Silica Aerogel: Synthesis, Characterization, Applications, and Recent Advancements

2200186 (1 of 22) © 2023 Wiley-VCH GmbH
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Review
Silica Aerogel: Synthesis, Characterization, Applications,
and Recent Advancements
Adib Bin Rashid,* Shariful Islam Shishir, Md. Azim Mahfuz, Md. Tanvir Hossain,
and Md Enamul Hoque*
A. B. Rashid, S. I. Shishir, Md. A. Mahfuz, Md. T. Hossain
Industrial and Production Engineering Department
Military Institute of Science and Technology (MIST)
Dhaka 1216, Bangladesh
E-mail: adib@me.mist.ac.bd
M. E. Hoque
Department of Biomedical Engineering
Military Institute of Science and Technology (MIST)
Dhaka 1216, Bangladesh
E-mail: enamul1973@gmail.com
The ORCID identification number(s) for the author(s) of this article
can be found under https://doi.org/10.1002/ppsc.202200186.
DOI: 10.1002/ppsc.202200186
1. Introduction
Aerogels (Figure 1) are the most well-known solid porous mate-
rials because of their tiny pores, huge surface area, and high
optical frequency transmission rate. Silica aerogels, in par-
ticular, have grown in popularity due to their unique combina-
tion of low conductivity of heat (0.01 W m−1 K−1), highly porous
(having 99%), high optical frequency remittance (greater
than 99%) in the range of visibility, high specific surface area
(>1000 m2 g−1), a low constant factor of dielectric characteristic
(1.0–2.0), low sound velocity (100 m s−1), and low refractive
index (1.05).[1]
Samuel Stephens Kistler developed the notion of replacing
the liquid phase with gas with just a small amount of gel
Silica aerogels have drawn considerable attention due to their low density
(almost 95% of the total volume is composed of air), hydrophobicity, optical
transparency, low conductivity of heat, and large surface to volume ratio.
Sol–gel processing is used to prepare aerogels from molecular precursors.
To replace the pore fluid with air while retaining the solid network, a super-
critical drying process (the most frequent approach) is used. However, recent
technologies use atmospheric pressure to allow for liquid removal followed
by chemical alteration of the gel’s internal layer, which leaves only a silica
network with a porous structure filled with air. This study discusses the
sol–gel method for preparing silica aerogels and their various drying pro-
cesses. Furthermore, various areas of applications of silica aerogels, including
electronics, construction, aerospace, purification of water and air, sensing,
catalyst, biomedical, absorbent, food packing, textile, etc., are also discussed.
Lastly, this review provides a perception of the recent scientific progress
along with the futuristic development of silica aerogel.
shrinkage in the 1930s and invented silica
aerogels. A team led by Prof. S. J. Teichner
at the University of Claude Bernard in
Lyon, France, rediscovers aerogels using
Kistler’s approach, which is laborious and
time-consuming. This new interest in
aerogels does not date back to 1968. They
used the sol–gel process to form a soluble
gel and did supercritical drying to extract
water from the gel.[2]
A typical silica aerogel created using
tetramethyl-orthosilicate (TMOS), or
tetraethyl-orthosilicate (TEOS) is hydro-
philic, which means that it is easily
affected by moisture. A first hydrolysis
step is required to hydrolyze portions of
the SiOCH3 (TMOS) or SiOCH2CH3
(TEOS) groups, resulting in SiOH.
Afterward, the polymerization occurs
owing to condensation reactions between
two SiOH groups or one SiOH and
one Si–OR group, resulting in a SiOSi linkage group. There
are some unreacted SiOH and Si–OR groups in the sol–gel
matrix.[4]
The hydroxyl and alkoxyl groups can exert considerable inter-
molecular pressures on water. Over time, water vapor in the
air around aerogels may be adsorption into the aerogel matrix,
resulting in the degradation of the nanostructure.[5]
It is possible to produce aerogels from silica sol–gels in a
variety of methods, including freeze-drying, ambient drying,
and supercritical drying. Using a high-temperature technique,
a metal mold in a hydraulic hot press is used to extract the sol-
vent from the pores of the sol–gel matrix using rapid supercrit-
ical extraction (RSCE).[6]
Before, the RSCE technique was mostly used for the hydrolysis
and polycondensation processes of silica aerogels made from
TMOS, with methanol as the solvent water as the hydrolysis
medium. Monolithic aerogels are possible within 3 h after pre-
cursor synthesis, but we prefer a six to 8 h chilling method to allow
for more progressive cooling of the final aerogel monoliths.[7]
The TEOS-based recipe was used by Rao and Bhagat and
it was shown that this RSCE construction approach might be
used to create silica aerogel monoliths.[8]
This RSCE method
has several benefits, including the low processing time, absence
of solvent interchange, and the possibility of being increased
proportionally.
However, TEOS-based recipes for silica aerogel produc-
tion have additional advantages. TEOS is less costly than
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TMOS, by a large margin. In addition, ethanol is the result of
TEOS hydrolysis and polycondensation processes rather than
methanol.[9]
TMOS-based aerogels, which emit methanol rather than eth-
anol, may be more desirable for commercial applications that
need process scale-up. For potential commercial applications,
the RSCE method and the using the TEOS as the silica source
make this technique appealing.[10]
Precursors such as TMOS and TEOS were traditionally used
in these preparation methods. As a result of the high quan-
tity of ethanol that must be used to dilute silica sol, alcogel is
almost impossible to be generated in the traditional one-step
technique.[11]
The two-step approach was proposed as a solution. The pre-
cursors (TMOS or TEOS) are partly hydrolyzed by an acid cata-
lyst and condensed in the first phase of the two-step method.[12]
Finally, a solvent that is nonalcoholic such as acetone, acetoni-
trile, or ether, is employed to complete the hydrolysis and fixa-
tion process, forming silica alcogel.
First-step factors such as temperature and time have been
shown to influence the characteristics of ultralow-density silica
aerogels. For example, some researchers have looked at using
a variety of precursors, while others have focused on changing
the synthesis parameters. According to Wagh et al. TMOS pro-
duces aerogels with smaller holes and more surface area than
the TEOS precursor.[13] They compared it to three other precur-
sors: TEOS, TMOS, and PEDS (polyethoxy disiloxanes) synthe-
sized by Zhou et al.[14]
Research on the properties of silica aerogels has been signifi-
cant. When Pierre and Pajonk compiled their study, they looked
at all relevant studies to see how the sol–gel matrix is created,
how it ages, and how it is extracted for aerogels.[15,16]
Silica aero-
gels may be used in several ways, such as energy storage, sen-
sors, chemical adsorption, thermal insulation, biomedical, and
shock absorption, supporting the growth of sectors like envi-
ronmental protection, building, aerospace, and transportation
engineering sector.[17,18]
The present study thus discusses the synthesis of silica
aerogels using the sol–gel method, drying processes, and its
diverse uses in contemporary industrial development and sci-
entific research, taking into account the surprising features of
aerogels.
2. Synthesis of Silica Aerogel
The synthesis of silica aerogel consists of three significant steps
by which a standard aerogel structure can be obtained. Step-1:
preparation of the gel (sol–gel method), step-2: aging the gel
after preparation, step-3: drying the gel.
2.1. Preparation of Aerogel
In the production of silica aerogels, silicon alkoxides are the pri-
mary precursors. Si(OCH3)4 (tetramethoxysilane), Si (OC2H5)4
(tetraethoxysilane), and SiOn(OC2H5)4 (polyethoxy disiloxane)
are the most frequently used chemicals. Tetramethoxysilane
and polyethoxy disiloxane have much lower thermal conduc-
tivity than tetraethoxysilane aerogel monoliths in terms of the
conductivity spectrum.[19]
When it comes to producing high-
quality clear aerogels, tetraethoxysilane is the best option.
2.1.1. Sol–Gel Approach
The sol–gel process is regarded as a useful technique for
altering substrate surfaces. The sol–gel method’s most essential
feature is its ability to produce large surface areas and stable
surfaces.[20] There is a direct correlation between experimental
circumstances and the chemical and physical characteristics of
materials produced by the sol–gel technique. Soluble gels are
formed by hydrolysis of the precursor in acidic or basic solu-
tions, followed by polycondensation of the hydrolyzed product.
