CN111498857B - Process for the preparation of silica aerogels - Google Patents
Process for the preparation of silica aerogels Download PDFInfo
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- CN111498857B CN111498857B CN202010142667.0A CN202010142667A CN111498857B CN 111498857 B CN111498857 B CN 111498857B CN 202010142667 A CN202010142667 A CN 202010142667A CN 111498857 B CN111498857 B CN 111498857B
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- C01B33/00—Silicon; Compounds thereof
- C01B33/113—Silicon oxides; Hydrates thereof
- C01B33/12—Silica; Hydrates thereof, e.g. lepidoic silicic acid
- C01B33/14—Colloidal silica, e.g. dispersions, gels, sols
- C01B33/157—After-treatment of gels
- C01B33/159—Coating or hydrophobisation
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- C01B33/00—Silicon; Compounds thereof
- C01B33/113—Silicon oxides; Hydrates thereof
- C01B33/12—Silica; Hydrates thereof, e.g. lepidoic silicic acid
- C01B33/14—Colloidal silica, e.g. dispersions, gels, sols
- C01B33/157—After-treatment of gels
- C01B33/158—Purification; Drying; Dehydrating
- C01B33/1585—Dehydration into aerogels
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- C01P2006/10—Solid density
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- C01P2006/32—Thermal properties
Abstract
A method for preparing silicon dioxide aerogel, mainly to the solvent replacement process in preparing silicon dioxide aerogel, carry on the operation of adding solvent "the circulating filtration way continuously", make wet gel circulate and replace (change and wash) continuously through a large amount of alcohol solvents and organic solvents in relatively short time, therefore can accelerate (reduce) the time of the wet gel solvent replacement operation by a wide margin, meanwhile, the alcohol solvent or organic solvent that leads out of the reaction vessel will pass the filtration procedure, in order to separate wet gel and moisture or alcohol solvent, then alcohol solvent and organic solvent enter this reaction vessel again and carry on solvent replacement, carry on the circulating filtration continuously fast so, can promote the effect that the wet gel solvent replaces by a wide margin.
Description
[ technical field ] A method for producing a semiconductor device
The present invention relates to a method for preparing silica aerogel, and more particularly, to a method for preparing silica aerogel by performing a "continuous circulation filtration" process of adding a solvent in a solvent replacement process for preparing silica aerogel.
[ background of the invention ]
Aerogels (aerogels) generally refer to light-weight nano solid materials in which nano-scale super-wei particles are mutually aggregated to form a nano porous network structure, and gaseous dispersion media are filled in the pores of the network. Different aerogels are prepared by different methods, but the preparation processes are different greatly, and a wet gel (wet gel) is prepared by a sol-gel method, and then the wet gel is subjected to solvent replacement and supercritical drying to obtain the corresponding aerogel. The aerogel is characterized in that: (a) the porosity is very high; can reach 99.8 percent (b) of nanometer level holes (2-50 nm) and three-dimensional nanometer skeleton particles (2-5 nm); (c) high specific surface area up to 1000m2(ii)/g; (d) low density (as low as 0.003 g/cm)3(ii) a (e) The thermal conductivity is extremely low, can be as low as 0.013W/(mK) under the normal temperature, and is lower than the heat conduction number of air; (f) low strength, high brittleness, and low density due to high specific surface area and porosity, resulting in low strength. Aerogel is a super insulating material in the modern high-tech industry, and is used in many industries. Kistler, s.s.kistler, stanford university, usa first produced silica aerogel (SiO) using water glass by a sol-gel process and supercritical drying technique2aerogels), also known as "silicon aerogels".
The conventional methods for preparing silica aerogels have been disclosed in various countries, for example: US10377637B2 "" Aerogel and manufacturing method of thermal of "" CN103818912B "" a method for preparing silica Aerogel with low density and large specific surface area under normal pressure "", CN104030301B "" a silica Aerogel material and a preparation method thereof "", CN105776234A "" a supercritical drying method for purifying water glass Aerogel by sodium metaaluminate modified resin "", CN106745000A "" a preparation method for water glass-based silica Aerogel "", CN106865558A "" a method for preparing silica Aerogel under normal pressure "" and the prepared silica Aerogel "", CN109133070A, "a method for preparing hydrophobic silica aerogel at ultra-fast atmospheric pressure using water glass as raw material", CN104071797B, "a method for preparing silica aerogel coating with low density and large specific surface area at room temperature under atmospheric pressure using water glass", I561561, "aerogel particles and preparation method thereof", I516447, "aerogel and manufacturing method thereof", and I588209 "modified hydrophobic aerogel material" are all the same.
