Facile Synthesis of Dual Modal Pore Structure Aerogel with Enhanced Thermal Stability

Abstract

Regarding the preparation of aerogels by the co-precursor method, the skeleton collapse caused by its low strength is one of the key problems that needs to be solved urgently. In this study, vinyl-functionalized silica aerogel was prepared under atmospheric drying conditions (APD) with vinyltriethoxysilane (V) and water glass (W) as co-precursors. The performance of aerogels varied with the components of co-precursors. When the V:W ratio was 0.8, the aerogel had excellent properties of low thermal conductivity (0.0254 W/(m·K)), super hydrophobicity (hydrophobic angle of 160°), high specific surface area (890.76 m²/g), high porosity (96.82%), and low density (0.087 g/cm³). Test results of SEM and BET showed that the V:W ratio affected the pore structure. When the V:W ratio was around 0.8, the aerogel had a dual modal pore structure composed of both small (6–8 nm) and large (20–30 nm) mesopores, which could contribute to enhance the skeleton strength of the aerogel. On the other hand, the addition of vinyltriethoxysilane promoted the skeleton stability by reducing the capillary force. The vinyltriethoxysilane and water glass as novel co-precursor combinations can provide guidance for the preparation of aerogels under APD conditions.

1. Introduction

Silica aerogel is a new material with excellent properties, such as ultra-low thermal conductivity and density, extremely high specific surface area and porosity [1,2,3,4,5,6], which has the potential to be widely used in thermal insulators, adsorption agents, extractants, catalysts, etc. [7]. However, silica aerogel is usually prepared by supercritical drying process, but it has disadvantages such as requiring the use of expensive equipment and exhibiting poor safety performance, which restricts its practical production and large-scale applications [8,9]. Recently, the preparation of silica aerogel by the atmospheric drying (APD) method, with the advantages of safety and low cost, has made rapid progress and become a trend to replace the supercritical drying method [10,11,12,13].

During APD, the condensation reaction of silanol groups usually occurs on the surface of aerogels, and the condensation could easily lead to shrinkage or collapse of the gel skeleton under the action of surface energy and capillary pressure. In this case, the thermal conductivity and other properties of the aerogel are dramatically degenerated [12,14]. Relevant studies have reported that the surface modification technology can effectively alleviate the condensation reaction and stabilize the skeleton structure to improve the performance of aerogels [1,15].

There are two commonly used surface modification technology in APD: the surface derivatization method and co-precursor method [16]. The surface derivatization method has the characteristics of excellent product performance and stability. In the process of surface derivatization, the wet gel is soaked with a large number of solvents and modifiers before APD. In the co-precursor method, based on the cross-linking reaction of precursors, hydrophilic hydroxyl groups are replaced with hydrophobic groups [15,17,18]. The co-precursor includes a traditional precursor material (such as water glass, ethyl orthosilicate (TEOS), and methyl orthosilicate (TMOS)), and the other new type of organosilicon source (e.g., trimethylsilane, triethoxysilane, phenylsilane, etc.). The co-precursor method has the advantages of a simple process, strong operability, and short preparation period [4,14]. However, the commonly used co-precursor combinations have the characteristics of low cross-linking reaction degree, poor skeleton structure strength, and prone to collapse, leading to problems of high density, high thermal conductivity, and poor hydrophobic performance of the prepared aerogel [19,20]. Therefore, developing new co-precursor combinations with high cross-linking reaction degrees, is an effective way to improve the skeleton strength and the aerogel performance.

Vinyltriethoxysilane (VTES) is a material with high degree of cross-linking reaction, low toxicity, and high hydrophobicity [14,18,21]. It has been reported that high-strength skeletons could be synthesized using VTES as a raw material [22,23,24]. Thus, VTES is expected to be applied in the co-precursor method to enhance the framework strength and prevent the framework collapse during APD, so as to obtain aerogels with excellent performance. Compared with traditional organic silicon sources (TMOS, TEOS), the water glass selected in this work has the advantages of being cheap, easy to obtain, non-toxic and harmless. Overall, these two co-precursors are expected to be suitable for the mass production of aerogels

In this study, VTES and water glass (W) were selected as the co-precursor combinations to prepare vinyl-functionalized silica (VF-SiO₂) aerogel under APD. The effects of the ratio of VTES:W on the properties of aerogel were investigated, including thermal conductivity, hydrophobicity, bulk density and porosity. Then, based on the characterization results of N₂ adsorption and desorption, scanning electron microscopy (SEM) and Fourier-transform infrared spectroscopy (FT-IR), as well as the data of the surface energy and capillary pressure by theoretical formula, the mechanism by which the ratio of VTES and W in the co-precursor combination affected the performance of VF-SiO₂ aerogel was analyzed.

