aerogel. °C [35]. A schematic illustration of the proposed carbothermal reduction reaction mechanism for silica To confirm that the surface methyl groups of the silica aerogel were the carbon source for the aerogels is shown in Figure7. The surface of the hydrophobic silica aerogel was modiﬁed to contain carbothermal reduction of silica, the surface methyl groups were removed by calcining the silica methyl groups. During the carbothermal reduction of the aerogel, the C-H bonds were broken, whereas aerogel at 600 °C for 3 h in air. The resulting hydrophilic silica aerogel powder was mixed with carbon the tetrahedral environments of silicon and carbon were maintained. It can be inferred that amorphous black, and the carbothermal reduction was performed as described in Section 2. For comparison, a hydrogenated SiC or hydrogenated Si-C-O (silicon oxycarbide) has a cristobalite form at temperatures commercial quartz powder was purchased from Junsei (extra pure, Japan) and used for an additional between 1000 and 1400 C[38]. During this process, condensation reactions between –Si(CH ) and carbothermal reduction◦ experiment. 3 3 neighboring –Si(CH ) groups occur, which lead to the formation of CH and H from the consumed Figure 6 illustrates3 3 the XRD patterns of the powder samples synthesized4 2 from the hydrophobic CH groups [39]. Above 1400 C, SiC particles nucleate and grow toward the inside of the silica silica3 aerogel, aerogel without◦ surface methyl groups (hydrophilic silica aerogel), and quartz aerogel particles via a gas-phase reaction between SiO and CO, as has typically been observed for powders. The carbothermal reductions of the hydrophobic silica aerogel (Figure 6a,b) and other the carbothermal reduction of silica. It appears that the surface methyl groups not only act as the samples (Figure 6c,d) were performed at 1525 and 1550 °C, respectively. As described above, the carbon source but also serve as a template for the formation of the SiC particles [40–42]. As observed powder sample synthesized from the hydrophobic silica aerogel containing surface methyl groups in Figures6 and7, the SiC particles maintained the spherical morphology of the silica aerogel powder. was single-phase β-SiC, although the C/SiO2 ratio was 0.5 and the reduction temperature was 1525 Figure8 shows the SEM images of the silica aerogel and β-SiC powders synthesized from the SiO -C °C. By contrast, the XRD analysis indicates that the powder sample synthesized from the hydrophilic2 mixtures at various reduction temperatures. The C/SiO ratio was 0.5. Based on Figure8a, the starting silica aerogel is α-cristobalite. Characteristic peaks corresponding2 to the crystalline β-SiC phase could silica aerogel had spherical morphology and signiﬁcant aggregation cannot be observed. The particle not be observed. This phenomenon suggests that the surface methyl groups play important roles in size was estimated to be 5 to 10 µm. An interesting feature can be observed in Figure8b–e, that is, the enhancement of the carbothermal reduction reaction of silica. With respect to quartz, α- the synthesized β-SiC powders exhibit spherical morphology, regardless of the reduction temperature. cristobalite remained in the synthesized powder, although the reduction temperature and C/SiO2 ratio were higher than those of the hydrophobic silica aerogel. Appl. Sci. 2020, 10, x FOR PEER REVIEW 7 of 11

Appl. Sci. 2020, 10, 6161 7 of 11 Appl. Sci. 2020, 10, x FOR PEER REVIEW 7 of 11

Figure 6. XRD patterns of the powder samples synthesized from (a) the hydrophobic silica aerogel (C/SiO2 ratio of 0.5), (b) the hydrophobic silica aerogel (C/SiO2 ratio of 3), (c) the hydrophilic silica

aerogel (C/SiO2 ratio of 1), and (d) the quartz (C/SiO2 ratio of 3).

