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![State of the art comparison of the polyimide and polyimide-composite aerogels properties, all the references cited in the figure were listed in Table S1, [9,10,14,15,17,19,22,27,34-60]: E modulus as a function of aerogel density (a), dependence of thermal conductivity on aerogel density (b) and elastic compressive modulus (c), and thermal conductivity as a function of specific surface area (d).](profile/Wim-Malfait-2/publication/360006814/figure/fig2/AS:1169684766429184@1655885988115/State-of-the-art-comparison-of-the-polyimide-and-polyimide-composite-aerogels-properties.png)
State of the art comparison of the polyimide and polyimide-composite aerogels properties, all the references cited in the figure were listed in Table S1, [9,10,14,15,17,19,22,27,34-60]: E modulus as a function of aerogel density (a), dependence of thermal conductivity on aerogel density (b) and elastic compressive modulus (c), and thermal conductivity as a function of specific surface area (d).
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Polymer aerogels are a promising, non-brittle alternative to silica aerogel, but are limited by their very poor high-temperature stability. Polyimide is widely known for its high heat-resistance, however, a high degree of volume shrinkage is common for polyimide aerogels after exposure to temperatures above 200 °C. Here, we present the aerogel-in-a...
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... is decreased thermal conductivity. Polyimide aerogel generally has very good mechanical properties, but its thermal conductivity is always higher than silica aerogel (Table S3). In this study, the pure polyimide aerogels display a thermal conductivity between 22.0 and 24.0 mW m -1 K 1 , among the best reported for polyimide aerogels (Table S3, Fig. 5), and below that of standing air (26.0 mW m -1 K 1 ) due to the Knudsen effect in the polyimide aerogel mesopores. The addition of silica aerogel reduces the thermal conductivity to below 20.0 mW m -1 K 1 , with the lowest value of 17.5 mW m -1 K 1 with 35 wt% silica loading (relative to the total dry mass). The thermal gradient of the ...
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... comprehensive dataset of density, modulus, specific surface area, and thermal conductivity of polyimide and polyimide composite aerogels was compiled (Table S3) to benchmark the PI-silica aerogel composites developed here (Fig. 5). In this dataset, the polyimide composite aerogels cover a wide variety of nanofillers, e.g. clay, silica, graphene oxide, cellulose, and glass fibers etc. The first Ashby plot describes the density dependence of the compressive modulus E (Fig. 5a). Most aerogel materials display a strong, power-law dependence, E ∝ ρ α , which appears ...
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... aerogels was compiled (Table S3) to benchmark the PI-silica aerogel composites developed here (Fig. 5). In this dataset, the polyimide composite aerogels cover a wide variety of nanofillers, e.g. clay, silica, graphene oxide, cellulose, and glass fibers etc. The first Ashby plot describes the density dependence of the compressive modulus E (Fig. 5a). Most aerogel materials display a strong, power-law dependence, E ∝ ρ α , which appears as a straight line on a log-log plot [61]. However, the properties of polyimide aerogel are influenced by many factors, i.e. a wide variety of starting precursors, crosslinkers, and imidizers, as well as gel formation and gel drying solvents [9]. ...
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... (i) of the samples after heat treatment. density, our polyimide and polyimide-silica composite aerogels display a moderate E modulus, which is not surprising considering it is prepared from a 'soft' ODA chain, which according to the literature, results in a relatively low Young's modulus [9]. The density dependence of thermal conductivity (λ) (Fig. 5b) displays a very broad positive correlation. Perhaps most relevant are the lowest thermal conductivity samples. Only a few polyimide and composite aerogels are reported with thermal conductivity below 20.0 or even 25.0 mW m -1 K 1 . Still, the best performing samples for a given density, i.e. the low λ boundary of the dataset, describe ...
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... composite aerogels, the heterogeneous aerogel-in-aerogel composites presented here display the lowest λ of 17.5 mW m -1 K 1 . In terms of application, a simultaneously low λ and high E modulus is optimal: the composite aerogel Pyr_40_3 is in one of the best scenarios for thermal insulation applications, with a very low λ at intermediate E C (Fig. 5c). The dependence of thermal conductivity on the surface area is highly scattered (Fig. 5d), most likely because many of the materials have thermal conductivities well above 30.0 mW m -1 K 1 , and in this range, any possible reductions of gas phase conduction through the Knudsen effect, which correlates with mesoporosity and surface ...
