- Email: [email protected]
silicon carbide composite aerogels
Journal of Non-Crystalline Solids 358 (2012) 3150–3155
Contents lists available at SciVerse ScienceDirect
Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/ locate/ jnoncrysol
Facile synthesis of resorcinol–formaldehyde/silica composite aerogels and their transformation to monolithic carbon/silica and carbon/silicon carbide composite aerogels Yong Kong, Ya Zhong, Xiaodong Shen ⁎, Sheng Cui, Meng Yang, Kaiming Teng, Junjun Zhang College of Materials Science and Technology, Nanjing University of Technology, Nanjing 210009, PR China
a r t i c l e
i n f o
Article history: Received 6 June 2012 Received in revised form 31 August 2012 Available online 5 October 2012 Keywords: Aerogels; Resorcinol–formaldehyde/silica composite; Silicon carbide; Carbothermal reduction
a b s t r a c t Resorcinol–formaldehyde/silica composite (RF/SiO2) gels were synthesized in one pot by simply mixing the monomers and dried under supercritical carbon dioxide to form RF/SiO2 aerogels. Carbon/silica composite (C/SiO2) and carbon/silicon carbide composite (C/SiC) aerogels were formed from the RF/SiO2 aerogels after carbonization and carbothermal reduction. The as-prepared C/SiC products exhibited a preserved monolithic morphology similar to the original templates and were composed of carbon particles and α-SiC nanocrystals. The C/SiC specimen possessed a BET surface area of 892 m2/g and a porosity of 94.8%, both of which were significantly higher than the BET surface area and porosity of C/SiO2 and RF/SiO2 aerogels. The resulting C/SiC monolith was stable up to temperatures near 550 °C, which is almost 150 °C higher than what C/SiO2 aerogels can tolerate. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Aerogels are unique porous materials with a distinctive microstructure consisting of pores and particles in the nanometer size range. Recently, increased attention has been given to monolithic aerogels due to their combined compact integral structure and porous microstructure that exhibits low density, good mechanical behavior, large internal void space and high specific surface area in a material [1–6]. Among these, carbon/silicon carbide composite (C/SiC) aerogels have excellent properties, including chemical and thermal stability, high conductivity, high surface area and high porosity [4], and can potentially be used as adsorbents, thermal insulators or electrode materials [7–10]. In the past, binary carbonaceous silica aerogels were used as precursors of carbon/silica composite (C/SiO2) aerogels for the synthesis of porous SiC by carbothermal reduction [2–4,11–13]. However, the sol–gel processes employed to form hybrid gels were complicated and time-consuming. In these techniques, tetraethylorthosilicate (TEOS) and tetramethylorthosilicate (TMOS) were usually used as a silicon source, and acid and alkali were involved as catalysts. If catalysts are not involved, neither the polycondensation of resorcinol and formaldehyde nor the hydrolysis and polymerization of Si(OR)4 can take place. Anhydrous sodium carbonate was frequently used in the preparation of RF gels, and silica gels could be prepared using acid–base catalyzed two-step or base catalyzed single-step ⁎ Corresponding author. Tel.: +86 25 83587235; fax: +86 25 83221690. E-mail addresses: [email protected] (Y. Kong), [email protected] (X. Shen). 0022-3093/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jnoncrysol.2012.08.029
methods. Therefore, the conflict of catalysts is a considerable problem in the process of forming hybrid gels. Moreover, there is a very different gelation time between RF gels and silica gels. Therefore, to form carbon– silica hybrid gels, the silica sol and carbonaceous sol had to be prepared separately, and the processing goes through a multiple-step sol–gel process. As a matter of fact, a structurally uniform monolithic hybrid gel is hard to prepare if resorcinol, formaldehyde and Si(OR)4 are only used as reactants. Ke Chen et al. [2] proposed a way to synthesize RF/SiO2 gels by introducing acetonitrile to the system. RF sol was first prepared using hydrochloric acid as a catalyst, and then the TEOS solution was added into the RF sol, and lastly HF was added to obtain the hybrid gels. Nicholas Leventis et al. [3] used acrylonitrile instead of resorcinol and formaldehyde as a carbon source to synthesize hybrid gels, which is more complicated than the method that was reported by Ke Chen et al. 3-(Aminopropyl)triethoxysilane (APTES) and (3-aminopropyl) trimethoxysilane (APTMS) were commonly used as amino-functionalized modifiers in the synthesis of porous silica [14–20], but have never been used solely as silica sources. Recently, we have reported a method of preparing nanoporous amine-based sorbent using only APTES as a silica source [21], but the process was still complex and timeconsuming. Therefore, according to the modified technology process, we propose a facile synthesis of resorcinol–formaldehyde/silica composite (RF/SiO2) aerogels. Without mixing two sols (silica sol and RF sol), RF/SiO2 gels were synthesized in one pot by simply mixing the monomers. Only three reactants (APTES, resorcinol and formaldehyde) and a solvent (anhydrous alcohol) were involved in the sol–gel process, and no catalysts were required. C/SiO2 and C/SiC aerogels were formed after thermal treatment at different temperatures.