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 °C [38]. During this process, condensation reactions between – Si(CH3)3 and neighboring –Si(CH3)3 groups occur, which lead to the formation of CH4 and H2 from the consumed CH3 groups [39]. Above 1400 °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 theFigure carbothermal 7. Surface structure reduction of the of hydrophobicsilica. It appears silica that aerogel the used surface in this methyl study. groups not only Figure 7. Surface structure of the hydrophobic silica aerogel used in this study. act as the carbon source but also serve as a template for the formation of the SiC particles [40–42]. As The particle size and morphology of the SiC powder samples obtained at different C/SiO ratios observedFigure in 8 showsFigures the 6 andSEM 7, images the SiC of particles the silica ma aerogelintained and the β-SiC spherical powders morphology synthesized of fromthe2 silica the wereaerogel analyzed powder. using SEM. Figure9 shows the SEM images of the β-SiC powders synthesized from SiO2-C mixtures at various reduction temperatures. The C/SiO2 ratio was 0.5. Based on Figure 8a, the SiO -C mixtures with C/SiO ratios of 0.5 and 3. The carbothermal reduction was performed at 1525 C starting2 silica aerogel had spherical2 morphology and significant aggregation cannot be observed. The◦ for 1 h. The comparison of Figure9a,b shows that the morphology of the SiC particles was completely particle size was estimated to be 5 to 10 μm. An interesting feature can be observed in Figure 8b–e, different. At a C/SiO ratio of 0.5, the SiC particles were spherical. By contrast, a high C/SiO ratio of 3 that is, the synthesized2 β-SiC powders exhibit spherical morphology, regardless of the2 reduction yielded an agglomerated structure consisting of fine SiC particles. In addition, nanofiber-like SiC was temperature. observed between the fine SiC particles. The BET specific surface areas of the SiC powders prepared 2 using C/SiO2 ratios of 1 and 3 were estimated to be 6.4 and 12.7 m /g, respectively. This shows that the specific surface areas increase with the C/SiO2 ratio. These results suggest that the C/SiO2 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. The oxygen contents in the SiC powder prepared at 1525 ◦C for 5 h with C/SiO2 ratios of 0.5 and 3 were 0.36 and 0.05 wt.% for the samples after the heat treatment in air, respectively. Almost-pure SiC powders were obtained when Figure 7. Surface structure of the hydrophobic silica aerogel used in this study. the C/SiO2 ratio was 3. Figure 8 shows the SEM images of the silica aerogel and β-SiC powders synthesized from the SiO2-C mixtures at various reduction temperatures. The C/SiO2 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 μm. An interesting feature can be observed in Figure 8b–e, that is, the synthesized β-SiC powders exhibit spherical morphology, regardless of the reduction temperature. Appl. Sci. 2020, 10, 6161 8 of 11 Appl. Sci. 2020, 10, x FOR PEER REVIEW 8 of 11

FigureFigure 8.8. SEMSEM imagesimages ofof ((aa)) thethe startingstarting sphericalspherical silicasilica aerogelaerogel andand b-SiCb-SiC powderspowders synthesizedsynthesized fromfrom thethe SiOSiO22–C mixturesmixtures atat ((bb)) 14501450 ◦°C,C, ((cc)) 14751475 ◦°C,C, ((dd)) 15001500 ◦°C,C, andand ((ee)) 15251525◦ °CC forfor 33 h.h. TheThe CC/SiO/SiO22 ratioratio Appl. Sci.waswas 2020 0.5.0.5., 10 , x FOR PEER REVIEW 9 of 11

