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Abbreviation (ISO4): Prog Chem      Editor in chief: Jincai ZHAO

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Progress in Chemistry >

2025 , Vol. 37 >Issue 11: 1688 - 1703

DOI: https://doi.org/10.7536/PC20250406

Review

Silica Composite Aerogels

  • Zichun Lin 1, 2 ,
  • Xinyue Wang 1, 2 ,
  • Qing Xu 1, 2 ,
  • Hongjuan Duan , 1, 2, * ,
  • Haijun Zhang 1, 2
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  • 1 Key Laboratory of Advanced Refractories， Wuhan University of Science and Technology， Wuhan 430081， China
  • 2 International Joint Laboratory on New Technologies of Refractories， Wuhan University of Science and Technology， Wuhan 430081， China
*

Received date: 2025-04-07

  Revised date: 2025-06-24

  Online published: 2025-10-25

Supported by

National Natural Science Foundation of China(52272021)

National Natural Science Foundation of China(U23A20559)

National Natural Science Foundation of China(52232002)

Natural Science Foundation of Wuhan(2024040701010051)

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Abstract

Silica composite aerogels， characterized by their extremely low density， high specific surface area， and remarkable porosity， have found extensive applications in high-temperature kilns， the oil and gas sector， aerospace， and various other advanced domains. Firstly， silica aerogels that have been composited through inorganic and organic compositing were thoroughly reviewed in this paper， as well as fiber reinforcement， including a comparative analysis of their thermal conductivity， compressive strength， porosity， density， and other significant physical properties. Secondly， the most recent strategies for additive manufacturing of silica composite aerogels are summarized. Finally， the challenges related to the fabrication and performance of silica composite aerogels and proposed future research directions for their advancement was addressed by this paper.

Contents

1 Introduction

2 Inorganic composite silica aerogel and preparation strategy

2.1 Aluminum oxide composite silica aerogel

2.2 Carbon composite silica aerogel

2.3 Non-oxide composite silica aerogel

3 Organic composite silica aerogel and preparation strategy

3.1 Polymer composite silica aerogel

3.2 Non-polymerzied composite silica aerogel

4 Fiber reinforced silica aerogel and preparation strategy

4.1 Carbonfiber

4.2 Glass fiber

4.3 Other inorganic fibers

4.4 Organic fiber

5 Additive manufacturing strategies for silica composite aerogels

6 Forntier application

6.1 Aerospace

6.2 Energy saving

6.3 Battery thermal management

7 Conculusion and outlook

Key words： silica aerogel; inorganic composite; organic composite; fiber reinforced; additive manufacturing

Cite this article

Zichun Lin , Xinyue Wang , Qing Xu , Hongjuan Duan , Haijun Zhang . Silica Composite Aerogels[J]. Progress in Chemistry, 2025 , 37(11) : 1688 -1703 . DOI: 10.7536/PC20250406

1 Introduction

Silica aerogels, as a type of novel lightweight insulation material, possess characteristics such as low density (0.005–0.5 g/cm³), high porosity (85%–99%), high specific surface area (400–700 m²/g), and low thermal conductivity (0.001–0.1 W/(m·K)), and have thus attracted extensive attention from researchers[1-5]. The conventional preparation process for silica aerogels involves first using the sol–gel method to prepare a silica sol. Through catalysis by acid or base, electrophilic and nucleophilic reactions are induced with the silicon source, respectively, thereby altering the potential and viscosity of the silica sol and promoting self-condensation of the sol. The sol is then allowed to stand in different solvents or environments to ensure the robust growth of its three-dimensional network structure. Finally, the gel is dried, with the solvent in the wet gel being replaced by a gas to yield the aerogel[6-7]. A schematic diagram of the synthesis process and reaction mechanism is shown in Figure 1.
图1 二氧化硅气凝胶的制备流程： （a） 具体制备过程； （b）硅溶胶网络的产生机理

Fig.1 Synthesis process of silica aerogels. （a） Specific synthesis process. （b） Mechanism of silica sol network generation

However, traditional silica aerogels undergo particle sintering at temperatures above 800 ℃, leading to pore collapse[8].Zhou et al.[9] investigated the properties of silica aerogels by holding them at 800 ℃ for 12 hours. The study showed that the density of the silica aerogel increased from 0.2 g/cm3 to 0.5 g/cm3, and the linear shrinkage in the thickness direction reached 25.5% after 12 hours of heat treatment. At this point, the mesoporous structure of the aerogel was destroyed, resulting in reduced thermal insulation performance. Compared to room temperature, the thermal conductivity of pure silica aerogels at 800 ℃ increased by 62%, which falls far short of meeting the temperature requirements (1400 ℃) and stability demands of industrial kiln insulation materials[10].
Silica composite aerogels not only retain the low thermal conductivity of pure silica aerogels, but also enhance their operating temperature and compressive strength through compounding with organic and inorganic materials, enabling their application in areas such as spacecraft thermal insulation walls, high-temperature insulation suits, and industrial kilns[11-13]. To improve the high-temperature and pressure-resistant performance of silica aerogels, numerous researchers have conducted studies on silica aerogel compounding[14], fiber reinforcement, and preparation methods[15]. The main advantages and disadvantages of silica composite aerogels are currently summarized in Table 1.
表1 二氧化硅复合气凝胶的优缺点

Table 1 Advantages and disadvantages of silica aerogel composite

Aerogel Advantage Disadvantage
Inorganic composite High service temperature， structure stability Limit service temperature， high brittleness
Organic composite Complex heat transfer path， high-stiffness network Carbonize， high deformation
Fiber reinforce Shielding effect， distribute stress Poor interface composite， increase heat transfer
3D printing Malleable， expand applications High cost， low efficiency
Based on this, this article reviews silica composite aerogels, compares the compressive strength and thermal insulation performance of inorganic, organic, and fiber-reinforced composite aerogels, and summarizes their physical properties such as porosity and pore volume; reviews additive manufacturing strategies for silica composite aerogels; and finally identifies challenges associated with silica composite aerogels, proposes improvement methods, and outlines future development directions for silica aerogels.

2 Inorganic Composite Silica Aerogel Materials and Their Preparation Strategies

Inorganic materials (such as alumina and graphene) can enhance the mechanical properties and high-temperature resistance of silica aerogels through interfacial interactions and phase transformation inhibition, thereby broadening the application scope of SiO2aerogels[16-17]. Figure 2shows that at high temperatures, Al2O3and SiO2form a mullite phase with excellent high-temperature resistance and mechanical properties, inhibiting sintering between pores, preserving higher porosity, thereby suppressing phonon heat transfer, reducing thermal conductivity, and enhancing compressive strength. A summary of the materials and properties of inorganic-composite SiO2aerogels is provided in Table 2.
图2 无机物复合二氧化硅气凝胶性能增强的原理示意图

Fig.2 Schematic representation of the principle of performance enhancement of inorganic composite silica aerogels

表2 无机物复合二氧化硅气凝胶材料与性能对比a）

Table 2 Comparison of inorganic composite silica aerogel materials and properties a）