Y. Kong et al. / Journal of Non-Crystalline Solids 358 (2012) 3150–3155
2. Experimental 2.1. Chemicals APTES, resorcinol (R), formaldehyde (F, 37% w/w aqueous solution) and anhydrous alcohol (C2H5OH) were used as raw materials. All of the reagents and solvents were of analytical grade and used as received without further purification. 2.2. Preparation of RF/SiO2 aerogels Scheme 1 shows the synthesis route of RF/SiO2 aerogels and their conversion to C/SiO2 and C/SiC aerogels. Resorcinol, formaldehyde, APTES and alcohol were mixed in a pot at room temperature, with R:F:APTES:C2H5OH prepared at a molar ratio of 1:2:1:60. Subsequently, the compound was transferred into polypropylene molds (48 mm in inner diameter) and placed into an air oven at 60± 0.1 °C. The liquid gelled within 70 min. After gelation, the wet gel was demolded, aged at 75±0.1 °C for 24 h and simultaneously washed with ethanol every 8 h to remove water and residual chemicals. After solvent exchange, the alcohol gels were dried in an autoclave (HELIX 1.1 system, Applied Separations, Inc., Allentown, PA) with supercritical fluid CO2 to form RF/SiO2 aerogels. 2.3. Formation of C/SiO2 and C/SiC aerogels The thermal treatment was performed in a tube furnace (72 and 80 mm inner and outer diameters of tube, respectively, 120 mm heating zone). RF/SiO2 aerogels were initially carbonized to form C/SiO2 aerogels
3151
under flowing argon (150 ±10 ml/min) at 800 °C for 3 h. Subsequently, the flow rate of argon was reduced to 60±10 ml/min, and the temperature was increased (1500 °C) for the carbothermal reaction and maintained for 5 h. Samples for analysis were removed at 800 °C and 1500 °C, by interrupting heating and by letting the tube furnace cool down to room temperature under flowing argon. 2.4. Measurement and characterization Bulk densities (ρb) were calculated from the weight and the physical dimensions of the samples. Skeletal densities (ρs) were determined by helium pycnometry using a Micromeritics AcuuPyc II 1340 instrument. Porosity was determined from the ρb and ρs values, porosity=1− ρb / ρs. The microstructure and energy spectrum of the specimens were surveyed by LEO-1530VP scanning electron microscopy (SEM). The phase composition of the sample was evaluated by ARL ARLX'TRA X-ray diffraction (XRD) using a Cu-Kα radiation. Transmission electron microscopy (TEM) was conducted using a JEOL JEM-2010 electron microscope. Surface areas, average pore diameter, pore volume and pore size distribution were measured by nitrogen adsorption/desorption porosimetry using a Micromeritics ASAP2020 surface area and pore distribution analyzer after the samples were degassed in a vacuum at 200 °C for 6 h. The specific surface area (σ) was calculated using Brunaur–Emmett–Teller (BET) and t-plot (for micropores) methods. By using the non-local density functional theory (NLDFT) model, the pore size distribution was derived from the desorption branch of isotherms, and the total pore volume was estimated from the adsorbed amount at a relative pressure p/p0 of 0.986. Thermogravimetric analysis (TGA) was performed using a NETZSCH STA449C thermogravimetric analyzer to determine the thermal stability under a constant air flow of 30 ml/min at a heating rate of 10 °C/min. The SiO2 and SiC contents were also determined by thermogravimetric analysis. The weight fraction of the remaining material was assumed to be pure stoichiometric SiO2 and SiC. 3. Results
Scheme 1. The synthesis of RF/SiO2 aerogels and their conversion to C/SiO2 and C/SiC aerogels.
Fig. 1 shows the photographs of RF/SiO2, C/SiO2 and C/SiC aerogels. The selected characterization data of samples are summarized in Table 1. Fig. 2 shows the XRD patterns of samples at different stages. The XRD spectra of RF/SiO2 and C/SiO2 aerogels are similar and have no visible diffraction peak that corresponds to the presence of amorphous silica and carbon. For the C/SiC aerogel, the peaks with 2θ values of 34°, 35.7°, 38.2°, 41.4°, 60°, 65.6°, 71.8°, 73.6°, and 75.5° correspond to the crystal planes of 101, 102, 103, 104, 110, 109, 116, 203 and 0012, respectively, for moissanite (6H of α-SiC) (PDF# 29-1131). No other crystalline phases of silica, carbon or other impurities were detected. Analysis of the peaks using the Scherrer equation indicates that the average crystallite size of SiC is approximately 15 nm. Fig. 3(a)–(c) shows the SEM images of RF/SiO2, C/SiO2 and C/SiC aerogels. The SEM photographs at higher magnification have been incorporated as insets. RF/SiO2 and C/SiO2 aerogels exhibit the disordered, porous structures of a typical colloidal gel. The particles of RF/SiO2 and C/SiO2 aerogels are spherical. On the contrary, the particles of C/SiC aerogels are non-spherical and indistinguishable, which are similar to the staghorn coral. The EDX spectrum (Fig. 3(d)–(e)) demonstrates that the oxygen decreased after carbothermal reduction processes, indicating that the SiO2 was deoxidized and transformed to SiC. Transmission electron microscopy (TEM) was performed to further understand the crystal structure and microstructure of C/SiC aerogels. From the TEM image (Fig. 4(a)), we can see that the SiC nanoparticles and carbon particles have been composited. As observed from the high-resolution transmission electron image (HRTEM, Fig. 4(b)), the lattice fringe, with a spacing of approximately 0.235 nm, corresponds
3152
Y. Kong et al. / Journal of Non-Crystalline Solids 358 (2012) 3150–3155
Fig. 2. XRD data of RF/SiO2 aerogels (a), C/SiO2 aerogels (b) and C/SiC aerogels (c).
loop in the IUPAC classification, suggesting that they are mesoporous. All pore structure characterization data including the porosity of the samples are summarized in Table 2. Pore-size distribution has a clear transformation throughout the process, shown as insets in Fig. 5. Pore-size distribution of RF/SiO2 aerogels presents a broad continuous distribution from 1 nm to 70 nm, with a peak near 9 nm. Relative to RF/SiO2 aerogels, the pores in the range of 3–10 nm disappear and many micropores appear around 1.8 nm in the C/SiO2 aerogels. There are only three narrow peaks near 1.9 nm, 13.5 nm and 21.6 nm in the pore-size distribution curve of the C/SiC aerogels. Fig. 6 shows the TGA curves for the C/SiO2 and C/SiC aerogels in air. For comparison, data for the pure carbon aerogel are also included. As expected, combustion of the pure carbon aerogel begins at 380 °C, and the material is completely consumed by 525 °C. The onset mass loss of free carbon for the C/SiO2 composite is 400 °C, which is close to that of carbon aerogels. For the C/SiO2, complete oxidation of the carbon occurs at 640 °C. Further improvements in thermal stability are observed in the C/SiC composite. Mass loss does not begin until the temperature reaches 550 °C, which is approximately 150 °C higher than the temperature at which this occurs in C/SiO2, and complete oxidation of carbon does not occur until 650 °C. 4. Discussion Fig. 1. The photographs of RF/SiO2 aerogels (a), C/SiO2 aerogels (b) and C/SiC aerogels (c).
to the 103 crystal plane of α-SiC (PDF# 29-1131). Selected area electron diffraction (SAED) patterns of C/SiC aerogels are shown as inset in Fig. 4(a), and the crystal planes are marked. The pore structures of the RF/SiO2, C/SiO2 and C/SiC aerogels are evaluated using nitrogen adsorption/desorption analysis. Fig. 5 shows the nitrogen adsorption/desorption isotherms of RF/SiO2, C/SiO2 and C/SiC aerogels. They are all type IV curves with a type H1 hysteresis
Table 1 Selected property of samples. Sample
Linear shrinkage (%)a
Mass loss (%)b
Bulk density, ρb (g/cm3)
Skeletal density, ρs (g/cm3)
RF/SiO2 C/SiO2 C/SiC
3.1 ± 0.1 28 ± 0.5 33.8 ± 0.2
– 48.1 ± 0.2 65.7 ± 0.3
0.101 ± 0.002 0.139 ± 0.004 0.117 ± 0.001
1.4454 ± 0.0005 1.9139 ± 0.0006 2.2706 ± 0.0002
a b
Relative to the molds (48 mm diameter). Mass loss relative to RF/SiO2.