In the above-mentioned former case, there are different aerogel preparation methods, but they must go through two steps of "replacement" and "modification", and may go ahead of "replacement", or "modification", or may go ahead of "replacement" and "modification" together. In the above mentioned former, in US10377637B2, the solvent replacement and modification operations of the gel are performed at 55 ℃ for 1-12 hr by using "n-hexane + isopropanol + hexamethyldisiloxane"; CN103818912B subjecting the gel to 50 deg.C water bath replacement for 3 times with ethanol (or isopropanol, acetone) and 8hr water bath modification for 2 times with n-hexane at 50 deg.C for 8 hr; CN104030301B the gel is subjected to solvent replacement operation at 40 deg.C for 3hr with ethanol (or propanol) + sulfuric acid, and then subjected to solvent modification operation with n-hexane + hexamethyldisilazane for 6 hr; CN105776234A performing solvent replacement operation of gel with ethanol at 50-70 deg.C for 8-24 hr, and then performing modification operation at 45 deg.C for 7-9 hr; CN106745000A subjecting the gel to solvent replacement operation of water bath heating at 45 deg.C for 2-4 hr with "pH 6.5 ethanol (or propanol)", and then to modification operation of "n-hexane + trimethylchlorosilane (10-60 mL)" for 50-70 deg.C for 12 hr; CN106865558A the gel is modified by n-hexane + silazane for 1-6 hr, and then replaced by sulfuric acid or hydrochloric acid in water bath at 60 deg.C for 6 hr; CN109133070A grinding the gel, sequentially performing solvent replacement and modification at 40 deg.C by using deionized water (1-2 hr), ethanol (0.5-1 hr), modifier (standing for 0.5-1 hr, and collecting the upper layer) and n-hexane (20-40 min); CN104071797B the solvent of the gel is replaced by ethanol in a closed container under the condition of 45 ℃ water bath for 8hr, then the modification operation is carried out by n-hexane and 20 mL/time of trimethylchlorosilane under the condition of 45 ℃ water bath, each time of reaction lasts for 2hr and 3-4 times; i561561 cleaning aerogel particles with "mixture of ethanol and water", vacuum drying aerogel particles at high temperature, performing hydrophobic modification by heating the mixture to vaporize the organic solvent, adding a chloracylated organic molecule to react with the hydroxyl groups of the aerogel particles to make the aerogel particles hydrophobic, and dispersing solvent to prevent the aerogel particles from being broken; i516447 uses "ethanol + n-hexane" to perform solvent replacement operation at 50 deg.C for 24hr, and then uses "n-hexane + trimethylchlorosilane" to perform modification operation at 50 deg.C for 24 hr.
In the conventional silica aerogel preparation method, two operations of "replacement" and "modification" are performed, which may be the "replacement" step first and the "modification" step later; it is also possible that the "upgrading" step is performed first and the "replacing" step is performed later; it is also possible to combine the operations of "replacement" and "modification". It is understood that, in any conventional silica aerogel preparation method, whether "replacement" or "modification" or "replacement + modification" is performed by "container immersion", that is, a reaction container, also called an action container, with a suitable volume is prepared, and the reaction container can be closed or incompletely closed depending on the nature of the organic solvent, and then the wet gel is placed in the reaction container, and then a selected suitable solvent is injected to perform the desired "replacement" and "modification" operations, wherein the "replacement" operation is particularly important because the "replacement" operation takes a long time and a large amount of solvent (hence, also called "solvent replacement" operation) is used, so that the "replacement" operation can be shortened and the replacement effect can be improved, thereby facilitating the preparation of silica aerogel.
[ summary of the invention ]
The invention aims to provide the following steps: 1. a method for producing a silica aerogel is provided which can greatly accelerate (reduce) the time required for the wet gel solvent replacement operation. 2. Provides a method for preparing silica aerogel with greatly improved substitution effect. 3. Provides a preparation method of silica aerogel which is convenient to recycle and achieve the effects of saving cost, protecting environment and saving energy.
