CN112299428B - Ultra-low-density transparent silicon dioxide aerogel capable of being cleaned later, and preparation method and application thereof - Google Patents
Ultra-low-density transparent silicon dioxide aerogel capable of being cleaned later, and preparation method and application thereof Download PDFInfo
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Abstract
The invention relates to an ultra-low density transparent silicon dioxide aerogel capable of being cleaned later, and a preparation method and application thereof. The method comprises the following steps: carrying out a grading sol-gel reaction on a system consisting of methyl orthosilicate, citric acid, water and a first organic solvent and a system consisting of long-chain alkyl trimethoxy silane, perfluorinated long-chain alkyl trimethoxy silane, polyvinylpyrrolidone, 1, 4-dioxane solution of ammonia and a second organic solvent to obtain wet gel; placing the wet gel in a mother solution containing high-concentration ammonia, and aging at high temperature to obtain a structure-enhanced alcogel; and (3) carrying out gradient solvent replacement on the structure-enhanced alcogel, and then drying at normal pressure to prepare the ultra-low-density transparent silicon dioxide aerogel capable of being cleaned later. The silicon dioxide aerogel prepared by the invention has the characteristics of super-hydrophobicity and super-oleophobic property, has unique post-cleaning and repeated use performance, and has great application value in the fields of new-generation deep space exploration, high-energy physics and the like.
Description
Technical Field
The invention belongs to the technical field of nano porous materials, relates to a preparation method of silicon dioxide aerogel, and particularly relates to ultra-low density transparent silicon dioxide aerogel capable of being cleaned later, and a preparation method and application thereof.
Background
The ultra-low density transparent silicon dioxide aerogel is an important member of a silicon dioxide aerogel family, and has the ultra-low density (10-50 mg/cm) which is not possessed by the conventional silicon dioxide aerogel3) And the most significant characteristics of ultra-high transparency (light transmittance is 85-95%), so that the ultra-low density transparent aerogel has important scientific research and practical application values in the deep space exploration field, and the capture of the spatial high-speed particles is the most common application scene of the ultra-low density transparent aerogel in the deep space exploration field. On the one hand, ultra low density of twoThe silica aerogel has ultrahigh porosity, a three-dimensional network porous solid framework formed by silica nanoparticles is almost completely occupied by air, so that high-speed particles can easily penetrate into a porous solid material, and gradually decelerate to achieve the purpose of 'soft landing' of the high-speed particles in the process of continuously weak mutual collision with a low-density aerogel framework, and meanwhile, the low-density characteristic can effectively reduce the load of a detector, so that the emission and recovery cost of the detector can be reduced in the aerospace industry, and the safety of a system can be greatly improved. On the other hand, under the condition that the particle size and the hole size of the silica aerogel are controlled to be small enough, the scattering effect of the aerogel on light can be effectively reduced, and the ultralow-density silica aerogel is endowed with excellent light transmittance, so that the observation, the positioning and the extraction of the captured particles under the naked eye or the microscope are facilitated.
By analyzing the planet dust particles captured by the aerogel medium, the characteristic information such as the movement speed, the microstructure, the composition, the life information and the distribution trend of the particles can be obtained, which is very helpful for revealing the rules of planet evolution, solar system development and earth life origin. The space particle capturing device made of high-transparency low-density silica aerogel is carried by a space ship launched in the United states in 1999, comet dust particles are brought back by the space particle capturing device in the return voyage in 2006, and the space particle capturing device is the first time that human beings collect solid samples in celestial bodies except the moon and provides important support for exploring the formation of a solar system.
Therefore, the preparation of ultra-low density transparent silica aerogels has attracted extensive attention from researchers. For example, in the chinese patent application CN105271263A, a silica sol system formed by co-hydrolyzing an organic silicon source and hexamethyldisiloxane is used, a gel formed by catalyzing with hydrofluoric acid is subjected to supercritical drying, and the density of the silica aerogel prepared by the method is 15-100 mg/cm3And the transparency and the hydrophobicity are good. Chinese patent application CN108328621A is prepared by depositing a thin silica layer on a carbon aerogel skeleton by chemical vapor deposition, removing the carbon aerogel template, and then performing vapor phase hydrophobization with perfluorooctyl trichlorosilane to obtain silica aerogelThe density can be as low as 25mg/cm3The light transmittance was 50.35%. It is worth noting that the ultra-low density transparent silica aerogel cannot be completely operated in an ultra-clean room during the preparation, assembly, transportation and use processes, and pollutants such as dust in the environment inevitably fall and gather on the surface of the aerogel, which greatly affects the cleanliness and transparency of the aerogel and even greatly interferes with the capture of space dust and the analysis of final results, and no relevant research has been paid attention to the problem at present.
Therefore, the development of a control technology for the surface of the aerogel is urgently needed, an ultra-low density transparent aerogel with a post-cleaning functional characteristic is prepared, and the requirement of a new generation of deep space exploration on an aerogel material with high surface cleanliness requirement is met.
Disclosure of Invention
In order to solve the technical problems in the prior art, the invention provides an ultra-low density transparent silicon dioxide aerogel capable of being cleaned later, and a preparation method and application thereof. The density of the ultra-low density transparent silicon dioxide aerogel prepared by the method is 20-60 mg/cm3The light transmittance is 84% -92%, the anti-pollution performance is strong, most importantly, after the aerogel is used for a period of time, if pollutants such as dust exist on the surface, the aerogel can be automatically cleaned under the assistance of low surface tension solvents such as n-hexane, and the surface can reach high degree of cleanness again.
The present invention provides, in a first aspect, a method for preparing a post-cleanable ultra-low density transparent silica aerogel, the method comprising the steps of:
(1) uniformly mixing methyl orthosilicate, citric acid and water by using a first organic solvent to obtain a first reaction system, and sequentially carrying out prehydrolysis reaction and distillation treatment on the first reaction system to obtain a precursor liquid A;
(2) uniformly mixing the precursor liquid A, long-chain alkyl trimethoxy silane, perfluoro-long-chain alkyl trimethoxy silane and polyvinylpyrrolidone by using a second organic solvent to obtain a second reaction system, and carrying out prehydrolysis reaction on the second reaction system to obtain a mixed liquid B;
(3) dropwise adding a 1, 4-dioxane solution of ammonia into the mixed solution B to perform a gel reaction to obtain wet gel;
(4) placing the wet gel in a mother solution containing high-concentration ammonia, and aging at high temperature to obtain a structure-enhanced alcogel;
(5) and carrying out gradient solvent replacement on the structure-enhanced alcogel, and then drying at normal pressure to prepare the ultra-low-density transparent silicon dioxide aerogel capable of being cleaned later.
Preferably, in the step (1), the temperature of the prehydrolysis reaction is 50-70 ℃, and the time of the prehydrolysis reaction is 10-16 h; in the step (1), the temperature of the distillation treatment is 80-90 ℃; and/or in the step (2), the temperature of the prehydrolysis reaction is 50-70 ℃, and the time of the prehydrolysis reaction is 1.5-3 hours.
Preferably, the molar ratio of the methyl orthosilicate to the citric acid is 1: (0.001-0.1) is preferably 1: 0.04; and/or the molar ratio of the methyl orthosilicate to the water is 1: (2-6) is preferably 1: 3; and/or the first organic solvent is one or more of methanol, ethanol, acetonitrile and acetone, preferably the first organic solvent is methanol; and/or the molar ratio of the methyl orthosilicate to the first organic solvent is 1: (2-5) is preferably 1:3.
