Applied Surface Science 437 (2018) 321–328

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Applied Surface Science

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Full length article

Mechanical performance and thermal stability of glass fiber

reinforced silica aerogel composites based on co-precursor method by
freeze drying
Ting Zhou, Xudong Cheng ∗ , Yuelei Pan, Congcong Li, Lunlun Gong, Heping Zhang ∗
State Key Laboratory of Fire Science, University of Science and Technology of China, Hefei, Anhui 230027, PR China

a r t i c l e i n f o a b s t r a c t

Article history: In order to maintain the integrity, glass fiber (GF) reinforced silica aerogel composites were synthesized
Received 26 September 2017 using methltrimethoxysilane (MTMS) and water glass co-precursor by freeze drying method. The com-
Received in revised form posites were characterized by scanning electron microscopy, Brunauer-Emmett-Teller analysis, uniaxial
12 December 2017
compressive test, three-point bending test, thermal conductivity analysis, contact angle test, TG-DSC
Accepted 18 December 2017
Available online 27 December 2017
analysis. It was found that the molar ratio of MTMS/water glass could significantly affect the proper-
ties of composites. The bulk density and thermal conductivity first decreased and then increased with
the increasing molar ratio. The composites showed remarkable mechanical strength and flexibility com-
Keywords:
GF/aerogel composites pared with pure silica aerogel. Moreover, when the molar ratio is 1.8, the composites showed high specific
Co-precursor surface area (870.9 m2 /g), high contact angle (150◦ ), great thermal stability (560 ◦ C) and low thermal con-
Freeze drying ductivity (0.0248 W/m·K). These outstanding properties indicate that GF/aerogels have broad prospects
Mechanical properties in the field of thermal insulation.
Thermal stability © 2017 Elsevier B.V. All rights reserved.

1. Introduction exchanges and hazardous organic solvent. Co-precursor could be

a good choice without further surface modification. Rao et al.
Silica aerogels are nanostructured materials with fascinating prepared MTMS/ TMOS aerogel with thermal stability of 277 ◦ C
characteristics, such as extremely low density (0.03–0.20 g/cm3 ), and thermal conductivity of 0.069 W/m·K [11]. Nadargi et al. pre-
high specific surface area (>1000 m2 /g), low thermal conductivity pared TEOS based aerogel using trimethylmethoxysilane (TMMS),
(0.005–0.021 W/m·K), which have drawn substantial attention in dimethyldimethoxysilane (DMMS) and methyltrimethoxysilane
thermal insulation, aerospace application, catalytic supports, etc (MTMS) as co-precursor, respectively, with low thermal stability
[1,2]. Pan et al. obtained hybrid aerogels based on MTMS/water (300 ◦ C) and high thermal conductivity (0.065 W/m·K) [12]. From
glass with low thermal conductivity and high thermal stability the above, although the co-precursor aerogels were successfully
[3]. But the low strength, fragility and brittleness of the aero- obtained by co-precursor method, the aerogels prepared using this
gel restrict the practical application. Lots of researches have been way all presented poorer thermal stability and higher thermal con-
made to improve the mechanical properties using fibers as support- ductivity than that of pure MTMS aerogels or pure TEOS aerogels.
ing skeletons. Inorganic and organic fibers, including mineral [4], Lots of efforts have been done to improve the thermal insulation
ceramic [5,6], aramid [7] and glass fibers [8,9], can decrease the bulk properties by researchers. In the present work, the thermal stabil-
size of aerogel and improve the compressive strength. Glass fiber, ity of aerogels is improved based co-precursor method with organic
a kind of noncombustible fiber, possess high compressive fracture and inorganic silicon source hybridization.
strength, which is suited for fragile aerogels as the reinforcement. It is known that supercritical drying is the earliest method
Chang et al. synthesized flexible aerogel-glass fiber composites by used to prepare silica aerogel [13]. However, it is difficult for
ambient drying with improved mechanical properties [10]. large-scale commercialization requiring intensive energy, high-
Another critical factor, which limits the aerogels’ applica- cost facility and high pressure. Thus, ambient pressure drying has
tion, is the surface modification process with arduous solvent aroused widespread concern [14,15]. But it involves repetitive sol-
vent exchange and surface modification before drying to obtain
hydrophobic aerogel. These toxic solvents are harmful to envi-
∗ Corresponding author. ronment and human. Freeze drying is another promising method
E-mail addresses: zhanghp@ustc.edu.cn, chengxd@ustc.edu.cn (H. Zhang). which is based on sublimation of excess solvent. Ren et al. found

