Introduction

Aerogels with low density, high specific surface area, low thermal conductivity, and high-temperature stability are good candidates for thermal insulation and protections1,2,3,4. Damage-tolerant ceramic aerogels5 with cyclic elasticity and emerging thermo-mechanical properties such as near-zero/negative Poisson’s ratio and thermal expansion coefficient have attracted increasing attention in recent years6,7,8. These materials are very useful and urgently needed for applications at high temperatures and other harsh environments, yet their service temperature is still unsatisfactory. Oxide ceramic aerogels such as SiO2 and Al2O3 are the most studied material systems, with good stability up to ~800–900 °C9,10. Higher service temperatures often cause coarsening, collapse of the pore structure, volume contraction (sintering), and degradation of the aerogel properties. Non-oxide ceramic aerogels such as SiC, because of stronger covalent bonds and higher melting point, could further extend the service temperature range and enable new applications. The constitution of SiC aerogels can be SiC nanoparticles, nanorods, nanowhiskers, and nanowires. Among them, three-dimensionally (3D) connected nanowires offer SiC aerogels improved elastic properties, flexibility, and damage tolerance. They are able to provide low thermal conductivity (down to 0.014 W m−1 K−1)11, high stiffness (with specific modulus up to 24.7 kN m kg−1)12, and large temperature stability window (up to 1100 °C and down to liquid nitrogen temperature)13.

The excellent properties of SiC nanowire aerogels have stimulated the scientific research and industrialization, especially on the synthesis methods. Up to now, SiC nanowire aerogels have been reported to be successfully synthesized by sol-gel method14, carbothermal reduction15,16,17, pulsed laser deposition18, chemical vapor deposition19,20,21, freeze forming12,22, and three-dimension printing11. Many progresses have been made to obtain SiC nanowire aerogels with tunable microstructures, cyclic compressibility/stretchability, and other desirable thermo-mechanical properties. Nevertheless, the existing methods suffer from high cost, long synthesis cycle, complicated processing, and poor scalability. An inexpensive fast synthesis route is urgently needed for further development of SiC aerogels and their rapid penetration into industrial applications.

Combustion synthesis invented by Merzhanov et al.23,24 is a fast synthesis method with low cost, near zero energy consumption, and good scalability up to ton scale per batch (e.g., as demonstrated for SiC powders25). It is based on strong exothermic chemical reactions that can propagate rapidly through solid-state reactants26 and has been widely used to synthesize ceramic powders. For powders, the combustion synthesis parameters have been optimized towards high crystallinity, phase purity, equiaxial grains, fine size, narrow size distribution, minimal agglomeration, high compact density, and good sinterability. These characteristics are beneficial for follow-up ceramic processing and sintering. The application of combustion synthesis in synthesizing SiC powders was first explored by Pampuch et al.27 and later used by other researchers. Interestingly, there have been occasional reports that SiC nanowires and fibers were produced by combustion synthesis, if some raw chemicals (known as reaction promoters or activators) are present. For example, Nersisyan et al.28 suggested to use polytetrafluoroethylene (PTFE) to synthesise SiC powder and SiC wool by combustion synthesis. Huczko et al.29 used a mixture of Si-containing compounds, halogenated hydrocarbons, and PTFE as the raw materials and prepared one-dimensional (1D) nanostructures of cubic-phase silicon carbide (β-SiC) by combustion synthesis. Similarly, SiC nanowires have been obtained by reacting Si and PTFE in the calorimetric bomb30,31. Nevertheless, all previous reports ended up with loose powders and no 3D products in large-size bulk form can be directed made.

If these occasionally produced 1D nanostructures can be sinter-bonded into 3D bulk materials while maintaining high porosities, it seems plausible to directly produce SiC aerogels from one-step combustion synthesis. The proposal is reasonable given the fact that combustion process is accompanied by rapid self-heating and volumetric expansion of the gaseous phase. Indeed, combustion synthesis of high-porosity materials with significant expansion from initial to final form has been reported in the literature32. The application of combustion synthesis to direct synthesis of large-size bulk aerogels is the main progress to be reported in the present work. Remarkably, we demonstrated rapid production of high-performance SiC aerogels (e.g., with low thermal conductivity, negative Poisson’s ratio, and near-zero thermal expansion coefficient). SiC aerogels with 3D connected porous structures were obtained in a few seconds, with very low cost (>100 times cheaper than existing methods), and up to liter scale per batch in lab experiments. The process shows the feasibility of large-scale production of SiC aerogels with significantly reduced energy, time, and capital input. The technological breakthroughs and insights may encourage novel applications of combustion synthesis and lead to the invention of other bulk porous materials via renovated fast synthesis routes.

