Preparation of High-Temperature Resistant Aluminum-Doped Silica Aerogel from Aluminum Sol Source by Ambient Pressure Drying

doi:10.21203/rs.3.rs-3249068/v1

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Research Article

Preparation of High-Temperature Resistant Aluminum-Doped Silica Aerogel from Aluminum Sol Source by Ambient Pressure Drying

https://doi.org/10.21203/rs.3.rs-3249068/v1

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published 14 Nov, 2023

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Abstract The preparation of aluminum-doped silica aerogel (ASA) typically utilizes alumina alkoxides or inorganic aluminum salts as precursors usually hindered by high cost or precursors instability. In this study, we innovatively developed a high-temperature resistant ASA via the sol-gel and atmospheric pressure drying (APD) method, utilizing an inexpensive aluminum sol as the aluminum source, tetraethyl orthosilicate (TEOS) and basic silica sol as the composite silicon source. The effects of the Al/Si molar ratio and calcination temperature on the structure, morphology, and thermal stability of ASA were investigated. The results revealed that ASA-0.2 (with an Al/Si molar ratio of 0.2) exhibited a specific surface area of 616.3m2/g at room temperature, whereas ASA-0 was 508.2m2/g. After calcination at 1000°C for 2h, ASA-0.2 remained a significantly higher specific surface area (290.0m2/g) compared to ASA-0 (23.9m2/g). Mechanism analysis indicated that the addition of aluminum sol not only improved the strength of the aerogel skeleton but also inhibited the phase transition of silica, effectively enhancing the high-temperature resistance of the aerogel. The utilization of inexpensive and stable aluminum sol as a precursor presents a promising approach for the industrial production of heat-resistant aerogels.

Silica-based aerogel

Aluminum sol

High-temperature resistance

Ambient pressure drying

The preparation of aluminum-doped silica aerogels from aluminum sol as the aluminum source is discussed for the first time.
The appropriate amount of aluminum sol can increase the specific surface area and optimize the pore structure of the aluminum-doped silica aerogels.
The addition of aluminum sol significantly improves the high-temperature resistance of silica aerogels.

Aerogels are three-dimensional (3D) nano-porous materials, renowned for their low density, high porosity, and high specific surface area[1]. The unique special nanoparticle skeleton and pore structure make the aerogels have extremely low thermal conductivity (0.005~0.1W/m·k)[2]. Consequently, aerogels have been widely used in energy-saving buildings[3], aerospace[4], and chemical industry[5]. Since aerogel's initial preparation by Kistler in 1931, silica aerogel has been extensively studied[6]. In recent years, the industrialization of silica aerogels has expanded its application range, particularly in the fields of thermal insulation[7]. However, a major obstacle to the use of silica aerogel lies in the high surface activity characteristic of their nanostructures. This activity increases the size of the nanoparticles and damages the pore structure, resulting in sintering at a temperature higher than 650 ℃[8,9].

Efforts to improve the stability of silica aerogel in high-temperature environments have led to the incorporation of high-temperature resistant fibers, such as mullite whiskers, alumina powders, glass, or carbon fibers [10-13]. These additives can effectively maintain integrity and structural stability while also inhibiting sintering. However, these fiber-reinforced heat-resistant silica aerogels are usually produced via supercritical drying (SCD) or freeze drying (FD)[14,12,15]. These methods, although effective, present challenges in terms of cost and safety, which highlights the importance of the atmospheric pressure drying (APD) method in practical silica aerogel applications. Moreover, achieving uniform dispersion of fibers or whiskers in sol systems proves difficult even with mechanical or ultrasonic dispersion methods. The different shrinkage rates between the fibers and aerogels can generate large residual stresses, leading to severe cracking at elevated temperatures. Furthermore, the introduction of fibers typically increases the density, thermal conductivity, and internal stress of silica aerogels, potentially compromising their effectiveness [16].

Addressing the poor high-temperature performance of silica aerogels to enhance the bonding energy and the stability of the inorganic ionic crystal structure. This realization has led researchers to develop a series of multicomponent composite aerogels. It has been observed that the incorporation of high melting point oxide elements such as alumina, zirconia, titania, vanadium, and zinc into the silica aerogel precursor significantly enhances the heat resistance of the materials[17]. Among them, SiO₂-Al₂O₃aerogel exhibits superior thermal stability. SiO₂-Al₂O₃aerogel is generally prepared by sol-gel method using organosilicons (TEOS, TMOS, and MTMS), organic aluminum alkoxides (ASB and AIP) [18-20]and hydrated inorganic aluminum salts (Al(NO₃)₃·9H₂O and AlCl₃·6H₂O) [21,22] as precursors. The preparation method using organic aluminum alkoxides relies on SCD, often requiring chelating agents to control the high reactivity during hydrolysis and other complex chemical reactions [12,23]. In contrast, the method using hydrated inorganic aluminum salts is simpler and safer because it depends on APD. Al/Si ratio is a key parameter influencing the structure and thermal stability of SiO₂-Al₂O₃aerogel However, loss of Al³⁺ ions and partial flocculation of inorganic aluminum salts may occur during the sol-gel process, especially under alkaline catalytic conditions. To counteract these challenges, identifying suitable aluminum sources to attain a uniformly distributed and stable aluminum-doped silica aerogel system via the APD method is a promising approach.

