Preparation method of sepiolite fiber reinforced silica aerogel

Abstract

The present invention relates to the technical field of silica aerogels, and in particular to a method for preparing sepiolite fiber-reinforced silica aerogel. The steps are as follows: S1: pre-removing impurities from sepiolite ore to obtain crude purified sepiolite; S2: performing hydrothermal acid treatment on the crude purified sepiolite to obtain acidified sepiolite; S3: subjecting the acidified sepiolite to freeze-thaw cycles; S4: drying the sepiolite after the freeze-thaw cycles after washing; S5: grinding the dried sepiolite filter cake into a grinding jar to make it fluffy, loosening the agglomerated fiber bundles, and obtaining a nano-sepiolite fiber sample. The present invention provides a method for preparing sepiolite fiber-reinforced silica aerogel, which is prepared by hydrothermal acid treatment of nano-sepiolite fibers, which are introduced into SiO2 sol as a skeleton, and prepared into SEP‑NF / SiO2 aerogel blocks by gelation, hydrophobic modification and solvent replacement, and drying at normal pressure.

Description

Technical Field

[0001] The present invention relates to the technical field of silica aerogels, and in particular to a method for preparing sepiolite fiber-reinforced silica aerogels. Background Art

[0002] Silica (SiO2) aerogel is a three-dimensional nanoporous amorphous material. Due to its excellent properties such as high specific surface area, high porosity, low thermal conductivity, and high insulation, it has become a new type of nanomaterial, making SiO2 aerogel play an important role in thermal insulation, energy saving, adsorption, catalysis, sound insulation and other fields.

[0003] Currently, silica aerogels are often prepared using supercritical drying methods, which consume a lot of energy, require high equipment, and are somewhat dangerous. Therefore, researchers at home and abroad have tried to use low-cost, high-safety atmospheric pressure drying processes. However, the products are mostly broken slag or fragmented aerogels, which greatly limits its engineering application.

[0004] Fiber doping and toughening are the preferred methods for improving the performance and yield of aerogels. Commonly used fibers include carbon fibers and glass fibers, but these micron-sized fibers have fewer constraining interfaces on the aerogel body and can form their own thermal pathways. This improves the mechanical properties of the composite, but their ability to inhibit shrinkage cracking is not significant, and their thermal insulation properties are compromised. Nanofibers have a high specific surface area and can form a denser network within the aerogel. Their large interface area provides stronger constraints on aerogel shrinkage, reducing drying shrinkage during the preparation process and increasing yield. If they can be effectively composited with the aerogel's own skeleton, they can achieve both mechanical toughening and maintain its excellent low thermal conductivity, achieving a composite enhancement effect superior to that of various micron-sized fibers.

[0005] Sepiolite (SEP) is a magnesium-rich fibrous silicate containing natural water, with a theoretical total surface area of ​​900m 2 / g, of which the internal pore specific surface area is 500m 2 / g, resistant to high temperatures of 1200℃, and heat-insulating, environmentally friendly and inexpensive; its crystal is a 2:1 layered structure composed of silicon oxide tetrahedron and magnesium oxide octahedron, with axial through pores, which is easy to be nano-processed by ion exchange.

[0006] An existing patent (CN111020940A) discloses a rapid solvent replacement device, comprising a solvent pool, a conveying mechanism, and an ultrasonic vibration generator; the solvent pool contains a replacement solvent; the conveying mechanism is used to transport the replacement solvent in the solvent pool to a wet gel felt; and the ultrasonic vibration generator generates ultrasonic waves that act on the replacement solvent molecules and the water molecules in the wet gel felt, thereby accelerating the solvent replacement rate. The present invention has a simple structure. The ultrasonic waves generated by the ultrasonic vibration generator act directly on the replacement solvent molecules, and through the replacement solvent, on the water molecules in the wet gel felt, greatly increasing the activity of both molecules and thereby significantly improving the replacement rate.

[0007] An existing patent (CN111684024B) discloses a corrosion-resistant sol-gel, which is the reaction product of an organosilane, a metal alkoxide, an acid, and a chromium (III) salt and / or a lanthanide salt, wherein the solubility of the lanthanide salt at 23°C is about 1 gram or more per gram of sol-gel. The lanthanide salt comprises a cation and a ligand. The cation can be lanthanum, cerium, praseodymium, neodymium, promethium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, lutetium, scandium, yttrium, cobalt, calcium, strontium, barium, and zirconium. The ligand can be nitrate, trifluoromethanesulfonate, sulfate, phosphate, hydroxide, or a hydrate thereof. The chromium (III) salt comprises a cation and a ligand. The cation is chromium (III), and the ligand can be nitrate, trifluoromethanesulfonate, sulfate, phosphate, hydroxide, or a hydrate thereof.

[0008] Taking advantage of the physical and chemical properties of sepiolite, sepiolite nanofibers (SEP-NF) are used to dope and toughen aerogels, which have the following effects: (1) a supporting effect to prevent shrinkage, collapse and cracking; (2) a toughening effect to improve the toughness of the composite aerogel; and (3) a thermal insulation effect to have no negative impact on the thermal conductivity of the aerogel, so that the nanofiber-reinforced aerogel composite material block has both mechanical strength and thermal insulation properties.

[0009] To this end, we designed a preparation method of sepiolite fiber reinforced silica aerogel to provide another technical solution to the above technical problems. Summary of the Invention

[0010] Based on this, it is necessary to provide a method for preparing sepiolite fiber reinforced silica aerogel in order to solve the technical problems mentioned in the background technology.

