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

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Progress in Chemistry >

2024 , Vol. 36 >Issue 2: 234 - 243

DOI: https://doi.org/10.7536/PC230623

14

Research Advances on High-Temperature Infrared Modification of Oxide Aerogels

  • Ruiming Huang ,
  • Yonggang Jiang , * ,
  • Fengqi Liu ,
  • Junzong Feng ,
  • Liangjun Li ,
  • Jian Feng
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  • Science and Technology on Advanced Ceramic Fibers and Composites Laboratory, College of Aerospace Science and Engineering, National University of Defense Technology, Changsha 410073, China
* Corresponding author e-mail:

Received date: 2023-06-25

  Revised date: 2023-09-15

  Online published: 2024-01-08

Supported by

Hunan Province Key R&D Plan(2022GK2027)

Natural Science Foundation of Hunan Province(2023JJ30632)

National Key Research and Development Program of China(2022YFC2204403)

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Abstract

Oxide aerogel is a novel nano-porous material with ultra-low thermal conductivity. In particular, it can be used in spaceflight applications and other thermal management fields. Currently, with high infrared transmittance, most of the common pure oxide aerogels, such as silica and alumina, have no advantages in high-temperature insulation because of their intrinsic property. However, electromagnetic radiation in the near-infrared region is the main mode of heat conduction at high temperatures, accordingly, a large amount of electromagnetic radiation will pass through aerogel and lead to the rapid increase of thermal conductivity. Therefore, to meet the requirement of thermal insulation at higher temperature, it is necessary to reduce the radiative heat transfer. Based on the research status, this paper reviewed the main progress of improving high temperature insulation of oxide aerogel by adding opacifier, fiber and adjusting the structure and morphology. Moreover, the future research direction has prospected.

Contents

1 Introduction

2 Application of opacifiers in infrared modification of aerogels

2.1 TiO2 opacifier

2.2 SiC opacifier

2.3 Carbon materials opacifier

2.4 Other opacifier

3 Application of fiber in infrared modification of aerogels

3.1 Glass fiber

3.2 ZrO2 fiber

3.3 Mullite fiber

3.4 Modified fiber

4 Application of structure/morphology change in infrared modification of aerogels

4.1 Multiple-layer aerogel insulation materials

4.2 Lamellar aerogels

4.3 Nanofiber aerogels

5 Conclusion and outlook

Key words： oxide aerogel; infrared modification; composite material; high temperature insulation; opacifier; fiber

Cite this article

Ruiming Huang , Yonggang Jiang , Fengqi Liu , Junzong Feng , Liangjun Li , Jian Feng . Research Advances on High-Temperature Infrared Modification of Oxide Aerogels[J]. Progress in Chemistry, 2024 , 36(2) : 234 -243 . DOI: 10.7536/PC230623

1 Introduction

The rapid development of new high-speed aircraft meets the needs of human beings in the fields of aerospace and deep space exploration. However, with the increase of Mach number and flight time, the lack of thermal protection system will lead to extreme high temperature environment inside and outside the vehicle[1]. Therefore, the research on lightweight and thermal insulation materials has become one of the key links for aircraft to break through technical barriers.
Oxide aerogel materials are a new type of nano-solid materials with the characteristics of porosity, light weight and thermal insulation, which are formed by the nano-skeleton structure composed of oxide structural units (pearl chains, nanorods, nanosheets, etc.) And the uniform dispersion of gas in the nano-pores around the skeleton[2,3]. According to different synthesis methods, three kinds of structural units can be obtained, including nanoparticles, fibers and lamellae[2][4][5,6]. The skeleton structure of these units increases the heat conduction path and limits the mean free path of phonons, which together reduce the heat transfer of solids; The existence of nano-pore size not only weakens the influence of thermal convection, but also inhibits the movement of gas molecules, which leads to the decrease of gas heat transfer performance, so pure aerogel has extremely low thermal conductivity (such as the thermal conductivity of SiO2 aerogel is as low as about 0.012 W/ (m ∙ K) at room temperature), which has a good application prospect in aerospace, high temperature kiln and other fields[7,8][9]. However, as the temperature increases, according to the Stefan-Boltzmann formula, the relationship between the radiant heat of an object and the temperature is:
Φ = εAσT4
Where Φ is the radiant heat, A is the radiant surface area, σ is the Boltzmann constant, and T is the temperature. It can be seen from the above formula that the radiant heat is proportional to the fourth power of the temperature, so the radiant heat transfer becomes the main heat transfer mode at high temperature. However, infrared radiation (wavelength of 3 ~ 8 μm) is extremely easy to penetrate through the common pure aerogel thermal insulation materials, and even leads to the doubling of the thermal conductivity of pure aerogel at high temperature, resulting in a sharp decline in thermal insulation performance, which limits its application in high temperature environment[10⇓⇓⇓~14]. The radiative thermal conductivity of the material can be expressed as:[12]
Krad = 16n2σT3/(3β)
Where Krad is the radiative thermal conductivity, n is the effective refractive index of the porous material, σ is the Boltzmann constant, T is the temperature, and β is the extinction coefficient. It can be seen from the above formula that in order to improve the thermal insulation performance of oxide aerogel at high temperature, the most effective way is to increase the extinction coefficient (β) of the material, thereby reducing the radiative thermal conductivity （Krad）.
At present, the commonly used method is to add opacifiers and fibers to aerogel to prepare aerogel composites, so as to reduce its infrared radiation transmittance at high temperature[15⇓~17][18⇓~20]. In addition, the researchers also explored the effects of changing the structure and morphology of aerogels on the thermal insulation performance of aerogels at the macro and micro levels[21⇓~23]. In this paper, the research results of infrared modified oxide aerogels at home and abroad in recent years were reviewed from the above three kinds of infrared shading modification methods of aerogels, and the future research directions of infrared shading modification were prospected.