In order to achieve the above object, the present invention particularly provides a method for preparing silica aerogel, which comprises the following steps:
(1) mixing an inorganic silicon source and pure water in a container to form a solution;
(2) hydrolyzing the solution by cation exchange resin to obtain silicic acid solution;
(3) adding an alkaline catalyst to the silicic acid solution to obtain a wet gel;
(4) mincing the wet gel;
(5) adding the crushed wet gel into an alcohol solvent for aging for 1-12 hr;
(6) carrying out solvent replacement on the aged wet gel, and dividing the operation into two stages, wherein in the first stage, the aged wet gel is placed in a reaction container, an alcohol solvent is added, and the water in the pores of the wet gel is replaced by the alcohol solvent; the second stage is to replace alcohol solvent and add organic solvent into the reaction container of wet gel to replace the alcohol solvent in the pores of wet gel;
(7) surface modification is carried out on the wet gel after solvent replacement, in which a mixed solution of a siloxane modifier and an organic solvent is added into a reaction vessel of the wet gel after solvent replacement and the organic solvent to modify the wet gel from original hydrophilicity to hydrophobicity;
(8) drying the hydrophobic wet gel obtained after modification by adopting a normal pressure drying method, and preparing the silicon dioxide aerogel in a stepped heating mode;
the method is characterized in that: in the solvent replacement process of step 6, the operation of adding alcohol solvent and organic solvent is performed in a 'continuous circulation filtration mode', wherein a liquid inlet of a liquid inlet pipeline is arranged above a reaction container with wet gel arranged inside and is communicated with the reaction container, the bottom of the reaction container is connected with a liquid outlet pipeline through a liquid outlet, a filter device is arranged at the liquid outlet, the liquid outlet pipeline is communicated with a solution storage tank, a power source is arranged between the solution storage tank and the liquid inlet pipeline, the power source has the functions of sucking the liquid outlet pipeline and pushing the liquid inlet pipeline, the alcohol solvent with proper volume is stored in the solution storage tank in a first section, the organic solvent with proper volume is stored in the solution storage tank in a second section, and the alcohol solvent and the organic solvent are enabled to be in the reaction container, the liquid outlet pipeline and the solution storage tank under the action of the power source, A circulation is formed in the power source and the liquid inlet pipe, so that in the first stage, the water in the pores of the wet gel is displaced by the alcohol solvent, and in the second stage, the alcohol solvent in the pores of the wet gel is displaced by the organic solvent.
In the preparation method of the silica aerogel, in the step (1), 40 to 50 mass% of sodium silicate (commonly called as water glass) in solid content is used as an inorganic silicon source, the modulus of the sodium silicate diluted by pure water is 2.53 to 3.33, and the solid content is 8 to 20 wt%.
In the preparation method of the silica aerogel, a silicic acid solution with pH value of 2-3 is obtained in the step (2).
In the above-mentioned preparation method of silica aerogel, the alkaline catalyst in step (3) can be ammonium hydroxide (NH)4OH) or sodium hydroxide (NaOH), and adding 1M of the alkaline catalyst into silicic acid solution until the pH value of silicic acid is 4-8 to obtain wet gel.
In the above-mentioned method for preparing silica aerogel, in step (4), the wet gel is preferably crushed by a crusher at a speed of 300 to 1000 rpm.
In the preparation method of the silicon dioxide aerogel, the alcohol solvent in the step (5) is ethanol, isopropanol or methanol, the volume ratio of the alcohol solvent to the silicic acid solution is 1:1, and the wet gel after being crushed is aged for 1-12 hr at the temperature of 25-80 ℃.
In the above method for preparing silica aerogel, the alcohol solvent in step (6) is ethanol, isopropanol or methanol, the alcohol solvent is added into the reaction vessel of the wet gel, and then stirred at a speed of 300-1000 rpm for 1-12 hr at 25-80 ℃, and the operation of replacing water with the alcohol solvent is repeated for many times until the alcohol solvent concentration in the solution storage tank is reduced to a predetermined value (for example, the predetermined value is 5%, but the predetermined value is adjustable and not limited); the organic solvent is n-hexane, acetone or cyclohexane, the volume ratio of the organic solvent to the silicic acid solution is 1:1, the wet gel is added into the reaction vessel, stirred at the speed of 300-1000 rpm for 1-12 hr at the temperature of 25-80 ℃, and the operation of replacing the alcohol solvent with the organic solvent is repeated for a plurality of times until the concentration of the organic solvent in the solution storage tank is reduced to a set value (for example, the set value is 5%, but the set value is adjustable and not limited).