Results and discussion

Synthesis route

For combustion synthesis, Si-C is a weakly exothermic system. Its adiabatic temperature (1600–1700 K) is lower than the empirical criterion of self-sustained combustion reactions (1800 K)23. Therefore, combustion synthesis of SiC under normal conditions is difficult to achieve. Extra energy supply or effective reaction promoters can increase the reaction activity and decrease the reaction energy barrier33. In the present work, PTFE was used as the reaction promoter and gasifying agent. The production of a large amount of gas phase during the reaction helps the formation of porous products34. To enable flash synthesis of SiC nanowire aerogels in the bulk form, we proposed to use fine Si particles and high-molecular-weight PTFE as the raw materials for the following reasons. First, the chemical reaction between Si and PTFE is highly exothermic with a fast combustion velocity35,36. (Calorific value of reactants is shown in Supplementary Table 1.) A finer Si particle size has more surface area and can have more contact with other reactants, thus increasing the reactivity37. Second, the reaction could have multiple steps:

$${{{\rm{Low}}}} \, {{{\rm{temperature}}}}: 2n{{{\rm{Si}}}}(s)+{(-{{{{\rm{C}}}}}_{2}{{{{\rm{F}}}}}_{4}-)}_{n}=2n{{\mbox{SiF}}}_{2}(g)+2n{{{\rm{C}}}}(s)$$
(1)
$${{{\rm{Intermediate}}}} \, {{{\rm{temperature}}}}: 2{{\mbox{SiF}}}_{2}(g)+{{{\rm{C}}}}(s)={\mbox{Si}}{{{\rm{C}}}}(s)+{{\mbox{SiF}}}_{4}(g)$$
(2)
$${{{\rm{High}}}} \, {{{\rm{temperature}}}}: {{{\rm{Si}}}}(g)+{{{\rm{C}}}}(s)={\mbox{Si}}{{{\rm{C}}}}(s)$$
(3)

in which C and SiC are always in the solid form38,39. As a result, the C produced by Eq. (1) and the SiC produced by Eqs. (2) and (3) may inherit the morphological feature of the reactant PTFE. Here PTFE serves as the carbon source and we noticed a critical role of its polymerization degree, or equivalently, the molecular weight (see details in Supplementary Methods). Accordingly, if PTFE with long molecular chains and extensive inter-chain entanglements was selected as the reactant, SiC nanowire aerogels may be directly obtained in one-step combustion process.

With the above considerations and appropriately chosen raw materials, we succeeded in igniting Si-PTFE combustion that reached to a peak pressure of >57 atm in 2 s, a peak temperature of >1800 °C in 4 s, and a volume expansion of 1076% after cooling down to room temperature (Fig. 1a, b). Figure 1c shows the snapshots of the early combustion process (0.30 s to 0.93 s) inside the reaction chamber. The propagating rate of the combustion front is estimated to be 30 cm s−1 (see details in Supplementary Methods). During the reaction, the duration of high-temperature step above 1600 °C is about 4.69 s. So the estimated volumetric expansion rate was 1.56 L s−1. Figure 1d, e show the dramatic volume expansion from the initial reactant powders to the final product as compared in the same reaction boat (more details in Supplementary Fig. 1). The prepared SiC aerogels exhibited a homogeneous internal structure, a lightweight nature, and self-supporting characteristics (Fig. 1f, with a density of ~12 mg cm−3). (The non-uniform color at the surface is due to impurities such as rust from the direct contact with the inner wall of the stainless-steel reactor chamber and does not affect the properties of the synthesized SiC aerogels.) Our flash synthesis, once ignited, is self-sustaining and requires no additional energy input. It is highly time- and cost-effective. As detailed in Supplementary Note 1, we estimated a synthesis time of 0.5 h per batch and a synthesis cost of 0.7 $ L−1, which are >10 times faster and >100 times cheaper than the existing synthesis methods of aerogels (Fig. 1g)16,17,20,40,41,42,43,44. Since the volume of the obtained aerogels is compatible with the reactor chamber, the macroscopic size and shape of the SiC aerogels are also highly controllable.