The particle size and morphology of the SiC powder samples obtained at different C/SiO2 ratios were analyzed using SEM. Figure 9 shows the SEM images of the β-SiC powders synthesized from SiO2-C mixtures with C/SiO2 ratios of 0.5 and 3. The carbothermal reduction was performed at 1525 °C for 1 h. The comparison of Figure 9a,b shows that the morphology of the SiC particles was completely different. At a C/SiO2 ratio of 0.5, the SiC particles were spherical. By contrast, a high C/SiO2 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/SiO2 ratios of 1 and 3 were estimated to be 6.4 and 12.7 m2/g, respectively. This shows that the specific surface areas increase with the C/SiO2 ratio. These results suggest that the C/SiO2 ratio affects the nucleation and growth mechanisms during the synthesis of Figure 9. SEM images of the SiC powders synthesized from SiO2–C mixtures with C/SiO2 ratios of (a) SiC powder. SEM images of the SiC powders synthesized from SiO2–C mixtures with C/SiO2 ratios of (0.5a) 0.5and and (b) (3b at) 3 1525 at 1525 °C forC 1 for h. 1The h. Thepowder powder samples samples were were heat-treated heat-treated in air in at air 1000 at 1000°C for C1 forh after 1 h To elucidate the effect◦ of the porous structure of the SiC on oxidation, the oxygen◦ content in the reduction.after reduction. SiC powder samples was determined using an oxygen–nitrogen analyzer. The oxygen contents in the SiC powder prepared at 1525 °C for 5 h with C/SiO2 ratios of 0.5 and 3 were 0.36 and 0.05 wt.% for The overallmolar ratio carbothermal of carbon reductionin the SiO reaction2-C mixture for silica,is of great which importance is a solid (SiObecause2)–solid the (C)CO reaction,gas that the samples after the heat treatment in air, respectively. Almost-pure SiC powders were obtained is shownproduced in Equation by Equations (1). At (1) low or temperature,(2) was sufficiently the carbothermal supplied. reductionThe high reactionC/SiO2 ratio can be(3) written leads to in an as increasefollowswhen the [ 43in C/SiO]: the partial2 ratio waspressure 3. of gaseous species, such as SiO(g) and CO(g); consequently, SiC The overall carbothermal reduction reaction for silica, which is a solid (SiO2)–solid (C) reaction, particles form and grow, as illustratedSiO2(s) in+ EquatiC(s) onsSiO(g) (3) and+ CO(g) (4). The nanofiber-like SiC observed (2)in → Figureis shown 9b inmight Equation be due (1). to Atthe low reaction temperature, expressed the in carbothermal Equation (4). reduction reaction can be written in SiO(g) + 2C(s) SiC(s) + CO(g) (3) as follows [43]: →

4. ConclusionsAt a high temperature and highSiO PSiO2(s)/ P+CO C(s)ratio, → SiO(g) whisker- + CO(g) or nanofiber-like SiC can be grown (2) on the SiC nuclei based on Equation (4), which is similar to the CVD of SiC [35,44]. The SiC particles β-SiC powders were synthesized by the carbothermal reduction of methyl-bearing surface- obtained through Equation (3) serve as nucleation→ sites for Equation (4). modified silica aerogel/carbon mixtures.SiO(g) + Based2C(s) on SiC(s) the +XRD CO(g) analysis, single-phase β-SiC was (3) synthesizedAt a high at 1525temperature °C from and the highsilica P aerogel/carbonSiO/PCO ratio, whisker- mixture or with nanofibe a C/SiOr-like2 ratio SiC ofcan 3. beThe grown surface on SiO(g) + 3CO(g) SiC(s) + 2CO2(g) (4) methylthe SiC groupsnuclei basedthat are on covalentlyEquation (4), bonded which to is thesimilar→ silica to aerogelthe CVD serve of SiC as [35,44].a carbon The source SiC particles for the carbothermalobtainedThe morphologiesthrough reduction Equation ofreaction; the (3) SiC serve therefore, particles as nucleation ditheffer, fo asrmationsites shown for temperatureEquation in Figure (4).9a,b, for whichSiC is indicateslower. The that direct the bondformation between mechanisms silicon and for carbon the SiC atoms particles in the di surfffer.ace At methyl a C/SiO groupratio results of 0.5, thein the morphologies enhanced kinetics of the SiO(g) + 3CO(g) → SiC(s) + 2CO2 2(g) (4) ofsynthesized the carbothermal SiC particles reduction are reacti similaron. to The that results of the show starting that the silica C/SiO aerogel,2 ratio suggesting plays an important that the role SiC in bothThe the morphologies reduction of theof the formation SiC particles temperatur differ,e as for shown SiC and in theFigure control 9a,b, of which the morphology indicates that of thethe formation mechanism relies on Equation (1), that is, the solid–solid reaction. The solid SiO2 particles synthesizeddirectlyformation react mechanisms SiC with particles. carbon for from At the a the C/SiOSiC surface particles2 ratio methyl of differ. 0.5, groups, SiC At maintainsa wherebyC/SiO2 ratio the spherical originalof 0.5, SiC the spherical powder morphologies wasmorphology observed of the ofsynthesized the silica aerogels.SiC particles By contrast are similar, agglomerates to that of thewith starting fine SiC silica part iclesaerogel, were suggesting synthesized that from the theSiC at a C/SiO2 ratio of 0.5. SiOformation2-C mixture mechanism at a C/SiO relies2 ratioon Equation of 3. This (1), phenomenonthat is, the solid–solid can be attributed reaction. Theto different solid SiO formation2 particles The molar ratio of carbon in the SiO2-C mixture is of great importance because the CO gas that mechanismsdirectly react for with SiC, carbon which fromdepend the on surface the silica–carbon methyl groups, ratio ofwhereby the starting spherical silica/carbon SiC powder mixture. was is produced by Equations (1) or (2) was sufficiently supplied. The high C/SiO2 ratio (3) leads to an Almostobserved pure at a SiC C/SiO powder2 ratio (99.95%) of 0.5. was obtained when the silica aerogel/carbon mixture at a C/SiO2 ratio of 3 was reduced at 1525 °C for 5 h.