In the preparation method of the silica aerogel, the mixed solution of the siloxane modifier and the organic solvent is added in the step (7), and the mixture is stirred at the speed of 300-1000 rpm for 1-16 hr at the temperature of 25-80 ℃, wherein the siloxane modifier is hexamethyldisilazane, methyltrimethoxysilane or tetraethoxysilane, and the molar ratio of the siloxane modifier to silicic acid is 1: 0.35-1.67.
In the above-mentioned preparation method of silica aerogel, the step-wise heating manner in step (8) is performed at 60 deg.C, 150 deg.C and 230 deg.C for 1hr each.
In the above-mentioned method for preparing silica aerogel, the solution storage tank is filled with the alcohol solvent and the organic solvent for concentration detection by an external detecting instrument.
In the above method for preparing silica aerogel, the pH value obtained in step (2) is preferably: 2.4-2.6 of silicic acid solution.
The invention adopts a continuous circulation filtration mode that the solvent is rapidly circulated into and out of the reaction vessel and is continuously formed by a filtration procedure when the solvent is discharged, and has the advantages that:
(1) the wet gel is continuously and circularly replaced (exchanged and washed) by a large amount of alcohol solvent and organic solvent in a relatively short time, so that the time of the wet gel solvent replacement operation can be greatly accelerated (reduced).
(2) The alcohol solvent and the organic solvent which are led out of the wet gel reaction container are filtered to separate the wet gel from the solution, so that the wet gel is prevented from entering a liquid outlet pipeline, the solution is pushed into the reaction container to carry out solvent replacement on the wet gel, and the continuous circulating filtration is carried out, so that the replacement effect is greatly improved.
[ description of the drawings ]
FIG. 1 is a flow chart showing the steps of preparing silica aerogel according to the present invention.
FIG. 2 is a flow chart showing the steps of the solvent replacement operation in the production process of the present invention.
FIG. 3 is a schematic view showing the structure of the solvent replacement operation in the production process of the present invention.
The numbers in the figures illustrate the following:
[ detailed description ] embodiments
The present invention relates to a method for preparing silica aerogel, as shown in fig. 1, the method of the present invention mainly comprises the following steps:
(1) mixing an inorganic silicon source and pure water in a container to form a solution, wherein sodium silicate (commonly called water glass) with a solid content of 40-50 mass% is used as the inorganic silicon source, the modulus of the pure water for diluting the sodium silicate (commonly called water glass) is 2.53-3.33, and the solid content is 8-20 (8-20 wt%).
(2) Hydrolyzing the solution with cation exchange resin to obtain silicic acid solution with pH of 2-3, preferably pH of 2.4-2.6.
(3) Catalyzing the alkaliAdding agent into the silicic acid solution to obtain wet gel, wherein the alkaline catalyst can be ammonium hydroxide (NH)4OH) or sodium hydroxide (NaOH), preferably silicic acid solution, to a silicic acid base pH of 4 to 8 to obtain a wet gel.
(4) The wet gel is stirred and crushed, preferably the stirring speed of a stirrer is 300 to 1000 rpm;
(5) adding an alcohol solvent into the crushed wet gel, wherein the alcohol solvent is ethanol, isopropanol or methanol, adding the alcohol solvent and the silicic acid solution in a volume ratio of 1:1, and aging the crushed wet gel at the temperature of 25-80 ℃ for 1-12 hr.
(6) The method comprises the steps of (1) carrying out solvent replacement on aged wet gel, further dividing the wet gel into two stages of operation, and referring to fig. 2, in the first stage, putting the aged wet gel into a reaction container, adding an alcohol solvent, and replacing water in pores of the wet gel by the alcohol solvent, wherein the alcohol solvent is ethanol, isopropanol or methanol, the reaction container of the wet gel can be stirred at the speed of 300-1000 rpm for 1-12 hours at the temperature of 25-80 ℃ after the alcohol solvent is added, so that the replacement effect is improved, and the operation of replacing water by the alcohol solvent can be repeated for many times until the concentration of the alcohol solvent is reduced to a set value (for example, the set value is 5%, but the set value can be adjusted and is not limited); in the second stage, after the displacement of the alcohol solvent, an organic solvent is added into the reaction vessel of the wet gel to displace the alcohol solvent in the pores of the wet gel, wherein the organic solvent is n-hexane, acetone or cyclohexane, the volume ratio of the organic solvent to the silicic acid solution is 1:1, the reaction vessel of the wet gel is stirred at a speed of 300-1000 rpm for 1-12 hr at a temperature of 25-80 ℃ after the organic solvent is added, so that the displacement effect is improved, and the operation of displacing the alcohol solvent with the organic solvent can be repeated for many times until the concentration of the organic solvent is reduced to a set value (for example, the set value is 5%, but the set value can be adjusted and is not limited).