https://doi.org/10.1016/j.apsusc.2017.12.146
0169-4332/© 2017 Elsevier B.V. All rights reserved.
322 T. Zhou et al. / Applied Surface Science 437 (2018) 321–328

Fig. 1. Experimental illustration of the preparation of glass fiber-reinforced aero-

gels. Fig. 2. Photograph of the three-point bending test.

an easy way to prepare monolithic inorganic oxide aerogels under 48h at vacuum pressure. Finally, the glass fiber-reinforced silica
vacuum conditions [16]. Sanosh et al. successfully synthesized sil- aerogels were obtained.
ica cryogel-glass fiber blankets by vacuum freeze drying [17]. In
this study, we employed freeze drying method to reduce capillary 2.2. Methods of characterization
pressure and obtain versatile glass fiber reinforced silica aerogel
composites. The density of the GF/aerogel was calculated based on its
weight to volume ratio. The microstructure of the compos-
2. Experimental ite was studied by field emission scanning electron microscope
(SEM, SIRION200, FEI). The specific surface areas and pore size
2.1. Materials and preparation distributions (PSD) were estimated by Brunauer-Emmett-Teller
(BET) analysis and Barrett-Joyner-Halenda (BJH) method (Tristar II
Glass fibers (diameter of 8–15 ␮m) used as reinforcement were 3020M, Micromeritics Instrument Corporation, USA), respectively.
purchased from Sinopharm Chemical Reagent Co., Ltd. (China). The Uniaxial compression test and three-point bending test were
precursors were water glass (Qingdao Dongyue Sodium Silicate Co., performed using Electronic dynamic and static fatigue testing
Ltd., China) and MTMS (Aladdin). Other chemicals including Tert- machine (E3000K8953, Instron). The size for uniaxial compression
butyl alcohol, hydrochloric acid and ammonia were also purchased test is 25 mm × 25 mm×12 mm. The rectangular samples with the
from SCRC. 0.1 mol/L HCl (aq) and 0.5 mol/L NH4 OH (aq) were pre- dimension of 85 mm × 15 mm×12 mm were prepared for three-
pared and used as acid and base catalysts in the sol-gel process, point bending test. The test was shown in Fig. 2 and the loading
respectively. All agents were chemical pure grade. rate was set as 2 mm/min. The flexural modulus is calculated from
The experimental procedure for the preparation is illustrated Eqs. (1) – (3).
in Fig. 1. Firstly, the precursors of MTMS and water glass were
hydrolyzed separately. The MTMS and solvent was stirred for 3PL
f = (1)
30 min to ensure sufficient hydrolysis under acidic condition. The 2bd2
molar ratio of MTMS: Tert-butyl alcohol: H2 O: HCl was fixed at 6Dd
εf = (2)
1:2.97:15.8:5.7 × 10−3 . Water glass was diluted with deionized L2
water passing through amberlite ion exchanged resin to replace f 2 − f 1
the Na+ with H+ ion. The molar ratio of MTMS/water glass was Ef = (3)
εf 2 − εf 1
defined as X, and the values of X are 1.8, 1.3, 1, 0.6 and 0, respec-
tively. MTMS and water glass sol mixed together and added NH4 OH In which, ␴f is the flexural stress, in MPa; ␧f is the flexural strain in
solution to adjust the pH to 6. Secondly, the resulting alcosol was outer surface; Ef is the flexural modulus, in MPa; P is the load, in N;
quickly poured into a rectangular polyethylene plastic mold with L is the support span, set as 60 mm; D is the maximum deflection
the dimensions of 87 mm × 57 mm×20 mm and the glass fiber of the center of the specimen, in mm; b and d are the width and
was completely immersed in the alcosol. Then the condensation thickness of the specimen, in mm, respectively.
occurred and the gel generally formed in 30 min. The GF/aerogels TG-DSC (SDT Q600, TA) was employed to test the thermal sta-
aged with Tert-butyl alcohol for 3 days to strengthen the skeleton. bility operating in oxygen atmospheres with a heating rate of
At last, the GF/aerogels were placed in the laboratory freeze dryer 10 ◦ C/min from room temperature to 800 ◦ C. The hydrophobic-
for 8 h at a low temperature of -80 ◦ C and then were dried for about ity of the samples was measured with contact angle instrument
 T. Zhou et al. / Applied Surface Science 437 (2018) 321–328 323