[0011] In order to solve the above technical problems, the present invention adopts the following technical solutions:

[0012] A method for preparing sepiolite fiber reinforced silica aerogel, comprising the following steps:

[0013] S1: Pre-removing impurities from the sepiolite ore to obtain crude purified sepiolite;

[0014] S2: subjecting the crude purified sepiolite to a hydrothermal acid treatment to obtain acidified sepiolite;

[0015] S3: freeze-thaw cycles of acidified sepiolite;

[0016] S4: drying the sepiolite after freeze-thaw cycles after washing;

[0017] S5: placing the dried sepiolite filter cake into a grinding tank to grind it into fluffy particles, loosening the agglomerated fiber bundles, and obtaining a nano-sepiolite fiber sample;

[0018] S6: The sol was added to sepiolite nanofibers with different mass fractions and dispersed evenly by ultrasonication. Then, 1 mol / L NH3·H2O was added dropwise under magnetic stirring to adjust the pH to 6-8, and the gel was allowed to stand.

[0019] S7: For step S6, after the gel is completed, use anhydrous ethanol to seal it, seal it and place it in an oven at 50°C for aging for 48 hours. After completion, pour out the anhydrous ethanol;

[0020] S8: Pour trimethylchlorosilane modifier into the gel obtained in step S7, seal it and place it in a 25°C oven for 24 hours for hydrolysis and polycondensation modification, then pour out the liquid portion, then seal and clean and replace the solvent twice with n-hexane at 50°C, each time for 12 hours, and finally pour out the n-hexane liquid;

[0021] S9: placing the gel obtained in step S8 in a constant temperature oven and drying step by step, drying at 60° C. for 24 h, 80° C. for 2 h, and 120° C. for 1 h, to obtain nano-sepiolite reinforced silica aerogel.

[0022] As a preferred embodiment of the method for preparing the sepiolite fiber-reinforced silica aerogel provided by the present invention, in step S1, the sepiolite ore is pre-treated to remove impurities to obtain crudely purified sepiolite, and the steps are as follows:

[0023] Add sepiolite into water with a mass ratio of sepiolite to water of 1:10, stir thoroughly to disperse the sepiolite in the water, let it stand and soak for 24 hours, and then stir it magnetically for 2 hours; let it stand and settle naturally, take the upper suspension and filter it, wash it repeatedly with deionized water three times, and dry it at 85°C to constant weight.

[0024] As a preferred embodiment of the method for preparing the sepiolite fiber-reinforced silica aerogel provided by the present invention, in step S2, the crude purified sepiolite is subjected to hydrothermal acid treatment to obtain acidified sepiolite, and the steps are as follows:

[0025] The crude purified sepiolite was mixed with hydrochloric acid solution at a solid-liquid ratio of 1:50, and 20wt% oxalic acid was added, with a hydrochloric acid concentration of 3mol / L. The mixed solution was magnetically stirred evenly, placed in an autoclave, and hydrothermally reacted in an oven at 200°C for 8h. After cooling to room temperature, it was ultrasonicated for 20min.

[0026] As a preferred embodiment of the method for preparing the sepiolite fiber-reinforced silica aerogel provided by the present invention, in step S3, the acidified sepiolite is subjected to a freeze-thaw cycle, and the steps are as follows:

[0027] The hydrous acidified sepiolite was frozen into a solid at -20°C, thawed and ultrasonicated for 30 min, and the freeze-thaw cycles were repeated three times.

[0028] As a preferred embodiment of the method for preparing the sepiolite fiber-reinforced silica aerogel provided by the present invention, in step S4, the sepiolite after freeze-thaw cycle is washed and then dried, and the steps are as follows:

[0029] After freeze-thaw cycles, the sepiolite is washed three times to become weakly acidic or neutral, and the filter cake is then placed in an oven and dried to a constant weight.

[0030] As a preferred embodiment of the method for preparing the sepiolite fiber-reinforced silica aerogel provided by the present invention, in the step S6, 0wt%-25wt% of sepiolite nanofibers are added to the sol and ultrasonically dispersed uniformly, and then 1 mol / L NH3·H2O is added dropwise under magnetic stirring to adjust the pH to 6-8, and the gel is allowed to stand.

[0031] It can be seen without a doubt that the above-mentioned technical solution of this application can definitely solve the technical problem to be solved by this application.

[0032] At the same time, through the above technical solutions, the present invention has at least the following beneficial effects:

[0033] The present invention provides a method for preparing sepiolite fiber-reinforced silica aerogel. Nano-sepiolite fibers are prepared by hydrothermal acid treatment, which are introduced into SiO2 sol as a skeleton. After gelation, hydrophobic modification, and solvent replacement, SEP-NF / SiO2 aerogel blocks are prepared by normal pressure drying. The sepiolite fibers effectively inhibit the shrinkage and cracking of the aerogel blocks during normal pressure drying, and a block aerogel without crack defects can be obtained. The introduction of the fibers has no significant effect on the three-dimensional disordered spherical porous network structure of SiO2, but the pore distribution of the gel changes. With increasing fiber content, the pore volume, porosity, and specific surface area decrease, while the density increases. However, the fiber skeleton forms large pores, which increases the average pore size of the aerogel. The specific surface area of ​​aerogels with a fiber content of less than 10wt% is greater than that of pure aerogels.

[0034] Low content of sepiolite fiber can further reduce the thermal conductivity of aerogel to 0.0128W / (m·k), which is lower than that of pure SiO2 aerogel. The thermal conductivity of aerogel with a content above 5wt% increases with the increase of fiber content, but is still lower than the thermal conductivity of air at room temperature.