It is possible to create a 3D network by dispersing solid
nanoparticles in an agglomeration of liquid. Brinker et al. gave
a detailed description of the procedure.[12]
Aerogels are simply a gel’s solid structure, separated from
the liquid medium that it normally resides in.[22] Nanoparti-
cles for silica aerogels are produced in a liquid.[23] Silica alkox-
ides are the primary precursors of silica aerogels. The most
often used silanes are tetra-methoxy-silane Si(OCH3)4, tetra-
ethoxysilane Si(OC2H5)4 or TEOS, and polyethoxy disiloxane
SiOn(OC2H5)4-2n or PEDS-Px.[24] Dai et al. used ionic liquids as
solvents for inorganic polymeric processes to create new inor-
ganic materials.[21]
Ionic liquids are a special kind of solvent with low vapor
pressure and a wide range of characteristics. They have recently
been shown to be better solvents for numerous chemical pro-
cesses. In alcoholic aqueous solutions, tetra-alkyl orthosilicates
are hydrolyzed and condensed to produce gels (Figure 2). The
solvents evaporate, producing gel shrinkage before a stable sol–
gel network forms. Longer aging tends to lower pore volume
to that of the matching xerogel. However, too little aging time
may promote gel network instability, leading to gel network
collapse during solvent extraction. Controlling the aging dura-
tion is thus crucial to aerogel synthesis. Ionic liquids allow
Figure 1. Silica aerogel samples. Reproduced with permission.[3] Copy-
right 2018, Elsevier.
Figure 2. Solvent extraction mechanism by sol–gel method. Reproduced
with permission.[21]
Copyright 1996, Royal Society of Chemistry.
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for extended aging durations without shrinking the gel net-
work. Their low vapor pressure inhibits solvent evaporation,
and strong ionic strength accelerates aggregation. The benefits
of employing ionic liquids as solvents for aerogel synthesis
depend on their capacity to generate homogenous solutions
with aerogel precursors.
Chao et al. used PEDS as a precursor and did the sol–
gel method using a PEDS–water–ethanol combination with
NH4OH as a catalyst to make silica aerogels.[25] NH4OH and
distilled water were combined with PEDS, ethanol, and NH4OH
at room temperature. The mixture was transferred to a circular
glass container in the next step. A day after aging at 60 °C, the
glass tube and alcogels were supercritically dulled with ethanol
at 300 °C while maintaining a pressure of roughly 136 bar cre-
ated by the solvent vapor. That is when silica aerogels of mono-
lithic and fracture-free ultralow density were produced.
Babiaczuk et al. dissolved polyvinyl alcohol (PVA) powder (60
and 100 mg mL−1) in 95 °C water for ≈23 h in the beginning.[26]
The dissolving was accomplished with 250 rpm continuous
stirring to generate a homogenous PVA solution (Figure 3).
The solution was then cooled to 60 °C. After 23 min of
churning, the uniform liquid was put into cylindrical molds
and sealed with parafilm.[27] The samples were maintained at
21°Cfor24h.Thesol–gelreactionandphaseseparationoccurred.
The outcome was white light blue monolithic SiO2–PVA
hybrid gels. To avoid cracking, the monoliths were soaked in
methanol (3–4 times) before drying. The samples were then
autoclaved (Paar Mini BenchTop Boiler 4563) to replace the
methanol in the alcogels with CO2, which is liquid. Finally,
supercritical carbon dioxide drying was undertaken (37 °C,
90 bar), and, consequently, hybrid SiO2–PVA aerogels were
produced.
Linhares et al. formed a sol–gel structure by hydrolysis
of precursors, condensed into primary particles, evolution
through the evolving solution, and aggregation into bigger sec-
ondary particles, which connect in a continuous layer with fluid
in the interstices.[28]
In acidic circumstances, the hydrolysis process is faster,
and the rate-determining step is condensation/gelation,
favoring the synthesis of tiny oligomers containing reactive
Si–OH groups. Proton donors preferentially attack oxygen
atoms in acidic media, whereas slow hydrolysis is the rate-
determining process in basic media. With proton acceptors
present, alkaline circumstances speed up condensation, and
the hydrolyzed species are rapidly absorbed into bigger and
denser colloidal silica particles. Poco et al. have found that
more than 3 h are generally required to complete all aerogel
preparation steps.[29]
This includes filling molds and gelation, heating to the crit-
ical point of entrained liquid, decompression of supercritical
fluid, cooling to room temperature, and dismantling molds.
So far, they have utilized this method to create silica aerogel
monoliths up to 20 cm in diameter and 3 cm thick. They are
confident in the process’ ability to handle substantially greater
batches.
2.2. Aging
After the sol reaches the gel stage, several unreacted alkoxide
groups are still in the gel’s silica spine. There must be enough
time for the silica network to be strengthened by managing
the overlaying solution’s pH, concentration, and water content.
Hydrolysis and condensation may proceed. Gel structure can
change due to various factors, such as aging and the dissolving
of microscopic particles into bigger ones. Ethanol–siloxane mix-
tures are often used in aging methods. To eliminate any leftover
water from the pores, the gel should be cleaned with ethanol
and heptane once it has aged.[30]
2.3. Dryings
Aerogel manufacture necessitates the use of drying equipment.
The most prevalent method is a drying procedure known as
ambient pressure drying or supercritical drying. Capillary ten-
sion cannot be avoided while drying at ambient pressure. This
can be done if you remove pore fluids above the critical tem-
perature and pressure.
2.3.1. Supercritical Drying
The earliest and most frequent technique of drying aerogels
is supercritical drying. Capillary forces are shown by drying
gels at a key point. Due to the surface tension in the gel pores,
evaporation produces concave menisci. Constant compres-
sion induces pore collapse, whereas tension causes gel body
collapse. Supercritical drying in an autoclave prevents the gel
from surface tension. The autoclave must be heated to a crit-
ical degree to reduce surface tension. This is repeated until the
autoclave’s pressure equals that of the atmosphere. Methanol
is the most common supercritical drying solvent for aerogels.
High-temperature and low-temperature supercritical drying are
two types of process. Less shrinking of the gel is achieved by
high-temperature supercritical drying.
Figure 3. Synthesis procedure SiO2–PVA hybrid aerogels. Reproduced with permission.[26]
Copyright 2020, Elsevier B.V.
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Estok et al. produced aerogel in a confined mold inside a
hydraulic hot press utilizing a quick supercritical extraction
process in a contained mold.[32] Tetrahedron associates provided
a 24-ton hydraulic hot press for this project. A stainless-steel
mold with 16 wells measuring 1.9 cm in diameter and 1.9 cm
deep was produced with dimensions of 11 cm × 11 cm × 1.9 cm.
Before usage, a nonstick spray was applied to the mold. Gasket
material was needed to seal the decay before it could be
processed.
Anderson et al. used a hydraulic hot press and a bespoke
steel two-part mold to accomplish supercritical extraction in the
rapid supercritical extraction method.[31] During the first step of
gelation, the mold is heated to 38 °C and stabilized by adjusting
the hot press limiting strength to 94 kN for 2 to 7 h (Figure 4).
Because the inclusion of methyltrimethoxysilane (MTMS),
ethyltrimethoxysilane, and phenyltrimethoxysilane delay
hydrolysis and condensation, the duration of this step is deter-
mined by the amount of coprecursor in the precursor mixture,
for pure trimethylene oxide (TMO), only 2 min were needed.
By contrast, other gels required anything from 3 to 7 h to dry
off completely. Step 2 involves heating the mold from 38 to
288 °F over 3 h. Condensation is expected to persist for the pre-
dictable future. Step 3 (equilibration) requires 30 min for the
system to equilibrate and reach a supercritical state with all of
the solvents. This reduction in restraining force occurs within
30 min. A “seal” is broken, and monolithic aerogels remain
in the mold when the solvent is removed. Yoda and Ohshima
heated the autoclave to 553 °C at a rate of 40 °C per hour.[33]
For 3 h, the temperature remained at that level. Even with the
addition of water, these circumstances were deemed supercriti-
cally. Condensed alcohol vapor was emitted from the autoclave
and cooled. The temperature was maintained at 553 K while
the pressure fell at a rate of 5 MPa h−1. More than 12.5, 12.5
to 11.0, 11.0 to 8.0, 8.0 to 3.0, and 3.0 to 0.1 MPa for methanol
were the negative pressure areas of condensed alcohol (“solvent
eliminated”).