(SL200K, USA). The information of chemical bonds was given by can be seen from Eqs. (4) and (5), the hydroxyl groups presented
Fourier transform infrared spectroscopy (Nicolet 8700, TFS, USA). on the surface of precursors after sufficient hydrolysis. Hydrolyzed
Thermal conductivity (␭) was determined by thermal conductivity MTMS still remained one non-hydrolysable –CH3 group. Then the
measurement (TC 3000E, Xiaxi technology, China). hydrolyzed MTMS reacted with the hydrolyzed water glass and the
–CH3 groups can successfully adhere to the surface of the aerogel.
The formation of the gel skeleton is described in Eq. (6). The intro-
duction of MTMS is equal to the conventional surface modification
so that the network is strong enough to reduce volume shrinkage.

3. Results and discussion

3.2. Structure analysis
3.1. Reaction process
Fig. 3 gives the microstructures of different samples. Fig. 3(a)
The fibers act as supporting skeleton, which are just a physical shows silica aerogel matrix adhered to the glass fibers tightly. The
combination with silica aerogels [18]. The main chemical reactions diameter of a singer glass fiber is approximately 17 ␮m. Fig. 3(b)–(f)
are the hydrolysis and condensation of MTMS and water glass. As show the SEM photographs of samples G1–G5 which is the

Fig. 3. Glass fiber surface covered with aerogel (a), aerogel nanoparticles on the fiber surface prepared with: X = 1.8 (b), X = 1.3 (c), X = 1 (d), X = 0.6 (e), X = 0 (f).
324 T. Zhou et al. / Applied Surface Science 437 (2018) 321–328

Fig. 4. N2 adsorption-desorption isotherms of five GF/aerogels prepared with vari- Fig. 5. Pore size distributions of five GF/aerogels prepared with various molar ratio.
ous molar ratio.

magnification of aerogel matrix on the glass fiber. The aerogel is different. The specific surface area reaches the maximum value of
consisted of nanoparticles and it reveals that the introduction of 870.9 m2 /g when X = 1. But the sample G5 prepared by pure water
glass fiber does not influence the three-dimensional nanoporous glass without any MTMS has minimum surface area of 487.2 m2 /g
structure of the aerogel. With the decrease of MTMS/water-glass compared with other samples. This is because the capillary pres-
molar ratio, the size of the pores is smaller and the distribution of sure will occur when the solvent is removed through sublimation.
nanoparticles is more uniform. Seen from the Fig. 3(a) and (b), the The capillary differential pressure can give rise to structure collapse
large pores and aggregated clusters are formed with high X value. and be calculated according to the following equation:

2 cos 
3.3. Pore size distribution P= (7)
r
Table 1 lists a summary of pore structure including specific sur- In which, P is capillary pressure, in Pa; ␴ is surface tension of the
face area, pore volume and average pore size. It is obvious that the solvent, in N; ␪ is the contact angle and r is the meniscus of radius.
pore volume and average pore size decrease with the increase of Generally, solvent exchange and surface modification can reduce
MTMS in aerogels. But the trend of specific surface area is a little the capillary pressure. The samples G5 without any surface mod-

Fig. 6. Uniaxial compression test of the GF/aerogels.

 T. Zhou et al. / Applied Surface Science 437 (2018) 321–328 325

Table 1
Pore parameters of different samples.

