Method for preparing ultrathin aerogel by supercritical drying of carbon dioxide

Abstract

The invention belongs to the technical field of aerogel preparation, and relates to a method for preparing ultrathin aerogel by supercritical drying of carbon dioxide, which comprises the following steps: preparing a basic catalytic liquid, preparing silica sol, forming and aging ultrathin wet gel, performing surface modification, pretreating a drying system, performing limited loading and sealing, performing flexible gradient replacement and drying, and performing gradient depressurization and material taking. According to the method, limited loading is carried out by introducing a breathable constraint clamp, and a segmented gradient pressure increasing and decreasing carbon dioxide supercritical drying process introduced at an initial low speed is matched, so that macroscopic curling deformation caused by internal stress release of the ultrathin sample is forcibly eliminated; the mechanical damage of turbulent shearing to a fragile film skeleton is effectively avoided by utilizing fine fluid control of slow starting and quick rising, and meanwhile, the preparation period is remarkably shortened by utilizing the quick mass transfer characteristic of an ultrathin sample; the prepared aerogel film is high in flatness, free of macroscopic curl, complete in structure and free of cracks, and is suitable for the fields of flexible heat insulation and precision optics.

Description

Technical Field

[0001] This invention belongs to the field of aerogel preparation technology, specifically relating to a method for efficiently preparing ultrathin aerogel films with high flatness and high flexibility using supercritical carbon dioxide fluid technology. Background Technology

[0002] Aerogels are a class of ultralight porous materials composed of a nanoscale solid framework and a continuous gas phase. They possess extremely low density, ultra-high porosity, and excellent thermal insulation properties, and are hailed as one of the most promising functional materials. Among them, inorganic aerogels, represented by silica (SiO2), are widely used in building energy conservation, aerospace, microelectronic packaging, and flexible thermal management due to their mature preparation processes and good thermal stability. With the miniaturization and flexibility of electronic devices, the demand for ultrathin aerogels (or aerogel films) with thicknesses ranging from micrometers to millimeters (typically ≤1 mm) in thermal insulation pads, battery separators, and flexible wearable devices is increasing dramatically.

[0003] Currently, the preparation of aerogels typically involves sol-gel, solvent replacement, surface modification, and drying processes. Among these, supercritical carbon dioxide drying technology has become the mainstream process to avoid skeletal collapse due to its ability to eliminate gas-liquid interfacial tension. For example, invention CN105727851A discloses a supercritical drying method for silicon-aluminum composite aerogels. Although the aerogel structure is preserved through solvent replacement and hydrophobic modification, its process is mainly aimed at conventional bulk composite materials and does not address stress control in ultrathin morphologies.

[0004] Despite some progress in the preparation of bulk aerogels, significant technical bottlenecks remain in practical applications. Specifically, existing equipment and processes still fall short in terms of drying uniformity and structural retention. For example, invention CN102491326A discloses an apparatus and method for drying supercritical fluids, but in practical applications, this often leads to uneven pore structure distribution and insufficient structural strength in the resulting aerogels, and the drying and separation efficiency needs improvement. Similarly, the system disclosed in invention CN117398934A also suffers from the problem of being unable to directly and stably prepare aerogels with both excellent thermal insulation properties and high structural stability.

[0005] However, when the aforementioned existing technologies are directly applied to the preparation of ultrathin aerogels (materials with a large aspect ratio), these defects are further amplified, leading to specific process challenges. On the one hand, due to the extremely low stiffness of ultrathin wet gels, the "insufficient structural strength" problem mentioned in existing technologies (such as CN102491326A) directly manifests as severe macroscopic curling, warping, or even self-rolling into a cylindrical shape in such samples, making the product unsuitable for spread-out use. On the other hand, existing processes (such as CN105727851A) typically employ a relatively crude fluid injection method, and their conventional flow rates easily generate turbulent shear forces on the fragile ultrathin film surface, causing the film to tear or break, making it difficult to obtain intact samples. Furthermore, existing technologies typically set long supercritical thermal cycling times to ensure sufficient solvent replacement for bulk samples. If such long-cycle processes designed for bulk materials are directly applied to ultrathin samples with extremely short diffusion paths, it will inevitably result in a huge waste of time and energy consumption, leading to a severe mismatch between preparation efficiency and energy consumption. Therefore, there is an urgent need for a supercritical drying method for aerogels with higher dimensional stability and intelligent process control to solve the above-mentioned technical problems.

[0006] Therefore, there is an urgent need for a supercritical carbon dioxide drying method specifically designed for the properties of ultrathin aerogels, which can overcome the defects of insufficient strength and uneven pore structure in existing technologies, effectively solve the macroscopic curling problem, and significantly improve the preparation efficiency. Summary of the Invention

[0007] This invention aims to address the problems of easy curling and deformation, susceptibility to fluid impact damage, and low preparation efficiency of ultrathin aerogels during the drying process in existing technologies. It provides a method for preparing ultrathin aerogels through supercritical carbon dioxide drying. The aerogel films prepared by this method exhibit high flatness, no macroscopic curling, intact structure without cracks, and have a short preparation cycle and low energy consumption, making them suitable for flexible thermal insulation and precision optics applications.