As mentioned above, to prepare monolithic C/SiO2 and C/SiC aerogels, the hybrid gels were synthesized firstly. The traditional methods of forming hybrid gels were complicated and timeconsuming. The reasons were analyzed above. Beyond that, the compatibility of the reagents must be also considered. It is found in the experiment that the system presented a poor mutual solubility if all the regents (R, F, TEOS, C2H5OH and H2O) were mixed together at first. And the system consisting of resorcinol, formaldehyde, APTES and ethanol does not have that trouble, and it is also found that the sol– gel reaction did not come up without formaldehyde. Most importantly, the amino groups of APTES are an “internal catalyst” for the condensation of resorcinol and formaldehyde [22]. The gelation was strongly accelerated in the presence of APTES. It could be explained by the formation of intramolecular hydrogen bonds in the hydrolyzed species of APTES. The formation of such a cyclic intermediate could provide an energetically favorable reaction path, thereby accelerating the sol–gel process [23]. The results of the experiment and characterization show that the role of APTES is not just that of a catalyst in the system. It is also involved in the network formation process. It is probable that the hydrolyzed species of APTES not only condense with themselves, but also react with the intermediate from the reaction of resorcinol and formaldehyde. All of that provides a one-step sol–gel process and offers
Y. Kong et al. / Journal of Non-Crystalline Solids 358 (2012) 3150–3155
3153
Fig. 3. SEM images of RF/SiO2 aerogels (a), C/SiO2 aerogels (b), and C/SiC aerogels (c) and the EDX spectra (d, e) of C/SiO2 aerogels and C/SiC aerogels. Insets: the corresponding images at a higher magnification.
an even homogeneous distribution between silica and carbon phases, which is in favor of carbothermal reduction. Most carbon gels reported in the literature were prepared with resorcinol as carbon source, and the other isomers of resorcinol (pyrocatechol and catechol) have been little used. Organic aerogels obtained from the polymerization reaction of pyrocatechol with formaldehyde have been reported [24]. In our work, pyrocatechol is available, but the gels cannot form while using catechol. However, the solubility of pyrocatechol in water is very poor relative to resorcinol. The strength of the resulting RF/SiO2 gels derived from resorcinol is lower than that derived from pyrocatechol. Resorcinol is preferred in our system. Although C/SiO2 and C/SiC aerogels exhibited significant weight loss and shrinking relative to RF/SiO2 aerogels, they preserved their monolithic morphology. The mass loss approaches 50% during the carbonization process, but the density of the C/SiO2 aerogels exhibited a
substantial increase relative to the RF/SiO2 aerogels due to the significant amount of shrinkage that occurs. The density decreased after carbothermal reduction owing to the consumption of carbon in the process of carbothermal reduction. As described, the particles of the C/SiC aerogels are non-spherical and indistinguishable after carbothermal reduction, which are similar to the staghorn coral. It may result from the fact that a SiC-coating formed on the carbon particles and the particles are bonded by SiC, because the carbothermal reduction occurs more easily between the interface of SiO2 and the carbon particles [2]. In combination with the XRD patterns, the selected area electron diffraction (SAED) rings and the HRTEM image reveal that the microstructure of the SiC nanoparticles is an alpha phase structure. Anyhow, there is no evidence of existence of β-SiC. However, the mesoporous SiC products synthesized from C/SiO2 composites by carbothermal reduction are generally β phase [3,4,25–27]. β-SiC reportedly starts forming at
3154
Y. Kong et al. / Journal of Non-Crystalline Solids 358 (2012) 3150–3155
Fig. 4. TEM image (a, b), SAED patterns (inset in (a)), and HRTEM image (c) of C/SiC aerogels.
1250 °C from C/SiO2 aerogels under dynamic Ar-flow [25]. Similarly, a small amount of 6H-SiC reportedly starts forming from 1600 °C [2]. For our carbothermal work, SiC cannot be detected below 1500 °C. But owing to the nanometer effect, the generated SiC can transform to the alpha phase as soon as the SiC forms according to the characterization data. The BET specific surface areas of RF/SiO2, C/SiO2 and C/SiC aerogels increase from 384 m 2/g to 430 m 2/g and 892 m 2/g, respectively, as shown by the increased volume of N2 adsorbed. More micropores (b2 nm) that can contribute to the increase in surface area appear after thermal treatment [3]. Simultaneously, the pore volume of micropores and mesopores increases after heat treatment. These indicate that the process of charring and carbothermal reduction can lead to an increase of micropores and mesopores, due to the pyrolysis of organics and the consumption of carbon. However, it can be worked out that a sample with porosity in the range of 93–95% and a bulk density of 0.101–0.139 g/cm 3 should have a total porosity in the range of 6–10 cm 3/g, which is much higher than the test result. So it is worth noting that the total mesoporosity measured from the N2 adsorption analysis is underestimated largely