Transparent, flame retardant and machinable cellulose/silica composite aerogels with nanoporous dual network for energy-efficient buildings

doi:10.21203/rs.3.rs-3742276/v1

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Transparent, flame retardant and machinable cellulose/silica composite aerogels with nanoporous dual network for energy-efficient buildings

https://doi.org/10.21203/rs.3.rs-3742276/v1

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published 17 Sep, 2024

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The envelope structure with high light transmittance accounts for an increasing proportion of building energy consumption, which is one of the shortcomings of energy conservation and emission reduction. Cellulose-based aerogel has become a research hotspot because of its low thermal conductivity and good mechanical properties. However, most cellulose-based aerogels are opaque and flammable limiting their applications. Herein, cellulose/silica composite aerogels (CAS) with "organic-inorganic" structures were fabricated by two-step sol-gel method, spin-coating technique and supercritical CO₂ drying, using the ionic liquid 1-allyl 3-methylimidazolium chloride salt to dissolve the Cotton pulp, followed by the addition of tetraethylorthosilicate (TEOS) and methyltriethoxysilane (MTES) co-precursors into the cellulose gels. The synthesis mechanism, microstructure, mechanical and thermal properties of as-prepared aerogels samples were investigated. The obtained CAS have low density (0.093–0.170 g/cm³), high specific surface area (660.87-1089.70 m²/g), and high mechanical property (compressive strength of 18.74 MPa, tensile strength as high as 1.54 MPa, and bending tests above 500 times). In particular, the CAS4 shows the lowest thermal conductivity (0.0188 W·m^− 1·K^− 1), good thermal stability (> 331°C), high transparency (91.7%) and excellent flame retardancy. In addition, the self-designed aerogels glasses model was placed in a real outdoor environment for 5 hours. The results showed that the temperature difference between the inside and outside of the aerogels glasses model was as high as 12 ℃ under the thermal equilibrium state. Thus, the as-prepared high-performance cellulose/silica composite aerogels may increase the role of aerogels glasses in the building envelope and have promising applications in transparent energy-efficient construction and thermal insulation.

cellulose/silica composite aerogels

high transparency

machinable property

flame retardant

thermal insulation

With the continuous acceleration of urbanization and the continuous improvement of popular quality of life, the loss of building energy consumption is becoming increasingly serious and the energy crisis is intensifying. People's demand for green and energy-saving buildings is growing, and it is urgent to make every effort to solve the problem of building energy consumption (Liu et al. 2021). At present, the heat transfer coefficient of traditional glass is large, resulting in a building envelope energy consumption of 40% of the overall building energy consumption (Garnier et al. 2015, Mi et al. 2016), and energy-efficient glass for low-energy buildings is a breakthrough point to solve the problem. Since aerogels has various excellent physical properties, aerogels glass has attracted much attention in recent years (Li et al. 2020). Silica aerogels have been used in electrical (Ghaffari-Mosanenzadeh et al. 2022), thermal (Dorcheh and Abbasi 2008), optical (Cuce et al. 2014), and chemical applications due to their unique properties such as low density (Shao et al, 2014), high specific surface area, low refractive index, and low speed of sound. In particular, they also have very good optical properties and can be used in translucent envelopes (Buratti et al. 2021). This combination opens up interesting possibilities for the renovation of buildings with glazed areas and even for the construction of new buildings (Baetens et al. 2011). Silica aerogels have been used in a wide range of architectural applications over the last two decades. However, their application of architectural glass has been limited due to their low strength, brittleness, low light transmittance (particle or powder), and more difficult preparation of flexible large blocks (Fig. S1) (Dorcheh et al. 2008). Therefore, cellulose-based aerogels have a wide range of applications in energy-efficient buildings due to their lightweight, flexibility, porosity, and renewability.