Fig. 1: Flash synthesis of SiC aerogels.
figure 1

a Schematics for the flash synthesis of SiC aerogels. b Pressure and temperature profiles during the flash synthesis. c Snapshots of unstable propagating combustion wave for the chosen Si-PTFE reaction. d–e Photographs of the initial reactant powders before combustion synthesis and the final product after combustion synthesis in the same reaction boat. f Photograph of a bulk SiC aerogel (volume: ~2.5 L). g Comparison of the synthesis time and cost between our method and the ones reported in the literature16,17,20,40,41,42,43,44.

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Morphology and structure

The synthesized aerogels are β-SiC (X-ray diffraction, XRD in Fig. 2a). Under scanning electron microscope (SEM), they have a lamellar structure (Fig. 2b) at the mesoscale and a 3D connected network structure (Fig. 2c) at the nanoscale. The lamellar structure is likely to form due to the deformation of the expanding aerogels constrained by the limited size of the reaction chamber (see the proposed mechanism in Supplementary Note 2). The bulk density is ~12 mg cm−3, corresponding to 99.6% porosity (Supplementary Fig. 6). Benefiting from the rapid gas-solid reaction and the fast growth rate, the SiC nanowires are inter-twinned with load-bearing nodes (Fig. 2d). The diameters of the SiC nanowires are in the range of 20–300 nm (Supplementary Fig. 7). Their length varies from a few to tens of microns. The nanowires have apparent traces of helical growth (Supplementary Fig. 8), due to stress/strain accommodation by rotation along the growth direction in the synthesis36,45. High-resolution transmission electron microscope (HRTEM) image in Fig. 2e shows a characteristic distance of 0.25 nm, corresponding to (111) plane spacing of β-SiC. The surface of the SiC nanowires has an amorphous SiOx layer with a thickness of ~5 nm (Fig. 2f–h). The amorphous SiOx surface phase is further verified by Fourier transform infrared spectrometer (FTIR, Supplementary Fig. 9) and X-ray photoelectron spectroscopy (XPS, Fig. 2i–l). The formation of SiOx is due to surface oxidation/passivation from residue oxygen in the raw materials and in the reactor chamber. (The oxygen content of the initial Si powders is 1.5 wt% and the oxygen content of the synthesized SiC aerogels is 4.6 wt%.).

Fig. 2: Morphology and structure of SiC aerogels.
figure 2

a XRD. b Low-magnification SEM. c High-magnification SEM. d TEM. e HRTEM. f, g TEM showing the amorphous surface SiOx layer. Inset of f, Selected area diffraction showing β-SiC phase. h Energy-dispersive X-ray spectroscopy (EDS) mapping showing uniform C, O, and Si distribution. The O signal is attributed to the surface SiOx layer. i–l XPS survey spectrum i and Si 2p j, C 1 s k, and O 1 s l spectra.

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Compressibility, elasticity, and flexibility

The synthesized SiC aerogels, because of high porosity and damage tolerance, can be easily cut into different shapes and sizes (Supplementary Fig. 10). They have a negative Poisson’s ratio υ of −0.09 ± 0.02 (average and standard deviation calculated from three measurements) and a near-zero linear thermal expansion coefficient α (Fig. 3a; α is negative at elevated temperatures, e.g., −1.01 × 10−6 K−1 at 800 °C), both being metamaterial properties that have been considered only possible for precisely designed structures6,46,47. Our demonstrations of negative υ and near-zero/negative α in inexpensive aerogels are thus remarkable.

Fig. 3: Mechanical properties of SiC aerogels.
figure 3

a Thermal expansion coefficient at various temperatures. b σ-ε curves with maximum strains of 20, 40, 60, and 80% at room temperature. c Cyclic σ-ε curves with maximum strains of 40% at room temperature. d σ-ε curves with maximum strains of 20, 40, 60, and 80% at −196 °C. e Cyclic σ-ε curves with maximum strains of 40% at −196 °C. f σ-ε curves with maximum strains of 40% at room temperature, after repeated heating cycles to 1200 °C in air. g, h Photos showing facile bending g and curling h of SiC aerogel papers.