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... the lowest λ of 17.5 mW m -1 K 1 . In terms of application, a simultaneously low λ and high E modulus is optimal: the composite aerogel Pyr_40_3 is in one of the best scenarios for thermal insulation applications, with a very low λ at intermediate E C (Fig. 5c). The dependence of thermal conductivity on the surface area is highly scattered (Fig. 5d), most likely because many of the materials have thermal conductivities well above 30.0 mW m -1 K 1 , and in this range, any possible reductions of gas phase conduction through the Knudsen effect, which correlates with mesoporosity and surface area, are masked by the high solid, hence high overall conductions. As expected, λ values ...
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In this study, the mechanical, thermal and porosity properties of mortar samples containing aerogel and silica fume under different curing conditions were investigated. For this purpose, 0%, 0.25% and 0.50% by weight of silica aerogel as a cement additive and 10% silica fume as an industrial waste material were incorporated in the cement mixtures....
Citations
... For example, the peaks at 1780, 1720, and 1380 cm −1 confirm the presence of the imide ring. Solid-state NMR studies on polyimide aerogels prepared with near-identical formulations confirmed the chemical structure as well [36,51]. Polyimide in general and polyimide aerogels are well-known for their outstanding thermal stability. ...
... For polyimide aerogels, the density dependence of thermal conductivity has not been studied in great detail. Our recent papers on polyimide-silica(-fiber) aerogel composites also hint at a U-shaped density dependence, but the interpretation is complicated by the other components in the composites [25,51]. In addition, a previous parameter study on polymer concentration reported thermal conductivities above 30 mW/(m·K) for all investigated densities (0.080-0.014 g/cm 3 ), with no clear minimum as a function of density [69]. ...
Polyimide aerogels display excellent mechanical strength, high thermal stability, low thermal conductivity, and outstanding dielectric properties. Typically, the synthesis of polyimide aerogels involves the polycondensation of dianhydride and diamine into poly(amic acid) (PAA) oligomers, which are then cross-linked and chemically imidized into polyimide. The stoichiometry of dianhydride and diamine determines the number of repeat units and length of the PAA oligomers, which in turn determines the cross-linking density. Despite the critical role of polymer concentration and number of repeating units in determining the microstructure and properties of polyimide aerogels, few detailed studies exist on these two parameters. Here, we synthesized and characterized 16 polyimide aerogel formulations from the common monomers biphenyl-3,3′,4,4′-tetracarboxylic dianhydride (BPDA) and 4,4′-oxydianiline (ODA), with different repeat units (n = 5, 15, 30, 45) and total polymer concentrations (4, 7, 10, 13 wt%). An increased polymer concentration accelerated gelation and enhanced the mechanical performance of aerogels, but surprisingly, it also led to higher volumetric shrinkage during aging, solvent exchange, and supercritical drying (SCD). Specific surface areas (SSAs) reached a maximum at intermediate polymer concentrations. A shorter oligomer chain length, i.e., a higher cross-linking density, led to moderately higher SSAs (between 320 and 400 m²/g) and reduced shrinkage, resulting in lower densities for a given polymer concentration. The density dependence of the thermal conductivity exhibits a pronounced U-shaped curve with a minimum in thermal conductivity of 21–23 mW/(m·K) between 0.080 and 0.120 g/cm³, with somewhat lower values for more highly cross-linked aerogels. This systematic study of polyimide aerogels forms the basis for designing polyimide aerogels with tailored properties for targeted applications.
Graphical Abstract
... For instance, Zuo et al. effectively reduced the shrinkage from 27.8 vol.% to 21.1 vol.% by incorporating graphene and montmorillonite into PI aerogels [22]. Kantor et al. also observed a significant reduction in shrinkage upon the addition of silica aerogel powder into the PI aerogel matrix [24]. The volume shrinkage of the aerogel-in-aerogel composite with 28 wt.% silica aerogel powder added was reduced by 40% due to the high shape preservation capability of silica aerogel. ...
