Synthesis and characterization of silica aerogels dried under ambient pressure bed on water glass

Journal of Non-Crystalline Solids 410 (2015) 58–64

Contents lists available at ScienceDirect

Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/ locate/ jnoncrysol

Synthesis and characterization of silica aerogels dried under ambient pressure bed on water glass Song He a, Dongmei Huang b, Haijiang Bi a, Zhi Li a, Hui Yang a, Xudong Cheng a,⁎ a b

State Key Laboratory of Fire Science, University of Science and Technology of China, Hefei, Anhui 230027, PR China College of Quality and Safety Engineering, China Jiliang University, Hangzhou, Zhejiang 310018, PR China

a r t i c l e

i n f o

Article history: Received 7 October 2014 Received in revised form 6 December 2014 Accepted 7 December 2014 Available online xxxx Keywords: Water glass; Sol–gel process; Silica aerogel; Ambient pressure; DMF

a b s t r a c t In this paper, we report the experimental results on the synthesis of water glass based silica aerogels, which were dried under ambient pressure. Water glass was hydrolyzed and condensed in water using HCl as the catalyst. To minimize shrinkage during drying process, N,N-dimethylformamide (DMF), acting as drying control chemical additive (DCCA), was introduced. Before the ambient pressure drying, solvent exchange and surface modification were completed. In order to get hydrophobic aerogel, trimethylchlorosilane (TMCS) was used to modify the hydrophilic hydrogel surface. Here a large amount of TMCS can be saved compared with that in single step solvent exchange/surface modification method. The effects of DMF on the physical and textural properties of the resulting aerogels were investigated. When the molar ratio of Si in water glass to DMF is 2.23, the synthesized silica aerogels have better properties. Characterized by FT-IR, SEM, BET, etc., the resulting aerogels have welldeveloped mesoporous structure (mean pore size of ~ 15 nm) with super hydrophobicity (contact angle of 161°) and excellent absorption capacity of organic liquids. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Silica aerogels are low density, translucent and thermal insulating material consisting of nanoparticle building blocks, networked together to form an open, highly porous structure. They have a large surface area (500–1500 m 2 /g), high porosity (80–99%) and low bulk density (0.03–0.35 g/cm3). Due to these unusual properties, aerogels have attracted much attention for they have large potential to be applied in many fields. Silica aerogels modified with methyl (−CH3) groups can function very well in the sorption of organics, and their adsorption capacities are 10 times higher than those of activated carbon [1–3]. Silica aerogel beads (micron-sized) can easily be filled into hollow spaces and provide high thermal resistance. Further, silica aerogels can serve as thermal super-insulators in solar energy systems, refrigerators, thermal flasks [4–6], internal confinement fusion (ICF) targets for thermonuclear fusion reactions [7], very efficient catalysts and catalytic supports [8,9]. The extremely low density and high surface area of monolithic aerogels provide an opportunity to improve the performance of various metal-oxide-based devices, including gas and bio-sensors, batteries, heterogeneous catalysis devices, and low dielectric constant materials for integrated circuits (low-k dielectrics) [10–13]. Despite these advantages, aerogels are still not routinely found in our daily life, because they are fragile, collapse easily and difficult to prepare in a large-scale industrial production setting. ⁎ Corresponding author. E-mail address: [email protected] (X. Cheng).

http://dx.doi.org/10.1016/j.jnoncrysol.2014.12.011 0022-3093/© 2014 Elsevier B.V. All rights reserved.