[0008] This invention provides the following technical solution: a method for preparing ultrathin aerogels by supercritical carbon dioxide drying, comprising the following steps: Step 1: Preparation of basic catalyst solutions: Prepare acidic catalyst, basic catalyst, and surface treatment catalyst reagent respectively.

[0009] Step 2, Preparation of silica sol: Mix silicon source, solvent and acid catalyst, adjust pH to 2~5, stir at room temperature for 0.5h-3h to hydrolyze and obtain silica sol.

[0010] Step 3, Ultrathin Wet Gel Forming and Aging: The silica sol and solvent prepared in Step 2 are mixed and injected into a mold of a limited thickness or coated onto a substrate. Surface modifiers and alkaline catalysts are added, and the mixture is allowed to stand and gel to form an ultrathin wet gel, which is then aged.

[0011] Step 4, Surface modification: Add surface modifier and surface treatment catalyst to the ultrathin wet gel prepared in step 3, seal and let stand for modification treatment.

[0012] Step 5: Pre-treatment of the drying system.

[0013] Step 6, Restricted Loading and Sealing: The ultrathin wet gel obtained after surface modification in Step 4 is placed in a breathable constraint fixture to form a planar physical constraint structure. Then, the gel and fixture are immersed in a container containing organic solvent, and the whole thing is placed into the drying system and sealed.

[0014] Step 7, Flexible Gradient Replacement and Drying: Carbon dioxide is introduced into the drying system, and air is removed and solvent is replaced by segmented gradient pressure increase; first, the pressure increase rate is controlled to increase from 0 MPa to the carbon dioxide storage tank pressure, then to the supercritical pressure, and finally to the preset drying pressure to carry out supercritical thermal cycling.

[0015] Step 8, Gradient pressure reduction and material removal: Gradient pressure is reduced to normal pressure to remove the ultrathin aerogel.

[0016] Preferably, in step 1, the acidic catalyst includes concentrated sulfuric acid, hydrochloric acid, nitric acid, hydrofluoric acid, oxalic acid, and citric acid; the alkaline catalyst includes ammonia, alkanolamine, sodium hydroxide, and urea; and the surface treatment catalytic reagent includes hydrochloric acid, trimethylchlorosilane, hexamethyldisiloxane, and hexadecyltrimethylammonium bromide.

[0017] Preferably, in step 2, the silicon source includes: coal gangue, water glass, methyl orthosilicate, methyltriethoxysilane, methyltrimethoxysilane, vinyltrimethoxysilane + methyltrimethoxysilane, tetraethoxysilane + methyltrimethoxysilane.

[0018] Solvents include: ethanol and water.

[0019] The molar ratio of the solvent to the silicon source is 100~5:5~100.

[0020] The volume ratio of the acid catalyst to the solvent is 0.5 to 8.0.

[0021] The mass fraction of SiO2 in the prepared silica sol is 5% to 85%.

[0022] Preferably, in step 3: The materials of the mold or substrate include: polytetrafluoroethylene, stainless steel, silicone rubber, plastic, and PET release film.

[0023] Surface modifiers include: hexamethyldisiloxane, trimethylchlorosilane, hexamethyldisilazane, polydimethylsiloxane, and tertiary amines.

[0024] More preferably, in step 3, the molar ratio of solvent to silica sol is 2~85, the volume ratio of solvent to surface modifier is 1~18; an alkaline catalyst is added to adjust the pH to 7~9; the static gelation time is 0.1~6 h, and the drying and aging time is 0.5~12 h.

[0025] Preferably, in step 6: The breathable restraint fixture includes one or more of the following: stainless steel wire mesh, porous ceramic plate, and polytetrafluoroethylene porous membrane.

[0026] Restricted loading methods include: clamping the ultrathin wet gel between two layers of breathable constraint clamps, or placing it in a layered structure of breathable constraint clamps, using the clamps to apply planar constraint force to the gel surface to inhibit gel curling.

[0027] Organic solvents include: ethanol, isopropanol, n-hexane, trimethylchlorosilane, dimethylformamide, and tetrahydrofuran.

[0028] Preferably, in step 7: Carbon dioxide is produced at temperatures between 35 and 55 °C.

[0029] During the process of introducing carbon dioxide to increase the pressure from 0 MPa to the pressure in the carbon dioxide storage tank, the pressurization rate is 0.01~0.1 MPa / min.

[0030] During the process of increasing the pressure from the carbon dioxide storage tank to the supercritical pressure, the pressurization rate is 0.05~0.5 MPa / min.

[0031] During the process of increasing the pressure from supercritical pressure to the preset drying pressure, the drying pressure is 7~30 MPa and the pressure increase rate is 0.2~1.0 MPa / min.

[0032] During the thermal cycling phase, maintain the temperature and pressure for 0.5 to 4 hours.

[0033] More preferably, in step 8, the drying system is subjected to constant temperature and uniform pressure reduction. During the reduction from the preset drying pressure to the supercritical pressure, the pressure reduction rate is 0.05~2.5 MPa / min. During the reduction from the supercritical pressure to the carbon dioxide storage tank pressure, from the carbon dioxide storage tank pressure to 3 MPa, and from 3 MPa to atmospheric pressure, the pressure reduction rate is 0.01~2.5 MPa / min.

