A method for preparing a sioc aerogel

Abstract

The application relates to the technical field of aerogel preparation, in particular to a SiOC aerogel preparation method. The method comprises the following steps: sequentially dissolving tetraalkoxysilane and alkyltrialkoxysilane in a mixed solvent of deionized water and ethanol, and adjusting the pH value to 1-4; adding a catalyst (hexamethylenetetramine) and a dry control agent (N,N-dimethylformamide) into the mixed solution, and then respectively performing an aging reaction and solvent replacement; finally, obtaining a polysiloxane precursor aerogel through normal-pressure drying; placing the polysiloxane precursor aerogel in a pyrolysis furnace for high-temperature treatment to obtain SiOC aerogel doped with free carbon; and placing the SiOC aerogel in a muffle furnace for high-temperature calcination to remove the free carbon. The SiOC aerogel preparation method aims to solve the problems of complex process and low efficiency in the preparation of SiOC aerogel through normal-pressure drying, and realizes the purposes of efficient controllability, low cost and facilitation of industrial production in the SiOC aerogel preparation process.

Description

Technical Field

[0001] This invention relates to the field of aerogel preparation technology, and specifically to a method for preparing SiOC aerogel. Background Technology

[0002] Aerogels are solid materials with a fibrous network structure formed by interconnected colloidal particles or cross-linked polymer molecules, with gas filling the gaps in the network. Kistler first reported the synthesis of SiO2 aerogels in the 1930s. In subsequent development, SiO2 aerogels have gained popularity due to their high specific surface area (500–1200 m²). 2 High porosity (>85%), low density (0.02–0.20 g / cm³), high porosity (>85%), low density (0.02–0.20 g / cm³). 3 And low thermal conductivity (<0.035 W·m) -1 ·K -1 SiO2 aerogels possess properties such as low dielectric constant (1.0–2.0) and low refractive index (~1.05), making them widely used in thermal / acoustic resistance, adsorption, and catalysis. However, SiO2 aerogels are brittle, have low strength, and thin pore walls. Sintering at temperatures above 500–600℃ causes the pore structure to collapse, forming a dense structure and thus losing its thermal insulation function. To overcome these shortcomings, researchers often introduce heterogeneous elements to form hybrid or composite aerogels to improve their temperature resistance. Extensive research includes SiOC aerogels, SiO2 / Al2O3 aerogels, SiO2 / ZrO2 aerogels, and SiO2 / TiO2 aerogels, among others.

[0003] SiOC ceramics refer to hybrid structures formed in amorphous SiO2 where some oxygen (O) is replaced by carbide [C(Si)4] segments. Compared to pure SiO2, the introduction of carbon (C) into the Si-O-Si chain structure significantly improves the temperature resistance and anti-sintering properties of this hybrid system. Dense SiOC glass can remain stable in air at temperatures as high as 1200℃, even 1500℃. As a member of the SiOC ceramic family, porous SiOC ceramics have attracted widespread attention due to their lightweight, high specific surface area, and high-temperature resistance. Among the many methods for preparing porous SiOC ceramics, the sol-gel method for preparing SiOC aerogels, with its advantages of low reaction temperature and controllable element content, has become a research hotspot.

[0004] In the traditional process of preparing SiOC aerogels, supercritical drying is typically used to maintain the original morphology of the gel framework, eliminating gas-liquid interfacial tension and preventing network collapse. However, supercritical drying equipment has drawbacks such as high energy consumption, high cost, high risk, and unsuitability for large-scale component fabrication. Atmospheric pressure drying has attracted widespread attention due to its simplicity, low cost, and suitability for industrial-scale mass production. However, the presence of gas-liquid interfacial tension inevitably leads to problems such as gel framework shrinkage and cracking. Therefore, how to achieve atmospheric pressure drying for SiOC aerogel preparation has become a pressing technical problem to be solved in this field.

[0005] Therefore, the inventors have provided a method for preparing SiOC aerogel. Summary of the Invention

[0006] (1) Technical problems to be solved

[0007] This invention provides a method for preparing SiOC aerogel, which solves the technical problems of complex and inefficient SiOC aerogel preparation process by atmospheric pressure drying.