[0035] Infrared spectroscopy confirmed that trimethylchlorosilane effectively achieved hydrophobic modification of the aerogel; the contact angle increased with increasing sepiolite fiber content, indicating that sepiolite fibers improved the hydrophobicity of the aerogel;

[0036] Sepiolite fiber can improve the thermal stability of aerogel. When the fiber content is 25%, the thermal weight loss of aerogel at 1000℃ is as low as 7.29%, and the thermal stability is greatly improved. The mechanical properties of sepiolite fiber aerogel increase with the increase of fiber content, with the compressive strength reaching 1.20MPa and the modulus reaching 5.04MPa. The aerogel has excellent flame retardant properties, with LOI values ​​greater than 80 and UL-94 reaching V-0 level. After ablation at 1300℃, the bulk aerogel structure remains stable without deformation. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the following briefly introduces the drawings required for use in the description of the embodiments. Obviously, the drawings described below are some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0038] Figure 1 This is a process flow chart for preparing sepiolite fiber according to the present invention;

[0039] Figure 2 Schematic diagram of the preparation process of SEP-NF / SiO2 aerogel of the present invention;

[0040] Figure 3 This is a comparison diagram of the SEP-NF / SiO2 aerogel modification of the present invention;

[0041] Figure 4 Schematic diagram of the static contact angle of the SEP-NF / SiO2 aerogel of the present invention;

[0042] Figure 5 This is a SEM morphology characterization experiment diagram of the present invention;

[0043] Figure 6 This is a BET characterization experiment diagram of the pore structure of the present invention;

[0044] Figure 7 This is an experimental diagram of the mechanical properties of the SEP-NF / SiO2 aerogel of the present invention;

[0045] Figure 8Schematic diagram of thermal conductivity of SEP-NF / SiO2 aerogel with different dosages of the present invention;

[0046] Figure 9 Schematic diagram of the thermal stability experiment of the present invention;

[0047] Figure 10 The SEM images of pure SiO2 aerogel and SEP-NF / SiO2 aerogel treated at different temperatures of the present invention are shown;

[0048] Figure 11 Schematic diagram of a combustion test experiment of the aerogel of the present invention;

[0049] Figure 12 Schematic diagram of the vertical combustion test of the SEP-NF / SiO2 aerogel of the present invention;

[0050] Figure 13 Schematic diagram of the ablation test experiment of the present invention. DETAILED DESCRIPTION

[0051] In order to make the purpose, technical solutions and advantages of the present invention more clearly understood, the present 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 only used to explain the present invention and are not intended to limit the present invention.

[0052] In order to enable those skilled in the art to better understand the solutions of the present invention, the technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings.

[0053] It should be noted that, in the absence of conflict, the embodiments of the present invention and the features and technical solutions therein may be combined with each other.

[0054] It should be noted that similar reference numerals and letters denote similar items in the following drawings, and therefore, once an item is defined in one drawing, it does not need to be further defined or explained in subsequent drawings.

[0055] Reference Figures 1-13 , a preparation method of sepiolite fiber reinforced silica aerogel.

[0056] 1. Preparation method: the steps are as follows:

[0057] S1: Pre-removing impurities from the sepiolite ore. Add a certain amount of water to the sepiolite, with a mass ratio of 1:10, and stir thoroughly to disperse the sepiolite in the water. Allow to soak for 24 hours, then subject to magnetic stirring for 2 hours. Allow to settle naturally, filter the upper suspension, and repeatedly wash three times with deionized water. Dry at 85°C to constant weight to obtain crudely purified sepiolite.

[0058] S2: The crude purified sepiolite is subjected to hydrothermal acid treatment to obtain acidified sepiolite. The crude purified sepiolite is mixed with a hydrochloric acid solution at a solid-to-liquid ratio of 1:50, and 20 wt% oxalic acid is added, wherein the hydrochloric acid concentration is 3 mol / L. The mixed solution is magnetically stirred and placed in an autoclave. The mixture is hydrothermally reacted in an oven at 200°C for 8 hours. After cooling to room temperature, the mixture is ultrasonically treated for 20 minutes to obtain the acidified sepiolite.

[0059] S3: freeze-thaw cycle the acidified sepiolite. Freeze the acidified sepiolite at -20°C to solid form, thaw, and sonicate for 30 minutes. Repeat the freeze-thaw cycle for more than three times.

[0060] S4: Drying the sepiolite after the freeze-thaw cycle after washing. After the freeze-thaw cycle sepiolite is washed three times, it becomes weakly acidic or neutral, and then the filter cake is placed in an oven and dried to a constant weight.

[0061] S5: placing the dried sepiolite filter cake into a grinding jar to grind it into fluffy particles, loosening the agglomerated fiber bundles to obtain a nano-sepiolite fiber sample, and marking the obtained nano-sepiolite fiber sample as SEP-NF;

[0062] S6: The sol was added with sepiolite nanofibers of different mass fractions and ultrasonically dispersed uniformly. Then, 1 mol / L NH3·H2O was added dropwise under magnetic stirring to adjust the pH to 6-8, and the gel was allowed to stand.

[0063] The steps for making sol are as follows:

[0064] Tetraethyl orthosilicate (TEOS) / ethanol (EtOH) / water (H2O) at a molar ratio of 1:8:4 was magnetically stirred for 15 min;

[0065] After mixing evenly, add 1 mol / L HCl dropwise to adjust the pH of the mixed solution to 2-3;

[0066] The mixture was hydrolyzed for 30 min under magnetic stirring, and a sol was formed after the hydrolysis was completed.