[0034] This invention also discloses an ultrathin aerogel, which is prepared according to the above-described preparation method. The thickness of the ultrathin aerogel is 10 μm to 2000 μm, and the aspect ratio is 20 to 5000. Here, the aspect ratio is the ratio of the aerogel's diameter to its thickness, and it is an important indicator for evaluating the characteristics of ultrathin aerogels. A higher aspect ratio indicates that the aerogel has a large dimension in the planar direction while being relatively thin, which is of great significance for its applications in heat insulation pads, battery separators, and flexible wearable devices. When the aspect ratio is in the range of 20 to 5000, the ultrathin aerogel can better leverage its advantages of flexibility and high flatness, adapting to different application scenarios.

[0035] Preferably, the thickness of the ultrathin aerogel is 600 μm to 1500 μm.

[0036] The beneficial effects of this invention are: 1. This invention can significantly improve the flatness of aerogel and eliminate aerogel curling deformation. This invention introduces a breathable constraint fixture for "confined space drying", which physically forces the macroscopic curling tendency of ultrathin samples caused by the release of internal stress. Combined with gradient drying process, it effectively solves the problem of easy self-curling and warping of aerogel films in the prior art. The resulting product is flat and smooth, and easy to process and bond in subsequent processes.

[0037] 2. This invention features high process efficiency, significantly reduced energy consumption, and is suitable for ultra-thin samples with large specific surface area and short mass transfer path. This invention optimizes the supercritical thermal cycle parameters, shortening the drying cycle from more than ten hours to less than four hours, significantly improving production efficiency, reducing energy costs, and making it suitable for mass production.

[0038] 3. The aerogel structure prepared by the method of the present invention is intact, preventing mechanical damage; the present invention adopts a targeted "slow start and fast rise" flexible gradient pressurization strategy, which strictly limits the pressurization rate in the early stage of fluid introduction, avoiding the damage of fragile film by turbulent shear force, and achieving the maintenance of the integrity of the skeleton structure without tearing even at extremely thin thickness (as low as 10μm).

[0039] 4. The aerogel prepared by the method of the present invention maintains excellent thermal insulation and optical properties. The resulting ultrathin aerogel completely retains the nanoporous network structure, has a high specific surface area, and has excellent thermal insulation properties and good light transmittance (or optical uniformity), meeting the needs of precision instruments and flexible electronics for high-performance thermal insulation materials. Attached Figure Description

[0040] Figure 1 This is a cross-sectional SEM image of an ultrathin aerogel prepared by supercritical carbon dioxide drying according to the present invention. Figure 2The nitrogen isothermal adsorption / desorption curve and pore size distribution curve of the ultrathin aerogel of the present invention are shown. Figure 3 This is a structural schematic diagram of the breathable restraint clamp and restricted loading method of the present invention; Figure 4 This is an optical photograph of the ultrathin aerogel prepared in Example 1 of the present invention; Figure 5 Photograph of the aerogel that has curled and deformed as prepared for Comparative Example 1 (i.e., without the use of a breathable constraint fixture). Detailed Implementation

[0041] The related technologies of the present invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0042] like Figures 1-5 As shown, the method for preparing ultrathin aerogels by supercritical carbon dioxide drying in this embodiment includes the following steps: Step 1: Preparation of the basic catalyst solution. Add acid to the solvent, stir until homogeneous, and store for use as an acidic catalyst; add base to the solvent, stir until homogeneous, and store for use as a basic catalyst; add acid to the solvent, stir until homogeneous, and store for use as a surface treatment catalyst. The acidic catalysts include: concentrated sulfuric acid (H₂SO₄), hydrochloric acid (HCl), nitric acid (HNO₃), hydrofluoric acid (HF), oxalic acid, and citric acid. The basic catalysts include: ammonia water (NH₃·H₂O), alkanolamines, sodium hydroxide (NaOH), and urea. The surface treatment catalysts include: hydrochloric acid (HCl), trimethylchlorosilane (TMCS), hexamethyldisiloxane (HMDSO), and hexadecyltrimethylammonium bromide (CTAB).

[0043] Different types of catalysts were prepared to meet the reaction requirements at different stages of aerogel preparation: acidic catalysts were used for sol hydrolysis to adjust the pH and obtain a homogeneous silica sol; basic catalysts were used for gelation to accelerate the condensation of silanols to form a three-dimensional framework, while controlling the pore structure and mechanical properties; surface-treatment catalysts were used for framework surface modification to form a hydrophobic layer to prevent framework collapse during the drying process. By preparing and storing these catalysts separately, they can be precisely added in subsequent steps, achieving controllable preparation of ultrathin aerogels with homogeneous sols, uniform pore structures, stable frameworks, and high flatness.