Funding: This work was supported by LG Chem. This research was supported by the Ministry of Trade, Industry & Energy (MOTIE), Korea Institute for Advancement of Technology (KIAT), through the Encouragement Program for The Industries of Economic Cooperation Region (P0002149).

Conflicts of Interest: The authors declare no conflict of interest.

References

1. Shcherban, N.D. Review on synthesis, structure, physical and chemical properties and functional characteristics of porous silicon carbide. J. Ind. Eng. Chem. 2017, 50, 15–28.

2. Kim, K.; Hahn, Y.; Lee, S.; Choi, K.; Lee, J.-H. Mechanical Properties of Cf/SiC Composite Using a Combined Process of Chemical Vapor Infiltration and Precursor Infiltration Pyrolysis. J. Korean Ceram. Soc. 2018, 55, 392–399. Appl. Sci. 2020, 10, 6161 9 of 11 increase in the partial pressure of gaseous species, such as SiO(g) and CO(g); consequently, SiC particles form and grow, as illustrated in Equations (3) and (4). The nanoﬁber-like SiC observed in Figure9b might be due to the reaction expressed in Equation (4).

4. Conclusions β-SiC powders were synthesized by the carbothermal reduction of methyl-bearing surface-modified silica aerogel/carbon mixtures. Based on the XRD analysis, single-phase β-SiC was synthesized at 1525 ◦C from the silica aerogel/carbon mixture with a C/SiO2 ratio of 3. The surface methyl groups that are covalently bonded to the silica aerogel serve as a carbon source for the carbothermal reduction reaction; therefore, the formation temperature for SiC is lower. The direct bond between silicon and carbon atoms in the surface methyl group results in the enhanced kinetics of the carbothermal reduction reaction. The results show that the C/SiO2 ratio plays an important role in both the reduction of the formation temperature for SiC and the control of the morphology of the synthesized SiC particles. At a C/SiO2 ratio of 0.5, SiC maintains the original spherical morphology of the silica aerogels. By contrast, agglomerates with fine SiC particles were synthesized from the SiO2-C mixture at a C/SiO2 ratio of 3. This phenomenon can be attributed to different formation mechanisms for SiC, which depend on the silica–carbon ratio of the starting silica/carbon mixture. Almost pure SiC powder (99.95%) was obtained when the silica aerogel/carbon mixture at a C/SiO2 ratio of 3 was reduced at 1525 ◦C for 5 h.

Author Contributions: Conceptualization, K.-J.L. and H.H.; methodology, Y.K., Y.H.K. and S.W.B.; analysis, K.-J.L., Y.K. and Y.H.K.; writing—original draft preparation, K.-J.L. writing—review and editing, H.H.; supervision, H.H.; project administration, H.H. and S.W.B. All authors have read and agreed to the published version of the manuscript. Funding: This work was supported by LG Chem. This research was supported by the Ministry of Trade, Industry & Energy (MOTIE), Korea Institute for Advancement of Technology (KIAT), through the Encouragement Program for The Industries of Economic Cooperation Region (P0002149). Conﬂicts of Interest: The authors declare no conﬂict of interest.