Aluminum sol, also known as aluminum oxide sol or boehmite sol, is typically prepared industrially through the aluminum-hydrochloric acid process. Its applications span a variety of fields including inorganic fiber manufacturing, refractory, textile, and paper industry. Aluminum sol forms a relatively stable colloidal solution consisting of positively charged hydrated aluminum oxide particles dispersed in water. This stability is attributed to the electrical double-layer structure of aluminum sol, in which electrostatic interactions between the small particles and negatively charged acid ions (Cl^- or NO₃^-) in the solution prevent the gelatinous from coalescing and form a stable sol system[24,25]. So far, there is a research gap in the synthesis of SiO₂-Al₂O₃aerogel using aluminum sol as the aluminum source, no relevant studies have been reported.

In this work, the aluminum-doped silica aerogel (ASA) was successfully synthesized by the sol-gel and APD methods. We chose TEOS and alkaline silica sol as silicon co-precursors and aluminum sol as the aluminum source. Alkaline silica sol is an aqueous colloidal solution of nano-size SiO₂colloidal particles typically stabilized by a small amount of cationic sodium. These particles feature a surface covered by a substantial amount of reactive Si-OH groups. Introducing the alkaline silica sol into the mixture serves two purposes. First, it reduces cost by replacing some of the TEOS as the silicon source. Second, it elevates the pH value functioning as an alkali catalyst. Our team has previously experimented with different process parameters including silica sol/TEOS molar ratio, ethanol/silica sol and H₂O/silica sol molar ratio, as well as TMCS/n-Hexane volume ratio[26]. Furthermore, the effects of aluminum sol/TEOS molar ratio on the thermal stability of composite aerogels were studied.

2.1 Raw materials and chemical reagents

TEOS, ethanol (EtOH, 99%), and trimethylchlorosilane (TMCS) were purchased from Sinopharm Chemical Reagent Corporation (China); alkaline silica sol (pH value: 9.9; solid content: 26.8%; particle size: 8-10 nm; Guo Lian Technology CO., Ltd) diluted by ETOH and water, alkaline silica sol: EtOH: water molar ratio was 1:5:1; Aluminum sol (pH value: 4.3; solid content: 20.0%; particle size: 10-15 nm; Hangzhou Jiupeng New Materials Co., Ltd); n-Hexane (Aladdin industrial corporation), deionized water, HCl solution (0.5 M).

2.2 Synthesis of ASA

The gel preparation process can be delineated into two principal stages. Initially, TEOS was dissolved into water and EtOH at ambient temperature, followed by the addition of 0.5M HCl to attain a pH ranging between 2 and 3. The molar ratio of TEOS: water: EtOH was maintained at 1:2:9. Once the TEOS was fully dissolved, varying proportions of aluminum sols were introduced under constant stirring for 10 min. The molar ratio of aluminum sol to TEOS (denoted as Al/Si, Al corresponds to aluminum oxide) was controlled to be 0, 0.05, 0.1, 0.2, 0.3, 0.35, and 0.4, resulting in the respective gel samples ASA-0 to ASA-0.4. In the second phase, alkaline silica sol was incorporated into the mixture. After a 5 min stirring, the mixed sol was transferred into a polyethylene mold for gelation under atmospheric conditions. The gels were allowed to age for 12h at 45 ℃ and then followed by immersion in a 4:1 solution of EtOH and TEOS for 24h to reinforce their structure. Subsequently, the gels were immersed in n-hexane for solvent exchange, and the n-hexane was replaced twice every 12 hours at room temperature. The gels were then subjected to hydrophobic surface modification by placing them in a 1:9 volume solution of TMCS and n-Hexane for 24h. TMCS interaction with the Si-OH group rendered the gels hydrophobic, while the residual unreacted TMCS and HCl produced from TMCS-water interaction remained within the gel pores. To eliminate these residual components, the gels were cleaned with n-Hexane twice for 48 hours, refreshing the n-Hexane every 12 hours. Finally, the hydrophobic gels were dried at 60 ℃ for 8h yielding the monolithic ASA.

2.3 Characterization

The bulk density of ASA was derived from the mass-to-volume ratio by the drainage method (hydrometer method). Brunauer-Emmett-Teller (BET) and Barrett-Joyner-Halenda (BJH) methods were used to analyze the specific surface area and pore size distribution of the aerogels. These methods relied on N₂ adsorption isotherms measured at 77K (JW-BK112, JWGB, China). To observe the micro-morphology and chemical composition of the samples, we employed a transmission electron microscope /energy dispersive X-ray spectroscopy (TEM/EDS, JEM-2100F, Japan). An X-ray powder diffractometer (XRD, D/MAX-RB, Japan) was used for characterizing the crystalline phases present in the aerogels. The chemical composition of the aerogels was then investigated by a Fourier transform infrared spectroscopy (FTIR, Nexus, Nicolet, USA). Thermogravimetry and differential thermal analysis (TG-DTA, STA449F3, NETZSCH, Germany) were used to analyze the thermal stability of aerogels in air atmosphere, from room temperature to 1000℃ at a heating rate of 10℃/min.