Precursor Composite Sources Specific Surface
area
（m2/g）		 Porosity
（%）		 Pore
Value
（cm3/g）		 Density
（g/cm3）		 λ
（W·m-1·K-1）		 Compressive Strength
（MPa）/Strain（%）		 Contact Angel（°） Ref
TEOS Aluminum Oxide Aluminum sol 616 - - - - - - 20
MTMS 3-tert-Butylaluminum - - - 0.22 0.023 - - 21
TEOS Aluminum Oxide Aluminum sol 422 76.4 1.49 0.256 0.116 - - 22
TEOS AlCl3 - - - - 0.024 - - 23
TEOS ASB 232 94.8 4.543 0.207 0.035 0.79/16.4 - 24
TEOS Carbon Graphene 1096 - 2.67 0.105 0.018 - - 30
MTMS Graphene oxide - - - - 0.049 0.08/80.0 - 31
TEOS CNT 492 95.8 12.5 0.061 0.031 - - 32
MTMS CNT 446.33 - 3.84 0.23 0.178 1.36/4.3 - 33
APTES CNT 463.4 95 - 0.613 0.031 35/65.6 - 34
MTMS， DEMS Catechol， Formaldehyde 774 - - 0.118 0.044 2.09/44.2 - 35
TEOS Particles DMF， Ammonia solution 566 - 1.12 2.25 0.04 20.3/15.0 - 39
Octaphenylsilsesquioxane Polyhedral Oligomeric Sesquicone - - - - 0.028 26.4/82.6 158 40

a） λ： Thermal conductivity； TEOS： ethyl orthosilicate； MTMS：methyltrimethoxysilane； TMCS： trimethylsilyl chloride； ASB： aluminum tri-sec-butoxide； DMF： N，N-dimethylformamide CNT： carbon nanotubes； APTES： （3-aminopropyl）triethoxysilane； DEMS： diethoxymethylsilane

2.1 Alumina-silica aerogel composite

Alumina features a high melting point and excellent chemical stability, making it an excellent raw material for preparing high-temperature materials. By introducing metal oxides into the silica framework via metal alcohols, metal sols, or metal salts, or by replacing the original hydroxyl groups, more stable chemical bonds can be formed with Si to suppress the phase transition of pure silica at high temperatures and enhance the thermal stability of silica aerogels[18-19]..
Gao et al.[20]prepared Al2O3-SiO2 composite aerogels by fully hydrolyzing tetraethoxysilane and then introducing alumina sols in varying proportions. The results showed that the aerogel retained an amorphous three-dimensional network structure after calcination at 1000 ℃, with a specific surface area retention rate of 47.1%. The microstructure of the aerogel at room temperature was similar to that after calcination at 1000 ℃ (Fig. 3), and the introduction of alumina sols enhanced the high-temperature stability of the aerogel.
图3 （a） 铝硅比为0.2时的气凝胶SEM图像；（b） 1000 ℃煅烧后的铝硅比为0.2时的气凝胶SEM图像[20]

Fig.3 （a） SEM image of the aerogel at an aluminum/silicon ratio of 0.2； （b） SEM image of the aerogel at an Al-Si ratio of 0.2 after sintering at 1000 ℃[20]

To further validate the stabilizing effect of aluminum sol on the high-temperature thermal insulation performance of silica aerogels, Li et al.[21]used methyl triethoxysilane as a precursor, mixed it with 3-tert-butylaluminum after the addition of nitric acid, and prepared Al2O3-SiO2composite aerogels via the sol-gel method and high-temperature calcination. The study showed that after calcination at 800 ℃, the aerogel had a density of 0.22 g/cm3and a thermal conductivity of 0.0232 W/(m·K). The introduction of Al2O3inhibited sintering of the aerogel and enhanced its thermal stability.
Yang et al.[22]used ethyl orthosilicate as the precursor, alumina sol as the additive, and chitosan as the binder, and prepared Al2O3-SiO2composite aerogels via the sol-gel method combined with ambient pressure drying. The study showed that the aerogel has a density of 0.256 g/cm3, a specific surface area of 422 m2/g, and a pore volume of 1.49 cm3/g. After heat treatment at 500 ℃ for 2 hours, the aerogel still exhibits a thermal conductivity of 0.116 W/(m·K). The introduction of alumina ensures that the aerogel undergoes no significant phase transition upon calcination at 950 ℃, indicating good stability of the aerogel.
To further enhance the service temperature of Al2O3-SiO2composite aerogels, Lv et al.[23]used ethyl orthosilicate and aluminum chloride as precursors to synthesize a wet gel via a hydrothermal method. By employing supercritical drying and thermochemical reconfiguration—where organic matter is carbonized at a constant temperature to support the aerogel structure—they obtained C-Al2O3-SiO2composite aerogels. Studies have shown that this aerogel has a thermal conductivity of 0.024 W/(m·K). The addition of alumina prevents significant sintering of the silica aerogel at 1200°C.
In summary, existing scoring systems have limited predictive capabilities for bleeding events, and their results are inconsistent[25,30,33].Al2O3-SiO2The structure of composite aerogels significantly influences their thermal insulation and compressive strength. He et al.[24]used aluminum sec-butoxide and ethyl orthosilicate as precursors, employed a unidirectional freezing method, and then used a vacuum impregnation technique to obtain a wet gel. The wet gel retained the ice crystals formed during directional freezing. Drying the wet gel in an ambient air environment yielded an Al2O3-SiO2composite aerogel. Studies have shown that this aerogel has a porosity of 94.8%, a specific surface area of 231.5 m2/g, a compressive strength of 1.660 MPa, and a thermal conductivity of 0.0349 W/(m·K). The introduction of organoaluminum alcohol enables the silica aerogel to maintain a stable structure even at 1200 ℃, and its unique honeycomb-like nanoporous structure enhances the aerogel's strength.
Adding metal oxides can, to a certain extent, inhibit grain sintering and phase transformation at high temperatures, and the underlying mechanism can be explained through computational simulations. Yang et al.[25]used molecular dynamics to reveal the melting and sintering mechanisms of silica aerogels at high temperatures at the nanoscale, particle skeleton scale, and bulk material scale, and developed a thermal conductivity model. The study shows that this model can predict the melting process of metal oxides at high temperatures, thereby advancing the development of silica aerogels composite with metal oxides.
Based on the high-temperature resistance of alumina, the sintering of silica composite aerogels under high-temperature conditions is suppressed, which increases the service temperature of the composite aerogels and ensures their low thermal conductivity at high temperatures. However, alumina-composite silica aerogels have relatively low compressive strength, and their mechanical properties still need to be improved.

