Highly transparent low-scattering silica aerogel and preparation method and application thereof

Abstract

The application discloses high-transparent low-scattering silica aerogel and a preparation method and application thereof, and belongs to the technical field of nano-optical materials. The preparation comprises the following steps: 1) mixing and stirring tetramethoxysilane, methanol, water and an alkaline catalyst until they are uniform, wherein the molar ratio of the tetramethoxysilane and the alkaline catalyst is 1:0.00589-0.00597, and the molar ratio of the tetramethoxysilane, the methanol and the water is 1:7.29-7.43:4.07-4.17; 2) injecting the mixed solution into a mold, and standing at room temperature to form a wet gel; then demolding and aging the gel; and 3) finally performing supercritical drying, thereby obtaining the high-transparent low-scattering silica aerogel. The preparation method is simple, the obtained silica aerogel realizes a nano-level uniform structure and ultrahigh light transmittance, and has low density, high heat insulation and infrared barrier properties; meanwhile, the method has process stability and long-term optical-thermal stability of the product, and has a wide application prospect.

Description

Technical Field

[0001] This invention belongs to the field of nano-optical materials technology, specifically relating to a highly transparent, low-scattering silica aerogel, its preparation method, and its applications. Background Technology

[0002] Silica aerogels, due to their nanoporous structure, possess extremely low thermal conductivity and ultra-high specific surface area, making them widely used in heat insulation, catalysis, and other fields. However, most conventionally prepared silica-based aerogels are typically translucent or opaque, severely limiting their application in high-transmittance scenarios such as high-end optical devices and transparent heat-insulating windows. The main reasons for this optical performance bottleneck include the following:

[0003] First, during the drying process, the network structure of aerogels is prone to shrinkage and cracking due to the strong capillary forces, forming unavoidable macropores. These structural defects become strong light scattering centers, significantly reducing the optical transparency of the material. Specifically, existing aerogels generally contain scattering centers of about 20 nm and density fluctuations, resulting in visible light transmittance of less than 80% at a thickness of 10 mm, accompanied by strong blue haze (Haze>30%), directly weakening the solar radiation entering optical elements or solar collector absorbers.

[0004] Secondly, conventional supercritical or atmospheric pressure drying processes lack linear pressure control during the depressurization phase, and instantaneous pressure differences can easily induce millimeter-scale cracks. These cracks not only further reduce light transmittance by 15%–20%, but also become leakage channels for heat convection and radiation, weakening the thermal insulation properties of the material.

[0005] Third, the surface of the aerogel is rich in silanol groups, which undergo dehydration condensation under conditions of ≥250 ℃, resulting in enhanced Rayleigh scattering, which reduces the transmittance at a wavelength of 400 nm to below 85%, and causes long-term yellowing and structural embrittlement of the material.

[0006] Finally, traditional aerogels have limited ability to suppress thermal radiation at high temperatures (>120 ℃), making it difficult to achieve high-temperature (>200 ℃) thermal utilization under non-concentrating conditions. To achieve heat collection temperatures above 200 ℃, existing technologies often require the introduction of vacuum cavities or spectrally selective surfaces, which not only increases system complexity, cost, and maintenance difficulty, but also introduces reliability issues such as vacuum leakage and coating aging.

[0007] Therefore, there is an urgent need for a comprehensive solution that has a transmittance of ≥87% in the visible light band, can raise the absorber temperature to ≥200 ℃ under non-focusing, non-vacuum, and non-selective coating conditions, and also has long-term optical and thermal stability. Summary of the Invention

[0008] The purpose of this invention is to provide a highly transparent, low-scattering silica aerogel and its preparation method. The resulting silica aerogel achieves a nanoscale uniform structure and ultra-high light transmittance; it also obtains low density, high heat insulation and infrared blocking properties; at the same time, it has process stability and long-term optical-thermal stability, and has broad application prospects.