2.3.2. Ambient Pressure Drying
There are several drawbacks to supercritical drying for gels,
including cost, process continuity, and safety. Brinker et al.
devised and presented a commercially viable drying technology
called ambient pressure dry to produce silica aerogels. As the
last step, ambient pressure evaporation is used. Ambient pres-
sure drying is the most used method for reducing production
costs.[12]
Quignard et al. dried the microspheres by immersion in a
series of sequential ethanol–water baths with increasing alcohol
concentrations (10%, 30%, 50%, 70%, 90%, and 100%) for
15 min each in a rising alcohol concentration (10%, 30%, 50%,
70%, 90%, and 100%).[34]
Finally, the microspheres are dried in Polaron 3100 equip-
ment under supercritical CO2 circumstances (74 bar, 31.5 °C)
using supercritical CO2. Cai and Shan employed methanol
(MeOH) as a solvent and oxalic acid and ammonia (NH3H2O)
as catalysts in a two-step acid-base sol–gel synthesis followed by
ambient pressure drying to produce silica aerogels.[35]
Sol–gel and ambient pressure drying were used by Feng
et al. to produce silica aerogel with a large surface area. It was
used as the predecessor of silica aerogel, which was made from
water glass made from rice husks. It was found that the char-
acteristics of silica aerogel were affected by the synthesis condi-
tions, specifically the modulus of water glass.[36] After washing
the wet gels, Yun et al. dried them at ambient pressure in a fur-
nace at 80 °C for 24 h and finally at 120 °C for 12 h to produce
silica aerogels.[37]
By way of Zhao et al. liquid in aerogel could be evaporated
at ambient pressure without the silica skeleton structure col-
lapsing because of the avoidance of significant acid corrosion
on the equipment during the drying process.[38] By drying the
prepared aerogel at ambient pressure, Wu et al. discovered
that it had an outstanding hydrophobic property. Utilizing low-
cost ambient pressure drying with fly ash and trona ore as raw
ingredients. Highly porous and hydrophobic silica aerogel is
created in this way.[39]
2.3.3. Freeze Drying
Cryovacuum drying aerogels may also be dried using a
technique known as freeze-drying. The pore fluid is frozen
and subsequently vaporized under a vacuum in this pro-
cedure. In order to solidify the gel network, this approach
requires a long aging time. A solvent with a lower thermal
expansion and a higher sublimation pressure should be
used instead.
To achieve low thermal conductivity and great thermal
stability, Pan et al. employed various molar ratios (5.1–0) of
MTMS/water-glass co-precursor, followed by freeze-drying
(Figure 5), and created hybrid aerogels of silica and carbon.[40]
Experimental findings show that the MTMS/water-
glass molar ratio greatly influences the hybrid aerogels’
Figure 4. Mold and press synopsis for processing of RSCE aerogels.
Reproduced with permission.[31] Copyright 2009, Springer Nature.
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characteristics. After 8 h of drying at −80 °C in a laboratory
freeze drier, the aerogels were vacuum-dried for roughly 48 h.
Glass fiber-reinforced silicate aerogels were finally produced by
Zhou et al.[41]
3. Property Analysis of Silica Aerogel
3.1. Thermogravimetric Analysis
As shown in Figure 6, thermogravimetric analysis (TGA) was
used by Zhao et al. to study the thermal stability of aerogels
using a scanning temperature range of 50–650 °C. High-energy
bonds were discovered, such as SiO, SiC, and CH bonds,
which were ascribed to chemical property stability.[42]
The loss of mass up to 300 °C is caused by SiOH bonds
that persisted after the sol–gel process.[43] Most of these bonds
were changed to the SiOSi network, while unreacted group
reactions at high temperatures did not affect mechanical char-
acteristics or hydrophobicity. As a result, the mass loss from
room temp to 300 °C is smaller than the major CH3 mass
loss from 350 to 600 °C when the temperature hits 350 °F.[44]
Maghsoudi and Motahari used a Mettler Toledo TGA1
ultra-micro-balance equipment to conduct their TGA testing
(Figure 7). All of the tests were carried out in accordance with
ASTM E 1131-14.
Figure 5. Procedure to produce silica aerogels based on co-precursor. Reproduced with permission.[40] Copyright 2017, Elsevier.
Figure 6. The TG curve of the silica aerogel in the air atmosphere. Repro-
duced with permission.[42] Copyright 2018, Elsevier.
Figure 7. TGA test results for silica aerogel composites. Reproduced with
permission.[45]
Copyright 2017, John Wiley and Sons.
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The nitrogen flow rate was kept constant throughout
the trials at 50 mL min−1. For each experiment, the sample
mass was 55 mg. (In each experiment, the samples weighed
55 mg.) Maghsoudi and Motahari used a Mettler Toledo TGA1
ultra-micro-balance equipment to conduct their TGA testing.[46]
All of the tests were carried out in accordance with ASTM E
1131-14. The nitrogen flow rate was kept constant throughout
the trials at 50 mL min−1.[45]
3.2. Optical Properties Analysis
Fu et al. studied the optical characteristics of optically thick
silica aerogel in the visible, near-infrared, and infrared wave-
length ranges. The fiber-loaded silica aerogel sample with a
thickness of 1.10 mm was created using the sol–gel technique
and the supercritical drying procedure (Figure 8).
In comparison, a silica fiber sample was made with the
same chemical component, thickness, and physical structures.
To examine the radiative properties of samples, a basic two-
flux radiative model was utilized. In the wavelength range of
0.38–15 µm, the spectral normal-hemispherical reflectance,
transmittances, and normal emittances of silica aerogel and
silica fiber samples were measured and compared.[47]
3.3. Density and Porosity Analysis
According to Figure 9, the density of virgin silica aerogel pro-
duced in this investigation by Huang et al. is comparable with
earlier results. Density follows a similar pattern when heated to
950 and 980 °C. After 7 h of heating at these two temperatures,
the highest densities were 400 and 600 kg m−3, respectively.[48]
After 4 h of heating, the density varies between 330 and
600 kg m−3. It takes ≈3 h for the density of silica aerogel
to climb linearly at 1000 °C. The density drops to roughly
410 kg m−3 after 3 h of heating. Within the first 2 h, the den-
sity increased at a similar pace for samples heated to 950 and
980 °C as it did for those heated to 1000 °C.[49]
3.4. Hardness Analysis
The composite hardness of silica aerogel (SA)/unplasticized
polyvinyl chloride (UPVC) was tested by Eskandari et al. and
is shown in Figure 10. When silica aerogel is added to UPVC
in greater quantities, we see a rise in hardness. Compared to a
clean sample, the hardness of UPVC is increased by 10–24% by
adding 0.5–3% SA.[50] The mineral composition of silica aerogel
justifies this increase in hardness. For composites to be hard,
they must be filled with inorganic fillers.[51]
3.5. Flammability Analysis
Flexible glass fiber (GF)/aerogels composites were created
using the sol–gel process and dried at ambient pressure.[52]
Research by Li et al. looked at the microstructural and mechan-
ical properties as well as the thermal insulation and flamma-
bility of composite materials with the various “mole ratios of
H2O:TEOS” denoted as S (changing from 2 to 6). As the S value
Figure 8. Scanning electron microscopy (SEM) image of the cross-sec-
tion of silica aerogel sample. Reproduced with permission.[47]
Copyright
2015, Elsevier.
Figure 9. The structure of silica aerogel is represented by a) measured density and b) porosity. Reproduced with permission.[49] Copyright 2017, Elsevier.
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increased, the composites’ density decreased significantly at
first but after that very little.
In Figure 11, E1, E2, E3, E4, and E5 represent the five dif-
ferent samples of GF/aerogel composite under an incident heat
flux of 35 kW m−2, and H1, H2, H3, H4, H5 represent the five
different samples of GF/aerogel composite under an incident
heat flux of 60 kW m−2 in accordance with different S values.
There was the minimum density (0.142 g cm−3), the biggest
surface area (963.88 m2 g−1), the average pore size (8.61 nm),
and the highest specific compressive properties of samples with
S = 3. This work lowered the GF/aerogel composites’ thermal
conductivity to 0.0232 W m−1 K−1.[53]
4. Applications of Silica Aerogel
Aerogel’s skeleton and porous morphology may be controlled at
the nanoscale to provide a variety of remarkable physical char-
acteristics, including as ultralow density, ultrastrong adsorp-
tion capacity, ultrahigh specific surface area, and thermal
conductivity. Due to these characteristics, aerogels are an excel-
lent material class for a variety of potential implementations
(Figure 12).
4.1. Construction Materials
Construction materials like glass, buildings, coating, etc., are
being modified to endure more thermal wavelengths with the
help of aerogel.[54] Silica aerogel is not directly used in them.
But used as a raw composite material with other fibers like
glass, graphene, cellulose, etc. Some experimental composite
materials on different functions of construction done by var-
ious novel experimenters are described below.
4.1.1. Thermal Insulation
At atmospheric pressure, silica aerogel has been discovered
to have the lowest heat conductivity of any other insulator so
far documented (Figure 13). In addition, it is generally known
that air has poor heat conductivity (0.025 W m−1 K−1). Before
the development of silica aerogel by Kistler and Caldwell,[56]
no solid (nonporous) had been found to have heat conductivity
as low as air. Aerogel has a much lower thermal conductivity
than air, which is explained further by the Knudsen effect.[56]
According to the Knudsen effect, the smaller the pore size of
aerogel, the less thermal conductivity it has because trapped
gas cannot travel freely through the pore.[57]
Hasan et al. found that the bulk/monolith silica aerogel’s
thermal conductivity is significantly influenced by its density
and porosity, as seen in Figure 14a,b. As shown in Figure 14a,
the thermal conductivity of aerogel falls with decreasing den-
sity because heat transmission through conduction is lowered
owing to the reduction in solid volume.[58] By contrast, as seen
in Figure 14b, porosity will exhibit the reverse tendency. As the
porosity increases, so does the heat conductivity.[59] In gen-
eral, the air has a lower heat conductivity than solids, leading
to this trapped air rise. As previously stated, heat conduction
is hindered by increased porosity, which reduces the solid
network.[60]
He and Xie discovered that the solid nanoscale framework
and the complicated structure of aerogel both have the potential
Figure 10. Hardness and impact strength of UPVC/SA composites versus
SA content. Reproduced with permission.[51]
Copyright 2016, John Wiley
and Sons.