Silica aerogels are produced by removing the entrapped solvent from the wet gel while maintaining the integrity and high porosity of the gel. Supercritical fluid drying and ambient pressure drying are the most commonly used drying methods. Several researchers have used supercritical organic solvents for drying [3,10,11,14–18] to synthesize silica aerogel. Generally, fluids under supercritical pressures have almost zero surface tension. And this avoids the collapse and shrinkage caused by the surface tension force associated with the removal of the fluid. However, supercritical drying, involving high pressures (5–10 MPa), requires more power and high quality of manufacturing equipments. Some researchers have prepared silica aerogel using tetraethoxysilane (TEOS) under ambient pressure [19–21]. However TEOS is a rather expensive organic reagent as precursor. To reduce the preparation cost of silica aerogel, water glass has been chosen as a much cheaper precursor. Based on water glass, aerogels have been synthesized by researchers through sol–gel process to form a gel under ambient pressure drying [22–26]. Sodium ion should be removed from the water glass solution to improve the optical transmission of the final aerogel product [26]. The pore size distribution (PSD) is a key factor that judges and decides the quality of silica aerogels. And the DCCA was studied to have the influence on PSD of silica aerogels [27]. Several researchers have investigated the DCCA's effect with different agents [28–33] in recent years. In this work, to obtain a well distributed pore structure, DMF, acted as DCCA, has been introduced and its effect on the properties of water glass based silica aerogel was studied, which has not been investigated before. The aerogels with low density, low shrinkage and high porosity have been obtained by adding appropriate DMF. The pore size distribution became

S. He et al. / Journal of Non-Crystalline Solids 410 (2015) 58–64

uniform. Besides, much less amount of TMCS was used to completely modify the gel compared with previous research [34]. 2. Experimental procedure 2.1. Sample preparation Silica aerogels were prepared by a single-step sol–gel process followed by ambient pressure drying. The precursor used for preparation of hydrogels was water glass (wt: 34%, QDSS (Qingdao Dongyue Sodium Silicate CO., Ltd, China), Na2O::SiO2 = 1::3.33). Other agents including hydrochloric acid (HCl), DMF, ethanol (EtOH), n-hexane and trimethylchlorosilane (TMCS) were purchased from SCRC (Sinopharm Chemical Reagent CO., Ltd, China). Water glass was diluted with deionized water. The hydrolysis was carried out with the participation of HCl (5 M), before which DMF was added to the solution. The sol was stirred for 1 min in room temperature and the pH is about 6.2. The hydrolysis reaction led to the formation of silicic acid with the probable byproduct of NaCl. The gelation occurred within 15 min. To age and remove the sodium, the gel was soaked with enough deionized water in a beaker, which was put in 45 °C water bath in 10 h. The water was removed by filtration. Ethanol was added into the gel to exchange water within the pores of hydrogel twice in 12 h and the ethanol was replaced by n-hexane in 6 h. The whole exchange process was carried out in 45 °C water bath and the solvent was removed by filtration. The surface chemical modification of the gel was carried out by adding the silylating mixture of TMCS: hexane with the volume ratio of 15% to the gel under the condition of 45 °C water bath in 12 h. Then the silylated gel was dried in ambient pressure at 60 °C for 2 h and finally dried at 100 °C to get aerogel. The aerogel was weighted every 2 h to make sure that the aerogel was dried completely. 2.2. Characterization The bulk density of the aerogel sample was calculated by measuring its mass to volume ratio. The porosity of the aerogels was calculated using the following formula: porosity ¼

  ρ 1− a  100% ρs

ð1Þ

where ρa is the bulk density of aerogel and ρs is the skeleton density of the silica aerogels. Generally the value of ρs is 2.2 g/cm3. The content of sodium in the aerogel was measured by Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-AES). The contact angle measurements were performed using a KSV (Helsinki, Finland) CAM 200 contact angle goniometer at ~ 20 °C. The thermal stability of aerogel sample was studied by the thermal gravimetric and differential thermal analysis (TG-DTA) using SDT Q600 (TA Instruments, USA). TG-DTA can also help to determine the oxidation temperature of –CH3 group contained in the aerogel, that is, the temperature up to which the silica aerogel samples retain its hydrophobicity. The hydrophobic nano-porous silica aerogels with a weight of 6 mg were heat-treated in air, from room temperature (25 °C) up to 800 °C, with a controlled heating rate of 10 °C·min−1. The microstructure and morphology of the aerogels were studied using Field Emission Scanning Electron Microscope (FESEM) (SIRION 200, FEI, USA). The specific surface areas were determined by nitrogen sorption isotherms with standard Brunauer–Emmett–Teller (BET) analysis (Tristar II 3020 M, Micromeritics Instrument Corporation, USA). The cumulative pore volume was calculated from the N2 adsorption–desorption profiles. The average pore diameters and pore size distributions (PSD) were estimated by the Barrett–Joyner–Halenda (BJH) method (Tristar II 3020 M, Micromeritics Instrument Corporation, USA). The surface chemical modification of aerogels was studied using Fourier Transform Infrared