[0044] Step 2: Preparation of silica sol. The silicon source, solvent, and acidic catalyst are mixed in a specific molar ratio, the pH is adjusted to 3-4, and the mixture is stirred at room temperature for 2 hours to obtain the silica sol. The silica sol ratio is: n(solvent) : n(silicon source) = (100-5) : (5-100), and the mass fraction of SiO2 in the sol is 5-85%. Silicon sources include: coal gangue, water glass, methyl orthosilicate (TMOS), methyltriethoxysilane (MTES), methyltrimethoxysilane (MTMS), vinyltrimethoxysilane + methyltrimethoxysilane, and tetraethoxysilane + methyltrimethoxysilane (TEOS + MTMS). Solvents include: water, ethanol, isopropanol, n-hexane, trimethylchlorosilane, dimethylformamide, and tetrahydrofuran.

[0045] Step 3: Ultrathin Wet Gel Molding and Aging. Silica sol and solvent are mixed and added to a mold of limited thickness (such as a shallow groove mold, coating machine, or casting equipment). The SiO2 concentration is controlled, and the mixture is stirred at room temperature for 30 min. A portion of surface modifier is added, and the prepolymer is stirred for another 30 min to obtain the prepolymer. An alkaline catalyst is then added to adjust the pH to 8-9, and the mixture is stirred for 2 min before being allowed to gel. After gelation, the mixture is sealed and placed in an oven at 60 °C for a certain period of time for aging. The mold materials include: polytetrafluoroethylene, stainless steel, silicone rubber, and plastic. The surface modifiers include: hexamethyldisiloxane (HMDSO), trimethylchlorosilane (TMCS), hexamethyldisilazane (HMDZ), polydimethylsiloxane (PDMS), and tertiary amines. The surface modifiers are mainly used to introduce hydrophobic functional groups onto the surface of the wet gel skeleton, reducing the hydroxyl content and thus reducing skeleton collapse and volume shrinkage caused by capillary forces during drying. These modifiers exhibit good silanol reactivity, forming stable Si–R bonds with Si–OH without significantly affecting the pore structure and specific surface area of ​​the aerogel, thus balancing skeletal stability, hydrophobicity, and microporous structure uniformity. The purpose of static aging of the ultrathin wet gel at 60 °C is to promote further condensation between silanols and strengthen the three-dimensional skeletal network, improving the gel's crosslinking degree and mechanical strength. This is crucial for preventing damage to the ultrathin sample during subsequent operations.

[0046] Step 4, Surface modification. Add the remaining surface modification and surface treatment catalysts to the ultrathin wet gel, seal it, and place it in an oven at 60 °C for a certain period of time to perform surface modification.

[0047] Step 5, Pre-treatment of the drying system. Set the drying kettle to the preset temperature of 45 ℃, heat it to the preset temperature of 45 ℃, and turn on the condenser to cool it down.

[0048] Step 6, Constrained Loading and Sealing. The surface-modified ultrathin wet gel is laid flat in a breathable constraint fixture (such as a stainless steel mesh, porous ceramic plate, or polytetrafluoroethylene porous membrane) to form a planar physical constraint structure. Then, the entire aerogel, along with the fixture, is immersed in a container containing organic solvents, placed in a drying autoclave, and sealed. The organic solvents include: ethanol, isopropanol, n-hexane, trimethylchlorosilane, dimethylformamide, and tetrahydrofuran. Introducing the breathable constraint fixture and constrained loading is to apply physical planar constraints to the ultrathin wet gel during the drying process, forcibly counteracting the macroscopic curling tendency caused by the release of internal stress, and ensuring the flatness of the resulting aerogel film.

[0049] Step 7, carbon dioxide replacement and pressurization. Start the carbon dioxide booster pump and adjust the frequency to allow the carbon dioxide fluid in the circulating storage tank to pass through the booster pump. The carbon dioxide fluid then enters the heater for heating, raising its temperature to 35–55 °C. The carbon dioxide flow rate is 15–30 L / h, and the pressure in the drying vessel is equal to that in the carbon dioxide storage tank, typically 4.2–6.5 MPa. Control the flow rate into the drying vessel using a flow rate control valve to maintain a constant temperature and pressure, keeping the temperature inside the vessel at 35–55 °C. Increase the pressure from the carbon dioxide storage tank to the supercritical pressure, controlling the pressure increase rate at 0.05–0.5 MPa / min (slow start to prevent membrane rupture). Increase the pressure from the supercritical pressure to the preset drying pressure (7–30 MPa), controlling the pressure increase rate at 0.2–1.0 MPa / min, until the pressure in the drying vessel reaches the preset drying pressure. Open the outlet valve of the drying vessel to allow the solvent in the wet gel to separate and be collected at the bottom of the drying vessel. Maintain the temperature and pressure of the drying vessel and perform supercritical thermal cycling for 0.5–4 hours. h (efficient cycling for ultrathin samples).