(7) And surface modification of the wet gel after solvent replacement, namely adding a mixed solution of a siloxane modifier and an organic solvent into a reaction vessel of the wet gel after solvent replacement and the organic solvent, stirring at the speed of 300-1000 rpm at 25-80 ℃ for 1-16 hours to modify the wet gel to change the original hydrophilicity into hydrophobicity, wherein the siloxane modifier can be hexamethyldisilazane, methyltrimethoxysilane or tetraethoxysilane, the molar ratio of the siloxane modifier to silicic acid is 1: 0.35-1.67, and the wet gel can be separated from the solution after modification is finished, and the solution can be recycled for other places.
(8) Drying the hydrophobic wet gel obtained after modification by adopting a normal pressure drying method, wherein the drying is carried out for 1hr in a stepped heating mode at 60 ℃, 150 ℃ and 230 ℃, so as to prepare the silicon dioxide aerogel with low thermal conductivity and high hydrophobicity.
The above is a brief description of the preparation method of silica aerogel according to the present invention, wherein the process of step 6 (solvent replacement) is the most important step for preparing silica aerogel, so the present invention improves the process of step 6 (solvent replacement) to accelerate (reduce) the operation time of solvent replacement and improve the effect of solvent replacement.
Referring to fig. 3, in the process of step 6 (solvent replacement), the conventional solvent immersion (no matter the alcohol solvent in the first stage or the organic solvent in the second stage) is changed to a "continuous circulation filtration method", for example, a liquid inlet 21 of a liquid inlet pipeline 20 above a reaction vessel 10 with wet gel is connected to the reaction vessel 10, a liquid outlet pipeline 30 is connected to the bottom of the reaction vessel 10 through a liquid outlet 31, a filtration device is disposed at the liquid outlet 31 for mainly separating wet gel from solvent and preventing wet gel from entering the liquid outlet pipeline 30 through the liquid outlet 31, so that a filter screen 32 is disposed at the simplest, the liquid outlet pipeline 30 is connected to a solution storage tank 40, and then a power source 50 (e.g., a motor-operated pressurizer) is disposed between the solution storage tank 40 and the liquid inlet pipeline 20, therefore, when wet gel (wet gel) is put into the reaction vessel 10 and solvent (no matter alcohol solvent of the first stage or organic solvent of the second stage) is added, the solvent forms a circulation in the reaction vessel 10, the liquid outlet pipe 30, the solution storage tank 40, the power source 50 and the liquid inlet pipe 20 by the action of the power source 50 for pumping the liquid outlet pipe 30, the solution storage tank 40 and the liquid inlet pipe 20, the solution (no matter alcohol solvent of the first stage or organic solvent of the second stage) is filtered by the filter screen 32 before passing through the liquid outlet pipe 30, so as to ensure that the wet gel stays in the reaction vessel 10 to separate the wet gel and water or alcohol solvent, then the solution enters the solution storage tank 40, is pumped into the reaction vessel 10 after passing through the power source 50 and the liquid inlet pipe 20, the quick circulation is repeated, so that the wet gel is replaced by a large amount of solvent in a short time, the method of rapidly circulating the solution into and out of the reaction vessel 10 and filtering the solution is a "continuous circulation filtration method" to accelerate the separation and removal of water (i.e., accelerate the removal of water for the first stage) or to separate and remove an alcohol solvent (i.e., accelerate the removal of an alcohol solvent for the second stage).