3.1 Physical properties of the ASA

The gelation time, bulk density, and pore structure of aerogels prepared by altering the aluminum sol/TEOS molar ratio are presented in Table 1. It can be seen that with an increase in Al/Si molar ratio from 0 to 0.4, the gelation time increases from 46 min to 194 min while the bulk density of the ASA increases from 0.119 g·cm^-3 to 0.187 g·cm^-3. The significant increase in gelation time is caused by two reasons. Firstly, the aluminum sol used in this work is acidic and its pH value is 4.3. For the hydrolyzed TEOS, the more aluminum sol is used the lower the pH value of the mixture, which is unfavorable to the dehydration condensation reaction of the silicate monomer hydrolysis products and consequently prolongs gelation time. Secondly, for more aluminum sols, the spacing between the active particles after hydrolysis is larger which also leads to an increase in the time to form the gel network. Simultaneously, the bulk density of ASA shows an increasing trend with the increase in Al/Si molar ratio, and this result obtained was consistent with some previous reports on the preparation of aerogels with the addition of inorganic aluminum sources[27]. It was also found that the flowability of the mixed sol decreases when the Al/Si molar ratio exceeds 0.3, and the mixture almost lost its flowability at the Al/Si molar ratio of 0.4, which is unfavorable to the gel reaction.

Al content significantly impacts the pore structure of ASA. The specific surface area, pore volume, and average pore diameter of ASA-0 are 508.266 m²∙g⁻¹, 3.058 cm³∙g⁻¹, and 18.752nm, respectively. As the Al/Si molar ratio escalates from 0 to 0.4, the average pore diameter of ASA decreases first and then increases. This trend indicates that the appropriate amount of aluminum sol can play a role in refining the pore size. Contrarily, the specific area and pore volume display an inverse pattern when the aluminum sol content increases. This phenomenon might stem from the fact that the volume of the composite precursor is not greatly affected by the lesser addition of aluminum sol. Still, the electrostatic attraction between the aluminum sol and particles is intense, thereby shrinking the pore size and enhancing the specific area. However, an increase in the aluminum sol additive amount introduces a complication. An excessive volume of water filling the gel pores becomes detrimental to the solvent replacement and modification process of the wet gel. The capillary forces are amplified by the moisture present within the pores during the APD process, leading to an increase in gel volume shrinkage. When the capillary forces are excessive, they can even trigger a structural collapse, resulting in a deterioration of the pore structure.

Table 1 Density and pore structure characteristics of Al₂O_3-SiO₂ aerogels with different Al to Si molar ratios.

Sample	Gelation time (min)	Bulk density(g·cm^-3)	Specific surface area(m²∙g⁻¹)	Pore volume(cm³∙g⁻¹)	Average pore diameter (nm)
ASA-0	46	0.119	508.266	3.058	18.752
ASA-0.05	63	0.124	544.149	3.122	14.624
ASA-0.1	84	0.130	571.661	3.083	13.932
ASA-0.2	114	0.129	616.317	3.272	13.762
ASA-0.3	147	0.141	554.842	2.996	14.170
ASA-0.35	174	0.162	527.462	2.649	14.112
ASA-0.4	194	0.187	526.311	2.348	14.226

The N₂ adsorption-desorption isotherms and pore diameter distributions of ASAs are shown in Fig. 1. The adsorption-desorption isothermals exhibited in Fig. 1(a) are classified as type IV adsorption curves according to the IUPAC classification, indicating that ASAs are typical mesoporous materials[28]. The curves rise slowly at the relative pressure 0.1-0.8, and the N₂ adsorption is mainly monolayer adsorption; Under high pressure (relative pressure at 0.8-1), the adsorption capacity increases rapidly, and the adsorbed gas coalesces into liquid in the pore resulting in capillary condensation; Due to the incomplete desorption, the residual adsorption capacity in the desorption curves is greater than the adsorption capacity at the same pressure, and the desorption curves lag behind the adsorption curves. The adsorption-desorption curve shows an H3 hysteresis loop, indicating that the porous structure is composed of silt-type mesopore pores[29]. Moreover, it can be seen that the ASAs have relatively wide pore size distributions from Fig. 1(b), mainly in the range of 20nm to 50nm.

3.2 Morphology, crystalline and chemical structure of the ASAs

We performed TEM, EDS, XRD, and FTIR analysis of ASAs to further explore the effect of aluminum sol doping on the samples. APD samples are monolithic as shown in Fig. 2(a). Fig. 2(b-d) presents the TEM images of samples for ASA-0, ASA-0.2 and ASA-0.35. It can be seen that the aerogels possess a typical nano-porous morphology, the size of the continuous network skeleton particles is uniform and the pearl necklace aerogel particles constitute a sophisticated three-dimensional network structure. The ASA-0 is characterized by secondary particles that form coarse spheres, where the connections between these large round particles are notably weak. However, upon the introduction of an aluminum sol, the secondary particles of ASA-0.2 shrink in size, leading to tighter inter-particle connections and increased cross-linking. This alteration enhances the skeletal strength of the aerogel, thus improving its overall structural integrity. It is noteworthy that the pore distribution of ASA-0 and ASA-0.2 is relatively uniform, and the nanopores remain intact. With an increase of aluminum sol, the 3D network skeleton of ASA-0.35 becomes loose and the pores increase obviously. The significant change in pore structure is due to two reasons. Firstly, the hydrolyzed TEOS is known to condense at a pH above the isoelectric point of silicon dioxide under normal conditions. The addition of excessive aluminum sol makes the mixture lower pH value which is unfavorable to the condensation reaction. Secondly, excessive aluminum sol makes the silicon content per unit volume decrease and the spacing of Si-OH in the condensation process increase, leading to increase porosity of the aerogel skeleton. The distributions of Si, O, and Al elements in ASA-0.2 and ASA-0.35 were characterized by EDS, and the results are shown in Fig. 3. The spectrums present that Si, O, and Al elements are uniformly distributed in the aerogel particles. It is worth noting that the Al/Si molar ratios of samples ASA-0.2 and ASA-0.35 are 0.12 and 0.21, respectively. These ratios are much lower than the theoretical values, indicating that the loss of Al occurred in the process of solvent exchange.

