High Temperature Resistance and Heat Insulation of Oxide Aerogels

Qing Xu; Xinyue Wang; Weijie Cai; Hongjuan Duan; Haijun Zhang; Shaoping Li

doi:10.7536/PC240208

Progress in Chemistry >

2024 , Vol. 36 >Issue 10: 1520 - 1540

DOI: https://doi.org/10.7536/PC240208

综述

High Temperature Resistance and Heat Insulation of Oxide Aerogels

Qing Xu ¹ ,
Xinyue Wang ¹ ,
Weijie Cai ¹ ,
Hongjuan Duan ^,¹^,^* ,
Haijun Zhang ^,¹^,^* ,
Shaoping Li ²

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¹ State Key Laboratory of Refractories and Metallurgy, Wuhan University of Science and Technology,Wuhan 430081, China
² Hubei Three Gorges Laboratory, Yichang 443007, China

^*e-mail: duanhongjuan@wust.edu.cn (Hongjuan Duan);

zhanghaijun@wust.edu.cn (Haijun Zhang)

Received date: 2024-02-19

Revised date: 2024-07-14

Online published: 2024-09-15

Supported by

National Natural Science Foundation of China(52272021)

National Natural Science Foundation of China(52232002)

Hubei Three Gorges Laboratory Open/Innovation Fund(SK232006)

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Abstract

Oxide aerogel is one type of three-dimensional nano porous material, which has the advantages of high porosity, high specific surface area, low thermal conductivity, high melting point and so on. Moreover, oxide aerogel always shows excellent high-temperature resistance and thermal insulation performance. Thus, in this paper,the research progress of heat-resistant oxide aerogels including silica, alumina, zirconia aerogels, binary and multi-component and their composite counterparts are reviewed. The preparation method and performance of oxide aerogels are summarized, the existing problems are pointed out, and the application of oxide aerogels in the field of high temperature thermal insulation is prospected.

Contents

1 Introduction

2 Preparation of oxide aerogel

2.1 Preparation method

2.2 Drying method

3 SiO₂ aerogel

3.1 Precursor of SiO₂ aerogel

3.2 Pretreatment of SiO₂ aerogel

3.3 SiO₂ composite aerogel

4 Al₂O₃ aerogel

4.1 Precursor of Al₂O₃ aerogel

4.2 Structural control of Al₂O₃ aerogels

4.3 Al₂O₃ composite aerogel

5 ZrO₂ aerogel

5.1 Precursor of ZrO₂ aerogel

5.2 Structural control ZrO₂ aerogels

5.3 ZrO₂ composite aerogel

6 Two component and multi-component oxide aerogel

6.1 Two component oxide aerogel

6.2 Multi-component oxide aerogel

7 Conclusion and outlook

Key words： oxide aerogel; high temperature resistance; heat insulation

Cite this article

Qing Xu , Xinyue Wang , Weijie Cai , Hongjuan Duan , Haijun Zhang , Shaoping Li . High Temperature Resistance and Heat Insulation of Oxide Aerogels[J]. Progress in Chemistry, 2024 , 36(10) : 1520 -1540 . DOI: 10.7536/PC240208

1 Introduction

With the development of aerospace technology, the flight speed of hypersonic vehicles has exceeded 5 Mach numbers. Due to intense aerodynamic heating, when the flight speed reaches 5 Mach numbers, the surface temperature of the vehicle is about 1273 K; when the flight speed increases to 6 Mach numbers, the surface temperature of the vehicle can reach 1667 K; subsequently, for every increase of 1 Mach number in flight speed, the surface temperature of the vehicle increases by approximately 500 K^[1]. However, the pilots and precision instruments inside the vehicle need to operate at temperatures between 293 K and 313 K. The development of thermal protection materials that enable vehicles to overcome harsh service environments has become a hot topic in current research^[2,3].

Aerogels have advantages such as high porosity (~99.8%), high specific surface area (~1200 m²/g)^{[4⇓⇓⇓⇓⇓⇓⇓~12]}. Some of the pores in aerogels (<50 nm) are smaller than the average free path of air molecules (70 nm), which can effectively suppress gas convection. Moreover, aerogels have low solid content and low solid-phase heat transfer efficiency, making them possess extremely low thermal conductivity (up to 0.013 W/(m·K))^[13], and they exhibit excellent thermal insulation effects, playing a good role in the aerospace field^[14]. Among them, oxide aerogels have excellent thermal stability and chemical stability under oxygen atmosphere, making them an excellent candidate material for use in high-speed flight vehicles in high-temperature oxygen-rich environments.

2 Preparation of Oxide Aerogels

2.1 Preparation of Oxide Aerogels

The preparation methods of oxide aerogels generally include sol-gel method, epoxy compound addition method, and inorganic sol-gel method. As shown in Fig. 1.

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图1 (a) 溶胶-凝胶法；（b）环氧化物添加法；（c）无机溶胶-凝胶法^[15]

Fig. 1 (a) Sol-gel method. (b) Epoxide addition method and (c) Inorganic sol-gel method for preparing oxide aerogels^[15]

2.1.1 Sol-gel method

The sol-gel method is a process in which organic salts hydrolyze under the action of a catalyst to generate colloidal particles with small particle size and high surface energy, and then form a gel through spontaneous condensation^[16]. As shown in Fig. 1a, part of the organic alkoxide (M—(O—R)_n, where M represents Si, Al, Zr, Ti, etc., and R represents an alkyl) hydrolyzes to form hydroxyl groups (—OH), and M atoms bond with hydroxyl groups to form M—OH. These combine through dehydration reactions, causing oxygen atoms to bridge different M atoms to form M—O—M. The other part of the unhydrolyzed organic alkoxides has M atoms connected to alkoxy groups (—OR) forming M—OR, which reacts with the hydroxyl groups formed by partial hydrolysis via de-alcoholization, also causing oxygen atoms to bridge different M atoms to form M—O—M. These M—O—M and hydroxyl groups form HO—M—O—M—OH oxide colloids, which continuously crosslink to form a network, ultimately resulting in the formation of a gel.

2.1.2 The Epoxidized Oil Addition Method

The epoxy addition method is a technique that uses inorganic salts as precursors and epoxides as gel aids, undergoes ring-opening addition reactions, and forms gels through hydrolysis and condensation reactions^[17]. The epoxy addition method includes three processes: hydration, hydrolysis, and condensation, as shown in Figure 1b. Inorganic salts mainly exist initially as hydrated ions [M(H₂O_x)]ⁿ⁺; subsequently, the hierarchical hydrolysis reaction of hydrated ions occurs spontaneously, generating hydrogen ions and making the solution slightly acidic. Epoxides consume hydrogen ions slowly via ring-opening nucleophilic addition reactions. After adding epoxides, the hydrolysis equilibrium shifts to the right, producing ions with hydroxyl groups. Subsequently, different hydroxylated ions are connected by oxygen bridges to form M—O—M, followed by dehydration condensation. Since the catalytic hydrolysis rate of epoxides is low, it can maintain the initial acidic condition and gently promote condensation. These two conditions reduce the condensation rate, facilitating the formation of stable colloids. Finally, sol particles crosslink and assemble into a gel framework. Zhao et al.^[18] prepared La₂Zr₂O₇ aerogels using hydrated zirconium nitrate pentahydrate and hydrated lanthanum nitrate hexahydrate as precursors by the epoxy addition method. Research shows that this La₂Zr₂O₇ aerogel has a specific surface area of 413.2 m²/g and retains its porous framework structure after heat treatment at 1000 ℃.

2.1.3 Inorganic Sol-gel Method

The inorganic sol-gel method is a technique for preparing aerogels by adding soluble polymers to the epoxy-based addition method, suitable for low-valent metal oxide. Du et al.^[19] added a small amount of polyacrylic acid to the epoxy system, which activated the carboxyl center and increased the nucleation rate. After rapid nucleation, the residual ion concentration was reduced to slow down the growth rate, successfully preparing oxides gels of various main elements such as Li(Ⅰ), Al(Ⅲ), Ca(Ⅱ), Ti(Ⅳ), Cr(Ⅲ), Mn(Ⅱ), Fe(Ⅲ), Co(Ⅱ), Ni(Ⅱ), Cu(Ⅱ), Zn(Ⅱ), Zr(Ⅳ), Mo(Ⅳ), Cd(Ⅱ), and Ta(Ⅳ).

In summary, the sol-gel method has been widely used for the preparation of SiO₂, Al₂O₃, and ZrO₂ aerogels. However, due to the difficulty in preparing organic salts and their rapid hydrolysis rate, which makes it difficult to control the sol-gel process, many oxide aerogels are unsuitable for preparation by the sol-gel method. The hydrolysis of inorganic salts is relatively mild and easy to control, and the epoxide addition method is suitable for preparing oxide aerogels with inorganic salts as precursors. The inorganic sol-gel method is conducive to the formation of a three-dimensional network structure of metal oxide in low-valence states, enabling the preparation of low-valence metal element oxide aerogels.

2.2 Drying Methods

The drying method affects the pore structure and properties of aerogels, and it is crucial to select the appropriate drying method. The commonly used drying methods for oxide aerogels are supercritical drying, freeze drying, and ambient pressure drying. In addition, microwave drying, which can achieve rapid drying, has been developed in recent years.

2.2.1 Supercritical Drying

Supercritical drying is a method of drying in which the liquid in wet gels is replaced with supercritical fluids such as ethanol or liquid carbon dioxide and then exhausted. Lermontov et al.^[20] prepared SiO₂ aerogels using methyl orthosilicate as the precursor by the supercritical ethanol drying process. The study showed that the supercritical temperature of ethanol was 243 ℃, the pressure was 6.3 MPa, and the specific surface area of the obtained SiO₂ aerogel was 580 m²/g. Shafi et al.^[21] used tetraethyl orthosilicate as the precursor and replaced ethanol with CO₂ as the drying medium in the supercritical drying process to prepare SiO₂ aerogels. The study showed that the supercritical temperature of liquid carbon dioxide was 31 ℃, the pressure was 7.3 MPa, and the thermal conductivity of the obtained SiO₂ aerogel was 0.0179 W/(m·K), with a specific surface area of 1102 m²/g.

The use of organic solvents under high-temperature and high-pressure conditions requires higher equipment standards, and the flammability of organic solvents themselves also brings safety issues. Compared with organic solvents, liquid carbon dioxide has a lower supercritical temperature, and the chemical groups on the gel surface are relatively stable, reducing the danger during the drying process [22]. However, supercritical CO₂ drying has a long cycle, the solvent replacement process is complicated, and the solvents to be replaced also need to be miscible with liquid carbon dioxide, making it difficult to prepare aerogels.

2.2.2 Constant-pressure Drying

Constant-pressure drying, also known as drying under environmental pressure, was employed by Zhang et al.^[23]to prepare SiO₂ aerogels using silica and methyltrimethoxysilane as precursors. The study showed that the prepared SiO₂ aerogel had a specific surface area of 113 m²/g, a bulk density of 0.36 g/cm³, a compressive strength of 0.92 MPa, and a thermal conductivity of 0.056 W/(m·K). Constant-pressure drying is characterized by low equipment requirements, simple operation, and low cost. However, it suffers from significant sample volume shrinkage and cracking due to capillary forces acting on the skeleton. Therefore, efforts should be made to minimize these capillary forces during the process^[24]. In the constant-pressure drying process, the solution in the wet gel is typically replaced with another low-surface-energy solvent before drying to reduce capillary forces. Additionally, the wet gel can be subjected to aging treatment to strengthen the skeleton structure or undergo hydrophobic modification to mitigate the impact caused by the destruction of pore structures due to solvent evaporation^{[25⇓⇓⇓⇓⇓~31]}.