2 Application of opacifier in infrared modification of aerogel

The opacifier particles have strong radiation scattering and radiation absorption characteristics, and can effectively inhibit high-temperature radiation heat transfer, so that the high-temperature thermal insulation performance of the aerogel is improved[24]. As early as 1995, Kuhn et al. Studied the effect of different kinds of opacifier doping on the properties of SiO2 aerogel[12]. The experimental results show that the extinction coefficient of SiO2 aerogel increases after doping, which is beneficial to the improvement of high temperature thermal insulation performance. In this paper, the application of opacifiers such as TiO2, SiC and carbon materials in the field of infrared modification of oxide aerogels is introduced.

2.1 TiO2 opacifier

Fig. 1 is a schematic diagram of an infrared shading mechanism of a TiO2. The TiO2 absorbs electromagnetic radiation of a fixed frequency band through energy level transition and converts the electromagnetic radiation into self heat energy for storage; When the infrared radiation is projected on the material, the TiO2 can absorb the infrared radiation and form infrasound, which scatters the original radiation energy and weakens the external infrared radiation intensity, and the TiO2 can effectively absorb the radiation in the infrared band of 2 ~ 8 μm[25]. At a certain temperature, TiO2 can be transformed into different crystal forms, mainly including rutile, anatase and brookite. Among them, rutile TiO2 has better thermal stability and higher refractive index, so rutile TiO2 is usually selected as infrared opacifier[26,27].
图1 TiO2遮光机理示意图[16]

Fig. 1 Schematic diagram of shading mechanism of TiO2[16]

Sun Dengke found that the thermal conductivity of pure SiO2 aerogel increased to about 0.060 W/ (m ∙ K) at 500 ℃[25]; However, Wang et al. Used TiO2 as opacifier and ceramic fiber as reinforcement to prepare TiO2 doped SiO2 aerogel composite by sol-gel and supercritical drying methods, and measured the thermal conductivity of the modified SiO2 aerogel composite by transient hot-wire method.The measurement results show that the TiO2 doped SiO2 aerogel composite with a density of 260 kg/m3 can still maintain a low thermal conductivity of only 0.038 W/ (m ∙ K) at 527 ° C[28].
Liu et al. Prepared 8 wt%TiO2 doped SiO2 gel by a two-step acid-base catalytic synthesis method, and after surface modification by a mixed solvent of trimethylchlorosilane and n-hexane, the aerogel composite was obtained by ambient pressure drying method[29]. The thermal conductivities of pure aerogel and doped aerogel were measured by transient hot-plate method, and the thermal conductivities were 0.021 and 0.0274 W/ (m ∙ K) at room temperature, respectively; The thermal conductivity of pure aerogel is close to 0.05 W/ (m ∙ K) at 700 ℃, while the thermal conductivity of doped aerogel is as low as 0.0426 W/ (m ∙ K).
Zhang et al. Impregnated the TiO2 doped Y2O3-SiO2 sol into mullite fibers, and after gelation, aging, drying, and heat treatment at 1000 ° C, the TiO2 doped Y2SiO5 aerogel composite with a thermal conductivity of 0.042 W/ (m ∙ K) at room temperature was obtained[30]. The results show that the room temperature thermal conductivity does not change significantly with the increase of the doping amount of TiO2, indicating that TiO2 has no significant effect on reducing the room temperature thermal conductivity. The thermal insulation performance of the material was tested by heating the single side at 1200 ℃ for 1500 s, and it was found that the back surface temperature of the Y2SiO5 aerogel composite was 477 ℃ before doping, and decreased to 457, 346 and 351 ℃ with the increase of TiO2 doping content to 5 wt%, 10 wt% and 15 wt%, respectively, indicating that the addition of TiO2 significantly improved the high temperature thermal insulation performance of the material.
Yang et al. Prepared TiO2 and Si3N4 co-doped SiO2 aerogel composite by adding Si3N4 and TiO2 powder into silica sol.The Si3N4 is used to support the pore structure of aerogel and avoid crystallization at high temperature, and the TiO2 is used to reduce the thermal conductivity of aerogel at high temperature[31]. The study shows that the Si3N4/SiO2 aerogels with different TiO2 doping amount have similar thermal conductivity at low temperature. The thermal conductivity of Si3N4/SiO2 aerogel is about 0.45 W/ (m ∙ K) when the temperature rises to 1300 ° C, and it decreases by 35% to about 0.3 W/ (m ∙ K) with the increase of TiO2 doping from 0 to 20 wt%.