Preferably, the long-chain alkyl trimethoxy silane is one or more of hexyl trimethoxy silane, octyl trimethoxy silane, decyl trimethoxy silane, dodecyl trimethoxy silane and tetradecyl trimethoxy silane, and preferably, the long-chain alkyl trimethoxy silane is decyl trimethoxy silane; and/or the molar ratio of the methyl orthosilicate to the long-chain alkyl trimethoxy silane is 1: (0.2-0.5) is preferably 1: 0.4; the perfluorinated long-chain alkyl trimethoxy silane is one or more of perfluorohexyl trimethoxy silane, perfluorooctyl trimethoxy silane and perfluorodecyl trimethoxy silane, and preferably is perfluorodecyl trimethoxy silane; and/or the molar ratio of the methyl orthosilicate to the perfluorinated long-chain alkyl trimethoxy silane is 1: (0.02-0.1) is preferably 1: 0.04.
Preferably, the second organic solvent is one or more of n-pentanol, cyclopentanol, n-hexanol and cyclohexanol, and preferably, the second organic solvent is n-pentanol; and/or the molar ratio of the methyl orthosilicate to the second organic solvent is 1: (8-20) is preferably 1: 12; and/or the molar ratio of the methyl orthosilicate to the polyvinylpyrrolidone is 1: (0.001-0.006) is preferably 1: 0.003.
Preferably, the methyl orthosilicate is mixed with NH contained in the 1, 4-dioxane solution of ammonia in the step (3)3In a molar ratio of 1: (0.002-0.008) is preferably 1: 0.005.
Preferably, in step (4): the mother liquor containing high-concentration ammonia is a 1, 4-dioxane solution of ammonia with the concentration of 0.8-1.2 mol/L; and/or the amount of the mother liquor containing high-concentration ammonia is 2-6 times of the volume of the wet gel; and/or the aging temperature is 60-90 ℃, the aging time is 2-5 days, preferably, the aging temperature is 80 ℃, and the aging time is 3 days.
Preferably, the gradient solvent is replaced by: sequentially using pure ethanol, ethanol and n-hexane in a volume ratio of (7.5-8.5): (1.5-2.5) the volume ratio of the mixed solvent, ethanol and n-hexane is (5.5-7): (3-4.5) the volume ratio of the mixed solvent, ethanol and n-hexane is (3-4.5): (5.5-7), wherein the volume ratio of the mixed solvent to the ethanol to the n-hexane is (1.5-2.5): (7.5-8.5) respectively soaking the structural enhancement alcogel in the mixed solvent and pure hexane for 12-36 hours; and/or the temperature of the normal pressure drying is 20-60 ℃, the time of the normal pressure drying is 1-4 days, preferably, the temperature of the normal pressure drying is 40 ℃, and the time of the normal pressure drying is 2 days.
The present invention provides, in a second aspect, a post-cleanable ultra-low density transparent silica aerogel produced by the production method of the first aspect of the present invention; preferably, the post-cleanable ultra-low density transparent silica aerogel has one or more of the following properties: the density of the ultra-low-density transparent silicon dioxide aerogel capable of being cleaned afterwards is 20-60 mg/cm3(ii) a The light transmittance of the ultra-low-density transparent silica aerogel capable of being cleaned after being cleaned is 84-92%; the surface of the ultra-low density transparent silicon dioxide aerogel capable of being cleaned afterwards is hydrophobic and oleophobic, the contact angle to water is 158.3 degrees, and the contact angle to oil is 161.7 degrees; the post-cleanable ultra-low density transparent silica aerogel has unique post-cleanable and re-usability properties.
The invention provides an application of the ultra-low density transparent silicon dioxide aerogel which can be cleaned after prepared by the preparation method in the first aspect in the new generation deep space exploration field or the high energy physical field.
Compared with the prior art, the invention at least has the following beneficial effects:
(1) compared with the ultra-low density transparent silica aerogel prepared by other prior art, the surface of the aerogel simultaneously shows the dual characteristics of super-hydrophobicity and super-oleophobicity, and the goal can be achieved at the sol-gel stage without subsequent surface modification processes of hydrophobization and oleophobicity, so that the process operation is simplified, and the defect of uneven hydrophobization caused by the hydrophobization by a solution method or a gas phase method is overcome. On one hand, the achievement of the aim is based on the graded hydrolysis strategy of the methyl orthosilicate, the long-chain alkyl trimethoxy silane and the perfluoro-long-chain alkyl trimethoxy silane, and the problem that the silicon source (the methyl orthosilicate) is separated from the organic solvent and water due to the direct mixed hydrolysis reaction of the methyl orthosilicate, the long-chain alkyl trimethoxy silane and the perfluoro-long-chain alkyl trimethoxy silane is solved. Methyl orthosilicate can be uniformly dispersed and fully hydrolyzed in a solvent with larger polarity, such as methanol, but long-chain alkyl trimethoxy silane and perfluoro long-chain alkyl trimethoxy silane are better dispersed and hydrolyzed in a solvent with smaller polarity, such as n-amyl alcohol, if methyl orthosilicate, long-chain alkyl trimethoxy silane and perfluoro long-chain alkyl trimethoxy silane are directly mixed in a single methanol, a single amyl alcohol or a methanol/amyl alcohol mixed solvent, the dispersion and hydrolysis of the three can not be considered, and the methyl orthosilicate is fully hydrolyzed in methanol by adopting a graded hydrolysis strategy, the methanol is removed after the hydrolysis is completed, and the long-chain alkyl trimethoxy silane and the perfluoro-long-chain alkyl trimethoxy silane added into the methanol are continuously and fully hydrolyzed in n-amyl alcohol, so that the problem that sol-gel cannot occur simultaneously is solved.
(2) The ultra-low density transparent silica aerogel prepared by the invention has strong solvent intrusion resistance because the surfaces of the silica particles composing the aerogel are completely covered by hydrophobic long-chain alkane and oleophobic perfluoro long-chain alkane, which is attributed to the regulating effect of citric acid and polyvinylpyrrolidone on the growth of sol particles. In the present invention, the trace amount of water for reaction exists in the form of clusters in the second organic solvent solution (e.g. n-amyl alcohol solution) containing polyvinylpyrrolidone, the hydrolysate of methyl orthosilicate tends to transfer into the water clusters to undergo condensation reaction to form the core of silica sol, and the hydrolysate of long-chain alkyltrimethoxysilane and perfluoro-long-chain alkyltrimethoxysilane tends to remain in the second organic solvent phase (e.g. n-amyl alcohol phase) first, and after the silica sol core is formed, it undergoes condensation reaction with the silicon hydroxyl group on the surface of the core to form a hydrophobic and oleophobic shell layer.
(3) The ultra-low density transparent silica aerogel prepared by the invention adopts a normal pressure drying mode, overcomes the problems of long drying period, large equipment dependence, large safety risk, high production cost and the like existing in supercritical drying, has great economic value for large-scale production of low-cost ultra-low density transparent aerogel, and greatly promotes the application of the ultra-low density transparent aerogel in the fields of transparent heat insulation, solar heat collection systems, building glass and the like.
(4) The ultra-low density transparent silica aerogel prepared by the invention has unique post-cleaning and repeated use performance, and solves the technical problems that pollutants such as dust in the environment can inevitably fall and gather on the surface of the aerogel in the preparation, assembly, transportation and use processes of the existing transparent silica aerogel, so that the cleanliness and transparency of the aerogel are greatly influenced, and even the capture of space dust and the analysis of final results are greatly interfered. The aerogel prepared by the invention has excellent solvent invasion resistance and normal-pressure drying characteristics, the aerogel with surface pollution can be soaked or washed by using a low-surface-tension solvent such as normal hexane, and the surface of the aerogel can reach a high degree of cleanness again after normal-pressure drying, and the unique post-cleanability has great significance for capturing high-energy particles in deep space detection and reusing the transparent aerogel.