References

1. Shcherban, N.D. Review on synthesis, structure, physical and chemical properties and functional characteristics of porous silicon carbide. J. Ind. Eng. Chem. 2017, 50, 15–28. [CrossRef]

2. Kim, K.; Hahn, Y.; Lee, S.; Choi, K.; Lee, J.-H. Mechanical Properties of Cf/SiC Composite Using a Combined Process of Chemical Vapor Infiltration and Precursor Infiltration Pyrolysis. J. Korean Ceram. Soc. 2018, 55, 392–399. [CrossRef] 3. Seo, Y.; Kim, Y.; Nishimura, T.; Seo, W.S. High-temperature strength of a thermally conductive silicon carbide ceramic sintered with yttria and scandia. J. Eur. Ceram. Soc. 2016, 36, 3755–3760. [CrossRef] 4. Chen, H.; Wang, X.; Xue, F.; Huang, Y.; Zhou, K.; Zhang, D. 3D printing of SiC ceramic, Direct ink writing with a solution of preceramic polymers. J. Eur. Ceram. Soc. 2018, 38, 5294–5300. [CrossRef] 5. Bernardo, E.; Fiocco, L.; Parcianello, G.; Storti, E.; Colombo, P. Advanced ceramics from preceramic polymers modified at the nano-scale: A review. Materials 2014, 7, 1927–1956. [CrossRef] 6. Prochazka, S.; Scanlan, R.M. Effect of boron and carbon on sintering of SiC. J. Am. Ceram. Soc. 1975, 58, 72. [CrossRef] 7. Syväjärvi, M.; Ma, Q.; Jokubavicius, V.; Galeckas, A.; Sun, J.; Liu, X.; Jansson, M.; Wellmann, P.; Linnarsson, M.; Runde, P.; et al. Cubic silicon carbide as a potential photovoltaic material. Sol. Energy Mater. Sol. Cells 2016, 145, 104–108. [CrossRef] 8. Kim, J.G.; Jeong, J.H.; Kim, Y.; Makarov, Y.; Choi, D.J. Evaluation of the change in properties caused by axial and radial temperature gradients in silicon carbide crystal growth using the physical vapor transport method. Acta Mater. 2014, 77, 54–59. [CrossRef] 9. Sciti, D.; Bellosi, A. Effects of additives on densification, microstructure and properties of liquid-phase sintered silicon carbide. J. Mater. Sci. 2000, 35, 3849–3855. [CrossRef] Appl. Sci. 2020, 10, 6161 10 of 11