2.2 SiC opacifier

SiC has strong infrared active polar vibration, excellent infrared emission performance in the wavelength range of 3 ~ 8 μm, excellent stability and high refractive index at high temperature, and the effective extinction coefficient of the material can be greatly improved by adding SiC into aerogel[32]. In addition, SiC has more excellent opacifying properties than TiO2 opacifying agents, so researchers usually add SiC into aerogels to reduce the high temperature thermal conductivity of aerogels[33].
Pang et al. Studied the effect of different doping amounts of SiC particles on the thermal conductivity of SiO2 aerogel in the hot surface temperature range of 135 – 827 ° C[34]. It is found that the thermal conductivity of the aerogel without SiC particles is 0.04634 ~ 0.1852 W/ (m ∙ K), while the thermal conductivity of the aerogel with 1% and 5.84% (volume fraction) SiC particles is 0.03111 ~ 0.03998 and 0.03131 ~ 0.03685 W/ (m ∙ K), respectively, which indicates that the addition of SiC can effectively reduce the thermal conductivity. At the same time, comparing the thermal conductivity of 1% and 5% doping, it may be due to the increase of solid heat transfer caused by the increase of doping, which leads to a slight increase in low temperature thermal conductivity; At high temperature, the proportion of radiation heat transfer increases, so the increase of SiC content is more conducive to the reduction of high temperature thermal conductivity.
As the process shown in Fig. 2, Wang et al. Prepared SiO2 aerogel by using 3D printing technology, and doped SiC in it to improve the thermal conductivity of aerogel at medium and high temperatures[35]. The thermal conductivity of the pure aerogel prepared by 3D printing is 0.028 W/ (m ∙ K) at room temperature and 0.106 W/ (m ∙ K) when the temperature is increased to 400 ℃, which is an increase of 278.6%; At the same temperature, the thermal conductivities of 20 wt% SiC-doped aerogel are 0.072 and 0.111 W/ (m ∙ K), respectively, which are only increased by 54.2%. The increase of thermal conductivity of aerogel doped with SiC at room temperature is mainly due to the increase of solid heat transfer caused by SiC particles, but the increase of thermal conductivity at high temperature is strongly inhibited by SiC.
图2 (a) 3D打印SiO2气凝胶流程图； (b, c) 打印气凝胶实物图[35]

Fig. 2 (a) 3D printing of SiO2 aerogel flow chart; (b, c) physical diagram of printiong aerogel[35]

2.3 Carbon material opacifier

Carbon materials (such as carbon black, graphene, carbon nanotubes, etc.) have the characteristics of absorbing and scattering radiation, so they have good shading properties. Zeng et al. Studied the effect of doped carbon black on the properties of SiO2 aerogel by establishing a theoretical model[36]. It is found that when the temperature rises from 27 ℃ to 327 ℃, the thermal conductivities of the materials before and after doping are 0. 088 ~ 0.143 and 0. 059 ~ 0.0113 W/ (m ∙ K), respectively, and the ratio of the thermal conductivities of the two materials at high temperature reaches 10, so the shading effect of carbon black at high temperature is more significant. It is also found that the optimum theoretical doping amount of carbon black varies linearly with temperature.
Zhao et al. Used Monte Carlo method and Mie scattering theory to study the effect of carbon black doping on improving the infrared transmittance of aerogel, and found that when the particle size of carbon black was 2 μm, the comprehensive shading performance was the best[37]; It is found that at 327 ℃, with the increase of the volume fraction of carbon black, the solid thermal conductivity increases while the radiation thermal conductivity decreases, and when the volume fraction of carbon black is 1%, the sum of the solid thermal conductivity and the radiation thermal conductivity is the smallest.
Zhu et al. Successfully prepared graphene-doped SiO2 aerogels with different mass ratios by adding graphene into the sol[38]. It is found that when the doping amount of graphene increases from 0 to 0.1 wt%, the thermal conductivity of aerogel decreases as a whole, and the lowest is only 0.0184 W/ (m ∙ K). Because the content of graphene is very small, the solid thermal conductivity of the material is hardly affected; At the same time, its unique band structure and π-π conjugation make it have broad spectrum absorption, so it can reduce the radiation thermal conductivity; However, the lamellar structure of graphene makes it vulnerable to external influences to form clusters, as shown in Figure 3. When the doping amount is increased to 0.15 wt%, the thermal conductivity is increased.
图3 石墨烯掺杂量对热导率的影响[38]