and a large fraction of macroporosity is present in the samples. Pore-size distribution curves suggest that the biggish pores gradually disappear and the pore-size distribution presents narrower pores in the specific scope after heat treatment. For the C/SiO2 aerogels, the mass loss below 125 °C is due to water and organic impurities adsorbed in the inner surface of the pores. This improvement in thermal stability of C/SiC aerogels suggests that the SiC can supply an effective barrier to oxygen diffusion. The products gain weight after the free carbon is expended, denoting that the oxidation of SiC in air has occurred. The materials retain 40.5% and 57.5% of the original mass for C/SiO2 and C/SiC aerogels, respectively. It is remarkable that the content of SiC in C/SiC composites exhibits a noticeable increase relative to that of SiO2 in C/SiO2 aerogels, which can be explained by the reaction mechanism. On the basis of the reaction mechanism of carbothermal reduction [2–4,11–13], the SiC content in C/SiC aerogels is 60.6 wt.%, approaching the test value. The Si, C, and O contents in C/SiO2 and C/SiC composites are shown in Table 3. 5. Conclusions In conclusion, a simple method for the synthesis of RF/SiO2 aerogels was demonstrated. After carbonization and carbothermal reduction, RF/ SiO2 aerogels were converted to monolithic C/SiO2 and C/SiC aerogels. XRD and TEM analyses indicated that nanocrystalline α-SiC was formed after the carbothermal reaction. The as-synthesized C/SiC aerogels
showed mesostructures with high surface area and high pore volume. Additionally, the monoliths exhibited good anti-oxidation properties. This new class of materials can be applied in technologies such as catalysis, thermal isolation and energy storage where a high surface area, accessible pore volume, and high temperature resistance are desired. Acknowledgment This work was supported by the National Defense Preliminary Research Foundation (613120020020202) of China and the Graduate Student Innovation Plan (CXLX11_0343) of Jiangsu province. We also acknowledge the support from the Priority Academic Program Development of Jiangsu Higher Education Institution (PAPD) and the Program for Changjiang Scholars and Innovative Research Team in University (PCSIRT). References [1] A. Pons, Ll. Casas, E. Estop, E. Molins, K.D.M. Harris, M. Xu, J. Non-Cryst. Solids 358 (2012) 461–469. [2] N. Leventis, A. Sadekar, N. Chandrasekaran, C. Sotiriou-Leventis, Chem. Mater. 22 (2010) 2790–2803. [3] K. Chen, Z. Bao, A. Du, X. Zhu, G. Wu, J. Shen, B. Zhou, Microporous Mesoporous Mater. 149 (2012) 16–24. [4] M.A. Worsley, J.D. Kuntz, J.H. Satcher Jr., T.F. Baumann, J. Mater. Chem. 20 (2010) 4840–4844. [5] N. Leventis, S. Mulik, X. Wang, A. Dass, V.U. Patil, C. Sotiriou-Leventis, H. Lu, G. Churu, A. Capecelatro, J. Non-Cryst. Solids 354 (2008) 632–644. [6] H. Xu, H. Zhang, Y. Huang, Y. Wang, J. Non-Cryst. Solids 356 (2010) 971–976. [7] J. Lee, S. Yoon, S.M. Oh, C.H. Shin, T. Hyeon, Adv. Mater. 12 (2000) 359–362. [8] J. Lee, J. Kim, Y. Lee, S. Yoon, S.M. Oh, T. Hyeon, Chem. Mater. 16 (2004) 3323–3330. [9] Y. Mastai, S. Polarz, M. Antonietti, Adv. Funct. Mater. 12 (2002) 197–202. [10] D. Lee, P.C. Stevens, S.Q. Zeng, A.J. Hunt, J. Non-Cryst. Solids 186 (1995) 285–290. [11] X. Li, X. Chen, H. Song, J. Mater. Sci. 44 (2009) 4661–4667. [12] X.K. Li, L. Liu, Y.X. Zhang, S.D. Shen, S. Ge, L.C. Ling, Carbon 39 (2001) 159–165. [13] J.Q. Hu, Q.Y. Lu, K.B. Tang, B. Deng, R.R. Jiang, Y.T. Qian, W.C. Yu, G.E. Zhou, X.M. Liu, J.X. Wu, J. Phys. Chem. B 104 (2000) 5251–5254. [14] S. Cui, W. Cheng, X. Shen, M. Fan, A. Russell, Z. Wu, X. Yi, Energy Environ. Sci. 4 (2011) 2070–2074. [15] L.D. White, C.P. Tripp, J. Colloid Interface Sci. 232 (2000) 400–407. [16] T. Borrego, M. Andrade, M.L. Pinto, A.R. Silva, A.P. Carvalho, J. Rocha, C. Freire, J. Pires, J. Colloid Interface Sci. 344 (2010) 603–610. [17] J.C. Hicks, R. Dabestani, A.C. Buchanan III, C.W. Jones, Inorg. Chim. Acta 361 (2008) 3024–3032. [18] J. Mondal, A. Modak, A. Bhaumik, J. Mol. Catal. A: Chem. 335 (2011) 236–241. [19] S. Hamoudi, A. El-Nemr, K. Belkacemi, J. Colloid Interface Sci. 343 (2010) 615–621. [20] M. Alnaief, I. Smirnova, J. Non-Cryst. Solids 356 (2010) 1644–1649. [21] L. He, M. Fan, B. Dutcher, S. Cui, X. Shen, Y. Kong, A.G. Russel, P. McCurdy, Chem. Eng. J. 189–190 (2012) 13–23. [22] N. Hüsing, U. Schubert, Chem. Mater. 11 (1999) 451–457. [23] B.V. Zhmud, J. Sonnefeld, J. Non-Cryst. Solids 195 (1996) 16–27. [24] C. Moreno-Castilla, M.B. Dawidziuk, F. Carrasco-Marín, Z. Zapata-Benabithe, Carbon 49 (2011) 3808–3819.
Y. Kong et al. / Journal of Non-Crystalline Solids 358 (2012) 3150–3155
3155
Fig. 6. TG curves of RF/SiO2, C/SiO2 and C/SiC aerogels in air.
Table 3 Component of C/SiO2 and C/SiC aerogels. Sample Si (wt.%)
Fig. 5. Nitrogen adsorption/desorption isotherms for RF/SiO2 aerogels (a), C/SiO2 aerogels (b) and C/SiC aerogels (c). Insets: the corresponding pore size distribution.
[25] G. Jin, X. Guo, Microporous Mesoporous Mater. 60 (2003) 207–212. [26] X. Yuan, J. Lü, X. Yan, L. Hu, Q. Xue, Microporous Mesoporous Mater. 142 (2011) 754–758. [27] Z. Liu, W. Shen, W. Bu, H. Chen, Z. Hua, L. Zhang, L. Li, J. Shi, S. Tan, Microporous Mesoporous Mater. 82 (2005) 137–145.