Polyimide (PI) aerogels, renowned for their nano-porous structure and exceptional performance across a spectrum of applications, often encounter significant challenges during fabrication, primarily due to severe shrinkage. In this study, we innovatively incorporated ceramic fibers of varying diameters into the PI aerogel matrix to enhance the shape stability against shrinkage. Thestructure of the resulting ceramic fiber-reinforced PI (CF-PI) aerogel composites as well as their performance in thermal decomposition, thermal insulation, and compression resistance were characterized. The results revealed that the CF-PI aerogel composites dried by supercritical ethanol achieved greatly reduced shrinkage as low as 5.0 vol.% and low thermal conductivity ranging from 31.2 mW·m−1·K−1 to 35.3 mW·m−1·K−1, showcasing their excellent performance in shape stability and thermal insulation. These composites also inherited the superior residue-forming ability of ceramic fibers and the robust mechanical attributes of PI, thereby exhibiting enhanced thermal stability and compression resistance. Besides, the effects of different drying conditions on the structure and properties of CF-PI aerogels were also discussed. The coupling use of supercritical ethanol drying with the addition of ceramic fibers is preferred. This preferred condition gives birth to low-shrinkage CF-PI aerogel composites, which also stand out for their integrated advantages include high thermal stability, low thermal conductivity, and high mechanical strength. These advantages attribute to CF-PI aerogel composites substantial potential for a wide range of applications, particularly as high-performance thermal insulation materials for extreme conditions.
... Therefore, how to solve the design and manufacture of composite materials to achieve compatibility of various properties is also a huge challenge. Most recently, aerogels have been widely studied due to their low density, hydrophobicity, and excellent thermal insulation properties, and have great application potential in aerospace [29][30][31][32]. The SiO 2 aerogel insulation composite material prepared by the fiber reinforced method, the aerogel as a continuous phase can effectively inhibit the heat transfer of gas and solid, the addition of fiber can provide a strong support for the aerogel to provide a strong support, so that the composite material has strong strength and toughness, while effectively inhibiting the radiation heat conduction at high temperature, so as to obtain the aerogel insulation composite material with both high heat insulation effect and good mechanical properties [33][34][35]. ...
... In contrast, vacuum freeze-drying emerges as a promising alternative method due to its cost-effectiveness and environmental friendliness. Zou et al. [96] synthesized Polyimide/carboxyl functionalized multi-walled carbon nanotube (PI/MWCNTs/COOH) composite aerogels featuring an aligned slitlike channel structure through unidirectional freeze-drying. The incorporation of carbon nanotubes enhances the rigidity of polyimide molecular chains, reducing its liquidity and increasing its char yield to 56.1 wt% at 800 • C. Zhao et al. [97] reported a series of all-aromatic anisotropic polyimide aerogels (p-phenylenediamine (PDA)) fabricated by random freeze-drying, exhibiting a thermal conductivity of 0.0559 W/(m·K). ...
High-temperature aerogels have garnered significant attention as promising insulation materials in various industries such as aerospace, automotive manufacturing, and beyond, owing to their remarkable thermal insulation properties coupled with low density. With advancements in manufacturing techniques, the thermal resilience of aerogels has considerable improvements. Notably, polyimide-based aerogels can endure temperatures up to 1000 °C, zirconia-based aerogels up to 1300 °C, silica-based aerogels up to 1500 °C, alumina-based aerogels up to 1800 °C, and carbon-based aerogels can withstand up to 2500 °C. This paper systematically discusses recent advancements in the thermal insulation performance of these five materials. It elaborates on the temperature resistance of aerogels and elucidates their thermal insulation mechanisms. Furthermore, it examines the impact of doping elements on the thermal conductivity of aerogels and consolidates various preparation methods aimed at producing aerogels capable of withstanding temperatures. In conclusion, by employing judicious composition design strategies, it is anticipated that the maximum tolerance temperature of aerogels can surpass 2500 °C, thus opening up new avenues for their application in extreme thermal environments.
... The observed trend in this The pore volume and average pore size were determined using the BJH method from N 2 desorption data. Uncertainties: BET surface areã 50 m 2 /g; pore volume and average pore diameter with a relative fluctuation of 5% [66] c,d ...