10. Wang, X.; Cai, D.; Zhang, H. Increase of SiC sublimation growth rate by optimizing of powder packaging. J. Cryst. Growth 2007, 305, 122–132. [CrossRef] 11. Hayun, S.; Paris, V.; Mitrani, R.; Kalabukhov, S.; Dariel, M.P.; Zaretsky, E.; Frage, N. Microstructure and mechanical properties of silicon carbide processed by Spark Plasma Sintering (SPS). Ceram. Int. 2012, 38, 6335–6340. [CrossRef] 12. Kim, J.G.; Jung, E.J.; Kim, Y.; Makarov, Y.; Choi, D.J. Quality improvement of single crystal 4H SiC grown with a purified β-SiC powder source. Ceram. Int. 2014, 40, 3953–3959. [CrossRef] 13. Omidi, Z.; Ghasemi, A.; Bakhshi, S.R. Synthesis and characterization of SiC ultrafine particles by means of sol-gel and carbothermal reduction methods. Ceram. Int. 2015, 41, 5779–5784. [CrossRef] 14. Barbouche, M.; Zaghouani, R.B.; Benammar, N.E.; Khirouni, K.; Ezzaouia, H. Synthesis and characterization of 3C-SiC by rapid silica carbothermal reduction. Int. J. Adv. Manuf. Technol. 2017, 91, 1339–1345. [CrossRef] 15. Kim, J.; Choi, S.; Kim, J.; Choi, S.C. Characteristics of silicon carbide nanowires synthesized on porous body by carbothermal reduction. J. Korean Ceram. Soc. 2018, 55, 285–289. [CrossRef] 16. Kwon, W.T.; Kim, S.R.; Kim, Y.; Lee, Y.J.; Won, J.Y.; Park, W.K.; Oh, S.C. Effect of temperature and carbon contents on the synthesis of β-SiC from silicon sludge by direct carbonization method. Mater Sci. Forum. 2012, 724, 45–48. [CrossRef] 17. Yang, Y.; Lin, Z.; Li, J. Synthesis of SiC by silicon and carbon combustion in air. J. Eur. Ceram. Soc. 2009, 29, 175–180. [CrossRef] 18. Colombo, P.; Mera, G.; Riedel, R.; Soraru, G.D. Polymer-derived ceramics: 40 years of research and innovation in advanced ceramics. J. Am. Ceram. Soc. 2010, 93, 1805–1837. [CrossRef] 19. Andrievski, R.A. Synthesis, structure and properties of nanosized silicon carbide. Rev. Adv. Mater Sci. 2009, 22, 1–20. 20. Yazdanfar, M.; Pedersen, H.; Sukkaew, P.; Ivanov, I.G.; Danielsson, Ö.; Kordina, O.; Janzén, E. On the use of methane as a carbon precursor in Chemical Vapor Deposition of silicon carbide. J. Cryst. Growth 2014, 390, 24–29. [CrossRef] 21. Galvagno, S.; Portofino, S.; Casciaro, G.; Casu, S.; d’Aquino, L.; Martino, M.; Russo, A.; Bezzi, G. Synthesis of beta silicon carbide powders from biomass gasification residue. J. Mater. Sci. 2007, 42, 6878–6886. [CrossRef] 22. Kevorkijan, V.M.; Komac, M.; Kolar, D. Low-temperature synthesis of sinterable SiC powders by carbothermic

reduction of colloidal SiO2. J. Mater. Sci. 1992, 27, 2705–2712. [CrossRef] 23. Ba˘gci,C.; Arik, H. Synthesis of SiC Powders by carbothermal reduction of enriched brown sepiolite with carbon black. J. Mater. Eng. Perform. 2013, 22, 958–963. [CrossRef] 24. Shimoo, T. Carbon removal and oxidation of SiC powder synthesized by carbothermic reduction of silica. J. Ceram Soc. Jpn. 1991, 99, 768–773. [CrossRef] 25. Parmentier, J.; Patarin, J.; Dentzer, J.; Vix-Guterl, C. Formation of SiC via carbothermal reduction of a carbon-containing mesoporous MCM-48 silica phase: A new route to produce high surface area SiC. Ceram. Int. 2002, 28, 1–7. [CrossRef] 26. Meng, G.W.; Cui, Z.; Zhang, L.D.; Phillipp, F. Growth and characterization of nanostructured β-SiC via

carbothermal reduction of SiO2 xerogels containing carbon nanoparticles. J. Cryst. Growth 2000, 209, 801–806. [CrossRef] 27. Lee, K.; Kim, Y.H.; Lee, J.K.; Hwang, H.J. Fast synthesis of spherical silica aerogel powders by emulsion polymerization from water glass. ChemistrySelect 2018, 3, 1257–1261. [CrossRef] 28. Sarawade, P.B.; Kim, J.K.; Hilonga, A.; Quang, D.V.; Jeon, S.J.; Kim, H.T. Synthesis of sodium silicate-based hydrophilic silica aerogel beads with superior properties: Effect of heat-treatment. J. Non-Cryst. Solids 2011, 357, 2156–2162. [CrossRef] 29. Bhagat, S.D.; Kim, Y.H.; Suh, K.H.; Ahn, Y.S.; Yeo, J.G.; Han, J.H. Superhydrophobic silica aerogel powders with simultaneous surface modification, solvent exchange and sodium ion removal from hydrogels. Microporous Mesoporous Mater. 2008, 112, 504–509. [CrossRef] 30. Rao, A.P.; Rao, A.V.; Pajonk, G.M. Hydrophobic and physical properties of the two step processed ambient pressure dried silica aerogels with various exchanging solvents. J. Sol-Gel Sci. Technol. 2005, 36, 285–292. [CrossRef] 31. He, S.; Huang, Y.; Ghen, G.; Feng, M.; Dai, H.; Yuan, B.; Chen, X. Effect of heat treatment on hydrophobic silica aerogel. J. Hazard. Mater. 2019, 362, 294–302. [CrossRef][PubMed] Appl. Sci. 2020, 10, 6161 11 of 11