2.2.3 freeze-drying

The freeze-drying process is a method of drying that freezes the solvent in the structure into a solid state and sublimates it from the solid to the gaseous state under a high vacuum environment. Zhou et al.^[32]prepared glass fiber-reinforced SiO₂ aerogel composites using methyltrimethoxysilane and sodium silicate as co-precursors by the freeze-drying method. The study showed that the specific surface area of this composite material was 870.9 m²/g, the thermal conductivity was 0.0248 W/(m·K), and the contact angle was 150°. Lin et al.^[33]prepared organic-inorganic hybrid aerogels using tetraethyl orthosilicate and 3-glycidoxypropyltrimethoxysilane as co-precursors, gelatin as a gelling agent, mica and montmorillonite as fire-resistant fillers, zinc borate and ammonium polyphosphate as fluxes through the freeze-drying method. The study showed that the thermal conductivity of this aerogel was 0.04821 W/(m·K), and the usage temperature could reach up to 1300℃. Freeze-drying can form an ordered structure, making it have stronger mechanical properties and better thermal insulation performance. Vacuum freeze-drying eliminates capillary force, with no shrinkage before and after drying. However, during the freezing process, some weakly cross-linked gel frameworks are prone to damage, and the final obtained samples are mostly powder^[34,35].

2.2.4 Microwave Drying

The heat for atmospheric pressure drying is transferred from the surface of the material to its interior, resulting in uneven heat distribution and making the skeleton prone to shrinkage and collapse. The energy of microwave drying is transmitted through the material, ensuring uniform heat distribution. Bonnardel et al.^[36] used tetraethyl orthosilicate as the precursor and prepared SiO₂ aerogels by microwave drying. Studies have shown that this SiO₂ aerogel has a thermal conductivity as low as 0.0135 W/(m∙K). Microwave drying helps more accurately control the drying process, with most liquids evaporating before leaving the gel, which shortens the drying time and reduces production costs.

Supercritical drying produces aerogels with high porosity, large specific surface area, and minimal volume shrinkage. However, the equipment for supercritical drying is expensive, the operation complex, and the risks high. During vacuum freeze-drying, there is no gas-liquid interface, reducing the surface energy interaction between the liquid solvent and the solid skeleton structure. However, due to the slow sublimation of solids and the need for a vacuum condition, both product size and production efficiency are subject to significant limitations. Atmospheric pressure drying can realize large-scale production of aerogels and is the most promising drying method, but it needs to address issues such as skeleton collapse caused by capillary force. Microwave drying, based on atmospheric pressure drying, effectively alleviates problems like volume shrinkage and skeleton collapse caused by solvent evaporation, and has potential application prospects in the drying methods for preparing aerogels.

3 SiO₂ Aerogel

Silica aerogels are porous materials constructed from SiO2 nanoparticles, where the [SiO4] tetrahedral structures in the colloid connect to form a three-dimensional network. Due to their high porosity (porosity > 95%, up to 99% or more^[37]), silica aerogels possess extremely low thermal conductivity, which can be as low as 0.015 W/(m·K), lower than the thermal conductivity of air under the same conditions (0.023 W/(m·K)). This makes silica aerogels one of the commonly used thermal insulation materials with broad application prospects in the field of thermal insulation^{[38⇓⇓⇓~42]}. For detailed preparation methods and properties of silica aerogels, see Table 1.

表1 SiO₂气凝胶的制备方法与性能^a)

Table 1 Preparation method and properties of SiO₂ aerogel^a)

Precursor	Drying method	Modified way	Service temperature /℃	Specific surface area/ (m²/g)	Thermal conductivity/ (W/(m·K))	Ref.
TMOS	Supercritical drying	−	−	−	0.0135 (RT)	44
Silica sol		Hydrothermal	1200	156 (RT) 148 (1200 ℃)	−	48
MTMS		Insert flexible ether group	−	−	0.0159 (RT)	50
TEOS		Add ZrCl₄/AlCl₃	1200	653.67 (RT ZrCl₄) 524.32 (RT AlCl₃)	−	53
P-VTMS		Add TPU	−	2145 (RT)	0.026 (RT)	54
TEOS		Doped with Y₂O₃	900	1010.4 (RT) 643.8 (900 ℃)	−	56
Silica sol	Atmospheric pressure drying	−	700	333.43 (RT)	0.019 (RT) 0.044 (600 ℃)	46
Nano SiO₂ aqueous liquid slurry		Add fiberglass felt	−	−	0.02846 (100 ℃) 0.05457 (300 ℃) 0.09367 (500 ℃) 0.1506 (700 ℃)	47
TMOS		Age	500	595 (RT)	−	25
TEOS、HMDZ		Solvent displacement	−	750 (RT)	0.07 (RT)	26
TEOS		Solvent displacement	−	530 (RT)	−	27
TEOS		Hydrophobic modification	−	973 (RT HMDS) 1067 (RT TMCS)	−	29
TEOS、MTMS		−	300	364.5 (RT) 895.5 (300 ℃)	−	30
TEOS		Vapor deposition BN	700	526 (RT) 252 (RT BN)	0.083 (RT) 0.090 (BN)	31
TEOS		Adding SiO₂ nanofibers and hydrophobic modification	−	624.19 (RT)	0.021 (RT)	52
Nano SiO₂ aqueous liquid slurry		Add fiberglass felt and SiC	−	−	0.1334 (700 ℃)	47
TEOS		Doped with Y₂O₃	727	917.5 (RT)	0.051 (RT) 0.080(727 ℃)	55
TEOS	Freeze-drying	Electrospinning	1100		0.024 (RT) 0.036 (300 ℃)	44

a) TEOS：Ethyl orthosilicate；TMOS：Methyl silicate；MTMS：Methyltrimethoxysilane；P-VTMS：Polyethylene based trimethoxysilane；TPU：Thermoplastic Polyurethane； HMDS：Hexamethyldisilazane；TMCS：Trimethylsilyl chloride；RT：Room Temperature

3.1 SiO₂
Precursor of Aerogel

3.1.1 Organic Precursor

Methyl orthosilicate and ethyl orthosilicate are the most commonly used organic precursors for SiO2 aerogels. Shi et al. [Ref. 43] obtained highly transparent SiO2 aerogels using methyl orthosilicate as the precursor, methanol as the solvent, and ammonia water as the catalyst via supercritical CO2 drying method. The study shows that with the increase of the mass fraction of methyl orthosilicate from 15% to 45%, the linear shrinkage decreases from 22%-27% to 5%-9%. The lowest thermal conductivity of 0.0135 W/(m·K) was achieved when the mass fraction of methyl orthosilicate was 15% or 25% and the concentration of ammonia water was 0.75 mol/L. Wang et al. [Ref. 44] prepared SiO2 fibers by electrospinning using ethyl orthosilicate as the precursor, then added them into silica sol, and finally fabricated SiO2 nanofiber aerogels with super elasticity by freeze-drying process. The study shows that the thermal conductivity of SiO2 nanofiber aerogels is 0.024 W/(m·K), and at 300 ℃ it is 0.036 W/(m·K). The volumetric density is 0.00025 g/cm3. The nanofibers in situ construct stable bonding structures during the freezing forming process, making them have isothermal super elasticity at −196 ℃ and 1100 ℃, as shown in Figure 2.

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图2 (a) SiO₂纳米纤维气凝胶的制造的示意图；（b） SiO₂纳米纤维气凝胶中纳米纤维之间的交联网络；（c） SiO₂纳米纤维气凝胶在丁烷喷灯（1100 ℃）和液氮（−196 ℃）的火焰中；（d） SiO₂纳米纤维气凝胶的微观结构^[44]

Fig. 2 (a) Schematic illustration showing the fabrication of SiO₂ nanofiber aerogel. (b) The crosslinking networks between the nanofibers of SiO₂ nanofiber aerogel. (c) SNF aerogels in flame of a butane blowtorch (1100 ℃) and liquid nitrogen (−196 ℃). (d) Microscopic architecture of an SiO₂ nanofiber aerogel^[44]

The reactivity of methyl orthosilicate is better than that of ethyl orthosilicate, with a faster hydrolysis rate and easier Si-OH polymerization [45]. However, methyl orthosilicate is expensive, and the methanol generated during hydrolysis is harmful to the environment and human health, while ethyl orthosilicate is inexpensive, environmentally friendly, and non-toxic [44].

3.1.2 Inorganic Precursor

The large-scale application of aerogels requires solving the problem of the high cost of precursors, and the cost of inorganic precursors is much lower than that of organic precursors.

Sun et al.^[46] prepared SiO₂ aerogel using colloidal silica as the precursor through ambient pressure drying. The study showed that the SiO₂ aerogel prepared when the mass fraction of colloidal silica was 20% had a uniform structure and could withstand high temperatures up to 700 ℃. The specific surface area of this SiO₂ aerogel was 333.43 m²/g, with thermal conductivities of 0.019 W/(m·K) at room temperature and 0.044 W/(m·K) at 600 ℃.

Liu et al.^[47] used aqueous nanosilica sol as the precursor and glass fiber felt as the reinforcing agent, and prepared nanosilica composite aerogels by atmospheric pressure drying. Studies have shown that when the solid content of the sol is 15%, the density of the composite aerogel is 0.24 g/cm³, with the lowest thermal conductivity, which are 0.02846, 0.05457, 0.09367, and 0.1506 W/(m·K) at 100, 300, 500, and 700 ℃, respectively.

Compared with the conventional sol-gel method, the sol-gel-hydrothermal method can significantly improve the heat resistance and mechanical properties of aerogels by changing their microstructures, which is an important method for preparing high-strength and high-temperature-resistant oxide aerogels [48]. Li et al. [49] used silicon sol as the precursor and treated the gel particles and network structure by hydrothermal method, then prepared SiO₂ aerogels by supercritical drying. The study showed that when the hydrothermal treatment time was 24 h and the temperature was 180 ℃, the aerogel could maintain almost unchanged pore size, pore volume, and structure after being heat-treated at 1200 ℃ for 30 min, with a shrinkage rate of only 5% and a specific surface area decreased from 156 m²/g to 148 m²/g. The high-temperature hydrothermal treatment conditions increased the average particle size of the SiO₂ aerogel nanoparticles and thickened the network framework; besides, the high temperature promoted the condensation between sol particles and enhanced the bonding force.

Inorganic precursors are inexpensive, but the specific surface area of the prepared samples is not as high as that of organic precursors. At present, the most commonly used precursor for preparing SiO₂ aerogels is still tetraethyl orthosilicate.

3.2 SiO₂ Pre-treatment of Aerogels

Atmospheric pressure drying can realize the large-scale preparation of SiO₂ aerogels. However, the untreated SiO₂ wet gels retain a lot of water solution in their pores and have a large number of hydroxyl groups on the surface of the gel skeleton. When directly subjected to atmospheric pressure drying, the capillary force generated exceeds the bearing limit of the skeleton structure, resulting in the shrinkage and structural collapse of the aerogel. Methods such as aging treatment, low surface energy solvent replacement, and hydrophobic modification can alleviate the problem of structural collapse of SiO₂ aerogels during atmospheric pressure drying. As shown in Figure 3.

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图3 （a）SiO₂气凝胶的Ostwald老化机制；（b）低表面能溶剂置换^[27]；（c）SiO₂气凝胶的疏水改性

Fig. 3 (a) Schematic diagram of Ostwald aging mechanism of SiO₂ aerogel. (b) Low surface tension solvent replacement^[27]. (c) Hydrophobic modification of SiO₂ aerogel

3.2.1 Aging Treatment

Aging leads to the continued reaction of particle dissolution and precipitation on the surface of the skeleton after the formation of the wet gel^[49]. Aging thickens the connections between particles, strengthens the network skeleton, and improves the strength of the SiO₂ aerogel.

Haereid et al.^[25] prepared SiO₂ wet gels using methyl orthosilicate as the precursor, and aged them with a mixture of tetraethyl orthosilicate solution and methanol at different concentrations. SiO₂ aerogels were then obtained by atmospheric pressure drying. The study showed that as the volume fraction of tetraethyl orthosilicate in the aging solution increased from 30% to 70%, the linear shrinkage after 48 hours decreased from 30% to 22%, the bulk density decreased from 0.65 g/cm³ to 0.5 g/cm³, and the specific surface area decreased from 685 m²/g to 595 m²/g.

The aging treatment effectively enhanced the network skeleton structure and reduced the shrinkage rate during the drying process. However, the thickening of the skeleton caused by SiO₂ particles will lead to a decrease in pore size and the disappearance of some small pores, resulting in a reduction in the specific surface area and porosity of the aerogel.