Cellulose, a natural linear macromolecule widely found in the cell wall structure of higher plants, is one of the most abundant green biomaterials. Aerogels made from cellulose are receiving more and more attention in the fields of energy-saving construction (Aguilar-Santana et al. 2020, Cuce et al. 2015), catalyst carriers (Koga et al. 2012), adsorbents (Zhao et al. 2020), biopharmaceuticals (Zhao et al. 2015), and water purification because of their renewable, biodegradable (Peng et al. 2016), environmentally friendly and hydrophilic nature of natural vitamins themselves (Klemm et al. 2005). The preparation process of cellulose aerogels is carried out in three main stages as follows (Cai et al. 2008). The first step is to dissolve the cellulose. In the second step, the cellulose is precipitated in a suitable liquid medium to penetrate the structure and displace the solvent, thus inducing gelation and solvent displacement. The final step is drying, which includes freeze-drying (Sescousse et al. 2011), supercritical CO₂ drying (Hoepfner et al. 2008), and ambient pressure drying (APD) (Shang et al. 2021). However, cellulose is usually difficult to dissolve in ordinary aqueous or organic solvents, mainly due to its strong inter- or intramolecular hydrogen bonding, high degree of polymerization, and high degree of crystallinity (Zhang et al. 2012). The applications of cellulose aerogels in buildings are limited, mostly result from their inherent flammability, higher thermal conductivity, and low mechanical properties. In order to improve the combination properties of cellulose aerogels and exploit their additional advantages, recent studies have explored physicochemical methods for surface modification including atomic layer deposition (ALD) (Korhonen et al. 2011), alkylene-dimer (AKD) modification (Russler et al. 2012), and nanoparticle deposition (Liu et al. 2012). However, although the above methods can promote the potential functional applications of cellulose aerogels, there are still some drawbacks in the preparation process, such as high synthesis cost and complicated process. Therefore, silica aerogels obtained by a simple and mature preparation process can be introduced by using the sols-gels method to improve the mechanical strength and flame retardancy of cellulose aerogels (Cai et al. 2012). Zhao et al. investigated the multiscale assembly of super-insulating silica aerogels in silicon-based nanocellulose scaffolds. The results showed that the new composite aerogels had lower thermal conductivity (≤ 0.020 W·m^− 1·K^− 1), higher mechanical strength (55% and 126% increase in Young’s modulus and tensile strength, respectively), and improved thermal insulation properties, and the as-prepared aerogels was important for structural stability and cost-effectiveness (Zhao et al. 2015). Feng et al. investigated silica-cellulose aerogels synthesized by using the regenerated cellulose and the precursor methyltrimethoxysilane (MTMS), which showed good thermal and acoustic insulation properties (Feng et al.2016). Chen et al. developed a silica-cellulose aerogel composed of silica aerogel nanoparticles and cellulose nanofibrils by using the sol-gel method and freeze-drying, corresponding to the properties of low density (0.055 ~ 0.06 g/cm³), compression resistance (compressive strength of 95.4 kPa), high surface area (900 m²/g), and low thermal conductivity (0.023 W·m^− 1·K^− 1) (Chen et al. 2021). However, most of cellulose/silica composite aerogels prepared by the above methods were opaque and inflexible (Ren et al. 2022). Thus, transparent and flexible cellulose-based aerogels materials are urgently needed for wider applications in energy-efficient buildings (Lamy-Mendes et al. 2021). Zu et al. prepared transparent ultra-flexible double cross-linked aerogels consisting of a single precursor-VMDMS or VMDES-by free radical polymerization followed by hydrolytic polycondensation, combined with ultra-low-cost and highly scalable APD technology, directly using alcohol as drying medium. The resulting aerogels and xerogels have low density (0.16–0.22 g/cm³), high transparency (> 80%), excellent processability, super-flexibility in compression (80% of compressive strain in 500 cycles) and ultraflexibility in bending (100 bending cycles) (Zu et al. 2018). Song et al. prepared sustainable lignocellulose-based films with good light management (high transparency of 78%, haze of 61%), through easy dissolution and regeneration of wood pulp and xylitol production (CRXP) (Song et al. 2021). Additionally, there are few research papers on the preparation of transparent cellulose/silica composite aerogels by using two silica sources as a co-precursor. The previous researches have shown that silicon-based aerogels with excellent comprehensive properties could be prepared by using dual-silicon source as co-precursor. He et al. improved the mechanical properties of silicon-based aerogels by introducing MTMS and TMCS dual silicon sources (He and Chen 2017). Cai et al. improved the hydrophobicity of composite aerogels by introducing a methyl silica source (Cai et al. 2018). Compared to a single silicon source, the co-precursor system for preparing functional silica-based aerogel can effectively save the post-processing time and correspondingly save the production cost, which is conducive to industrial production.

In this study, TEOS and MTES were used as the co-precursors, and sheep pulp was chosen as the regenerated fiber with a three-dimensional network. The transparent cellulose/silica composite aerogels with organic-inorganic structure were prepared by a two-step acid-base catalytic method, sol-gel process, spin-coating technique, and supercritical CO₂ drying. The as-prepared cellulose/silica composite aerogels can be made into different shapes (blocks, films, etc.) and thicknesses using different moulds and spin coating parameters, respectively. The resulting CAS samples have high light transmission and excellent mechanical properties, such as compressive, tensile and flexural properties, which are attributed to the ingenious combination of cellulose nanofiber reinforcement as a supportive backbone with the porous network microstructure of SiO₂ aerogels. The novel preparation approach of aerogels materials offers unlimited possibilities for the applications in energy-efficient buildings and thermal management system.