The operation of "placing a wet gel (wet gel) and adding a solvent into the reaction vessel 10" mentioned above means "filling a solvent into the reaction vessel 10", which is possible in many ways, and as the simplest expression of fig. 3, a plurality of solution tanks are provided at a high place, in which desired solvents are stored, for example: the alcohol solvent or the organic solvent, and different solvents are stored in different solution tanks, so when what solvent is needed, the liquid sending pipe is pulled from the solution tank storing the needed solvent to align with the reaction vessel 10, and then the liquid sending switch is turned on to inject the needed solvent into the reaction vessel 10, thereby achieving the operation of 'injecting the solvent into the reaction vessel 10'. However, the operation of "injecting a solvent into the reaction vessel 10" is a front-end operation of the present invention and is not within the scope of the present invention, and thus will not be described in detail.
The solution storage tank 40 can be introduced into the solution in the solution storage tank 40 by an external detection instrument 60 for concentration detection, mainly when the first stage of alcohol solvent replacement operation is performed, the detection instrument 60 detects the concentration of the alcohol solvent in the solution storage tank 40, when the concentration of the alcohol solvent in the solution storage tank 40 is detected to be reduced to a set value (for example, the set value is 5%, but the set value is adjustable and not limited), the operation is stopped and the alcohol solvent is removed, the removed alcohol solvent can be recycled by post-treatment (separation of water and the alcohol solvent), the organic solvent is injected into the reaction vessel 10, the continuous circulation filtration operation is restarted, when the second stage of organic solvent replacement operation is performed, the detection instrument 60 detects the concentration of the organic solvent in the solution storage tank 40, and when the concentration of the organic solvent in the solution storage tank 40 is detected to be reduced to a set value (for example, the set value is 5%, but the set value is adjustable and not limited), the operation is stopped and the organic solvent is removed, and likewise, the removed organic solvent may be subjected to a post-treatment (separation of the alcohol solvent from the organic solvent) to recover and reuse.
There are many possibilities for discharging the solution (for recycling and post-treatment), as shown in fig. 3, a drain pipe is disposed at the bottom of the solution storage tank 40, a recycling container is placed under the drain pipe, or a drain pipe is directly connected to a recycling tank, and then a drain switch is turned on to discharge the solution in the reaction vessel 10 to the recycling container or the recycling tank. However, the operations of solution post-processing and recycling are not included in the scope of the present invention, and therefore are not described in detail.
The invention aims at the step 6 (solvent replacement) in the preparation process of the silicon dioxide aerogel to carry out a continuous circulating filtration mode, and the effect is proved to be good through various experimental trials. In the invention, when the preparation of the silica aerogel is carried out, a continuous circulation filtration mode is carried out aiming at the step 6, and the characteristics of the final silica aerogel obtained by changing different working conditions are shown as (a) specific surface area: reach 649.29 to 727.79m2(ii)/g; (b) pore volume: 0.97-2.63 cm3Between/g; (c) the size of the holes is as follows: 5.80-14.80 nm; (d) drainage angle: 138-140 degrees; (e) density: 0.1150-0.1749 g/cm3A (c) is added; (f) porosity: 92.1-94.8%; (g) the heat transfer is as follows: 0.0299 to 0.0432W/mK; (h) the process time is 17-23 hr, which proves that the final silica aerogel has good properties.
Claims (11)
1. A preparation method of silica aerogel comprises the following steps:
(1) mixing an inorganic silicon source and pure water in a container to form a solution;
(2) hydrolyzing the solution by cation exchange resin to obtain silicic acid solution;
(3) adding an alkaline catalyst to the silicic acid solution to obtain a wet gel;
(4) mincing the wet gel;
(5) adding the crushed wet gel into an alcohol solvent for aging for 1-12 hr;
(6) carrying out solvent replacement on the aged wet gel, and dividing the operation into two stages, wherein in the first stage, the aged wet gel is placed in a reaction container, an alcohol solvent is added, and the water in the pores of the wet gel is replaced by the alcohol solvent; in the second stage, after the replacement of the alcohol solvent, adding an organic solvent into a reaction container of the wet gel to replace the alcohol solvent in pores of the wet gel;
(7) surface modification is carried out on the wet gel after solvent replacement, namely a mixed solution of a siloxane modifier and an organic solvent is added into a reaction container of the wet gel after solvent replacement and the organic solvent to modify the wet gel from original hydrophilicity to hydrophobicity;
(8) drying the hydrophobic wet gel obtained after modification by adopting a normal pressure drying method, and preparing the silicon dioxide aerogel in a stepped heating mode;
the method is characterized in that: in the solvent replacement process of step (6), the operation of adding alcohol solvent and organic solvent is performed in a 'continuous circulation filtration mode', wherein a liquid inlet of a liquid inlet pipeline is arranged above a reaction container with aged wet gel arranged inside and is communicated with the reaction container, the bottom of the reaction container is connected with a liquid outlet through a liquid outlet, a filtration device is arranged at the liquid outlet, the liquid outlet is communicated with a solution storage tank, a power source is arranged between the solution storage tank and the liquid inlet pipeline, the power source has the functions of sucking the liquid outlet pipeline and pushing the liquid inlet pipeline, the alcohol solvent with proper volume is stored in the solution storage tank in a first section, the organic solvent with proper volume is stored in the solution storage tank in a second section, and the alcohol solvent and the organic solvent are enabled to be in the reaction container under the action of the power source, A circulation is formed in the liquid outlet pipe, the solution storage tank, the power source and the liquid inlet pipe, so that in the first stage, the water in the pores of the wet gel is displaced by the alcohol solvent, and in the second stage, the alcohol solvent in the pores of the wet gel is displaced by the organic solvent.