Drawings
FIG. 1 is a reaction scheme for preparing a post-cleanable ultra-low density transparent silica aerogel according to example 1 of the present invention. In the figure: TMOS hydrolysis means hydrolysis of methyl orthosilicate, C8H17/C8F17TMOS hydrolysis means hydrolysis of long-chain alkyltrimethoxysilane and of perfluorinated long-chain alkyltrimethoxysilane.
FIG. 2 is a drawing showing the arrangement of a post-cleanable ultra-low density transparent silica Aerogel prepared in example 1 of the present invention on a sheet of paper filled with an Aerogel.
FIG. 3 is a photograph showing the contact angle with water of a post-cleanable ultra-low density transparent silica aerogel prepared in example 1 of the present invention.
FIG. 4 is a photograph of a contact angle of a post-cleanable ultra-low density transparent silica aerogel prepared in example 1 of the present invention with n-hexane.
Detailed Description
In order to make the objects, technical solutions and advantages of the present invention more apparent, the technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. It is to be understood that the embodiments described are only a few embodiments of the present invention, and not all embodiments. All other embodiments, which can be obtained by a person skilled in the art without any inventive step based on the embodiments of the present invention, are within the scope of the present invention.
The present invention provides, in a first aspect, a method for preparing a post-cleanable ultra-low density transparent silica aerogel, the method comprising the steps of:
(1) uniformly mixing methyl orthosilicate (TMOS), citric acid and water by using a first organic solvent to obtain a first reaction system, and sequentially carrying out prehydrolysis reaction and distillation treatment on the first reaction system to obtain a precursor liquid A; in the invention, for example, methyl orthosilicate (TMOS), citric acid and water are uniformly mixed by using a first organic solvent, a prehydrolysis reaction is carried out for 12 hours at 50-70 ℃, the first organic solvent and methanol generated by prehydrolysis of the methyl orthosilicate are completely distilled at 80 ℃, and a precursor liquid A is obtained; in the present invention, the time of the distillation treatment is, for example, terminated by complete distillation of the first organic solvent and methanol produced by prehydrolysis of the methyl orthosilicate;
(2) uniformly mixing the precursor liquid A, long-chain alkyl trimethoxy silane, perfluoro-long-chain alkyl trimethoxy silane and polyvinylpyrrolidone by using a second organic solvent to obtain a second reaction system, and carrying out prehydrolysis reaction on the second reaction system to obtain a mixed liquid B; in the invention, for example, the precursor liquid A, the long-chain alkyl trimethoxy silane, the perfluoro long-chain alkyl trimethoxy silane and the polyvinylpyrrolidone are uniformly mixed by using the second organic solvent, and a prehydrolysis reaction is carried out for 2 hours at 50-70 ℃ to obtain a mixed liquid B;
(3) dropwise adding a 1, 4-dioxane solution of ammonia into the mixed solution B to perform a gel reaction to obtain wet gel; in the present invention, for example, a 1, 4-dioxane solution of ammonia as a catalyst is added dropwise to the mixed solution B under stirring at room temperature, and a gelling reaction occurs to obtain a wet gel; in the invention, the slow dripping mode can fully reduce the reaction rate and ensure the controllable gelation of the silicon dioxide sol; in the present invention, the wet gel obtained is composed of silica sol particles whose surface is both hydrophobic and oleophobic; in the present invention, the 1, 4-dioxane solution of ammonia may be purchased directly from the market, for example; the invention selects 1, 4-dioxane solution of ammonia as a catalyst, the 1, 4-dioxane of the solvent has good compatibility with prehydrolysis products of methyl orthosilicate, long-chain alkyl trimethoxy silane and perfluoro long-chain alkyl trimethoxy silane in the system, and the problem that ammonia water or methanol solution of ammonia and the like are used as the catalyst to introduce water or methanol can be avoided to the greatest extent, which can cause phase separation, the occurrence of the phase separation can cause the particle size of silicon dioxide nano particles to be enlarged, the transparency of the aerogel is influenced if the particle size is light, the sediment of the silicon dioxide particles is generated if the particle size is heavy, and the aerogel with uniform structure and composition can not be prepared;
(4) placing the wet gel in a mother solution containing high-concentration ammonia, and aging at high temperature to obtain a structure-enhanced alcogel; the method utilizes the solution of high-concentration ammonia to age the mother liquor, so that the concentration of the catalyst is high, the reaction rate in the aging stage is high, the reaction degree is high, and the structural strength of the wet gel can be greatly improved;
(5) carrying out gradient solvent replacement on the structure-enhanced alcogel, and then drying at normal pressure to prepare ultra-low-density transparent silicon dioxide aerogel capable of being cleaned later; in the invention, for example, ethanol/n-hexane solution with different ratios is used for carrying out gradient solvent replacement on the structure-enhanced alcogel to obtain organogel soaked in n-hexane, and the organogel is placed into an oven for drying under normal pressure to obtain ultra-low density transparent silica aerogel which can be cleaned later.
According to the invention, a system consisting of methyl orthosilicate, citric acid, water and a first organic solvent and a system consisting of long-chain alkyl trimethoxy silane, perfluorinated long-chain alkyl trimethoxy silane, polyvinylpyrrolidone, 1, 4-dioxane solution of ammonia and a second organic solvent are subjected to a grading sol-gel reaction to obtain wet gel consisting of silica sol particles with hydrophobic and oleophobic surfaces; the staged hydrolysis strategy of the invention avoids the problem of phase separation of methyl orthosilicate, organic solvent and water caused by directly mixing methyl orthosilicate, long-chain alkyl trimethoxy silane and perfluoro long-chain alkyl trimethoxy silane together for hydrolysis reaction. In the invention, the methyl orthosilicate can be uniformly dispersed and fully hydrolyzed in a solvent with larger polarity such as methanol, however, long chain alkyl trimethoxy silane and perfluoro long chain alkyl trimethoxy silane are better dispersed and hydrolyzed in less polar solvent such as n-amyl alcohol, if methyl orthosilicate, long-chain alkyl trimethoxy silane and perfluoro long-chain alkyl trimethoxy silane are directly mixed in a single methanol, a single amyl alcohol or a methanol/amyl alcohol mixed solvent, the dispersion and hydrolysis of the three can not be considered, and the methyl orthosilicate is fully hydrolyzed in methanol by adopting a graded hydrolysis strategy, the methanol is removed after the hydrolysis is completed, and the long-chain alkyl trimethoxy silane and the perfluoro-long-chain alkyl trimethoxy silane added into the methanol are continuously and fully hydrolyzed in n-amyl alcohol, so that the problem that sol-gel cannot occur simultaneously is solved. In the invention, the combined action of the citric acid and the polyvinylpyrrolidone can effectively stabilize water clusters and avoid the generation of large-particle silicon dioxide caused by phase separation, and the citric acid and the polyvinylpyrrolidone can jointly play a role in regulating and controlling the growth of sol particles, so that the surfaces of the silicon dioxide particles forming the aerogel can be completely covered by the hydrophobic long-chain alkane and the oleophobic perfluorinated long-chain alkane, and the ultra-low-density transparent silicon dioxide aerogel prepared by the invention has strong solvent intrusion resistance.
The density of the ultra-low density transparent silicon dioxide aerogel prepared by the method is 20-60 mg/cm3The light transmittance is 84% -92%, the super-hydrophobic and super-oleophobic dual characteristics are shown, the anti-pollution performance is strong, the unique post-cleaning and repeated use performance is realized, even if pollutants such as dust exist on the surface of the aerogel, the aerogel can be automatically cleaned under the auxiliary soaking or washing of low surface tension solvents such as n-hexane and the like, the surface can reach a high degree of cleanness again after being dried under normal pressure, the unique post-cleaning performance is not reported yet, the unique post-cleaning performance has great significance for the transparency keeping and repeated use of the transparent aerogel, and the application value is great in the fields of new-generation deep space exploration, high-energy physics and the like.