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The negative υ is mainly due to microstructure rather than material composition48. The interior of SiC aerogels can be regarded as an open hinged network49, which is formed by a large number of concave (curved) pore wall units connected. Multiple adjacent pore walls are regarded as a basic folding element (Supplementary Fig. 11). During compression, the pore walls of this structural unit will undergo deflection deformation50, resulting in lateral contraction. Therefore, the SiC aerogels prepared by combustion synthesis always maintain a hyperbolic macrostructure during the compression process (Supplementary Fig. 12). The negative α of SiC aerogels synthesized by combustion is due to the release of thermal stress in the SiC nanowire network/frame. With the high temperature and high pressure during the combustion synthesis reaction, the SiC nanowires produced by the reaction mostly show buckling morphology. With the rapid cooling of the reaction system, the SiC aerogels produced a large thermalstress concentration. During the reheating process, the SiC nanowires bend inward and/or rotate due to the dual action of thermal expansion and thermal stress release, which causes the overall structure to shrink. When the shrinkage exceeds the intrinsic thermal expansion of the nanowires, SiC aerogels exhibit negative thermal expansion properties51. In addition, the ultra-high porosity of the aerogel (99.6%) provides enough space to accommodate the deformation of the nanowires, so the negative α of the SiC aerogels is close to zero.

To further evaluate the mechanical properties, we measured the compressive stress (σ)-compressive strain (ε) curves up to the maximum strain of 20, 40, 60, and 80% (Fig. 3b). Compared to brittle aerogels synthesized by traditional colloidal processes52,53, our SiC aerogels can recover the original shape after large-strain compression. The σ-ε response can be classified into two stages, including a linear elastic deformation regime at ε < ~20% (with more-or-less constant compressive modulus of 0.95 kPa) and a nonlinear hardening regime at ε > ~20% (with increasing maximum stress up to 8.24 kPa at ε = 80%). It has a hysteresis loop under loading/unloading and the hysteresis becomes larger under larger strains. Dynamic mechanical tests show that the storage modulus, loss modulus, and damping ratio weakly depend on the angular frequency between 20 Hz and 100 Hz (Supplementary Fig. 13). Meanwhile, the high compressibility can be maintained over extended cycles, e.g., with <10% degradation in compressive strength (Fig. 3c) and ~8.5% thickness decrease (compared to the original dimension; Supplementary Fig. 14) after 100 loading/unloading cycles over ε from 0 to 40%. Therefore, the synthesized SiC aerogels can be useful for repeated energy dissipation under mechanical shocks54,55. A whole bunch of aerogels can be self-supporting. The samples can be handled manually with ease as shown in Supplementary Fig. 15. The excellent mechanical properties of the synthesized SiC aerogels can be attributed the laminated mesoscale structure and the well-bonded nanowire nanoscale structure, which promotes uniform straining, mitigates stress concentration, and thus protects the overall architecture from catastrophic failure56.

The aerogels have excellent mechanical properties over a wide temperature range. Figure 3d shows the σ-ε curves at liquid nitrogen temperature (−196 °C), with maximum strain of 20, 40, 60, and 80%. The high compressibility is maintained, with slight increase in the modulus compared to that at room temperature. Cyclic compression tests in Fig. 3e demonstrate high reversibility is also maintained at liquid nitrogen temperature. To test the service stability at high temperatures, we heated the synthesized SiC aerogels to 1200 °C, measured the σ-ε curve after rapid cooling to room temperature, and repeated the practices over extended cycles. As shown in Fig. 3f, the aerogels become stiffer in the initial heating-cooling cycles and the modulus reaches the maximum at the 50 cycles. Further exposure to the heating-cooling cycles results in slight decrease in the modulus. Nevertheless, high compressibility and reversibility are ensured throughout the experiments. As further proof, Supplementary Fig. 16 shows that the SiC aerogels can be in situ compressed and then recover the original shape after unloading at 1100 °C. Lastly, the synthesized SiC aerogels can be manually handled to bend and curl and recover their original shape after unloading (Fig. 3g, h). These mechanical properties and the structural robustness would provide many opportunities for the practical application of SiC aerogels.