32. Shao, Z.D.; Cheng, X.; Zheng, Y.M. Facile co-precursor sol-gel synthesis of a novel amine-modified silica aerogel for high efficiency carbon dioxide capture. J. Colloid Interface Sci. 2018, 530, 412–423. [CrossRef] [PubMed] 33. Stojanovic, A.; Zhao, S.; Angelica, E.; Malfait, W.J.; Koebel, M.M. Three routes to superinsulating silica aerogel powder. J. Sol-Gel Sci. Technol. 2019, 90, 57–66. [CrossRef] 34. Bhagat, S.D.; Oh, C.; Kim, Y.; Ahn, Y.S.; Yeo, J.G. Methyltrimethoxysilane based monolithic silica aerogels via ambient pressure drying. Microporous Mesoporous Mater. 2007, 100, 350–355. [CrossRef] 35. Jeong, S.; Seo, W.; Jung, I.; Lee, K.J.; Hwang, H.J. Thermodynamic analysis of the synthesis of silicon carbide nanofibers from exfoliated graphite and amorphous silica. CrystEngComm 2014, 16, 2348–2351. [CrossRef] 36. Choi, H.; Lee, J. Continuous synthesis of silicon carbide whiskers. J. Mater. Sci. 1995, 30, 1982–1986. [CrossRef] 37. Eichhammer, Y.; Roeck, J.; Moelans, N.; Iacopi, F.; Blanpain, B.; Heyns, M. Calculation of the Au-Ge phase diagram for nanoparticles. Arch. Metall. Mater. 2008, 53, 1133–1139. 38. Bouillon, E.; Langlais, F.; Pailler, R.; Naslain, R.; Cruege, F.; Huong, P.V.; Sarthou, J.C.; Delpuech, A.; Laffon, C.; Lagarde, P.; et al. Conversion mechanisms of a polycarbosilane precursor into an SiC-based ceramic material. J. Mater. Sci. 1991, 26, 1333–1345. [CrossRef] 39. Soraru, G.D.; Babonneau, F.; Mackenzie, J.D. Structural evolutions from polycarbosilane to SiC ceramic. J. Mater. Sci. 1990, 25, 3886–3893. [CrossRef] 40. Xie, W.; Möbus, G.; Zhang, S. Molten salt synthesis of silicon carbide nanorods using carbon nanotubes as templates. J. Mater. Chem. 2011, 21, 18325–18330. [CrossRef] 41. Wu, Y.J.; Qin, W.; Yang, Z.X.; Wu, J.S.; Zhang, Y.F. Preparation of high-quality ß-SiC nanowhiskers by using carbon fibres as carbon source. J. Mater. Sci. 2004, 39, 5563–5565. [CrossRef] 42. Tony, V.; Voon, C.H.; Lee, C.C.; Lim, B.Y.; Arshad, M.M.; Gopinath, S.C.; Foo, K.L.; Ruslinda, A.R.; Hashim, U.; Nashaain, M.N. Novel synthesis of silicon carbide nanotubes by microwave heating of blended silicon dioxide and multi-walled carbon nanotubes: The effect of the heating temperature. Ceram. Int. 2016, 42, 17642–17649. [CrossRef] 43. Van Dijen, F.K.; Metselaar, R. The chemistry of the carbothermal synthesis of β-SiC: Reaction mechanism, reaction rate and grain growth. J. Eur. Ceram. Soc. 1991, 7, 177–184. [CrossRef] 44. Moshtaghioun, B.M.; Poyato, R.; Cumbrera, F.L.; de Bernardi-Martin, S.; Monshi, A.; Abbasi, M.H.; Karimzadeh, F.; Dominguez-Rodriguez, A. Rapid carbothermic synthesis of silicon carbide nano powders by using microwave heating. J. Eur. Ceram. Soc. 2012, 32, 1787–1794. [CrossRef]

© 2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).