2. Materials and Methods

2.1. Materials

The vinyltriethoxysilane (VTES, AR grade, Shanghai Aladdin Chemical Co., Shanghai, China) and water glass (W with wt.: 34%, Na₂:SiO₂ = 1:3.33, Jiashan County Yourui Refractory Co., Ltd., Jiaxing, China) were used as precursors, and a hydrochloric acid solution (HCl, Shanghai Macklin Biochemical Co., Ltd., Shanghai, China) and an ammonium hydroxide solution (NH₃·H₂O, 25–28%, Sinopharm Chemical Reagent Co., Ltd., Shanghai, China) were used as acid and base in the adjustment of the pH of the final solution. Ethanol (AR grade, Shanghai Aladdin Chemical Co., Shanghai, China) and de-ionized (DI) water were used as solvents throughout the entire process.

2.2. Preparation

A schematic representation of the steps of the sample preparation is shown in Figure 1. Firstly, the 20 mL of W precursor was added to 100 mL of DI water and 38 mL of HCl (1 mol/L), and the mixture was stirred and hydrolyzed at 40 °C for 2 h to form a solution called “W solution”. At the same time, the 27.7 mL of VTES, 23 mL of ethanol and 21.23 mL of DI water were hydrolyzed in 1 mol/L of HCl solution (adjusted to PH = 3) at 30 °C for 30 min. The resulting solution is called “V solution”. Next, the different volumes of V and W solution were taken to control the volume ratio between 0.2 and 2.0 (e.g., 20 mL W and 4 mL V solution, the volume ratio is 0.2). It was stirred continuously at 30 °C for about 30 min to mix evenly. Next, 0.1 mol/L NH₄OH was added to the mixture to adjust the pH to 7, and stirring was continued for 10 min, and the final solution was moved into the mold to start the gelation process. The gel started after 55 min at 30 °C and completed for approximately 4 h, followed by repeated solvent exchange with ethanol (three times). Finally, the wet gel was dried in an oven at 60 °C for 5 h. When the V:W volume ratios were 0.2, 0.4, 0.6, 0.8, 1.0, and 2.0, the gel samples were referred to as VW-0.2, VW-0.4, VW-0.6, VW-0.8, VW-1.0, and VW-2.0.

2.3. Characterizations

The microstructure, chemical structure and pore structure of the samples were analyzed by using Scanning Electron Microscopy (SEM, lTDLCS-4800, Shimadzu Company, Kyoto, Japan), Fourier-Transform Infrared spectroscopy (FT-IR, TENSOR27, Brock GMBH, Saarbrucken, Germany) and the NOVA 2200E surface area analyzer (BK112T, Beijing Jingwei Gaobo Science and Technology Co. LTD, Beijing, China) respectively. The latter characterization method followed the BET (Brunauer-Emmett-Teller) method.

Based on the transient plane heat source method, the thermal conductivity was measured based on the Swedish Hot Disk 2500S heat conductivity. In the experiment, the sample was dried to a constant weight and then put into a sealed bag to cool to room temperature. Second, the nickel helix probe was mounted on the polyimide film was sandwiched between the two samples. The measured temperature was 23 °C, atmospheric pressure, with a power of 30 mW, 40 s, and detection depth of 8 mm. Each sample was measured three times and the mean was calculated to represent the thermal conductivity values of the sample.

Moreover, the JGW-360 instrument was used to measure the contact angle of the sample, a water drop was placed on the surface of the sample, and the camera was used to take the angle between the interface of the sample and the water drop. The OCA 20 system determined the value of the angle, which was the contact angle of the sample.

To determine the specific structure of the copolymer aerogel skeleton, the silicon spectrum of the samples was tested using a Swiss company Bruker type AVANCE III HD 400 M solid-state NMR instrument (SSNMR, Solid-State Nuclear Magnetic Resonance). The sample is solid and does not need to be put into the quartz tube, avoiding the SiO₂ component in the quartz tube from interfering with the test results of the silica spectrum.