According to some preferred embodiments, in step (1), the temperature of the prehydrolysis reaction is 50-70 ℃ (e.g., 50 ℃, 55 ℃, 60 ℃, 65 ℃ or 70 ℃), and the time of the prehydrolysis reaction is 10-16 h (e.g., 10, 11, 12, 13, 14, 15 or 16 h); in the step (1), the temperature of the distillation treatment is 80-90 ℃ (for example, 80 ℃, 85 ℃ or 90 ℃) and is preferably 80 ℃; and/or in the step (2), the temperature of the pre-hydrolysis reaction is 50-70 ℃ (such as 50 ℃, 55 ℃, 60 ℃, 65 ℃ or 70 ℃), and the time of the pre-hydrolysis reaction is 1.5-3 h (such as 1.5, 2, 2.5 or 3 h). In the invention, the first organic solvent and the methanol generated by prehydrolysis of the methyl orthosilicate are preferably completely distilled at the temperature of 80-90 ℃, performing the distillation treatment at this temperature enables a minute amount of water not participating in the reaction to be present in the precursor liquid a, thus the precursor liquid A containing a slight amount of water exists in the form of clusters in the second reaction system containing polyvinylpyrrolidone, so that the hydrolysis product of methyl orthosilicate tends to be transferred into water clusters to undergo condensation reaction to form the core of the silica sol, whereas the hydrolysis products of long-chain alkyltrimethoxysilane and perfluorolong-chain alkyltrimethoxysilane tend to remain in the second organic solvent phase (e.g., the n-pentanol phase) first, after the silica sol core has formed, condensation reactions with the silicon hydroxyl groups of the core surface are facilitated, which facilitates the formation of a hydrophobic and oleophobic shell.
According to some preferred embodiments, the molar ratio of the methyl orthosilicate to the citric acid is 1: (0.001 to 0.1) (e.g., 1:0.001, 1:0.005, 1:0.008, 1:0.01, 1:0.02, 1:0.03, 1:0.04, 1:0.05, 1:0.06, 1:0.07, 1:0.08, 1:0.09, or 1:0.1) is preferably 1: 0.04; and/or the molar ratio of the methyl orthosilicate to the water is 1: (2-6) (e.g., 1:2, 1:2.5, 1:3, 1:3.5, 1:4, 1:4.5, 1:5, 1:5.5, or 1:6) is preferably 1: 3; and/or the first organic solvent is one or more of methanol, ethanol, acetonitrile and acetone, preferably the first organic solvent is methanol; and/or the molar ratio of the methyl orthosilicate to the first organic solvent is 1: (2-5) (e.g., 1:2, 1:2.5, 1:3, 1:3.5, 1:4, 1:4.5, or 1:5) is preferably 1: 3; in the present invention, it is preferable that the molar ratio of the methyl orthosilicate and the water is 1: (2-6), so that the full hydrolysis of the methyl orthosilicate can be effectively ensured, and the reaction degree is increased.
According to some more preferred embodiments, the molar ratio of the methyl orthosilicate, the citric acid, the water and the first organic solvent is 1: (0.001-0.1): (2-6): (2-5) is preferably 1:0.04:3: 3.
According to some preferred embodiments, the long chain alkyl trimethoxysilane is one or more of hexyl trimethoxysilane, octyl trimethoxysilane, decyl trimethoxysilane, dodecyl trimethoxysilane, tetradecyl trimethoxysilane, preferably, the long chain alkyl trimethoxysilane is decyl trimethoxysilane; and/or the molar ratio of the methyl orthosilicate to the long-chain alkyl trimethoxy silane is 1: (0.2 to 0.5) (e.g., 1:0.2, 1:0.25, 1:0.3, 1:0.35, 1:0.4, 1:0.45, or 1:0.5) is preferably 1: 0.4; the perfluorinated long-chain alkyl trimethoxy silane is one or more of perfluorohexyl trimethoxy silane, perfluorooctyl trimethoxy silane and perfluorodecyl trimethoxy silane, and preferably is perfluorodecyl trimethoxy silane; and/or the molar ratio of the methyl orthosilicate to the perfluorinated long-chain alkyl trimethoxy silane is 1: (0.02 to 0.1) (e.g., 1:0.02, 1:0.03, 1:0.04, 1:0.05, 1:0.06, 1:0.07, 1:0.08, 1:0.09, or 1:0.1) is preferably 1: 0.04.
In the present invention, it is preferable that the molar ratio of the methyl orthosilicate, the long-chain alkyltrimethoxysilane, and the perfluoro long-chain alkyltrimethoxysilane is 1: (0.2-0.5): (0.02-0.1), if the addition ratio of the long-chain alkyl trimethoxy silane to the perfluoro long-chain alkyl trimethoxy silane is too low, on one hand, the effect of regulating the sol-gel reaction cannot be achieved, and on the other hand, the effect of hydrophobizing and oleophobizing cannot be achieved, and the final result is that the hydrophobic and oleophobic ultra-low density transparent silica aerogel which can be cleaned later cannot be prepared; and if the addition ratio of the long-chain alkyl trimethoxy silane to the perfluoro long-chain alkyl trimethoxy silane is too high, phase separation is easy to occur, the size of the formed silicon dioxide nano particles is large if the silicon dioxide nano particles are light, the light transmittance of the aerogel is influenced, and precipitation can occur if the silicon dioxide nano particles are heavy, so that the aerogel cannot be prepared.
According to some preferred embodiments, the second organic solvent is one or more of n-pentanol, cyclopentanol, n-hexanol, cyclohexanol, preferably, the second organic solvent is n-pentanol; and/or the molar ratio of the methyl orthosilicate to the second organic solvent is 1: (8-20) (e.g., 1:8, 1:9, 1:10, 1:11, 1:12, 1:13, 1:14, 1:15, 1:16, 1:17, 1:18, 1:19, or 1:20) preferably is 1: 12; and/or the molar ratio of the methyl orthosilicate to the polyvinylpyrrolidone is 1: (0.001-0.006) (e.g., 1:0.001, 1:0.002, 1:0.003, 1:0.004, 1:0.005 or 1:0.006) is preferably 1: 0.003.
According to some more preferred embodiments, the molar ratio of the methyl orthosilicate, the citric acid and the polyvinylpyrrolidone is 1: (0.001-0.1): (0.001-0.006), under the preferable molar ratio, the citric acid and the polyvinylpyrrolidone can play a better stabilizing role in water clusters and a better regulating and controlling role in the growth of sol particles, and the preparation of the ultra-low density transparent silica aerogel capable of being cleaned after and consisting of silica particles with surfaces completely covered by hydrophobic long-chain alkane and oleophobic perfluoro long-chain alkane is facilitated.
According to some more preferred embodiments, the molar ratio of the methyl orthosilicate, the long-chain alkyltrimethoxysilane, the perfluorinated long-chain alkyltrimethoxysilane, the polyvinylpyrrolidone, the second organic solvent is 1: (0.2-0.5): (0.02-0.1): (0.001-0.006): (8-20).
According to some preferred embodiments, the solution of methyl orthosilicate and ammonia in 1, 4-dioxane in step (3) contains NH3In a molar ratio of 1: (0.002-0.008) (e.g., 1:0.002, 1:0.003, 1:0.004, 1:0.005, 1:0.006, 1:0.007, or 1:0.008) is preferably 1: 0.005.