Molar ratio of MTMS/water glass BET surface area(m2 /g) Pore volume (cm3 /g) Average pore size(nm)

G1 1.8 669.8 3.186 13.710

G2 1.3 740.9 3.091 13.369
G3 1 870.9 1.803 9.125
G4 0.6 852.4 1.549 8.186
G5 0 487.2 2.097 7.553

Fig. 8. Uniaxial compressive curve of five composites with various molar ratio: (G1)
Fig. 7. A typical uniaxial compression stress-strain curve of sample G2 at 25 ◦ C.
X = 1.8; (G2) X = 1.3; (G3) X = 1; (G4) X = 0.6; (G5) X = 0.

ification will have great shrinkage during freeze drying process

[19]. Table 2
The N2 adsorption-desorption isotherms curves of all five sam- Analytical data of compressive curve of GF/aerogels.

ples in Fig. 4 exhibit a typical type IV, which generally presents Molar ratio (X) ␴15% (kPa) ␴20% (kPa) Elastic modulus (kPa)
in the mesoporous materials. The same hysteresis loops of type H3
1.8 47.26 78.97 634.2
for five samples indicate the probable presence of silt-shaped pores 1.3 50.87 87.44 731.4
[20]. The results of the pore size distribution (PSD), measured by 1 85.28 154.93 1393.0
BJH method, are shown in Fig. 5. It is known that most of the pore 0.6 141.23 250.54 2186.2
0 128.88 288.10 3184.4
diameter distributed between 20–40 nm for all five samples. But
the highest dV/dlog (D) pore volume for G1 and G2 are only 0.91
and 1.28 cm3 /g, respectively. It is obviously lower than those of
G3-G5, which indicates more mesopores exist in aerogel with low
molar ratios. four parts. The curves showed better linearity when the strain is
from 15% to 20%. Table 2 listed the summaries of the stress at the
3.4. Mechanical properties strains of 15%, 20% and elastic modulus. Elastic modulus is used to
measure the degree of elastic deformation of the material. With the
Fig. 6 shows the photograph of typical uniaxial compression increasing value, the stress deformation of the material is increas-
test of the GF/aerogels. Fig. 6(a) is the initial stage of the test and ing, which indicates a great rigidity of the material [21]. As shown in
the compression machine could not contact the GF/aerogel tightly Table 2, the ␴15% , ␴20% and compressive elastic modulus increase
because of the unsmooth surface of the specimen. With higher with increasing molar ratio in which the composites show great
molar ratio of MTMS/water glass, the composites exhibited high elasticity at X = 1.8 and 1.3. Glass giber as a whole acted as sup-
deformability and recovered their original shape without breaking porting skeleton and retained the integrality of aerogel matrix. The
when the stress released. volume and mass were keeping constant in this study, so the dif-
From Fig. 7, it can be seen that the stress-strain curve of the ference of elastic modulus was caused by difference of MTMS. It is
GF/aerogels could be divided into four different stages: the contact indicated that the elasticity of material is better with more MTMS.
stage, the linear stage, the yielding stage and the densification stage Fig. 9 (a) shows the flexural strain-stress curves of the
[21]. The contact stage with low strains from 0 to 10% is caused GF/aerogels obtained by three-point bending test. It can be seen
by the uneven surface of the specimen. At the linear stage, the that the flexural stress continues to increase with the increasing
strain ranges from 10 to 20% and the slope of the curve remains flexural strain. The tested specimen was only bent without material
unchanged. The nanopores of silica aerogel act as the main bearing failure because glass fiber connected tightly layer by layer. Flex-
part while the fibers are only responsible for the integrity of the ural modulus (Ef ) is to characterize the stiffness of the material,
GF/aerogel [18]. At the yielding stage, the stress increases rapidly that is, the ability of the material to resist deformation. The Ef first
and fibers would be contributed to the main bearing part which increases steadily and then increases rapidly as the molar ratio
is different from the pure aerogels. At the densification stage, the increases in Fig. 9 (b). With the molar ratio increasing, the compos-
slope of curve rises significantly which is mainly due to the collapse ites become stiffer and the flexibility of the composites decreases
of aerogel and gradual densification of the porous structure. with the flexural modulus rising. The mass of glass fibers in five
The stress-strain curves of the five composites with various samples is the same. Therefore, the change in molar ratio is the key
molar ratio are shown in Fig. 8. All curves could be divided into factor affecting the elasticity of the material.
326 T. Zhou et al. / Applied Surface Science 437 (2018) 321–328