[0008] (2) Technical solution

[0009] This invention provides a method for preparing SiOC aerogel, comprising the following steps:

[0010] Tetraalkoxysilane and alkyltrialkoxysilane were dissolved sequentially in a mixed solvent of deionized water and ethanol, and the pH was adjusted to 1-4.

[0011] Hexamethylenetetramine and N,N-dimethylformamide were added to the above mixed solution, followed by aging reaction and solvent replacement, and finally dried under normal pressure to obtain polysiloxane precursor aerogel.

[0012] The polysiloxane precursor aerogel was subjected to high-temperature treatment in a pyrolysis furnace to obtain SiOC aerogel doped with free carbon.

[0013] The SiOC aerogel was placed in a muffle furnace and calcined at high temperature to remove free carbon.

[0014] Further, the molar ratio of the tetraalkoxysilane, the alkyltrialkoxysilane, the deionized water, the ethanol, and HCl is: (1-3):(0.1-0.5):(10-25):(3-7):(0.0001-0.001).

[0015] Furthermore, gelation and aging reactions occur at 40–100°C for 2–15 hours.

[0016] Furthermore, the aged wet gel is replaced with ethanol 1 to 3 times, each replacement lasting 10 to 20 hours, and then replaced with n-hexane 1 to 2 times, each replacement lasting 5 to 15 hours.

[0017] Further, the replaced wet gel is placed in a sealed drying oven and dried at normal pressure at a temperature controlled at 80-120°C for 6-12 hours to obtain the polysiloxane precursor aerogel.

[0018] Furthermore, the amount of hexamethylenetetramine used accounts for 0.1% to 5.0% of the solution mass fraction.

[0019] Furthermore, the amount of N,N-dimethylformamide used accounts for 1% to 15% of the solution mass fraction.

[0020] Furthermore, the pyrolysis furnace atmosphere is protected by Ar2 or N2, and the processing temperature is 1100–1200℃.

[0021] Furthermore, the heating rate of the pyrolysis furnace is 5–10 °C / min, and the holding time is 2 h.

[0022] Furthermore, the calcination temperature is 400–800℃, and the calcination time is 2–5 hours.

[0023] (3) Beneficial effects

[0024] In summary, this invention achieves a controllable and low-cost atmospheric pressure drying preparation process for SiOC aerogels by controlling the gel pore size through precursor molecular chain structural design. During the sol-gel reaction, hexamethylenetetramine is used as a base catalyst, utilizing its thermal decomposition under heating conditions to release NH3, thereby adjusting the pH value of the sol and avoiding the direct use of highly volatile and irritating ammonia water. This makes the entire reaction process more controllable and simpler. Attached Figure Description

[0025] To more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings used in the embodiments of the present invention will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0026] Figure 1 This is a schematic flowchart of a method for preparing SiOC aerogel provided in an embodiment of the present invention;

[0027] Figure 2 This is a microstructure morphology diagram of a SiOC aerogel provided in an embodiment of the present invention;

[0028] Figure 3(a) is a pore size distribution curve of a SiOC aerogel provided in an embodiment of the present invention, showing the micropore and mesopore sizes.

[0029] Figure 3(b) is a macropore size distribution curve of a SiOC aerogel provided in an embodiment of the present invention. Detailed Implementation

[0030] The embodiments of the present invention will be further described in detail below with reference to the accompanying drawings and examples. The following detailed description of the embodiments and the accompanying drawings are used to illustrate the principles of the present invention by way of example, but should not be used to limit the scope of the present invention, that is, the present invention is not limited to the described embodiments.

[0031] It should be noted that, unless otherwise specified, the embodiments and features described in this application can be combined with each other. This application will now be described in detail with reference to the accompanying drawings and embodiments.

[0032] In the description of this invention, it should be understood that the terms "upper," "lower," "front," "rear," etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings, or the orientation or positional relationship commonly used when the product of this invention is in use, or the orientation or positional relationship commonly understood by those skilled in the art. They are only used to facilitate the description of this invention and to simplify the description, and are not intended to indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on this invention.