59

Spectroscopy (FT-IR) (Nicolet 8700, Thermo Fisher Scientific, USA), which gave information about various chemical bonds, such as O\H, C\H, and Si\O\Si. For this purpose, the silica aerogels were ground into refined powders, mixed with KBr and pressed to form a sample pellet for FT-IR measurements. To measure the absorption capacity of the silica aerogels, the samples were immersed into various types of solvents/oils. The absorption process was very fast and generally reached equilibrium within a few minutes. Then the soaked samples were taken out and weighed after the aerogel surface was blotted using a filter paper to remove excess surface solvent/oil [35]. The absorption capacity (Q) was calculated from the mass gain using Eq. (2), Q ð%Þ ¼

ðW−W 0 Þ  100% ρL V 0

ð2Þ

where W0 and W are the weights of aerogels before and after absorption respectively, V0 is the aerogel volume, and ρL is the density of absorbed solvent/oil. The weight measurements of the aerogels with absorbed oil were carried out quickly to avoid evaporation of the solvent/oil. 3. Results and discussion Silica hydrogels were prepared by a single-step sol–gel process of deionized water (DI·H2O) diluted water glass in the presence of acid (hydrochloric acid) catalyst, as shown in the following reactions: Hydrolysis :

Na2 SiO3 þ H2 O þ 2HCl→SiðOHÞ4 þ 2NaCl

Condensation :

SiðOHÞ4 þ ðOHÞ4 Si→ðOHÞ3 Si–O–SiðOHÞ3 þ H2 O:

ð3Þ ð4Þ

When the liquid contained in the pores of hydrogel vaporised, the capillary pressure which can lead to pore collapse is occurred. It can be calculated with the following equation: p ¼ 2σcosθ=r

ð5Þ

where p is capillary pressure, σ is surface tension of solvent, θ is contact angle and r represents the capillary radius. To minimize the capillary pressure, the solvent exchange and surface modification process is necessary. Through comparison (Table 1), ethanol and n-hexane were chosen to be the exchange solvents, respectively. TMCS was the surface silylation reagent and the chemical reaction in the surface modification process is as follows: ðCH3 Þ3 SiCl þ ≡ Si–OH→ ≡ Si–O–SiðCH3 Þ3 þ HCl:

ð6Þ

After gelation, measured by ICP-AES, the weight content of sodium in the hydrogel is 1.24%, while it sharply decreased to 0.02% after the DI·H2O washing process. So the DI·H2O washing is an effective way to remove the sodium in the hydrogel. To determine the volume ratio of the solvent DI water to water glass, a series of experiments setting the volume ratio from 1 to 5 with increment of 1 were conducted, in which enough TMCS was added to the system to modify the gel. The porosity is presented in Fig. 1. It is clear that when the volume ratio is 3, the obtained aerogels have the highest porosity. And thus the volume ratio of DI water to water glass is fixed 3. Table 1 Surface tension of several solvents at 20 °C [36]. Solvent

Surface tension (10−3 N·m−1)

Isopropanol n-Hexane Acetone Ethanol Ethylene glycol Water

21.7 18.4 23.7 22.8 48.4 72.7

60

S. He et al. / Journal of Non-Crystalline Solids 410 (2015) 58–64 Table 2 Contact angle of silica aerogels under various molar ratios of TMCS to pore water. N Contact angle (°)

Fig. 1. Porosity of silica aerogel with no DMF.

