CN110818383A - m-HNTs-aluminum oxide-silicon dioxide composite aerogel and preparation method and application thereof - Google Patents
m-HNTs-aluminum oxide-silicon dioxide composite aerogel and preparation method and application thereof Download PDFInfo
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Abstract
The invention discloses an m-HNTs-alumina-silicon dioxide composite aerogel, a preparation method and application thereof, wherein the invention combines the advantages of two aerogel precursors of silicon dioxide and aluminum oxide, and adopts a means of combining a sol-gel method and supercritical drying to successfully prepare Al in order to make up the respective performance defects2O3/SiO2And (3) compounding the aerogel. Wherein SiO is2As a cross-linking phase, Al in order to obtain a three-dimensional network structure with high specific surface area junctions and porosity2O3As the reinforcing phase, the purpose is to reinforce the skeleton structure of the composite aerogel and to suppress shrinkage and densification of the composite aerogel under medium-high temperature conditions. In addition, m-HNTs is added in the experiment to be used as a nano fiber reinforced filler to prepare m-HNTs-Al2O3/SiO2The composite aerogel further improves the mechanical property and high temperature of the composite aerogelThe shrinkage resistance makes it have excellent heat resistance and ultralow high-temperature heat conductivity.
Description
Technical Field
The invention relates to the technical field of aerogels, in particular to m-HNTs (silane modified halloysite nanotube) -aluminum oxide-silicon dioxide composite aerogel and a preparation method and application thereof.
Background
SiO2However, the heat treatment of pure silica aerogel at the temperature higher than 600 ℃ can generate obvious phenomena of collapse and densification of a pore structure, which seriously influences the application of the silica aerogel in the field of heat preservation and heat insulation at the medium and high temperature (800 plus 1200 ℃), alumina has the characteristics of low heat conductivity coefficient, high catalytic activity, good heat resistance and the like, and a metastable alumina phase can be converted into α -Al with stable crystal at the temperature higher than 1150 DEG C2O3But the requirement of self-crosslinking condition is higher, so that the specific surface area of the prepared pure aerogel relative to the silica aerogel is lower, and the practical application is limited to a certain extent.
Disclosure of Invention
The invention aims to provide m-HNTs-alumina-silicon dioxide composite aerogel, a preparation method and application thereof aiming at technical defects in the prior art.
The technical scheme adopted for realizing the purpose of the invention is as follows:
the invention relates to an m-HNTs-alumina-silicon dioxide composite aerogel, wherein the m-HNTs are silane modified halloysite nanotubes with the mass percent of 4-20 wt%, the mass percent of alumina is 40-48 wt%, the mass percent of silicon dioxide is 40-48 wt%,the m-HNTs-Al2O3-SiO2The density of the composite aerogel is 0.20-0.30g/cm3Porosity of 97-99%, pore volume of 1.2-1.6cm3/g。
In the technical scheme, the mass percent of the m-HNTs is 10-15%, and preferably 15%.
In the technical scheme, the m-HNTs-alumina-silicon dioxide composite aerogel is prepared according to the following method:
and 2, adding tetraethoxysilane into an ethanol solution obtained by mixing absolute ethyl alcohol and deionized water, wherein the mass ratio of the absolute ethyl alcohol to the deionized water is (7-9): 1, stirring to mix uniformly, magnetically stirring for 15-35min at 30-50 ℃ to obtain silica sol, adding emulsified and ultrasonically dispersed m-HNTs, and uniformly dispersing to obtain m-HNTs-silica sol;
step 3, uniformly mixing the alumina sol obtained in the step 1 and the m-HNTs-silica sol obtained in the step 2 according to the mass ratio of 1 (1-2), standing for 10-30min to obtain m-HNTs-Al2O3-SiO2Compounding the wet gel;
step 4, dissolving sodium bicarbonate in an ethanol water solution to obtain a saturated soaking solution, and adding the m-HNTs-Al obtained in the step 32O3-SiO2Placing the composite wet gel into the soaking solution, soaking for 10-20min to remove excessive glacial acetic acid, and performing solvent replacement with anhydrous ethanol to obtain treated m-HNTs-Al2O3-SiO2Compounding the wet gel;
In the above technical scheme, the m-HNTs in step 2 is prepared by the following steps: adding halloysite nanotube HNTs into ethanol, performing ultrasonic dispersion to obtain HNTs suspension, adding isooctyltriethoxysilane into DMF solution, stirring, pouring into the HNTs suspension, wherein the mass ratio of isooctyltriethoxysilane to NHTs is 1 (3-5), reacting the obtained mixed solution in a water bath at 70-80 ℃ for 2-4h, washing with absolute ethanol, and performing vacuum drying for 5-7h to obtain m-HNTs.