[0067] Preferably, the sol is added with 0wt%-25wt% of sepiolite nanofibers and ultrasonically dispersed uniformly, and then 1 mol / L NH3·H2O is added dropwise under magnetic stirring to adjust the pH to 6-8, and the gel is allowed to stand;

[0068] Preferably, the sol is added with 5 wt% of sepiolite nanofibers and ultrasonically dispersed uniformly, and then 1 mol / L NH3·H2O is added dropwise under magnetic stirring to adjust the pH to 6-8, and the gel is allowed to stand;

[0069] Preferably, the sol is added with 10 wt% of sepiolite nanofibers and ultrasonically dispersed uniformly, and then 1 mol / L NH3·H2O is added dropwise under magnetic stirring to adjust the pH to 6-8, and the gel is allowed to stand;

[0070] Preferably, the sol is added with 15 wt% of sepiolite nanofibers and ultrasonically dispersed uniformly, and then 1 mol / L NH3·H2O is added dropwise under magnetic stirring to adjust the pH to 6-8, and the gel is allowed to stand;

[0071] Preferably, the sol is added with 20 wt% of sepiolite nanofibers and ultrasonically dispersed uniformly, and then 1 mol / L NH3·H2O is added dropwise under magnetic stirring to adjust the pH to 6-8, and the gel is allowed to stand;

[0072] Preferably, the sol is added with 25 wt% of sepiolite nanofibers and ultrasonically dispersed uniformly, and then 1 mol / L NH3·H2O is added dropwise under magnetic stirring to adjust the pH to 6-8, and the gel is allowed to stand;

[0073] S7: For step S6, after the gel is completed, use anhydrous ethanol to seal it, seal it and place it in an oven at 50°C for aging for 48 hours. After completion, pour out the anhydrous ethanol;

[0074] S8: Pour trimethylchlorosilane modifier into the gel obtained in step S7, seal it and place it in a 25°C oven for 24 hours for hydrolysis and polycondensation modification, then pour out the liquid portion, then seal and clean and replace the solvent twice with n-hexane at 50°C, each time for 12 hours, and finally pour out the n-hexane liquid;

[0075] Preferably, the trimethylchlorosilane modifier is diluted with n-hexane to obtain a trimethylchlorosilane modifier with a volume fraction of 10%.

[0076] S9: The gel obtained in step S8 was placed in a constant temperature oven and dried step by step, drying at 60°C for 24 hours, 80°C for 2 hours, and 120°C for 1 hour to obtain nano-sepiolite / silica aerogel, which was labeled as SEP-NF / SiO2.

[0077] 2. Test Experiment

[0078] 1. Using a scanning electron microscope (SEM, Zeiss-Sigma 300) manufactured by Zeiss, Germany, the conductive carbon glue sample was prepared and gold-sprayed to observe the microscopic morphology of the sample. The surface functional groups of the sample were characterized by a Nicolet 6700 intelligent Fourier transform infrared spectrometer from Thermo Fisher Scientific, USA. The dried sample was ground evenly with dried KBr and pressed into a pellet with a wave number range of 4000 cm -1 ~500cm -110 scans per group. Thermal analysis was performed using a German NETZSCH thermal analyzer (STA 449F3) with a temperature scanning range of room temperature to 1000°C, temperature accuracy of ±0.3°C, and temperature repeatability of ±0.2°C (<1000°C). A heating rate of 10°C / min was used under a nitrogen atmosphere. Static contact angles (with a 3μL water droplet) were measured using a SPCAX1 surface tension / dynamic contact angle meter from Beijing Hako Testing Instrument Co., Ltd., China, to characterize the hydrophobic properties of the aerogels. Compression strength tests were performed using an MTS universal testing machine (CMT2000) in the United States. The specimens were cylindrical and greater than 20 mm thick. The stress at 10% strain was used as the compressive strength of the aerogel samples. Thermal conductivity was measured using a Swedish Hot Disk TP 2500S thermal conductivity meter to characterize the thermal insulation properties of the aerogels. According to the GB / T 10294-2008 standard, the prepared aerogels had a diameter greater than 2 cm and a thickness greater than 200 μm. The flame retardancy index (the oxygen volume concentration at which a sample can sustain combustion in oxygen) was measured using an HC-2C limiting oxygen index tester, characterizing the flame retardancy of the aerogel. The flammability was evaluated using a CZF-4 vertical combustion tester.

[0079] 2. Infrared spectroscopy FT-IR characterization experiment

[0080] FT-IR absorption spectra of SEP-NF / SiO2 aerogel before and after modification are as follows: Figure 3 As shown, the antisymmetric stretching vibration and bending vibration of the -OH group are located at 3467 cm -1 and 1633cm -1 , which is attributed to the adsorbed water HOH in the gel and the silanol Si-OH on the gel surface. Comparison of the spectra before modification (a) and after modification (b) shows that the hydrophobically modified aerogel (b) has a peak at 3467 cm -1 The peak height and peak width at 1633cm -1 The absorption peak at 1076 cm is mainly due to the infrared absorption peak of Si-OH on the gel surface. -1 , 795cm -1 It is the typical absorption peak of SiO2. After modification, the adsorbed water decreases and the relative content of SiO2 increases, and both absorption peaks narrow and increase. The wave number is located at 2921cm -1 and 842cm -1 The absorption peaks are the vibration peaks of CH and the stretching vibration of Si-C. The newly appeared 842cm -1 Absorption peak, proving the introduction of Si-C, 2921 cm -1 The peak height at position was also significantly enhanced, confirming that TMCS introduced methyl groups. Before modification, the peak height was 2921 cm-1 The faint peaks indicate that there is still a trace amount of residual replacement solvent. The FT-IR spectrum fully proves that TMCS has achieved aerogel modification through hydrolysis and polycondensation.