Figure 11. Heat release rate (HRR) curves under different composites’ 35 and 60 kW m−2
radiant heat flux. Reproduced with permission.[53]
Copyright
2017, Elsevier.
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to limit solid heat transmission. Aerogel has a very low density
and a complicated 3D porous structure that is exceedingly com-
plex. As a result, this structure extends the heat transfer route
and complicates heat transmission through the solid matrix.
This results in a substantially lower solid heat transfer for
aerogel than for typical thermal insulation.[61]
To explain the increase in porosity and specific sur-
face area, Kwon et al. cited the spring-back phenomenon,
which occurs when thermally expanded gases inside gel-
network pores expand during heat treatment and cause the
gel to swell, as well as the weight loss caused by the oxida-
tion of organic groups in the gel. At ambient temperature,
the thermal conductivity of 5STA heat-treated at 350 °C was
0.0136 W m−1 K−1.[62]
It is becoming more common for aerogels to be used instead
of more traditional materials in a variety of applications. Lamy-
Mendes et al. summarized the most current developments in
silica aerogel integration into composites and building con-
structions. As an effective replacement for present construction
heat insulation components, aerogel-containing materials have
previously been widely developed and examined for building
applications such as blankets, panels, and aerogel integration in
cement mortar plaster casts.[63]
4.1.2. Acoustics Insulation/Devices
The silica aerogels and their cellulose frameworks were tested
for their sound-insulating abilities. The sound-generating
source was put both in and out of an insulating box in this
method.[64] Double-sided tape was used to attach aerogels to
all four corners of the container, ensuring a completely sealed
system. In all situations, the incidence sound signal was deter-
mined at the same space away from the sound source. It was
described as the discrepancy between the informed sound
strength and the actual sound strength. The ratio of the con-
sumed sound strength to the known sound intensity is what is
known as the sound absorption coefficient.[65]
Sound absorption and insulation in buildings have his-
torically relied on conventional materials like rock wool
and open-cell foam, but modern businesses and society are
searching for ecologically friendly alternatives with better
sound absorption and insulation.[66] Mazrouei-Sebdani et al.
have found that aerogels provide a way to combine thermal
insulation with desirable acoustical qualities. Aerogel sound
absorption and insulation are highly dependent on the mate-
rial’s production technique, density, and pore structure.
Acoustic waves are attenuated in an aerogel due to energy
loss as they go from the gas phase to the solid phase. This
decreases both the amplitude and velocity of the waves so
that they dissipate more rapidly. As a result, aerogels may be
useful for sound insulation.[67]
Samples from Merli et al. reveal a high absorbing coef-
ficient at low frequencies (not more than 2000 Hz) with low
absorbing coefficients at medium and high frequencies.
Because of the increased tortuosity of the sample, the high
absorbing coefficient moves to lower frequencies as sample
thickness rises, as predicted. The 25.4 mm thick monolithic
pane has a TL of 15 dB at 1600 Hz, which is a considerable
amount of acoustic insulation.[3]
Fernández-Marín et al. stated that, in acoustics, aerogels are
used by developers of efficient ultrasonic systems as imped-
ance-matching materials for anechoic rooms with sound-
dampening materials (Figure 15).
Because of their subwavelength resonances and high absorp-
tion efficiency, silica aerogel plates are great starting points for
Figure 12. Various applications of silica aerogel.
Figure 13. Aerogel glazing into the facade of a building. Reproduced with
permission.[55] Copyright 2015, Elsevier.
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developing novel membrane metamaterials. Membrane meta-
materials have gained a lot of attention in recent years for their
ability to manipulate acoustic waves.[68]
4.1.3. Solar Energy Material/Solar Window
As a transparent insulating material for passive solar com-
ponents like windows and solar walls, the goal is to employ
monolithic silica aerogel from this company. To safeguard
the monolithic silica aerogel and ensure optimal thermal
insulation, Chen et al. focused on unique architecture.
A depleted monolithic silica aerogel layer is sandwiched
between two sheets of glass, and a rim seal completes
the design. Each of the four prototypes has a distinct seal
design.[69] Due to the potential for twice the overall heat
transfer coefficient, special emphasis has been paid to cold
bridges at the perimeters. One of the most recent proto-
types featured an aerogel that was 150 kg m−3 in density and
had an 18 mm thickness. The heat exchange coefficient was
0.52 W m−2 K−l in the center and 0.57 W m−2 K−l overall. The
center and overall heat exchange coefficients would just be
0.37 W m−2 K−1 and 0.47 W m−2 K−1 if the aerogel density and
thickness were 100 kg m−3 and 20 mm. Aerogel windows
installed in a single-family home in Denmark have been
demonstrated in simulations to save 878 MI per year com-
pared to conventional double-pane windows.[70]
There are two kinds of aerogels: monolithic and granular. To
put it another way, the solar transmittance of monolithic silica
aerogel windows has been demonstrated to reach up to 0.8,
which is much greater than the maximum solar transmittance
of granular windows made from the same material found by
Berardi (Figure 16).
The manufacture of large sections of monolithic aerogels is
prone to fractures, which has limited the usage of monolithic
aerogel glazing systems.[72] It was created as part of the EU
project HILIT, which demonstrated the feasibility of creating
windows with a 0.66 W m−2 K−1 thermal conductivity and light
transmissibility over 0.8.[71]
Buratti and Moretti stated that a novel glazing system made
of aerogel might be a viable alternative to traditional windows
in high-glazed structures where severe regulations are in place
or where energy consumption is an issue. Aerogel panes may
be increased in thickness so that thin windows with thermal
transmittance (U-values) below 0.5 W m−2 K−1 can be produced
without significantly decreasing the solar factor or the daylight
transmittance.[74]
Buratti et al. placed the granular form of aerogel in a 15 mm
thick gap between the outer and inner layers of float glass, which
are both 4 mm thick (Figure 17). Two layers of transparent float
glass were used in window type 1 and window type 2 feature
solar control coating on the inside side of the exterior glass and
a Low-E. Aerogel grains were packed tightly between the glass
layers using mechanical vibration during manufacturing. Type
“a” silica aerogel granules were evaluated for the type 1 window
frame, whereas type “b” was studied for the type 2 frame. This
aerogel’s most current version, type “b,” has bigger granules and
so improves the material’s ability to reflect light.[73]
Figure 14. a) Variation of thermal conductivity as a function of density and b) porosity of monolith silica aerogel. Reproduced with permission.[60]
Copyright 2017, Springer Nature.
Figure 15. a) Photograph of the experimental configuration. b) Absorption as a function of the frequency. c) Complex plane representation of the reflec-
tion coefficient. Reproduced with permission.[68] Copyright 2019, AIP Publishing.
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4.2. Electrotechnics
In electronic devices, it is very important to separate all the elec-
trical components like capacitors, resistors, etc., because if the
electricity is distributed uniformly, it will face a serious short
circuit. As silica aerogel is the world’s lightest solid, it can be
used in any electrical component if modified properly per the
requirements. Research by various researchers on the electrical
applications of aerogel is being discussed.