[0009] To solve the above-mentioned technical problems, the present invention adopts the following technical solution:

[0010] This invention provides a method for preparing highly transparent, low-scattering silica aerogel, specifically including the following steps:

[0011] 1) Tetramethoxysilane (TMOS), methanol (MeOH), water, and alkaline catalyst are mixed and stirred until homogeneous, wherein: the molar ratio of TMOS to alkaline catalyst is 1:0.00589~0.00597; the molar ratio of TMOS, methanol, and water is 1:7.29~7.43:4.07~4.17; precise control of the alkaline catalyst simultaneously accelerates the hydrolysis and condensation reactions;

[0012] 2) After injecting the mixed solution obtained in step 1) into the mold, let it stand at room temperature to form a wet gel; then demold and allow the gel to age.

[0013] 3) The gel aged in step 2) is subjected to supercritical drying to obtain a highly transparent, low-scattering silica aerogel.

[0014] According to the above scheme, in step 1), the alkaline catalyst is NH3•H2O or NaOH.

[0015] According to the above scheme, in step 1), the stirring time is 5~15 minutes.

[0016] According to the above scheme, in step 2), the standing time at room temperature is 1~2 hours to allow it to gel.

[0017] According to the above scheme, in step 2), the gel is aged in methanol. Preferably, the aging time is 10-15 days.

[0018] According to the above scheme, in step 3), the supercritical drying process is as follows: the pressure is slowly reduced from 1300~1500 psi to ambient pressure at a depressurization rate of 80~100 psi / μL to complete the supercritical drying. Preferably, the supercritical fluid is CO2.

[0019] A highly transparent, low-scattering silica aerogel prepared by the above preparation method is provided.

[0020] According to the above scheme, the highly transparent, low-scattering silica aerogel has the following characteristics: particle size ≤ 7 nm; density 150~300 kg / m³. 3 Thermal conductivity ≤0.02 W / m·K; weighted transmittance in the visible light band (0.35-0.76μm) at a thickness of 10mm is ≥87%, with the highest transmittance reaching over 95%; transmittance in the mid- and far-infrared band (>2500 nm) is ≤5%.

[0021] This invention provides an application of the aforementioned highly transparent, low-scattering silica aerogel in building-integrated high-temperature solar water heating / steam systems, industrial process heat supply, solar seawater desalination, solar combined heat and power (PVT) system covers, or portable solar cookers.

[0022] This invention provides a method for preparing highly transparent, low-scattering silica aerogel. Through precise control of raw material ratios and drying processes, a silica aerogel with high transparency and low scattering properties was successfully prepared. The specific mechanism is as follows:

[0023] This invention, by strictly controlling the molar ratio of alkaline catalyst to TMOS within an extremely low range of 0.00589~0.00597, can simultaneously and smoothly accelerate the hydrolysis and condensation reactions of TMOS, avoiding localized excessively rapid nucleation and particle agglomeration caused by reaction rate mismatch. The formation process of the gel network is more uniform and controllable. Simultaneously, this invention uses methanol and water as a mixed solvent, where methanol effectively regulates the concentration and viscosity of the reaction system, while water ensures complete but not excessive hydrolysis. Furthermore, by limiting the appropriate ratio of TMOS to MeOH and water, a stable gel framework with fine and uniform pores is synergistically formed, laying the structural foundation for subsequent supercritical drying. The resulting stable gel framework has high strength and low internal stress, capable of withstanding the supercritical drying process employed in this invention, effectively avoiding millimeter-scale cracks and structural collapse caused by instantaneous capillary forces. At the same time, the precisely controlled synthesis process reduces density fluctuations in the aerogel framework and excessive residual surface silanol groups, ensuring structural integrity and the absence of macroscopic cracks.

[0024] Through the above process optimization, the resulting silica aerogel has fine and uniformly distributed pores with a particle size ≤7nm, significantly reducing optical losses caused by Mie scattering, and achieving a visible light weighted transmittance of over 87%. It also exhibits high porosity and low aerogel density (150~300kg / m³). 3It exhibits low thermal conductivity (≤0.02 W / m·K) and strong scattering and absorption in the mid- and far-infrared band (>2500 nm), resulting in a transmittance of ≤5% in this band and an extinction coefficient more than three orders of magnitude higher than that in the visible light band, creating a highly efficient "artificial greenhouse effect." Simultaneously, under high-temperature conditions ≥250℃, it can long-term suppress Rayleigh scattering enhancement, yellowing, and structural embrittlement caused by dehydration condensation, demonstrating excellent long-term optical and thermal stability.