Fig. 4(a)displays the XRD presents of ASAs with varying aluminum sol contents. The prepared ASA-0 does not show any distinct crystal structure diffraction peak. Instead, a diffuse diffraction peak package emerges between the corresponding crystalline SiO₂ characteristic peak 2θ=20-25°, suggesting that the fabricated SiO₂ aerogel is amorphous at room temperature. Although the crystal structure of ASAs does not undergo significant alterations after the aluminum sol addition, the diffraction peaks start appearing as the content increases. The ASA-0.35 sample reveals diffraction peaks at 14.5, 27.8, 38.4, 49.2, and 64.9°, corresponding to the boehmite crystal plane diffraction peaks of (020), (120), (031), (200), and (002), respectively. These are undesirable crystals formed due to their excess water content[30,31], implying a threshold exists for the amount of aluminum sol that can be added, and any excess cannot adequately react with the silicon source.

The acquired FTIR spectra of ASAs are shown in Fig. 4(b). The peaks at 1096 cm^-1 and 468 cm^-1 originate from the antisymmetric stretching vibration and bending vibration of the Si-O-Si bond, respectively[32]. Absorption peaks near 3437 cm^-1 and 1632 cm^-1 correspond to the -OH stretching vibration and the H-O-H bending vibration in H₂O[33], while The peak at 957 cm^-1 is due to the Si-OH stretching [34]. The peaks at 2964 cm^-1, 1257 cm^-1 and 847 cm^-1 are attributable to the Si-CH₃ vibration[35]. The hydrophobic modification process eliminates the hydrophilic -OH group from the wet gel skeleton, replacing it with the hydrophobic -CH₃. Compared to the spectrum of ASA-0, a new peak emerges at 610cm^-1 with the addition of aluminum sol. This peak corresponds to the bending vibration of the Al³⁺ octahedral structure [36], indicating that not all the Al elements are incorporated into the aerogel skeleton after the aluminum sol addition. Instead, they accumulate in another way, which is consistent with the XRD test results.

Drawing from the aforementioned analyses, we propose a growth mechanism for the composite gel. The precursor TEOS undergoes hydrolysis in an acidic environment to form Si-OH, which is then uniformly blended with the aluminum sol. The introduction of silica sol to the mixture elevates the pH which prompts the gel reaction. The nano-SiO₂ particles present in silica sol comprise disordered silica-oxygen tetrahedra, with the internal structure of the particles linked by Si-O-Si bonds. Meanwhile, Si atoms on the surface are connected with numerous -OH groups. The nanoparticles in the aluminum sol exhibit a double-layer structure. The inner layer consists of Al-O bonds, while the surface hydroxyl groups form hydrogen bonds to connect the interlayer. Both the nanoparticles in silica and aluminum sols can serve as nucleation sites. Simultaneously, the hydrolysis product Si-OH reacts with these exposed nucleation sites on the surface and undergoes self-polymerization, as well as mutual polymerization. This sequence of events culminates in the formation of a skeleton possessing a 3D structure.

3.3 Thermal stability of ASAs

Table 2 presents the specific surface areas of the ASAs after being calcined at 600, 800, and 1000 ℃ for 2h, respectively. As the calcination temperature rises, the specific surface area of ASA-0 decreases significantly, with a retention rate is merely 4.7% at 1000 ℃. An increase in calcination temperature after alumina sol addition similarly reduces the specific surface area of aerogel. However, the specific surface area remains significantly larger than that of ASA-0 at the same temperature. Notably, ASA-0.1, ASA-0.3, and ASA-0.5 show an increase in specific surface area at 600 ℃, while the other three samples remain almost unchanged. ASA-0.2 exhibits the highest specific surface area of 290.04 m²·g and a retention rate of 47.1% after being calcined at 1000 ℃ for 2h. As a result, the subsequent analysis will mainly focus on the ASA-0.2 sample because it exhibits the best thermal stability at 1000 ℃ among all the samples.

Sample	Specific surface area (25 ℃)/m²·g	Specific surface area (600 ℃)/m²·g	Specific surface area (800 ℃)/m²·g	Specific surface area (1000 ℃)/m²·g	Retention rate of specific surface area (1000 ℃)/%
ASA-0	508.266	349.676	35.347	23.912	4.7
ASA-0.05	544.149	537.651	371.912	224.332	41.2
ASA-0.1	574.982	591.871	371.769	248.147	43.2
ASA-0.2	616.317	598.781	363.463	290.040	47.1
ASA-0.3	530.567	544.044	338.579	248.719	46.9
ASA-0.35	527.462	579.419	331.782	224.332	42.5
ASA-0.4	523.587	514.432	329.291	216.577	41.4

Table 2 The specific surface area of Al₂O₃-SiO₂ aerogels with different Al/Si molar ratios at high temperature