4.2.1. Piezoelectric Device
An air-coupled piezoelectric transducer using silica aerogel as
the outer layer can operate at megahertz frequencies using
a process Gomez et al. developed that creates two matching
layers.[75] The aerogel film, investigated by scanning electron
microscope, has a nonuniform scattering and a fractured
structure. Aerogel’s acoustic characteristics may be deter-
mined by comparing experimental results to theoretical pre-
dictions made using a 1D model and matching the results
to a piezoelectric plate.[76] The aerogel sheet produces a dis-
tinct resonance focused at 3 MHz, despite being broken and
nonuniform. A model for a pair of transducers with identical
layers has been developed. The pitch-catch operation’s calcu-
lated reaction has been shown. It is possible to get a response
with a narrow band and great sensitivity.[75]
4.2.2. Microelectronics
The porous morphology, decreased polarity due to the exist-
ence of the siloxane network, and hydrophobic nature of
frozen smoke aerogel contribute to both the low dielectric con-
stant (2.16) and surface free energy (mJ m−2) with a high 20
shielding behavior of 94.4%, according to data from several
investigations.[77]
A new type of phase change microsphere with intriguing
electrical properties and the ability to store and release thermal
energy was developed by Wang et al. using graphene aerogel
templates and phase change materials, and it was success-
fully used as a thermal buffer for an electric circuit in micro-
electronic devices. This approach was used to create graphene
aerogel microspheres with very consistent size distribution and
perfect roundness.[78]
4.2.3. Microwave Electronics
In low-frequency microwave bands, Ślosarczyk et al. inves-
tigated the shielding effectiveness of silica aerogel with
unchanged and coated carbon fiber with nickel (from 8 to
18 GHz).[79] Habibullah et al. have presented a novel organic
aerogel derived from rice husk. Substances derived from
organic biocompatible origins are used to make the sug-
gested substrate’s basic components standard laboratory con-
ditions have been used to conduct the preparatory and testing
procedures.[80]
Shah et al. investigated a multicomponent magnetic aero-
gel’s microwave absorption properties and found that the
combination of organic and inorganic components yielded
good results.[81] In 1974, Cantin et al. created the first Cerenkov
radiation detector based on silica aerogels. Following the intro-
duction of aerogels into the scientific community, they have
been utilized or investigated for applications ranging from
laser experiments to sensors to thermal insulation to waste
management.[82]
4.2.4. Supercapacitor
Supercapacitors are a type of energy storage capacitor. Elec-
tronic differential lock control (EDLC) is made up of polarizing
electrodes and collectors that are separated by a gap through
an electrolyte. Polarizing electrodes are usually made of carbon
compounds.[83]
Supercapacitors based on SiO2 aerogel, manufactured by the
sol–gel technique, were explored for the first time by Du et al.
Because of their mesoporous structure and large surface area,
the SiO2 aerogels investigated in this article were selected for
further investigation.[84]
According to Korkmaz and Kariper, increasing the rough-
ness and surface area of electrodes increases the stored
charge in supercapacitors. In contrast to aerogel graphene
oxide, reduced graphene oxide may be made by chemists
more readily and cost-effectively. Even so, we are certain that
scientists will find a way to make graphene oxide aerogel
shortly.[85]
Figure 16. Two examples of aerogel windows with granular filled
have been alternated with traditional transparent windows: Detroit
School of Arts, MI, USA (left), and Nobel Halls at SUNY Stony
Brook, NY, USA (right). Reproduced with permission.[71] Copyright 2015,
Elsevier.
Figure 17. a) Window with double glazing and b) window with aerogel
between glass layers. Reproduced under terms of the CC-BY license.[73]
Copyright 2017, The Authors, published by MDPI.
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For supercapacitor applications, Pottathara et al. described
the basics as well as the most current advances and fabrica-
tions of graphene aerogels. Aerogel graphene fabrication
methods such as 3D printing and covalent cross-linking were
studied and shown in Figure 18. Graphene aerogels may be
used as electrode materials in electric double-layer capacitors
(EDLCs), either alone or in combination with other carbon
materials.[86]
4.2.5. Battery Applications
Vanadium oxide is a preferred cathode material for lithium-ion
batteries because of its high capacity for lithium insertion in
nonaqueous electrolytes. Vanadyl tri-isopropoxide was the pre-
cursor utilized by Salloux et al. to manufacture vanadium pen-
toxide (V2O5) aerogels. Lithium-ion intercalation in this aerogel
may reach 1.9 mol Li per mole. Compared to amorphous and
crystalline V2O5, the particular strengths of V2O5 aerogels
increased up to 1500 Wh kg−1.[83]
Chen-Yang et al. conducted research with and without silica
aerogel powder (SAP), and a variety of composite polymer elec-
trolytes have been examined as a function of salt and SAP con-
centration. Changes in the EO (ethyl oxide)/Li (lithium) ratio
and SAP concentration of polymer electrolytes affected the con-
ductivity of the polymer electrolytes.[91]
According to Lim et al. silica aerogel particles were included
in solid polymer electrolytes based on polyethylene oxide (PEO),
polymethyl methacrylate (PMMA), ethylene carbonate (EC),
LiClO4 to study the relationship between the electrolyte’s struc-
ture, lithium-ion conductivity, and thermal behavior. A combi-
nation of PMMA and PEO retards crystallization and lowers
the glass transition temperature, resulting in improved lithium-
ion conductivity, according to the X-ray diffraction (XRD), and
Fourier transforms infrared investigations.[92]
For the silica aerogel composite separator, Feng et al. used
hydrophobic silica aerogel and polypropylene separators. After
being folded 200 times, a 3D crosslinked network structure is
evenly distributed with the coating layer over the PP substrate
with little abscission, contributing to improved thermal stability
and electrolyte wettability.[93]
Silica aerogel being prepared without nitrogen gas pres-
sure, labeled as Aerogel-S1, has superior electrochemical per-
formance than silica aerogel being prepared with pressurized
20 bar nitrogen into the vessel and kept for 30 min at 255 °C
and slowly released, which is labeled as Aerogel–S4, according
to Shanmugam et al. This may be due to the mesoporous sam-
ple’s significant surface area contribution, which improves
electrolyte wettability and alloy accommodation inside the
mesopore structure.[94]
Zhang et al. successfully developed a modified polyacryloni-
trile silica aerogel separator, which is a modified polyacryloni-
trile/silica aerogel separator. In situ silica aerogel synthesis
and hydrolysis of the nitrile group increase the chemical sta-
bility of polyacrylonitrile (PAN) nonwoven in commonly used
electrolytes.[95]
Solid-polymer electrolytes, including PEO, polyvinylidene
fluoride (PVDF), and silica aerogel powder, were effectively pro-
duced by Yoon et al. Depending on the PEO:PVDF ratio, the
polymer:Li-salt ratio, and the silica aerogel content, the lithium-
ion thermodynamic properties of the solid polymer electrolytes
may vary.[96]
Figure 18. Illustration of supercapacitor. a) Structure for a pseudo-supercapacitor and EDLCs. Reproduced under terms of the CC-BY license.[87]
Copy-
right 2017, The Authors, published by Oxford University Press. b) 3D-printed graphene aerogel. Reproduced with permission.[88]
Copyright 2019, Else-
vier. c) Graphene balls for application in pseudocapacitor. Reproduced with permission.[89]
Copyright 2013, Springer Nature. d) Hybrid electrodes for
pseudocapacitors. Reproduced with permission.[90]
Copyright 2013, John Wiley and Sons.
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4.3. Aerospace and Defense Applications
As a part of the STS-47 (50th NASA space shuttle mission)
in September 1992, aerogels were transported into space
for testing as a particle capture medium and as a vehicle for
reentry.[97] Capture cells fitted with panels of silica aerogel with
dimensions of 10 cm × 10 cm × 1 cm and densities in the order
of 20 mg mL−1 were placed on the shuttle get away special
(GAS) payload canisters to house the sample return experiment
(SRE).
NASA expanded thermal insulation research on aerogel
due to its low conductivity. The Sojourner rover, silica aerogel
was first employed as an insulator on the Pathfinder mission.
Thermal protection for the main battery pack of the alpha par-
ticle X-ray spectrometer was achieved using composite boxes,
known as warm electronic boxes. Each day, the battery’s oper-
ating temperature was limited to a maximum of +55 °C for 5 h.
It was possible to obtain a temperature of 21 °C.[97] Aerogel was
reconsidered for the mars exploration rover’s spirit and oppor-
tunity in 2003 because of its effectiveness. Robots equipped
with radioisotope heating units could generate additional
heat.[98]
The net surface area of each GAS SRE was 0.165 m2. There
were no visible damages to the aerogels after their successful
launch and reentry into Earth’s atmosphere. During this pre-
liminary flight, at least four big hypervelocity particles were
caught (Figure 19). More than two dozen particles from STS-60
and many more from other GAS canisters were found later
on.[99] Recently, the panel of two aerogels has been installed on
the International Space Station (Figure 19a). After 18 months of
revolution and return to the earth, the aerogels have collected
several debris having different impact signatures and morphol-
ogies.[100] After being retrieved from the space station, a wide
range of impacts (0.1mm) were observed in the aerogel from
hard particles (metals) (Figure 19b) or paint flakes (Figure 19c)
As a result of their high thermal conductivity and low den-
sity, silica aerogels are an appealing option for a wide range
of thermal insulation applications in the aerospace industry
discussed by Randall et al. Several instances are shown in
Figure 20. In the mar’s sojourner rover, for example, the battery
packs are insulated. Electronics and the batteries were well-pro-
tected by the aerogel insulation for three months, which lasted
far longer than expected. An improved aerogel is sought for use
in aerospace applications, though. NASA is considering using
aerogels to insulate extravehicular activity suits on future Mars
human missions in order to keep astronauts safe.[101]
4.4. Gas-Phase Purification, Sensing, and Catalysis
Since they were first synthesized in the 1930s, silica aerogels
have been used in many industries. This review begins with
a short description of the underlying challenges driving the
motion of gases in silica aerogels and then provides an over-
view of the work done in gas purification, gas sensing, and
silica aerogels as catalysts for gas-phase reactions.