[0025] The beneficial effects of this invention are as follows:

[0026] 1. This invention provides a method for preparing highly transparent, low-scattering silica aerogel. By precisely controlling the ratio of alkaline catalyst to TMOS raw materials, and synergistically using a suitable ratio of TMOS to solvent MeOH and water, a stable gel framework suitable for supercritical drying is obtained. Combined with a supercritical drying process, a silica aerogel with high transparency and low scattering characteristics is successfully prepared. The resulting silica aerogel has a small particle size and a visible light weighted transmittance of over 87%, reaching over 95% in specific wavelengths. It has high porosity, with fine and uniformly distributed pores, exhibiting low density, high thermal insulation, and infrared blocking properties, forming a highly efficient "artificial greenhouse effect." Furthermore, the silica aerogel has a complete structure without macroscopic cracks. Under high-temperature conditions ≥250℃, it can long-term suppress Rayleigh scattering enhancement, yellowing, and structural embrittlement caused by dehydration condensation, demonstrating excellent long-term optical-thermal stability and broad application prospects.

[0027] 2. The preparation method of this invention obtains a silica aerogel with complete structure and excellent comprehensive performance by simply adjusting the raw material ratio and then using a supercritical drying process. The raw materials are inexpensive and readily available, the process is simple and mild, the process stability is good, and it has prospects for industrial application.

[0028] 3. The aerogel obtained by this invention can be directly applied to building-integrated high-temperature solar water heating / steam systems, industrial process heat supply, solar seawater desalination, solar combined heat and power (PVT) system covers, and portable solar cooking appliances. It achieves high-efficiency heat collection at ≥200℃ under non-concentrating, non-vacuum, and non-selective coating conditions, significantly reducing the initial installation and maintenance costs of the system. Attached Figure Description

[0029] Figure 1 The image shows the transmittance of the silica aerogel prepared in Example 1.

[0030] Figure 2 The image shows a scanning electron microscope (SEM) image of the silica aerogel prepared in Example 1 at 100 nm.

[0031] Figure 3 This is a physical image of the silica aerogel sample prepared in Example 1.

[0032] Figure 4 The image shows a scanning electron microscope (SEM) image of the silica aerogel prepared in Example 2 at 400 nm.

[0033] Figure 5 The image shows a scanning electron microscope (SEM) image of the silica aerogel prepared in Comparative Example 1 at 400 nm. Detailed Implementation

[0034] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0035] Example 1

[0036] A method for preparing highly transparent, low-scattering silica aerogel is provided, specifically including the following steps:

[0037] 1) Mix tetramethoxysilane (TMOS), methanol (MeOH), deionized water, and 79.5 mM ammonia (calculated as NH3•H2O) in a molar ratio of 1:7.38:4.13:0.00592 and stir at room temperature for 15 min until the solution is homogeneous; the deionized water includes the water in the ammonia solution and water added separately.

[0038] 2) After injecting the mixture obtained in step 1) into the mold, let it stand at room temperature for 2 hours to form a wet gel. After demolding, soak the wet gel in methanol for 12 days to age.

[0039] 3) The aged gel is subjected to supercritical drying with CO2 as the supercritical fluid. The pressure is slowly reduced from 1300 psi to ambient pressure at a decompression rate of 100 psi / μL to complete the supercritical drying, thus obtaining a highly transparent, low-scattering silica aerogel.

[0040] Characterization revealed that the aerogel exhibits a uniform spherical particle network structure. SEM measurements showed particle sizes ranging from 5.3 to 6.1 nm. At a thickness of 10 mm, the visible light transmittance reached a maximum of 96.5%, with a haze of only 1.9%. The visible light weighted transmittance was ≥91.0%. The thermal conductivity at 25℃ was as low as 0.019 W / (m•K). At a high temperature of 250℃, the transmittance at 400 nm was 88.2%. No significant yellowing was observed. The density was 220 kg / m³. 3 It fully meets the requirements of high transparency, low scattering, and high temperature resistance in high-temperature solar thermal utilization scenarios.