Furthermore, the calculations of porosity and shrinkage were performed using the equations presented below:

The porosity ($p$) of a sample can be calculated as a function of its skeletal density ($p_s$) and the bulk density ($p_b$) [25]:

$$p=\left(1-p_s/p_b\right)\times 100\%$$

(1)

where, $p_s$ is determined by using a pycnometer (Shanghai Guan Zhi Electric Technology Co., Ltd., Shanghai, China), and $p_b$ is the bulk density by using the following equation [26]:

$$p_b=\frac{{\sum }_{i=1}^N{m}_i}{{\sum }_{i=1}^N{V}_i}$$

(2)

where, $m_i$ is the mass of the sample, $V_i$ is the volume of the sample, and >3 (to ensure the accuracy of $p_{b }$).

Moreover, the shrinkage percentage (S) of the aerogel was defined by using equation [27]:

$$S=(1-\frac{D_{\mathrm{a}}}{D_{\mathrm{f}}})\times 100\%$$

(3)

where, $D_{\mathrm{a}}$ is the aerogel diameter and $D_{\mathrm{f}}$ is the diameter of the gelatinizing mold.

3. Results and Discussion

3.1. Performance Analysis

3.1.1. Thermal Conductivity

To clarify the thermal conductivity of the VF-SiO₂ aerogel, the curves of the thermal conductivity, bulk density, and shrinkage are presented in Figure 2. It is obvious that all three curves present a V shape, which is “protruding on both sides, sunken in the middle”. At the minimum, the VW-0.8 sample shows the lowest thermal conductivity with a value of 0.0254 W/(m·K). These results indicate that too high or too low a ratio of VTES in the samples will have a negative effect on the thermal conductivity of the VF-SiO₂ aerogel. In addition, it is worth noting that the density of the VW-0.6 and VW-0.8 samples are basically stable, but the shrinkage varies significantly. The reason may be due to the insertion of the non-hydrolyzed vinyl groups into the Si-O-Si polymer chain network, which results in a weakening of the cohesions in the chain. Thus, the three-dimensional skeleton structure experiences a certain degree of elasticity and flexibility.

3.1.2. Hydrophobicity

The hydrophobic behavior is an important performance parameter of the aerogel, and the measurement results of the different samples are shown in Figure 3. There is an obvious feature in these results. For VTES = 0, the water droplets will be quickly absorbed when it touches the surface of the sample, which is a strong indication of superhydrophilicity. It can also be directly observed that the contact angle increases with an increase in V:W ratio. The main reason is the circumstance that a VTES precursor with vinyl groups can modify the gel to increase its hydrophobicity. As can be seen in Figure 2, for the VW-0.8 sample, the contact angle reaches a maximum of 160°, which is the situation when basically the complete gel is covered by vinyl groups. On the other hand, the contact angle is just slightly decreased for the VW-1.0 and VW-2.0 samples, which is most probably due to the dominant hydrolysis and condensation reaction of VTES. As verified by SEM analysis, an aerogel with a micrometer structure has been formed. In this micrometer range, the air between the water droplets and the aerogel surface has become extruded to increases the contact area (between the droplet and the surface), which directly results in a decrease in the contact angle.

3.1.3. Comparison

Table 1. Comparison of aerogel samples that are prepared by using different co-precursors.

Precursor	Drying Method	Preparation Time	Thermal Conductivity	Hydrophobicity	Published Time
MTES + TEOS [28]	Supercritical drying	/	/	148°	2018
MTMS + CATB [16]	APD	10 h	0.0370 W/(m·K)	/	2019
MTMS + DMDMS [12]	APD	/	0.0384 W/(m·K)	/	2020
MTES + TEOS [17]	Microwave irradiation	19 h	/	168°	2021
MTES + TEOS [29]	APD (silane strengthening)	20 h	0.0223 W/(m·K)	/	2021
MTMS + TMOS [30]	Supercritical drying	4 d	/	141°	2018
VTES + MTES [31]	APD	7.5 d	0.0243 W/(m·K)	141°	2019
MTMS + TEOS [32]	APD (silane strengthening)	25 h	/	128°	2021
MTMS + TEOS [33]	APD (surface modification)	54 h	/	153.9°	2020
MTES [26]	APD	88 h	0.0526 ± 0.0008 W/(m·K)	/	2021
VTES + water glass (this work)	APD	9 h	0.0254 W/(m·K)	160° ± 2°	/

compares two important properties (thermal conductivity and hydrophobicity) of aerogels. It is easy to observed that the use of the MTES and TEOS as precursors has been the dominant direction for the preparation of aerogels in recent years. However, there is also a frequent problem. That is, the preparation time is long, and the drying method is harsh. In the present study, the VF-SiO₂ aerogel has its own unique characteristics in terms of raw materials, drying method, preparation period, comprehensive performance, etc. In fact, these are necessary conditions for large-scale aerogel preparation.