To enhance the thermal safety and preserve the excellent thermal insulation of hydrophobic silica aerogels (SA), sodium dodecyl sulfate (SDS) intercalated layered double hydroxides (LDH) was incorporated into SA by in situ doping to form SDS-LDH/SA composites. The intercalation modification by SDS extends the layer spacing of LDH and improves the dispersibility of LDH in SA, in favor of the combination between LDH and SA. The physical combination between the SA and SDS-LDH is demonstrated by FTIR analyses. As the content of SDS-LDH rises, the SDS-LDH/SA continues to exhibit a low density (0.11–0.20 g/cm³), low thermal conductivity (<26.8 mW/m/K), and large specific surface area (709.4–839.2 m²/g), ensuring excellent thermal insulation performance. It further finds that the SDS-LDH effectively absorbs heat and inhibits the thermal decomposition of SA. Therein, the onset temperature of thermal decomposition of the SA with 20% SDS-LDH is 114.0 °C higher than that of pure SA. Additionally, it also finds that the gross calorific values of the SDS-LDH/SA decrease with the SDS-LDH content, and all these gross calorific values are lower than that of the pure SA. Hence, SDS intercalated LDH presents significant effects on enhancing the thermal safety of hydrophobic SA without impairing the thermal insulation too much.
Graphical Abstract
... Kantor et al. [144] synthesized heterogeneous polyimide-silica aerogels with low shrinkage by adding silica aerogel particles into a polyimide sol. The polyimide-silica aerogels exhibited heterogeneous structures and have properties of a high surface area over 609 m 2 /g and a low thermal conductivity of 0.0175 W/m·K. ...
... As shown in Figure 17, the BTC solution and silica aerogel powders occurred gelation in the polypropylene container, and the samples were obtained through aging and supercritical drying. Kantor et al. [144] synthesized heterogeneous polyimide-silica aerogels with low shrinkage by adding silica aerogel particles into a polyimide sol. The polyimide-silica aerogels exhibited heterogeneous structures and have properties of a high surface area over 609 m 2 /g and a low thermal conductivity of 0.0175 W/m·K. ...
... The preparation of PI-silica composites. Reprinted with permission from Ref.[144]: Copyright 2022, Elsevier. ...
Silica aerogels are considered as the distinguished materials of the future due to their extremely low thermal conductivity, low density, and high surface area. They are widely used in construction engineering, aeronautical domains, environmental protection, heat storage, etc. However, their fragile mechanical properties are the bottleneck restricting the engineering application of silica aerogels. This review briefly introduces the synthesis of silica aerogels, including the processes of sol–gel chemistry, aging, and drying. The effects of different silicon sources on the mechanical properties of silica aerogels are summarized. Moreover, the reaction mechanism of the three stages is also described. Then, five types of polymers that are commonly used to enhance the mechanical properties of silica aerogels are listed, and the current research progress is introduced. Finally, the outlook and prospects of the silica aerogels are proposed, and this paper further summarizes the methods of different polymers to enhance silica aerogels.
... Note: a, b are obtained according to Eqs. (2) and (3) with 10% relative uncertainty [32]. Uncertainties: BET surface area around 50 m 2 /g; pore volume and average pore diameter, 5% relative [33]. ...
In this study, the surface chemistry, skeleton structure and thermal safety of three different methylsilyl modified silica aerogels (SAs) have been explored under heat treatment in an argon atmosphere. With the heat treatment temperature increasing to 700°C, the secondary particles aggregate, the silica skeletons get denser and the pore volumes diminish. Meanwhile, the excellent thermal insulation performance is still maintained, with the thermal conductivity not exceeding 30 mW⋅m − 1 ⋅k − 1. It finds the Si-(CH 3) 3 groups are converted to Si-(CH 3) 2 groups, and further transformed to Si-CH 3 groups. The generated and original Si-CH 3 groups can be maintained until 700°C, which constitutes the chemical basis for the high-temperature hydrophobicity. The improved thermal stability and reduced gross calorific values both verify that the thermal safety of the heat-treated SAs has been obviously enhanced. This study provides an engineering reference for the high-temperature thermal insulation application for hydrophobic SAs.
... However, conventional polymers such as PS and PP have limited maximum operating temperatures and relatively poor mechanical properties, which cannot meet the increasing requirements in diverse application scenarios [12,13]. Aromatic polyimides have been widely employed as dielectric materials for electronic packaging and electrical insulation in the microelectronics industry due to their superior thermal, mechanical, and dielectric properties [14][15][16][17][18]. Nevertheless, the k-value of conventional polyimides, such as commercial DuPont Kapton polyimide films, is typically between 3.1 and 3.5, which is insufficient for high-frequency microelectronics. ...