Materials and reagents

Cotton pulp (DP = 1400) was provided by Shangdong Henglian New Materials Co., Ltd (China). 1-Allyl 3-methylimidazole chloride salt (Amimcl) (purity > 99%), Tetraethylorthosilicate (TEOS), Methyltriethoxysilane (MTES), hydrochloric acid, ammonia were provided by Nanjing Wanqing Glass Instrument Co., Ltd (China). Absolsute ethyl alcohol (EtOH) was received from Wuxi City Yasheng Chemical Co., Ltd (China). Deionized water (H₂O) was supplied by Nanjing Wanqing Chemical glass ware & Instrument Co., Ltd (China). Cotton pulp was dried in a vacuum oven at 60 ℃ for 24 h before using. Deionized water was used for the preparation of solutions. All other chemicals were analytical grade reagents and used as received.

Preparation of transparent cellulose aerogels

The cotton pulp was crushed into small flakes, washed with deionized water, and then dried in a vacuum oven for spare parts. 2wt% of the cotton pulp was dissolved in AmimCl solvent at 80°C and stirred magnetically for 0.5-2 h to obtain homogeneous and transparent cellulose sols. After the defoaming treatment in a vacuum oven at 80°C for 30 min, the obtained sols were poured into molds, and when the sols dropped to room temperature, a 60wt% of aqueous AmimCl solution was poured into the molds to form a transparent cellulose gels at 4 ℃ constant temperature incubator. The cellulose alcohol gels were obtained by washing 3–5 times with anhydrous ethanol until no chloride ions were detected in silver nitrate. Thin film gels were produced by spin-coating process. Subsequently, the transparent cellulose aerogels samples (blocks or films) were obtained by supercritical CO₂ drying in a high-pressure autoclave (50°C, 10 MPa, HELIX 1.1 system, Applied Separations, Inc., Allentown, PA).

Preparation of transparent cellulose/silica composite aerogels

TEOS and MTES co-precursors were firstly dissolved in H₂O, EtOH, and the silica solution was obtained by magnetic stirring for 15–30 min at room temperature. The molar ratios of TEOS, MTES, H₂O, and EtOH were adjusted to 1: 1: 5: x. The variable x values were 40, 30, 20, 10, and 5, corresponding to 14, 18, 25, 38, and 52wt% of TEOS and MTES co-precursors, respectively. Subsequently, the preformed cellulose gels (blocks or films, referred to 2.2 preparation process) were immersed into silica solution. Meanwhile, hydrochloric acid (0.01 M) was used as a catalyst to adjust the pH value of the solution to 2–3. The cellulose gels were impregnated into silica sols at room temperature for 12–24 h. Then, the catalyst solution was prepared with a molar ratio of EtOH: H₂O = 5: 2.6, and ammonium hydroxide (0.1 M) was used as a basic catalyst to adjust the pH value of the solution to 8 ~ 9. The cellulose/silica composite gels were obtained after adding the catalyst solution for 6–12 h. Afterwards, the formed cellulose/silica composite gels were aged in ethanol at 50°C for 12 h and then washed with ethanol for 24 h at room temperature to remove impurities and water. Finally, the transparent cellulose/silica composite aerogels were dried by using supercritical CO₂ drying in a high-pressure autoclave (50°C, 10 MPa, HELIX 1.1 system, Applied Separations, Inc., Allentown, PA). The as-synthesized aerogels samples were denoted as CAS1, CAS2, CAS3, CAS4, and CAS5, and the corresponding thickness of each sample was approximately 0.5ཞ5 mm.