3.2. Microstructure

The SEM microstructures and morphologies of the different samples are shown in Figure 4. Obviously, the V/W ratio has a large impact on the aerogel network structure. The microstructure can roughly be divided into three types: (i) the particles which are so tightly connected with just a few visible pores (see Figure 4a,b). This is most probably due to the collapse of the skeleton, resulting in a more closely aligned structure. (ii) The aerogel skeleton is mainly composed of nanoparticles with a high degree of cross-linking (which is the key to the monolithic of the aerogels) (see Figure 4c,d). The Cassie-Baxter model used to describe the extreme wettability of a surface confirms that the presence of air increases its contact angle. Therefore, the contact angle is higher with an appropriate amount of VTES. This also explains why VW-0.8 has greater hydrophobicity. (iii) The aerogel skeleton is connected by micron-sized particles, and the assembly of these large particles forms a chain-like structure with huge gaps and poor structure crossing (see Figure 4e,f). The sample image (upper right of the SEM image) confirms this result.

3.3. Pore Characteristics

The adsorption/desorption isotherms of the different samples in addition to corresponding BJH pore diameter distributions are shown in Figure 5. The VW-0.2 and VW-0.4 samples exhibit a type I isotherm, which is an indication of the presence of an ink-bottle [29]. This phenomenon is due to the collapse of the skeleton structure under APD conditions. Furthermore, as the VTES content is increasing, the isotherms of VW-0.6 and VW-0.8 become similar to the typical IV with type H3 hysteresis loop [17,34], this is a characteristic of a mesoporous structure. In addition, both small (6–8 nm) and large (20–30 nm) mesopores in the VW-0.8 sample can be observed in the pore size distribution pattern (see Figure 5b). This is an indication of a dual modal pore structure of VW-0.8, which helps to prevent both excessive shrinkage and cracking of the gel. As the VTES content further increases, only III-type isotherms can be observed [29]. These are indications of predominantly micron-sized VW-1.0 and VW-2.0 structures with, in principle, no micropore filling and capillary condensation. This explains why the isotherm hysteresis loop and pore size distribution (2–5 nm) are unable to be clearly observed (Figure 5b).

Table 2. Physicochemical properties of the different samples.

	Surface Area (m2/g)	Pore Volume (cm3/g)	Mean Pore Size (nm)	Porosity (%)
VW-0.2	485.47 ± 9.5	0.970 ± 0.04	4.599 ± 0.4	40.12 ± 0.3
VW-0.4	641.68 ± 6.7	1.174 ± 0.36	6.273 ± 0.2	49.28 ± 0.1
VW-0.6	843.79 ± 4.4	1.872 ± 0.01	8.155 ± 1.3	78.43 ± 0.2
VW-0.8	890.76 ± 7.9	2.413 ± 0.13	10.597 ± 2.8	96.82 ± 0.1
VW-1.0	458.47 ± 8.8	1.122 ± 0.38	8.629 ± 0.4	94.35 ± 0.04
VW-2.0	125.84 ± 11.0	0.435 ± 0.16	8.230 ± 1.0	94.15 ± 0.04

lists the relevant physical parameters of various VF-SiO₂ aerogel samples. Obviously, the VW-0.8 sample has the largest specific surface area (890.76 m²/g), porosity (96.82%) and pore volume (2.413 cm³/g), which can be attributed to the appearance of the dual-mesoporous structure.

3.4. Capillary Pressure and Surface Energy

3.4.1. Capillary Pressure

As mentioned above, a high capillary pressure is a key factor in the restrictions of the high-performance aerogel during atmospheric drying. Non-polar vinyl groups have, in the present work, been inserted into the gel during the synthesis process. The silicon network has, thereby, achieved hydrophobicity in minimizing the disadvantage caused by capillary forces. With reference to the work by Scherer, Equation (4) [27], the capillary force is a characteristic function of the contact angle θ (between a water droplet and the substrate), the pore fluid/vapor surface tension $\gamma$ and the pore radius r.

$$P_C=-\frac{2\gamma \cos \theta }{r}$$

(4)

According to the equation above, the following results can be obtained:

Based on Equation (4) and

Table 3. Positive and negativity of the capillary force values.