3.2.2 Low Surface Energy Solvent Replacement

Low surface energy solvent replacement is to replace the high surface energy solution in the wet gel with a low surface energy organic solvent, thereby reducing the stress on the network framework during drying. Rao et al.^[26] prepared SiO₂ gels using ethanol-diluted tetraethyl orthosilicate and hexamethyldisilazane methylation reagents as precursors through water hydrolysis with oxalic acid and ammonia condensation. Different organic solvents such as hexane, cyclohexane, heptane, benzene, toluene, and xylene were used for solvent replacement, and SiO₂ aerogels were prepared by atmospheric pressure drying. The study showed that the sample treated with heptane had a shrinkage rate as low as 4%, porosity of 97%, thermal conductivity of 0.07 W/(m·K), and specific surface area of 750 m²/g. Moreover, the prepared SiO₂ aerogel exhibited excellent hydrophobic properties with a contact angle of approximately 165°.

Although the above method has lower energy consumption, it is costly due to the extensive use of organic solvents. Lu et al.^[27] prepared SiO₂ aerogels by using tetraethyl orthosilicate as the precursor, aging with ethanol for 24 hours, and then replacing the solvent with ammonium bicarbonate before atmospheric pressure drying. The study shows that as the mass fraction of ammonium bicarbonate increases from 3% to 25%, the specific surface area of the samples increases from 360 m²/g to 530 m²/g, and the average pore size increases from 2.7 nm to 6.1 nm. During the drying process, ammonium bicarbonate decomposes upon heating, producing CO₂ and NH₃ in the pores of the wet gel, thereby preventing the collapse of the pores during atmospheric pressure drying.

Solvent exchange effectively alleviates the problem of framework collapse in ambient pressure drying, but the process of solvent exchange is often time-consuming and complex.

3.2.3 Hydrophobic modification

The hydrophobic modification of SiO₂ aerogel can not only reduce the capillary force damage to the gel structure, but also decrease the moisture absorption ability of the aerogel. There are two common methods for hydrophobic modification of SiO₂ aerogel: one is to replace the hydrophilic hydroxyl functional groups with silane-based compounds such as trimethylchlorosilane, methyltrimethoxysilane, and hexamethyldisilazane^[28], so that a layer of silane groups covers the surface and internal pores of the gel, making the aerogel hydrophobic. The other method is to use organic silanes containing methyl or phenyl groups together with tetramethyl orthosilicate or tetraethyl orthosilicate as co-precursors.

Bi et al.^[29] used tetraethyl orthosilicate as the precursor and hydrophobic modifiers including trimethylchlorosilane and hexamethyldisilazane, and prepared hydrophobic SiO₂ aerogels by sol-gel method and ambient pressure drying. The results showed that the density of the SiO₂ aerogel modified by hexamethyldisilazane was 0.204 g/cm³, the contact angle was 128°, the specific surface area was 973 m²/g, and the average pore size was 7.57 nm. The density of the SiO₂ aerogel modified by trimethylchlorosilane was 0.115 g/cm³, the contact angle was 158°, the specific surface area was 1067 m²/g, and the average pore size was 13.40 nm.

Wu et al.^[30] prepared a flexible SiO₂ aerogel with hydrophobic properties using methyltrimethoxysilane and tetraethyl orthosilicate as co-precursors, hydrochloric acid and ammonia water as acid-base catalysts, and cetyltrimethylammonium chloride as a surfactant. After aging for 24 hours, the sol-gel was sequentially exchanged with ethanol and ethane solvents, surface-modified with a 10 vol% trimethylchlorosilane/ethane solution for 24 hours, and then washed three times with ethane. The aerogel was finally prepared by atmospheric pressure drying, as shown in Figure 4. Research results show that the optimal volume percentage of methyltrimethoxysilane is 60%, with a specific surface area of 364.5 m²/g. After heat treatment at 300 ℃, the microstructure becomes more uniform, the specific surface area reaches up to 895.5 m²/g, and the water contact angle reaches 153.9°, as shown in Figure 4.

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图4 (a) 常压干燥法制备柔性疏水二氧化硅气凝胶的示意图；将水滴置于干燥后的疏水二氧化硅气凝胶（b）与在300 ℃ （c）和400 ℃ （d）热处理后的疏水二氧化硅气凝胶光学图片；以MTMS和TEOS为共前驱体（MTMS体积百分比为60%），经TMCS表面改性后凝胶的SEM图像（e）和300 ℃热处理后（f）二氧化硅气凝胶的SEM图像及结构^[30]

Fig. 4 (a) Schematic diagram of forming flexible hydrophobic silica aerogel by APD process. Place the water drop on the dried hydrophobic silica aerogel (b) and the optical picture of hydrophobic silica aerogel after heat treatment at 300 ℃ (c) and 400 ℃ (d). SEM images of gel after surface modification of TMCS with MTMS and TEOS as co precursors (MTMS volume percentage is 60%) (e) and 300 ℃ heat treatment (f)^[30]

However, when the temperature exceeds 400 ℃, the hydrophobic functional groups fail, limiting the application of hydrophobic aerogels in high-temperature fields. By depositing a layer of hydrophobic substance on the surface of the aerogel via chemical vapor deposition, not only can the surface energy of the aerogel be reduced and the SiO₂ framework protected, but the porous structure of the aerogel can also be maintained. Zhao et al.^[31] prepared nitrogen-modified SiO₂ aerogels using tetraethyl orthosilicate as the precursor, deionized water and ethanol as solvents, oxalic acid and sodium hydroxide as acidic and basic catalysts, respectively, through atmospheric pressure drying. The study shows that after modification, the thermal conductivity of the aerogel increases from 0.083 W/(m·K) to 0.090 W/(m·K), and the specific surface area decreases from 526 m²/g to 252 m²/g. The hydrophobic performance of this aerogel is stable, with a contact angle of 144.3° and a failure temperature up to 700 ℃.

3.2.4 Polymer Reinforcement

By crosslinking polymers with the skeleton of SiO₂ aerogels or forming covalent bonds with the gel surface, the mechanical properties of SiO₂ aerogels can be enhanced. Rezaei et al.^[50] synthesized a new generation of polymer precursors for preparing organic/inorganic silica composite aerogels using trimethoxysilane as raw material, diethyl ether boron trifluoride as catalyst, anhydrous ethanol as solvent, and tert-butyl peroxide. Rezaei prepared polyether-based SiO₂ aerogels by opening-ring polymerization with ethyl alcohol as solvent and ammonia water as catalyst, followed by supercritical drying to insert flexible ether groups into the main chain. Studies have shown that the particle structure of polyether-based SiO₂ aerogels transforms into a novel non-particle continuous structure. The elastic compressive strain (10%) of non-particle polyether-based SiO₂ aerogels is much higher than that of typical SiO₂ aerogels (2%-4%), and the thermal conductivity is 0.0159 W/(m·K).

Aging treatment, low surface energy solvent replacement, hydrophobic modification and polymer reinforcement have all alleviated the shrinkage and collapse of the SiO2 aerogel framework. However, the process of hydrophobic modification is complicated, and the hydrophobic performance of the aerogel will fail at high temperatures; in aging treatment and low surface energy solvent replacement, the type, content and environmental conditions of the aging liquid and replacement solvent still need to be explored; polymer reinforcement will increase the thermal conductivity of the aerogel and reduce its thermal stability. Therefore, it is necessary to find a simpler method in the future to prepare SiO2 aerogels with non-shrinking gel frameworks, good hydrophobic properties and good mechanical properties.

3.3 SiO₂-based Composite Aerogel

3.3.1 Fiber-reinforced SiO₂ composite aerogel

The introduction of nanofibers into SiO₂ aerogels can not only improve the pore structure of the aerogels but also act as a supporting network skeleton structure to reduce volume shrinkage during the drying process, and reduce gas-phase thermal conduction to lower the thermal conductivity. Nanofibers can overcome the problem of poor bonding performance in aerogels, thereby effectively enhancing their mechanical properties. Moreover, the nanofibers interweave within the pores of the aerogels, supporting the pore diameter; simultaneously, nanofibers help suppress high-temperature radiative heat transfer, further reducing the high-temperature thermal conductivity of SiO₂ aerogels^[51].

Zheng et al.^[52]First, tetraethyl orthosilicate was used as the precursor, hydrochloric acid and ammonia water were used as acid-base catalysts to prepare SiO₂ wet gels; then, silica sol was used as the precursor, and SiO₂ nanofibers were obtained through electrospinning; finally, the two were compounded, and trimethylchlorosilane was used as the surface modifier, and SiO₂ nanofiber aerogel composites were prepared by atmospheric pressure drying. Studies have shown that the composite material has good flexibility and hydrophobic properties, with a thermal conductivity of 0.021 W/(m·K), porosity of 90.8%, and specific surface area of 624.19 m²/g. After heat treatment at 300 ℃, the hydrophobic angle of the aerogel can still reach 140 ℃.

Ishibin et al.^[53] utilized the phenomenon of acid-base imbalance in the precursor solution and prepared zirconia, alumina nanofiber-reinforced silica (SiO₂) composite aerogels through sol-gel method and supercritical drying, followed by heat treatment at 1200 ℃. The study showed that the crystalline phases of the zirconia and alumina nanofiber-reinforced SiO₂ composite aerogels were ZrSiO₄, ZrO₂, and α-Al₂O₃, with compressive strengths of 6.3 and 8.5 MPa, and specific surface areas of 653.67 and 524.32 m²/g, respectively. The zirconium and aluminum metal oxides and SiO₂ aerogel precursors were combined during gelation aging to achieve the integration of fibers with the aerogel matrix via high-temperature conversion.

Compared with inorganic nanofibers, polymer fibers have exhibited better mechanical strength and flexibility. Karamikamkar et al.^[54] prepared composite aerogels with a thermal conductivity of 0.026 W/(m·K) and a specific surface area as high as 2145 m²/g by supercritical drying, combining organic nanofibers of thermoplastic polyurethane nanofibers with polyvinyltrimethoxysilane as the silicone-based precursor. After adding thermoplastic polyurethane nanofibers, the overall shrinkage rate of the samples decreased from 27% to 7%.

The heat resistance of the fiber-reinforced aerogel composites is jointly determined by the aerogel and the fiber reinforcement, and currently, the nanofibers compatible with the SiO₂ aerogel network skeleton remain to be further optimized.

3.3.2 Composite aerogels with shading agents and SiO₂

Silica aerogels can transmit electromagnetic waves with wavelengths of 2 to 8 μm, so radiative heat transfer at high temperatures is the main mode of heat conduction in silica aerogels, which leads to a decrease in their thermal insulation performance at high temperatures. Moreover, due to their small particle size and high surface energy, they tend to sinter under high-temperature conditions, causing the aerogel to shrink, destroying its pore structure and reducing its specific surface area, thereby further deteriorating its thermal insulation performance. To reduce infrared radiation at high temperatures and suppress radiative heat transfer, it is necessary to add opacifiers to inhibit infrared radiation. Common opacifiers used in silica aerogels mainly include carbon black, SiC, ZrO₂, and Y₂O₃. Since carbon black has poor oxidation resistance at high temperatures, SiC, ZrO₂, and Y₂O₃ are typically used as opacifiers in composite aerogels under oxygen-rich high-temperature environments^[40].

Liu et al.^[47] used an aqueous nanosilica liquid slurry as the precursor and uniformly dispersed the aqueous liquid slurry with a SiO₂:SiC volume ratio of 1:13, preparing doped SiC light-blocking agent nanosilica composite aerogels by atmospheric drying method. The thermal conductivity of the composite aerogel decreased from 0.1506 W/(m·K) to 0.1334 W/(m⋅K) at 700 ℃.

Parale et al.^[55]prepared SiO₂ aerogels using ethyl orthosilicate as the precursor and Y₂O₃ doping by atmospheric pressure drying method. The study showed that when the molar ratio of Y₂O₃ to ethyl orthosilicate was 0.37, the room-temperature thermal conductivity of the SiO₂ aerogel was 0.051 W/(m·K), the thermal conductivity at 1000 K was 0.080 W/(m·K), the specific surface area was 917.5 m²/g, the bulk density was 0.076 g/cm³, and the porosity was 96%.

Zhang Junjun et al.^[56] prepared Y₂O₃-doped SiO₂ aerogels with tetraethyl orthosilicate as precursor and yttrium chloride hydrate as additive by supercritical CO₂ drying. The study shows that the specific surface area of SiO₂ aerogels doped with 10% Y₂O₃ can reach 1010.4 m²/g, and after heat treatment at 900 ℃ for 2 h, it is 643.8 m²/g, still maintaining the nanoscale porous structure, and possessing excellent high-temperature resistance performance.