The hydrophilic surface of hydrogel network became hydrophobic for the formation of alkoxytrimethylsilane (`Si\O\Si(CH3)3) (Eq. (6)). The surface modification of the gels was studied by measuring the contact angle of water droplets on the silica aerogel surface under various molar ratios of TMCS to pore water (denoted as N). As shown in Fig. 2, the contact angle reached to 161° at N = 0.0311, which indicated a super hydrophobicity of the silica aerogel. And with the further increase of N, the contact angle almost remains the same (Table 2). N is set 0.0311 which is much smaller than that used in single step solvent exchange/surface modification [34]. The super hydrophobicity of aerogel was attributed to the attachment of –Si(CH3)3 groups to the gel surface, which can be confirmed by the Fourier Transform Infrared Spectroscopy (FT-IR) analysis, as shown in Fig. 3. It is evident that apart from the Si–O–Si (absorption peak at about 470 cm− 1 and 1060 cm− 1 [37]), the modified silica aerogels show strong absorption peaks at about 850 cm − 1 and 1260 cm− 1, which respectively correspond to the terminal Si–CH3 groups [37]. The presence of these two absorption peaks in the FTIR spectra confirms the attachment of –Si–CH3 groups from the TMCS to the silica aerogel surface, indicating that the surface modification of the aerogels was successful. In addition, it is clear that the intensity of the bonds at ~ 3450 and ~ 1650 cm − 1 of –OH groups were decreased but not disappeared after surface modification [2, 37–40]. On one hand, the amount of Si–OH groups decreased after modified by TMCS (Eq. (6)), leading to a weaker absorption intensity of –OH. However, on the other hand, when conducting FT-IR measurement, silica aerogel samples were ground into refined powders, mixed with KBr and pressed to form sample pellet. KBr can easily absorb H2 O in the air whose –OH can also cause absorption peak at ~ 3450 and ~ 1650 cm− 1. The modified silica aerogel is hydrophobic for the surface modification in which process –CH3 groups are attached to the gel surface. When heated in a circumstance of high temperature, pyrolysis will happen to

Fig. 2. Water droplet placed on water glass-based silica aerogel surface (N = 0.0311).

0 83

0.0104 106

0.0207 135

0.0313 161

0.0415 160

the –CH3 groups. In this study, the aerogels were heated in a furnace at a range of temperatures from 25–800 °C in air to further investigate the thermal stability. The TG/DTA curve of synthesized silica aerogel is illustrated in Fig. 4. It can be observed that for modified aerogels, there presents an obvious exothermic peak at a temperature of ~400 °C, while as to the unmodified ones, no obvious exothermic peak can be seen. The –CH3 groups in the modified aerogels are oxidized at ~ 400 °C [41] and resulted in the sharp decrease in the gravity curve. Not alkylated by TMCS, there are considerable amount of hydroxyl (− OH) which is hydrophilic in the surface of unmodified aerogels, and thus much free water in the air was absorbed by the unmodified aerogels. The pronounced weight loss of unmodified silica aerogel before 200 °C owed to the evaporation of free water absorbed by the unmodified aerogels. And the weight loss upward 200 °C resulted from the condensation of hydroxyl that existed in the surface aerogel. While as to the well modified aerogel, the weight loss before 100 °C may be results from the evaporation of residual solvent and water molecules. To study the effect of DMF on the properties of silica aerogel, the molar ratio of Si to DMF was set as 3.34, 2.23, 1.11 and 0.83, corresponding to samples B2, B3, B4 and B5 respectively. The sample B1 was synthesized without DMF. The gelation time and physical properties of the silica aerogel are listed in Table 3. The specific surface area of the sample is as high as 680 m2/g which is much larger than the ones without DMF [22,25,26]. The polarity of DMF is relatively strong. The hydrogen bonds will form between hydroxyls (− OH) in orthosiliconic acid and DMF molecules which would impede the polycondensation of Si(OH)4. The condensation rate slowed down and the gelation process lasted longer, just as shown in Table 3. Fig. 5 shows that the silica aerogels' bulk density and volume shrinkage affected by the gelation time apparently. Either for the bulk density or volume shrinkage, the U-shaped curve can be clearly seen with the increase of gelation time. When there is no DMF, the Si(OH)4 molecules condensed quickly and disordered. While DMF was introduced, hydrogen bond will be formed between hydroxyls in Si(OH)4 and DMF molecules, then the condensation should be processed between the hydroxyls which were not hydrogen bonded with DMF. In this way, it can be considered that the condensation was conducted in some well-organized way.

Fig. 3. FTIR spectra of the synthesized water glass based silica aerogels.