	VW-0.2	VW-0.4	VW-0.6	VW-0.8	VW-1.0	VW-2.0
PC	-	-	-	+	+	+

, the following conclusions have been drawn. The capillary force, P_C, is negative for a contact angle of θ < 90. The hydrophilic samples VW-0.2, VW-0.4, and VW-0.6 are, hence, under tension. Moreover, P_C becomes positive for an increase in VTES ratio (i.e., for VW-0.8, VW-1.0, and VW-2.0). This indicates that a large amount of VTES can relieve the fluid tension during the evaporation process. It also explains why the thermal conductivity of a V/W aerogel, with a large VTES content, is significantly better than the thermal conductivity of a gel with a smaller VTES content.

3.4.2. Surface Energy

Based on the equations by Neumann (Equation (5)) and Young (Equation (6)) [27,35], the surface energy of the VF-SiO₂ aerogel has in the present study been theoretically estimated by using a semi-empirical method. The equations have the following expressions:

$${\gamma }_{LV}\cos \theta ={\gamma }_{SV}-{\gamma }_{SL}$$

(5)

$${\gamma }_{\mathrm{SL}}={\gamma }_{\mathrm{LV}}+{\gamma }_{\mathrm{SV}}-2\sqrt{{\gamma }_{\mathrm{LV}}{\gamma }_{\mathrm{SV}}}{e}^{-\beta ({\gamma }_{\mathrm{LV}}-{\gamma }_{SV})}{}^2$$

(6)

By combining these two equations, the following equation is obtained:

$$\cos \theta =2\sqrt{\frac{{\gamma }_{\mathrm{SV}}}{72.8}}{e}^{-0.000125 (72.8-{\gamma }_{\mathrm{SV}})}{}^2-1$$

(7)

Based on data presented in Figure 6, the surface energy of the VF-SiO₂ aerogel can, finally, be calculated by using Equation (7). The results are shown in Figure 7.

As is obvious in Figure 7, the presence of VTES does significantly lower the surface energy of the gel. The surface energy will initially decrease to a minimum for an increase in VTES content (from 40.95 to 0.044 mJ/m²). However, for the VW-0.8 sample there is the tendency for the energy to start increasing again. Thus, there is a slight increase in surface energy at higher VTES contents. It must here be stressed that a low surface energy is the key to the formation of aerogels with excellent performances.

In the present study, an VF-SiO₂ aerogel has been developed for which it is possible to tune the surface energy in a wide range (50.52 mJ/m²–0.25 mJ/m²). This result is, thereby, of a large guiding significance for the industrial production of VF-SiO₂ aerogels.

3.5. Chemical Structure

The FT-IR spectra of the samples modified by different V/W ratios can be seen in Figure 8. According to previous reports [36,37], the characteristic absorption peaks of the bending vibrations and symmetric stretching vibrations of Si-O-Si, appear at 454 cm⁻¹ and 779 cm⁻¹, respectively. Moreover, the characteristic peaks of the linear structure of Si-O-Si, appear at 1084 cm⁻¹. These observations indicate that the network structure of the VF-SiO₂ aerogel is mainly composed of Si-O-Si structures. By comparing the infrared spectra for the different aerogel samples, it is clear that an increase in VTES content causes a decrease in absorption peak intensity of Si-O-Si (which has a negative effect on the stability of the VF-SiO₂ aerogel structure [17]). Moreover, the presence of VTES in the samples can be determined in the spectra [38] at: (i) 540 cm⁻¹, (ii) 1413 cm⁻¹, (iii) 1640 cm⁻¹, and (iv) between 2970 cm⁻¹ and 3070 cm⁻¹. The peak at 540 cm⁻¹ can be traced from the torsional vibration of H in the vinyl groups. Moreover, the peak at 14 cm⁻¹ is attributed to the deformation vibration of =C-H. Finally, the peaks at 1640 cm⁻¹, and between 2970 cm⁻¹ and 3070 cm⁻¹ represent the stretching vibrations of the C=C and =C-H groups, respectively. In addition, there is a peak close to 3419 cm⁻¹ which indicates O-H stretching vibrations [19]. The intensity of this stretching vibration peak did at first decrease, and then slightly increase with an increase in the V:W ratio. The decrease in intensity is attributed to the effective replacement of O-H by the right amount of VTES. Additionally, the slight increase in intensity indicates the existence of active hydroxyl groups, which may have been formed by the aggregation of VTES. An aggregation of VTES leads to a larger exposure of O-H in the aerogel structure, and this result is consistent with the results from the contact angles measurements.