Fig. 5(a)-(d) presents the N₂ adsorption-desorption isotherms, pore diameter distribution curves, average pore diameters, and the pore volume curve of ASA-0.2 after being calcined at 1000 ℃ for 2h. All ASA-0.2 samples at different temperatures exhibit type Ⅳ isotherms with an H₃ hysteresis loop, indicating that the porous is primarily composed of slit-type mesoporous. The N₂ adsorption capacity of ASA-0.2 weakens as the calcination temperature increases and the hysteresis loop decreases significantly. In Fig. 5(b), the number of pores decreases after heat treatment, and the pore diameter tends to increase. This change is mainly attributed to the deformation of the aerogel skeleton under thermal stress, resulting in the formation of larger pores by interconnecting some of the smaller pores. However, even after treatment at 1000℃, the pore size of ASA-0.2 remains in the range of 20 to 50nm. This observation is consistent with Fig. 5(c), which shows that the average pore diameters of the samples range from 13 nm to 25 nm, indicating that ASA-0.2 can maintain a stable mesoporous structure up to 1000 ℃. Fig. 5(d) shows that the higher temperatures reduce the pore volume, but it still retains a relatively high-value 1.615cm³/g after calcination 1000 ℃.

Fig. 6(a-d) presents the TEM images of ASA-0.2 before and after calcination. At room temperature, the nano-secondary particles comprising the ASA-0.2 skeleton structure appear spherical, and they aggregate to form particle clusters, creating a loose 3D network structure with uniform pore size and distribution. Remarkably, the morphology of the sample has no obvious change after calcination at 600 and 1000 °C, and the skeleton structure remains similar to that before calcination. At 1000 ℃, a certain degree of sintering occurs in the sample, resulting in the transformation of the aerogel skeleton from loose to dense. The spherical particles are observed to stack closer together, forming larger particle clusters, but the 3D nano-porous structure of ASA-0.2 is preserved. In comparison, the TEM image of ASA-0 calcined at 1000 ℃ exhibits severe sintering, volume shrinkage, and destruction of the 3D network structure. These findings indicate that the addition of aluminum sol can enhance the high-temperature resistance of silica aerogel.

3.4 Thermal, crystalline, and chemical structure of ASA after calcination

To further investigate the impact of high temperatures on ASA, samples calcined at various temperatures were subjected to TG, XRD, and FTIR analysis. Fig. 7(a-c) shows the TG and DTA curves of ASA-0, ASA-0.2, and ASA-0.35, respectively. The TG curve of ASA-0 reveals three distinct stages. In the first stage, ranging from room temperature to 250 ℃, a total weight loss of 0.95% occurs, attributed to the evaporation of a small amount of organic solvent and water from the pores and surface of the sample. The second stage shows an exothermic peak at 353.3 ℃, corresponding to a mass loss of 4.04% within the temperature range of 250 ℃ to 420 ℃. This loss is due to the partial oxidation of the hydrophobic group Si-CH₃ to Si-OH[37]. As the temperature rises further, the Si-CH₃ oxidation reaction tends to complete in the third stage, spanning from 420 ℃ to 1200 ℃. At this point, the generated Si-OH undergoes a condensation reaction at high temperatures, forming Si-O-Si bonds[38], leading to a mass loss of 8.83%.

The TG curves of ASA-0.2 and ASA-0.35 also exhibit three stages. In contrast to ASA-0, the mass loss of these samples gradually increased with the addition of aluminum sol. Specifically, the total weight loss in the first stage increases, which may be attributed to the enhanced adsorption of water and organic solvents after the addition of aluminum sol. ASA-0.2 and ASA-0 show similar mass loss in the second stage, and the heat absorption peak corresponding to the Si-OH dehydration condensation reaction is more obvious in ASA-0.35, reaching a mass loss of 10.07%. The XRD analysis in section 3.2 indicates that the ASA-0.35 aerogel at room temperature contains a certain amount of boehmite(γ-AlOOH). Consequently, the dehydration reaction of γ-AlOOH begins to transform into γ-Al₂O₃, leading to an increase in mass loss in the second stage [39]. When the temperature continues to rise, the γ-AlOOH dehydration reaction tends to be complete. When the temperature exceeds 800 ℃, the mass of ASA-0.2 and ASA-0.35 decreases slightly, and the TG curves become smooth, indicating better thermal stability compared to ASA-0.

Fig. 7(d) illustrates the XRD patterns of ASA-0.2 at different temperatures. ASA-0.2 exhibits an amorphous structure at room temperature, and its crystal structure remains unchanged even after high-temperature calcination. Interestingly, unlike some previous studies on SiO₂-Al₂O₃ composite aerogels, there is no evidence of γ-Al₂O₃ crystallization in the curve at 1000 ℃. To further investigate the crystal structure of ASA-0.2 at high temperatures, we conducted an additional XRD test after calcination at 1100°C for 2 hours. The results reveal that ASA-0.2 maintains its original structure, and no mullite crystals formed between 1000 ℃ to 1100 ℃, as reported in other studies[40-42]. We hypothesize that this phenomenon is attributed to two reasons. Firstly, the Al content in ASA-0.2 is relatively low, inhibiting the formation of γ-Al₂O₃and mullite crystalline phase. Secondly, the Al is uniformly dispersed at the nanoscale throughout the aerogel, and integrates into the SiO2 lattice, effectively suppressing the crystal phase change after high-temperature heat treatment.