Table 2 Pore structure properties of aerogels. Sample
BET surface area, σ (m2/g)
Micropore surface area (m2/g)
Mesopore volume (cm3/g)
Micropore volume (cm3/g)
Porosity (%)
RF/SiO2 C/SiO2 C/SiC
384 430 892
77 182 229
1.4376 1.6762 2.6000
0.03159 0.08206 0.09810
93 92.7 94.8
Si (at.%)
C/SiO2
18.9±0.2
9.66±0.05
C/SiC
40.25± 0.07
22.4±0.1
C (wt.%) 59.5±0.5
C (at.%)
O (wt.%) O (at.%)
71.01±0.08 21.6± 0.3 59.75±0.06 77.6±0.7 0
19.33± 0.03 0
Contents lists available at SciVerse ScienceDirect
Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/ locate/ jnoncrysol
Facile synthesis of resorcinol–formaldehyde/silica composite aerogels and their transformation to monolithic carbon/silica and carbon/silicon carbide composite aerogels Yong Kong, Ya Zhong, Xiaodong Shen ⁎, Sheng Cui, Meng Yang, Kaiming Teng, Junjun Zhang College of Materials Science and Technology, Nanjing University of Technology, Nanjing 210009, PR China
a r t i c l e
i n f o
Article history: Received 6 June 2012 Received in revised form 31 August 2012 Available online 5 October 2012 Keywords: Aerogels; Resorcinol–formaldehyde/silica composite; Silicon carbide; Carbothermal reduction
a b s t r a c t Resorcinol–formaldehyde/silica composite (RF/SiO2) gels were synthesized in one pot by simply mixing the monomers and dried under supercritical carbon dioxide to form RF/SiO2 aerogels. Carbon/silica composite (C/SiO2) and carbon/silicon carbide composite (C/SiC) aerogels were formed from the RF/SiO2 aerogels after carbonization and carbothermal reduction. The as-prepared C/SiC products exhibited a preserved monolithic morphology similar to the original templates and were composed of carbon particles and α-SiC nanocrystals. The C/SiC specimen possessed a BET surface area of 892 m2/g and a porosity of 94.8%, both of which were significantly higher than the BET surface area and porosity of C/SiO2 and RF/SiO2 aerogels. The resulting C/SiC monolith was stable up to temperatures near 550 °C, which is almost 150 °C higher than what C/SiO2 aerogels can tolerate. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Aerogels are unique porous materials with a distinctive microstructure consisting of pores and particles in the nanometer size range. Recently, increased attention has been given to monolithic aerogels due to their combined compact integral structure and porous microstructure that exhibits low density, good mechanical behavior, large internal void space and high specific surface area in a material [1–6]. Among these, carbon/silicon carbide composite (C/SiC) aerogels have excellent properties, including chemical and thermal stability, high conductivity, high surface area and high porosity [4], and can potentially be used as adsorbents, thermal insulators or electrode materials [7–10]. In the past, binary carbonaceous silica aerogels were used as precursors of carbon/silica composite (C/SiO2) aerogels for the synthesis of porous SiC by carbothermal reduction [2–4,11–13]. However, the sol–gel processes employed to form hybrid gels were complicated and time-consuming. In these techniques, tetraethylorthosilicate (TEOS) and tetramethylorthosilicate (TMOS) were usually used as a silicon source, and acid and alkali were involved as catalysts. If catalysts are not involved, neither the polycondensation of resorcinol and formaldehyde nor the hydrolysis and polymerization of Si(OR)4 can take place. Anhydrous sodium carbonate was frequently used in the preparation of RF gels, and silica gels could be prepared using acid–base catalyzed two-step or base catalyzed single-step ⁎ Corresponding author. Tel.: +86 25 83587235; fax: +86 25 83221690. E-mail addresses: [email protected] (Y. Kong), [email protected] (X. Shen). 0022-3093/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jnoncrysol.2012.08.029
methods. Therefore, the conflict of catalysts is a considerable problem in the process of forming hybrid gels. Moreover, there is a very different gelation time between RF gels and silica gels. Therefore, to form carbon– silica hybrid gels, the silica sol and carbonaceous sol had to be prepared separately, and the processing goes through a multiple-step sol–gel process. As a matter of fact, a structurally uniform monolithic hybrid gel is hard to prepare if resorcinol, formaldehyde and Si(OR)4 are only used as reactants. Ke Chen et al. [2] proposed a way to synthesize RF/SiO2 gels by introducing acetonitrile to the system. RF sol was first prepared using hydrochloric acid as a catalyst, and then the TEOS solution was added into the RF sol, and lastly HF was added to obtain the hybrid gels. Nicholas Leventis et al. [3] used acrylonitrile instead of resorcinol and formaldehyde as a carbon source to synthesize hybrid gels, which is more complicated than the method that was reported by Ke Chen et al. 3-(Aminopropyl)triethoxysilane (APTES) and (3-aminopropyl) trimethoxysilane (APTMS) were commonly used as amino-functionalized modifiers in the synthesis of porous silica [14–20], but have never been used solely as silica sources. Recently, we have reported a method of preparing nanoporous amine-based sorbent using only APTES as a silica source [21], but the process was still complex and timeconsuming. Therefore, according to the modified technology process, we propose a facile synthesis of resorcinol–formaldehyde/silica composite (RF/SiO2) aerogels. Without mixing two sols (silica sol and RF sol), RF/SiO2 gels were synthesized in one pot by simply mixing the monomers. Only three reactants (APTES, resorcinol and formaldehyde) and a solvent (anhydrous alcohol) were involved in the sol–gel process, and no catalysts were required. C/SiO2 and C/SiC aerogels were formed after thermal treatment at different temperatures.