3.6. Solid-State NMR

To further determine the skeleton structure of the aerogel, solid-state NMR results are shown in Figure 9. According to previous reports [39,40], the peaks at −58 ppm and −67 ppm are T₂ and T₃ peaks of Si respectively, which correspond to C₂H₃Si(-O)₂(-OH)₁ and C₂H₃Si(-O)₃ in aerogel, and the latter (T₃) peak is stronger, indicating that most of the Si-OH in VTES and water glass have been completely hydrolyzed. VTES and water glass condense together into C₂H₃Si(-O)₃, and the weak T₂ peak is mainly due to the small amount of silicon hydroxyl group remaining. On the left side of the spectrum, there are two relatively small peaks at −105 ppm and −113 ppm, corresponding to Q₃ and Q₄ peaks, which can be regarded as Si(-O)₃(-OH)₁ and Si(-O)₄, which are structural fragments formed by self-polymerization of sodium silicate. The Q₄ peak is stronger than Q₃, mainly because of the high degree of hydrolysis of water glass, and only a small amount of silicon hydroxyl is unhydrolyzed.

3.7. Synthesis Mechanism

Based on the relevant literature and the above results [28,41,42], the formation process of the VF-SiO₂ aerogel has been analyzed.

In the process of hydrolysis, the V solution (V sol) and W solution (W sol) are, at first, separately produced, and the particle surfaces are thereafter adequately covered by silanol groups (Si-OH) (i.e., the Si atoms will become bonded to a large number of active hydroxyl molecules (see Figure 10)).

In the process of polycondensation, according to the FT-IR and solid-state NMR results, the skeleton of the aerogel is composed of the Si-O-Si bond. For the large W solution, the Si-O-Si peak is strong. The W sol gradually polymerizes through the Si-O-Si bond to form a three-dimensional network structure (see Figure 11). However, the -OH active group peak is strong, and the structure is collapsed under a strong surface energy (50.52 mJ/m²), which was also confirmed by the SEM. On the other hand, the Si-O-Si peak is weak for the large of V solution, indicating that the skeleton structure is weak, and a gel structure composed of chains with large gaps and particles appears. For an appropriate amount of W sol and V sol, the Si-O-Si bond is closely connected, and a certain amount of non-hydrolyzed vinyl group (derived from V) polymerization replaces the -OH group at the outer end of the skeleton. Under low surface energy conditions, the aerogel still has a stable skeleton structure.

4. Conclusions

With vinyltriethoxysilane and water glass as the co-precursor, the vinyl-functionalized silica (VF-SiO₂) aerogel was synthesized under APD, and the effects of the co-precursor on the properties of VF-SiO₂ aerogel were explored. The main conclusions can be drawn as follows:

(1)

The performances of VF-SiO₂ aerogel vary with the ratio of V:W. When V:W = 0.8, the prepared aerogel has low thermal conductivity (0.0254 W/(m·K)), low density (0.087 g/cm³), superhydrophilicity (160°), high surface area (890.76 m²/g), and high porosity (96.82%).

(2)

When V:W = 0.2 or 0.4, the particles connected to the skeleton are compact and the porosity is low; when V:W = 1.0 or 2.0, the particles connected to the skeleton have a chain structure with high porosity. However, when V:W = 0.6 or 0.8, the skeleton is composed of nanoparticles with high cross-linking degree, and it has a dual modal pore structure composed of both small (6–8 nm) and large (20–30 nm) mesopores. Therefore, the promotion of aerogel performance can be attributed to the improvement of pore structure.

(3)

By changing the content of VTES, the surface energy of aerogel can be adjusted in a wide range (50.52 mJ/m²–0.25 mJ/m²). Thus, the addition of VTES can reduce capillary force and improve skeleton stability during drying. This is one of the key reasons that VTES can prevent the collapse of aerogel skeleton under APD conditions.

(4)

VTES can rapidly condense on the surface of water glass to form a high degree of cross-linking reaction, thereby increasing the reaction degree of the system and improving the performance of aerogel.