S. He et al. / Journal of Non-Crystalline Solids 410 (2015) 58–64

Fig. 4. TG/DTA curves in air atmosphere for the water glass based silica aerogel.

And the formed hydrogel would have relatively uniform pore and particle size. On the other hand, the hydrogen bond will be destroyed by high energy, like high temperature water bath, 60 °C and 100 °C ambient pressure drying. Then the free hydroxyls, which were not condensed during the condensation process, would condense and destroy the existed gel microstructure. Therefore, when DMF is in excess (B4, B5), there would be considerable amount of hydroxyls that take part in the condensation, which can destroy the existed uniform pore structure. The difference of pores' size increases, so does the capillary pressure during ambient drying. Thus the pores will collapse. The aerogel's shrinkage aggravates and its density rises. The pore size distributions of the samples are presented in Fig. 6a. It can be seen that for all the samples, most of the pores are distributed in the size of ~230 Å. However, samples B4 and B5 have the broadest pore size distribution (PSD) (17 Å–1700 Å) among the five samples. As to B1, the pore volume peaks are not so distinct which means pores within a certain diameter interval largely coexist. There is a pronounced peak at a diameter of ~ 230 Å in the PSD curve of B3 and the pore volumes of other diameters are relatively small. Thus the PSD is the most uniform one. As mentioned above, the gelation time for sample B1 is the shortest. Condensation processed in a fast way and nonuniform network formed. When evaporated under ambient pressure, the big difference of capillary pressure operated on the pores made the pore structure collapse. However, when a moderate amount of DMF was added during the preparation process, the aerogels' PSDs have been apparently changed which is much narrower. When excess DMF was added, hydroxyls' hydrogen that bonded with DMF molecules would condense during high temperature water bath or ambient drying, and the existed pore structure would be destroyed. Fig. 6b compares the N2 sorption isotherms obtained from the silica aerogel samples. All the isotherms in Fig. 6b represent a type IV isotherm [42] (in the BDDT classification) with H3 type hysteresis loop [43,44], which indicates that they are a kind of material containing mesopores. The adsorption–desorption cycles of the given isotherms show a hysteresis loop in the high relative pressure region, which is generally attributed to the capillary condensation occurring in the mesopores [44].

61

Fig. 5. Effect of gelation time on bulk density and volume shrinkage of silica aerogels.

The microstructure of the aerogels can be seen in the scanning electron microscope images in Fig. 7, in which the aerogels' highly branched polymeric structure can be evidently observed. The pore sizes in sample B1 distribute in a rather broad range, while the PSD of sample B3 is the most uniform one among the five. For B1, the high condensation rate resulted in nonuniform pore size of hydrogel, and the difference of capillary pressure caused by ambient pressure drying collapse the pores. Thus some extremely big pores formed (Fig. 7a). As to B2 and B3, the PSDs gradually turn to uniform. For the addition of DMF resulted

Table 3 Gelation time and physical properties of the silica aerogel samples. Sample

Molar ratio of Si:DMF

Gelation time (min)

Specific surface area (m2/g)

Pore volume (cm3/g)

Average pore diameter (Å)

B1 B2 B3 B4 B5

∞ 3.34 2.23 1.11 0.83

0.8 2.3 6.1 10.5 11.2

469 551 680 583 565

1.8 2.3 2.8 3.3 2.2

156 136 131 136 138

Fig. 6. a) Pore size distribution and b) N2 adsorption–desorption isotherms of water glass based silica aerogel samples.

62

S. He et al. / Journal of Non-Crystalline Solids 410 (2015) 58–64

a. B1

b. B2

c. B3

d. B4

e. B5 Fig. 7. Scanning electron microscope images of aerogel samples.