Fig. 3 Effect of doping amount on thermal conductivity of graphene[38]

Nanoscale carbon materials can be distributed in the pore structure of aerogels to form a special microstructure and enhance the thermal insulation performance of aerogels[39,40]. Jiang et al. Added carbon nanotubes during the gelation process to prepare carbon nanotube-doped Al2O3-SiO2 aerogel composite[39]. It is found that the thermal conductivity at room temperature is reduced by 14.8% to about 0.05 W/ (m ∙ K) after doping 1 wt% carbon nanotubes compared with the undoped aerogel material; When the temperature reaches 1000 ℃, the thermal conductivity of the aerogel doped with carbon nanotubes is 0.179 W/ (m ∙ K), which is reduced by about 15%.

2.4 Other opacifiers

Materials such as ZrO2, Y2O3 and clay minerals also have an inhibitory effect on infrared radiation. Zhu Zhaoxian et al. Prepared in-situ ZrO2 doped mullite fiber reinforced Al2O3-SiO2 aerogel composite by adding zirconium source[13]. It is found that the thermal conductivity of the aerogel composite before and after ZrO2 doping is 0.098 and 0.076 W/ (m ∙ K) at 1500 ℃, respectively, which is reduced by about 22%.
Parale et al. Prepared Y2O3 doped SiO2 aerogel by ambient pressure drying method, and the results showed that the low temperature radiation thermal conductivity of aerogel before and after doping was similar, but when the temperature reached 727 ℃, the radiation thermal conductivity of aerogel after doping decreased by about 72%[41]; The total thermal conductivity was further studied, and it was found that the thermal conductivity of the aerogel doped with Y2O3 was lower than that of the undoped SiO2 aerogel from 127 ℃ to 727 ℃, for example, the thermal conductivity of the aerogel before and after doping was 0. 104 and 0. 080 W/ (m ∙ K) at 727 ℃, respectively.
Soorbaghi et al. Studied the thermal insulation properties of Cloisite-30 B clay mineral doped SiO2 aerogel composite[42]. The theoretical model shows that the extinction coefficient of clay minerals is higher than that of SiO2 aerogel, so the radiative thermal conductivity at high temperature decreases with the increase of content. In addition, 3 wt%, 5 wt% and 7 wt% clay minerals were added to the SiO2 aerogel, respectively, and the theoretical calculation results show that the minimum value of the radiative thermal conductivity is about 0.014 W/ (m ∙ K) at 702 ℃ with the increase of the clay mineral content. The effective thermal conductivity was tested, and it was found that the addition of clay minerals had little effect on the high temperature thermal conductivity of aerogel, which was mainly due to the high solid thermal conductivity of clay minerals, which did not play an ideal role in infrared suppression.
After adding most of the above opacifiers, the high temperature thermal conductivity of aerogel is significantly improved, which is fundamentally due to the absorption and scattering of high temperature infrared radiation by the opacifiers. However, due to the different densities of aerogel and opacifier, and the easy agglomeration of opacifier, it is difficult to prepare aerogel composites with uniform distribution of opacifier[43]. In addition, the increase of the doping amount of the opacifier often leads to the increase of the solid thermal conductivity of the material, so it is necessary to further control the geometric size and doping amount of the opacifier, and explore the technical ways to make it uniformly distributed in the aerogel, so as to achieve lower high temperature thermal conductivity.

3 Application of fiber in infrared modification of aerogel

Glass, ZrO2, mullite and other fibers can not only improve the mechanical properties of aerogels, but also inhibit the thermal radiation at high temperature, so they are widely used as the reinforcement of aerogels, but also as the shading modification of aerogel materials to improve the high temperature thermal insulation performance[44]. Guo et al. Found through theoretical research that the shading modification effect of the fiber is related to the material type, doping amount and diameter of the fiber[45]. In addition, the surface modification of the fiber is beneficial to further improve the shielding effect on infrared radiation.

3.1 Glass fiber

Glass fiber is a kind of multifunctional ceramic material with low thermal expansion coefficient, which is widely used in the preparation of aerogel composites[46]. Zhao et al. Established a theoretical model for the study of the thermal insulation performance of glass fibers and their composites based on the inclination, size and volume fraction of glass fibers[47]. Similar to the mechanism of opacifier, it is necessary to select materials with larger extinction coefficient as far as possible to reduce the influence of infrared radiation on aerogel composites. For SiO2 glass fiber, the fiber with diameter of 4 μm and inclination angle of 40 ° has the maximum extinction coefficient at high temperature, and the total extinction coefficient increases with the increase of fiber volume fraction from 0 to 10%. At the same time, the radiative thermal conductivity of aerogel composites doped with 5% fiber decreases from 0.5 W/ (m ∙ K) of pure aerogel to 0.02 ~ 0.07 W/ (m ∙ K) at 1027 ℃; The total thermal conductivity of glass fiber doped aerogel is reduced by more than 90% compared with that of pure aerogel.
Jiang et al. Added micro-glass fibers into SiO2 aerogel as reinforcement, and prepared aerogel composites by ambient pressure drying process, and studied the effect of micro-glass fiber doping amount on the thermal conductivity of the composites[48]. The thermal conductivity test results are shown in Figure 4. At the same temperature, the thermal conductivity of the composite decreases gradually with the increase of the volume fraction of fiber doping from 4.5% to 9.1%; The thermal conductivity at 650 ℃ decreases from 0.0265 W/ (m ∙ K) at 4.5% doping to 0.022 W/ (m ∙ K) at 9.1% doping.
图4 不同微玻璃纤维掺杂量(fc)下热导率随温度变化趋势[48]