In the technical scheme, after sintering at 1000 ℃, the density of the m-HNTs-alumina-silicon dioxide composite aerogel is 0.30-0.32g/cm3The specific surface area is 285-300m2Per g, average pore diameter of 10-11nm and pore volume of 1.13-1.19cm3/g。
In the technical scheme, the m-HNTs-Al2O3-SiO2The compression strength of the composite aerogel before sintering is 0.75-0.95 MPa, and the compression strength of the composite aerogel after sintering at 1000 ℃ is 1.45-1.65 MPa.
In the technical scheme, the m-HNTs-Al2O3-SiO2The thermal conductivity coefficient of the composite aerogel before sintering is 0.024-0.026W/mK, and the thermal conductivity coefficient after sintering at 1000 ℃ is 0.028-0.032W/mK.
In the technical scheme, the m-HNTs-Al2O3-SiO2The compression strength of the composite aerogel before sintering is 0.75-0.95 MPa, the compression strength of the composite aerogel after sintering at 1000 ℃ is 1.45-1.65 MPa, and the m-HNTs-Al2O3-SiO2The thermal conductivity coefficient of the composite aerogel before sintering is 0.024-0.026W/mK, and the thermal conductivity coefficient after sintering at 1000 ℃ is 0.028-0.032W/mK.
In another aspect of the invention, a preparation method of the m-HNTs-alumina-silicon dioxide composite aerogel is characterized by comprising the following steps:
and 2, adding tetraethoxysilane into an ethanol solution obtained by mixing absolute ethyl alcohol and deionized water, wherein the mass ratio of the absolute ethyl alcohol to the deionized water is (7-9): 1, stirring to mix uniformly, magnetically stirring for 15-35min at 30-50 ℃ to obtain silica sol, adding emulsified and ultrasonically dispersed m-HNTs, and uniformly dispersing to obtain m-HNTs-silica sol;
step 3, uniformly mixing the alumina sol obtained in the step 1 and the m-HNTs-silica sol obtained in the step 2 according to the mass ratio of 1 (1-2), standing for 10-30min to obtain m-HNTs-Al2O3-SiO2Compounding the wet gel;
step 4, dissolving sodium bicarbonate in an ethanol water solution to obtain a saturated soaking solution, and adding the m-HNTs-Al obtained in the step 32O3-SiO2Placing the composite wet gel into the soaking solution, soaking for 10-20min to remove excessive glacial acetic acid, and performing solvent replacement with anhydrous ethanol to obtain treated m-HNTs-Al2O3-SiO2Compounding the wet gel;
In another aspect of the invention, the application of m-HNTs in improving the mechanical property and the thermodynamic property of the alumina-silicon dioxide composite aerogel is characterized in that the m-HNTs is prepared by the following steps: adding halloysite nanotube HNTs into ethanol, performing ultrasonic dispersion to obtain HNTs suspension, adding isooctyltriethoxysilane into DMF solution, stirring, pouring into the HNTs suspension, wherein the mass ratio of isooctyltriethoxysilane to NHTs is 1 (3-5), reacting the obtained mixed solution in a water bath at 70-80 ℃ for 2-4h, washing with absolute ethanol, and performing vacuum drying for 5-7h to obtain m-HNTs.
Compared with the prior art, the invention has the beneficial effects that:
1. combining the advantages of two aerogel precursors of silicon dioxide and aluminum oxide, and adopting a sol-gel method and supercritical drying combined method to successfully prepare Al in order to make up the respective performance deficiencies2O3/SiO2And (3) compounding the aerogel.
2.SiO2As a cross-linking phase, Al in order to obtain a three-dimensional network structure with high specific surface area junctions and porosity2O3As the reinforcing phase, the purpose is to reinforce the skeleton structure of the composite aerogel and to suppress shrinkage and densification of the composite aerogel under medium-high temperature conditions.
3. Adding modified halloysite nanotubes (m-HNTs) as nano-fiber reinforced filler to prepare m-HNTs-Al2O3/SiO2The composite aerogel further improves the mechanical property and the high-temperature shrinkage resistance of the composite aerogel, so that the composite aerogel has excellent heat resistance and ultralow high-temperature heat conductivity.