[0033] Chinese patent CN113956044A discloses a method for preparing high-strength SiOC aerogel by atmospheric pressure drying. This method uses siloxane small molecule monomers as reactants and deionized water and anhydrous ethanol as mixed solvents. After acetic acid-catalyzed hydrolysis and pH adjustment with ammonia, a wet gel is formed. Ethanol and n-hexane are then used for replacement, and finally, high-strength SiOC aerogel (8.8–9.5 MPa) is prepared by atmospheric pressure drying. However, the free carbon in the final SiOC aerogel is not completely removed. Under high-temperature and aerobic conditions, the carbon will continue to oxidize and generate gas, which is detrimental to the practical application of the material. Furthermore, the high volatility of ammonia during the preparation process limits its feasibility in adjusting the pH value.

[0034] Chinese patent CN114195520A discloses a method for preparing high-temperature resistant SiOC aerogel, which also uses alkoxysilane as the reactant and water as the reaction solvent. In order to achieve normal pressure drying, it is necessary to reduce the surface tension of water, so a surfactant is introduced into the system. However, the critical micelle concentration of the surfactant is very precisely controlled, and its addition amount is very sensitive to the surrounding environment such as pH value and reaction temperature, which is not conducive to industrial application.

[0035] Figure 1This is a schematic flowchart of a method for preparing SiOC aerogel provided in an embodiment of the present invention, as shown below. Figure 1 As shown, the method may include the following steps:

[0036] S100: Tetraalkoxysilane and alkyltrialkoxysilane are dissolved in a mixed solvent of deionized water and ethanol in sequence, and the pH value is adjusted to 1-4.

[0037] S200. Add hexamethylenetetramine and N,N-dimethylformamide to the above mixed solution, then carry out aging reaction and solvent replacement, and finally obtain polysiloxane precursor aerogel by drying under normal pressure.

[0038] S300: The polysiloxane precursor aerogel is placed in a pyrolysis furnace for high-temperature treatment to obtain SiOC aerogel doped with free carbon.

[0039] S400: Place the SiOC aerogel in a muffle furnace and calcine it at high temperature to remove free carbon.

[0040] In the above embodiments, in step S100, the tetraalkoxysilane is at least one of tetramethyl orthosilicate and tetraethyl orthosilicate, and the alkyltrialkoxysilane is at least one of methyltriethoxysilane, methyltrimethoxysilane, ethyltriethoxysilane, ethyltrimethoxysilane, propyltrimethoxysilane, propyltriethoxysilane, phenyltriethoxysilane, phenyltrimethoxysilane, vinyltrimethoxysilane, vinyltriethoxysilane, dodecyltrimethoxysilane, dodecyltriethoxysilane, hexadecyltrimethoxysilane, hexadecyltriethoxysilane, octadecyltrimethoxysilane, and octadecyltriethoxysilane. The order in which the tetraalkoxysilane and alkyltrialkoxysilane are added is strictly required; the tetraalkoxysilane must be added first, followed by the alkyltrialkoxysilane, to avoid affecting their respective dissolution reactions.

[0041] The pH of mixed solutions is mainly adjusted using acidic substances, such as diluted strong or weak acids, typically diluted HCl solution.

[0042] In step S200, hexamethylenetetramine is used as a base catalyst. The decomposition reaction of hexamethylenetetramine under heating conditions and the release of NH3 are utilized, which avoids the direct use of ammonia water, which has strong volatility and irritation, making the whole reaction process more controllable and simpler.

[0043] As an optional implementation, the molar ratio of tetraalkoxysilane, alkyltrialkoxysilane, deionized water, ethanol, and HCl is (1-3):(0.1-0.5):(10-25):(3-7):(0.0001-0.001), the amount of hexamethylenetetramine is 0.1%-5.0% of the solution mass fraction, and the amount of N,N-dimethylformamide is 1%-15% of the solution mass fraction.

[0044] Specifically, by controlling the monomer ratio of the two alkoxysilane small molecules and the amount of drying control agent, the micro-macropores and mesopores were controlled. Furthermore, by replacing the solvent with one that has a lower surface tension, the capillary tension during the atmospheric pressure drying process was reduced. The entire preparation process is simpler, more efficient, and suitable for the preparation of large-sized components.