2.2 Carbon-Silica Composite Aerogel

Graphene, carbon nanotubes, and other nano-carbons possess characteristics of high specific surface area and high stiffness[26-29].When nano-carbons are incorporated into silica aerogels, they can suppress radiative heat transfer, reduce thermal conductivity, and enhance the mechanical properties of the aerogel.
Graphene’s high specific surface area enhances the interfacial interaction between the aerogel and graphene, resulting in strong bonding. Zhu et al.[30]combined tetraethoxysilane with graphene in different ratios to synthesize wet gels, which were then subjected to supercritical carbon dioxide drying to prepare graphene–silica composite aerogels in varying ratios. Studies have shown that when the mass ratio of graphene to silica gel in this composite aerogel is 1:10, its thermal conductivity is 0.0184 W/(m·K), its density is 0.105 g/cm3,and its specific surface area is 1096 m2/g. The unique bandgap properties, conjugated large π-bonds, and layered structure of graphene effectively reduce radiative heat transfer, thereby enhancing the thermal insulation performance of the composite aerogel.
However, the C—C bonds in graphene are stable, and its chemical bonding with silica is relatively weak. To address this issue, Zheng et al.[31]used more reactive graphene oxide, methyltrimethoxysilane, and methyldiethoxysilane as precursors, mixed them to form a wet gel, and then prepared a graphene oxide–silica composite aerogel by ambient pressure drying. Studies have shown that this aerogel has a thermal conductivity as low as 0.049 W/(m·K), a compressive strength five times that of pure silica aerogel, and excellent compressive resistance. The presence of graphene oxide helps form a cross-linked network, which reduces the thermal conductivity while enhancing the compressive strength.
Unlike the two-dimensional structure of graphene, the high aspect ratio of carbon nanotubes provides more reaction sites during aerogel formation, leading to greater physical bonding with the aerogel. Alyne et al.[32]incorporated carbon nanotubes into trimethoxysilane to prepare a wet gel, which, after ambient-pressure drying, yielded a carbon nanotube–silica composite silica aerogel. Studies have shown that this composite aerogel exhibits a three-dimensional porous structure composed of coarsely assembled particles, with a thermal conductivity of 0.031 W/(m·K), a specific surface area of 492.4 m2/g, and a pore volume of 14.7 cm3/g. The carbon nanotubes enable the three-dimensional silica network to grow along their periphery, thereby increasing the number of micropores in the aerogel and enhancing its thermal insulation performance.
The high-temperature stability of nanotubes can provide excellent support for the pore structure of composite aerogels at elevated temperatures. Jiang et al.[33]mixed tetraethoxysilane with an aluminum sol, and added 1,2-epoxypropane and carbon nanotubes (CNTs) to prepare a high-performance CNT-Al2O3-SiO2composite aerogel. The schematic diagram of its supporting mechanism is shown in Figure 4. Studies have shown that this aerogel exhibits a thermal conductivity of 0.178 W/(m·K) at 1000 ℃, a compressive strength of 1.36 MPa, a specific surface area of 446.33 m2/g, a pore volume of 3.84 cm3/g, and a density of 0.23 g/cm3. The incorporation of carbon nanotubes provides robust support for the aerogel’s pore structure, expanding the pore size while reducing thermal conductivity and enhancing compressive strength.
图4 碳纳米管支撑气凝胶多孔结构的示意图[33]

Fig.4 Schematic representation of carbon nanotube-supported aerogel porous structure[33]

Similarly, Alyne et al.[34]used methyltrimethoxysilane as a precursor and employed the sol-gel method, followed by ambient pressure drying, to prepare carbon nanotube–silica composite aerogels. The study showed that the aerogel has a porosity of 95.8%, a density of 0.613 g/m3,a specific surface area of 463.4 m2/g, and a thermal conductivity of 0.0312 W/(m·K). The presence of carbon nanotubes provides excellent support for the pore structure of the aerogel.
At high temperatures, silicon dioxide can react with carbon to form silicon carbide, thereby reinforcing the aerogel’s network structure. An et al.[35]used 3-aminopropyltriethoxysilane as the silicon precursor and formaldehyde and catechol as carbon sources to prepare C-SiC-SiO2composite aerogels via sol-gel supercritical drying and high-temperature calcination. Studies have shown that this aerogel has a density of 0.118 g/cm3, a thermal conductivity of 0.044 W/(m·K), a specific surface area of 774 m2/g, and a compressive strength of 2.09 MPa. At high temperatures, carbon combines with silicon and transforms into silicon carbide whiskers, strengthening the bonds between particles and forming a porous network structure.
When the content of graphene or carbon nanotubes is low, heat is difficult to transfer across graphite particles, which increases the interfacial thermal resistance of graphene within the silica aerogel framework and also hinders the thermal conductivity of the silica framework. When graphene or carbon nanotubes are cross-linked with silica, their intrinsic high strength can impede the movement of the aerogel framework under stress. This enables the aerogel to exhibit excellent thermal insulation performance and compressive strength. However, the production scale of graphene and carbon nanotubes is relatively small, and their preparation costs are high.

2.3 Non-oxide composite silica aerogel

Boron nitride particles and similar particulates exhibit excellent mechanical properties and structural stability[36-38].Dispersing these particles within a silica aerogel can effectively impede dislocation movement, thereby preventing the collapse of the porous structure.
Yang et al.[39]used tetraethoxysilane as the precursor, and diazine-dimethylformamide and ammonia solution as the solvent. After forming a wet gel through mixing, the material was sintered in a nitrogen atmosphere at 900 ℃ for 2 hours to obtain a Si3N4-SiO2 composite aerogel. The study showed that boron nitride particles are surrounded by aerogel nanoparticles. The specific surface area is 566 m2/g, the pore volume is 1.12 cm3/g, the density is 2.25 g/cm3, and the compressive strength is 20.3 MPa. The boron nitride particles enable the aerogel to maintain a good structure even at 1300 ℃.
However, this method requires high-temperature nitriding treatment, which is relatively costly. Therefore, inorganic polymers can be introduced to incorporate particles that enhance the aerogel. Long et al.[40]used octa(aminophenyl)-octamethylsilsesquioxane as a precursor to obtain a wet gel via sol-gel and ethanol aging, and then prepared particle-reinforced silica composite aerogels with polyhedral oligomeric silsesquioxane using supercritical carbon dioxide drying. Studies have shown that this aerogel has a compressive strength of 1.45 MPa, a hydrophobic contact angle of 158°, and exhibits superhydrophobicity. At the same time, under ambient temperature and pressure conditions, the thermal conductivity of this aerogel is 0.0285 W/(m·K). The alternating structure of rigid cores and flexible chains gives the aerogel a more stable structure; a schematic diagram is shown in Figure 5.
图5 硬核与柔性链交替连接的示意图[40]

Fig. 5 Schematic diagram of alternating hard core and flexible chain connections[40]

The introduction of particles enhances the thermal insulation and compressive strength of silica composite aerogels through interfacial interactions. However, how particles can be uniformly dispersed within the silica matrix and the underlying mechanisms by which particles influence the network structure remain to be investigated.