[0050] Step 8, Gradient depressurization and material removal. The drying vessel is subjected to constant temperature and uniform depressurization. The pressure is reduced from the preset drying pressure to the supercritical pressure, with a depressurization rate controlled at 0.05~2.5 MPa / min; from the supercritical pressure to the carbon dioxide storage tank pressure, with a depressurization rate controlled at 0.01~2.5 MPa / min; from the carbon dioxide storage tank pressure to 3 MPa, with a depressurization rate controlled at 0.01~2.5 MPa / min; and from 3 MPa to atmospheric pressure, with a depressurization rate controlled at 0.01~2.5 MPa / min. Controlling the pressure increase / decrease rate and employing gradient pressure increase / decrease is crucial to adapting to the characteristics of the low-strength nanoporous framework of the aerogel wet gel: if the pressure increase is too rapid or the rate is unstable, the sudden pressure change can easily impact the fragile nanoframework, leading to framework collapse, cracking, or even membrane tearing. Controlling different pressure increase rates in stages, combined with constrained loading, allows supercritical carbon dioxide fluid to gradually penetrate into the nanopores of the wet gel in a gentle manner, uniformly replacing the internal organic solvent. Meanwhile, by taking advantage of the rapid mass transfer characteristics of ultrathin samples, the thermal cycling time can be appropriately shortened, thereby significantly improving the preparation efficiency while ensuring the displacement effect.

[0051] By employing the aforementioned supercritical carbon dioxide drying process based on constrained loading and gradient control, the problems of easy curling and breakage of ultrathin aerogels were effectively solved. The prepared ultrathin aerogel is a semi-transparent film with a smooth and clean surface, free from macroscopic curling, and a specific surface area ≥850 m². 2 / g, thermal conductivity ≤0.020 W / (m·K), excellent structural stability and flexibility.

[0052] The present invention will be further illustrated below through different embodiments.

[0053] Example 1 (This example aims to prepare a highly flat SiO2 aerogel film with a thickness of approximately 600 μm) 1) Prepare the basic catalyst solution by adding 1 mL of concentrated sulfuric acid to 49 mL of ethanol, stirring evenly and storing for later use in preparing the acid catalyst; add 1 mL of ammonia water to 9 mL of ethanol, stirring evenly and storing for later use in preparing the alkaline catalyst; add 1 mL of hydrochloric acid to 19 mL of ethanol, stirring evenly and storing for later use in preparing the surface treatment catalyst. All solutions are stirred evenly and then stored.

[0054] 2) To prepare silica sol, TMOS, EtOH, acidic catalyst, and H2O were added to a flask in a certain molar ratio, where n(EtOH):n(TMOS) = 2 and v(H2SO4):v(EtOH) = 2. The pH was adjusted to 3, and the mixture was stirred at room temperature for 2 h to obtain silica sol.

[0055] 3) Ultrathin molding and aging: Silica sol, EtOH, and H2O are mixed in a specific ratio and poured into a shallow PTFE mold with a depth of 0.85 mm. The SiO2 concentration is controlled, and the mixture is stirred at room temperature for 5 min. Then, a certain volume of PDMS is added. The PDMS used for surface treatment is added in two parts: one part is added to the prepolymer before gelation, and the other part is added simultaneously with the catalyst HCl during surface treatment. Stirring is performed for 1.5 h (Note: Due to the thin liquid layer, rapid operation is required to prevent premature gelation) to obtain the prepolymer. Then, a certain volume of NH3·H2O is added to the prepolymer to adjust the pH to 9. After stirring evenly, the mixture is allowed to stand. After gelation is complete, the mixture is kept sealed. The ultrathin wet gel is placed in an oven at 60 ℃ and allowed to stand for a certain period of time for aging.

[0056] 4) Surface modification: Add a certain volume of PDMS and HCl to the mold, immerse the gel in the liquid, keep it sealed, place it in an oven, and let it stand for a certain time at 60 ℃ to carry out surface modification.

[0057] 5) Preheat the system. Set the temperature of the drying vessel to 55 ℃, and turn on the heating device to raise the temperature of the drying vessel to 55 ℃ at a rate of 5 ℃ / min. Turn on the condenser to keep the condenser's cooling temperature below 15 ℃.

[0058] 6) For restricted loading, carefully remove the modified ultrathin wet gel, lay it flat between two layers of 300-mesh stainless steel wire mesh, and fix the edges with spring clamps to form a planar constraint structure. Immerse the constraint structure in a carrier box pre-filled with the organic solvent isopropanol, open the lid of the drying vessel, place the entire structure into the supercritical drying vessel, close the lid, and seal it.

[0059] 7) Supercritical Drying: Close the outlet valve of the drying vessel, open the inlet valve, and start the carbon dioxide booster pump. First, in the initial introduction stage, slowly introduce carbon dioxide into the drying vessel, increasing the pressure from 0 MPa to the carbon dioxide storage tank pressure (approximately 5.5 MPa), strictly controlling the pressurization rate at 0.05 MPa / min. This stage is extremely slow to ensure the membrane does not vibrate or tear upon gas flow introduction. Then, in the critical transition stage, continue pressurizing from the carbon dioxide storage tank pressure to 7.3 MPa, controlling the pressurization rate at 0.1 MPa / min to slowly cross the phase transition point. Next, in the supercritical pressurization stage, increase the pressure from 7.3 MPa to 25 MPa, controlling the pressurization rate at 0.6 MPa / min. After entering the supercritical state, rapidly increase the pressure to improve efficiency. Once the pressure stabilizes, open the outlet valve and perform supercritical thermal cycling for 2 hours.