[0081] 3. Contact angle test experiment

[0082] In the process of preparing SiO2 aerogel by atmospheric pressure drying, hydrophobic modification is very important. The static contact angle test of SEP-NF / SiO2 composite aerogel was carried out using a contact angle meter. Each sample was tested 10 times. After discarding the data that deviated 10% from the mean, the average value was taken as the test result. The effect of SEP-NF content on the contact angle of SEP-NF / SiO2 is shown in the figure. Figure 4 As shown, all samples exhibited contact angles (CA) greater than 90°, indicating hydrophobicity, consistent with infrared analysis results. Within 20 wt% SEP-NF content, the contact angle increased with increasing SEP-NF content, reaching a maximum of 133°. This is primarily attributed to the inclusion of SEP-NF, which intermixes the spherical SiO2 network with fibrous morphology, positively impacting the surface geometry of the composite gel and increasing the contact angle (structural hydrophobicity). The superior hydrophobicity of SEP-NF / SiO2 facilitates normal pressure drying, reduces shrinkage, and lays the foundation for the preparation of intact bulk aerogels.

[0083] 4. SEM morphology characterization experiment

[0084] Figure 5 The macroscopic appearance of SEP-NF / SiO2 aerogel is shown in Figure 2. Figure 5 As can be seen from (b) and (e), when the fiber content is 0wt%, the pure SiO2 aerogel becomes transparent light blue, while the aerogel with added fibers becomes opaque white, which is due to the Rayleigh scattering caused by the difference in refractive index between the two. When the fiber content increases from 0wt% to 25wt%, the drying shrinkage of the aerogel gradually decreases, and the microcracks gradually disappear. It can be seen that the introduction of fibers has a more obvious inhibitory effect on shrinkage. After aging, the gel pores contain a large amount of ethanol. The direct addition of TMCS (without solvent replacement) can provide favorable conditions for subsequent normal pressure drying. TMCS is modified by covalent bonding with the hydroxyl groups on the gel surface through hydrolysis and condensation. The reaction is mild and does not interfere with the gel pore structure. The SEM morphology of SEP-NF / SiO2 aerogel is shown in Figure 2. Figure 5 (g) to (h) are shown in Figure. Figure 5 Middle (g) shows that the aerogel conforms to the characteristics of a three-dimensional disordered spherical porous network structure. The diameter of the SiO2 spheres is less than 100 nm, and the spherical particles are tightly connected to form a mesoporous network. Figure 5 (h) shows that the SEP-NF fibers are isolated by the SiO2 gel network and are distributed relatively evenly (not agglomerated). The doping of SEP-NF has no obvious effect on the appearance of the gel, and the fiber overlap is not obvious.

[0085] 5. BET characterization experiment of pore structure

[0086] Dynamic oscillatory rheological measurements showed that aging strengthened the alcohol gel, especially at high strain rates. Due to the Ostwald ripening process during aging, the specific surface area decreased with increasing aging time and temperature. With increasing aging time and aging temperature, the linear shrinkage and bulk density of the room temperature dried gel decreased, and the pore size and pore volume increased. The N2 adsorption-desorption isotherms of different doping levels of sepiolite / SiO2 aerogels are shown in Figure 2. Figure 6 As shown in the figure, it can be seen that both pure SiO2 aerogel and SEP-NF / SiO2 aerogel have type IV N2 adsorption and desorption isotherms, and there is an H3 type hysteresis loop in the medium and high pressure range, and the positions of the hysteresis loops of their curves are basically the same, indicating that the SEP-NF / SiO2 composite aerogel meets the characteristics of mesoporous materials. Figure 6 (b) shows the pore volume distribution of SEP-NF / SiO2 aerogels with different fiber content. The graph shows that the pore volume of pure SiO2 aerogel is mainly provided by 10-20nm pores. With the increase of sepiolite fiber content, the total pore volume decreases, the pore diameter corresponding to the maximum pore volume peak decreases (all less than 10nm), the pore volume of 2-3nm nanometers increases, and the pore diameter corresponding to the 10-20nm peak shifts toward smaller pores, reducing the pore volume. When the SEP-NF content is less than 15%, the pore volume is mainly provided by 10-20nm pores. When the SEP-NF content exceeds 15%, the pore volume is mainly provided by pores <10nm. The pore volume curve shows that SEP-NF can reduce the 10-20nm pore volume and increase the 2-3nm pore volume, refining the pores, but reducing the total pore volume.

[0087] Table 1 shows the BET characterization results of the aerogels. As the SEP-NF content increases from 0wt% to 25wt%, the specific surface area decreases overall, while the density increases from 0.168g / cm³ to 0.521g / cm³. Porosity decreases from 93.7% to 67.3%, attributed to the smaller interlayer pores of the SEP-NF itself than those of the SiO2 gel. However, the specific surface area increases at 5% and 10% SEP-NF content, driven by the SEP-NF surface-induced nucleation of SiO2, which increases the number of sol particles and refines the particles (similar to heterogeneous nucleation and grain refinement), resulting in a larger number of particles in the later gels. While the SEP-NF content shows no significant effect on pore size, the average pore size is larger than that of pure SiO2 gel. This can be explained by the micron-sized SEP-NF introducing a macroporous framework, which increases the average pore size, but the pore size remains in the nanopore range (less than 9nm).