3 Organic-Inorganic Composite Silica Aerogel Materials and Their Preparation Strategies

Traditionally, the network of silica is connected via Si—O bonds. However, compared to the bonds between organic molecules, these bonds have lower bond energy and are prone to dissociation and chain scission at high temperatures. This is one of the reasons why conventional silica aerogels tend to fail under high-temperature conditions. The incorporation of organic components can increase the bond energy within the SiO2aerogel network. By extending the chains, the bonding mode of the original silica aerogel network can be modified: the short-chain network, whose primary bonding is Si—O—Si, is transformed into a long-chain network with Si—O—R (where R represents a long-chain group). This enhances chain rigidity, reduces vibration, complicates the network structure of the aerogel, and hinders heat conduction. At the same time, it improves the mechanical properties of the composite aerogel; a schematic diagram is shown in Figure 6. A summary of the materials and properties of organic-composite SiO2aerogels is presented in Table 3.
图6 有机复合二氧化硅气凝胶的增强机理示意图

Fig.6 Schematic enhancement mechanism of organic composite silica aerogel

表3 有机物复合二氧化硅气凝胶材料与性能对比a）

Table 3 Comparison of organic composite silica aerogel materials and properties a）

Precursor Composite sources Specific
surface area
（m2/g）		 Porosity
（%）		 Pore
value
（cm3/g）		 Density
（g/cm3）		 λ
（W·m-1·K-1）		 Compressive
Strength
（MPa）/
Strain（%）		 Contact
angel （°）		 Ref
Silica aerogel powders Polymers 4，4'-Oxybisbenzenamine and 3，3'，4，4'-
Biphenyltetracarboxylic dianhydride		 609 - - - 0.018 - >150 41
MPTES PSA - 80 - 0.3 0.06 >1.50/0.5 - 42
Silica aerogel powders PVA - - - 0.13 0.019 - 150 43
TMOS PEG 303 - 0.19 0.093 0.023 - 136 44
Sodium Silicate Methyl methacrylate - - - - 0.019 - - 45
Olivine Polymers cellulose - - - - 0.027 - 135 46
MTMS cellulose 958 98 0.72 0.055 0.023 0.09/80 140 47
Liquid Water Glass Cellulose 630 - - - 0.021 58.00/20 - 48
Liquid Water Glass Bateria cellulose 729 - 2.7 - - - 148 49
TEOS PP - - - - - 2.02/70 145 50
TEOS， MTMS Non-polymers sorbital 1193 92.3 2.28 0.15 0.041 0.21/10 151 55
（3-glycidyloxypropyl）trimethoxysilane and （3-mercaptopropyl）trimethoxysilane， TEOS Epoxy-thiol 519 93.42 - 0.225 0.047 11.00/60 - 56
Liquid Water Glass DMAC 843 - 3.36 - - - - 57

a） MPTES： 3-mercaptopropyl triethoxysilane； PSA： polyarylacetylene； PVA： polyvinyl alcohol； PEG： polyethylene glycol； PP： polypropylene； BHNC： bifunctional hairy nanocellulose； APTES： （3-aminopropyl）triethoxysilane； TMOS： Tetramethyl orthosilicate； DMAC： N，N-dimethylacetamide

3.1 Polymer-composite silica aerogel

Based on the high thermal stability of polyimide, Kantor et al.[41]used silica aerogel powder as a precursor and 4,4′-diaminodiphenyl ether and biphenyltetracarboxylic dianhydride as additives to synthesize a wet gel. The polyimide–silica composite aerogel was then prepared via supercritical drying. Studies have shown that this aerogel has a specific surface area of 609 m2/g, a thermal conductivity of 0.0175 W/(m·K), a water contact angle greater than 150°, and exhibits superhydrophobicity on its surface. The aerogel features a fibrous network structure with fiber diameters ranging from 20 to 50 nm, which provides excellent thermal insulation performance for the aerogel.
However, polyimide has relatively low stiffness, resulting in a limited enhancement effect on the compressive strength of aerogels. Therefore, Zhao et al.[42]reacted mercaptopropyl triethoxysilane with silicon-containing polyarylacetylene at 105 ℃ under anhydrous and anaerobic conditions to synthesize a wet gel, which was then aged and freeze-dried to produce a silica composite aerogel. Studies have shown that this composite aerogel has a density of 0.30 g/cm3,a porosity of 80%, a thermal conductivity of 0.060 W/(m·K), and a compressive strength greater than 1.5 MPa. It features a honeycomb-like, porous, ordered three-dimensional network structure, which enhances its compressive strength and reduces its thermal conductivity.
Polymers containing hydroxyl groups can form a hydrogen-bonding network with silica, resulting in strong adhesion. Lee et al.[43]used silica aerogel powder as a raw material, added polyvinyl alcohol, and prepared a wet gel via impregnation. The polyvinyl alcohol–silica composite aerogel was then obtained through ambient-pressure drying. Studies have shown that this aerogel has a thermal conductivity of 0.0189 W/(m·K), a hydrophobic contact angle of 150°, and a density of 0.13 g/cm3.
Similarly, Arshad et al.[44]used tetramethyl orthosilicate and diisocyanate as precursors, mixed them with polyethylene glycol to synthesize a wet gel, and then obtained polyethylene glycol–silica composite aerogels through freeze-drying and heat treatment in a nitrogen atmosphere. Studies have shown that the aerogel has a specific surface area of 230 m2/g, a thermal conductivity of 0.023 W/(m·K), and a hydrophobic contact angle of 136°. Polyethylene glycol crosslinks the randomly dispersed silica particles, making the structure more stable and enhancing its thermal insulation performance.
The high thermal stability of polyester can provide excellent thermal insulation performance for aerogels. At the same time, the high functional group reactivity of polyester enables it to form strong bonds with the silica network. Ma et al.[45]immersed silica gel in a polymethyl methacrylate solution at 60 ℃ and used a thermally induced phase separation method to prepare a polymethyl methacrylate–silica composite aerogel. The study showed that, compared with silica aerogels without added polymethyl methacrylate, the compressive strength increased by 1400%, while the density increased by only 28%. The thermal conductivity at room temperature was 0.0192 W/(m·K). The thermally induced phase separation method effectively preserves the original mesoporous structure of the aerogel.
Cellulose's natural β-1,4-glycosidic bond network structure can effectively integrate with the silica network. Sarkar et al.[46]used sodium silicate as a precursor, mixed it with cellulose, and then placed the mixture in an oven for wet gel preparation and drying/aging. Using this method, a cellulose–silica composite aerogel was synthesized. Studies have shown that this aerogel has a thermal conductivity of 0.0271 W/(m·K), a hydrophobic contact angle of 135.4°, and exhibits surface hydrophobicity.
Chen et al.[47]used olivine and cellulose as raw materials to prepare cellulose–silica composite aerogels via a one-pot method. The study showed that the aerogel has a density of 0.055 g/cm3,a compressive strength of 94.5 kPa, a specific surface area of 958 m2/g, a thermal conductivity of 0.023 W/(m·K), and a porosity as high as 98%. After hydrophobic modification with trimethylchlorosilane, the hydrophobic contact angle of the aerogel reached 140°, exhibiting excellent hydrophobic properties.
Peng et al.[48]By adding cellulose to sodium silicate and conducting an in-situ reaction followed by ambient-pressure drying at room temperature, a cellulose–silica composite aerogel was synthesized. Studies have shown that this aerogel has a compressive strength of 58 MPa, a specific surface area of 630 m2/g, and a thermal conductivity of 0.021 W/(m·K). The aerogel prepared using this method exhibits a cross-linked layered structure, providing excellent mechanical properties that enable it to support a 500 g weight and lift 1.5 kg of water without deformation; see Figure 7.
图7 纤维素复合二氧化硅气凝胶的力学强度图像[48]

Fig.7 Mechanical strength images of cellulose-composite silica aerogels[48]