[0060] 8) Pressure Reduction and Material Removal: The drying vessel was subjected to constant temperature and uniform pressure reduction. The pressure was reduced from 25 MPa to atmospheric pressure, with the pressure reduction rate controlled at 0.5 MPa / min. After the pressure reduction was completed, the vessel was opened and the product was removed. An aerogel film with a thickness of approximately 600 μm was obtained. The sample was flat, without curling, and had good light transmittance.

[0061] Example 2 (This example aims to prepare a flexible aerogel sheet with a thickness of approximately 1500 μm) 1) Prepare the basic catalyst solution by adding 2 mL of nitric acid to 98 mL of isopropanol (IPA), stirring evenly and storing for later use in preparing the acid catalyst; add 2 mL of alkanolamine to 98 mL of isopropanol (IPA), stirring evenly and storing for later use in preparing the basic catalyst; add 1.5 mL of hydrochloric acid to 18.5 mL of isopropanol (IPA), stirring evenly and storing for later use in preparing the surface treatment catalyst. Stir evenly in all cases and then store.

[0062] 2) To prepare silica sol, MTES, IPA, acidic catalyst, and H2O were added to a flask in a certain molar ratio, where n(IPA):n(MTES)=3 and v(HNO3):v(IPA)=3. The pH was adjusted to 3.5, and the mixture was stirred at room temperature for 3 h to obtain silica sol.

[0063] 3) Ultra-thin molding and aging: Silica sol, IPA, and H2O were mixed in a specific ratio and coated onto a PET substrate using a precision blade coating method, controlling the wet film thickness to approximately 600 μm. A certain volume of TMCS was added (in two parts), and the mixture was stirred for 4 h to obtain a prepolymer. Then, a certain volume of alkanolamine was added to the prepolymer to adjust the pH to 8, and the mixture was stirred and allowed to stand. After gelation was complete, the mixture was sealed and aged at 60 °C.

[0064] 4) Surface modification: Add TMCS and HCl, and allow to stand at 60 °C for modification.

[0065] 5) Preheat the system. Set the temperature of the drying kettle to 55 ℃ and preheat it.

[0066] 6) For restricted loading, the ultrathin wet gel is peeled off from the substrate and placed between a porous ceramic plate and a polytetrafluoroethylene breathable membrane to form a layered constraint. It is then immersed in a mold containing isopropanol and sealed in a drying autoclave.

[0067] 7) Supercritical drying: Carbon dioxide is introduced, and air is removed. First, the initial introduction stage is entered, with the pressure increased from 0 MPa to the pressure in the carbon dioxide storage tank, and the pressure increase rate is controlled at 0.05 MPa / min. Then, the critical transition stage is entered, with the pressure in the carbon dioxide storage tank increased to 7.3 MPa, and the pressure increase rate is controlled at 0.2 MPa / min. Next, the supercritical pressure increase stage is entered, with the pressure increased from 7.3 MPa to 25 MPa, and the pressure increase rate is controlled at 0.8 MPa / min. Finally, the pressure and temperature are maintained, and supercritical thermal cycling is carried out for 3 hours.

[0068] 8) Pressure reduction and material collection: The pressure is gradually reduced to atmospheric pressure. A flexible aerogel sheet with a thickness of approximately 1500 μm is prepared, which remains straight and does not curl under natural conditions.

[0069] Morphology, structure, and performance testing (taking Example 1 as an example): 1. SEM micromorphological characterization The ultrathin aerogel prepared in Example 1 was analyzed by SEM, and the results are shown in the figure. Figure 1 .Depend on Figure 1 As can be seen, the ultrathin aerogel prepared by this invention exhibits a typical three-dimensional nanostructure, with uniform and densely arranged framework particles, and maintains a clear hierarchical pore network connecting mesopores and macropores. This indicates that, despite the use of confined loading and physical constraints, thanks to the "slow start, fast rise" gradient pressure control strategy, the fluid can still diffuse and displace smoothly within the confined space without causing crushing or densification blockage of the microstructure, thus fully preserving the high porosity characteristics of the aerogel.

[0070] 2. Nitrogen isothermal adsorption / desorption and pore size distribution test The ultrathin aerogel prepared in Example 1 was subjected to nitrogen physical adsorption tests, and its N2 isothermal adsorption / desorption curves and pore size distribution curves are shown in the figure. Figure 2 .Depend on Figure 2 (a) shows that the curve exhibits typical characteristics of a type IV adsorption isotherm, accompanied by an H3-type hysteresis loop, indicating the presence of abundant mesoporous structures within the material. The adsorption amount increases rapidly in the low relative pressure range, reflecting the material's high specific surface area (calculated BET specific surface area is 920 m² / g). Figure 2 (b) It can be seen that the pore size distribution exhibits a single-peak narrow distribution, with the peak value concentrated in the range of 10~20 nm. This reflects that within a confined space, the precise control and stable maintenance of the microporous structure of the ultrathin wet gel were achieved by precisely controlling the aging and surface modification processes.