[0088] Table 1 Pore characteristics of sepiolite fiber reinforced aerogels with different fiber contents

[0089]

[0090] Notably, at 5 wt% content, the composite aerogel's pore volume and porosity decreased by 31.5% and 5.5%, respectively, compared to pure SiO2 aerogel, while its pore diameter, density, and specific surface area increased by 223.5%, 3.0%, and 7.2%, respectively. This indicates that the increase in macropores actually reduced the pore volume and porosity, leading to a more compact structure, but increased the specific surface area. This suggests that SEP-NF itself carries a large number of nanopores below 3 nm, and surface nucleation further refines the SiO2 gel particles, increasing the total specific surface area. The macropores, likely representing randomly distributed pores within the long fiber skeleton structure, contribute to a 2.23-fold increase in total pore diameter.

[0091] 6. Mechanical properties test experiment

[0092] Specific surface area and density have a significant impact on the mechanical properties of SiO2 aerogels. Under a certain load, the necks of the nanoparticles in the bead-chain structure of the aerogel are prone to cracking due to the small contact surface under external forces, and the structure is irreversibly damaged and becomes fragmented or powdery. To address its defects such as low strength and high brittleness, it is necessary to introduce a reinforcing phase to prevent the aerogel from shrinking and cracking during drying, as well as stress damage under use conditions. SEP-NF has a continuous layered, flexible structure with good mechanical properties. Its surface is rich in silanols, which can covalently condense with the hydroxyl groups on the surface of SiO2 particles, achieving interfacial bonding through Si-O-Si, forming a supporting skeleton that improves the integrity and mechanical properties of SEP-NF / SiO2 aerogels.

[0093] Mechanical properties test results are as follows Figure 7 As shown, the compressive stress-strain curves (a) for all samples exhibit nonlinearity. The strength and modulus of SEP-NF significantly increase, with the modulus generally increasing with increasing fiber content. The compression process can be divided into three stages: linear (strain <10%), yielding and cracking (strain 10-100%), and densification (strain >100%). Strength (b) and modulus (d) calculated using the linear stage (strain <10%) show that both compressive strength and modulus increase linearly with increasing SEP-NF content. The sample with 25wt% SEP-NF content achieves a strength of 1.20 MPa and a modulus of 5.04 MPa. After the compression densification stage, the ultimate crushing stress values ​​of the samples are shown in graph (c). Strength increases essentially linearly with SEP-NF content (except for the 15wt% sample), reaching a maximum ultimate strength of 53.37 MPa (for the 25wt% sample).

[0094] Mechanical property tests revealed that SEP-NF provided structural support, significantly improving both strength and modulus. However, when SEP-NF content exceeded 25 wt%, it tended to agglomerate during wet gel preparation, resulting in insufficient composite gel homogeneity. This led to a deviation in the effect of SEP-NF content on the stress-strain curve (the curve was lower than that of the sample containing 20 wt%).

[0095] 7. Thermal performance test experiment

[0096] 7.1 Thermal conductivity

[0097] The thermal conductivity of aerogels is generally affected by factors such as the density and pore size of the solid, and is primarily composed of solid-state conduction, gaseous conduction, and radiative heat transfer. The solid portion of SiO2 aerogels is an amorphous structure devoid of free electrons, resulting in low solid-state conduction. The pore size in the gel is typically less than 70 nm, and the mean free path of air molecules at 23°C is approximately 66 nm. The Knudsen effect explains the weak gaseous heat conduction in aerogels. Radiative heat transfer primarily involves the aerogel's absorption of external radiation and its own external heat radiation at high temperatures. Because the pores within SiO2 aerogels are nanoscale and the material itself has a low bulk density, coupled with the presence of SEP nanofibers within the aerogel, the number of pore walls within the material is nearly "infinite," and each pore wall acts as a heat shield, creating a near-infinite heat shield effect, reducing radiative heat transfer to its lowest possible level.

[0098] Thermal conductivity of SEP-NF / SiO2 aerogel is as follows Figure 8 As shown, the thermal conductivity increases with increasing SEP-NF content, from 0.0140 W / (m·k) to 0.0228 W / (m·k). This is attributed to the high density and long length of sepiolite fibers, which form thermal bridges when fibers overlap. However, at 5 wt% SEP-NF, the thermal conductivity is 0.0128 W / (m·k) (the lowest value), lower than that of pure SiO2 aerogel, indicating improved aerogel insulation performance. The introduction of 5 wt% SEP-NF increases the aerogel's specific surface area to 676.811 cm2 / g (a 7.2% increase). This increases the adsorption and confinement area of ​​gas molecules on the pore walls, virtually eliminating collisions between air molecules (gaseous heat transfer). At the maximum content of 25 wt%, the thermal conductivity remains lower than the ambient temperature thermal conductivity of air, 0.0255 W / (m·k), indicating that the sepiolite fibers maintain the aerogel's excellent thermal barrier properties.