Introducing opacifiers to suppress radiative heat transfer can effectively reduce the high-temperature thermal conductivity of SiO₂ aerogel. Moreover, the commonly introduced high-temperature-resistant opacifiers such as SiC, ZrO₂, and Y₂O₃ can withstand temperatures above 2000 ℃, thereby improving the high-temperature resistance performance of SiO₂ aerogel.

In summary, researchers can prepare SiO₂ aerogels with high porosity and low thermal conductivity, which can be used at temperatures up to 1200 ℃. Hydrophobic modification, addition of fibers, and use of opacifiers have improved the hydrophobicity, mechanical properties, and high-temperature heat insulation performance of SiO₂ aerogels. In the future, it is necessary to further regulate the microstructure of SiO₂ aerogels and composite them with one or more novel high-temperature resistant materials to prepare SiO₂ aerogels with high strength, low thermal conductivity, and elasticity. The large-scale preparation of SiO₂ aerogels with excellent performance is the future development trend.

4 Al₂O₃ aerogel

The melting point of Al₂O₃ is 2054 ℃, and Al₂O₃ aerogel can still maintain its nanoscale porous structure at 1000 ℃, thus attracting great attention as a high-temperature thermal insulation material^[57]. For the preparation methods and properties of Al₂O₃ aerogel, see Table 2.

表2 Al₂O₃气凝胶的制备方法与性能^a)

Table 2 Preparation method and properties of Al₂O₃ aerogel^a)

Precursor	Drying method	Modified way	Service temperature /℃	Specific surface area/ (m²/g)	Thermal conductivity/ (W/(m·K))	Ref.
ASB	Supercritical drying	Add ceramic fiber felt	1000	-	0.022 (RT) 0.058 (600 ℃) 0.092 (1000 ℃)	58
AIP		-	-	376 (RT)	0.029 (RT)	59
ASB		ISWF Method Combined with SCFM and HMDS Gas Phase Modification	1300	152~261 (1200 ℃) 125~136 (1300 ℃)	0.05 (RT)	60
AlCl₃·6H₂O		Doped with Si	1200	283 (1000 ℃)	0.035 (RT)	75
AlCl₃·6H₂O		Doped with Si	1200	515 (RT) 217 (1000 ℃) 68 (1200 ℃)	0.025 (150 ℃) 0.121 (1200 ℃)	76
AIP		Carbon coated Al₂O₃ nanorods	1400	-	0037 (RT) 0.065 (1200 ℃)	77
AlCl₃·6H₂O		Doped with SrO	1200	122 (1200 ℃)	0.060 (RT)	80
AlCl₃·6H₂O		Doped with Y₂O₃	1000	380~400 (1000 ℃)	-	81
Al(NO₃)₃·9H₂O		Introducing SiO₂ fiber felt	900	-	0.028 (35 ℃) 0.033 (600 ℃)	83
AlCl₃·6H₂O		Adding mullite fibers	1400	-	0.058 (200 ℃) 0.152 (1400 ℃)	84
AlCl₃·6H₂O		Doped with TiO₂	1000	650 (RT)	0.136 (1000 ℃)	86
AlCl₃·6H₂O	Atmospheric pressure drying	-	1000	465 (RT)	-	60
Al(NO₃)₃·9H₂O	Freeze-drying	Preparation α- Al₂O₃ nanosheets bonded with silica sol	1600	-	0.029 (RT)	65
AlCl₃·6H₂O		Using chitosan as a template and using solution freezing drying calcination technology	1300	250 (RT)	-	69
Al₂O₃ nanorod sol		PVA bonding and doped with Si	1400	118 (RT) 39.12 (1400 ℃)	0.0246 (RT) 0.0949 (1000 ℃)	78
ASB	Freeze-drying	Adding TEOS and SiO₂ aerogel nanoparticles, Introducing inert inorganic molecular chains	1700	-	0.028 (RT)	79
ASB	-	Combining Blow Spinning and Atomic Layer Deposition (ALD)	900	-	0.022 (RT)	68

a) ASB：Aluminum sec-butoxide；AIP：Aluminum isopropoxide；ISWF：Acetone Aniline in situ Water Formation；SCFM：Supercritical Fluid Modification；HMDS：Hexamethyldisilazane；TEOS：Ethyl orthosilicate；RT：Room Temperature

4.1 Precursor of Al₂O₃ aerogel

4.1.1 Organic Precursor

The organic precursors of Al₂O₃ aerogels are mainly secondary butyl aluminum alcoholate or isopropyl aluminum alcoholate. Wang Hong et al.^[58] used secondary butyl aluminum alcoholate as the precursor, and obtained Al₂O₃ aerogel by supercritical ethanol drying after aging with anhydrous ethanol. Ceramic fiber felt was compounded in the sol-gel process to obtain Al₂O₃ composite aerogel. Studies have shown that after the sample is treated at 1000 ℃, the linear shrinkage rate of Al₂O₃ aerogel is 9.8%. The room temperature thermal conductivity of Al₂O₃ composite aerogel is 0.022 W/(m·K), 0.058 W/(m·K) at 600 ℃, and 0.092 W/(m·K) at 1000 ℃. Poco et al.^[59] used isopropyl aluminum alcoholate as the precursor, water and ethanol as solvents, and a mixed solution of acetic acid and methanol as catalysts. First, the sol-gel reaction process was controlled, and then Al₂O₃ aerogel was prepared by supercritical CO₂ drying. Studies have shown that the specific surface area of this Al₂O₃ aerogel is 376 m²/g, and the room temperature thermal conductivity is 0.029 W/(m·K).

Organic precursors have been widely used for the preparation of Al₂O₃ aerogels. However, the hydrolysis and polymerization reactions of organic precursors are difficult to control, the sol remains unstable, and precipitation phenomena easily occur. Moreover, these organic precursors are costly, flammable, and harmful to human health.

4.1.2 Inorganic Precursor

Inorganic precursors are cost-effective and the process is simple and controllable, making it easy to achieve large-scale production. The inorganic precursors for preparing Al₂O₃ aerogels are usually hexahydrate aluminum chloride and nonahydrate aluminum nitrate. Chen Heng et al.^[60] selected hexahydrate aluminum chloride as the precursor, propylene oxide as the gelation agent, and a mixed solution of anhydrous ethanol and deionized water as the reaction system, preparing Al₂O₃ aerogels through supercritical ethanol drying. Studies have shown that this Al₂O₃ aerogel has a density of 0.089 g/cm³, a specific surface area of 465 m²/g, an average pore diameter of 19.24 nm, and a pore volume of 2.37 cm³/g. However, after heat treatment at 1000 ℃, the specific surface area and pore volume of the aerogel significantly decreased.

To investigate the influence of two commonly used inorganic precursors on the preparation of Al₂O₃ aerogels, Baumann et al. ^[61] prepared Al₂O₃ aerogels by supercritical drying using aluminum chloride hexahydrate and aluminum nitrate nonahydrate as inorganic precursors and propylene oxide as a gelation agent. The study found that the gel structure obtained using aluminum nitrate nonahydrate was loose and the gelation rate was slow, with a microstructure composed of a network structure formed by spherical particle aggregation, and the phase composition was amorphous. However, under the same conditions, stable and transparent gels could be obtained using aluminum chloride hexahydrate, with a microstructure composed of a network skeleton structure formed by needle-like particle joints.

Inorganic precursors cannot form gels through the traditional sol-gel method; gel promoters such as propylene oxide need to be added to promote the formation of gels.

4.2 Al₂O₃ Aerogel Structure Regulation

The mechanical properties and high-temperature thermal insulation performance of Al₂O₃ aerogels are poor^[62,63], and to prepare Al₂O₃ aerogels with high strength and excellent high-temperature thermal insulation performance, it is necessary to regulate the structure of the aerogel.

Zu et al.^[64] used aluminum sec-butoxide as the organic precursor and ethanol as the supercritical fluid. The wet gel was placed in ethanol containing a certain amount of dissolved aluminum sec-butoxide and ethyl orthosilicate. By in-situ formation of acetone-aniline, combined with supercritical fluid modification and hexamethyldisilazane vapor modification, stacked sheet-structured Al₂O₃ aerogels were prepared through supercritical drying, as shown in Fig. 5. Research shows that hexamethyldisilazane reacts with the gel surface to introduce Si-CH₃ groups, which convert into SiO₂ particles at high temperature, inhibiting phase transformation and sintering of the Al₂O₃ aerogel. The thermal conductivity of the prepared Al₂O₃ aerogel is less than 0.05 W/(m·K), and after being held at 1200 and 1300 ℃ for 2 hours, the specific surface area reaches 152~261 m²/g and 125~136 m²/g respectively, with shrinkage rates of only 1% and 5%. The strength increases significantly by 120%, and it still exhibits excellent thermal insulation performance at 1300 ℃.

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图5 (a) 丙酮-苯胺原位水形成（ISWF）法溶胶-凝胶法；（b）耐高温氧化铝气凝胶原氧化铝颗粒改性机理示意图^[64]

Fig. 5 (a) Sol-gel process via acetone-aniline in situ water formation (ISWF) method. (b) Schematic representation of modification mechanism of primary alumina particles of heat-resistant alumina aerogel^[64]

Ji et al.^[65]prepared α-Al₂O₃ nanosheets using hydrated aluminum nitrate as precursor, and then fabricated Al₂O₃ aerogels by bonding the α-Al₂O₃ nanosheets with silica sol and freeze-drying. The research showed that due to the structure of mullite-covered alumina, this aerogel exhibited excellent thermal and chemical stability even at 1600 ℃; the thermal conductivity of this Al₂O₃ aerogel was 0.029 W/(m·K), and its compressive strength was 0.6 MPa. After calcination at 1600 ℃ for 30 min, the linear shrinkage rate was only 2.7%.

The above preparation process is relatively cumbersome, and the template method can easily prepare aerogels^[66,67]. Xu et al.^[68] used aluminum sec-butoxide as the precursor and polyvinylpyrrolidone sponge as the template, and prepared Al₂O₃ aerogels by combining blow spinning with atomic layer deposition. The study showed that this Al₂O₃ aerogel has certain elasticity, a thermal conductivity of 0.022 W/(m·K), and can still maintain its original shape after heat treatment at 900 ℃ for 2 hours. Liao et al.^[69] used aluminum chloride hexahydrate as the precursor and chitosan as the template, and prepared Al₂O₃ aerogels with layered structures using a solution-freezing-drying-calcination technique through the chitosan template method. The study showed that the specific surface area of this Al₂O₃ aerogel is 250 m²/g, and the volume density is about 0.009 g/cm³. This aerogel has a three-dimensional porous structure of the chitosan template, and can withstand high temperatures up to 1300 ℃.

In summary, researchers have enhanced the performance of Al₂O₃ aerogels by regulating their micro-morphology. The solution-freeze-drying-calcination technique eliminates processes such as sol-gel, aging, and solvent exchange, featuring simple operation; the template method can prepare metal oxide aerogels with desired microstructures by changing the used templates. These different micro-morphologies further endow Al₂O₃ aerogels with distinct properties, giving them broad development prospects in the field of high-temperature heat insulation.

4.3 Al₂O₃ Composite Aerogel

With the increase of temperature, Al₂O₃ aerogel will produce γ-AlOOH

phase transformation to α-Al₂O₃ ^[70]. After γ-AlOOH dehydroxylation, the activated atoms in the Al₂O₃ lattice will migrate and diffuse at high temperatures, O^2- changes from cubic to hexagonal close-packed, while randomly distributed Al³⁺ in octahedral or tetrahedral voids will uniformly distribute in octahedral voids, eventually transforming into stable α-Al₂O₃ ^[71]. AlOOH, γ-Al₂O₃, and δ-Al₂O₃ are all spinel structures, while α-Al₂O₃ is a hexagonal close-packed structure. Therefore, the volume shrinkage caused by α-phase transformation above 1000 ℃ will damage the structure, making it unable to be used under high-temperature conditions. Al₂O₃ aerogel will also cause sintering between Al₂O₃ particles at high temperatures ^[72], resulting in a decrease in surface energy and particle aggregation growth, reducing the specific surface area of Al₂O₃ particles. Therefore, the research on improving the thermal stability of Al₂O₃ mainly focuses on inhibiting α-phase transformation and high-temperature sintering.