Fig. 9. Flexural stress-strain curves (a) and flexural modulus (b) of the five composites with various molar ratios in three-bending bending test.

indicates that the amount of Si-CH3 functionalities declines with

the decrease in MTMS [23]. Particularly, the bonds of Si-CH3 groups
disappear when X = 0. Furthermore, the faint peak at 960 cm−1 is
attributed to the stretching vibrations of ≡Si-OH group for sam-
ples G4 and G5 [24]. The samples possess unreacted Si–OH from
orthosilicate with lower molar ratio of MTMS/water glass. From
above, it explains why the GF/aerogels show decreased contact
angle and demonstrates that samples G4 and G5 show hydrophilic-
ity. The images of contact angles are presented in Fig. 11.
Fig. 11 shows the water contact angles of GF/aerogels with dif-
ferent molar ratio of MTMS/water-glass. Fig. 11(a) shows a water
droplet on the top surface of aerogel blanket and the hydropho-
bicity is further quantified by contact angle. It is known from G1
to G5 that the increasing MTMS/water-glass molar ratio results
in an increase of contact angle, and the contact angle of 150◦ for
G1 indicates superhydrophobic properties. It is consistent with the
FTIR spectra analysis that the surface chemical groups ( Si CH3 )
Fig. 10. FTIR spectrum of five samples (G1–G5). from MTMS successfully replaced the hydroxyl groups. On the con-
trary, the samples of G3–G5 are hydrophilic which is caused by the
remained Si OH groups and G5 is completely hydrophilic with
3.5. FTIR analysis and hydrophobicity
no surface chemical group [25].

The FTIR spectrum given in Fig. 10 indicates the information

of chemical bonds. The strong adsorption peaks around 1100 cm−1
and 800 cm-1 are caused by the symmetric and asymmetric stretch- 3.6. Thermal properties
ing vibrations of Si-O-Si bonds, respectively [11]. For five samples,
the vibrations at 3435 cm−1 and 1631 cm−1 are due to residual –OH Fig. 12 shows that the bulk density of the GF/aerogels first
groups or the adsorbed water [22]. decreases and then grows up when X decreases from 1.8 to 0. The
The presence of Si-CH3 groups is derived from MTMS by thermal conductivity shows the same trend with the density. At a
vibrational bands at 1275 cm−1 and 917 cm−1 , and the intensity MTMS/water glass molar ratio of 1, the aerogel shows low density
decreases as the molar ratio of MTMS/water glass decreases. This 0.174 g/cm3 and low thermal conductivity 0.0213 W/m K.

Fig. 11. A water droplet on the surface of GF/aerogel (a), photograph of the contact angle test for the G1–G5 aerogel samples.
 T. Zhou et al. / Applied Surface Science 437 (2018) 321–328 327

(150◦ ), great thermal stability (560 ◦ C) and low thermal conduc-

tivity (0.0248 W/m K). The microstructure analysis presents the
interfacial adhesion with the glass fiber and aerogel matrix. The
silica aerogel composites show remarkable mechanical strength
and flexibility, which could endure large compressive and flexural
strain without structural destroyed. These outstanding character-
istics indicate that the obtained GF/aerogels by freeze drying based
on organic/inorganic co-precursor have significantly improved
mechanical properties and thermal insulation performance.