Fig. 4 Variation trend of thermal conductivity with temperature under different doping amounts(fc) of microglass fibers[48]

3.2 ZrO2 fiber

ZrO2 fiber has the advantages of high temperature stability, acid and alkali corrosion resistance, low thermal conductivity and high temperature oxidation resistance. Compared with glass fiber, it can be used in higher temperature oxidation environment.
He et al. Prepared the ZrO2 fiber felt /ZrO2-SiO2 aerogel composite with the morphology shown in Fig. 5 by impregnating the ZrO2 fiber felt into the ZrO2-SiO2 sol under vacuum, and after aging, ethanol replacement, and supercritical drying[49]. The thermal conductivity of the ZrO2 fiber felt /ZrO2-SiO2 aerogel composite was 0.0341 – 0.096 W/ (m ∙ K) in the temperature range of 20 – 1100 ° C.
图5 (a~c) ZrO2纤维增强ZrO2-SiO2气凝胶的SEM图片[49]

Fig. 5 (a~c)SEM image of ZrO2 fiber reinforced ZrO2-SiO2 aerogel[49]

In order to reduce the thermal conductivity of Al2O3-SiO2 aerogel at high temperature, Zhang et al. Prepared the skeleton material with ZrO2 fibers, and impregnated the skeleton material into Al2O3-SiO2 sol in vacuum atmosphere, and then prepared the composite material after aging and supercritical drying[50]. The thermal conductivity of the composite is 0.049 W/ (m ∙ K) at room temperature and 0.102 W/ (m ∙ K) at 1000 ℃, which is more than 50% lower than that of the pure Al2O3-SiO2 aerogel reported by Jiang et al. (0.21 W/ [m ∙ K] at 1000 ℃)[39].

3.3 Mullite fiber

Mullite fibers are composed of mullite crystallites, which have good high temperature stability, chemical stability, low thermal conductivity and creep resistance. The introduction of mullite fibers into aerogels can enhance radiation scattering and absorption to a certain extent, so they have been widely studied. In order to improve the mechanical properties and thermal insulation effect of SiO2 aerogel, Feng Jian et al. Used supercritical drying method to prepare mullite fiber reinforced SiO2 aerogel composite, which has a thermal conductivity of 0.017 W/ (m ∙ K) at 200 ℃, and can still maintain excellent high temperature thermal insulation performance at 800 ℃, with a thermal conductivity as low as 0.042 W/ (m ∙ K)[51].
He et al. Prepared mullite fibers into ceramic fiber blocks, which were impregnated into ZrO2-SiO2 sol, and the mullite fiber ceramic reinforced ZrO2-SiO2 aerogel composite was obtained after aging and supercritical drying treatment[52]. When the temperature increases from room temperature to 1200 ℃, the thermal conductivity ranges from 0.0524 to 0.182 W/ (m ∙ K), and the high temperature thermal insulation performance needs to be further improved. Liu et al. Prepared a highly hybrid ZrO2-SiO2 aerogel, also using mullite fibers as reinforcement, and found that the thermal conductivity was 0.026, 0.037 and 0.058 W/ (m ∙ K) at 600, 800 and 1000 ° C, respectively, when the mass ratio of ZrO2 to SiO2 was 86 ∶ 14, which could maintain a low thermal conductivity in a large temperature range[53]. Although the above two studies used mullite fibers to reinforce ZrO2-SiO2 aerogel, He et al. Made mullite fibers connect at the intersection point through process treatment, which increased the heat transfer path, resulting in the improvement of thermal insulation performance lower than that of the composite prepared by Liu et al[52][53].
Peng et al. Successfully prepared two kinds of aerogel composites by introducing mullite fibers into Al2O3-SiO2 aerogel and silicon modified boehmite aerogel synthesized by hydrothermal method, respectively[54,55]. The results show that both composites have low thermal conductivities of 0.082 and 0.093 W/ (m ∙ K) at 1200 ℃, respectively. The effects of temperature and fiber density on the thermal conductivity of fiber reinforced Al2O3-SiO2 aerogel composites were further investigated.It was found that when the temperature was below 1 100 ℃, the thermal conductivity changed non-monotonically with the increase of fiber density, but when the temperature was above 1 100 ℃, the thermal conductivity decreased monotonically with the increase of fiber density.
Liu et al. Added mullite fiber as reinforcement on the basis of carbon-coated Al2O3 nanorod aerogel, and prepared mullite fiber reinforced carbon-coated Al2O3 aerogel composite materials that can still show excellent thermal protection performance at high temperature (as shown in Fig. 6 B – d)[56]. When the fiber density is 0.15 g/cm3, the thermal conductivity of the composite is 0.055 W/ (m ∙ K) at 1200 ℃, while the thermal conductivity of the pure aerogel is 0.065 W/ (m ∙ K); With the increase of fiber density, the increase of gas and solid thermal conductivity is greater than the decrease of radiation thermal conductivity, so the overall thermal conductivity shows an upward trend.
图6 (a) 复合材料隔热机理图；(b~d) 材料隔热性能测试；(e) 测试前后样品形貌变化；(f) 与先前文献中其他气凝胶复合材料于1000 ℃高温热导率对比[56]