2. The method for preparing silica aerogel according to claim 1, wherein: in the step (1), sodium silicate (commonly called water glass) with the solid content of 40-50 mass% is used as an inorganic silicon source, the modulus of the sodium silicate diluted by pure water is 2.53-3.33, and the solid content is 8-20 wt%.
3. The method for preparing silica aerogel according to claim 1, wherein: and (3) obtaining a silicic acid solution with pH value of 2-3 in the step (2).
4. The method for preparing silica aerogel according to claim 1, wherein: the alkaline catalyst in the step (3) is ammonium hydroxide (NH)4OH) or sodium hydroxide (NaOH), and adding 1M of the alkaline catalyst into silicic acid solution until the pH value of silicic acid is 4-8 to obtain wet gel.
5. The method for preparing silica aerogel according to claim 1, wherein: and (4) stirring at the speed of 300-1000 rpm by a stirrer.
6. The method for preparing silica aerogel according to claim 1, wherein: and (3) adding an alcohol solvent which is ethanol or isopropanol or methanol into the mixture obtained in the step (5) and adding the alcohol solvent and the silicic acid solution according to the volume ratio of 1:1, and aging the crushed wet gel at the temperature of 25-80 ℃ for 1-12 hours.
7. The method for preparing silica aerogel according to claim 1, wherein: the alcohol solvent in the step (6) is ethanol, isopropanol or methanol, the alcohol solvent is added into the reaction container of the wet gel, then the wet gel is stirred for 1-12 hours at the temperature of 25-80 ℃ and the speed of 300-1000 rpm, the operation of replacing water by the alcohol solvent is repeated for many times until the concentration of the alcohol solvent in the solution storage tank is reduced to a set value; and the organic solvent is n-hexane, acetone or cyclohexane, the volume ratio of the organic solvent to the silicic acid solution is 1:1, the wet gel is added into the reaction vessel, then the wet gel is stirred for 1-12 hours at the temperature of 25-80 ℃ and the speed of 300-1000 rpm, and the operation of replacing the alcohol solvent by the organic solvent is repeated for many times until the concentration of the organic solvent in the solution storage tank is reduced to a set value.
8. The method for preparing silica aerogel according to claim 1, wherein: and (7) adding a mixed solution of a siloxane modifier and an organic solvent, and stirring at the temperature of 25-80 ℃ and the speed of 300-1000 rpm for 1-16 hours, wherein the siloxane modifier is hexamethyldisilazane, methyltrimethoxysilane or tetraethoxysilane, and the molar ratio of the siloxane modifier to silicic acid is 1: 0.35-1.67.
9. The method for preparing silica aerogel according to claim 1, wherein: drying at 60 deg.C, 150 deg.C and 230 deg.C for 1hr respectively in step (8).
10. The method for preparing silica aerogel according to claim 1 or 7, characterized in that: the solution storage tank is filled with the alcohol solvent and the organic solvent of the solution storage tank by an external detection instrument for concentration detection.
11. The method for preparing silica aerogel according to claim 3, wherein: preferably, the pH value obtained in step (2) is: 2.4-2.6 of silicic acid solution.
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