Bio-cellulose can also be used to fabricate composite silica aerogels. Shi et al.[49]used industrial water glass as a precursor, exchanged it with an acidic ion-exchange resin, and mixed the resulting product with bacterial cellulose to obtain bacterial cellulose-reinforced wet gels. After aging, titanium tetrachloride and tungstate solutions were added, and a solvothermal crystallization method was employed to prepare SiO2composite aerogels. Studies have shown that the composite aerogel has a specific surface area of 729 m2/g, a pore volume of 2.70 cm3/g, and a hydrophobic contact angle of 148°. Under heating conditions, bacterial cellulose decomposes into short-chain glucose, thereby acting as a pore-expanding agent in the aerogel. While maintaining a stable three-dimensional network structure, the aerogel exhibits larger pores.
Organic materials with unique frameworks can serve as benign carriers for silica aerogels, enabling them to form strong bonds with the silica aerogel matrix. Choi et al.[50]prepared polypropylene scaffolds using a thermally induced phase separation method and synthesized polypropylene–silica composite aerogels via the sol–gel process using methyltrimethoxysilane as a precursor, followed by supercritical drying. The study showed that the aerogel has a hydrophobic contact angle of 145° and a compressive strength as high as 2.02 MPa. The polypropylene scaffold enhances the compressive performance of the silica composite aerogel.
After the addition of polymers, silica particles can adsorb onto the surface of the long-chain polymers, forming a more robust and extensive network that enhances the mechanical properties of the aerogel[51-53].At the same time, the extensive polymer network can interpenetrate the network formed by silica particles, further enhancing the material’s strength and toughness. However, its performance at high temperatures still needs to be improved, and the incorporation of polymers leads to a certain degree of porosity reduction, which requires further refinement.

3.2 Non-polymer composite silica aerogel

Unlike polymers, non-polymers more frequently undergo grafting to modify the original hydroxyl groups, providing more reactive sites for the formation of long-chain networks and thereby making the aerogel’s network structure more stable[54]..
Based on sorbitol's polyhydroxy characteristics, it can exert a synergistic effect with silanes to form a well-structured network, thereby enhancing the performance of aerogels. Meti et al.[55]used a mixture of sorbitol and trimethylsilylmethane as a precursor, which was added to methanol for wet gel preparation. After aging, supercritical drying was employed to obtain a sorbitol–silica composite aerogel. Studies have shown that this aerogel has a specific surface area of 1193 m2/g, a compressive strength of 205.9 kPa, a pore volume of 2.2 cm3/g, a thermal conductivity of 0.041 W/(m·K), and thermal stability at 418°C. Structurally, the aerogel exhibits a unique cage-like network (see Figure 8), which significantly enhances the aerogel's stability.
图8 山梨醇复合二氧化硅气凝胶的SEM图像[55]

Fig.8 SEM image of sorbitol-silica composite aerogel[55]

Similarly, Dhavale et al.[56]used (3-glycidoxypropyl)trimethoxysilane and (3-mercaptopropyl)trimethoxysilane as co-precursors to synthesize wet gels via acid-base catalysis, and obtained epoxy-thiol composite silica aerogels after supercritical drying. Studies have shown that the aerogel has a thermal conductivity of 0.047 W/(m·K), a density of 0.225 g/cm3,a porosity of 93.42%, a specific surface area of 519 m2/g, and a compressive strength of 11 MPa.
Some organic compounds can act as pore-expanding agents, reducing solid-phase heat transfer and lowering thermal conductivity. Ma et al.[57]used industrial water glass as a precursor and obtained silica sol using styrene cation-exchange resin, which was then mixed with KBr, KI, and AgNO3solutions to prepare a wet gel. Using N, N-dimethylacetamide as a pore-expanding agent and combining it with supercritical drying, a silica composite aerogel was prepared. Studies have shown that this aerogel has a specific surface area of 843 m2/g and a pore volume of 3.36 cm3/g, whereas the aerogel without a pore-expanding agent has a specific surface area of only 446 m2/g and a pore volume of just 1.15 cm3/g. The prepared aerogel exhibits a uniform network structure, and the presence of N, N-dimethylacetamide slows down the gelation process, allowing for the formation of larger pores.
The introduction of non-polymers can provide more reaction sites for the silica network, enabling the formation of a more robust network. However, compared with polymers, the fixed cross-linking method may offer only limited enhancement in interfacial bonding strength in silica aerogels, thereby affecting their overall mechanical performance. Non-polymer-composite silica aerogels still have significant potential for further development.
Although organic composites can significantly enhance the mechanical properties of silica aerogels, the organic components soften at temperatures as low as 600°C, causing the composite aerogel to lose its original performance. Therefore, when using organic-composite silica aerogels, high temperatures should be avoided as much as possible.

4 Fiber-Reinforced Silica Aerogel Materials and Their Preparation Strategies

Adding one-dimensional materials such as fibers or whiskers to the gel system enables them to be uniformly distributed within the porous structure, thereby endowing the composite material with excellent performance. These fibers typically have small diameters and large specific surface areas, and can exist within silica aerogels, creating a shielding effect that scatters and absorbs infrared radiation. Moreover, the presence of the aerogel reduces contact between pure fiber materials, thereby minimizing solid-state heat conduction and lowering the thermal conductivity of the composite material. At the same time, external loads can be effectively dispersed within the fibers, which possess high stiffness and high modulus, thus enhancing the mechanical properties of the composite[58].A schematic diagram illustrating the enhanced thermal insulation and mechanical properties is shown in Figure 9.The materials and properties of fiber-reinforced SiO2 aerogels are summarized in Table 4.
图9 纤维复合二氧化硅气凝胶的增强机理示意图

Fig.9 Schematic enhancement mechanism of fiber composite silica aerogel

表4 纤维增强的二氧化硅气凝胶的性能对比a）

Table 4 Comparison of properties of silica aerogels with fiber reinforced a）

Precursor Composite sources Specific surface area
（m2/g）		 Porosity
（%）		 Pore value （cm3/g） Density （g/cm3） λ
（W·m-1·K-1）		 Compressive
strength
（MPa）/Strain（%）		 Contact angel （°） Ref
TEOS Carbon Fiber T700 polyacrylonitrile fiber - - - - 0.112 17.01/40 - 60
APTES CF （hydroxyl modification） 463.4 95 - 0.613 0.031 10.00/80 - 61
TEOS， TMOS Glass fiber 604 86 7.69 0.281 0.026
（664 ℃）		 0.75/55 150 64
Liquid Water Glass - - - 0.131 0.036 - 135 65
TEOS - 95 - 0.104 - - - 66
Slica Aerogel
Particles		 - - - - 0.022
（650 ℃）		 - - 67
TEOS Other Fiber Xonolite Fiber - - 4.13 0.126 0.028 - - 68
MTES， TEOS TiO2 nanotubes - - - - 0.118
（1000 ℃）		 - - 69
TEOS Silica Nanowires - - - 0.11 0.039 1.38/60 >140 70
TEOS ZrO2-SiO2 fiber 207.8 99.95 - - 0.029 - - 71
TEOS Organic
Fiber		 Aramid pulp 764 - 2.73 0.19 0.0261 - 145 72
TEOS， VTMS Aramid fibers 973.3 - - - 0.022 0.35/60 - 73
TEOS Kevlar pulp 619 84.5 - 0.208 0.026 0.55/25 156 74
Liquid Water Glass PP fibers 644 87 - - - - 120 75

a） CF： carvon fiber MTES： methyltriethoxysilane； VTMS： vinyltrimethoxysilane； PP： polypropylene