Fig. 6 (a) Mechanism schematic of thermal insulation of composite material; (b~d) insulation performance test of materials; (e) sample morphology changes before and after testing; (f) compared with the thermal conductivity of other aerogel composites in previous literature at 1000 ℃[56]

3.4 Modified fiber

Adding the fibers mentioned above and increasing the fiber density can inhibit infrared radiation to a certain extent. In order to further improve the thermal insulation performance of the material, the researchers also explored the reduction of the high temperature thermal conductivity of the material after the modification of the fibers and the fiber surface.
Silica fiber has poor thermal insulation performance at high temperature, so Yu et al. Grew one-dimensional rutile TiO2 nanowires on the surface of silica fiber in situ and compounded with Al2O3-SiO2 aerogel[57]. The experimental results show that the thermal conductivity of the composite is 0.026 ~ 0.071 W/ (m ∙ K) in the temperature range of 200 ~ 1100 ℃, while the thermal conductivity of the composite without surface treatment is 0.025 ~ 0.108 W/ (m ∙ K), indicating that the one-dimensional TiO2 array can effectively help reduce infrared radiation.
Xu et al. Prepared mullite fiber @ SiC reinforced Al2O3-SiO2 aerogel composite (morphology shown in Fig. 7) by using Al2O3-SiO2 aerogel as the matrix and SiC coated on the surface of mullite fiber as the reinforcing phase[58]. The SiC layer on the surface can effectively reduce the radiation heat transfer at high temperature. At 1000 ℃, the thermal conductivity of the composite is as low as 0.049 W/ (m ∙ K), while the thermal conductivity of the aerogel composite without fiber modification is 0.062 W/ (m ∙ K). The thermal conductivity is reduced by about 20% by coating the surface.
图7 SiC包覆莫来石纤维SEM图[58]

Fig. 7 SEM images of SiC-coated mullite fiber[58]

Zhang et al. Prepared hollow TiO2 sphere/mullite composite nanofibers by electrospinning, and prepared aerogel composite materials by directional freeze-drying technique after compounding with SiO2 sol, and grew TiO2 nanowires on the surface[59]. The thermal conductivity of the fiber-reinforced aerogel composite with TiO2 nanowires on the surface is 0.033 and 0.056 W/ (m ∙ K) at 25 and 1000 ℃, respectively, and the thermal conductivity at 1000 ℃ is 10% lower than that of the fiber-reinforced aerogel composite without TiO2 nanowires on the surface. The mechanism of thermal insulation enhancement is shown in Fig. 8. The TiO2 inside and outside the fiber together enhance the infrared radiation suppression effect, thereby improving the thermal insulation performance.
图8 改性莫来石纤维隔热机理图[59]

Fig. 8 Schematic diagram of thermal insulation mechanism of modified mullite fiber[59]

Ding et al. Prepared TiO2/ mullite fiber composite with cactus structure, growing TiO2 nanorods on the fiber surface, and filling SiC aerogel in the fiber skeleton to obtain the aerogel composite. The minimum thermal conductivity of the obtained sample was 0.151 W/ (m ∙ K) at 1400 ℃[60].
After the fiber is introduced into the aerogel to prepare the composite material, the high-temperature thermal conductivity of the system is improved to a certain extent, but there is still room for improvement. By means of phase and surface modification of the fiber, it is beneficial to further improve the reflection and scattering of the fiber to infrared radiation, and the high-temperature thermal conductivity of the material is further reduced[44,61,62].