According to some preferred embodiments, in step (4): the mother liquor containing high-concentration ammonia is a 1, 4-dioxane solution of ammonia with the concentration of 0.8-1.2 mol/L (such as 0.8, 0.9, 1, 1.1 or 1.2mol/L), and in the invention, the unit mol/L is also marked as M; the invention preferably adopts 1, 4-dioxane solution of high-concentration ammonia to carry out mother liquor aging, thus, solvents such as methanol, water and the like which are incompatible with the system can not be introduced in the aging reaction stage; and/or the amount of the mother liquor containing high concentration ammonia is 2-6 times (for example, 2, 3, 4, 5 or 6 times) the volume of the wet gel; and/or the temperature of the aging is 60 to 90 ℃ (e.g., 60 ℃, 65 ℃, 70 ℃, 75 ℃, 80 ℃, 85 ℃ or 90 ℃), and the time of the aging is 2 to 5 days (e.g., 2, 3, 4 or 5 days), preferably, the temperature of the aging is 80 ℃, and the time of the aging is 3 days.
According to some preferred embodiments, the gradient solvent is replaced with: sequentially using pure ethanol, ethanol and n-hexane in a volume ratio of (7.5-8.5): (1.5-2.5) the volume ratio of the mixed solvent, ethanol and n-hexane is (5.5-7): (3-4.5) the volume ratio of the mixed solvent, ethanol and n-hexane is (3-4.5): (5.5-7), wherein the volume ratio of the mixed solvent to the ethanol to the n-hexane is (1.5-2.5): (7.5-8.5) soaking the structural reinforced alcohol gel for 12-36 h (for example, 12, 14, 16, 18, 20, 22, 24, 26, 28, 30, 32, 34 or 36h) respectively with the mixed solvent and pure hexane. The invention finds that the gradient solvent replacement is preferably carried out, so that water remained in a gel system can be fully replaced to the maximum extent, the inner holes of the gel are filled with n-hexane solvent with low surface energy, and the drying pressure is reduced to the maximum extent in the normal pressure drying stage, so that the aerogel is obtained. The invention finds that if the n-hexane is directly used for replacement at the beginning, water with high surface can not be effectively replaced because the water and the n-hexane are completely insoluble, and the ethanol mutually soluble with the water is introduced at the beginning, so that the water can be effectively replaced by a gradient replacement strategy.
In the invention, the volume ratio of ethanol to n-hexane is (7.5-8.5): (1.5-2.5) as an example, the volume ratio of ethanol to n-hexane is (7.5-8.5): the mixed solvent of (1.5-2.5) is prepared from (7.5-8.5) by volume: (1.5-2.5) a mixed solvent of ethanol and n-hexane. In the present invention, pure ethanol for solvent substitution, each of the mixed solvents for solvent substitution, and pure hexane for solvent substitution are used in an amount of 5 to 10 times (e.g., 5, 6, 7, 8, 9, or 10 times) the volume of the structure-enhancing alcogel.
According to some specific embodiments, the gradient solvent displacement is: the structure-enhanced alcohol gel is sequentially soaked in pure ethanol, a mixed solvent of ethanol and n-hexane in a volume ratio of 8:2 (also referred to as ethanol/n-hexane (volume ratio of 8: 2)), a mixed solvent of ethanol and n-hexane in a volume ratio of 6:4 (also referred to as ethanol/n-hexane (volume ratio of 6: 4)), a mixed solvent of ethanol and n-hexane in a volume ratio of 4:6 (also referred to as ethanol/n-hexane (volume ratio of 4: 6)), a mixed solvent of ethanol and n-hexane in a volume ratio of 2:8 (also referred to as ethanol/n-hexane (volume ratio of 2: 8)), and pure hexane for 12-36 h (for example, 12, 14, 16, 18, 20, 22, 24, 26, 28, 30, 32, 34, or 36h) respectively.
According to some preferred embodiments, the temperature of the atmospheric drying is 20 to 60 ℃ (e.g., 20 ℃, 25 ℃, 30 ℃, 35 ℃, 40 ℃, 45 ℃, 50 ℃, 55 ℃ or 60 ℃), the time of the atmospheric drying is 1 to 4 days (e.g., 1, 1.5, 2, 2.5, 3, 3.5 or 4 days), preferably the temperature of the atmospheric drying is 40 ℃, and the time of the atmospheric drying is 2 days.
The present invention provides, in a second aspect, a post-cleanable ultra-low density transparent silica aerogel produced by the method of the invention described in the first aspect. The ultra-low density transparent silicon dioxide aerogel capable of being cleaned after prepared by the invention has strong anti-pollution performance, even if pollutants such as dust exist on the surface of the aerogel, the surface can reach high cleanliness degree again through secondary normal pressure drying under the auxiliary soaking or washing of low surface tension solvents such as normal hexane, and the unique cleanability after cleaning has great significance for the transparency maintenance and the repeated use of the transparent aerogel.
According to some preferred embodiments, the post-cleanable ultra-low density transparent silica aerogel has one or more of the following properties: the density of the ultra-low-density transparent silicon dioxide aerogel capable of being cleaned afterwards is 20-60 mg/cm3(ii) a The light transmittance of the ultra-low-density transparent silica aerogel capable of being cleaned after being cleaned is 84-92%; the surface of the ultra-low density transparent silicon dioxide aerogel capable of being cleaned afterwards is hydrophobic and oleophobic, the contact angle to water is 158.3 degrees, and the contact angle to oil is 161.7 degrees; the post-cleanable ultra-low density transparent silica aerogel has unique post-cleanable and re-usability properties.
The invention provides an application of the ultra-low density transparent silicon dioxide aerogel which can be cleaned after prepared by the preparation method in the first aspect in the new generation deep space exploration field or the high energy physical field.
The invention will be further illustrated by way of example, but the scope of protection is not limited to these examples.
Example 1
Respectively adding 15.2g of methyl orthosilicate (0.1mol), citric acid (0.004mol), 5.4g of water (0.3mol) and 9.6g of methanol (0.3mol) into a single-neck flask, magnetically stirring and uniformly mixing, heating to 60 ℃, carrying out reflux reaction (prehydrolysis reaction) for 12 hours at the temperature, changing a reflux device into a distillation device, heating to 80 ℃, and completely evaporating the methanol in the reaction solution to obtain a precursor solution A, wherein the raw materials of each component in the precursor solution A are methyl orthosilicate according to molar ratio: citric acid: water: the methanol is 1:0.04:3: 3.
adding 10.5g of decyl trimethoxy silane (0.04mol), 2.3g of perfluoro decyl trimethoxy silane (0.004mol), 7.2g of polyvinylpyrrolidone (molecular weight 24kDa, 0.0003mol) and 105.6g of n-amyl alcohol (1.2mol) into the precursor liquid A respectively, stirring and uniformly mixing by magnetic force, heating to 60 ℃, carrying out reflux reaction (prehydrolysis reaction) for 2 hours at the temperature to obtain a mixed liquid B, wherein the raw materials of each component in the mixed liquid B are methyl orthosilicate in molar ratio: decyl trimethoxy silane: perfluorodecyl trimethoxysilane: polyvinylpyrrolidone: n-pentanol is 1: 0.4: 0.04: 0.003: 12.
③ adding 10mL of 1, 4-dioxane solution of ammonia (NH contained in the 1, 4-dioxane solution of ammonia) with the concentration of 0.05M into the mixed solution B dropwise while stirring at room temperature3The amount of the substance (b) is 0.0005mol), continuously stirring for 5min after the dropwise addition is finished, pouring the sol solution into a metal mold, and carrying out a gel reaction to obtain wet gel.
Adding 300mL of 1, 4-dioxane solution with 0.1M ammonia concentration into the metal mold filled with the wet gel, transferring the mold into an oven with the temperature of 80 ℃ for aging for 3 days, and carefully taking out the structure-reinforced alcohol gel after the metal mold is cooled to the room temperature.