[0041] Figure 1The graph shows the transmittance of the silica aerogel prepared in Example 1. The horizontal axis represents wavelength (0.25-20 μm), and the vertical axis represents transmittance (%), overlaid with the AM 1.5 standard solar spectrum. The graph shows that the silica aerogel obtained in Example 1, with a thickness of 10 mm, has a weighted transmittance of ≥91% in the visible light band (0.35-0.76 μm) and ≤5% in the mid-to-far-infrared band (>2500 nm), demonstrating the characteristics of "high visible light transmission and low infrared leakage." The weighted visible light transmittance is calculated by integrating the transmittance at each wavelength with the corresponding solar radiation energy weight using the AM1.5D standard solar radiation spectrum (a universal reference spectrum for solar radiation on the Earth's surface) as the energy weight benchmark. This is the core optical performance indicator used to quantitatively characterize the light transmission capability of the aerogel under actual solar illumination conditions.

[0042] Figure 2 The image shows a scanning electron microscope (SEM) image of the silica aerogel prepared in Example 1. The image shows that at the 100 nm scale, the aerogel is a three-dimensional network of uniform spherical particles with a particle size of 5.3-6.1 nm, no agglomeration, and pores of 10-20 nm that are regularly distributed, confirming the low scattering structure.

[0043] Figure 3 The image shows the actual silica aerogel prepared in Example 1. The image shows that the aerogel (film / block) is colorless and transparent, with no visible cracks or bubbles, which directly demonstrates its high transparency and structural integrity.

[0044] Example 2

[0045] A method for preparing highly transparent, low-scattering silica aerogel is provided, specifically including the following steps:

[0046] 1) Mix tetramethoxysilane (TMOS), methanol (MeOH), deionized water, and a 79.7 mM sodium hydroxide aqueous solution (calculated as NaOH) in a molar ratio of 1:7.35:4.10:0.00590, and stir at room temperature for 5 minutes until the solution is clear; the deionized water includes the water in the sodium hydroxide aqueous solution and water added separately;

[0047] 2) Pour the mixture obtained in step 1) into a polytetrafluoroethylene mold and let it stand at room temperature for 2 hours to form a wet gel. After demolding, soak the wet gel in methanol for 12 days to age.

[0048] 3) After aging, the gel is subjected to supercritical drying. The supercritical fluid is CO2. During the supercritical drying stage, the pressure is slowly reduced from 1300 psi to ambient pressure at a decompression rate of 100 psi / μL to complete the supercritical drying and ensure that the structure does not collapse, thus obtaining a highly transparent and low-scattering silica aerogel.

[0049] Characterization revealed that this aerogel exhibits a typical three-dimensional layered network structure, lacking clearly defined spherical particle morphology. It is formed by the interweaving of continuous sheet-like gel skeletons, with elongated or near-elliptical pores. The average pore size is approximately 21.2 nm. At a thickness of 10 mm, the highest visible light transmittance is 95.5%, haze is 5.1%, and the visible light weighted transmittance is ≥87.2%. The thermal conductivity at 25℃ is 0.021 W / (m•K), and at a high temperature of 250℃, the transmittance at 400 nm is 89.1%. The density is 170 kg / m³. 3 It exhibits slight yellowing and is suitable for medium- and low-temperature insulation applications where flexibility and light transmittance requirements are not extremely high.

[0050] Figure 4 The image shows a scanning electron microscope (SEM) image of the sample at 400 nm in Example 1. At the 400 nm scale, the aerogel exhibits a typical three-dimensional layered network structure without clearly defined spherical particle morphology. It is a three-dimensional support network formed by the interweaving of continuous sheet-like gel skeletons with pores of 20-30 nm. The structure is less dense than that of Example 1, corresponding to slightly lower light transmittance and slightly higher thermal conductivity.