[0099] 7.2 Thermal stability

[0100] The thermal stability of SEP-NF / SiO2 aerogel at 1000℃ was investigated by comprehensive thermal analysis TG / DSC. Figure 9 .in, Figure 9(a) is the thermogravimetric TG curve of sepiolite ore and nanofibers. It can be seen that after the adsorbed water of SEP-NF is removed at around 100℃, it remains stable until 1000℃ (weight loss <2%), while the sepiolite ore loses about 30% of its weight at 600-800℃, mainly due to the release of crystal water. This shows that the preparation of SEP-NF destroys the hydrated crystal structure of the ore, leaving an ordered layered high-temperature stable structure dominated by silicon-oxygen tetrahedrons and without crystal water. Figure 9 This can be supported by the (h) SEM morphology. Figure 9 (b) shows the TG curves of SEP-NF / SiO2 aerogels at various contents. Near 100°C, the desorption of water adsorbed on the pore surface and residual solvent occurs; near 250°C, the thermal decomposition (pyrolysis) of the methyl groups introduced by TMCS modification is the main factor. The total thermal weight loss of pure SiO2 aerogel at 1000°C is as high as 47.98%, but the introduction of 5wt% SEP-NF significantly reduces this to 17.77%, a 63.0% reduction in weight loss. The total thermal weight loss further decreases with increasing SEP-NF content, with the 25wt% sample losing only 7.29% and exhibiting the best thermal stability. This indicates that SEP-NF significantly improves the thermal stability of aerogels, and this improvement steadily increases with increasing SEP-NF content.

[0101] Figure 9 (c) to (i) are TG-DSC curves of a single sample. From low to high temperature, the DSC curve can be divided into three sections: desorption, pyrolysis, and melting (desorption and pyrolysis have been described above), corresponding to the TG weight loss steps. The DSC curves of each sample basically contain the corresponding three peaks. The DSC peaks in the desorption section are all low, with desorption weight loss rates ranging from 4% to 6%, and there is no significant difference among the samples. In the pyrolysis section, the peak heat flux of pure SiO2 aerogel at 308.13℃ is about 4mW / mg, which is the highest. The desorption heat flux peaks of the other samples are all less than 1mW / mg, corresponding to their highest weight loss rates. This should be attributed to the relatively loose structure and large specific surface area of ​​the pure gel, as well as the high methyl coverage of the TMCS modification, which leads to its maximum pyrolysis weight loss of 41.16%, causing structural collapse and loss of thermal stability. The temperature of the fusion stage starts at about 500°C, which is caused by the endothermic melting of amorphous SiO2 particles. Small-sized particles with higher surface energy are easier to melt, sinter and coalesce. Among them, the 5wt% content sample has more fine particles and the largest specific surface area based on the aforementioned nucleation effect. It has the highest endothermic peak at 962.20°C, but it is only about 1.2mW / mg. It is possible that only the small-sized particles have collapsed, and the main structure can still remain stable.

[0102] In order to further verify the structural stability of bulk SEP-NF / SiO2 aerogel, samples with 0wt%, 10wt%, and 20wt% content were selected and calcined in a muffle furnace at 800℃ and 1000℃ for 3 hours, respectively, and the SEM morphology was observed. Figure 10 Pure SiO2 aerogel treated at 800℃ Figure 10 (a) to (b) and 1000℃ Figure 10 Comparison between (c) and (d) shows that the spherical particle morphology of the aerogel at 800℃ is well maintained, but at 1000℃, the particles melt and form a cluster, especially Figure 10 In the upper right part of (d), the particle morphology has obviously melted and transformed into agglomerates. Parallel comparison of 10wt% content sample Figure 10 (e)~(f) and Figure 10 In (g) to (h), SEP-NF / SiO2 aerogels were calcined at 800℃. Local sintering occurred only in the areas where the crosslinking between the fibers and the SiO2 particles was poor, forming larger particles. After calcining at 1000℃, the internal pore structure still existed, but slight edge melting occurred, and a small amount of SiO2 particles agglomerated into large diameter particles. Parallel comparison of the 20wt% content sample Figure 10 (i)~(j) and Figure 10 In (k) to (l), the SEP-NF / SiO2 aerogel has relatively large pores, which show little change after calcination. The particle morphology remains largely intact, and the flake-like SEP-NF is almost unchanged. This indicates that SEP-NF can reduce the high-temperature sintering of the aerogel SiO2 particles, maintain the three-dimensional pore structure, and maintain excellent high-temperature thermal insulation properties.

[0103] 8. Fire safety test

[0104] 8.1 Burning test (alcohol lamp)

[0105] Flame retardant performance is one of the important indicators of aerogel safety performance. The flame retardant properties of aerogel can be directly observed by burning with an alcohol lamp. Figure 11 As shown in the figure, both pure SiO2 aerogel and SEP-NF / SiO2 aerogel exhibited excellent flame retardancy when burned with the flame of an alcohol lamp. After 20 seconds of burning, none of the samples showed signs of burning. However, pure SiO2 aerogels experienced significant shrinkage and deformation after burning, exhibiting a curled shape. SEP-NF / SiO2 aerogels, however, maintained a flat, sheet-like shape, exhibiting no warping and a smooth surface. This demonstrates that SEP-NF can also improve the high-temperature dimensional stability of aerogel blocks.

[0106] 8.2 Flame retardancy test

[0107] The flame retardancy of aerogel was tested using the limiting oxygen index LOI and UL-94 flammability test. The results are as follows: Figure 12and Table 2. Under an oxygen concentration of 80%, the sample was subjected to a top-ignition test. After burning for 3 minutes, the specimen still did not ignite. Two 10-second vertical combustion tests were conducted, and the flames extinguished instantly. These results show that the LOI values ​​of the SEP-NF / SiO2 aerogels are all greater than 80, with a UL-94 rating of V-0, indicating a non-flammable material. In contrast, the LOI value of pure SiO2 aerogel is only 40 (it ignites at 40% oxygen concentration). This is attributed to its loose internal particle structure and large specific surface area, which results in an excessive amount of methyl (combustible) introduced by the MTCS modification. Despite the non-flammable SiO2 being the main component, a fire safety risk still exists. Therefore, the addition of only 5 wt% SEP-NF is sufficient to transform the aerogel into a non-flammable material.