Fifthly, putting the structural-enhanced alcogel into 1L ethanol for soaking for 24 hours, and discarding waste liquid; then, putting the alcohol gel into a mixed solvent of 1L ethanol/n-hexane (volume ratio is 8:2) for soaking for 24 hours, and discarding waste liquid; then, putting the alcohol gel into a mixed solvent of 1L ethanol/n-hexane (volume ratio is 6:4) for soaking for 24 hours, and discarding waste liquid; then, putting the alcohol gel into a mixed solvent of 1L ethanol/n-hexane (volume ratio is 4:6) to be soaked for 24 hours, and discarding waste liquid; then, putting the alcohol gel into a mixed solvent of 1L ethanol/n-hexane (the volume ratio is 2:8) for soaking for 24 hours, and discarding waste liquid; and finally, putting the alcogel into 1L of n-hexane for soaking for 24h, and discarding the waste liquid. And (3) putting the alcohol gel subjected to gradient solvent displacement into an oven at the temperature of 40 ℃ and drying for 2 days under normal pressure to obtain the ultra-low density transparent silicon dioxide aerogel capable of being cleaned later.
The outline of the ultra-low density transparent silica Aerogel which can be post-cleaned and is prepared by the embodiment is placed on a piece of paper full of Aerogel, as shown in fig. 2, it can be seen that the silica Aerogel prepared by the embodiment has ultra-high transparency; the photograph of the contact angle of the post-cleanable ultra-low density transparent silica aerogel prepared in this example with respect to water, as shown in fig. 3, shows that the contact angle with water (water contact angle) is 158.3 °; the photograph of the contact angle of the post-cleanable ultra-low density transparent silica aerogel prepared in this example with respect to n-hexane, as shown in fig. 4, was found to have a contact angle with oil (oil contact angle) of 161.7 °.
Example 2
Example 2 is essentially the same as example 1, except that:
in the step I, 15.2g of methyl orthosilicate (0.1mol), citric acid (0.001mol), 3.6g of water (0.2mol) and 9.6g of methanol (0.3mol) are respectively added into a single-neck flask, the mixture is stirred and mixed uniformly by magnetic force, the temperature is raised to 60 ℃, the reflux reaction (prehydrolysis reaction) is carried out for 12 hours at the temperature, the reflux device is changed into a distillation device, the temperature is raised to 80 ℃, the methanol in the reaction solution is completely evaporated out to obtain a precursor solution A, and the raw materials of all components in the precursor solution A are methyl orthosilicate according to molar ratio: citric acid: water: the methanol is 1: 0.01: 2: 3.
in the second step, 5.25g of decyl trimethoxy silane (0.02mol), 1.15g of perfluoro decyl trimethoxy silane (0.002mol), 2.4g of polyvinylpyrrolidone (molecular weight 24kDa, 0.0001mol) and 105.6g of n-amyl alcohol (1.2mol) are respectively added into the precursor liquid A, the mixture is stirred and mixed by magnetic force, the temperature is raised to 60 ℃, reflux reaction (prehydrolysis reaction) is carried out for 2h at the temperature, and a mixed liquid B is obtained, wherein each component raw material in the mixed liquid B is methyl orthosilicate according to molar ratio: decyl trimethoxy silane: perfluorodecyl trimethoxysilane: polyvinylpyrrolidone: n-pentanol is 1: 0.2: 0.02: 0.001: 12.
example 3
Example 3 is essentially the same as example 1, except that:
in the step I, 15.2g of methyl orthosilicate (0.1mol), citric acid (0.01mol), 10.8g of water (0.6mol) and 9.6g of methanol (0.3mol) are respectively added into a single-neck flask, the mixture is stirred and mixed uniformly by magnetic force, the temperature is raised to 60 ℃, the reflux reaction (prehydrolysis reaction) is carried out for 12 hours at the temperature, the reflux device is changed into a distillation device, the temperature is raised to 80 ℃, the methanol in the reaction solution is completely evaporated out to obtain a precursor solution A, and the raw materials of all components in the precursor solution A are methyl orthosilicate according to molar ratio: citric acid: water: the methanol is 1: 0.1: 6: 3.
in the second step, 13.125g of decyltrimethoxysilane (0.05mol), 5.75g of perfluorodecyltrimethoxysilane (0.01mol), 14.4g of polyvinylpyrrolidone (molecular weight 24kDa, 0.0006mol) and 105.6g of n-amyl alcohol (1.2mol) are respectively added into the precursor liquid A, the mixture is stirred and mixed by magnetic force, the temperature is raised to 60 ℃, and the reflux reaction (prehydrolysis reaction) is carried out for 2h at the temperature, so as to obtain a mixed liquid B, wherein the raw materials of each component in the mixed liquid B are methyl orthosilicate according to molar ratio: decyl trimethoxy silane: perfluorodecyl trimethoxysilane: polyvinylpyrrolidone: n-pentanol is 1: 0.5: 0.1: 0.006: 12.
example 4
Example 4 is essentially the same as example 1, except that:
in the step I, 15.2g of methyl orthosilicate (0.1mol), citric acid (0.001mol), 1.8g of water (0.1mol) and 9.6g of methanol (0.3mol) are respectively added into a single-neck flask, the mixture is stirred and mixed uniformly by magnetic force, the temperature is raised to 60 ℃, the reflux reaction (prehydrolysis reaction) is carried out for 12 hours at the temperature, the reflux device is changed into a distillation device, the temperature is raised to 80 ℃, the methanol in the reaction solution is completely evaporated out to obtain a precursor solution A, and the raw materials of all components in the precursor solution A are methyl orthosilicate according to molar ratio: citric acid: water: the methanol is 1: 0.01: 1:3.
in the second step, 2.625g of decyltrimethoxysilane (0.01mol), 0.575g of perfluorodecyltrimethoxysilane (0.001mol), 2.4g of polyvinylpyrrolidone (molecular weight 24kDa, 0.0001mol) and 105.6g of n-amyl alcohol (1.2mol) are respectively added into the precursor liquid A, the mixture is stirred and mixed by magnetic force, the temperature is raised to 60 ℃, reflux reaction (prehydrolysis reaction) is carried out for 2 hours at the temperature, and a mixed liquid B is obtained, wherein each component raw material in the mixed liquid B is methyl orthosilicate according to molar ratio: decyl trimethoxy silane: perfluorodecyl trimethoxysilane: polyvinylpyrrolidone: n-pentanol is 1: 0.1: 0.01: 0.001: 12.
example 5
Example 5 is essentially the same as example 1, except that:
in the step I, 15.2g of methyl orthosilicate (0.1mol), citric acid (0.02mol), 14.4g of water (0.8mol) and 9.6g of methanol (0.3mol) are respectively added into a single-neck flask, the mixture is stirred and mixed uniformly by magnetic force, the temperature is raised to 60 ℃, the reflux reaction (prehydrolysis reaction) is carried out for 12 hours at the temperature, the reflux device is changed into a distillation device, the temperature is raised to 80 ℃, the methanol in the reaction solution is completely evaporated out to obtain a precursor solution A, and the raw materials of all components in the precursor solution A are methyl orthosilicate according to molar ratio: citric acid: water: the methanol is 1: 0.2: 8: 3.
in the second step, 15.75g of decyltrimethoxysilane (0.06mol), 11.5g of perfluorodecyltrimethoxysilane (0.02mol), 19.2g of polyvinylpyrrolidone (molecular weight 24kDa, 0.0008mol) and 105.6g of n-amyl alcohol (1.2mol) are respectively added into the precursor liquid A, the mixture is stirred and mixed by magnetic force, the temperature is raised to 60 ℃, and the reflux reaction (prehydrolysis reaction) is carried out for 2h at the temperature, so as to obtain a mixed liquid B, wherein the raw materials of each component in the mixed liquid B are methyl orthosilicate according to molar ratio: decyl trimethoxy silane: perfluorodecyl trimethoxysilane: polyvinylpyrrolidone: n-pentanol is 1: 0.6: 0.2: 0.008: 12.
comparative example 1
Comparative example 1 is substantially the same as example 1 except that:
in step (c), 10mL of 0.05M methanolic ammonia solution (NH contained in the methanolic ammonia solution) was added dropwise to the mixture B at room temperature while stirring3The amount of the substance (b) is 0.0005mol), continuously stirring for 5min after the dropwise addition is finished, pouring the sol solution into a metal mold, and carrying out a gel reaction to obtain wet gel.