4 Application of Structure/Morphology Modification in Infrared Modification of Aerogel

4.1 Multilayer aerogel insulation

The reflective screen is made of a material with high reflectivity, and the spacer layer material has the characteristic of low thermal conductivity, and the reflective screen and the spacer layer are alternately arranged to form a multi-layer thermal insulation material[63]. The aerogel material has low thermal conductivity, which can effectively reduce the heat transfer of solid and gas, and the high reflectivity of the reflective screen can reduce the transmission of infrared radiation, thereby effectively reducing the impact of radiation heat transfer, so the combination of the two can achieve low solid, gas and radiation heat transfer at the same time.
Sheng et al. Prepared a multilayer thermal insulation material with Al2O3 fiber mat reinforced SiO2 aerogel as the spacer layer (the structure is similar to Fig. 9)[64]. At 176.85 ℃ and high vacuum （10−3Pa） environment, the thermal conductivity of the multilayer thermal insulation material prepared by alumina fiber is reduced by about 21.8% compared with that of the multilayer thermal insulation material prepared by pure alumina fiber, which is about 1.76×10−3W/(m∙K); At near space pressure (100 Pa), the thermal conductivity is reduced by about 24%. The temperature resistance test at 927 ℃ also shows excellent thermal insulation performance.
图9 多层隔热材料结构示意图

Fig. 9 Schematic diagram of multi-layer insulation material structure

Wang Miao et al. Prepared aerogel multilayer thermal insulation materials with aluminum silicate fiber reinforced SiO2 aerogel as the spacer layer and different metal foils (copper, stainless steel, titanium, tantalum, zirconium and molybdenum) as the reflective screen[65]. It is found that the thermal conductivity of the 15-layer aerogel multilayer composite with stainless steel as the reflective screen is about 0. 425 W/ (m ∙ K) at 1 000 ℃, which is 24. 56% and 34. 85% lower than that of the pure aerogel composite and the multilayer thermal insulation material, respectively. In addition, the effects of the number of layers and different types of metal foils on the thermal insulation performance were studied, and it was found that the 15-layer multilayer thermal insulation material had the lowest thermal conductivity at high temperature, and the copper foil showed excellent radiation reflection characteristics compared with other types of metal foils at 1000 ℃.
Li Jian et al. Combined the aerogel composite material with low thermal conductivity, the radiation-resistant dense panel layer and the flexible coating layer with protective effect to form a multi-layer thermal protection material[66]. The results show that the thermal conductivity of the multilayer insulation material is 0.028 ~ 0.035 W/ (m ∙ K) at room temperature, and only 0.074 ~ 0.090 W/ (m ∙ K) at 1200 ℃. In addition, the material was tested for 8 times at 1200 ℃ for 600 s, and it was found that the material was not damaged and the thermal conductivity did not increase significantly.

4.2 Lamellar aerogel

Compared with aerogels composed of nanoparticles, lamellar aerogels are anisotropic, which is beneficial to improve the disadvantage of heat retention. Due to the structural characteristics of multilayer aerogels, the modification of the surface of lamellar aerogels can help to enhance the scattering of phonons at the interface, thus further reducing the radiation heat transfer and improving the thermal insulation performance.
Zhu Zhen prepared Al2O3 aerogel with layered structure by directional freeze-drying technique using aluminum sec-butoxide as precursor[67]. The layered structure leads to different structures in different directions, so the heat conduction paths are also different, so the thermal conductivity is different in different directions. The room temperature thermal conductivity in the Z direction (parallel to the sheet) is 0.09463 W/ (m ∙ K), which is nearly twice as high as that in the y direction (perpendicular to the sheet). At the same time, the thermal conductivity in Z direction increases by about 34% and that in y direction increases by about 1.5 times at 800 ℃.
Ji et al. Prepared α-Al2O3 nanosheet-based biphasic aerogels by the process shown in Fig. 10 using silica sol as a high-temperature binder[68]. The thermal conductivity is 0.029 W/ (m ∙ K) at room temperature, and the back surface temperature is only 120 ℃ at 1600 ℃, which has good thermal insulation performance. The mullite phase generated in situ on the surface of the nanosheets at high temperature helps to reduce the radiative heat transfer.
图10 α-Al2O3纳米片制备过程示意图及其SEM图[68]

Fig. 10 Schematic illustration of the synthesis and the SEM image of α-Al2O3 nanosheets[68]