Y. Kong et al. / Journal of Non-Crystalline Solids 358 (2012) 3150–3155
2. Experimental 2.1. Chemicals APTES, resorcinol (R), formaldehyde (F, 37% w/w aqueous solution) and anhydrous alcohol (C2H5OH) were used as raw materials. All of the reagents and solvents were of analytical grade and used as received without further purification. 2.2. Preparation of RF/SiO2 aerogels Scheme 1 shows the synthesis route of RF/SiO2 aerogels and their conversion to C/SiO2 and C/SiC aerogels. Resorcinol, formaldehyde, APTES and alcohol were mixed in a pot at room temperature, with R:F:APTES:C2H5OH prepared at a molar ratio of 1:2:1:60. Subsequently, the compound was transferred into polypropylene molds (48 mm in inner diameter) and placed into an air oven at 60± 0.1 °C. The liquid gelled within 70 min. After gelation, the wet gel was demolded, aged at 75±0.1 °C for 24 h and simultaneously washed with ethanol every 8 h to remove water and residual chemicals. After solvent exchange, the alcohol gels were dried in an autoclave (HELIX 1.1 system, Applied Separations, Inc., Allentown, PA) with supercritical fluid CO2 to form RF/SiO2 aerogels. 2.3. Formation of C/SiO2 and C/SiC aerogels The thermal treatment was performed in a tube furnace (72 and 80 mm inner and outer diameters of tube, respectively, 120 mm heating zone). RF/SiO2 aerogels were initially carbonized to form C/SiO2 aerogels
3151
under flowing argon (150 ±10 ml/min) at 800 °C for 3 h. Subsequently, the flow rate of argon was reduced to 60±10 ml/min, and the temperature was increased (1500 °C) for the carbothermal reaction and maintained for 5 h. Samples for analysis were removed at 800 °C and 1500 °C, by interrupting heating and by letting the tube furnace cool down to room temperature under flowing argon. 2.4. Measurement and characterization Bulk densities (ρb) were calculated from the weight and the physical dimensions of the samples. Skeletal densities (ρs) were determined by helium pycnometry using a Micromeritics AcuuPyc II 1340 instrument. Porosity was determined from the ρb and ρs values, porosity=1− ρb / ρs. The microstructure and energy spectrum of the specimens were surveyed by LEO-1530VP scanning electron microscopy (SEM). The phase composition of the sample was evaluated by ARL ARLX'TRA X-ray diffraction (XRD) using a Cu-Kα radiation. Transmission electron microscopy (TEM) was conducted using a JEOL JEM-2010 electron microscope. Surface areas, average pore diameter, pore volume and pore size distribution were measured by nitrogen adsorption/desorption porosimetry using a Micromeritics ASAP2020 surface area and pore distribution analyzer after the samples were degassed in a vacuum at 200 °C for 6 h. The specific surface area (σ) was calculated using Brunaur–Emmett–Teller (BET) and t-plot (for micropores) methods. By using the non-local density functional theory (NLDFT) model, the pore size distribution was derived from the desorption branch of isotherms, and the total pore volume was estimated from the adsorbed amount at a relative pressure p/p0 of 0.986. Thermogravimetric analysis (TGA) was performed using a NETZSCH STA449C thermogravimetric analyzer to determine the thermal stability under a constant air flow of 30 ml/min at a heating rate of 10 °C/min. The SiO2 and SiC contents were also determined by thermogravimetric analysis. The weight fraction of the remaining material was assumed to be pure stoichiometric SiO2 and SiC. 3. Results
Scheme 1. The synthesis of RF/SiO2 aerogels and their conversion to C/SiO2 and C/SiC aerogels.
Fig. 1 shows the photographs of RF/SiO2, C/SiO2 and C/SiC aerogels. The selected characterization data of samples are summarized in Table 1. Fig. 2 shows the XRD patterns of samples at different stages. The XRD spectra of RF/SiO2 and C/SiO2 aerogels are similar and have no visible diffraction peak that corresponds to the presence of amorphous silica and carbon. For the C/SiC aerogel, the peaks with 2θ values of 34°, 35.7°, 38.2°, 41.4°, 60°, 65.6°, 71.8°, 73.6°, and 75.5° correspond to the crystal planes of 101, 102, 103, 104, 110, 109, 116, 203 and 0012, respectively, for moissanite (6H of α-SiC) (PDF# 29-1131). No other crystalline phases of silica, carbon or other impurities were detected. Analysis of the peaks using the Scherrer equation indicates that the average crystallite size of SiC is approximately 15 nm. Fig. 3(a)–(c) shows the SEM images of RF/SiO2, C/SiO2 and C/SiC aerogels. The SEM photographs at higher magnification have been incorporated as insets. RF/SiO2 and C/SiO2 aerogels exhibit the disordered, porous structures of a typical colloidal gel. The particles of RF/SiO2 and C/SiO2 aerogels are spherical. On the contrary, the particles of C/SiC aerogels are non-spherical and indistinguishable, which are similar to the staghorn coral. The EDX spectrum (Fig. 3(d)–(e)) demonstrates that the oxygen decreased after carbothermal reduction processes, indicating that the SiO2 was deoxidized and transformed to SiC. Transmission electron microscopy (TEM) was performed to further understand the crystal structure and microstructure of C/SiC aerogels. From the TEM image (Fig. 4(a)), we can see that the SiC nanoparticles and carbon particles have been composited. As observed from the high-resolution transmission electron image (HRTEM, Fig. 4(b)), the lattice fringe, with a spacing of approximately 0.235 nm, corresponds
3152
Y. Kong et al. / Journal of Non-Crystalline Solids 358 (2012) 3150–3155
Fig. 2. XRD data of RF/SiO2 aerogels (a), C/SiO2 aerogels (b) and C/SiC aerogels (c).
loop in the IUPAC classification, suggesting that they are mesoporous. All pore structure characterization data including the porosity of the samples are summarized in Table 2. Pore-size distribution has a clear transformation throughout the process, shown as insets in Fig. 5. Pore-size distribution of RF/SiO2 aerogels presents a broad continuous distribution from 1 nm to 70 nm, with a peak near 9 nm. Relative to RF/SiO2 aerogels, the pores in the range of 3–10 nm disappear and many micropores appear around 1.8 nm in the C/SiO2 aerogels. There are only three narrow peaks near 1.9 nm, 13.5 nm and 21.6 nm in the pore-size distribution curve of the C/SiC aerogels. Fig. 6 shows the TGA curves for the C/SiO2 and C/SiC aerogels in air. For comparison, data for the pure carbon aerogel are also included. As expected, combustion of the pure carbon aerogel begins at 380 °C, and the material is completely consumed by 525 °C. The onset mass loss of free carbon for the C/SiO2 composite is 400 °C, which is close to that of carbon aerogels. For the C/SiO2, complete oxidation of the carbon occurs at 640 °C. Further improvements in thermal stability are observed in the C/SiC composite. Mass loss does not begin until the temperature reaches 550 °C, which is approximately 150 °C higher than the temperature at which this occurs in C/SiO2, and complete oxidation of carbon does not occur until 650 °C. 4. Discussion Fig. 1. The photographs of RF/SiO2 aerogels (a), C/SiO2 aerogels (b) and C/SiC aerogels (c).
to the 103 crystal plane of α-SiC (PDF# 29-1131). Selected area electron diffraction (SAED) patterns of C/SiC aerogels are shown as inset in Fig. 4(a), and the crystal planes are marked. The pore structures of the RF/SiO2, C/SiO2 and C/SiC aerogels are evaluated using nitrogen adsorption/desorption analysis. Fig. 5 shows the nitrogen adsorption/desorption isotherms of RF/SiO2, C/SiO2 and C/SiC aerogels. They are all type IV curves with a type H1 hysteresis
Table 1 Selected property of samples. Sample
Linear shrinkage (%)a
Mass loss (%)b
Bulk density, ρb (g/cm3)
Skeletal density, ρs (g/cm3)
RF/SiO2 C/SiO2 C/SiC
3.1 ± 0.1 28 ± 0.5 33.8 ± 0.2
– 48.1 ± 0.2 65.7 ± 0.3
0.101 ± 0.002 0.139 ± 0.004 0.117 ± 0.001
1.4454 ± 0.0005 1.9139 ± 0.0006 2.2706 ± 0.0002
a b
Relative to the molds (48 mm diameter). Mass loss relative to RF/SiO2.