[0051] Comparative Example 1

[0052] A method for preparing silica aerogel is provided, specifically including the following steps:

[0053] 1) Mix tetramethoxysilane (TMOS), methanol (MeOH), deionized water, and 79.5 mM ammonia (calculated as NH3•H2O) in a molar ratio of 1:7.36:4.11:0.00787, and stir at room temperature for 3 minutes until the mixture is homogeneous; the deionized water includes the water in the ammonia solution and water added separately.

[0054] 2) Pour the mixture obtained in step 1) into a mold; let it stand at room temperature for 1.5 hours to form a wet gel. After demolding, soak the wet gel in methanol for 12 days to age.

[0055] 3) After aging, the gel is subjected to supercritical drying. The supercritical fluid is CO2. During the supercritical drying stage, the pressure is slowly reduced from 1300 psi to ambient pressure at a decompression rate of 100 psi / μL to complete the supercritical drying and ensure that the structure does not collapse, thus obtaining silica aerogel.

[0056] Characterization revealed that due to excessive NH3•H2O content, the aerogel exhibited severe agglomeration of spherical particles, with particle size increasing to 7.6–8.8 nm. At a thickness of 10 mm, the visible light weighted transmittance decreased to 81.6%, haze increased to 8.7%, thermal conductivity at 25°C rose to 0.023 W / (m•K), and at a high temperature of 250°C, the transmittance at 400 nm was 82.1%, with a density of 350 kg / m³.3 The yellowing was obvious, and the performance exceeded the core indicators of the patent, confirming the necessity of precisely controlling the amount of NH3•H2O.

[0057] Figure 5 The image shows a scanning electron microscope (SEM) image of the silica aerogel prepared in Comparative Example 1. The image shows that at the 400 nm scale, the spherical particles are severely aggregated to form blocky regions (>50 nm), and the pores are irregular, which directly reflects the structural defects caused by the high amount of NH3•H2O.

[0058] It should be understood that those skilled in the art can make improvements or modifications based on the above description, and all such improvements and modifications should fall within the protection scope of the appended claims.

Claims

1. A method for preparing a highly transparent, low-scattering silica aerogel, characterized in that, Specifically, the following steps are included: 1) Mix tetramethoxysilane, methanol, water and alkaline catalyst and stir until homogeneous, wherein: the molar ratio of tetramethoxysilane to alkaline catalyst is 1:0.00589~0.00597; the molar ratio of tetramethoxysilane, methanol and water is 1:7.29~7.43:4.07~4.17; 2) After injecting the mixed solution obtained in step 1) into the mold, let it stand at room temperature to form a wet gel; then demold and allow the gel to age. 3) The gel aged in step 2) is subjected to supercritical drying to obtain a highly transparent, low-scattering silica aerogel.

2. The preparation method according to claim 1, characterized in that, In step 1), the alkaline catalyst is NH3•H2O or NaOH.

3. The preparation method according to claim 1, characterized in that, In step 1), the stirring time is 5-15 minutes; in step 2), the standing time at room temperature is 1-2 hours.

4. The preparation method according to claim 1, characterized in that, In step 2), the gel is aged in methanol.

5. The preparation method according to claim 4, characterized in that, The aging time is 10-15 days.

6. The preparation method according to claim 1, characterized in that, In step 3), the supercritical drying process is as follows: the pressure is slowly reduced from 1300~1500 psi to ambient pressure at a depressurization rate of 80~100 psi / μL to complete the supercritical drying.

7. The preparation method according to claim 1, characterized in that, In the supercritical drying process, the supercritical fluid is CO2.

8. A highly transparent, low-scattering silica aerogel prepared by the preparation method according to any one of claims 1-7.

9. The silica aerogel according to claim 8, characterized in that, The highly transparent, low-scattering silica aerogel has the following characteristics: particle size ≤ 7 nm; density 150~300 kg / m³. 3 Thermal conductivity ≤ 0.02 W / m·K; weighted transmittance in visible light band ≥ 87% at a thickness of 10 mm; transmittance in mid- and far-infrared band ≤ 5%.

10. The application of the high-transparency, low-scattering silica aerogel of claim 8 in building-integrated high-temperature solar water heating / steam systems, industrial process heat supply, solar seawater desalination, solar combined heat and power system covers, or portable solar cookers.

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