4.1 Carbon fiber

Carbon fibers possess characteristics such as light weight, high specific strength, and high specific stiffness. When introduced into silica aerogels, they can provide support for the porous three-dimensional network structure[59].
Based on the high thermal insulation performance of carbon fibers, Cheng et al.[60]used 3-aminopropyl triethoxysilane and dissolved isopropanol and hexamethylenetetramine in ethylene glycol to prepare a common precursor solution, which was then transferred into T700 carbon-quartz fiber fabric. A wet gel was obtained via vacuum impregnation, and subsequent drying at room temperature and ambient pressure yielded a carbon-quartz fiber-reinforced SiO2composite aerogel. Studies have shown that this material has a thermal conductivity of 0.112 W/(m·K) and a compressive strength of 17.01 MPa. During ablation with an oxyacetylene flame at 2000 ℃, the composite aerogel still retained a relatively intact morphology, and the internal temperature was significantly reduced; see Figure 10.Carbon fibers are uniformly distributed within the silica gel, forming a well-interconnected network that provides structural support for the overall material, thereby endowing the composite aerogel with excellent performance.
图10 （a） 2000 ℃氧乙炔火焰烧蚀前后的烧蚀表面； （b） 烧蚀过程中气凝胶内部距烧蚀表面不同距离的温度[60]

Fig.10 （a） Ablated surfaces before and after ablation with an oxyacetylene flame at 2000 ℃； （b） temperatures inside the aerogel at different dista

However, the interfacial bonding between carbon fibers and silica is relatively weak. Therefore, Liu et al.[61]used ethyl orthosilicate as a precursor, employed the sol-gel method with hydroxyl-modified carbon fibers, and prepared carbon fiber-reinforced SiO2composite aerogels after ambient-pressure drying. Studies have shown that this aerogel has an average pore size of approximately 1.8 nm, a compressive strength of 10.0 MPa, a thermal conductivity of 0.0375 W/(m·K), and a water contact angle of 154.5°, exhibiting superhydrophobic properties.
Due to the intrinsic strength of carbon fiber, using it as a support material results in excellent pressure resistance. However, the production process for carbon fiber is relatively complex, and its performance can easily degrade under extremely high-temperature oxidation (1500 ℃). Moreover, the production cost of carbon fiber is relatively high.

4.2 Glass fiber

Glass fibers provide excellent structural support, effectively enhancing the strength of polymers, and their manufacturing processes are well-established with relatively low production costs[62-63].
Shafi et al.[64]Using tetraethoxysilane and trimethylsilane as co-precursors, a wet gel was synthesized via the sol-gel method, and a glass fiber-reinforced silica aerogel was prepared after supercritical drying. The study showed that the aerogel has a hydrophobic contact angle of 150°, exhibiting superhydrophobic properties, and a thermal conductivity of 0.0258 W/(m·K) at 664 ℃. The presence of glass fibers imparts a mesoporous three-dimensional network structure to the silica aerogel and reduces the exposure of hydrophilic groups, thereby enhancing its hydrophobicity.
Similarly, Duan et al.[65]synthesized a wet gel by dispersing silica particles in an aqueous polyvinyl alcohol solution containing glass fibers, and prepared a glass fiber-reinforced silica composite aerogel via freeze-drying. Studies have shown that this aerogel has a thermal conductivity of 0.0356 W/(m·K), a density of 0.131 g/cm3,a hydrophobic contact angle of 135°, and exhibits excellent hydrophobicity. Silica and glass fibers were successfully cross-linked to form an interwoven multi-level pore structure, with glass fibers providing support and enabling the material to withstand pressure equivalent to 75,000 times its own weight; however, the bonding between glass fibers and the silica matrix relies solely on physical interactions.
To address this issue, Chandradass et al.[66]used water glass as the silicon source and modified glass wool with an alumina sol. After mixing, they synthesized a wet gel and then prepared silica aerogels via ambient-pressure drying, investigating the properties of silica aerogels prepared at different silane concentrations. The study showed that glass wool modified with alumina sol exhibits good interfacial adhesion. When the tetraethoxysilane concentration is 25%, the resulting aerogel has the lowest density (0.104 g/cm3)and the highest porosity (95%). Glass fibers with a diameter of 2.5 μm are interconnected with silicon dioxide clusters, which strengthens the porous structure; however, long glass fibers tend to concentrate stress and are prone to agglomeration.
To address this issue, Jiang et al.[67]used tetraethoxysilane as the precursor, incorporated microglass fibers for improvement, and synthesized wet gels via acid-base catalysis, followed by ambient-pressure drying. They successfully prepared microglass-fiber-reinforced silica aerogels, with the preparation process shown in Figure 11.Studies have shown that the thermal conductivity at 650 ℃ is 0.022 W/(m·K). The microglass fibers are uniformly distributed within the pores, supporting the structure while imparting excellent properties to the gel.
图11 常温干燥的微玻璃纤维复合二氧化硅气凝胶纳米复合材料的制备示意图[67]

Fig.11 Schematic overview of synthesizing ambient-dried microglass fibers-silica aerogel nanocomposites[67]

The preparation process for glass fibers is well-established, and they exhibit excellent mechanical properties, effectively enhancing the mechanical performance of silica aerogels while reducing thermal conductivity. However, compared to carbon fibers, glass fibers have slightly inferior compressive strength, and the interfacial bond strength between glass fibers and silica aerogels may be insufficient, potentially leading to structural failure risks during long-term use.