[0108] Table 2 Combustion performance of SEP-NF / SiO2 aerogel

[0109]

[0110] 8.3 Ablation test (propane spray gun)

[0111] After LOI and UL-94 tests, SEP-NF / SiO2 aerogel showed no combustion phenomenon, so the aerogel was further subjected to a 1-minute ablation test. The results are as follows: Figure 13 , including (a) propane torch ablation test; (b) front view of the sample after ablation. During the ablation process at a temperature of up to 1300°C, the sample still did not show any combustion phenomenon, the block shape remained intact, and there was no change in the back-fire surface. The sample had a significant flame barrier effect. Mainly based on high-temperature resistant SEP-NF as the reinforcing phase, larger pores were introduced into the gel (Table 1), forming a composite structure with SiO2 spherical particles filled in the macroporous skeleton. Under ablation conditions, the stable skeleton support reduced the melting and agglomeration of SiO2 particles, prevented the structure from melting, instability and collapse, and improved the ablation stability.

[0112] The preferred embodiments of the present invention disclosed above are intended only to help illustrate the present invention. The preferred embodiments do not describe all details in detail, nor do they limit the present invention to the specific embodiments described. Obviously, many modifications and variations are possible based on the contents of this specification. These embodiments are selected and described in detail in this specification to better explain the principles and practical applications of the present invention, thereby enabling those skilled in the art to better understand and utilize the present invention. The present invention is limited only by the claims and their full scope and equivalents.

Claims

1. A method for preparing nano-sepiolite fiber reinforced silica aerogel, characterized in that: Here are the steps: S1: Pre-removing impurities from the sepiolite ore to obtain crude purified sepiolite; S2: subjecting the crude purified sepiolite to a hydrothermal acid treatment to obtain acidified sepiolite; S3: freeze-thaw cycles of acidified sepiolite; S4: drying the sepiolite after freeze-thaw cycles after washing; S5: placing the dried sepiolite filter cake into a grinding tank to grind it into fluffy particles, loosening the agglomerated fiber bundles, and obtaining a nano-sepiolite fiber sample; S6: The sol was added to nano-sepiolite fibers with different mass fractions and dispersed evenly by ultrasonication. Then, 1 mol / L NH3·H2O was added dropwise under magnetic stirring to adjust the pH to 6-8, and the gel was allowed to stand. S7: For step S6, after the gel is completed, use anhydrous ethanol to seal it, seal it and place it in an oven at 50°C for aging for 48 hours. After completion, pour out the anhydrous ethanol; S8: Pour trimethylchlorosilane modifier into the gel obtained in step S7, seal it and place it in a 25°C oven for 24 hours for hydrolysis and polycondensation modification, then pour out the liquid portion, then seal and clean and replace the solvent twice with n-hexane at 50°C, each time for 12 hours, and finally pour out the n-hexane liquid; S9: placing the gel obtained in step S8 in a constant temperature oven and drying it step by step, drying it at 60° C. for 24 h, 80° C. for 2 h, and 120° C. for 1 h to obtain nano-sepiolite fiber reinforced silica aerogel; In step S3, the acidified sepiolite is subjected to a freeze-thaw cycle, and the steps are as follows: The hydrous acidified sepiolite was frozen solid at -20℃, thawed and ultrasonicated for 30min, and then frozen and thawed three times; In step S4, the sepiolite after freeze-thaw cycle is dried after washing, and the steps are as follows: After freeze-thaw cycles, the sepiolite is washed three times to a weakly acidic or neutral state, and the filter cake is dried in an oven to a constant weight; In step S6, 5 wt% to 25 wt% of nano-sepiolite fibers are added to the sol and ultrasonically dispersed uniformly. Then, 1 mol / L NH3·H2O is added dropwise under magnetic stirring to adjust the pH to 6 to 8, and the gel is allowed to stand.

2. The method for preparing a nano-sepiolite fiber reinforced silica aerogel according to claim 1, characterized in that: In step S1, the sepiolite ore is pre-treated to remove impurities to obtain crude purified sepiolite, and the steps are as follows: Add sepiolite into water with a mass ratio of sepiolite to water of 1:10, stir thoroughly to disperse the sepiolite in the water, let it stand and soak for 24 hours, and then stir it magnetically for 2 hours; let it stand and settle naturally, take the upper suspension by suction filtration, and repeatedly wash it three times with deionized water, and dry it at 85°C to constant weight.

3. The method for preparing a nano-sepiolite fiber reinforced silica aerogel according to claim 1, characterized in that: In step S2, the crude purified sepiolite is subjected to hydrothermal acid treatment to obtain acidified sepiolite, and the steps are as follows: The crude purified sepiolite was mixed with hydrochloric acid solution at a solid-liquid ratio of 1:50, and 20wt% oxalic acid was added. The hydrochloric acid concentration was 3mol / L. The mixed solution was magnetically stirred and placed in an autoclave. The mixture was hydrothermally reacted in an oven at 200℃ for 8h, cooled to room temperature, and then ultrasonicated for 20min.

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