As mentioned above, to prepare monolithic C/SiO2 and C/SiC aerogels, the hybrid gels were synthesized firstly. The traditional methods of forming hybrid gels were complicated and timeconsuming. The reasons were analyzed above. Beyond that, the compatibility of the reagents must be also considered. It is found in the experiment that the system presented a poor mutual solubility if all the regents (R, F, TEOS, C2H5OH and H2O) were mixed together at first. And the system consisting of resorcinol, formaldehyde, APTES and ethanol does not have that trouble, and it is also found that the sol– gel reaction did not come up without formaldehyde. Most importantly, the amino groups of APTES are an “internal catalyst” for the condensation of resorcinol and formaldehyde [22]. The gelation was strongly accelerated in the presence of APTES. It could be explained by the formation of intramolecular hydrogen bonds in the hydrolyzed species of APTES. The formation of such a cyclic intermediate could provide an energetically favorable reaction path, thereby accelerating the sol–gel process [23]. The results of the experiment and characterization show that the role of APTES is not just that of a catalyst in the system. It is also involved in the network formation process. It is probable that the hydrolyzed species of APTES not only condense with themselves, but also react with the intermediate from the reaction of resorcinol and formaldehyde. All of that provides a one-step sol–gel process and offers
Y. Kong et al. / Journal of Non-Crystalline Solids 358 (2012) 3150–3155
3153
Fig. 3. SEM images of RF/SiO2 aerogels (a), C/SiO2 aerogels (b), and C/SiC aerogels (c) and the EDX spectra (d, e) of C/SiO2 aerogels and C/SiC aerogels. Insets: the corresponding images at a higher magnification.
an even homogeneous distribution between silica and carbon phases, which is in favor of carbothermal reduction. Most carbon gels reported in the literature were prepared with resorcinol as carbon source, and the other isomers of resorcinol (pyrocatechol and catechol) have been little used. Organic aerogels obtained from the polymerization reaction of pyrocatechol with formaldehyde have been reported [24]. In our work, pyrocatechol is available, but the gels cannot form while using catechol. However, the solubility of pyrocatechol in water is very poor relative to resorcinol. The strength of the resulting RF/SiO2 gels derived from resorcinol is lower than that derived from pyrocatechol. Resorcinol is preferred in our system. Although C/SiO2 and C/SiC aerogels exhibited significant weight loss and shrinking relative to RF/SiO2 aerogels, they preserved their monolithic morphology. The mass loss approaches 50% during the carbonization process, but the density of the C/SiO2 aerogels exhibited a
substantial increase relative to the RF/SiO2 aerogels due to the significant amount of shrinkage that occurs. The density decreased after carbothermal reduction owing to the consumption of carbon in the process of carbothermal reduction. As described, the particles of the C/SiC aerogels are non-spherical and indistinguishable after carbothermal reduction, which are similar to the staghorn coral. It may result from the fact that a SiC-coating formed on the carbon particles and the particles are bonded by SiC, because the carbothermal reduction occurs more easily between the interface of SiO2 and the carbon particles [2]. In combination with the XRD patterns, the selected area electron diffraction (SAED) rings and the HRTEM image reveal that the microstructure of the SiC nanoparticles is an alpha phase structure. Anyhow, there is no evidence of existence of β-SiC. However, the mesoporous SiC products synthesized from C/SiO2 composites by carbothermal reduction are generally β phase [3,4,25–27]. β-SiC reportedly starts forming at
3154
Y. Kong et al. / Journal of Non-Crystalline Solids 358 (2012) 3150–3155
Fig. 4. TEM image (a, b), SAED patterns (inset in (a)), and HRTEM image (c) of C/SiC aerogels.
1250 °C from C/SiO2 aerogels under dynamic Ar-flow [25]. Similarly, a small amount of 6H-SiC reportedly starts forming from 1600 °C [2]. For our carbothermal work, SiC cannot be detected below 1500 °C. But owing to the nanometer effect, the generated SiC can transform to the alpha phase as soon as the SiC forms according to the characterization data. The BET specific surface areas of RF/SiO2, C/SiO2 and C/SiC aerogels increase from 384 m 2/g to 430 m 2/g and 892 m 2/g, respectively, as shown by the increased volume of N2 adsorbed. More micropores (b2 nm) that can contribute to the increase in surface area appear after thermal treatment [3]. Simultaneously, the pore volume of micropores and mesopores increases after heat treatment. These indicate that the process of charring and carbothermal reduction can lead to an increase of micropores and mesopores, due to the pyrolysis of organics and the consumption of carbon. However, it can be worked out that a sample with porosity in the range of 93–95% and a bulk density of 0.101–0.139 g/cm 3 should have a total porosity in the range of 6–10 cm 3/g, which is much higher than the test result. So it is worth noting that the total mesoporosity measured from the N2 adsorption analysis is underestimated largely and a large fraction of macroporosity is present in the samples. Pore-size distribution curves suggest that the biggish pores gradually disappear and the pore-size distribution presents narrower pores in the specific scope after heat treatment. For the C/SiO2 aerogels, the mass loss below 125 °C is due to water and organic impurities adsorbed in the inner surface of the pores. This improvement in thermal stability of C/SiC aerogels suggests that the SiC can supply an effective barrier to oxygen diffusion. The products gain weight after the free carbon is expended, denoting that the oxidation of SiC in air has occurred. The materials retain 40.5% and 57.5% of the original mass for C/SiO2 and C/SiC aerogels, respectively. It is remarkable that the content of SiC in C/SiC composites exhibits a noticeable increase relative to that of SiO2 in C/SiO2 aerogels, which can be explained by the reaction mechanism. On the basis of the reaction mechanism of carbothermal reduction [2–4,11–13], the SiC content in C/SiC aerogels is 60.6 wt.