[0071] 3. Macroscopic morphology and flatness analysis (verification of core advantages) Optical photographs of the ultrathin aerogel prepared in Example 1 are shown below. Figure 4 .Depend on Figure 4It can be seen that, through the method of the present invention (i.e., using...) Figure 3 The aerogel film prepared by the air-permeable constraint fixture (under restricted loading) exhibits a light blue, semi-transparent appearance with an extremely smooth and clean surface, regular edge contours, and no visible macroscopic curling, warping, or cracks. This is because the air-permeable constraint fixture applies planar physical constraints during the drying process, forcibly counteracting the curling tendency of the ultrathin sample caused by the release of internal stress; at the same time, the slow pressure increase strategy in the initial stage (0.05 MPa / min) effectively avoids the impact of fluid turbulence on the fragile film, thus achieving high integrity and high flatness at a thickness of only 600 μm.

[0072] 4. Comparative experiments and creative verification To verify the necessity of the process of this invention, the aerogel prepared in Comparative Example 1 (i.e., without using a breathable constraint fixture, all other conditions are the same) was compared, and its optical photograph is shown below. Figure 5 .Depend on Figure 5 As can be seen, in the absence of physical constraints, the ultrathin aerogels of the same formulation underwent severe spontaneous curling after drying, exhibiting a "roller-like" cylindrical shape. Furthermore, the curling stress caused some edges to crack, making them unsuitable for flat application. Figure 6 further illustrates a visual comparison of the macroscopic morphology of Example 1 (flat film) and Comparative Example 1 (curled cylinder). This significant difference directly confirms the outstanding substantial features and significant progress of the constrained loading and gradient drying synergistic process proposed in this invention in solving the problem of ultrathin aerogel curling and deformation.

[0073] The above only presents the performance testing results of TMOS. However, the acidic catalysts used in this application include: concentrated sulfuric acid (H2SO4), hydrochloric acid (HCl), nitric acid (HNO3), hydrofluoric acid (HF), oxalic acid, and citric acid; the alkaline catalysts include: ammonia (NH3·H2O), alkanolamines, sodium hydroxide (NaOH), and urea; the surface treatment catalysts include: hydrochloric acid (HCl), trimethylchlorosilane (TMCS), hexamethyldisiloxane (HMDSO), and hexadecyltrimethylammonium bromide (CTAB); and the silicon sources include: coal gangue, water glass, and methyl orthosilicate. The components include: esters (TMOS), methyltriethoxysilane (MTES), methyltrimethoxysilane (MTMS), vinyltrimethoxysilane + methyltrimethoxysilane, TEOS + MTMS; solvents include: water, ethanol, isopropanol, n-hexane, trimethylchlorosilane, dimethylformamide, tetrahydrofuran; molds include: polytetrafluoroethylene, stainless steel, silicone rubber, plastic; surface modifiers include: HMDSO, TMCS, HMDZ, PDMS, tertiary amines; the second organic solvent includes: ethanol, isopropanol, n-hexane, trimethylchlorosilane, dimethylformamide, tetrahydrofuran. The breathable constraint fixtures include: stainless steel mesh, porous ceramic plates, or polytetrafluoroethylene porous membranes.

[0074] In summary, this invention addresses the characteristics of ultrathin aerogels (especially thin film materials with thicknesses in the micrometer to millimeter range)—namely, their tendency to curl, break, and rapid mass transfer. It creatively introduces a breathable constraint fixture for confined loading, combined with a segmented gradient supercritical carbon dioxide drying process using a slow initial induction. This combined process forcibly eliminates macroscopic curling deformation caused by internal stress release in ultrathin samples through physical planar constraints. The precise fluid control of "slow start, fast rise" effectively avoids mechanical damage to the fragile film skeleton caused by turbulent shearing. Simultaneously, the rapid mass transfer characteristics of ultrathin samples significantly shorten the preparation cycle. Ultimately, an ultrathin aerogel with a smooth, crack-free, curl-free surface and excellent thermal insulation properties is produced.

[0075] It should be emphasized that the above are merely preferred embodiments of the present invention and are not intended to limit the present invention in any way. Any simple modifications, equivalent changes and alterations made to the above embodiments based on the technical essence of the present invention shall still fall within the scope of the technical solution of the present invention.

Claims

1. A method for preparing ultrathin aerogels by supercritical carbon dioxide drying, characterized in that, Includes the following steps: Step 1: Preparation of basic catalyst solutions: Prepare acidic catalyst, basic catalyst, and surface treatment catalytic reagent separately; Step 2, Preparation of silica sol: Mix silicon source, solvent and acidic catalyst, adjust pH to 2-5, stir at room temperature for 0.5-3 hours to hydrolyze and obtain silica sol; Step 3, Ultrathin wet gel forming and aging: The silica sol and solvent prepared in step 2 are mixed and injected into a mold of a limited thickness or coated on a substrate. Surface modifier and alkaline catalyst are added, and the mixture is allowed to stand and gel to form an ultrathin wet gel, which is then aged. Step 4, Surface modification: Add surface modifier and surface treatment catalyst to the ultrathin wet gel prepared in step 3, seal and let stand for modification treatment; Step 5: Pre-treatment of the drying system; Step 6, Restricted Loading and Sealing: The ultrathin wet gel obtained after surface modification in Step 4 is placed in a breathable constraint fixture to form a planar physical constraint structure. Then, the fixture and the gel are immersed in a container containing organic solvent, and the whole thing is placed into a drying system and sealed. Step 7, Flexible Gradient Replacement and Drying: Carbon dioxide is introduced into the drying system, and air is removed and solvent is replaced by segmented gradient pressurization; first, the pressurization rate is controlled to increase from 0 MPa to the carbon dioxide storage tank pressure, then to the supercritical pressure, and finally to the preset drying pressure to carry out supercritical thermal cycling. Step 8, Gradient pressure reduction and material removal: Gradient pressure is reduced to normal pressure to remove the ultrathin aerogel.