Acknowledgments

The work was supported by Anhui Programs for Science

and Technology Development (No. 1604a0902175), Fundamen-
Fig. 12. Bulk density and thermal conductivity of GF/aerogels with the increasing
tal Research Funds for the Central Universities (Grant No.
X.
WK2320000032) and the Open Project Program of State Key Labo-
ratory of Fire Science (No. HZ2017-KF12).
Fig. 13 presents the TG-DSC analysis in air atmosphere of two
samples with different molar ratio 1.8 and 0, respectively. The pro-
cess in Fig. 13(a) can be divided into three stages. The specimen References
first loses weight at a low temperature which is lower than 200 ◦ C
because of the evaporation of residual solvent and absorbed water [1] P.C. Thapliyal, K. Singh, Aerogels as promising thermal insulating materials:
[26]. The second stage is the condensation of Si OH groups. There an overview, J. Mater. (2014), 127049 (127010 pp.)-127049 (127010 pp.).
[2] F. Shi, L. Wang, J. Liu, Synthesis and characterization of silica aerogels by a
is a dramatic exothermic peak at the third stage which is consid- novel fast ambient pressure drying process, Mater. Lett. 60 (2006) 3718–3722.
ered to be the oxidation of Si-CH3 groups. One of the key points [3] Y. Pan, S. He, L. Gong, X. Cheng, C. Li, Z. Li, Z. Liu, H. Zhang, Low
to measure the thermal stability is the initial temperature of the thermal-conductivity and high thermal stable silica aerogel based on
MTMS/water-glass co-precursor prepared by freeze drying, Mater. Des. 113
exothermic reaction. The thermal stability temperature is about (2017) 246–253.
560 ◦ C and the hydrophobicity turns into the hydrophilicity with [4] S.K. Rajanna, M. Vinjamur, M. Mukhopadhyay, Mechanism for formation of
the decomposition of –CH3 groups when exceeding 560 ◦ C. hollow and granular silica aerogel microspheres from rice husk ash for drug
delivery, J. Non-Cryst. Solids 429 (2015) 226–231.
It can be seen from Fig. 13(b) that only two stages in TG curve [5] D. Shi, Y. Sun, J. Feng, X. Yang, S. Han, C. Mi, Y. Jiang, H. Qi, Experimental
when the molar ratio is 0. There is no exothermic peak because no investigation on high temperature anisotropic compression properties of
–CH3 group exists in aerogel without any surface modification. The ceramic-fiber-reinforced SiO2 aerogel, Mater. Sci. Eng. Struct. Mater. Prop.
Microstruct. Process. 585 (2013) 25–31.
first weight loss stage is 15.97% between 30 ◦ C and 230 ◦ C which [6] A. Karout, P. Buisson, A. Perrard, A.C. Pierre, Shaping and mechanical
is larger than that in Fig. 13(a). It is caused by the dehydration of reinforcement of silica aerogel biocatalysts with ceramic fiber felts, J. Sol-Gel
hydroxyl groups on the surface of aerogel. Sci. Technol. 36 (2005) 163–171.
[7] Z. Li, L. Gong, X. Cheng, S. He, C. Li, H. Zhang, Flexible silica aerogel composites
strengthened with aramid fibers and their thermal behavior, Mater. Des. 99
4. Conclusions (2016) 349–355.
[8] J.-J. Zhao, Y.-Y. Duan, X.-D. Wang, B.-X. Wang, An analytical model for
combined radiative and conductive heat transfer in fiber-loaded silica
Glass-fiber (GF) reinforced silica aerogel composites based
aerogels, J. Non-Cryst. Solids 358 (2012) 1303–1312.
on MTMS/water glass were synthesized by freeze drying. The [9] Y. Liao, H. Wu, Y. Ding, S. Yin, M. Wang, A. Cao, Engineering thermal and
microstructure, thermal stability, hydrophobicity and mechanical mechanical properties of flexible fiber-reinforced aerogel composites, J.
Sol-Gel Sci. Technol. 63 (2012) 445–456.
properties of the GF/aerogels with different molar ratio of MTMS
[10] C.-Y. Kim, J.-K. Lee, B.-I. Kim, Synthesis and pore analysis of aerogel-glass fiber
to water glass (1.8–0) were systematically measured and ana- composites by ambient drying method, Colloid Surf. A 313 (2008) 179–182.
lyzed. The density and thermal conductivity first decreases and [11] A.V. Rao, M.M. Kulkarni, D.P. Amalnerkar, T. Seth, Surface chemical
then grow up with the increase of molar ratio. When the molar modification of silica aerogels using various alkyl-alkoxy/chloro silanes, Appl.
Surf. Sci. 206 (2003) 262–270.
ratio is 1.8, the obtained composites have outstanding performance [12] D.Y. Nadargi, R.R. Kalesh, A.V. Rao, Rapid reduction in gelation time and
with high specific surface area (870.9 m2 /g), high contact angle impregnation of hydrophobic property in the tetraethoxysilane (TEOS) based