Characterization

The chemical structures were characterized using a Fourier transform infrared (FTIR) spectrometer (Nicolet IS 10). The spectral range was from 400 to 4000 cm^− 1 with an accuracy of 4 cm^− 1. The microstructure of the samples was analyzed by transmission electron microscopy (TEM, FEITF20, USA) and scanning electron microscopy (SEM, Gemini300, Zeiss, Germany) equipped with an energy dispersive X-ray spectrometer (EDS). X-ray diffraction (XRD) patterns were obtained using CuKa radiation (k = 0.15406 nm) from a Rigaku Smart Lab-9000 Science Smart Lab. X-ray photoelectron spectroscopy (XPS) was carried out using Axis Ultra DLD equipped with Al Ka (1486.6 eV). The Brunauer-Emmett-Teller (BET) specific surface areas and pore distribution were measured by using a V-sorb 2800P after pretreatment for 6 h at 120 ℃. Thermal conductivity was obtained by the transient planar heat source method (Hot Disk). Transparency was measured by UV-Vis spectrometer within the visible wavelength range of 400–800 nm. Compression and tensile tests were carried out on Instron 5569 universal testing machine. The thermal stability of CA and CAS was investigated by thermogravimetric mass spectrometry (TG-DSC). The apparent density was obtained from\(q=m/v\), the apparent density of the material (g/cm³), m, the mass of the material (g), V, the volume of the material (cm³).

Preparation and morphological/chemical characterization of aerogels

Figure 1 shows the preparation process and synthesis mechanism of the transparent cellulose/silica composite aerogels. The cellulose block/film gels were firstly prepared and then impregnated into co-precursors (TEOS/MTES) to prepare cellulose/silica composite aerogels via a two-step sol-gel process followed by supercritical CO₂ drying. It is well-known that anions in the ionic dissolve cellulose by forming strong hydrogen bonds with hydroxyl hydrogen atoms in cellulose (Zhang et al. 2017). On the contrary, the cations have little solubilization effect on cellulose (Fig. S5a). As can be seen in Fig. S5b, the co-precursors (TEOS/MTES) undergo hydrolysis reaction, and the hydrolyzed siloxy groups generate hydroxyl groups linking to silicon atoms (Si(OH)₄, CH₃-Si(OH)₃), respectively. Subsequently, the silica sols particles will gradually penetrate into the internal pore structures of the cellulose wet gels, displacing the original ionic solution. After catalyzed by alkaline agent, the hydrolyzed monomers undergo polycondensation and crosslink with each other. Generally, the condensation rate of Si(OH)₄ molecules is much higher than that of CH₃-Si(OH)₃ molecules in the mixed precursor solution, which leads to a preferential condensation of Si(OH)₄ molecules to form primary particles (He et al. 2017). In addition, the unreacted hydroxyl groups in co-precursors will further bond with the hydroxyl groups on the surface of the cellulose wet gels, forming the final cellulose/silica composite gels dual network structures. Table S1 shows the summary of the textural properties of aerogels samples with different SiO₂ content. The spin-coating process, equipment and representative samples (CAS wet gels and aerogels films) are supplemented in Fig. S2-4. With the increase of silica content, the densities of aerogel samples increase from 0.058 g/cm³ of pure CA to 0.17 g/cm³ of CAS5. Oppositely, the volume shrinkage rates of aerogels samples decrease gradually from 22.9% (CA) to 10.1% (CAS5). Therefore, the feasibility of this study prepared the transparent cellulose/silica composite aerogel has been proved both theoretically and practically.

Figure 2a shows the SEM images of CA and CAS aerogels and the corresponding macro digital photographs. It is evident that both CA and CAS exhibit a uniform three-dimensional porous structure. Meanwhile, all aerogels samples exhibit outstanding integrity and transparency. By contrast, the pure CA aerogels show an intricate three-dimensional network consisting of inter-layered and interwoven multi-scale pore structures. The average pore sizes of CAS aerogels are obviously smaller than that of CA aerogels, mainly due to the introduction of a large number of spherical nanoscale silica aerogels nanoparticles, forming a more uniform composite structure. It is found that with the increase of silica content, the morphology gradually changes from filamentous networks (CA) to pearl-necklace (CAS1-3), then to spherical particles aggregation state (CAS4-5). Furthermore, the evolution of pore structure also has a certain influence on the transparency of the aerogel sample, and the smaller and more uniform the pore size, the better the light transmission (as shown in physical photograph). In order to further observe the micro-morphological combination of cellulose and silica particles, the TEM image shows that nanoscale silica particles loaded on the surface of fibrous micrometer cellulose monomer (as seen in Fig. S6). Therefore, the addition of silica aerogel particles makes the pore structure of the composite aerogel more uniform. Meanwhile, the fibrous cellulose aerogel also plays a role in supporting the network skeleton structure, reducing the shrinkage and improving the flexibility of cellulose/silica composite aerogel. The EDS spectrum of the selected CAS4 is revealed in Fig. 2b. The specific element content distribution is shown in Fig. 2b, which basically corresponds to the molar ratio of CAS4. In addition, the uniform distribution of C, O and Si elements on the surface of the CAS4 further proves that the cellulose micronano fibrous monomers were equably covered by silica aerogel particles.