4.4.1. Gas-Phase Purification
It is possible to remove contaminants from gases using
three main methods: filtration, sorption, and destruction. It
is still possible to remove bigger particles from gas streams
using filtering using silica aerogels. To remove gaseous con-
taminants, sorption is only helpful in a restricted number
of cases and depends on the surface’s silica aerogel with
functionalization.[102]
Chen et al. revealed that titania-coated silica aerogel parti-
cles might provide potential air purification photocatalysts. To
begin, ambient pressure drying of olivine silica resulted in the
successful production of silica aerogel. Thermally treated aero-
gels were then employed as the photocatalytic support for the
titanium dioxide. Finally, a novel photocatalyst has been tested
for its ability to remove NOX from the atmosphere.[103]
Things are moving at a breakneck pace when it comes to
making porous materials using sol–gel methods. Because of
the large surface area and porosity of porous materials play an
important role in applications including adsorption, sensing,
and catalysis.[104]
A study by Akimov shows that Exhaust gases from automo-
biles may be cleaned using aerogel after they exit the exhaust
pipe. In this example, aerogel is enriched with metal oxides
such as copper and aluminum. As heterogeneous catalysts,
they reduce the NOX content in exhaust gases. With a high
Figure 19. a) Two-panel aerogels on the sensor device have been installed
on the international space station, b) Hard particles or debris (metal).
c) Soft debris (paint or polymer). Reproduced under terms of the CC-BY
license.[100] Copyright 2013, The Authors, published by Hindawi.
Figure 20. Aerospace applications of aerogels. Reproduced with permis-
sion.[101] Copyright 2011, American Chemical Society.
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porosity, aerogel provides a vast contact area between the cata-
lyst and the gas.[105]
4.4.2. Gas-Phase Sensing
Silica aerogels provide several advantages as a sensor platform
for gaseous species. Because they have 99% porosity, they are
mostly gas-filled (on a volumetric basis). Their refraction index
is approximately comparable to air, allowing light to easily
pass through their volume. Because of their enormous interior
surface areas, they may absorb and attach functional groups
to analytes, making detecting them easier. Depending on the
chemical and the area in which they are employed, their sur-
faces might be hydrophilic or hydrophobic. When produced
using the sol–gel technique, it is possible to include many
different colors, fluorescence, and even biological receptors
(e.g., enzymes or bacteria) into their design, allowing them to
be used in a number of applications. Powder, thin layers, and
monolithic shapes are all possible, providing a wide range of
design possibilities. With this in mind, they are capable of being
hybridized with organic polymers that may be used to provide
them with additional mechanical and electrical properties.[102]
Toluene molecules in solution and nitrogen molecules in the
gas phase infiltrated mesoporous silica aerogels showed effec-
tive, coherent anti-Stokes Raman scattering in experiments
by Konorov et al. Silica aerogels are an ideal host for Raman-
active gases and liquids, which can be detected and analyzed
by coherent anti-Stokes Raman scattering, allowing the devel-
opment of gas and condensed-phase sensors for chemical and
biological species, including pollutants and aerosols, and sug-
gesting an intriguing nanoscale elucidation.[106]
Leventis et al. observed that neither the liquid phase pro-
cessing nor the supercritical drying of sol–gel generated mate-
rials leached a guest covalently bound to the silicate framework.
Doping levels may now be precisely controlled thanks to this
quantitative adjustment. As a result of their optical transpar-
ency mass transfer, large surface-to-volume ratios, and ability
to be altered by molecular and particulate guests, aerogels
are good substrates for chemical sensor development and
testing.[107]
Cyt. C contained in aerogels without nanoparticles preserves
structural integrity and responsiveness to nitric oxide, resulting
in the simpler manufacturing of these bioactive aerogels, as
shown by Harper-Leatherman et al.[108]
4.4.3. Gas-Phase Catalyst
According to Kearby and Swann, the original excitement for
aerogels as catalyzers had faded considerably by 1940.[109] By
using the novel aerogel photoreactor to conduct a gas phase
photocatalytic oxidation of trichloroethylene, Cao et al. dis-
covered superior results in photocatalytic oxidation efficiency.
Further improvement of the aerogel catalyst might be achieved
by increasing the amount of active TiO2 domains, increasing
the permeability of the aerogel block, and overall decreasing
UV scattering by amorphous materials.[110]
Dunn et al. devel-
oped, described, and assessed cobalt loadings on silica aerogel
for Fischer–Tropsch synthesis. The catalysts were produced by
adding Co(NO3)26H2O to a tetra methoxy silane-filled gel. The
aerogels were made by drying the gels in supercritical ethanol.
It turns out that the cobalt is either present as minute particles
(50–70 nm in diameter) or needles after reduction by hydrogen,
as seen by transmission electron microscopy.[111]
A novel photocatalyst Ag/TiO2/CA developed by Jafari et al.
was employed to degrade toluene in polluted air streams, with
varying mass ratios of TiO2 and Ag used. According to XRD,
SEM, and energy-dispersive X-ray, titanium dioxide nanoparti-
cles were evenly scattered on the aerogel surface, and Ag nano-
particles were well spread between them.[112]
While Ryu et al. reported on constructing an active and
selective catalyst for the aqueous phase hydrodeoxygenation
(APHDO) reaction, this study examined the endurance of the
catalyst owing to the harsh conditions. APHDO of 1-propanol
(1-PrOH) was carried out using Pt catalysts based on crystal-
line Nb2O5xH2O, amorphous SiO2, Al2O3, and Nb2O5 (niobium
pentoxide) aerogels. Support materials that exhibited poor con-
version rates were changed into TT–Nb2O5, quartz, and boe-
hmite during the reaction.[113]
4.5. Biomedical Applications
As a result of their many beneficial characteristics, aerogels
are gaining popularity across a wide range of industries, from
building to medical. Nonmedical uses of aerogels have been
largely ignored over the last several decades even though
numerous aerogel materials, organic, inorganic, or hybrid in
nature, are biocompatible.
4.5.1. Wound Care
Wound treatment is a lengthy process that begins at the
moment of injury and lasts until the patient is well enough to
return home. Therefore, an ideal wound care solution must
maintain the wound interface wet, enable gaseous exchange,
function as an antimicrobial barrier, and clear excess exudates.