Fig. 13. The TG-DSC curves of two different GF/aerogels prepared with: (a) X = 1.8; (b) X = 0.
328 T. Zhou et al. / Applied Surface Science 437 (2018) 321–328

silica aerogels using NH4F catalyzed single step sol–gel process, J. Alloys [20] F. Rojas, I. Kornhauser, C. Felipe, J.M. Esparza, S. Cordero, A. Domínguez, J.L.
Compd. 480 (2009) 689–695. Riccardo, Capillary condensation in heterogeneous mesoporous networks
[13] A.V. Rao, R.R. Kalesh, Comparative studies of the physical and hydrophobic consisting of variable connectivity and pore-size correlation, Phys. Chem.
properties of TEOS based silica aerogels using different co-precursors, Sci. Chem. Phys. 4 (2002) 2346–2355.
Technol. Adv. Mater. 4 (2003) 509–515. [21] X. Yang, Y. Sun, D. Shi, J. Liu, Experimental investigation on mechanical
[14] J.L. Gurav, A.V. Rao, D.Y. Nadargi, H.-H. Park, Ambient pressure dried properties of a fiber-reinforced silica aerogel composite, Mater. Sci. Eng. A 528
TEOS-based silica aerogels: good absorbents of organic liquids, J. Mater. Sci. (2011) 4830–4836.
45 (2009) 503–510. [22] S.A. Mahadik, F. Pedraza, V.G. Parale, H.-H. Park, Organically modified silica
[15] J.L. Gurav, A.V. Rao, U.K.H. Bangi, Hydrophobic and low density silica aerogels aerogel with different functional silylating agents and effect on their
dried at ambient pressure using TEOS precursor, J. Alloys Compd. 471 (2009) physico-chemical properties, J. Non-Cryst. Solids 453 (2016) 164–171.
296–302. [23] H. Yu, X. Liang, J. Wang, M. Wang, S. Yang, Preparation and characterization of
[16] L. Ren, S. Cui, F. Cao, Q. Guo, An easy way to prepare monolithic inorganic hydrophobic silica aerogel sphere products by co-precursor method, Solid
oxide aerogels freeze-drying, Angew Chem. Int. Ed. Engl. 53 (2014) State Sci. 48 (2015) 155–162.
10147–10149. [24] S. Zong, W. Wei, Z. Jiang, Z. Yan, J. Zhu, J. Xie, Characterization and comparison
[17] S.K. Padmanabhan, E. Ul Haq, A. Licciulli, Synthesis of silica cryogel-glass fiber of uniform hydrophilic/hydrophobic transparent silica aerogel beads:
blanket by vacuum drying, Ceram. Int. 42 (2016) 7216–7222. skeleton strength and surface modification, RSC Adv. 5 (2015) 55579–55587.
[18] Z. Li, X. Cheng, S. He, X. Shi, L. Gong, H. Zhang, Aramid fibers reinforced silica [25] C.C. Li, X.D. Cheng, Z. Li, Y.L. Pan, Y.J. Huang, L.L. Gong, Mechanical, thermal
aerogel composites with low thermal conductivity and improved mechanical and flammability properties of glass fiber film/silica aerogel composites, J.
performance, Compos. A: Appl. Sci. Manufact. 84 (2016) 316–325. Non-Cryst. Solids 457 (2017) 52–59.
[19] J. He, X.L. Li, D. Su, H.M. Ji, X.J. Wang, Ultra-low thermal conductivity and high [26] H. Liu, W. Sha, A.T. Cooper, M. Fan, Preparation and characterization of a novel
strength of aerogels/fibrous ceramic composites, J. Eur. Ceram. Soc. 36 (2016) silica aerogel as adsorbent for toxic organic compounds, Colloids Surf. A:
1487–1493. Physicochem. Eng. Asp. 347 (2009) 38–44.