The N₂ adsorption–desorption isotherms and BJH pore size distribution of the as-prepared CA and CAS aerogels are described in Fig. 3. The relevant data are summarized in Table S1. All isotherms are Type IV based on the IUPAC classification, reflecting the characteristic of mesoporous materials. The desorption cycles of the isotherms show a hysteresis loop for the six samples, which is generally caused by the capillary condensation that occurs in the mesopore. The rapid adsorption process in the low-pressure region (0-0.1) is caused by the micropores inside the aerogel matrix (Sehaqui et al. 2011). As shown in Table S1, the specific surface area of CAS samples is significantly higher than that of CA samples, mainly due to the network structures of CAS samples with high porosity. With the increase of silicon content, the specific surface area of CAS samples show an increasing trend (from 660.87 m²/g to 1089.70 m²/g), as shown by the change in volume of N₂ adsorbed. From the pore size distribution, the average pore sizes of the CAS samples gradually become smaller from 15.20 nm to 9.89 nm, resulting from the gradually denser skeleton structures of the CAS (as shown in Fig. 2a). It is worth noting that a small number of micropores (2–10 nm) appeared in the CAS5 sample, presumably due to the local agglomeration of particles in the composite aerogel (corresponding to SEM image of CAS5, in Fig. 2a). Moreover, these newly generated micropores may also be one of the reasons for further improving the specific surface area of CAS5 sample. Previous studies have also demonstrated that an appropriate addition of silica or other inorganic nanoparticle can effectively improve the non-uniform macroporous structure of cellulose aerogel, forming a more uniform mesoporous structure of cellulose-based composite aerogel (Cai et al. 2012, Wan and Li 2015).

The FTIR spectra of the as-prepared aerogels samples are described in Fig. 4a. The spectrum of CA is significantly distinct from those of CAS. The two broad peaks at 3410 and 1650 cm^− 1 are assigned to the stretching vibrations of hydroxyl groups of cellulose and the presence of absorber H₂O, respectively. However, these two bands are less evident in the spectrum of the CAS samples. This is because the fact that the continuous consumptions of the hydroxy groups on the surface of cellulose and silicon co-precursors during the hydrolysis-polycondensation process, resulting in the decrease of the hydrophilic characteristics of the final CAS samples. In addition, the characteristic peaks of cellulose are also observed at 2898, 1347, 1156 and 898 cm^− 1, which are related to C-H stretching vibrations, C-H deformations, C-O-C asymmetric stretching and β-glucosidic bonds (Ashori et al. 2012). Compared to CA, there are two new characteristic peaks of 795 and 446 cm⁻¹ in CAS, corresponding to Si-O-Si bending and symmetric stretching vibrations, respectively (Zu et al. 2018). It is worth noting that the intensities of these characteristic peaks become obvious with the increase of silica content. To some extent, the presence and change of silica aerogel in CAS composites can be demonstrated by FTIR spectra. XPS spectroscopy is employed to further observe the chemical composition of the CA and selected CAS4 in Fig. 4(c-f). As shown in Fig. 4c, the C, O, and Si elements signals can be clearly detected from the spectra. Two characteristic peaks, Si-O-Si (532.5 eV) and SiO₂ (532.9 eV) are clearly visible in the O1s spectrum. The presence of Si-O-Si groups confirms the occurrence of hydrolysis and polycondensation reactions in TEOS and MTES co-precursors. As for the C1s spectra, two characteristic peaks, C-Si (286.2 eV) and C-C (284.8 eV) are also discernible. The analyses of XPS further confirm the FTIR test results. Figure 4b shows the XRD of CA and representative CAS4. Previous studies have shown that the amount of silica aerogel has no effect on its amorphous state. As shown in Fig. 4b, both CA and CAS4 display the relative broad diffraction peaks at 22.40° (2θ), indicating the presence of amorphous cellulose and silica aerogels particles in as-prepared aerogels samples (Jiao et al. 2018, Zhang et al. 2020). Besides, the introduction of silica aerogel do not change the shape of the original CA diffraction peak, but the diffraction peak of the cellulose/silica composite (CAS4) becomes wider. Therefore, it can be inferred that the CAS samples are comprised of amorphous fibrous cellulose and spherical silica particles with more uniformly dispersed state (as shown in Fig. 2a).