In addition, it should be devoid of harmful components, have
appropriate mechanical properties, be compatible with biocom-
ponents, and be simple to remove after use. A biodegradable
and nonadhesive version may also be available.[114]
Lu et al. have found that porosity of 93%, bulk density of
0.02 g cm−3, water absorption ratio of 3000%, cytotoxicity, and
high biocompatibility was found in the NCF/collagen aerogels,
indicating the material’s potential as a biological scaffold and
wound dressing.[115]
Also, Sani et al. synthesized hydroxyapatite-encapsulated
silica aerogel from rice husk ash (RHA) using the sol–gel
ambient-pressure drying process to improve its performance in
implant and wound care applications.[116]
A new study by Raman et al. suggests that hybrid Ca-alginate
aerogels with Zn2+
and Ag+
additions may have ideal proper-
ties for wound dressing applications, such as absorbing excess
fluid and antiseptic behavior high cell and tissue tolerance
and controlled compound release that help the wound healing
process.[117]
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4.5.2. Drug Delivery
Preventing the degradation of drugs in transit by the body’s
physiological medium is a primary goal of these formulations,
which should be able to precisely and predictably manage drug
release.[118] To minimize side effects and reduce the dosage of
cytotoxic drugs, it is essential that they reach their target loca-
tion. Drug particles with increased specific surface area, those
dispersed on porous substrates, or those with polymorphic
forms may all aid in this process.[119]
In a composite created by Giray et al. hydrophobic and
hydrophilic pharmaceuticals may be included in the core of
aerogel and PEG hydrogel shell, separately to permit the suc-
cessive flow of hydrophobic and hydrophilic drugs. To speed up
the release of a drug within the aerogel core, the PEG hydrogel
network may be broken down using pH- or thermal PEG hydro-
gels in the composite structure.[120]
An ideal aerogel density of 0.12 gr cm−3 has been discov-
ered by Mohammadian et al. In this study, the effect of density
on the concentration of ketoprofen loading on a silica aerogel
was determined. Aerogel density may be used to modulate the
loading of ketoprofen on silica aerogels. Therefore, the appro-
priate density may be set based on the kind of medicine, the
aims of drug administration, and the drug dose.[121]
Smirnova et al. also concluded that aerogels might be effi-
ciently employed as medication delivery devices. In the case
of pharmaceuticals that need to be taken orally, hydrophilic
aerogels may be an ideal carrier material. An alternative to
micronization, which is presently employed to produce a quick
release of pharmaceuticals, may be administered using this
technology.[122]
4.5.3. Cardiovascular Implantable Devices
Medical implants have traditionally used polymers. They are
found in a variety of medical devices, including prosthetic heart
valves, pacemakers, stents, and reservoir medicine delivery sys-
tems. In addition, a number of studies have been published on
its use in the construction of scaffolds for tissue engineering
for wound healing. A curious thing about polymer-based
aerogels is that they are just now being identified as possible
materials for comparable purposes despite their extraordinary
properties. One of the most common blood-implanted devices
is an artificial heart valve, which is used to replace defective or
damaged valves.[114]
When implanted biomaterials come into touch with blood,
they may cause thrombosis, as shown by Nita et al. in their
study. Aerogels for cardiovascular implantable devices (e.g.,
valves) must meet several requirements in addition to their spe-
cific biomechanical properties, such as low inertia, biocompat-
ibility, and hemocompatibility, to prevent plasma protein depo-
sition or adsorption on the surface, which may trigger an acute
immune response. They must also meet these requirements.[123]
Recent work by Yin and Rubenstein at Oklahoma State Uni-
versity has looked into how well different types of aerogels work
in the cardiovascular system. At the beginning of their work,
they used a type of aerogel called a surfactant-template poly-
urea-nano encapsulated macroporous silica aerogel.[124]
4.5.4. Tissue Engineering Substrates
Organ and tissue substitutes are the primary focus of tissue
engineering, which is a broad term. Biocompatibility with
the target tissues and their constituent cells is essential for
successful tissue replacement. When it comes to tissue engi-
neering, aerogels have a number of physical advantages. One of
their most important advantages for tissue engineering applica-
tions is their high and adaptable porosity.[114]
Quraishi et al. found that mechanical characteristics are a
significant consideration in tissue engineering. At two distinct
depressurization rates, the mechanical properties of alginate–
lignin aerogels were tested. This research shows that alginate–
lignin aerogels may be categorized as low-stiffness materials in
both the dry and wet stages of the matter.[125]
The pH range of 7.2–7.4 is ideal for developing biomate-
rial scaffolds for tissue engineering. Ge et al. hypothesized
that combining silica aerogel and polycaprolactone (PCL) to
generate a PCL–silica aerogel composite material may resist
the acidic state that occurs from PCL degradation and hence
enhance the tissue indemnity microenvironment to enable cell
development and tissue regeneration to take place.[126]
Through cell culture and the Thiazolyl Blue Tetrazolium
Bromide test, Lu et al. examined biocompatibility, cytotoxicity,
etc. At the start of the process, the matrix has just one cell of
each kind. However, after five days of cultivation, multicel-
lular spheroids ≈96.79% of the cells were active on average,
demonstrating that the aerogel scaffold has no detrimental
effect on cell growth and morphology and has the potential to
be employed as a cell culture substratum and a cell transport
mechanism.[115]
To put it another way, the aerogel made by Mallepally et al.
has an extremely large surface area and pore volume compared
to freeze-dried scaffolds. Cell culture and tissue engineering
may benefit from the unique properties of silk fibroin (SF)
aerogel scaffolds. For human foreskin fibroblast cell adhesion
and viability, a microporous aerogel scaffold was necessary.
They are compatible with human cells, and the scaffolding
of SF aerogel promotes their growth via supercritical CO2
processing.[127]
4.5.5. Biosensors
Because of its high porosity and capacity to retain a wide range
of compounds, silica aerogels might be a suitable matrix for
creating biological sensors. Using a photoluminescence-based
aerogel based on silica as an active element, they pioneered the
optical detection of oxygen. As a result of this treatment, the
interior surface of the silica aerogel became oxygen-deficient
silica (SiOX). For example, fluorophores in the SiO2 lattice were
responsible for absorbing and emitting the UV–vis wavelengths
of visible light. Molecular O2 interacted with an excited fluoro-
phore and quenched; This was how the optical sensor worked.
ATP5O (human ATP synthase) and CANX were chosen as
DNA targets for a 3D aerogel biochip to detect nucleotide acids
(Homo sapiens calnexin).[119]
Figure 21 depicts a nonspecific DNA target recognition test
performed by Li et al. on the produced aerogel. Ten milliliters
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of the ATP5Oc probe, immobilized on the aerogel, detected
the human gene proteasome subunit alpha type-5 (PSMA5).
With eight duplicate dots on each slide, Li et al. measured the
PSMA5 target concentration at 0, 40, 120, and 360 nm from the
left to the right slides. The positive (bottom left) was Sp5-cy3
(10 mm), while the negative (bottom right) was a hybridiza-
tion buffer (bottom right). The low background level supports
the lack of nonspecific sequence identification in our aerogel
biochips.[128]
Recently, researchers have focused their attention on aerogel-
based biosensors with electrochemical sensing. An aerogel
mass transport improvement might increase aerogel-based bio-
sensors’ efficacy, as Yang et al. discovered, by allowing target
molecules to reach more active areas inside the aerogels. The
aerogel’s mechanical strength may be increased by employing
aerogels directly.[129]
According to Sani et al. tyrosinase encapsulated silica aerogel
(TESA) can eliminate 80% of the phenol in an aqueous solu-
tion after only 3 h of exposure. TESA’s exceptional reusability
is shown by the fact that after ten uses, phenol removal is
only 60%. Finally, the research shows that encapsulation in
silica aerogel considerably improves the stability of tyrosi-
nase in acidic and basic environments, making TESA an ideal
nanosensor for removing phenol from water.[130]
4.6. Environmental Applications
The use of silica aerogel for the absorption of crude oil from
water has not been extensively researched.[131] To absorb crude
oil from oil and saltwater mixtures, powdered CF3-functional-
ized aerogels were used by Reynold et al. (CH3O)4Si was syn-
thesized using the sol–gel method with CF3(CH2)2Si(OCH3)3
in CH3OH and NH4OH as the catalyzer. To create hydro-
phobic aerogels, supercritical drying was used using CH3OH.
Although the functionalizing agent concentration varied, aero-
gels removed all the oil from water, as illustrated in Figure 22.
As much as 234 times its weight, Reynolds et al. found that the
Figure 21. Nonspecific molecular recognition test on the aerogel biochips. Reproduced with permission.[128] Copyright 2010, Elsevier.
Figure 22. Picture of a) TiO2 aerogel situated in oil, b) a magnet can carry away TiO2 aerogel, and c) the water surface was transported on the oil layer.
Reproduced with permission.[133] Copyright 2014, Elsevier.
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synthetic aerogel was able to absorb oil. Fe3O4 nanoparticles in
the aerogel make it magnetic and easy to remove using an exte-
rior magnetic field.[132]
The magnetic cellulose/TiO2 aerogel’s oil absorption ability
was tested by Chin et al. by dispersing it in an oil/water
combination.
Because of their hydrophobicity and oleophilic (Figure 22a),
magnetic cellulose/TiO2 aerogels have been shown to float over
the oil layer. This is due to the hydrophobic TiO2 coating on
the cellulose aerogel. After the absorption trials, the aerogel
was shown to absorb oil quickly and be readily removed using a
magnet, as shown in Figure 22b. When the aerogel was exposed
to oily waters, it absorbed most of it.[133]
Renjith et al. observed that oil absorption increases with the
sorbate’s viscosity and specific gravity. As opposed to diesel oil,
high-viscosity engine oil was more easily absorbed. Figure 23
shows that the oil-selective MF-SiAG (weighing just 0.02 g)
completely absorbs the oil (0.45 g) and the absorption is fin-
ished in less than 90 s (full absorption capacity of 0.66 g). The
residual water is completely free of any oil residues, as shown
in Figure 23.[134]
Pawer et al. carried out a series of organic solvent and oil
absorption experiments according to Figure 24. This was fol-
lowed by gradual immersion and subsequent stirring at 25 °C
of a 2-by-2-cm piece of polyethylene terephthalate (PET-01)
aerogel until equilibrium was achieved. It was removed,
the oil was wiped away, and the aerogel sample’s weight was
measured.[135]
These intriguing materials are the subject of an in-depth
analysis by Jatoi et al., which examines several environmental
cleanup strategies. As a result, aerogel’s superior applicability
relies on its usage as an excellent adsorbent material to remove
harmful volatile organic chemicals from the atmosphere and
industry. An intriguing adsorbent for water treatment, aerogel
successfully lowers oil spills, other harmful organic solvents,
and heavy metal ions released into water sources from indus-
trial and municipal waste.[136]
4.7. Food Packaging
Packaging materials using silica aerogel reinforcement have
also been suggested for food contact applications. Using
PVA and silicon dioxide (SiO2), Chen et al. created food pack-
aging films with enhanced thermal insulation and oxygen
obstacle properties.[137]
Particles made of silica aerogel have been utilized by Ven-
taka Prasad et al. to enhance the mechanical characteristics
of PLA/sisal composites. They conclude that improved inter-
facial bonding between them the polymeric matrix and sisal
filament boosted the mechanical properties of the final food
packing materials.[138]
Aragón-Gutierrez et al. produced transparent films with a
thickness of between 100 and 200 nm. Thanks to the processing
method employed in producing silica aerogel-reinforced plas-
ticized PLA formulations. Even after adding silica aerogel, the
films remained transparent, in keeping with previous studies.