in the formation of hydrogen bond, which made the condensation of free hydroxyls (not hydrogen bonded with DMF) in Si(OH)4 in an ordered way. And the clusters or pore sizes become homogeneous. For B4 and B5, excess DMF resulted in much hydroxyls that hydrogen bonded with DMF, which will condense with each other under high energy condition, such as water bath or ambient pressure drying. The existed pores, particles or clusters would be destroyed by the second condensation. The particles or clusters are larger and the pores are not uniform in Fig. 7d and e. After being modified by TMCS, methyl groups were attached to the surface, and the silica aerogels are hydrophobic and oleophylic. Thus the silica aerogels can absorb organic liquids, such as n-hexane from

water quickly (Fig. 8a). Presented in Fig. 8b, the absorption capacity of the silica aerogels were measured using ethanol, n-hexane and gasoline, respectively. Silica aerogels of 1 cm3 can absorb more than 0.9 ml organic liquids. Compared with the silica aerogels using TEOS as precursor [2], the absorption capacity of the sample B3 in this study is obviously higher. The aerogels' absorption efficiency increased significantly. 4. Conclusion Super hydrophobic ( = 161°) and oleophylic silica aerogels were successfully synthesized under ambient pressure using single step sol– gel polymerization of water glass. The volume ratio of deionized water

S. He et al. / Journal of Non-Crystalline Solids 410 (2015) 58–64

63

Fig. 8. A layer of n-hexane labeled by Oil Red O on top of the water was absorbed by a piece of silica aerogel within 40 s; b) comparison of absorption capacity with other material.

to water glass was fixed at 3 and the proper molar ratio of TMCS to pore water was studied and set to be 0.0311, which is much lower than previous research. The TG/DTA analysis revealed that the aerogels retain their hydrophobic nature up to a maximum temperature of 400 °C. The DMF was added to slow down the condensation speed and obtain a uniform distribution of the aerogel pores. However, excess DMF can speed the condensation to a certain extent and the distribution of pores and particles is not so nice. When the molar ratio of Si to DMF is 2.23, the obtained aerogels have better properties: relatively low bulk density (0.089 g/cm3), large surface area (680 m2/g) and high pore volume (3.3 cm3/g). The aerogels' absorption capacity of organic liquids is relatively high: more than 0.9 ml of organic liquids (ethanol, n-hexane and gasoline) can be absorbed by per cubic centimeter of as-prepared silica aerogel. Acknowledgment The research is partly supported by the fund (No.HZ2012-KF03) in State Key Laboratory of Fire Science, University of Science and Technology of China. References [1] C. Wingfield, L. Franzel, M.F. Bertino, N. Leventis, Fabrication of functionally graded aerogels, cellular aerogels and anisotropic ceramics, J. Mater. Chem. 21 (2011) 11737–11741. [2] J.L. Gurav, A.V. Rao, D.Y. Nadargi, H.-H. Park, Ambient pressure dried TEOS-based silica aerogels: good absorbents of organic liquids, J. Mater. Sci. 45 (2010) 503–510. [3] B. Lin, S. Cui, X. Liu, Y. Liu, X. Shen, G. Han, Preparation and adsorption property of phenyltriethoxysilane modified SiO2 aerogel, J. Wuhan Univ. Technol. Mater. Sci. Educ. 28 (2013) 916–920. [4] A. Venkateswara Rao, R.R. Kalesh, Comparative studies of the physical and hydrophobic properties of TEOS based silica aerogels using different co-precursors, Sci. Technol. Adv. Mater. 4 (2003) 509–515. [5] X.D. Wang, D. Sun, Y.Y. Duan, Z.J. Hu, Radiative characteristics of opacifier-loaded silica aerogel composites, J. Non-Cryst. Solids 375 (2013) 31–39. [6] L.F. Su, L. Miao, S. Tanemura, G. Xu, Low-cost and fast synthesis of nanoporous silica cryogels for thermal insulation applications, Sci. Technol. Adv. Mater. 13 (2012) 035003.