Optical properties of aerogels

The optical performances of CA and CAS samples with different thicknesses are in Fig. 5. As shown in Fig. 5a, all the as-prepared aerogels samples have good light transmittance (physical photos in Fig. 2a). With the increasing of wavelength (400–800 nm), the light transmittance of the aerogel sample exhibits an increasing trend. The CA displays a transmittance of 78.2% at wavelength of 800nm, indicating comparatively high transparency in relation to other cellulose aerogels (Cai et al. 2012, Guzun et al. 2014, Cai et al. 2009). Meanwhile, the curves of transmittance of the CAS samples show a trend of first increasing and then decreasing (from 45.8–80.6%). By contrast, the CAS4 sample exhibits the best transparency properties, which is mainly attributed to its uniform pore structure and the suitable size of secondary particles (Zu et al. 2018, Nakanishi et al. 2020). Similar reports have proved that the silica content has an effect on the transparency of cellulose-silica composite aerogel. For example, Cai et al. synthesized the cellulose-silica composite aerogels with the transmittance of 84% (39wt% of silica aerogel) (Cai et al. 2012). Moreover, the light transmittance of the CAS samples with different thickness was investigated. The highly transparent wet gels and aerogels of CAS samples (circle, 0.5 mm and square, 1 mm) are shown in Fig. 4S. To further demonstrate the influence of thickness on the transmittance of CAS aerogel, the representative CAS4 samples are prepared with gradient thicknesses of 0.5, 1, 2, 3 and 5 mm, and the photos of aerogels are shown in Fig. 5c. Obviously, with the increase of thickness, the transmittance of the CAS4 samples decrease gradually. In contrast, the transparency of the samples (thickness, ༜2 mm) is higher, and the CAS4 of 0.5 mm shows the highest transparency of 91.7%, which is far higher than the requirement of 70% transparency of architectural glass. Compared to the glass-air interface, the minimal refractive index differential of the CAS4-air interface leads to significantly less light reflection enhancing the light transmission for CAS4 within the visible spectral range (Salamati et al. 2020).

Mechanical Properties and Thermal Stability

The mechanical properties of the as-prepared cellulose/silica composite aerogels are explored by compression, tensile, and bending tests. Due to their unique molecular and nanoscale structures, the composite aerogels exhibit excellent mechanical properties, including high compression, substantial tensile toughness, and notable bending elasticity. Figure 6a visually shows the considerable bending elasticity of the composite aerogel and the tested sample could still keep the intact structure after more than 500 times bending tests. The stress-strain curves of the CAS samples are illustrated in Fig. 6b. During compression, all composite aerogels exhibit the typical deformation behavior seen in porous materials. Remarkably, the inorganic core composed of Si-O bonds shows resistance to deformation, allowing the composite aerogels to withstand higher stresses without brittle damage. Simultaneously, the cross-linking of cellulose with organic side chains and the micro- and nanopore structure increases the cross-link density of the aerogels, which also contributes to the strength of the aerogels (Wang et al. 2020). At low strains, aerogels exhibit linear elasticity due to the irreversible collapse of the porous structure, with a plateau of plastic deformation at moderate strains. At strains above about 40%, the sharp increase in stress indicates the presence of a dense state after the complete collapse of the porous structure. The increase in silica significantly boosts compressive strength. Simultaneously, the composite aerogels containing high levels of silica exhibit good toughness, with the ability to endure compression until strains exceed 60%. Figure 6(c) displays the maximum tensile strengths of CA and CAS4, measuring 4.07 MPa and 1.54 MPa, respectively. It is found that the composite aerogels become less flexible with the addition of silica nanoparticles, but still retains a degree of flexibility, making it suitable for applications such as windows and energy-efficient buildings (Abraham et al. 2023). Moreover, as shown in Fig. 6d, the excellent mechanical properties enable the composite aerogels to be easily shaped with a knife, highlighting their practical workability. In addition, the reinforcement mechanisms of silicon source co-precursors on cellulose aerogels are shown in Fig. 6(e ~ g). We propose that the co-precursors (TEOS, MTES), and cellulose create an "organic-inorganic combination" structure, featuring a sturdy inorganic core and flexible organic side chains. During the testing processes of compression, stretching and bending, the inorganic core consisting of Si-O bonds resists deformation, while the organic side chains formed by cellulose exhibit significant flexibility. Therefore, the dual nature allows the cellulose/silica composite aerogels to withstand high stresses without becoming brittle.