[0045] As an optional implementation method, a gelation and aging reaction is carried out at 40–100°C for 2–15 hours. The aged wet gel is replaced with ethanol 1–3 times, each time for 10–20 hours, and then replaced with n-hexane 1–2 times, each time for 5–15 hours. The replaced wet gel is placed in a sealed drying oven and dried at normal pressure at 80–120°C for 6–12 hours to obtain a polysiloxane precursor aerogel.

[0046] As an optional implementation, the pyrolysis furnace atmosphere is protected by Ar2 or N2, the processing temperature is 1100-1200℃, the heating rate is 5-10℃ / min, and the holding time is 2h.

[0047] As an optional implementation method, the calcination temperature is 400–800℃, and the calcination time is 2–5 hours. High-temperature calcination in an aerobic environment (usually air) aims to remove free carbon. The microscopic morphology of the SiOC aerogel prepared after calcination is as follows: Figure 2 As shown in Figure 3, the pore size distribution of SiOC aerogel is as follows. The adsorption-desorption curve in Figure 3(a) is a type IV curve, exhibiting typical mesoporous characteristics. A clear distribution of pore sizes below 10 nm is observed, and the specific surface area is as high as 393.5 m². 2 / g, Figure 3(b) shows that the macropore size distribution is in the range of hundreds of nanometers to several micrometers, and this pore structure also ensures the smooth progress of atmospheric pressure drying.

[0048] Example 1

[0049] Tetramethyl orthosilicate and phenyltriethoxysilane were dissolved in a mixed solution of deionized water and ethanol, and the pH was adjusted to 2 by introducing HCl. The molar ratio of tetramethyl orthosilicate, phenyltriethoxysilane, deionized water, ethanol, and HCl was 1:1:54.5:21.3:0.098. After stirring at room temperature until clear, stirring was continued for 4 hours to ensure complete hydrolysis. Then, 0.1% hexamethylenetetramine (HMTA) was added to the solution, and after stirring until clear, 1% N,N-dimethylformamide (DMF) was introduced as a drying control agent. After stirring for 30 minutes, the solution was poured into a sealed mold and cured at 100°C for 12 hours. After cooling to room temperature, the wet gel was replaced twice with ethanol and once with n-hexane, with each replacement lasting 10 hours. Finally, the wet gel was dried at room temperature and pressure for 3 days to obtain the precursor siloxane aerogel.

[0050] The prepared siloxane aerogel was placed in a tube furnace and subjected to pyrolysis at 1000℃ under an Ar2 atmosphere to obtain a dark-colored SiOC aerogel.

[0051] The dark SiOC aerogel was placed in a muffle furnace and calcined at 800°C for 2 hours in air atmosphere to obtain white SiOC aerogel.

[0052] The obtained white SiOC aerogel has a pore size distribution of 100–1000 nm and a porosity of 85%.

[0053] Example 2

[0054] Tetramethyl orthosilicate and methyltrimethoxysilane were dissolved in a mixed solution of deionized water and ethanol, and the pH was adjusted to 3 by introducing HCl. The molar ratio of tetramethyl orthosilicate, methyltrimethoxysilane, deionized water, ethanol, and HCl was 1:0.5:9.58:24.48:0.044. Then, 3.0% (w / w) of hexamethylenetetramine (HMTA) was added to the solution, and after stirring until clear, 8.0% (w / w) of N,N-dimethylformamide (DMF) was introduced as a drying control agent. After stirring for another 30 min, the mixture was poured into a sealed mold and cured at 120°C for 12 h. After cooling to room temperature, the wet gel was replaced twice with ethanol and once with n-hexane, each replacement lasting 10 h. Finally, the wet gel was dried at room temperature and pressure for 3 days to obtain the precursor siloxane aerogel.

[0055] The prepared siloxane aerogel was placed in a tube furnace and subjected to pyrolysis at 1100℃ under an Ar2 atmosphere to obtain a dark-colored SiOC aerogel.

[0056] The dark SiOC aerogel was placed in a muffle furnace and calcined at 1000℃ in air for 2 hours to obtain a white SiOC aerogel.

[0057] The obtained white SiOC aerogel has a pore size distribution of 100–1000 nm and a porosity of 80%.