Thermal properties and stability

A key application of aerogels is for thermal insulation. Low thermal conductivity and good high-temperature stability are required. To visualize the excellent thermal insulation properties of the synthesized SiC aerogels, a fresh petal was placed on top of an aerogel cloth (~3 mm thick) heated by alcohol lamp underneath (Fig. 4a). No damage was observed after 10 min continuous heating. The SiC aerogels with ~12 mg cm−3 density have a thermal conductivity of 0.027 ± 0.002 W m−1 K−1 at room temperature (Fig. 4b). By modifying the synthesis conditions, we can obtain SiC aerogels with lower densities of 6.4 mg cm−3 and 8.7 mg cm−3, which have lower thermal conductivities of 0.023 ± 0.004 W m−1 K−1 and 0.025 ± 0.001 W m−1 K−1, respectively. Elevating the temperature slightly increases the thermal conductivity (Fig. 4c; 0.061 ± 0.008 W m−1 K−1 at 200 °C, 0.088 ± 0.012 W m−1 K−1 at 400 °C, and 0.125 ± 0.007 W m−1 K−1 at 600 °C for ~12 mg cm−3 density aerogel) yet they can still offer superior thermal insulation. The insulation performance is directly related to the thickness of the aerogel block (Supplementary Fig. 17). If placed on a hotplate, the temperature rise is slower for the 14-mm-thick sample than thinner samples (Fig. 4d), and the steady-state temperature on the top surface of the aerogel is also lower for thicker samples (Fig. 4e). To evaluate under more extreme conditions, we exposed a 20 mm-thick SiC aerogel to butane burner flame (Fig. 4f; flame temperature about 1100 °C). The temperature difference between the top and bottom surfaces is about 900 °C, indicating excellent thermal insulating properties at high temperatures (Supplementary Fig. 18). Meanwhile, the aerogel remains stable, with no observable damage or shape change. More strict long-term stability tests were next conducted. With a holding time of 0.5 h, we observed no phase transition for SiC aerogels up to 1100 °C in air and up to 1700 °C in Ar (Supplementary Fig. 19), as well as minimal microstructure change (Supplementary Fig. 20) and volume shrinkage (Supplementary Fig. 21) up to 1000 °C in air. Therefore, our SiC aerogels can offer superior high-temperature insulations in both oxidizing (up to 1000 °C for long-term service and higher temperature for transient service) and non-oxidizing atmospheres (>1700 °C for long-term service) over existing materials (detailed comparison listed in Supplementary Note 3).

Fig. 4: Thermal insulation performance of SiC aerogels.
figure 4

a Photos showing a petal placed on a 3-mm-thick SiC aerogel heated by an alcohol lamp. b Room-temperature thermal conductivities of SiC aerogels with different densities. (Error bars represent standard deviation, n = 3 independent replicates.) c Thermal conductivities of the SiC aerogels (density: ~12 mg cm−3) at elevated temperatures. (Error bars represent standard deviation, n = 3 independent replicates.) d Temperature profiles as a function of heating time of the SiC aerogels with different thicknesses on a hot plate. e Radiation temperature profiles at the top surface of the SiC aerogels in d. f Optical and infrared images of SiC aerogels exposed to butane burner flame (flame temperature: ~1100 °C) for 5 min.

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To summarize, we invented a flash synthesis method to produce large-piece bulk SiC aerogels with unprecedentedly low cost (>100 times cheaper than existing methods), high throughput (>10 times faster than existing methods), and minimal energy consumption (self-sustaining after ignition). The synthesized inexpensive SiC aerogels have not only low density, low thermal conductivity, and wide service temperature range, but also metamaterial properties such as negative Poisson’s ratio and negative thermal expansion coefficient. The further expansion of the equipment and scale-up of the synthesis are very important and we are very optimistic about large-scale production of SiC aerogels using the method reported here. We believe the technological breakthrough would enable a wider range of applications for ceramic aerogels in daily life, industry, and extreme-condition explorations. Before ending, we would like to point out three issues for future research and development. First, the synthesized SiC aerogels may provide multi-functional properties such as electromagnetic wave absorption, self-cleaning, and wastewater treatment. Such possibilities have been briefly explored (Supplementary Notes 4-6). Second, the fast combustion synthesis route of ceramic aerogels can be extended to other material systems, such as nitrides, carbonitrides, oxides, MAX phases, and their composites. As further support, we show in Supplementary Note 7 that TiC nanowires can be readily obtained from combustion synthesis like the process reported in this study. Thus, our work offers ample opportunities in exploring combustion synthesis for novel products. Lastly, we emphasize that the waste gas, some being rather toxic, should be properly handled in lab and industrial combustion synthesis processes. For example, some SiF4 was produced in the combustion synthesis reaction between Si and PTFE, which had been properly removed by venting through a base solution.

Methods

Materials and synthesis

Importantly, safety needs to be assessed before the experiment. The combustion synthesis of the Si-PTFE system will result in a significant increase in pressure, which will present a considerable challenge for the pressure vessel. Accordingly, when experimenting again, it is advisable to begin with a relatively small quantity of reactants. The reaction process will yield a toxic fluoro-silicon gas; thus, the gas product must be treated with lye before discharge.