In the rapidly growing area of high-frequency communications, polyimide films with ultralow dielectric constant and dielectric loss, adequate insulating strength, and recyclability are in high demand. Using a synthesized soluble fluorinated polyimide, a series of recyclable porous dielectric films with varying porosities were fabricated in this study through nonsolvent-induced phase separation. By manipulating the mass ratio of the binary solvent used to dissolve the polyimide, the shape, size, and size distribution of the pores generated throughout the polyimide matrix can be accurately regulated. The porosity and average pore size of the as-prepared porous films were adjustable between 71% and 33% and between 9.31 and 1.00 μm, respectively, which resulted in a variable dielectric constant of 1.51–2.42 (100 kHz) and electrical breakdown strength of 30.3–119.7 kV/mm. The porous sPI film with a porosity rate of 48% displayed a low dielectric constant of 2.48 at 10 GHz. Coupled with their superior thermal stability, mechanical characteristics, and recyclability, these porous polyimide films are highly promising for constructing high-frequency microelectronic devices.
... Hence, the fabrication of aerogels with an outstanding combination of mechanical properties and high-temperature resistance via a simple method is still a great challenge. Among all kinds of organic constituents, the imide ring exhibits the highest initial decomposition temperature of 550 °C [25,26], which donates high-temperature stability for polyimide (PI) chemical structure. In addition, the chain structure constructed by imide rings also imparts PI with superior mechanical properties and favorable chemical resistance. ...
... In addition, the chain structure constructed by imide rings also imparts PI with superior mechanical properties and favorable chemical resistance. Even though the chemical structure is rather stable, a common problem for PI aerogels with dominated mesoporosity: the extensive volume shrinkage and structural collapse when exposed to moderately high temperatures (> 150 °C), is well below the thermal decomposition temperature of the polyimide itself [25]. The preparation of PI includes two steps. ...
Polymer-reinforced silica aerogel is one of the promising strategies to improve the weak mechanical properties of silica aerogel. However, their high-temperature applications (> 300 °C) are often plagued by the inherent thermal decomposition of polymer additives, which intensively affects the practical applications in the high-temperature use scenarios. In this paper, we used trace polyimide (PI) to modify the microstructure of polymethylsilsesquioxane (PMSQ) aerogel by co-gelating polyamic acid salt and methyltrimethoxysilane in an aqueous solution. The drying and thermal imidization of polyamic acid can be achieved simultaneously during the ethanol supercritical fluid drying process, and a special neuronal network-like silica aerogel structure was built through the templating of polymer additives. As-prepared PMSQ-PI hybrid aerogel presents a dominated mesoporous structure and inherits methyl hydrophobe ligands from methylsiloxane precursor, which leads to superior properties of ultra-low thermal conductivity (0.016 W·m⁻¹ K⁻¹), super-hydrophobicity (water contact angle of 151°), high flexibility (compressibility of 80% and radial deflection of 27%), excellent machinability, and high-temperature (400 °C to 1050 °C) resistance. This new class of materials opens up new perspectives for high-temperature-resistant insulators by using strong silica-polymer hybrid aerogels.
Graphical Abstract
... Theoretically, the difference in the structure of diamine and dianhydride precursors used in polyimide will affect the microstructure and properties of aerogels [15]. The schematic diagrams corresponding to the linear and cross-linked PAAS structures using different dianhydrides, diamines and crosslinking agent as shown in Figure 1a,b. ...
A series of high-performance linear and cross-linked polyimide (PI) aerogels with different molecular structures have been successfully synthesised using the freeze-drying process. In this study, the comprehensive regulation of microstructure, thermodynamic, thermal insulation and dielectric properties of PI aerogels are achieved by controlling the rigid/flexible structure and composition of polymerised monomers. The increase in rigidity of PI molecular structure could promote the formation of denser pores, which is beneficial to improve the thermodynamic and thermal insulation properties of aerogels. Notably, the cross-linked PI aerogel prepared by introducing cross-linking agent (tris(4-aminophenyl) amine, [TPA]) into linear PI exhibits high thermal stability (Td5% > 560°C), excellent ultralow permittivity (εr = 1.31, f = 10⁶ Hz) and good thermal insulation property (k = 0.056 W/m · K). This innovative strategy promotes the wider application of the cross-linked polyimide aerogel in the field of integrated circuits and aerospace exploration. © 2022 The Authors. IET Nanodielectrics published by John Wiley & Sons Ltd on behalf of The Institution of Engineering and Technology.