Flame retardancy

The flame retardant properties of the various aerogels samples are evaluated through ignition burning tests, as shown in Fig. S7 and Fig. 7(a-b). Figure 7(a-b) shows the comparative state of the CA and CAS4 samples on a burning candle. It is found that the CA sample is easily ignited with a strong flame and disappears after a few seconds, mainly due to the composition of cellulose aerogel is mostly organic matter. Instead, the ignited CAS4 sample self-extinguished in about 5 seconds, maintaining its intact block structure. Thus, the presence of inorganic silica aerogel nanoparticles can significantly improve the flame retardant property of the cellulose-based aerogels. Additionally, as shown in Fig. S7, it can be clearly observed that the higher the content of silica aerogel, the better the flame retardancy of composite aerogel. The flame-retardant process and mechanism of cellulose/silica composite aerogel is further investigated, as shown in Fig. 7c. Because the silica sol is introduced into the macroporous structure of cellulose wet gel, the composite aerogel with a double network structure is finally formed after supercritical CO₂ drying process. This dual network structure of the cellulose/silica composite aerogel forms a robust insulation and gas barrier layer during the burning process. The insulating layer slows down the heat transfer within the cellulose aerogel system, thereby retarding the combustion of composite aerogels. Simultaneously, the gas barrier layer efficiently inhibits the volatilization of combustible components, reducing their contact with oxygen and further delaying the effective combustion of composite aerogels. Conversely, the gas barrier layer proficiently impedes the volatilization and escape of combustible components, diminishing their interaction with oxygen and thus extending the combustion delay of composite aerogels. Additionally, the high temperature resistant structure of inorganic silica aerogel supports the formation of a porous carbonization layer, which can be used as an effective barrier layer. Moreover, this porous carbonization layer further retards the heat transfer, the volatilization and escape of combustible components.

TG and DSC analyses are further carried out in an air environment to study the effect of silica aerogels nanoparticles on the thermal oxidation and thermal stability of CA and CAS samples, as shown in Fig. 4(g, h). Comparatively speaking, the CAS samples show higher thermal stability than that of the CA sample. The first stage from room temperature to 100°C, the CAS samples show a weight loss of approximately 5%, due to the evaporation of physically adsorbed H₂O and residual solvent. At this stage, the mass loss of CA is slightly larger (about 10%), mainly because of the presence of micron-sized pores. In general, because of the nano-sized porous structure and high porosity, H₂O, CO₂ and solvent adsorbed in the porous structure of the sample cannot be removed completely during the supercritical CO₂ drying process. A significant mass loss of all the aerogels samples occurs at 250–350°C, which corresponded to the continuous thermal decomposition of organic components of cellulose polymer. Immediately, another significant mass loss occurs in the temperature range of 350–500°C, probably due to the further consumption of organic groups in composite aerogel, such as -CH₃ groups. For the last stage (above 500°C), all the TG curves of the samples tend to smooth and stabilization, while the corresponding final residual mass of 1.63%, 48.14%, 64.43%, 68.67%, 71.78% and 73.41%, respectively. Specifically, the T_onset (the onset temperature during the decomposition of the samples) has changed to a higher temperature with the increasing of silica content. The T_onset for the CA is 278°C, which continuously increases to maximum temperature of 331°C for the CAS5 sample. Additionally, the CA aerogel displays a significant secondary degradation stage, completely degrading at approximately 500°C. This degradation behavior aligns with prior reports on cellulose and cellulose aerogels (Aggarwal et al. 1997, Yuan et al. 2016). Compared to CA, all the CAS samples show a higher T_onset (325ཞ331°C) and residual mass (48.14ཞ73.41%), which indicate that silica nanoparticles can effectively improve the thermal stability of cellulose/silica composites aerogels. Moreover, inorganic porous silica aerogel used as an efficient heat-resistant barrier can delay the thermal decomposition rate of cellulose polymers and further improve the flame-retardant property of composite aerogels, as shown in Fig. 7c. Therefore, the as-prepared CAS with excellent thermal stability and heat-insulating property is a promising candidate for a variety of applications in high temperature environments.