Fig .7(e) displays the FTIR spectrum of ASA-0.2 calcined at different temperatures for 2h. The Si-CH₃groups (2964cm^-1, 1257cm^-1, and 847cm^-1) in the skeleton of ASA-0.2 are destroyed during the high-temperature calcination process, leading to the complete disappearance of the absorption peaks corresponding to C-H bonds (2964cm^-1and 1257cm^-1) from the graph at 600 ℃. Moreover, the content of Si-OH (957cm^-1) in the sample increases above 600 ℃ which is attributed to the oxidation of a portion of Si-CH₃ to form Si-OH. A Si-OH condensation reaction occurs at the heat treatment temperature is further raised, resulting in the formation of Si-O-Si bonds, which weakens the characteristic peaks of Si-OH. Upon comparison, it is observed that the absorption peak near 1632cm^-1 is enhanced in the heat-treated sample. This increase is attributed to the adsorption of water by the sample, making it completely hydrophilic after high-temperature heat treatment, leading to moisture absorption from the air after a period of time. Additionally, we note that the absorption peak at 610cm^-1 of the sample calcined at 600 °C almost disappears due to the thermal decomposition of γ-AlOOH, which is consistent with the TG analysis.

According to the sintering mechanism, high-temperature sintering of aerogels is driven by two main factors: (1) Thermodynamically, phase transition plays a significant role; (2) Kinetically, diffusion of aerogel particles at high temperatures is an important driving force. The influence of high temperature on ASA-0 can be attributed to the following two reasons. Firstly, Si-CH3 on the surface of the aerogel is oxidized to form Si-OH, which subsequently undergoes condensation to form Si-O-Si bonds at high temperatures. This process leads to the sintering of the skeleton particles and disruption of the 3D network structure. Secondly, due to the aerogel's high surface energy, surface diffusion of particles occurs at high temperatures. Continuous particle diffusion increases the particle diameter and contact area, resulting in severe necking and collapse of the pore structure. However, when aluminum sol is added, the nanoparticles provide numerous nucleation sites and enhance the crosslinking degree of the aerogel, making the composite aerogel skeleton more stable at high temperatures. Consequently, the composite aerogel exhibits higher heat resistance compared to pure SiO₂ aerogel. The addition of aluminum sol serves two purposes. Firstly, it inhibits the surface driving force between particles by promoting aerogel particle growth and reinforcing the skeleton. Secondly, the uniform distribution of Al in the micron-sized SiO₂particles effectively inhibits phase transition.

The aluminum doped silica aerogels (ASA) were prepared by sol-gel atmospheric pressure drying method using aluminum sol, TEOS, and basic silica sol as co-precursors. The effects of the Al/Si molar ratio and calcination temperature on the properties of ASA were investigated. It was found that the highest specific surface area of ASA-0.2 was 616.3m²∙g⁻¹ at room temperature, and the specific surface area retention rate after calcination at 1000°C for 2h was 47.1% significantly higher than that of ASA-0 (4.7%). Moreover, the composite aerogel had a three-dimensional network structure in which the aluminum element was uniformly distributed, and ASA-0.2 still maintained an amorphous structure after calcination at 1100℃. The addition of aluminum sol could effectively inhibit the surface driving force between particles and the occurrence of phase transition, which made ASA-0.2 have better high-temperature resistance performance. Our work shows that low-cost and stable raw materials can be used to prepare aerogels with high thermal stability by a simple atmospheric pressure drying process, which is a useful exploration for industrializing the production of heat-resistant aerogels in the future.

Acknowledgments

The authors would like to express our gratitude to the State Key Laboratory of Advanced Technology for Materials Synthesis and Processing at Wuhan University of Technology for facilitating the tests. We are also indebted to Prof. Hongyi Jiang for his invaluable guidance throughout the research process. Additionally, our sincere thanks go to the students within our group for their continual assistance and support.

Declaration of Funding

The authors declare that no funds, grants, or other support were received during the preparation of this manuscript.