This comparative example, the introduction of methanol during the gel reaction, resulted in local phase separation of some free long chain alkyl trimethoxy silane, hydrolysis products of perfluoro long chain alkyl trimethoxy silane, resulting in the growth of large size silica nanoparticles, thereby affecting the transparency of the aerogel produced.
Comparative example 2
Comparative example 2 is substantially the same as example 1 except that:
in the first step, 15.2g of methyl orthosilicate (0.1mol), 5.4g of water (0.3mol) and 9.6g of methanol (0.3mol) are respectively added into a single-neck flask, the mixture is stirred and mixed evenly by magnetic force, the temperature is raised to 60 ℃, the reflux reaction (prehydrolysis reaction) is carried out for 12 hours at the temperature, the reflux device is changed into a distillation device, the temperature is raised to 80 ℃, the methanol in the reaction liquid is completely evaporated out, and a precursor liquid A is obtained, wherein the raw materials of each component in the precursor liquid A are methyl orthosilicate according to the molar ratio: water: the methanol is 1: 3: 3.
in the preparation process of the comparative example, citric acid is not introduced, so that water clusters cannot be stabilized, and on one hand, phase separation occurs, so that large-particle silicon dioxide is generated, and the transparency of the prepared aerogel is influenced; on the other hand, the surface hydrophobicity and oleophobicity of silica sol particles cannot be controlled.
Comparative example 3
Comparative example 3 is substantially the same as example 1 except that:
in the second step, 10.5g of decyl trimethoxy silane (0.04mol), 2.3g of perfluoro decyl trimethoxy silane (0.004mol) and 105.6g of n-amyl alcohol (1.2mol) are respectively added into the precursor liquid A, the mixture is stirred and mixed by magnetic force, the temperature is raised to 60 ℃, reflux reaction (prehydrolysis reaction) is carried out for 2h at the temperature, and a mixed liquid B is obtained, wherein the mixed liquid B comprises methyl orthosilicate according to molar ratio: decyl trimethoxy silane: perfluorodecyl trimethoxysilane: n-pentanol is 1: 0.4: 0.04: 12.
in the preparation process of the comparative example, polyvinylpyrrolidone is not introduced, so that water clusters cannot be stabilized, and on one hand, phase separation occurs, so that large-particle silicon dioxide is generated, and the transparency of the prepared aerogel is influenced; on the other hand, the surface hydrophobicity and oleophobicity of silica sol particles cannot be controlled.
Comparative example 4
Comparative example 4 is substantially the same as example 1 except that:
in the step I, 15.2g of methyl orthosilicate (0.1mol), citric acid (0.004mol), 5.4g of water (0.3mol) and 9.6g of methanol (0.3mol) are respectively added into a single-neck flask, the mixture is stirred and mixed uniformly by magnetic force, the temperature is raised to 60 ℃, the reflux reaction (prehydrolysis reaction) is carried out for 12 hours at the temperature, the reflux device is changed into a distillation device, the temperature is raised to 100 ℃, the methanol and the water in the reaction solution are completely distilled out to obtain a precursor solution A, and the raw materials of each component in the precursor solution A are methyl orthosilicate in molar ratio: citric acid: water: the methanol is 1:0.04:3: 3.
the precursor liquid A obtained by the comparative example has no trace water, no water clusters exist in the preparation process, the prehydrolysis products of the methyl orthosilicate, the long-chain alkyl trimethoxy silane and the perfluoro long-chain alkyl trimethoxy silane can not perform ordered reaction, rapid gelation is caused, and the particle size and the surface characteristics of the prepared aerogel can not be effectively controlled.
Comparative example 5
Comparative example 5 is substantially the same as example 1 except that:
in step (iv), the metal mold containing the wet gel is transferred to an oven at 80 ℃ for aging for 3 days, and after the metal mold is cooled to room temperature, the structure-enhancing alcogel is carefully removed.
The comparative example did not age in aging mother liquor containing high concentration ammonia, the wet gel was very low in aging degree and weak in structural strength, and the aerogel cracked during subsequent drying at normal pressure, and a complete block could not be obtained.
Comparative example 6
Comparative example 6 is substantially the same as example 1 except that:
in the fifth step, the structural reinforced alcogel is put into 1L ethanol for soaking for 24h, and waste liquid is discarded; replacing 1L of ethanol solvent and continuing to soak for 24 h; then 1L of ethanol solvent is replaced to continuously soak for 24 h; then, putting the alcogel into 1L of n-hexane for soaking for 24h, and discarding waste liquid; replacing 1L of n-hexane and continuing to soak for 24 h; then 1L of n-hexane is replaced to continuously soak for 24 h. And (3) putting the alcohol gel subjected to solvent replacement into an oven at the temperature of 40 ℃ and drying for 2 days under normal pressure to obtain the silicon dioxide aerogel.
In the preparation process of the comparative example, the solvent replacement is insufficient, water remains in the gel system, and the gel system has high surface energy, so that the pressure inside gel holes is high in the subsequent normal-pressure drying, the obtained aerogel generates cracks, and a complete block cannot be obtained.
Comparative example 7
Adding 15.2g of methyl orthosilicate (0.1mol), citric acid (0.004mol), 5.4g of water (0.3mol), 9.6g of methanol (0.3mol), 10.5g of decyl trimethoxy silane (0.04mol), 2.3g of perfluoro decyl trimethoxy silane (0.004mol) and 7.2g of polyvinylpyrrolidone (molecular weight 24kDa, 0.0003mol) into a single-neck flask, magnetically stirring and uniformly mixing, heating to 60 ℃, carrying out reflux reaction (prehydrolysis reaction) for 12 hours at the temperature, changing the reflux device into a distillation device, raising the temperature to 80 ℃, completely evaporating the methanol in the reaction liquid to obtain a precursor liquid, wherein each component raw material in the precursor liquid is methyl orthosilicate in molar ratio: citric acid: water: methanol: decyl trimethoxy silane: perfluorodecyl trimethoxysilane: polyvinylpyrrolidone is 1:0.04:3: 3: 0.4: 0.04: 0.003.
② same as the third step in the embodiment 1.
③ same as the procedure (iv) in example 1.
The procedure was repeated except for the same procedure used in EXAMPLE 1.
The comparative example has serious phase separation in the preparation process, so that the silicon dioxide nano particles have large sizes and extremely poor light transmission, the surface of the prepared aerogel does not have excellent hydrophobic and oleophobic performances, and the obtained aerogel has a plurality of cracks and has incomplete appearance.
Table 1: the performance indexes of the silica aerogels prepared in examples 1 to 5 and comparative examples 1 to 7.
Respectively spraying the same amount of dust pollutants on the surfaces of the silica aerogels prepared in the embodiments 1 to 5 and the comparative examples 1 to 7, respectively soaking and washing the polluted silica aerogels in an n-hexane solvent, and drying the polluted silica aerogels at normal pressure again to obtain the silica aerogels; the performance indexes of the silica aerogel after washing and drying of each example and each comparative example are shown in table 2.
Table 2: the performance indexes of the silica aerogels in examples 1 to 5 and comparative examples 1 to 7 after cleaning and drying.