Drawings
FIG. 1 is pure SiO2Aerogel and different Al2O3Content of Al2O3/SiO2SEM image of composite aerogel (0 wt% for a, 15 wt% for b, and 50 wt% for c).
FIG. 2 shows pure SiO after heat treatment at 1000 deg.C2Aerogel and Al2O3/SiO2Aerogel (Al)2O3Content 40 wt.%) SEM image.
FIG. 3 is Al2O3/SiO2Aerogel and m-HNTs-Al2O3/SiO2Scanning electron microscope images of aerogel before and after 1000 ℃ heat treatment.
FIG. 4 pure SiO2Aerogel, Al2O3/SiO2Composite aerogel and m-HNTs-Al2O3/SiO2N of composite aerogel2Adsorption-desorption curve (a) and pore size distribution curve (b).
FIG. 5 shows the heat-treated pure SiO2 aerogel, Al2O3/SiO2 compositeAerogels and m-HNTs-Al2O3/SiO2N2 adsorption-desorption curve (a) and pore size distribution curve (b) of the composite aerogel.
FIG. 6 shows pure SiO2 aerogel, Al2O3/SiO2 composite aerogel, m-HNTs-Al after heat treatment at 1000 deg.C2O3/SiO2Bar graph of compressive strength of composite aerogel.
FIG. 7 shows pure SiO2 aerogel, Al2O3/SiO2 composite aerogel, m-HNTs-Al before and after heat treatment at 1000 deg.C2O3/SiO2Bar graph of thermal conductivity of the composite aerogel.
Detailed Description
The invention is described in further detail below with reference to the figures and specific examples. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.
Experiment raw materials required for the experiment:
experimental Main sol precursors of Tetraethoxysilane (TEOS) and aluminum sec-butoxide (ASB) are purchased from Guangdong fine chemical research Co., Ltd, Tianjin; the analytical pure medicines such as absolute ethyl alcohol (EtOH), glacial acetic acid, sodium bicarbonate, hydrochloric acid, ammonia water, deionized water and the like are provided by Baiotai scientific and technological development Limited company of Tianjin. The experiment was carried out using commercially available reagents without further purification.
The required laboratory equipment and equipment are listed in table 1.
TABLE 1 Experimental instrumentation
The characterization method comprises the following steps:
scanning Electron Microscope (SEM): the sample cut into small pieces was attached to a sample stand with a conductive adhesive, subjected to gold plating treatment with a precision etching plating apparatus (Gatan-682 type), and then the inside and outside shapes of the sample aerogel were observed under an accelerating voltage of 5.0kV using a field emission scanning electron microscope (S-4800 type) of Hitachi (Hitachi) corporation to obtain photographs.
And (3) analyzing and characterizing the pore structure: a specific surface area and pore diameter tester (3H-2000PS1 model) of Beijing Behcard instruments science and technology Limited performs a nitrogen adsorption-desorption experimental test on a sample. The samples were dried in advance at 150 ℃ for 3h under vacuum before testing. The specific surface area of the analysis sample was calculated according to the Brunauer-Emmett-Teller (BET) method, and the pore size distribution was calculated according to the Barrett-Joyner-Halenda (BJH) method.
Preparation of alumina sol:
in the preparation process of the composite gel, the preparation of the alumina sol is very difficult, mainly because the alkoxy group in the secondary butanol aluminum has very strong electronegativity, and under the condition that the secondary butanol aluminum is directly contacted with water, aluminum atoms are very easy to be attacked by nucleophilicity, and are hydrolyzed with free-OH in the water at a very high hydrolysis speed, so that alumina precipitates are generated, and uniform and clear alumina sol cannot be obtained.
The main effect of adding glacial acetic acid is to perform chelation with aluminum sec-butoxide before the aluminum sec-butoxide is rapidly hydrolyzed with water, occupying the hydrolysis space site of aluminum sec-butoxide, so that the aluminum sec-butoxide cannot be fully hydrolyzed. The main reaction process between acetate ions and aluminum sec-butoxide in the sol is as follows:
analysis of the reaction scheme of aluminum sec-butoxide and glacial acetic acid shows that acetate ions can be chelated with aluminum sec-butoxide in a bidentate manner to replace one alkoxy group in aluminum sec-butoxide, so that a steric site is occupied by aluminum atoms, and a relatively stable complex or a reticular macromolecular group is formed. The generated complex has strong inertia to hydrolysis and condensation reaction, and can effectively inhibit the growth and agglomeration of nano particles, so the hydrolysis degree of the aluminum sec-butoxide can be controlled by adjusting the content of the glacial ethanol, and the aim of preparing uniform and clear alumina sol is fulfilled.