Li CD, Chen ZF, Dong WF, Lin LL, Zhu XM, Liu QS, Zhang Y, Zhai N, Zhou ZH, Wang YH, Chen BM, Ji YX, Chen XQ, Xu XC, Yang YF, Zhang HT (2021) A review of silicon-based aerogel thermal insulation materials: Performance optimization through composition and microstructure. Journal of Non-Crystalline Solids 553:120517. doi:10.1016/j.jnoncrysol.2020.120517
Villasmil W, Fischer LJ, Worlitschek J (2019) A review and evaluation of thermal insulation materials and methods for thermal energy storage systems. Renew Sust Energ Rev 103:71-84. doi:10.1016/j.rser.2018.12.040
Liu YC, Wu HJ, Zhang YH, Yang JM, He FA (2020) Structure characteristics and hygrothermal performance of silica aerogel composites for building thermal insulation in humid areas. Energy Build 228:110452.110451-110452.110458. doi:10.1016/j.enbuild.2020.110452
Dou LY, Zhang XX, Cheng XT, Ma ZM, Wang XQ, Si Y, Yu JY, Ding B (2019) Hierarchical Cellular Structured Ceramic Nanofibrous Aerogels with Temperature-Invariant Superelasticity for Thermal Insulation. ACS Appl Mater Interfaces 11 (32):29056-29064. doi:10.1021/acsami.9b10018
Feng JZ, Su BL, Xia HS, Zhao SY, Gao C, Wang LK, Ogbeide O, Feng J, Hasan T (2021) Printed aerogels: chemistry, processing, and applications. Chem Soc Rev 50 (6):3842-3888. doi:10.1039/c9cs00757a
Liu GW, Zhou B, Du A, Shen J, Yu QJ (2013) Effect of the thermal treatment on microstructure and physical properties of low-density and high transparency silica aerogels via acetonitrile supercritical drying. J Porous Mat 20 (5):1163-1170. doi:10.1007/s10934-013-9699-x
Lamy-Mendes A, Pontinha ADR, Alves P, Santos P, Duraes L (2021) Progress in silica aerogel-containing materials for buildings' thermal insulation. Constr Build Mater 286:122815.122811-122815.122832. doi:10.1016/j.conbuildmat.2021.122815
Guo JF, Tang GH, Jiang YG, Cai HF, Feng J, Feng JZ (2021) Inhibited radiation transmittance and enhanced thermal stability of silica aerogels under very-high temperature. Ceram Int 47 (14):19824-19834. doi:10.1016/j.ceramint.2021.03.321
Huang DM, Guo CN, Zhang MZ, Shi L (2017) Characteristics of nanoporous silica aerogel under high temperature from 950 C-omicron to 1200 C-omicron. Materials & Design 129:82-90. doi:10.1016/j.matdes.2017.05.024
An L, Wang JY, Petit D, Armstrong JN, Li CN, Hu Y, Huang YL, Shao ZF, Ren SQ (2020) A scalable crosslinked fiberglass-aerogel thermal insulation composite. Appl Mater Today 21:100843. doi:10.1016/j.apmt.2020.100843
Linhares T, de Amorim MTP, Duraes L (2019) Silica aerogel composites with embedded fibres: a review on their preparation, properties and applications. J Mater Chem A 7 (40):22768-22802. doi:10.1039/c9ta04811a
Liu RL, Dong X, Xie ST, Jia T, Xue YJ, Liu JC, Jing W, Guo AR (2019) Ultralight, thermal insulating, and high-temperature-resistant mullite-based nanofibrous aerogels. Chem Eng J 360:464-472. doi:10.1016/j.cej.2018.12.018
Zhang HM, Feng JZ, Li LJ, Jiang YG, Feng J (2022) Preparation of a carbon fibre-reinforced carbon aerogel and its application as a high-temperature thermal insulator. RSC Adv 12 (22):13783-13791. doi:10.1039/d2ra00276k
Li MM, Chen X, Li XT, Dong J, Zhao X, Zhang QH (2022) Controllable Strong and Ultralight Aramid Nanofiber-Based Aerogel Fibers for Thermal Insulation Applications. Adv Fiber Mater 4 (5):1267-1277. doi:10.1007/s42765-022-00175-2
Peng F, Jiang YG, Feng J, Cai HF, Feng JZ, Li LJ (2021) Thermally insulating, fiber-reinforced alumina-silica aerogel composites with ultra-low shrinkage up to 1500 ℃. Chem Eng J 411:128402. doi:10.1016/j.cej.2021.128402
Zhong Y, Zhang JJ, Wu XD, Shen XD, Cui S, Lu CH (2017) Carbon-fiber felt reinforced carbon/alumina aerogel composite fabricated with high strength and low thermal conductivity. J Sol-Gel Sci Technol 84 (1):129-134. doi:10.1007/s10971-017-4485-x
Wu Y, Wang XD, Shen J (2023) Metal oxide aerogels for high-temperature applications. J Sol-Gel Sci Technol 106 (2):360-380. doi:10.1007/s10971-021-05720-w
Shalygin AS, Kozhevnikov IV, Gerasirnov EY, Andreev AS, Lapina OB, Martyanov ON (2017) The impact of Si/Al ratio on properties of aluminosilicate aerogels. Microporous Mesoporous Mat 251:105-113. doi:10.1016/j.micromeso.2017.05.053
Wu LA, Qiao XS, Cui S, Hong ZL, Fan XP (2015) Synthesis of monolithic aerogel-like alumina via the accumulation of mesoporous hollow microspheres. Microporous Mesoporous Mat 202:234-240. doi:10.1016/j.micromeso.2014.10.015
Zu GQ, Shen J, Zou LP, Wang WQ, Lian Y, Zhang ZH, Du A (2013) Nanoengineering Super Heat-Resistant, Strong Alumina Aerogels. Chem Mat 25 (23):4757-4764. doi:10.1021/cm402900y
Ji XF, Zhou Q, Qiu GB, Yue CS, Guo M, Chen FQ, Zhang M (2018) Preparation of monolithic silica-based aerogels with high thermal stability by ambient pressure drying. Ceram Int 44 (11):11923-11931. doi:10.1016/j.ceramint.2018.03.261