[7] P.B. Wagh, S.V. Ingale, S.C. Gupta, New technology for rapid processing and moulding of silica aerogel materials in prescribed shapes and sizes and their characterization, JSGST 58 (2011) 481–489. [8] Y.N. Kim, G.N. Shao, S.J. Jeon, S.M. Imran, P.B. Sarawade, H.T. Kim, Sol–gel synthesis of sodium silicate and titanium oxychloride based TiO2–SiO2 aerogels and their photocatalytic property under UV irradiation, Chem. Eng. J. 231 (2013) 502–511. [9] C. Daniel, S. Longo, R. Ricciardi, E. Reverchon, G. Guerra, Monolithic nanoporous crystalline aerogels, Macromol. Rapid Commun. 34 (2013) 1194–1207. [10] J.C.H. Wong, H. Kaymak, S. Brunner, M.M. Koebel, Mechanical properties of monolithic silica aerogels made from polyethoxydisiloxanes, Microporous Mesoporous Mater. 183 (2014) 23–29. [11] G. Liu, B. Zhou, A. Du, J. Shen, Q. Yu, Effect of the thermal treatment on microstructure and physical properties of low-density and high transparency silica aerogels via acetonitrile supercritical drying, J. Porous. Mater. 20 (2013) 1163–1170. [12] N. Bheekhun, A.R. Abu Talib, M.R. Hassan, Aerogels in aerospace: an overview, Adv. Mater. Sci. Eng. 2013 (2013) 1–18. [13] G. Liu, B. Zhou, Synthesis and characterization improvement of gradient density aerogels for hypervelocity particle capture through co-gelation of binary sols, JSGST 68 (2013) 9–18. [14] A. Cabañas, E. Enciso, M. Carmen Carbajo, M.J. Torralvo, C. Pando, J.A.R. Renuncio, Studies on the porosity of SiO2-aerogel inverse opals synthesised in supercritical CO2, Microporous Mesoporous Mater. 99 (2007) 23–29. [15] F. He, H. Zhao, X. Qu, C. Zhang, W. Qiu, Modified aging process for silica aerogel, J. Mater. Process. Technol. 209 (2009) 1621–1626. [16] D. Sanli, C. Erkey, Monolithic composites of silica aerogels by reactive supercritical deposition of hydroxy-terminated poly(dimethylsiloxane), ACS Appl. Mater. Interfaces 5 (2013) 11708–11717. [17] P.R. Aravind, L. Ratke, M. Kolbe, G.D. Soraru, Gels dried under supercritical and ambient conditions: a comparative study and their subsequent conversion to silica–carbon composite aerogels, JSGST 67 (2013) 592–600. [18] M. Nogami, S. Hotta, K. Kugimiya, H. Matsubara, Synthesis and characterization of transparent silica-based aerogels using methyltrimethoxysilane precursor, JSGST 56 (2010) 107–113. [19] T.Y. Wei, T.F. Chang, S.Y. Lu, Y.C. Chang, Preparation of monolithic silica aerogel of low thermal conductivity by ambient pressure drying, J. Am. Ceram. Soc. 90 (2007) 2003–2007. [20] A. Hilonga, J.-K. Kim, P.B. Sarawade, H.T. Kim, Low-density TEOS-based silica aerogels prepared at ambient pressure using isopropanol as the preparative solvent, JAllC 487 (2009) 744–750. [21] D.B. Mahadik, A.V. Rao, R. Kumar, S.V. Ingale, P.B. Wagh, S.C. Gupta, Reduction of processing time by mechanical shaking of the ambient pressure dried TEOS based silica aerogel granules, J. Porous. Mater. 19 (2012) 87–94. [22] F. Shi, L. Wang, J. Liu, Synthesis and characterization of silica aerogels by a novel fast ambient pressure drying process, Mater. Lett. 60 (2006) 3718–3722.