In particular, the light transmittance of the present invention refers to the light transmittance at 550nm of a silica aerogel sample having a thickness of 10mm, and the light transmittance at 550nm is used as an index, because the human eye is most sensitive to visible light at 550 nm. In the present invention, transparency is represented by light transmittance, and the greater the light transmittance, the higher the transparency of the silica aerogel is.
Finally, it should be noted that: the above examples are only intended to illustrate the technical solution of the present invention, but not to limit it; although the present invention has been described in detail with reference to the foregoing embodiments, it will be understood by those of ordinary skill in the art that: the technical solutions described in the foregoing embodiments may still be modified, or some technical features may be equivalently replaced; and such modifications or substitutions do not depart from the spirit and scope of the corresponding technical solutions of the embodiments of the present invention.
Claims (18)
1. A method for preparing an ultra-low density transparent silica aerogel which can be post-cleaned, characterized in that the method comprises the following steps:
(1) uniformly mixing methyl orthosilicate, citric acid and water by using a first organic solvent to obtain a first reaction system, and sequentially carrying out prehydrolysis reaction and distillation treatment on the first reaction system to obtain a precursor liquid A; the precursor liquid A contains trace water; the molar ratio of the methyl orthosilicate to the citric acid is 1: (0.001 to 0.1); the molar ratio of the methyl orthosilicate to the water is 1: (2-6); the molar ratio of the methyl orthosilicate to the first organic solvent is 1: (2-5);
(2) uniformly mixing the precursor liquid A, long-chain alkyl trimethoxy silane, perfluoro-long-chain alkyl trimethoxy silane and polyvinylpyrrolidone by using a second organic solvent to obtain a second reaction system, and carrying out prehydrolysis reaction on the second reaction system to obtain a mixed liquid B; the molar ratio of the methyl orthosilicate to the long-chain alkyl trimethoxy silane is 1: (0.2 to 0.5); the molar ratio of the methyl orthosilicate to the perfluorinated long-chain alkyl trimethoxy silane is 1: (0.02-0.1); the molar ratio of the methyl orthosilicate to the polyvinylpyrrolidone is 1: (0.001 to 0.006); the molar ratio of the methyl orthosilicate to the second organic solvent is 1: (8-20);
(3) dropwise adding a 1, 4-dioxane solution of ammonia into the mixed solution B to perform a gel reaction to obtain wet gel;
(4) placing the wet gel in a mother solution containing high-concentration ammonia, and aging at high temperature to obtain a structure-enhanced alcogel;
(5) carrying out gradient solvent replacement on the structure-enhanced alcogel, and then drying at normal pressure to prepare ultra-low-density transparent silicon dioxide aerogel capable of being cleaned later; the gradient solvent displacement is: sequentially using pure ethanol, ethanol and n-hexane in a volume ratio of (7.5-8.5): (1.5-2.5) the volume ratio of the mixed solvent, ethanol and n-hexane is (5.5-7): (3-4.5) the volume ratio of the mixed solvent, ethanol and n-hexane is (3-4.5): (5.5-7), wherein the volume ratio of the mixed solvent to the ethanol to the n-hexane is (1.5-2.5): (7.5-8.5) respectively soaking the structural enhancement alcogel in the mixed solvent and the pure hexane for 12-36 hours.
2. The method of claim 1, wherein:
in the step (1), the temperature of the prehydrolysis reaction is 50-70 ℃, and the time of the prehydrolysis reaction is 10-16 h;
in the step (1), the temperature of the distillation treatment is 80-90 ℃; and/or
In the step (2), the temperature of the prehydrolysis reaction is 50-70 ℃, and the time of the prehydrolysis reaction is 1.5-3 hours.
3. The production method according to claim 1 or 2, characterized in that:
the molar ratio of the methyl orthosilicate to the citric acid is 1: 0.04; and/or
The molar ratio of the methyl orthosilicate to the water is 1: 3; and/or
The first organic solvent is one or more of methanol, ethanol, acetonitrile and acetone; and/or
The molar ratio of the methyl orthosilicate to the first organic solvent is 1:3.
4. The production method according to claim 3, characterized in that:
the first organic solvent is methanol.
5. The production method according to claim 1 or 2, characterized in that:
the long-chain alkyl trimethoxy silane is one or more of hexyl trimethoxy silane, octyl trimethoxy silane, decyl trimethoxy silane, dodecyl trimethoxy silane and tetradecyl trimethoxy silane; and/or
The molar ratio of the methyl orthosilicate to the long-chain alkyl trimethoxy silane is 1: 0.4;
the perfluorinated long-chain alkyl trimethoxy silane is one or more of perfluorohexyl trimethoxy silane, perfluorooctyl trimethoxy silane and perfluorodecyl trimethoxy silane; and/or
The molar ratio of the methyl orthosilicate to the perfluorinated long-chain alkyl trimethoxy silane is 1: 0.04.
6. The method of claim 5, wherein:
the long-chain alkyl trimethoxy silane is decyl trimethoxy silane.
7. The method of claim 5, wherein:
the perfluoro long-chain alkyl trimethoxy silane is perfluoro decyl trimethoxy silane.
8. The production method according to claim 1 or 2, characterized in that:
the second organic solvent is one or more of n-pentanol, cyclopentanol, n-hexanol and cyclohexanol; and/or
The molar ratio of the methyl orthosilicate to the second organic solvent is 1: 12; and/or
The molar ratio of the methyl orthosilicate to the polyvinylpyrrolidone is 1: 0.003.
9. The method of claim 8, wherein:
the second organic solvent is n-pentanol.
10. The production method according to claim 1 or 2, characterized in that:
the methyl orthosilicate and NH contained in the 1, 4-dioxane solution of ammonia in the step (3)3In a molar ratio of 1: (0.002-0.008).
11. The method of manufacturing according to claim 10, wherein:
the methyl orthosilicate and NH contained in the 1, 4-dioxane solution of ammonia in the step (3)3Is 1: 0.005.
12. The production method according to claim 1 or 2, characterized in that, in step (4):
the mother liquor containing high-concentration ammonia is a 1, 4-dioxane solution of ammonia with the concentration of 0.8-1.2 mol/L; and/or
The using amount of the mother liquor containing high-concentration ammonia is 2-6 times of the volume of the wet gel; and/or
The aging temperature is 60-90 ℃, and the aging time is 2-5 days.
13. The method of manufacturing according to claim 12, wherein:
the temperature of the aging is 80 ℃, and the time of the aging is 3 days.
14. The production method according to claim 1 or 2, characterized in that:
the temperature of the normal pressure drying is 20-60 ℃, and the time of the normal pressure drying is 1-4 days.
15. The method of claim 14, wherein:
the temperature of the normal pressure drying is 40 ℃, and the time of the normal pressure drying is 2 days.
16. A post-cleanable ultra-low density transparent silica aerogel produced by the production method of any one of claims 1 to 15.
17. The post-cleanable, ultra-low density transparent silica aerogel according to claim 16, wherein the post-cleanable, ultra-low density transparent silica aerogel has one or more of the following properties:
the density of the ultra-low-density transparent silicon dioxide aerogel capable of being cleaned afterwards is 20-60 mg/cm3；
The light transmittance of the ultra-low-density transparent silica aerogel capable of being cleaned after being cleaned is 84-92%;
the surface of the ultra-low density transparent silicon dioxide aerogel capable of being cleaned afterwards is hydrophobic and oleophobic, the contact angle to water is 158.3 degrees, and the contact angle to oil is 161.7 degrees;
the post-cleanable ultra-low density transparent silica aerogel has unique post-cleanable and re-usability properties.
18. Use of a post-cleanable ultra-low density transparent silica aerogel produced by the production method according to any one of claims 1 to 15 in a new generation of deep space exploration field or high energy physical field.
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