4.3 Other inorganic fibers

Compared with fibers such as carbon fiber and glass fiber, other nanomaterials used as supports can also exhibit superior performance.
The high-temperature resistance of fibrous wollastonite provides aerogels with excellent thermal stability. Li et al.[68]prepared fibrous wollastonite via a hydrothermal method. By adding fibrous wollastonite to an alkaline silica source and employing ambient-pressure drying, a silica composite aerogel reinforced with fibrous wollastonite was obtained. Studies have shown that this aerogel has a density of 0.126 g/cm3,a pore volume of 4.132 cm3/g, and a thermal conductivity of 0.0285 W/(m·K). The fibers are physically embedded within the pores of the silica, providing structural support and endowing the aerogel with superior thermal insulation performance.
In addition, titanium dioxide nanorods exhibit high rigidity and can also support the aerogel’s network structure, maintaining its skeletal stability at high temperatures. Ji et al.[69]used mullite nanosheets as structural building blocks, tetraethyl orthosilicate as the precursor, and added titanium dioxide nanorods to their surface. After mixing, the mixture was cast into a wet gel, which was then prepared into a mullite nanosheet–titanium dioxide nanorod–reinforced silica composite aerogel via freeze-drying followed by calcination at 1000 ℃. Studies have shown that this aerogel has a thermal conductivity of 0.0356 W/(m·K) at room temperature. After calcination at 1500 ℃ for 30 minutes, the shrinkage rate is only 2%. The interwoven distribution of mullite nanosheets and titanium dioxide nanorods creates small pores in the aerogel’s original porous structure, which helps enhance its thermal insulation performance.
Although the high stiffness of fibrous fibrous silica and titanium dioxide nanorods can effectively support the pore structure of aerogels at high temperatures, stress concentration may occur at the interfaces, making microcracks more likely. In contrast, the high aspect ratio of silica nanowires can provide excellent mechanical properties for aerogels and exhibit better compatibility with the matrix, without creating additional thermal bridging effects. Wang et al.[70]introduced silica nanowires into tetraethoxysilane and trimethylsilane precursors, synthesized wet gels via the sol-gel method, and then prepared silica nanowire-reinforced silica composite aerogels through ambient pressure drying. Studies have shown that when the silica nanowire content is 11%, the compressive strength is 1.379 MPa, the density is 0.11 g/cm3,and the thermal conductivity at room temperature is 0.039 W/(m·K). At the same time, its microstructure remains unchanged after calcination at 1000 ℃. The silica nanowires are uniformly distributed within the network formed by the spherical packing of silica particles, reducing the thermal conductivity, while the strong interfacial bonding also enhances the compressive strength of the aerogel.
Similarly, Xing et al.[71]used tetraethoxysilane and ZrO2, SiO2electrospun fibers and tetraethoxysilane as precursors to synthesize wet gels via chemical swelling and the sol-gel method. After high-temperature drying at 800 ℃, a silica composite aerogel reinforced by electrospun nanofiber supports was prepared. Studies have shown that its thermal conductivity is 0.029 W/(m·K), the pore volume is 0.24 cm3/g, the specific surface area is 207.86 m2/g, and the porosity reaches as high as 99.55%. With the support of the electrospun fibers, the aerogel can still maintain a stable porous structure at 1400 ℃.
Compared with glass and carbon fibers, these inorganic fiber-composite aerogels also exhibit good mechanical properties and low thermal conductivity. However, the fabrication process for these inorganic fibers is relatively complex and costly, and their compatibility with the matrix still requires further investigation.

4.4 Organic fiber

Compared with inorganic fibers such as carbon and glass, organic fibers can provide good support for aerogels while reducing production costs.
The high modulus of aramid fibers provides aerogels with excellent compressive strength. Li et al.[72]used tetraethoxysilane as a precursor and prepared aramid fiber-reinforced silica aerogel composites via the sol-gel method. The composite has a density of 0.19 g/cm3,a hydrophobic contact angle of 138.5°, and a thermal conductivity of 0.026 W/(m·K).
The distribution morphology of aramid fibers directly affects the mechanical properties of aerogels. Li et al.[73]used the sol-gel method, with tetraethoxysilane as the precursor, to reinforce silica aerogels using aramid fibers. Studies have shown that aramid fibers are distributed within the pores as a support, giving the material good toughness, with a compressive strength of 0.35 MPa and a thermal conductivity of 0.0227 W/(m·K).
Similarly, Ghica et al.[74]used the sol-gel method, employing tetraethoxysilane and vinyltrimethoxysilane as precursors. After adding Kevlar pulp to obtain a wet gel, followed by aging and ambient-pressure drying, a Kevlar fiber-reinforced silica composite aerogel was produced. Studies have shown that this aerogel maintains structural stability at 550 ℃, with a thermal conductivity of 0.026 W/(m·K). The aerogel exhibits an interconnected three-dimensional network, with Kevlar fibers embedded within the silica matrix, forming a strong bond with the silica base and thereby enhancing the silica aerogel.
Compared with the high stiffness of aramid fibers, the low modulus of polypropylene fibers can form a looser network structure within the aerogel, disrupting the heat flow transmission path and thereby reducing thermal conductivity. Jadhav et al.[75]used low-cost industrial-grade water glass as a precursor and prepared polypropylene fiber-reinforced silica aerogels using the sol-gel method. Studies have shown that after heat treatment at 270 ℃, the aerogel exhibits a contact angle of 120°, a porosity of 87%, and a specific surface area of 644 m2/g. Fibers with a diameter of 10 μm are randomly distributed within the pores of the aerogel, providing effective structural support.
Organic fibers can effectively reduce costs and enhance the structural stability of silica aerogels, but compared to inorganic fibers, their high-temperature resistance remains relatively poor, which limits their application in high-temperature environments.

5 Additive Manufacturing Strategies for Silica Composite Aerogels

Although the sol-gel method can produce high-performance silica composite aerogels, it suffers from long preparation cycles and difficulties in fabricating complex-shaped aerogels, which severely limit its applications. As one of the additive manufacturing strategies, 3D printing can leverage the solidification of raw materials to shorten the preparation time of silica composite aerogels and, through digital design, construct aerogels of various shapes, thereby meeting diverse high-temperature insulation and energy-saving needs and demonstrating promising application prospects[76-78]..
Wang et al.[79]Using methyl trimethoxy silane and silica sol as precursors, a 3D printing process was conducted at 100 ℃, followed by high-temperature curing of the slurry and supercritical drying to prepare silica aerogels. Studies have shown that the resulting aerogel has a density of 0.25 g/cm3,a specific surface area of 272 m2/g, and a thermal conductivity of 0.032 W/(m·K).
In the application of thermal insulation and heat preservation, Wang et al.[80]used polyamic acid and silica aerogel powder as precursors, employed 3D printing technology, and leveraged the polymerization-induced solidification effect to prepare polyimide–silica composite aerogels after supercritical drying. Studies have shown that this aerogel has a specific surface area of 518 m2/g and a linear shrinkage rate of less than 5% at 400 ℃. The 3D-printed aerogel, while maintaining excellent thermal insulation properties, can adapt to irregular heat sources and sustain good thermal insulation performance (Figure 12).
图12 （a） 在不规则形状的加热源上安装3D打印气凝胶； （b） 气凝胶在加热源上的冷面和热面的温度随时间的变化情况[80]

Fig.12 （a） 3D printed aerogel on an irregularly shaped heating source； （b） temperature of the cold and hot sides of an aerogel on a heating source as a function of time[80]

Photocuring technology offers a new approach to the preparation of 3D-printed silica composite aerogels. Wang et al.[81]used silica aerogel particles as precursors and employed UV-assisted 3D printing combined with supercritical drying to prepare photosensitive resin–silica composite aerogels; a schematic is shown in Figure 13.Studies have shown that this aerogel has a density of 0.256 g/cm3,a specific surface area of 269.3 m2/g,a pore volume of 1.82 cm3/g,a compressive strength of 0.28 MPa, and a thermal conductivity of 0.038 W/(m·K). This method achieves rapid gel curing while preserving the excellent properties of the aerogel.
图13 紫外辅助3D打印的复合气凝胶制备示意图[81]

Fig.13 Schematic illustration of UV-assisted extrusion printing of aerogel composites[81]

The method of preparing silica composite aerogels using 3D printing technology effectively addresses issues inherent in the sol-gel method, such as long preparation cycles, susceptibility to cracking during drying, and difficulties in subsequent shape processing and molding. However, for 3D-printed silica composite aerogels, the cost remains higher compared to traditional insulation materials.

6 Cutting-edge applications

After compounding, the aerogel’s thermal insulation and mechanical properties are enhanced, its physical structure is optimized, and its applicability is expanded, giving it promising application prospects. The main cutting-edge applications currently include the following.