4.3.1 Doped Elements in Al₂O₃ Composite Aerogels

The commonly used method to suppress the α-phase transformation is doping with Si elements. Si ions occupy the tetrahedral vacancies in γ-Al₂O₃, reducing the total number of vacancies; Al—OH on the surface of Al₂O₃ is replaced by less mobile Si—OH, and Si—OH further reacts with other hydroxyl groups to form Si—O—Si or Si—O—Al, which can firmly embed in the surface of Al₂O₃ and thus suppress the α-phase transformation. The doping of Si can also enhance anisotropy, reduce neck contact between particles, thereby inhibiting the nucleation and grain growth of α-Al₂O₃, and Si suppresses the surface diffusion of Al atoms, transforming into mullite phase at high temperature, consuming part of the Al₂O₃ for α-phase transformation, ultimately suppressing the α-phase transformation [73, 74]. Tian et al. [75] prepared Si-doped Al₂O₃ composite aerogels with Al∶Si = 9∶1 as precursors using hexahydrate aluminum chloride and tetraethyl orthosilicate, followed by adding propylene oxide and supercritical drying. Studies show that its room-temperature thermal conductivity is 0.035 W/(m·K), and its linear shrinkage rate is only 5% at 1200 ℃, with a specific surface area of 283 m²/g. No α-Al₂O₃ was detected during heating, and Al—O—Si bonds entered the structure of Al₂O₃ aerogel, hindering its phase transformation. Wang et al. [76] prepared Si-doped quartz fiber-reinforced Al₂O₃ composite aerogels using hexahydrate aluminum chloride and ethyl orthosilicate as precursors, followed by adding propylene oxide and supercritical drying. Studies show that the thermal conductivity of the composite aerogel is 0.025 W/(m·K) at 150 ℃ and 0.121 W/(m·K) at 1200 ℃; the specific surface area of the aerogel at room temperature is 515 m²/g, and after heat treatment at 1000 and 1200 ℃ for 2 h, the specific surface areas are 217 and 68 m²/g respectively.

The size of Al₂O₃ nanorods is between zero-dimensional nanoparticles and one-dimensional nanofibers. They can not only enhance structural stability through self-overlapping but also introduce nanopore structures to effectively suppress heat transfer^[77]. Liu et al.^[78] used Al₂O₃ nanorod sol as the precursor, mixed with tetraethyl orthosilicate and PVA solution thoroughly, and prepared Si-doped Al₂O₃ nanorod aerogels (SARAs) via freeze-drying and thermal treatment, as shown in Fig. 6. The study shows that this aerogel has a volume density as low as 0.008 g/cm³, a room temperature thermal conductivity of 0.0246 W/(m·K), and a thermal conductivity of 0.0949 W/(m·K) at 1000 ℃. The aerogel exhibits excellent thermal stability and can withstand high temperatures up to 1400 ℃; however, when reaching 1400 ℃, its linear shrinkage rate is 14.13%, and its specific surface area decreases from 118 m²/g to 39.12 m²/g.

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图6 (a) Si掺杂Al₂O₃纳米棒气凝胶制备示意图；（b） Si掺杂Al₂O₃纳米棒气凝胶在干燥后和在不同温度下处理后的照片；（c） Si掺杂Al₂O₃纳米棒气凝胶在1400 ℃下的SEM图像^[78]

Fig. 6 (a) Schematic diagram of preparation of Si doped Al₂O₃ nanorod aerogels; (b) Macroscopic photographs of Si doped Al₂O₃ nanorod aerogel after drying and after treatment at different temperatures; (c) SEM image of Si doped Al₂O₃ nanorod aerogel at 1400 ℃ ^[78]

Xu et al.^[79]prepared a sol with the ability to spin into nanofibers using aluminum sec-butoxide as the aluminum source, sec-butoxide as the solvent, and adding tetraethyl orthosilicate, acetylacetone, and dimethylformamide. Nanofiber membranes were then obtained through electrospinning and thermal treatment. The nanofiber membrane was immersed and stacked in an aluminosilicate binder solution composed of aluminum chloride, isopropyl aluminum, tetraethyl orthosilicate, and SiO₂ aerogel particles as the upper layer, and in an aluminosilicate binder solution without SiO₂ aerogel particles as the lower layer. After freeze-drying and high-temperature thermal treatment, a SiO₂-bonded Al₂O₃ nanofiber ceramic aerogel was prepared, as shown in Fig. 7. Research shows that the thermal conductivity of this aerogel is approximately 0.028 W/(m·K). Due to the effective protection of inert inorganic molecular chains in the sol, the reaction between alumina and silica was avoided, successfully delaying the α-phase transition temperature of Al₂O₃ to 1400 ℃. During the electrospinning process, the rapid hydrolysis and condensation of molecular chains formed stable chemical crosslinks at the contact points of the nanofibers after curing, giving the prepared aerogel excellent flexibility. When the strain was 50%, the aerogel could still recover to its initial shape after being burned in a high-temperature flame of 1700 ℃ for 10 minutes, without structural damage; after burning for 30 minutes, due to the increase in grain size, the microstructure of the aerogel changed slightly, with a volume shrinkage rate of about 15%; after 60 minutes, due to the continuous growth of grain size, the microstructure of the aerogel was destroyed.

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图7 (a) 陶瓷气凝胶的制备工艺；陶瓷气凝胶（b）上半层和（c）下半层的微观结构^[79]

Fig. 7 (a) Preparation process of ceramic meta-aerogel. Microstructure of (b) upper half layer and (c) lower half layer of ceramic meta-aerogel^[79]

Alkaline earth metals, rare earth elements, and others can also react with Al₂O₃ in the solid phase to form compounds that inhibit the α-phase transformation of Al₂O₃, increase its sintering temperature, and thereby enhance the thermal stability of Al₂O₃ aerogels. Sun et al.^[80] prepared Sr-doped Al₂O₃ aerogels using hexahydrate aluminum chloride and strontium nitrate as precursors via supercritical drying. The study showed that after heat treatment at 1200 ℃, a SrO·6Al₂O₃ phase formed in the aerogel with 5% (mass fraction) SrO doping, inhibiting the Al₂O₃ phase transition. This Sr-doped Al₂O₃ aerogel had a specific surface area of 122 m²/g, which was 113 m²/g higher than that of pure Al₂O₃ aerogels under the same conditions. Zhou Jiejie et al.^[81] prepared Y₂O₃-doped Al₂O₃ aerogels using hexahydrate aluminum chloride and hexahydrate yttrium chloride as precursors via supercritical drying. The study indicated that after heat treatment at 1000 ℃, the amorphous structure of the 5.0% Y₂O₃-Al₂O₃ aerogel remained unchanged without phase transition, with a specific surface area of 380–400 m²/g, significantly higher than that of pure Al₂O₃ aerogels (174 m²/g). The doping of Y₂O₃ enhanced the high-temperature specific surface area and thermal stability of Al₂O₃ aerogels.

Dopant elements suppress the structural collapse of Al₂O₃ aerogels caused by α-phase transition at 1000 ℃, and the usage temperature of Al₂O₃ aerogels can reach 1200 ℃.

4.3.2 Fiber-reinforced Al₂O₃ aerogel

Introducing aerogel into the fiber felt can reduce the fiber-to-fiber contact heat transfer, and most of the solid heat transfer occurs through the aerogel with low thermal conductivity. In addition, the aerogel also fills the gaps between fibers, inhibiting the gas heat transfer. Moreover, as shown in Figure 8, the addition of fibers can enhance the strength of Al₂O₃ aerogel.

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图8 纤维-纤维和气凝胶-纤维传热模型^[82]

Fig. 8 Heat transfer models of fiber and aerogel-fiber^[82]

Wen et al.^[83] used nonahydrated aluminum nitrate as the precursor, and first prepared Al₂O₃ aerogel by supercritical drying method. Then, the Al₂O₃ aerogel was added to SiO₂ fiber felt to fabricate an Al₂O₃-SiO₂ fiber composite material. The study showed that the thermal conductivity of the composite material was 0.028 W/(m·K) at 35 ℃ and 0.033 W/(m·K) at 600 ℃, the crystalline transition temperature was 900 ℃, the Young's modulus was 6 MPa, and it also had hydrophobicity (~136°).

Sun Jingjing et al.^[84] prepared Al₂O₃ aerogel composite high-temperature thermal insulation tiles using hexahydrate aluminum chloride as the precursor and mullite fiber as the reinforcing phase by sol-gel and supercritical drying. The study showed that at room temperature, the compressive strength of the composite thermal insulation tile was 1.48 MPa, and the thermal conductivity was 0.058 W/(m·K) at 200 ℃. After heat treatment at 1400 ℃ for 30 minutes, the linear shrinkage was only 2%, and the thermal conductivity was 0.152 W/(m·K).

4.3.3 Al₂O₃ Aerogel with Light-Blocking Agent

The thermal conductivities of Al₂O₃ aerogel are very low at room temperature for both solid-phase and gas-phase heat transfer, but its high infrared radiation transmittance at high temperatures [^[85]], so it is necessary to reduce the high-temperature thermal conductivity of Al₂O₃ aerogel by adding opacifiers.

Titanium dioxide has strong infrared extinction ability at wavelengths below 8 μm and can reduce radiative heat conduction at higher temperatures. Gao et al.^[86] prepared Al₂O₃ aerogels and TiO₂-Al₂O₃ aerogels with spherical TiO₂ particles at a molar fraction of 10% as additives, using hexahydrate aluminum chloride and butyl titanate as precursors and propylene oxide as a gelation agent through supercritical drying. The study showed that the specific surface area of the aerogel doped with spherical TiO₂ particles was 650 m²/g, which was greater than that of pure Al₂O₃ aerogels, which was 326 m²/g. The thermal conductivity of the composite aerogel at 1000 °C was 0.136 W/(m·K). After heat treatment, the spherical particles transformed into fibrous particles, thereby improving the strength of the aerogel.

The introduction of fibers can support the skeleton structure of Al₂O₃ aerogel at high temperatures, and the addition of a light shield agent can reduce radiative heat conduction at higher temperatures, which effectively reduces the high-temperature thermal conductivity of Al₂O₃ aerogel. However, it also increases the solid-phase heat conduction to some extent, resulting in a slight increase in the room-temperature thermal conductivity of the aerogel.

Liu et al.^[77] used aluminum isopropoxide as the aluminum source and water and acetic acid as solvents. Aluminum oxide nanorod sol was first synthesized via a hydrothermal method, which was then added to resorcinol-formaldehyde (RF) sol composed of resorcinol, formaldehyde, sodium carbonate, and water. After gelation, aging, solvent exchange, supercritical drying, and carbonization in an argon atmosphere, carbon-coated Al₂O₃ nanorod aerogels were successfully prepared, as shown in Fig. 9. The results showed that the aerogel had a high porosity of 95.4% and a low bulk density of 0.086 g/cm³, withstanding temperatures of up to 1500 ℃ in an argon atmosphere and 1400 ℃ in air. It exhibited excellent high-temperature thermal stability and heat insulation performance (room temperature thermal conductivity of 0.037 W/(m·K), 0.065 W/(m·K) at 1200 ℃). Additionally, the aerogel had excellent specific strength (specific compressive strength of 69.83 kN·m/kg), wear resistance, and superhydrophobicity (water contact angle of 156° after 1000 wear cycles). The introduction of the carbon layer not only reduced the contact between nanorods to prevent sintering but also stabilized the grain size and alleviated the α phase transition of Al₂O₃, thus fully preserving the three-dimensional porous network structure of the aerogel.