2. The method for preparing ultrathin aerogels by supercritical carbon dioxide drying according to claim 1, characterized in that, In step 1, the acidic catalyst includes concentrated sulfuric acid, hydrochloric acid, nitric acid, hydrofluoric acid, oxalic acid, and citric acid; the alkaline catalyst includes ammonia, alkanolamine, sodium hydroxide, and urea; and the surface treatment catalytic reagent includes hydrochloric acid, trimethylchlorosilane, hexamethyldisiloxane, and hexadecyltrimethylammonium bromide.

3. The method for preparing ultrathin aerogels by supercritical carbon dioxide drying according to claim 1, characterized in that, In step 2, the silicon source includes: coal gangue, water glass, methyl orthosilicate, methyltriethoxysilane, methyltrimethoxysilane, vinyltrimethoxysilane + methyltrimethoxysilane, tetraethoxysilane + methyltrimethoxysilane; The solvents include: ethanol and water; The molar ratio of the solvent to the silicon source is 100~5:5~100; The volume ratio of the acidic catalyst to the solvent is 0.5 to 8.0; The mass fraction of SiO2 in the prepared silica sol is 5% to 85%.

4. The method for preparing ultrathin aerogels by supercritical carbon dioxide drying according to claim 1, characterized in that, In step 3: The materials of the mold or substrate include: polytetrafluoroethylene, stainless steel, silicone rubber, plastic, and PET release film; The surface modifiers include: hexamethyldisiloxane, trimethylchlorosilane, hexamethyldisilazane, polydimethylsiloxane, and tertiary amines.

5. The method for preparing ultrathin aerogels by supercritical carbon dioxide drying according to claim 4, characterized in that, In step 3, the molar ratio of the solvent to the silica sol is 2-85, the volume ratio of the solvent to the surface modifier is 1-18, an alkaline catalyst is added to adjust the pH to 7-9, the static gelation time is 0.1-6 h, and the drying and aging time is 0.5-12 h.

6. The method for preparing ultrathin aerogels by supercritical carbon dioxide drying according to claim 1, characterized in that, In step 6: The breathable constraint fixture includes one or more of the following: stainless steel wire mesh, porous ceramic plate, and polytetrafluoroethylene porous membrane; The restricted loading method includes: clamping the ultrathin wet gel between two layers of breathable constraint clamps, or placing it in a layered structure of breathable constraint clamps, and using the clamps to apply planar constraint force to the gel surface to inhibit gel curling. The organic solvents include: ethanol, isopropanol, n-hexane, trimethylchlorosilane, dimethylformamide, and tetrahydrofuran.

7. The method for preparing ultrathin aerogels by supercritical carbon dioxide drying according to claim 1, characterized in that, In step 7: The temperature of the carbon dioxide is 35~55℃; During the process of introducing carbon dioxide to increase the pressure from 0 MPa to the pressure in the carbon dioxide storage tank, the pressurization rate is 0.01~0.1 MPa / min; During the process of increasing the pressure from the carbon dioxide storage tank to the supercritical pressure, the pressurization rate is 0.05~0.5 MPa / min; During the process of increasing the pressure from supercritical pressure to the preset drying pressure, the drying pressure is 7~30 MPa, and the pressure increase rate is 0.2~1.0 MPa / min; During the thermal cycling phase, the temperature and pressure are maintained for 0.5 to 4 hours.

8. The method for preparing ultrathin aerogels by supercritical carbon dioxide drying according to claim 7, characterized in that, In step 8, the drying system is subjected to constant temperature and uniform pressure reduction. During the process of reducing the pressure from the preset drying pressure to the supercritical pressure, the pressure reduction rate is 0.05~2.5 MPa / min; During the period from supercritical pressure to carbon dioxide storage tank pressure, from carbon dioxide storage tank pressure to 3 MPa, and from 3 MPa to atmospheric pressure, the pressure reduction rate was 0.01~2.5 MPa / min.

9. An ultrathin aerogel, characterized in that, The ultrathin aerogel is prepared by the method according to any one of claims 1 to 8, wherein the thickness of the ultrathin aerogel is 10 μm to 2000 μm and the aspect ratio of the ultrathin aerogel is 20 to 5000.

10. The ultrathin aerogel according to claim 9, characterized in that, The thickness of the ultrathin aerogel is 600μm to 1500μm.

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