6.1 Aerospace

Thermal insulation is currently the most representative application area for silica-based composite aerogels. After improvements, these composite aerogels can be used in extreme-temperature environments such as aerospace. Shu et al.[82]used ethyl orthosilicate and aluminum chloride as precursors to prepare Al2O3-SiO2composite aerogels via the sol-gel method combined with supercritical drying. The study showed that at 1000 ℃, the cold side of the aerogel remained at only 98.6 ℃, and the aerogel exhibited no dimensional change after annealing at 1200 ℃. Huang et al.[83]used ZrO2nanofibers and silica sol as precursors to prepare ZrO2-reinforced silica composite aerogels via a vacuum impregnation method. The study demonstrated that this aerogel achieved a low thermal conductivity of 0.023 W/(m·K) at 1100 ℃ and could withstand being run over by a car without deforming.
The high-temperature volume stability and low thermal conductivity of silica composite aerogels make them suitable for use in aerospace and high-temperature applications.

6.2 Energy conservation

Aerogels with low thermal conductivity can serve as insulation materials for buildings at room temperature, thereby reducing the temperature difference between the interior and exterior of a building and achieving excellent energy-saving effects.
Zhao et al.[84]Using ethyl orthosilicate and glass fibers as precursors, a silica aerogel composite with glass fibers was prepared via the sol-gel process combined with supercritical drying. Numerical simulations indicate that the silica-composite aerogel can provide better indoor thermal insulation in cold regions; compared to the traditional insulation material polystyrene, it can increase indoor temperature by 11.8 ℃, successfully predicting the energy-saving applications of silica-composite aerogels. Sun et al.[85]Using methyl orthosilicate, TiO2, and silver nanorods as precursors, a composite silica aerogel was formed through the sol-gel method combined with supercritical drying, achieving a low thermal conductivity of 0.041 W/(m·K) at 127 ℃. Simulations and experiments have verified the energy-saving performance of the composite aerogel in a 50 m2 lunar base, showing that, compared to glass, the composite aerogel can reduce energy consumption by 87.3%.
Therefore, silica composite aerogels have promising application prospects in the field of energy conservation and can replace traditional materials.

6.3 Battery Thermal Management

Another important application of silica composite aerogels is as an insulating layer between battery modules. He et al.[86]used hexamethyldisiloxane and glass fibers as precursors to prepare glass fiber-reinforced silica composite aerogels via a sol-gel process combined with ambient-pressure drying. Studies have shown that this aerogel enables each module of a lithium-ion battery to operate below 180 ℃, inhibiting heat transfer between lithium battery modules and thereby ensuring the battery’s lifespan. Similarly, Li et al.[87]used ethyl orthosilicate and aluminum chloride as precursors, combined with mullite fibers, to prepare mullite fiber-reinforced Al2O3-SiO2composite aerogels via a sol-gel process and ambient-pressure drying, which show promising application prospects in thermal management control for lithium-ion batteries. Research indicates that after this aerogel is applied to lithium-ion batteries, the maximum temperature difference between modules can reach 526 ℃, effectively suppressing catastrophic heat transfer in the battery; a schematic diagram of its function is shown in Figure 14.
图14 莫来石纤维增强的Al2O3-SiO2复合气凝胶用于电池热管理的示意图[87]

Fig.14 Schematic illustration of Al2O3-SiO2 composite aerogel with mullite fiber reinforced used in battery thermal management[87]

Based on the low thermal conductivity of aerogels, they can be used in various electronic devices, such as chips and batteries, to impede heat transfer between these devices and thereby extend their lifespan.

7 Conclusion and Outlook

Silica aerogels, with their outstanding properties, exhibit great potential in fields such as aerospace, energy conservation, and battery thermal management, offering broad application prospects and making them a current hotspot in research. However, the high brittleness and insufficient high-temperature resistance of silica aerogels limit their practical applications. Researchers have achieved certain advancements in mechanical strength, high-temperature resistance, and preparation methods through approaches such as inorganic compounding, organic compounding, fiber reinforcement, and optimization of preparation processes.
(1) Inorganic-composite silica aerogels inhibit sintering between pore structures at high temperatures through phase transitions, enabling the use of composite aerogels at elevated temperatures (>800 ℃). At the same time, the introduction of inorganic materials optimizes the aerogel’s structure to a certain extent, enhancing its compressive strength.
(2) By extending the chains in organic–silica aerogel composites, chain stiffness is enhanced, thereby improving the mechanical properties of the composite aerogel. In addition, the long-chain organic network complicates the heat transfer path, resulting in a reduced thermal conductivity of the aerogel at a given temperature.
(3) Fiber-reinforced silica aerogels enhance mechanical strength by introducing high-stiffness fibers that disperse the stress borne by the aerogel. The presence of fibers creates a shielding effect, effectively impeding certain forms of radiative heat transfer and thereby reducing thermal conductivity to some extent.
(4) 3D printing has expanded the methods for preparing aerogels. By leveraging the curing of slurries under different conditions and digital manufacturing, it has enabled the preparation of irregular aerogels, offering a new approach to the high-precision fabrication of silica composite aerogels.
However, in practical applications, the following issues still exist.
(1) Inorganic composites still cannot overcome the high brittleness of aerogels; their mechanical properties remain poor, and phase separation at the interfaces is prone to occur, leading to reduced thermal insulation performance.
(2) The low flash point of the organic material itself causes the composite aerogel to undergo carbonization at high temperatures, preventing it from maintaining thermal stability under such conditions; organic materials generally have high toughness, resulting in relatively large deformation under high stress.
(3) The interfacial bonding between the fibers and the silica matrix is poor, leading to stress concentration and delamination at the interface. This prevents the aerogel from being used for a long period, and an excessive amount of fibers may increase solid-phase heat conduction.
(4) In terms of production cost, 3D-printed aerogels are still more expensive than traditional thermal insulation materials, and the curing and subsequent shaping of inorganic composite silica aerogels during 3D printing are relatively challenging.
Based on the above advantages and disadvantages, we believe that in the foreseeable future, the main directions for the development and application of silica composite aerogels are as follows.
(1) Rationally adjust the proportions of each component in the inorganic composite aerogel to control the formation of high-temperature-resistant phases during synthesis, ensuring that the aerogel’s pore structure remains stable at high temperatures; balance the relationship between the thermal conductivity and performance of the inorganic composite aerogel, enabling its extensive use in aerospace, converter linings, and thermal insulation for irregular heat sources.
(2) For organic composite silica aerogels, it is necessary to explore organic frameworks with high strength and high thermal stability, increase their carbonization temperature, and enable their use in building energy-saving systems at medium and high temperatures.
(3) Chemically modify the fibers to create a chemically bonded interface, ensuring strong adhesion between the fibers and the matrix while minimizing stress concentration at the fiber–aerogel interface. In addition, enhance the dispersion of fibers within the matrix to reduce the bridging heat effect of the fibers, enabling their use in battery thermal management.
(4) Investigate the curing and shaping conditions of 3D-printed silica composite aerogels to achieve one-step curing, thereby enhancing the precision and production efficiency of 3D-printed aerogels and further enabling their use in thermal insulation applications for irregular heating elements.

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