Si powders (Si, purity: \(\ge\)99.99%, \({d}_{50}\)=3.0 μm, Fuzhou Hokin Chemical Technology Co., Ltd, China) and PTFE powders ((C2F4)n, purity: \(\ge\)99.99%, \({\bar{M}}_{n}=92,720,000\), \({d}_{50}\)=10 μm, Dongguan Yupin Plasticizing Co., Ltd, China) were mixed with a molar ratio of 3:1 in a monomer casting nylon jar (500 ml) with zirconia balls (3 mm in diameter) using ball milling for 2 h. The grinding medium is anhydrous ethanol (C2H6O, AR, Sinopharm Chemical Reagent Co., Ltd, China), and the mass ratio of mixed powder to ethanol is 1:1. The ball to mixture ratio was 5:1. The rotational speed was 360 rpm.

The mixed, dried, and sieved materials were placed in a stainless-steel reaction boat and transferred to a reaction chamber with a volume of 30 L and a maximum pressure of 8 MPa. The chamber was then vacuumed to −0.09 MPa, followed by injecting Ar gas to 1 MPa. The combustion reaction is ignited by a resistively heated tungsten wire. The tungsten wire is placed at one end of the boat containing the reactants. AC is then passed through the tungsten wire, which heats the reactants locally (of a volume around 1 cm3) for ignition. After initiation, the chemical reaction propagates through the entire reactants in the form of a rapidly moving combustion wave. The schematics in Supplementary Fig. 31 show the equipment, including the position of the tungsten wires for ignition. The temperature and pressure of the chamber were monitored during the process.

The temperature was measured by C type W-Re5/26 thermocouples (\(\phi\)= 0.5 mm \(\times\) 80 mm; temperature measurement accuracy: ±1% t) attached to a four-channel data acquisition system (the maximum acquisition rate is not less than 25 ms each time). The temperature in the reactor was measured with tungsten-rhenium (W-Re) thermocouples. The hot end of the W-Re thermocouple was placed near the center of the reactant powders. The reaction pressure is recorded by the pressure display of the reactor and collected at 10 points per second. When the reaction was finished, the temperature was cooled, and the pressure was restored to ambient condition, the synthesized large-size bulk aerogel was taken out for further characterization and analysis. The propagation rate was measured by frames of video recording of combustion process and comparing the time difference between two thermocouples with known spacing to reach the highest temperature.

Characterizations

Phases were characterized by XRD (D8 Focus, Bruker, Germany) with Cu Kα radiation. Microstructures were characterized by SEM (S-4800, Hitachi, Japan) and TEM (JEM-2100F, JEOL, Japan). Densities were calculated by the weight and the geometry. Thermogravimetric analysis was conducted on a DSC/TGA thermal analyzer (STA 6000, PerkinElmer, USA). Chemical analysis was conducted using FTIR (Excalibur 3100, Varian, USA) and XPS (ESCALAB 250Xi, Thermo Fisher Scientific, UK). The oxygen content of the initial Si powders and the SiC aerogels using an oxygen and nitrogen analyzer (EMGA-820, HORIBA, Japan). N2 adsorption/desorption was measured using a Quadrasorb SI-MP instrument (Quantachrome, USA) at 77 K.

Mechanical properties

Poisson’s ratio was calculated from the geometry changes under mechanical loading, with a maximum compressive strain of 25%. Poisson’s ratio was measured three times to calculate the average and standard deviation. The linear thermal expansion coefficient was measured using a thermal dilatometer (L75VD1600, Linses, German). SiC aerogels were tested under nitrogen atmosphere with a sample length (L) of 3 mm, test temperature from room temperature to 800 °C, a heating rate of 10 K min−1, a contact pressure of 50 mN, and instrument resolution of 0.03 nm. Mechanical tests were performed using a dynamic mechanical analysis (DMA) instrument (TA Q800, USA).

Thermal insulation properties

Thermal transport properties were tested on a Hot Disk instrument (TPS2500S, Switzerland) under transient mode by ISO 220072:2015. The aerogels cut into cubes with a length and width of 4 cm \(\times\) 3 cm and thickness of 5 mm were measured in the isotropic module of bulk type by the Kapton insulated nickel sensor (7577 F1) with a radius of 2.0 mm. The heating power was 4 mW, and the measuring time was selected as 5 s. Three measurements were conducted to calculate the average and standard deviation. Sample temperatures were monitored using an infrared thermal imager (VarioCAM HD head, InfraTec, Germany).