Composites have been hailed as a viable material for food
packaging because of their high-barrier qualities. Thermal sta-
bility and hardness are advantages of inorganic materials like
silica.[139]
Nešić et al. created food packaging material made of pectin
and pectin/TiO2 nanocomposite aerogels by sol–gel synthesis,
which was then examined by a variety of methods, including
microscopy, textural and thermal analyses, and mechanical and
antibacterial testing.[140]
The pectin aerogel was an excellent
control in terms of thermal stability, mechanical strength, and
antibacterial activity. TiO2 significantly enhances mechanical
Figure 23. Oil absorption by MF-SiAG composite. Reproduced with permission.[134] Copyright 2021, Elsevier.
Figure 24. Performance of PET-01 aerogel as an oil absorbent. Repro-
duced with permission.[135]
Copyright 2021, Elsevier.
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performance, thermal stability, and antibacterial activity under
dark and UV light.[141]
4.8. Textile Applications
The incorporation of silica aerogel particles into a PAN solu-
tion by Bhuiyan et al. led to the effective development of an
electrospun nanofibrous membrane (Figure 25). PAN–silica
aerogel NFs were sandwiched between viscose nonwoven layers
to provide thermally comfortable protective apparel. An aerogel
membrane with well-dispersed, randomly deposited aerogel
particles was investigated morphologically to see whether they
may offer useful protection with their functional qualities. For
liquid chemical penetration prevention, the functionalized
nanofibers embedded in nonwoven textiles could retain more
chemicals.[142]
5. Recent Advancements in Aerogels
The 3D porous silica aerogel monoliths (SAMs) with unique
physical and chemical features offer a new age of technology.
However, this material’s poor mechanical characteristics in
large-scale manufacture are still a major hurdle to overcome.
The mechanical characteristics of SAMs have been improved in
several ways to meet this challenge and investigate their poten-
tial uses in diverse communities.
5.1. Multifunctional Graphene Aerogels
Carbon allotropes such as 2D graphene have attracted great
attention in materials research because of their unique phys-
icochemical characteristics. Self-assembled 2D graphene
sheets must be transformed into 3D graphene aerogels
with unique shapes and functionalities to advance practical
applications. Incorporating polymers, nanoparticles, and
working components into graphene aerogels (GAs) further
enhances their wide range of uses. Gadgets have a big sur-
face area, excellent compressibility, and extensibility, as well
as electrical solid conductivity. Efficacious electrodes for bat-
teries, supercapacitors, and sensors/actuators have all made
use of these.[143]
Progress in the synthesis of 3D GA and 3D GA photocata-
lyst composites was summarized by Long et al. Several tech-
niques have been investigated for their synthesis, including
hydrothermal, chemical vapor deposition, and chemical
oxidation. Known for its unique 3D porous structure, sub-
stantial specific surface area, and excellent adsorption
capacity,[144] GA is a very desirable material. GA’s high con-
nectivity, superior conductivity, and other desirable proper-
ties are made possible by interlaminar stacking and hydrogen
bonding.[145]
To create aerogels based on carbon nanotube (CNT) and
reduced graphene, hydrothermal and freeze-dry methods
were used by Lv et al. resulting in an extremely porous
3D structure with CNTs firmly attached to the graphene
nanosheet. In the 18–26.5 GHz frequency band, aerogels
with exceptionally low densities may obtain increased die-
lectric loss ability. At 22.4 GHz, the minimum reflection
loss value at 1.7 mm approaches 31.0 dB when four weight
percent of standard CNTs@GA aerogel is spread into
the polydimethylsiloxane matrix, and an efficient absorp-
tion bandwidth spans the whole observed frequency range
(18–26.5 GHz).[146]
One possible adsorbent for oily and organic wastewater treat-
ment done by Zhou et al. is a solvent-free NC/Al2O3 aerogel
with a 3D porous network and ultralight density of 5.1 mg cm3.
An aerogel with high porosity (99.09%) and strong adsorption
capability made from nanocellulose and nanoalumina is best
made with a weight ratio of 1:0.25.[147]
Wang et al. found that the directed freeze-drying approach
produced a construction of the cellulose nanofibers (CNFs)
aerogel with high mechanical characteristics, a large specific
surface area, and good hydrophilicity. The CNFs aerogel’s
highest adsorption capacity was 440.60 mg g−1
at a concentra-
tion between 5 and 50 mg L−1
. In keeping with Langmuir, static
adsorption was shown to be a monolayer and homogenous
process.[148]
Figure 25. Thermal protection by nonwoven fabric containing PAN–silica aerogel. Reproduced with permission.[142] Copyright 2020, Springer Nature.
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5.2. Nanocellulose Aerogels
Nanocellulose can be used in wider areas that include energy
storage, adsorption (oils, organic solvents, heavy metals, etc.),
thermal insulation, biomedical applications, and so on.[149–154]
5.3. Polysaccharide Aerogels
Polysaccharides draw attention due to their high porosity,
outstanding properties, and durability, which tend to their
numerous applications. Due to their porous nature, poly-
saccharide aerogel can be applied in drug industries, water
wastage purification, dressing wounds, air filtration, etc. Fab-
rication of porous structures opens a new door to work with
silica aerogels. Normally silica aerogels are brittle materials.
But due to fabrication, it makes the aerogels harder and more
sustainable. More development has been done on cellulose
and starch as it is more sustainable, and durability increases
with the fabrication of aerogel’s porous structure. In aerogel
formation, synthesis plays a vital role. Various recent appli-
cations can be discussed. Among them, the most important
applications are oil absorption, thermal insulation, food
packaging, wound dressing, supercapacitors, oil removal,
air filtration, fire resistance, etc. It becomes possible due to
its porous structure and fabrication of the porous structure
(Figure 26).[155]
5.4. Bio-Based Aerogels
Bio-based aerogels are being researched extensively by Yang
et al. because of their nontoxic, recyclable, and highly absor-
bent qualities, as well as their ability to be used in a variety of
applications. As for aerogel’s future research directions, carbon
aerogel has already been recognized as the most promising.
Additional studies are needed to determine how bio-based aero-
gels fare in harsh settings in order to improve their potential
for oil–water separation.[156]
The adsorption of CO2 on adsorbent materials at low tem-
peratures must be improved in order to address energy and
environmental concerns while also providing chances for eco-
nomic development and social impact found by Verma et al.
Until now, the literature on these topics has been lacking in
Figure 26. Formation of nanocellulose aerogel. Reproduced with permission.[149] Copyright 2021, John Wiley and Sons.
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specificity. Bio-based adsorbents that have been chemically
modified to absorb CO2 at low temperatures were also exam-
ined in this work. There are also promising options for CO2
adsorption using bio-based (polysaccharides) aerogels.[157]
Cao et al. showed a bio-based, low-fire-hazard, and super-
elastic aerogel with no hazardous cross-linkers with good
thermal insulation and oil absorption properties. In the absence
of crosslinkers, the resulting bio-aerogel had unique aniso-
tropic and wave-shaped cellular networks, which resulted in
super elasticity. Hydrophobicity and strong fire retardancy were
supplied by hydrophobic and nonflammable silane coating
layers for hydrophobic chitosan aerogels, opening the way for a
solution to the long-standing difficulties of diverse biopolymer
aerogels’ moisture sensitivity and flammability.[158]
6. Conclusions
Aerogel exhibits remarkable mechanical, optical, thermal,
and acoustic capabilities due to its solid network and nanoscale
pores filled with air. The chosen precursors and the opti-
mized sol–gel parameters determine the final aerogel product’s
physical qualities. Alcogels are dried using freeze, ambient
pressure, or supercritical drying processes, depending on the
commercial use of the aerogel. A significant chemical alteration
creates a new environment for researching aerogel properties.
Ambient pressure drying methods will certainly reduce the cost
of industrial preparation, making aerogels more competitive.
The primary use of silica aerogels is in thermal insulation of
various types. Additionally, SiO2 aerogels offer several physical
and ecological benefits over most other materials on the market
(nontoxic, nonflammable, and simple to dispose of).
Conflict of Interest
The authors declare no conflict of interest.
Keywords
hydrophobicity, silica aerogel, sol–gel, supercritical drying
Received: November 13, 2022
Revised: January 29, 2023
Published online:
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