Insulation performance

Lightweight nanoporous aerogels are known for their excellent optical and thermal insulation properties, and have great application potential in energy-saving buildings. As shown in Fig. 8a, the transparent aerogel glass windows used in the building can not only be efficiently penetrated by the bright sunlight, but also maintain the balance of indoor temperature for a long time, providing people with a comfortable living environment all year round. The light-transmitting, heat-insulating aerogel glass can significantly reduce energy consumption for heating in winter and cooling in summer, which is fully in line with the concept of global green ecology. Figure 8c displays an aerogel glass window model placed in an outdoor environment to further investigate the real effect of heat insulation using two miniature thermometers. We clearly found that after prolonged exposure to intense sunlight, the temperature on the thermometer placed outside quickly rose from the initial 30°C to 49°C. Oppositely, the temperature inside the model showed a slow rising trend and eventually stabilized at 37°C. The specific comparative temperatures changes over time are shown in Fig. 8b. Additionally, Fig. 4i shows the thermal conductivities of the CAS samples. With the increase of silica aerogel content, the thermal conductivity of CAS shows a significant decreasing trend. Particularly, the CAS4 sample possesses the lowest thermal conductivity of 0.0188 W·m^− 1·K^− 1, mainly due to the uniform mesoporous structure of the composite aerogel. Moreover, the comparative cold surface temperatures of the CA and CAS4 samples were measured on a heating platform set at 130°C by using an infrared thermal imager, as shown in Fig. 8(d-f). By contrast, the selected CAS4 sample (3mm) shows better heat-insulating property at ambient temperature of 22°C, with the cold surface temperature rising from 35.7°C to 51.4°C in 5 min, while the CA sample (3 mm) increases from 43.8°C to 72.7°C. The corresponding curves of cold surface temperatures with the test time are shown in Fig. S9. In order to make the as-prepared composite aerogel used in different scenarios, the cold surface temperatures of CAS4 samples with different thicknesses and shapes were also tested, and the results showed that the samples still had good thermal insulation properties, as shown in Fig. 8f. Some previous studies have shown that the practical applications of the cellulose-based aerogels depend on their comprehensive properties (Chen et al. 2021, Cai et al. 2018, Fu et al. 2018, Kunjalukkal Padmanabhan et al. 2021, Demilecamps et al. 2015, Yuan et al. 2017). The comparison of comprehensive properties of the as-prepared CAS with other representative aerogels materials are further carried out, and the detailed dates are in Fig. S10 and Table S2. The CAS4 aerogels exhibit a combination of high strength, low density, insulation properties, specific surface area, thermal stability, and transparency. This overall performance, coupled with the cost-effectiveness of cellulose, opens the door to a wide range of potential applications.

In summary, cellulose/silica composite aerogels with an “organic-inorganic” dual network porous structure were successfully synthesized. The cellulose block/film gels were firstly prepared and then impregnated into co-precursors (TEOS/MTES) to prepare cellulose/silica composite aerogels via a two-step sol-gel process followed by supercritical CO₂ drying. The obtained CAS showed outstanding light transmittance, excellent flame retardancy, machinability and thermal insulation. Especially, the resulting CAS4 sample exhibited the best light transmittance (91.7%, 0.5 mm) and lowest thermal conductivity (0.0188 W·m^− 1·K^− 1). The corresponding improved thermal insulation performance of the practical application was further demonstrated by a self-designed aerogels glasses model (ΔT༞12 ℃). Compared with the reported cellulose-based aerogels composite materials, the as-prepared CAS have higher comprehensive properties. Consequently, the as-prepared cellulose/silica composite aerogels, which integrate many unique physical and chemical properties, offer a broad scope of the potential applications in energy-saving buildings and thermal management materials.

Acknowledgments The authors acknowledge the financial support from the National Key Research and Development Plan (2023YFB3812301), National Natural Science Foundation of China (52272300, 52202367), the Major Science and Technology Projects in the National Building Materials Industry (202201JBGS18-01) and Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).

Supporting information Preparation process of cellulose/silica composite aerogel films; Hydrolytic condensation process of double silicon source (TEOS and MTES); TEM images of the CAS4; Combustion processes of CAS1, CAS2, CAS3 and CAS5; Summary of the textural properties of aerogels samples with different SiO₂ content; Self-designed aerogels glasses model fabricated by using the CAS4 films with 0.5 mm thickness; Comparison of comprehensive properties of the as-prepared CAS with other representative aerogels materials.

Author contributions All authors contributed to the study conception and design. Writing - original draft, data curation, and formal analysis were performed by JS, JH and YZ. Conceptualization and wring - review & editing were performed by JZ, SP, ZX, SC and XS. All authors read and approved the final manuscript.

Funding This work was financially supported by the National Key Research and Development Plan (2023YFB3812301), National Natural Science Foundation of China (52272300, 52202367), and the Major Science and Technology Projects in the National Building Materials Industry (202201JBGS18-01).

Conflict of interest The authors declare no competing interests.

Ethical approval We declare that there is no financial or personal relationship between us and other people or organizations that may have an inappropriate impact on this work. Consent has been taken from all the authors for participation in this study.

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Transparent, flame retardant and machinable cellulose/silica composite aerogels with nanoporous dual network for energy-efficient buildings

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