[0058] Example 3

[0059] Tetraethyl orthosilicate and vinyltrimethoxysilane were dissolved in a mixed solvent of deionized water and ethanol, and the pH was adjusted to 2 by introducing HCl. The molar ratio of tetraethyl orthosilicate, vinyltrimethoxysilane, deionized water, ethanol, and HCl was 1:0.5:31.38:12.26:0.06. Then, 3.0% (w / w) of hexamethylenetetramine (HMTA) was added to the solution, and after stirring until clear, 15% (w / w) of N,N-dimethylformamide (DMF) was introduced as a drying control agent. Stirring was continued for 30 min, and the mixture was poured into a sealed mold and cured at 140°C for 12 h. After cooling to room temperature, the wet gel was replaced twice with ethanol and once with n-hexane, each replacement lasting 8 h. Finally, the wet gel was dried at room temperature and pressure for 3 days to obtain the precursor siloxane aerogel.

[0060] The prepared siloxane aerogel was placed in a tube furnace and subjected to pyrolysis at 1000℃ under an Ar2 atmosphere to obtain a dark-colored SiOC aerogel.

[0061] The dark SiOC aerogel was placed in a muffle furnace and calcined at 1000℃ in air for 2 hours to obtain a white SiOC aerogel.

[0062] The obtained white SiOC aerogel has a pore size distribution of 150–500 nm and a porosity of 82%.

[0063] It should be noted that the various embodiments in this specification are described in a progressive manner, and the same or similar parts between the various embodiments can be referred to mutually. Each embodiment focuses on describing the differences from other embodiments. The present invention is not limited to the specific steps and structures described above and shown in the figures. Furthermore, for the sake of brevity, detailed descriptions of known methods and techniques are omitted here.

[0064] The above are merely embodiments of this application and are not intended to limit the scope of this application. Various modifications and variations can be made to this application by those skilled in the art without departing from the scope of the invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principle of this application should be included within the scope of the claims of this application.

Claims

1. A method for preparing SiOC aerogel, characterized in that, The method includes the following steps: Tetraalkoxysilane and alkyltrialkoxysilane were dissolved sequentially in a mixed solvent of deionized water and ethanol, and the pH was adjusted to 1-4. Hexamethylenetetramine and N,N-dimethylformamide were added to the above mixed solution, followed by aging reaction and solvent replacement. Finally, the polysiloxane precursor aerogel was obtained by drying under normal pressure. The polysiloxane precursor aerogel was subjected to high-temperature treatment in a pyrolysis furnace to obtain SiOC aerogel doped with free carbon. The SiOC aerogel was placed in a muffle furnace and calcined at high temperature to remove free carbon. The molar ratio of the tetraalkoxysilane, the alkyltrialkoxysilane, the deionized water, the ethanol, and HCl is (1-3):(0.1-0.5):(10-25):(3-7):(0.0001-0.001); the amount of hexamethylenetetramine is 0.1%-5.0% of the solution mass fraction, and the amount of N,N-dimethylformamide is 1%-15% of the solution mass fraction; Gel formation and aging reactions occur at 40–100°C for 2–15 hours.

2. The method for preparing SiOC aerogel according to claim 1, characterized in that, The aged wet gel is replaced with ethanol 1 to 3 times, each time for 10 to 20 hours, and then replaced with n-hexane 1 to 2 times, each time for 5 to 15 hours.

3. The method for preparing SiOC aerogel according to claim 1, characterized in that, The replaced wet gel was placed in a sealed drying oven and dried at normal pressure at a temperature controlled at 80-120℃ for 6-12 hours to obtain the polysiloxane precursor aerogel.

4. The method for preparing SiOC aerogel according to claim 1, characterized in that, The pyrolysis furnace atmosphere is protected by Ar2 or N2, and the processing temperature is 1100~1200℃.

5. The method for preparing SiOC aerogel according to claim 4, characterized in that, The heating rate of the pyrolysis furnace is 5-10℃ / min, and the holding time is 2h.

6. The method for preparing SiOC aerogel according to claim 1, characterized in that, The calcination temperature is 400–800℃, and the calcination time is 2–5 hours.

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