Wu XD, Shao GF, Cui S, Wang L, Shen XD (2016) Synthesis of a novel Al2O3-SiO2 composite aerogel with high specific surface area at elevated temperatures using inexpensive inorganic salt of aluminum. Ceram Int 42 (1):874-882. doi:10.1016/j.ceramint.2015.09.012
Nesterov NS, Shalygin AS, Pakharukova VP, Glazneva TS, Martyanov ON (2019) Mesoporous aerogel-like Al-Si oxides obtained via supercritical antisolvent precipitation of alumina and silica sols. J Supercrit Fluids 149:110-119. doi:10.1016/j.supflu.2019.03.014
Shojaie-Bahaabad M, Taheri-Nassaj E, Naghizadeh R (2009) Effect of yttria on crystallization and microstructure of an alumina-YAG fiber prepared by aqueous sol-gel process. Ceram Int 35 (1):391-396. doi:10.1016/j.ceramint.2007.11.010
Tan HB, Ding YP, Yang JF (2010) Mullite fibers prepared from an inorganic sol-gel precursor. J Sol-Gel Sci Technol 53 (2):378-383. doi:10.1007/s10971-009-2106-z
Li M, Jiang HY, Xu D, Hai O, Zheng W (2016) Low density and hydrophobic silica aerogels dried under ambient pressure using a new co-precursor method. Journal of Non-Crystalline Solids 452:187-193. doi:10.1016/j.jnoncrysol.2016.09.001
Yao JQ, Gao XD, Wu YQ, Zhao X, Li XM (2022) High-temperature resistant ambient pressure-dried aluminum doped silica aerogel from inorganic silicon and aluminum sources. Ceram Int 48 (11):15006-15016. doi:10.1016/j.ceramint.2022.02.029
Jadhav SB, Makki A, Hajjar D, Sarawade PB (2022) Synthesis of light weight recron fiber-reinforced sodium silicate based silica aerogel blankets at an ambient pressure for thermal protection. J Porous Mat 29 (4):957-969. doi:10.1007/s10934-022-01231-3
Zhang XY, Chen ZF, Zhang JX, Ye XL, Cui S (2021) Hydrophobic silica aerogels prepared by microwave irradiation. Chem Phys Lett 762:138127. doi:10.1016/j.cplett.2020.138127
Cao FC, Ren LL, Li XA (2015) Synthesis of high strength monolithic alumina aerogels at ambient pressure. RSC Adv 5 (23):18025-18028. doi:10.1039/c5ra00424a
Zu GQ, Shen J, Wei XQ, Ni XY, Zhang ZH, Wang JC, Liu GW (2011) Preparation and characterization of monolithic alumina aerogels. Journal of Non-Crystalline Solids 357 (15):2903-2906. doi:10.1016/j.jnoncrysol.2011.03.031
Cai HF, Jiang YG, Feng J, Zhang SZ, Peng F, Xiao YY, Li LJ, Feng JZ (2020) Preparation of silica aerogels with high temperature resistance and low thermal conductivity by monodispersed silica sol. Materials & Design 191:108640. doi:10.1016/j.matdes.2020.108640
Wu LN, Huang YD, Wang ZJ, Liu L, Xu HF (2010) Fabrication of hydrophobic alumina aerogel monoliths by surface modification and ambient pressure drying. Appl Surf Sci 256 (20):5973-5977. doi:10.1016/j.apsusc.2010.03.104
Eris G, Sanli D, Ulker Z, Bozbag SE, Jonas A, Kiraz A, Erkey C (2013) Three-dimensional optofluidic waveguides in hydrophobic silica aerogels via supercritical fluid processing. J Supercrit Fluids 73:28-33. doi:10.1016/j.supflu.2012.11.001
Mahadik DB, Lee YK, Chavan NK, Mahadik SA, Park HH (2016) Monolithic and shrinkage-free hydrophobic silica aerogels via new rapid supercritical extraction process. J Supercrit Fluids 107:84-91. doi:10.1016/j.supflu.2015.08.020
Tatsuro H, Laiyuan C, Toshihiko O, Toyohiko S, Kenzi S, Toshiaki M (1999) A novel alumina catalyst support with high thermal stability derived from silica-modified alumina aerogel, Catalysis Letters 58 (2-3):89-92. doi:10.1023/a:1019069631852
Bhagat SD, Rao AV (2006) Surface chemical modification of TEOS based silica aerogels synthesized by two step (acid-base) sol-gel process. Appl Surf Sci 252 (12):4289-4297. doi:10.1016/j.apsusc.2005.07.006
Bhagat SD, Kim YH, Ahn YS, Yeo JG (2007) Rapid synthesis of water-glass based aerogels by in situ surface modification of the hydrogels. Appl Surf Sci 253 (6):3231-3236. doi:10.1016/j.apsusc.2006.07.016
Maldonado M, Oleksiak MD, Chinta S, Rimer JD (2013) Controlling Crystal Polymorphism in Organic-Free Synthesis of Na-Zeolites. J Am Chem Soc 135 (7):2641-2652. doi:10.1021/ja3105939
Osaki T, Nagashima K, Watari K, Tajiri K (2007) Silica-doped alumina cryogels with high thermal stability. Journal of Non-Crystalline Solids 353 (24-25):2436-2442. doi:10.1016/j.jnoncrysol.2007.04.016
Shalygin AS, Kozhevnikov IV, Gerasirnov EY, Andreev AS, Lapina OB, Martyanov ON (2017) The impact of Si/Al ratio on properties of aluminosilicate aerogels, Microporous and mesoporous materials: The offical journal of the International Zeolite Association 251:105-113. doi:10.1016/j.micromeso.2017.05.053
4Xia CK, Hao MY, Liu WH, Zhang XY, Miao Y, Ma C, Gao F (2023) Synthesis of Al2O3-SiO2 aerogel from water glass with high thermal stability and low thermal conductivity. J Sol-Gel Sci Technol 106 (2):561-571. doi:10.1007/s10971-023-06085-y

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Preparation of High-Temperature Resistant Aluminum-Doped Silica Aerogel from Aluminum Sol Source by Ambient Pressure Drying

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