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图9 (a) RF包覆Al₂O₃纳米棒气凝胶、碳包覆Al₂O₃纳米棒气凝胶和Al₂O₃纳米棒气凝胶制备示意图；（b） Al₂O₃纳米棒和RF层之间的强耦合界面示意图；（c） RF包覆Al₂O₃纳米棒气凝胶，（d）碳包覆Al₂O₃纳米棒气凝胶和 (e) Al₂O₃纳米棒气凝胶的SEM图像^[77]

Fig. 9 (a) Preparation schematic diagram of RF coated Al₂O₃ nanorod aerogel, carbon coated Al₂O₃ nanorod aerogel and Al₂O₃ nanorod aerogel. (b) Schematic illustration of the strong interfacial coupling between Al₂O₃ nanorods and RF layers. (c) SEM images of RF coated Al₂O₃ nanorod aerogels, (d) carbon coated Al₂O₃ nanorod aerogels and (e) Al₂O₃ nanorod aerogels^[77]

In summary, researchers have improved the high-temperature thermal insulation performance and mechanical properties of Al₂O₃ aerogels through element doping modification, fiber compositing, preparing precursors into fibers, adding light-shielding agents, carbon coating, and other methods. However, further adjustment of the micro-morphology, pore structure, types of fibers, and light-shielding agents in Al₂O₃ aerogels is still needed to regulate the heat transfer mechanism and ultimately enhance the high-temperature thermal insulation performance and mechanical properties of Al₂O₃ aerogels.

5 ZrO₂ Aerogel

ZrO₂ aerogels are three-dimensional nanoscale porous materials assembled from randomly organized ZrO₂ nanoparticles. ZrO₂ has a high melting point of 2715 ℃, low thermal conductivity, and active centers for acid and base reactions^[87]. The three-dimensional structure formed by assembling ZrO₂ nanoparticles, namely ZrO₂ aerogels, exhibits excellent properties such as corrosion resistance, wear resistance, and superior high-temperature electrical conductivity^[88]. For more information on the preparation methods and partial characterization of ZrO₂ aerogels, see Table 3.

表3 ZrO₂气凝胶的制备方法与性能^a)

Table 3 Preparation method and properties of ZrO₂ aerogel^a)

Precursor	Drying method	Modified way	Service temperature /℃	Specific surface area/(m²/g)	Thermal conductivity/ (W/(m·K))	Ref.
ZBO	Supercritical drying	-	500	178 (RT)	-	90
(C₅H₈O₂)₄·Zr		Doped with SiO₂	1000	-	0.026 (600 ℃) 0.037 (800 ℃) 0.058 (1000 ℃)	91
ZrO(NO₃)₂		Introducing formamide	800	514.5 (RT)	-	93
ZrOCl₂·8H₂O		Electrolytic method	-	640 (RT)	-	94
ZrO(NO₃)₂		-	1000	223 (1000 ℃)	-	96
ZrO(NO₃)₂		Alcohol water heating method	1000	675.6 (RT)	-	97
ZrOCl₂		Doped with La	1200	107(1000 ℃)		106
ZrO(NO₃)₂	Atmospheric pressure drying	-	-	645 (RT)	-	98
ZrO(NO₃)₂	Atmospheric pressure drying	Add formamide and heat with alcohol and water	-	619 (RT)	-	99
ZrO(NO₃)₂	Freeze-drying	Gel casting process	900	-	-	101

a) ZBO：N-butanol zirconium；RT：Room Temperature

5.1 ZrO₂Precursor of Aerogel

5.1.1 Organic Precursor

ZrO₂The commonly used organic precursors for aerogels are zirconium acetylacetonate^[89]and zirconium n-butoxide^[90], and the addition of an acidic catalyst^[91]can effectively regulate the rate of hydrolysis and condensation in the sol-gel process.

Zhu et al.^[90] used zirconyl butanoate as precursor and prepared ZrO₂ aerogel by supercritical ethanol drying. The research results show that the specific surface area of this ZrO₂ aerogel is 178 m²/g, and the highest operating temperature is 500 ℃.

However, metal alcoholates are expensive, and it is difficult to control the hydrolysis and condensation rate of zirconium alcoholate precursors. Affected by temperature, it is difficult to obtain a uniform network structure, which reduces the physical properties of ZrO₂ aerogels.

5.1.2 Inorganic Precursor

Inorganic precursors need to be prepared using epoxides as gel aids to produce ZrO₂ aerogels^[92], and the epoxides can form stable ZrO₂ sols with protons in Zr⁴⁺ complexes, ultimately forming a ZrO₂ gel network.

Zhu et al.^[93] prepared ZrO₂ aerogels using zirconyl nitrate as the raw material, yttrium nitrate as the stabilizer, a mixed solvent of alcohol and water, formamide to adjust the pore size of the gel, and propylene oxide as the gel promoter by means of supercritical drying. The study showed that the specific surface area of the obtained ZrO₂ aerogel was 514.5 m²/g, and it could withstand a temperature up to 800 ℃.

The reaction process for preparing aerogels by electrochemistry combined with supercritical drying is milder than other methods, and it is easy to form aerogels with high specific surface area^[94]. Zhao et al.^[94] prepared ZrO₂ aerogels by room-temperature electrolysis of zirconium chloroxide solution combined with the supercritical drying process. The study showed that the specific surface area of this ZrO₂ aerogel was 640 m²/g, and the average pore size was 9.7 nm.

However, due to insufficient solvent exchange, the residual anions (Cl^- and/or NO₃^-) are harmful to both the supercritical drying reactor and human health^[95]. Therefore, researchers combined organic precursors with epoxides to prepare ZrO₂ aerogels by supercritical drying. Liu et al.^[96] synthesized ZrO₂ aerogels using zirconium acetacetonate as the precursor and propylene oxide as the gelation agent. Studies have shown that after being treated at 1000 ℃, this ZrO₂ aerogel has a specific surface area of 236 m²/g and exhibits good thermal stability.

Currently, the most commonly used precursor for preparing ZrO₂ aerogels is zirconyl nitrate. The specific surface area of ZrO₂ aerogels prepared by it can reach above 500 m²/g.

5.2 ZrO₂
Structural Regulation of Aerogels

The ZrO₂ aerogels prepared by traditional preparation methods have poor high-temperature performance, so it is necessary to develop other more effective methods for preparing ZrO₂ aerogels.

The ZrO₂ aerogel prepared by the alcohol-water heating method has a high specific surface area and fine, uniform particles^[97]. Wu Zhigang et al.^[97] used zirconyl nitrate as the precursor, added an appropriate amount of redistilled deionized water and anhydrous ethanol to dissolve and form a mixed solution, which was refluxed in a constant-temperature water bath at 80 ℃, and prepared the ZrO₂ aerogel by supercritical drying method. Studies have shown that the specific surface area of this ZrO₂ aerogel is 675.6 m²/g, and the proportion of tetragonal phase is 86% at 700 ℃; at 1000 ℃, the particle size of the ZrO₂ aerogel is greater than 30 nm, and 30% of ZrO₂ is tetragonal phase.

ZrO₂ aerogels prepared by supercritical drying are usually in the form of small blocks, particles or even powders, which are extremely difficult to shape. Therefore, the development of ambient pressure drying technology has become an important direction for research and application of ZrO₂ aerogels^[98]. Guo Xingzhong et al.^[98] prepared ZrO₂ aerogels by using zirconium nitrate as precursor and propylene oxide as gelation agent through ambient pressure drying. The study showed that the specific surface area of this ZrO₂ aerogel was 645.0 m²/g.

Li-Qing Yan et al.^[99] prepared ZrO₂ aerogels by using zirconyl nitrate as the precursor, propylene oxide as the gel promoter, formamide as the drying control chemical additive, and tetraethyl orthosilicate ethanol solution as the gel surface modifier solution via atmospheric pressure drying. The study showed that the ZrO₂ aerogel prepared under the process conditions of Zr : formamide : propylene oxide = 1:1:12 and alcohol-water ratio = 3:1 had a specific surface area of 619 m²/g.

The freeze gel forming process is a method that combines gel casting and freeze forming^[100]. Shen Lin et al.^[101] used zirconium nitrate as the precursor and adopted an acid-base two-step catalytic method, combining the advantages of aqueous gel casting and freeze forming methods. ZrO₂ aerogels were prepared using the freeze gel forming process and freeze drying method. Studies have shown that this ZrO₂ aerogel can withstand up to 900 ℃.

The high melting point and low thermal conductivity of ZrO₂ make it possible to prepare ZrO₂ aerogels with good high-temperature heat insulation performance, but the current pure ZrO₂ aerogel still has a service temperature below 1000 ℃, which cannot meet the requirements of the aerospace field.

5.3 Composite ZrO₂ Aerogel

ZrO₂ has three crystal forms: monoclinic zirconia (m-ZrO₂), tetragonal zirconia (t-ZrO₂), and cubic zirconia (c-ZrO₂). During the heat treatment process, a phase transformation occurs from m-ZrO₂

to t-ZrO₂

to c-ZrO₂

and finally to liquid-phase ZrO₂ phase transition [¹⁰²]. Tetragonal ZrO₂ has advantages such as low thermal conductivity and high mechanical strength. During the phase transition from the monoclinic ZrO₂ to tetragonal ZrO₂, compressive stress and tensile stress are generated, which cannot be offset by the elastic deformation and plastic deformation of the matrix, leading to cracking of ZrO₂ aerogels, severely limiting the application of ZrO₂ aerogels.

Adding Y₂O₃, MgO, CaO, and rare earth elements, etc. to ZrO₂ aerogel can make it partially or fully stabilized cubic ZrO₂ or tetragonal ZrO₂ [103 ⇓~105].

Wei Chu et al.^[106] prepared La-ZrO₂ composite aerogels doped with 10 wt% lanthanum using zirconium oxychloride and lanthanum chloride as precursors via supercritical ethanol drying. The study showed that after heat treatment at 1000 ℃, the linear shrinkage of the composite aerogel was 25%, the specific surface area was 107 m²/g, and the grain size was 8.6 nm, all of which were superior to those of pure ZrO₂ aerogels. After heat treatment at 1200 ℃, it remained tetragonal ZrO₂ without transformation into monoclinic phase. Lanthanum doping could effectively suppress the crystallization, growth, and phase transition of ZrO₂ particles, improving their thermal stability.

The oxygen vacancies generated by Y₂O₃, although they can stabilize the tetragonal phase of ZrO₂, also promote the sintering of ZrO₂. Introducing SiO₂ into zirconia does not cause the generation of oxygen vacancies. Therefore, Liu et al.^[107] prepared ZrO₂-SiO₂ aerogels doped with 14% SiO₂ mass fraction using zirconium acetylacetonate and ethyl orthosilicate as precursors through supercritical drying. The study showed that the thermal conductivities of the prepared ZrO₂-SiO₂ aerogels at 600, 800, and 1000 ℃ were (0.026 ± 0.001), (0.037 ± 0.001), and (0.058 ± 0.002) W/(m·K), respectively.

At present, the research on ZrO₂ aerogels is less than that on SiO₂ aerogels and Al₂O₃ aerogels. However, ZrO₂ has the characteristics of high melting point and low phonon scattering, which gives it better high-temperature thermal insulation performance. Currently, the thermal stability of the prepared ZrO₂ aerogel materials is generally poor, and the characteristics of this material have not been fully utilized, with relatively few applications in high-temperature thermal insulation, and the mechanism study of thermal stability is also scarce. Therefore, further exploration of the mechanism of thermal stability of ZrO₂ aerogel materials and the utilization of their properties are of great research value.

6 Binary and Multicomponent Oxide Aerogels

The porous structure of single-component oxide aerogels is very fragile, and the high-temperature phase transition further hinders its application at elevated temperatures. Therefore, binary and multi-component oxide aerogels have become the focus of researchers' attention. This not only can effectively improve their stability at high temperatures but also endows oxide aerogels with characteristics such as elasticity and processability. For the preparation methods and some properties of various binary and multi-component oxide aerogels, please refer to Table 4.