4.3 Nanofiber aerogel

Nanofiber aerogel has both excellent mechanical and high temperature thermal insulation properties, which is one of the current research hotspots. Similar to fibers, in order to suppress radiation penetration more effectively, the surface modification of nanofibers by using shading materials has gradually become the focus of attention.
Zhang et al. Prepared ZrO2-Al2O3 and ZrO2-SiO2 nanofiber aerogels, both of which have good mechanical properties and high temperature thermal insulation ability. The thermal conductivities of the two aerogels are 0.032 and 0.0268 W/ (m ∙ K) at room temperature, respectively. The former has a maximum working temperature of 1300 ℃, which has a good application prospect as a thermal insulation material[69,70]. The thermal conductivity of the latter also increases to 0.11 W/ (m ∙ K) with increasing temperature up to 900 ℃.
After the shading material is prepared into nanowires, the thermal conductivity is greatly reduced under the combined action of a large number of stacking faults, the thermal resistance of the interface between the nanowires and the interface between the nanowires and the air[23]. Therefore, coating the surface of nanofiber aerogel with shading materials to prepare nanofiber aerogel with shell-core structure may help to improve its high temperature thermal insulation performance. Liu et al. Used Al2O3 nanorods as the basic unit to prepare porous ceramic aerogels by a dual-template method[71]. The thermal conductivity of the porous ceramic aerogel is as low as 0.0246 W/ (m ∙ K) at room temperature and 0.0949 W/ (m ∙ K) at 1000 ℃. In addition, in order to improve the high infrared transparency and other shortcomings of Al2O3 nanorod aerogel, Liu et al. Prepared Al2O3 nanorod sol by hydrothermal method, and mixed it with resorcinol-formaldehyde solution to prepare aerogel. After carbonizing the obtained resorcinol-formaldehyde-coated Al2O3 nanorod-like aerogel, carbon-layer coated Al2O3 nanorod-like aerogels were prepared, and the specific preparation process is shown in Fig. 11[22]. It is found that the thermal conductivity of the rod-like aerogel with 10% carbon content is only 0.065 W/ (m ∙ K) at 1200 ℃.
图11 Al2O3纳米棒的包覆过程示意图[22]

Fig. 11 Schematic diagram of coating process of Al2O3 nanorods[22]

The multilayer aerogel thermal insulation material has a multilayer reflective screen, so that the transmission of infrared radiation can be effectively avoided, the thermal resistance is improved, and higher thermal insulation performance is obtained. The nanosheets allow the aerogel to be anisotropic, thereby meeting specific requirements and avoiding heat retention. Fiber aerogel can not only improve the mechanical strength, but also reduce the thermal conductivity of solid and gas to a certain extent. However, due to the infrared transparency of fiber material itself, the radiation thermal conductivity increases rapidly at high temperature, which is not conducive to the use of high temperature insulation, thus limiting its use at high temperature to a certain extent.

5 Conclusion and prospect

Oxide aerogel materials have great development potential and application prospects in the field of thermal insulation because of their ultra-low thermal conductivity at room temperature, and their infrared transparency at high temperature is their intrinsic defect. The addition of opacifier can effectively inhibit the penetration of infrared radiation, and the combination with fibers and the change of aerogel structure/morphology are beneficial to reduce the radiation heat transfer to a certain extent. However, there is still much room for improvement in the infrared shading modification system for oxide aerogels. In order to further improve the high temperature thermal insulation performance of aerogels, the follow-up research directions may include:
(1) Optimization and modification of opacifier and doping method. The addition of opacifier can suppress part of the infrared radiation, but the high temperature thermal conductivity of the opacifier doped aerogel material system is still high. Firstly, the effects of the type, particle size, volume fraction and doping method of the opacifier on different aerogel materials need to be further studied. Secondly, the design of opacifiers such as shell-core structure may complement the defects of opacifiers to a certain extent, so that it can reduce the thermal conductivity at high temperature. On the other hand, because the opacifier is easy to form agglomeration, it is necessary to select a more appropriate process to make the opacifier uniformly distributed in the aerogel and further reduce the thermal conductivity.
(2) Fiber material design and its geometric parameter selection. The addition of fiber can reduce the thermal conductivity at high temperature to a certain extent, but it is limited by the defects of the material itself (for example, the maximum service temperature of quartz fiber is generally not more than 1000 ℃), which limits its use at higher temperatures. Therefore, modifying the surface of the fiber (such as coating and surface growth of nanowires) and directly compounding opacifier particles into the matrix of the fiber to further reduce the high temperature thermal conductivity may be one of the main development directions in the future. In addition, the response of the size of the fiber to the thermal conductivity still needs to be further studied.
(3) Structure/morphology design of aerogel materials. The change of structure/morphology has a certain influence on the thermal conductivity, so the modification of aerogel materials from this point of view can also increase its service temperature. Firstly, for multilayer aerogel composites, the optimal solution between thermal conductivity and mass can be obtained by selecting the appropriate reflective material and determining the number of layers. Secondly, the change of aerogel morphology is beneficial to the realization of some special functions, but most of the current work does not reduce the high temperature thermal conductivity, and even leads to the increase of high temperature thermal conductivity to a certain extent, so it is necessary to further study how to further reduce the high temperature thermal conductivities of aerogels by controlling the morphology and size.
(4) Coordinated development of computer simulation and experiment. With the development of computer performance, computational materials science has been gradually applied to guide the development process of materials. At present, some researchers have explored the thermal insulation modification of aerogel through simulation, but the research on improving the high temperature thermal insulation performance of aerogel at home and abroad is still based on trial-and-error experimental methods. Secondly, some experimental results can not explain the reasons for the performance changes, so it is necessary to explore the modification mechanism fundamentally by combining the simulation results. Therefore, the combination of simulation and experiment may help to meet the needs of exploring new and efficient infrared opacifier systems and infrared modification methods in the future.

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