%, approaching the test value. The Si, C, and O contents in C/SiO2 and C/SiC composites are shown in Table 3. 5. Conclusions In conclusion, a simple method for the synthesis of RF/SiO2 aerogels was demonstrated. After carbonization and carbothermal reduction, RF/ SiO2 aerogels were converted to monolithic C/SiO2 and C/SiC aerogels. XRD and TEM analyses indicated that nanocrystalline α-SiC was formed after the carbothermal reaction. The as-synthesized C/SiC aerogels
showed mesostructures with high surface area and high pore volume. Additionally, the monoliths exhibited good anti-oxidation properties. This new class of materials can be applied in technologies such as catalysis, thermal isolation and energy storage where a high surface area, accessible pore volume, and high temperature resistance are desired. Acknowledgment This work was supported by the National Defense Preliminary Research Foundation (613120020020202) of China and the Graduate Student Innovation Plan (CXLX11_0343) of Jiangsu province. We also acknowledge the support from the Priority Academic Program Development of Jiangsu Higher Education Institution (PAPD) and the Program for Changjiang Scholars and Innovative Research Team in University (PCSIRT). References [1] A. Pons, Ll. Casas, E. Estop, E. Molins, K.D.M. Harris, M. Xu, J. Non-Cryst. Solids 358 (2012) 461–469. [2] N. Leventis, A. Sadekar, N. Chandrasekaran, C. Sotiriou-Leventis, Chem. Mater. 22 (2010) 2790–2803. [3] K. Chen, Z. Bao, A. Du, X. Zhu, G. Wu, J. Shen, B. Zhou, Microporous Mesoporous Mater. 149 (2012) 16–24. [4] M.A. Worsley, J.D. Kuntz, J.H. Satcher Jr., T.F. Baumann, J. Mater. Chem. 20 (2010) 4840–4844. [5] N. Leventis, S. Mulik, X. Wang, A. Dass, V.U. Patil, C. Sotiriou-Leventis, H. Lu, G. Churu, A. Capecelatro, J. Non-Cryst. Solids 354 (2008) 632–644. [6] H. Xu, H. Zhang, Y. Huang, Y. Wang, J. Non-Cryst. Solids 356 (2010) 971–976. [7] J. Lee, S. Yoon, S.M. Oh, C.H. Shin, T. Hyeon, Adv. Mater. 12 (2000) 359–362. [8] J. Lee, J. Kim, Y. Lee, S. Yoon, S.M. Oh, T. Hyeon, Chem. Mater. 16 (2004) 3323–3330. [9] Y. Mastai, S. Polarz, M. Antonietti, Adv. Funct. Mater. 12 (2002) 197–202. [10] D. Lee, P.C. Stevens, S.Q. Zeng, A.J. Hunt, J. Non-Cryst. Solids 186 (1995) 285–290. [11] X. Li, X. Chen, H. Song, J. Mater. Sci. 44 (2009) 4661–4667. [12] X.K. Li, L. Liu, Y.X. Zhang, S.D. Shen, S. Ge, L.C. Ling, Carbon 39 (2001) 159–165. [13] J.Q. Hu, Q.Y. Lu, K.B. Tang, B. Deng, R.R. Jiang, Y.T. Qian, W.C. Yu, G.E. Zhou, X.M. Liu, J.X. Wu, J. Phys. Chem. B 104 (2000) 5251–5254. [14] S. Cui, W. Cheng, X. Shen, M. Fan, A. Russell, Z. Wu, X. Yi, Energy Environ. Sci. 4 (2011) 2070–2074. [15] L.D. White, C.P. Tripp, J. Colloid Interface Sci. 232 (2000) 400–407. [16] T. Borrego, M. Andrade, M.L. Pinto, A.R. Silva, A.P. Carvalho, J. Rocha, C. Freire, J. Pires, J. Colloid Interface Sci. 344 (2010) 603–610. [17] J.C. Hicks, R. Dabestani, A.C. Buchanan III, C.W. Jones, Inorg. Chim. Acta 361 (2008) 3024–3032. [18] J. Mondal, A. Modak, A. Bhaumik, J. Mol. Catal. A: Chem. 335 (2011) 236–241. [19] S. Hamoudi, A. El-Nemr, K. Belkacemi, J. Colloid Interface Sci. 343 (2010) 615–621. [20] M. Alnaief, I. Smirnova, J. Non-Cryst. Solids 356 (2010) 1644–1649. [21] L. He, M. Fan, B. Dutcher, S. Cui, X. Shen, Y. Kong, A.G. Russel, P. McCurdy, Chem. Eng. J. 189–190 (2012) 13–23. [22] N. Hüsing, U. Schubert, Chem. Mater. 11 (1999) 451–457. [23] B.V. Zhmud, J. Sonnefeld, J. Non-Cryst. Solids 195 (1996) 16–27. [24] C. Moreno-Castilla, M.B. Dawidziuk, F. Carrasco-Marín, Z. Zapata-Benabithe, Carbon 49 (2011) 3808–3819.
Y. Kong et al. / Journal of Non-Crystalline Solids 358 (2012) 3150–3155
3155
Fig. 6. TG curves of RF/SiO2, C/SiO2 and C/SiC aerogels in air.
Table 3 Component of C/SiO2 and C/SiC aerogels. Sample Si (wt.%)
Fig. 5. Nitrogen adsorption/desorption isotherms for RF/SiO2 aerogels (a), C/SiO2 aerogels (b) and C/SiC aerogels (c). Insets: the corresponding pore size distribution.
[25] G. Jin, X. Guo, Microporous Mesoporous Mater. 60 (2003) 207–212. [26] X. Yuan, J. Lü, X. Yan, L. Hu, Q. Xue, Microporous Mesoporous Mater. 142 (2011) 754–758. [27] Z. Liu, W. Shen, W. Bu, H. Chen, Z. Hua, L. Zhang, L. Li, J. Shi, S. Tan, Microporous Mesoporous Mater. 82 (2005) 137–145.
Table 2 Pore structure properties of aerogels. Sample
BET surface area, σ (m2/g)
Micropore surface area (m2/g)
Mesopore volume (cm3/g)
Micropore volume (cm3/g)
Porosity (%)
RF/SiO2 C/SiO2 C/SiC
384 430 892
77 182 229
1.4376 1.6762 2.6000
0.03159 0.08206 0.09810
93 92.7 94.8
Si (at.%)
C/SiO2
18.9±0.2
9.66±0.05
C/SiC
40.25± 0.07
22.4±0.1
C (wt.%) 59.5±0.5
C (at.%)
O (wt.%) O (at.%)
71.01±0.08 21.6± 0.3 59.75±0.06 77.6±0.7 0
19.33± 0.03 0