表4 二元和多元氧化物气凝胶的制备方法与性能^a)

Table 4 Preparation method and properties of two component and multi-component oxide aerogel^a)

Precursor	Drying method	Modified way	Service temperature/℃	Specific surface area/ (m²/g)	Thermal conductivity/ (W/(m·K))	Ref.
ASB、TEOS	Supercritical drying	-	1200	97~116 (1200 ℃)	-	108
Water glass、AlCl₃		Al:Si=0.37	1200	613 (RT) 11.8 (1000 ℃)	0.029 (RT) 0.121 (1200 ℃)	109
ASB、TEOS		Mullite fibers impregnated with SiC coating	1000	-	0.049 (1000 ℃)	110
AlCl₃·6H₂O、TEOS		Impregnated ZrO₂ fibers	800	-	0.049 (RT)	111
TEOS、Al(NO₃)₃·9H₂O		Aluminum silicate fiber reinforcement	1200	600 (RT) 40 (1300 ℃)	0.026 (RT)	112
ZrOCl₂·8H₂O、TEOS		Supercritical fluid deposition	1000	551 (RT) 50 (1300 ℃)	-	113
ZrOCl₂、TEOS		ZrO₂ fiber reinforcement	-	-	0.0235~0.0296 (RT)	114
ZrOCl₂、TEOS		Impregnated mullite fiber	1200	-	0.0524 (RT)	115
ZrOCl₂·8H₂O、TEOS		Multiple impregnation of ZrO₂-SiO₂ sol	-	-	0.0231~0.0306 (RT)	116
TEOS、AlCl₃·6H₂O、ZrOCl₂·8H₂O		-	800	-	0.05 (RT) 0.26 (1000 ℃)	117
TEOS、AlCl₃·6H₂O、MgCl₂·6H₂O		-	800	-	0.06 (RT) 0.3 (1000 ℃)	119
TEOS、AlCl₃·6H₂O、ZrOCl₂·8H₂O、MgCl₂·6H₂O		-	1200	597.22 (RT) 358.53 (1000 ℃) 84.44 (1200 ℃)	0.03 (RT)	119
AlCl₃·6H₂O、AIP、C₈H₁₂O₈·Zr	Freeze-drying	Gel casting process	1300	-	0.1602~0.1623 (RT)	118
AlCl₃·6H₂O、AIP、Zr(CO₃)₂	-	electrospinning	1300	-	0.03166 (RT)	117

a) ZBO：N-butanol zirconium；AIP：Aluminum isopropoxide；TEOS：Ethyl orthosilicate；RT：Room Temperature

6.1 Binary Oxide Aerogels

Currently, common binary oxide aerogels include Al₂O₃-SiO₂ aerogels, ZrO₂-SiO₂ aerogels, and Al₂O₃-ZrO₂ aerogels, etc.

6.1.1 Al$_{2}$O$_{3}$-SiO$_{2}$ aerogel

Introducing SiO2 into Al2O3 aerogel improves the high-temperature stability of Al2O3 and makes up for the defect of low usage temperature of SiO2 aerogel. The Al2O3-SiO2 material system has been extensively studied.

Jian Feng et al.^[108] prepared Al₂O₂-SiO₂ aerogels using aluminum tert-butoxide and tetraethyl orthosilicate as precursors through supercritical ethanol drying. The study showed that when the silicon mass fraction in the aerogel was 6.1%-13.1%, the specific surface area after heat treatment at 1200℃ was 97-116 m²/g. When the temperature exceeded 600℃, the Al₂O₃-SiO₂ aerogel transformed from crystalline boehmite and amorphous SiO₂ to amorphous γ-Al₂O₃ and SiO₂. When the temperature exceeded 1200℃, it further transformed into mullite. In the Al₂O₃-SiO₂ aerogel structure, there were simultaneously Al-O-Al, Si-O-Si, and Al-O-Si. The addition of Si changed the morphology of the Al₂O₃ aerogel, and as the Si content increased, the aerogel gradually transformed from needle-like or elongated shapes to spherical particles of SiO₂.

Xia et al.^[109] prepared Al₂O₂-SiO₂ aerogels using sodium silicate and aluminum chloride as precursors by supercritical drying. The study showed that the Al₂O₃-SiO₂ aerogel with an Al/Si molar ratio of 0.37 had a specific surface area of 613 m²/g at room temperature and a thermal conductivity of 0.029 W/(m·K); after calcination at 1000 ℃, the specific surface area was 11.8 m²/g, and after calcination at 1200 ℃, the thermal conductivity was 0.121 W/(m·K). In the heat treatment process, mullite was generated, which improved the thermal stability.

Xu et al.^[110] synthesized an Al:Si = 3:1 Al₂O₃-SiO₂ sol using tetraethyl orthosilicate and secondary butyl aluminum alcohol as precursors, ethanol as solvent, and acetic acid as catalyst; then, adding ethyl acetoacetate to control the hydrolysis rate, the Al₂O₃-SiO₂ sol was used to impregnate the SiC-coated mullite fibers under vacuum conditions, as shown in Figure 10; after the sol solidified, the wet gel was aged at room temperature for 48 hours and then supercritically dried to prepare an opaque Al₂O₃-SiO₂ aerogel composite material. Studies have shown that the thermal conductivity of this Al₂O₃-SiO₂ aerogel composite material is 0.049 W/(m·K) at 1000 ℃, and the mullite fiber effectively blocks the infrared radiation of the composite material.

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图10 SiC包覆莫来石纤维的SEM图像^[110]

Fig. 10 SEM images of SiC-coated mullite fiber^[110]

The thermal stability of ZrO₂ fibers is better than that of SiO₂ fibers, and its thermal conductivity is lower than that of mullite fibers. Zhang et al.^[111] prepared Al₂O₃-SiO₂ sol by using hexahydrate aluminum chloride and tetraethyl orthosilicate as precursors, ethanol and propylene oxide as solvent and catalyst, respectively; then the Al₂O₃-SiO₂ sol was impregnated into porous ZrO₂ fiber network under vacuum atmosphere, and finally Al₂O₃-SiO₂ aerogel/ZrO₂ fiber composites with lamellar microstructure and crack-free were obtained by supercritical ethanol drying and vacuum extrusion forming technology. The research shows that the compressive strength of this composite material is 1.22 MPa in the z direction and the thermal conductivity is 0.049 W/(m·K).

Alumina-silicate nanofibers possess advantages such as ultralight weight, high-temperature resistance, and excellent mechanical properties. Hao et al.^[112] prepared alumina-silicate nanofibers with a diameter of 170 nm using tetraethyl orthosilicate, isopropyl aluminum, and nonahydrate aluminum nitrate as precursors through electrospinning. Then, by adding alumina-silicate nanofibers as precursors along with tetraethyl orthosilicate and nonahydrate aluminum nitrate, they fabricated nanofiber-reinforced Al₂O₃-SiO₂ aerogels via supercritical drying. Research shows that the compressive strength of this nanofiber-reinforced Al₂O₃-SiO₂ aerogel (0.44 MPa) is higher than that of pure Al₂O₃-SiO₂ aerogel (0.16 MPa). The specific surface area is 600 m²/g, the bulk density is 0.15 g/cm³, and the thermal conductivity is 0.026 W/(m·K). The addition of alumina-silicate nanofibers slows down the mullite formation of alumina and silica. When the heat treatment temperature rises to 1200 ℃, the specific surface area is 40 m²/g, and the alumina-silicate nanofibers play a supporting role in the structure, thereby slowing down sintering.

6.1.2 ZrO₂-SiO₂aerogel

ZrO₂ has higher heat resistance and can be used as a light shield for SiO₂ aerogel; besides, SiO₂ can effectively restrict the crystal growth and phase transformation of ZrO₂. However, the nanoscale porous structure of ZrO₂-SiO₂ aerogel is weak and will collapse and break under relatively low stress.

Wang et al.^[113] used zirconyl chloride octahydrate as the precursor and citric acid as a green organic acid gelation agent, and added different amounts of tetraethyl orthosilicate during the supercritical drying process. ZrO₂-SiO₂ composite aerogels were prepared by the supercritical fluid deposition method. The study showed that the composite aerogel had a specific surface area of 551 m²/g, remained amorphous phase at 800 ℃, transformed into tetragonal phase at 1000 ℃ with a specific surface area of 50 m²/g.

Hou et al.^[114] used tetraethyl orthosilicate and zirconium oxychloride as precursors to disperse ZrO₂ fibers in a ZrO₂-SiO₂ solution with a molar ratio of ZrO₂ to SiO₂ of 1:1. Supercritical drying was employed to prepare fiber-reinforced ZrO₂-SiO₂ aerogel composites. Studies have shown that the composite aerogels with fiber mass fractions ranging from 0 to 15% possess low density (0.16–0.33 g/cm³), low thermal conductivity (0.0235–0.0296 W/(m·K)), and high compressive strength (0.36–0.82 MPa). The addition of ZrO₂ fibers effectively enhanced their nanostructure.

He et al.^[115] used mullite fiber as the matrix and zirconia-silica aerogel prepared with tetraethyl orthosilicate and zirconium oxychloride as precursors as filler, and prepared a novel aerogel/fiber ceramic composite material by vacuum impregnation and supercritical drying methods. Research shows that the compressive strength of this aerogel/fiber ceramic composite material is 1.05 MPa, about twice that of mullite fiber and ten times that of pure aerogel; the room temperature thermal conductivity is 0.0524 W/(m·K), and it has good thermal insulation performance at 500-1200 ℃.

Reinforcement materials such as fibers and whiskers are typically dispersed in the precursor of aerogels through mechanical stirring, which leads to non-uniform distribution during the gelation process and easily disrupts the continuous nanoscale structure of the aerogels. Moreover, the difference in shrinkage rates between the reinforcement materials and aerogels at high temperatures can increase the occurrence of cracks and defects. Hou et al.^[116] used tetraethyl orthosilicate and octahydrate zirconium oxychloride as silicon and zirconium precursors, respectively, and propylene oxide as a gelation agent. They repeatedly immersed the ZrO₂-SiO₂ wet gel into the ZrO₂-SiO₂ sol, and prepared self-reinforced ZrO₂-SiO₂ aerogels via supercritical drying method, as shown in Figure 11. The study shows that with the increase in the number of immersions, the density of the aerogel increases from 0.16 g/cm³ to 0.46 g/cm³, and the mechanical strength rises from 0.51 MPa to 3 MPa. Meanwhile, the thermal conductivity first decreases from 0.0235 W/(m·K) to 0.0231 W/(m·K), then increases to 0.0306 W/(m·K). Multiple immersions can effectively eliminate large pores and defects within the aerogel matrix, improving the mechanical properties of the ZrO₂-SiO₂ aerogels.

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图11 (a) 浸渍法制备ZrO₂-SiO₂气凝胶工艺流程图；不同浸渍次数的0次（b），1次（c），2次（d），3次（e）自增强ZrO₂-SiO₂气凝胶SEM图像^[116]

Fig. 11 (a) Flow chart of the preparation processing for ZrO₂-SiO₂ aerogel fabricated by impregnation method.SEM of the nanostructure self-reinforcing ZrO₂-SiO₂ aerogels with different impregnation times. (b) ZrO₂-SiO₂-0; (c): ZrO₂-SiO₂-1; (d): ZrO₂-SiO₂-2; (e): ZrO₂-SiO₂-3^[116]

6.1.3 Al$_{2}$ O$_{3}$ -ZrO$_{2}$ aerogel

The high-temperature phase transition of Al₂O₃ aerogel and ZrO₂ aerogel limits their high-temperature performance, but the composite of the two can prepare Al₂O₃-ZrO₂ aerogels with better high-temperature performance.

Li et al.^[117] prepared Al₂O₃-ZrO₂ nanofiber aerogels using aluminum chloride hexahydrate, isopropyl alcohol aluminum, and zirconium carbonate as precursors by electrospinning method. The study showed that the Al₂O₃-ZrO₂ nanofiber aerogel had a thermal conductivity of 0.03166 W/(m·K) at room temperature and a bulk density of 0.01721 g/cm³. This Al₂O₃-ZrO₂ nanofiber aerogel could maintain flexibility in the temperature range of -196 to 1300 ℃ and showed no structural damage after 10 minutes under butane torch flame, demonstrating excellent high-temperature thermal insulation performance.

However, the aerogels prepared only with two kinds of oxides have relatively weak bonding strength, which leads to the degradation of the aerogel stability. Disodium hydrogen phosphate is widely used as a high-temperature adhesive in ceramic industry and fireproof material manufacturing. When nanofibers are heat-treated in an oxygen atmosphere, disodium hydrogen phosphate can form an alumino-phosphate network (Al-O-P-O-Al) between the nanofibers, thereby enhancing the high-temperature stability of ceramic materials^[118]. GaO et al.^[118] successfully prepared ZrO₂-Al₂O₃ composite aerogels by using disodium hydrogen phosphate as the high-temperature binder, aluminum chloride hexahydrate, isopropyl aluminum, and zirconium acetate as precursors, and adopting the freeze-drying method. Studies show that the thermal conductivity and thermal diffusivity of this ZrO₂-Al₂O₃ composite aerogel are 0.1602-0.1623 W/(m·K) and 0.3488-0.4702 mm²/s respectively. After being exposed to a flame at 1300 ℃ for 15 minutes, the 1.4 cm thick composite aerogel can quickly cool down to 32.7-38.4 ℃ within 60-70 seconds. With the increase of disodium hydrogen phosphate mass fraction from 0.5% to 1.5%, the thermal conductivity and thermal diffusivity first increase and then decrease. The aerogel produces stable phosphate products AlPO₄ and ZrP₂O₇, improving its high-temperature resistance and flame retardancy; the formed polyphosphate network (Al-O-P-O-Al) enhances the cross-linking property, as shown in Fig. 12.