64

S. He et al. / Journal of Non-Crystalline Solids 410 (2015) 58–64

[23] S.K. Hong, M.Y. Yoon, H.J. Hwang, S. Ehrman, Fabrication of spherical silica aerogel granules from water glass by ambient pressure drying, J. Am. Ceram. Soc. 94 (2011) 3198–3201. [24] P.B. Sarawade, J.-K. Kim, A. Hilonga, H.T. Kim, Influence of aging conditions on textural properties of water-glass-based silica aerogels prepared at ambient pressure, Korean J. Chem. Eng. 27 (2010) 1301–1309. [25] U.K.H. Bangi, I.-K. Jung, C.-S. Park, S. Baek, H.-H. Park, Optically transparent silica aerogels based on sodium silicate by a two step sol–gel process and ambient pressure drying, Solid State Sci. 18 (2013) 50–57. [26] J.L. Gurav, A.V. Rao, A.P. Rao, D.Y. Nadargi, S.D. Bhagat, Physical properties of sodium silicate based silica aerogels prepared by single step sol–gel process dried at ambient pressure, JAllC 476 (2009) 397–402. [27] D. HARANATH, A.V.R.A.P.B.W., Influence of DCCAs on optical transmittance and porosity properties of TMOS silica aerogels, J. Porous. Mater. 6 (1999) 55–62. [28] X.L. Li, Y.N. Jiao, H.M. Ji, X.H. Sun, W.P. Feng, H.L. Li, Modulated preparation of monolithic ZrO2 aerogel, Rare Metal Mater. Eng. 42 (2013) 755–757. [29] G. Andrade-Espinosa, V. Escobar-Barrios, R. Rangel-Mendez, Synthesis and characterization of silica xerogels obtained via fast sol–gel process, Colloid Polym. Sci. 288 (2010) 1697–1704. [30] Z.J. Xu, L.H. Gan, Y.C. Pang, L.W. Chen, Preparation of Al2O3 bulk aerogels by nonsupercritical fluid drying technology, Acta Phys. -Chim. Sin. 21 (2005) 221–224. [31] B. Lu, M. Song, D. Zhang, Q. Zhou, J. Sun, K. Hu, Effect of formamide on microstructure of TiO2 aerogel, J. Cent. South Univ. (Sci. Technol.) 44 (2013) 75–82. [32] S.W. Park, B.K. Jang, Y.J. Oh, Structural control of ambient drying silica aerogel by drying control chemical additive, J. Jpn. Inst. Metals 73 (2009) 608–612. [33] C.E. Kim, J.S. Yoon, H.J. Hwang, Synthesis of nanoporous silica aerogel by ambient pressure drying, JSGST 49 (2008) 47–52.

[34] S.W. Hwang, H.H. Jung, S.H. Hyun, Y.S. Ahn, Effective preparation of crack-free silica aerogels via ambient drying, JSGST 41 (2007) 139–146. [35] T. Wu, M. Chen, L. Zhang, X. Xu, Y. Liu, J. Yan, et al., Three-dimensional graphenebased aerogels prepared by a self-assembly process and its excellent catalytic and absorbing performance, J. Mater. Chem. A 1 (2013) 7612–7621. [36] Surface tension, Available from: http://en.wikipedia.org/wiki/Surface_tension. [37] J.L. Gurav, A.V. Rao, U.K.H. Bangi, Hydrophobic and low density silica aerogels dried at ambient pressure using TEOS precursor, JAllC 471 (2009) 296–302. [38] H. Liu, W. Sha, A.T. Cooper, M. Fan, Preparation and characterization of a novel silica aerogel as adsorbent for toxic organic compounds, Colloids Surf. A Physicochem. Eng. Asp. 347 (2009) 38–44. [39] J. Li, J. Cao, L. Huo, X. He, One-step synthesis of hydrophobic silica aerogel via in situ surface modification, Mater. Lett. 87 (2012) 146–149. [40] A.-Y. Jeong, S.-M. Koo, D.-P. Kim, Characterization of hydrophobic SiO2 powders prepared by surface modification on wet gel, JSGST 19 (2000) 483–487. [41] U.K.H. Bangi, A. Venkateswara Rao, A. Parvathy Rao, A new route for preparation of sodium-silicate-based hydrophobic silica aerogels via ambient-pressure drying, Sci. Technol. Adv. Mater. 9 (2008) 035006. [42] P.B. Sarawade, J.K. Kim, A. Hilonga, D.V. Quang, H.T. Kim, Synthesis of hydrophilic and hydrophobic xerogels with superior properties using sodium silicate, Microporous Mesoporous Mater. 139 (2011) 138–147. [43] S.J. Gregg, K.S.W.S., Adsorption Surface Area and Porosity, 2nd ed. Academic Press, London, 1982. [44] S.W. Kenneth, R.T.W. Sing, Physisorption hysteresis loops and the characterization of nanoporous materials, Adsorpt. Sci. Technol. 22 (2004) 773–782.