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图12 不同AHP含量的ZrO₂-Al₂O₃复合气凝胶涂层^[118]

Fig. 12 Preparation of ZrO₂-Al₂O₃ composite aerogel coating with different AHP content^[118]

6.2 Multicomponent Oxide Aerogels

With the continuous development of binary aerogels, researchers have found that ternary and quaternary aerogels can also improve high-temperature resistance performance.

Liu Zuhansuperscript[119] used tetraethyl orthosilicate, aluminum chloride hexahydrate, zirconyl chloride octahydrate, and magnesium chloride hexahydrate as precursors, propylene oxide as a gelation agent, and prepared ternary composite aerogels of Si∶Al∶Zr=3∶12∶1 (SiO₂-Al₂O₃-ZrO₂), Si∶Al∶Mg=1∶2∶4 (SiO₂-Al₂O₃-MgO), and Si∶Al∶Mg∶Zr=3∶9∶12∶1 (SiO₂-Al₂O₃-MgO-ZrO₂) through supercritical drying. The research showed that the density of the SiO₂-Al₂O₃-ZrO₂ ternary composite aerogel was 0.09 g/cm³, and its thermal conductivity was 0.05 W/(m·K). After heat treatment at 800℃, the linear shrinkage rate was 5.2%; after heat treatment at 1000℃, the linear shrinkage rate was 42.4%, and the thermal conductivity was 0.26 W/(m·K); after heat treatment at 1200℃, the linear shrinkage rate reached 53.6%. The thermal conductivity of the SiO₂-Al₂O₃-MgO ternary composite aerogel was 0.06 W/(m·K). After heat treatment at 800℃, the linear shrinkage rate was 3.6%; after heat treatment at 1000℃, the linear shrinkage rate was 45.8%, and the thermal conductivity was 0.3 W/(m·K); after heat treatment at 1200℃, the shrinkage rate was 61.7%. The specific surface area of the SiO₂-Al₂O₃-MgO-ZrO₂ quaternary composite aerogel was 597.22 m²/g, and its thermal conductivity was 0.03 W/(m·K). After heat treatment at 800℃, the linear shrinkage rate was 7%; after heat treatment at 1000℃, the linear shrinkage rate was 21%, and the specific surface area was 358.53 m²/g; after heat treatment at 1200℃, the shrinkage rate was 34%, and the specific surface area was still 84.44 m²/g. The shrinkage rate of the quaternary composite aerogel was lower than that of the two ternary composite aerogels.

In summary, the binary and ternary aerogels improve the high-temperature thermal insulation performance of oxide aerogels. However, the maximum operating temperature is still around 1200 ℃. Even if the types and amounts of elements are adjusted, the effect is still limited. Adding reinforcement phases and regulating microstructures are the key to improving the high-temperature thermal insulation properties of binary and ternary aerogels.

7 Summary and Prospect

Oxide aerogels have been widely used in high-temperature thermal insulation materials because of their unique advantages. However, they suffer from poor mechanical properties, particle sintering, phase transformation under high temperature leading to structural collapse, and degradation of thermal insulation performance due to water absorption in humid environments. Researchers have alleviated these problems through methods such as doping, fiber reinforcement, adding opacifiers, hydrophobic modification, and preparing binary and multiple oxide aerogels. Although certain achievements have been made, many issues remain in the preparation of high-performance high-temperature thermal insulation oxide aerogels. The future development direction and research focus of high-temperature thermal insulation oxide aerogels should concentrate on the following six aspects:

(1) The mechanical properties of oxide aerogels are relatively poor, and they need to be combined with appropriate fiber materials or other reinforcing phases to overcome this deficiency.

(2) The maximum operating temperature of oxide aerogels still needs to be further increased, and their highest temperature resistance can be improved by microstructural design and by the method of composite with other materials.

(3) Al₂O₃, ZrO₂, etc. will undergo phase transformation at high temperature. During the phase transformation process, volume contraction and expansion will occur, leading to a decrease in the high-temperature thermal insulation performance of the material. Additives need to be added to increase the phase transition temperature so that it becomes a structurally stable aerogel material.

(4) Oxide aerogel particles are prone to sintering. It is necessary to change their particle microstructure and develop oxide aerogels with excellent high-temperature thermal insulation performance.

(5) Partially oxidized aerogels are highly susceptible to moisture absorption in humid environments, which leads to an increase in thermal conductivity and a decrease in thermal insulation performance. To address this issue, it is necessary to prepare hydrophobic oxidized aerogels.

(6) The precursor types and ratios, drying methods, and preparation conditions of oxide aerogels can significantly affect their performance. More exploration is needed to develop new preparation methods for the production of oxide aerogels with superior performance.

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Abstract

Cite this article

1 Introduction

2 Preparation of Oxide Aerogels

2.1 Preparation of Oxide Aerogels

图1 (a) 溶胶-凝胶法；（b）环氧化物添加法；（c）无机溶胶-凝胶法[15]

2.1.1 Sol-gel method

2.1.2 The Epoxidized Oil Addition Method

2.1.3 Inorganic Sol-gel Method

2.2 Drying Methods

2.2.1 Supercritical Drying

2.2.2 Constant-pressure Drying

2.2.3 freeze-drying

2.2.4 Microwave Drying

3 SiO2 Aerogel

表1 SiO2气凝胶的制备方法与性能a)

3.1 SiO2Precursor of Aerogel

3.1.1 Organic Precursor

图2 (a) SiO2纳米纤维气凝胶的制造的示意图；（b） SiO2纳米纤维气凝胶中纳米纤维之间的交联网络；（c） SiO2纳米纤维气凝胶在丁烷喷灯（1100 ℃）和液氮（−196 ℃）的火焰中；（d） SiO2纳米纤维气凝胶的微观结构[44]

3.1.2 Inorganic Precursor

3.2 SiO2 Pre-treatment of Aerogels

图3 （a）SiO2气凝胶的Ostwald老化机制；（b）低表面能溶剂置换[27]；（c）SiO2气凝胶的疏水改性

3.2.1 Aging Treatment

3.2.2 Low Surface Energy Solvent Replacement

3.2.3 Hydrophobic modification

3.2.4 Polymer Reinforcement

3.3 SiO2-based Composite Aerogel

3.3.1 Fiber-reinforced SiO2 composite aerogel

3.3.2 Composite aerogels with shading agents and SiO2

4 Al2O3 aerogel

表2 Al2O3气凝胶的制备方法与性能a)

4.1 Precursor of Al2O3 aerogel

4.1.1 Organic Precursor

4.1.2 Inorganic Precursor

4.2 Al2O3 Aerogel Structure Regulation

图5 (a) 丙酮-苯胺原位水形成（ISWF）法溶胶-凝胶法；（b）耐高温氧化铝气凝胶原氧化铝颗粒改性机理示意图[64]

4.3 Al2O3 Composite Aerogel

4.3.1 Doped Elements in Al2O3 Composite Aerogels

图6 (a) Si掺杂Al2O3纳米棒气凝胶制备示意图；（b） Si掺杂Al2O3纳米棒气凝胶在干燥后和在不同温度下处理后的照片；（c） Si掺杂Al2O3纳米棒气凝胶在1400 ℃下的SEM图像[78]

图7 (a) 陶瓷气凝胶的制备工艺；陶瓷气凝胶（b）上半层和（c）下半层的微观结构[79]

4.3.2 Fiber-reinforced Al2O3 aerogel

图8 纤维-纤维和气凝胶-纤维传热模型[82]

4.3.3 Al2O3 Aerogel with Light-Blocking Agent

5 ZrO2 Aerogel

表3 ZrO2气凝胶的制备方法与性能a)

5.1 ZrO2 Precursor of Aerogel

5.1.1 Organic Precursor

5.1.2 Inorganic Precursor

5.2 ZrO2Structural Regulation of Aerogels

5.3 Composite ZrO2 Aerogel

6 Binary and Multicomponent Oxide Aerogels

表4 二元和多元氧化物气凝胶的制备方法与性能a)

6.1 Binary Oxide Aerogels

6.1.1 Al$_{2}$O$_{3}$-SiO$_{2}$ aerogel

图10 SiC包覆莫来石纤维的SEM图像[110]

6.1.2 ZrO2-SiO2aerogel

图11 (a) 浸渍法制备ZrO2-SiO2气凝胶工艺流程图；不同浸渍次数的0次（b），1次（c），2次（d），3次（e）自增强ZrO2-SiO2气凝胶SEM图像[116]

6.1.3 Al$_{2}$ O$_{3}$ -ZrO$_{2}$ aerogel

图12 不同AHP含量的ZrO2-Al2O3复合气凝胶涂层[118]

6.2 Multicomponent Oxide Aerogels

7 Summary and Prospect

References

图1 (a) 溶胶-凝胶法；（b）环氧化物添加法；（c）无机溶胶-凝胶法^[15]

3 SiO₂ Aerogel

表1 SiO₂气凝胶的制备方法与性能^a)

3.1 SiO₂
Precursor of Aerogel

图2 (a) SiO₂纳米纤维气凝胶的制造的示意图；（b） SiO₂纳米纤维气凝胶中纳米纤维之间的交联网络；（c） SiO₂纳米纤维气凝胶在丁烷喷灯（1100 ℃）和液氮（−196 ℃）的火焰中；（d） SiO₂纳米纤维气凝胶的微观结构^[44]

3.2 SiO₂ Pre-treatment of Aerogels

图3 （a）SiO₂气凝胶的Ostwald老化机制；（b）低表面能溶剂置换^[27]；（c）SiO₂气凝胶的疏水改性

3.3 SiO₂-based Composite Aerogel

3.3.1 Fiber-reinforced SiO₂ composite aerogel

3.3.2 Composite aerogels with shading agents and SiO₂

4 Al₂O₃ aerogel

表2 Al₂O₃气凝胶的制备方法与性能^a)

4.1 Precursor of Al₂O₃ aerogel

4.2 Al₂O₃ Aerogel Structure Regulation

图5 (a) 丙酮-苯胺原位水形成（ISWF）法溶胶-凝胶法；（b）耐高温氧化铝气凝胶原氧化铝颗粒改性机理示意图^[64]

4.3 Al₂O₃ Composite Aerogel

4.3.1 Doped Elements in Al₂O₃ Composite Aerogels

图6 (a) Si掺杂Al₂O₃纳米棒气凝胶制备示意图；（b） Si掺杂Al₂O₃纳米棒气凝胶在干燥后和在不同温度下处理后的照片；（c） Si掺杂Al₂O₃纳米棒气凝胶在1400 ℃下的SEM图像^[78]

图7 (a) 陶瓷气凝胶的制备工艺；陶瓷气凝胶（b）上半层和（c）下半层的微观结构^[79]

4.3.2 Fiber-reinforced Al₂O₃ aerogel

图8 纤维-纤维和气凝胶-纤维传热模型^[82]

4.3.3 Al₂O₃ Aerogel with Light-Blocking Agent

5 ZrO₂ Aerogel

表3 ZrO₂气凝胶的制备方法与性能^a)

5.1 ZrO₂Precursor of Aerogel

5.2 ZrO₂
Structural Regulation of Aerogels

5.3 Composite ZrO₂ Aerogel

表4 二元和多元氧化物气凝胶的制备方法与性能^a)

图10 SiC包覆莫来石纤维的SEM图像^[110]

6.1.2 ZrO₂-SiO₂aerogel

图11 (a) 浸渍法制备ZrO₂-SiO₂气凝胶工艺流程图；不同浸渍次数的0次（b），1次（c），2次（d），3次（e）自增强ZrO₂-SiO₂气凝胶SEM图像^[116]

图12 不同AHP含量的ZrO₂-Al₂O₃复合气凝胶涂层^[118]