When the content of the glacial acetic acid is lower, the glacial acetic acid can not play a role in inhibiting the rapid hydrolysis and condensation of the aluminum sec-butoxide, and aluminum hydroxide precipitate can still be formed in the mixed solution; when the content of the glacial acetic acid is high, the excessive glacial acetic acid can perform substitution reaction with three alkoxy groups of the sec-aluminum butoxide, on one hand, the gel time process can be caused, the composite preparation process flow is long, on the other hand, the excessive acetate in the gel can react with the formed alumina gel to generate aluminum acetate, and the formed three-dimensional network structure of the gel is damaged in the subsequent surface modification and solvent replacement processes. In addition, the proportion of the absolute ethyl alcohol plays a critical role in the rapid preparation of the alumina sol, the aluminum sec-butoxide is hydrolyzed too fast and the sol formation time is too long due to the excessively high proportion of the absolute ethyl alcohol, and the reactants cannot be subjected to sufficient chemical reaction due to the too low proportion of the absolute ethyl alcohol, so that the quality of the alumina sol is influenced. Table 2 shows the raw material ratio of alumina sol.
TABLE 2 Al prepared from different raw material ratios (molar ratios)2O3Sol characteristics and gel time
Comparative example 1
Al2O3-SiO2Preparing aerogel:
step 2, adding 2g of ethyl orthosilicate into an ethanol solution obtained by mixing 4g of anhydrous ethanol and 0.5g of deionized water, stirring to uniformly mix the solution, and magnetically stirring for 20min under the condition of a water bath at 35 ℃ to obtain silica sol;
step 3, stirring and mixing the two sols obtained in the step 1 and the step 2 according to the mass ratio of 1:1, and uniformly mixingThen quickly pouring into a cylindrical and square polytetrafluoroethylene mould to obtain Al within ten minutes2O3-SiO2Compounding the wet gel.
And 4, dissolving sodium bicarbonate in an ethanol water solution to obtain a saturated solution, putting the wet gel into the solvent, soaking for 10min to remove excessive glacial acetic acid, and then replacing the solvent with absolute ethanol for multiple times.
And 5, putting the obtained wet gel into supercritical drying equipment, wherein the drying temperature and the drying pressure are respectively set to be 45 ℃ and 8.0 MPa. Firstly, the wet gel is soaked in liquid carbon dioxide for about 1h, then the flow valve is opened to break the extraction equilibrium state, and the extracted solvent is discharged, and the pressure is kept unchanged all the time in the process. The drying time can be determined according to the amount of the composite wet gel, and the drying time in the experiment is about 8 h. Finally, after the test is finished, the gas in the equipment is discharged to ensure that the pressure reaches the normal pressure state, and the autoclave of the equipment is opened to take out the dried sample to obtain Al2O3-SiO2An aerogel.
FIG. 1 is pure SiO2Aerogel and different Al2O3Content of Al2O3/SiO2SEM image of composite aerogel (0 wt% for a, 15 wt% for b, and 50 wt% for c). From 1a we can see that pure SiO is in the micro-morphology2The aerogel is formed by mutually connecting and stacking spherical nano particles to form a three-dimensional porous structure; along with Al in the aerogel component composition2O3With increasing content (fig. 1b, 1c), the spherical particles gradually change to irregular polyhedral particles, the diameter of the particles increases, and the nano three-dimensional porous structure of the aerogel is still maintained. The reason for this change in morphology is probably that during the gel formation, the Al-OH component is first encapsulated in SiO2The surfaces, subsequently cross-linked to each other, form a porous structure.
In the sol-gel process, the junction area between the gel frameworks (called the neck region) is the dissolved and precipitated SiO formed and grown by the agglomeration of particles during gelation2At a certain point of time. For this reason, in SiO2The aerogel spherical particles have only a few Si-O-Si bonds fused together between these neck regions, which makes the gel network very weak. The coating of the alumina can change the particle form of the surface nano particles, thereby increasing the contact area of the neck region and improving the SiO to a certain extent2The inherent brittleness of aerogels also limits SiO2The aerogel is heat treated above 600 ℃ to sinter the collapse of the neck region.
FIG. 2 is pure SiO2Aerogel, Al2O3Al in an amount of 50 wt%2O3/SiO2Scanning electron microscope picture (a is SiO) of composite aerogel after high-temperature heat treatment for 2h2Aerogel is treated at 800 ℃ for 2h, and b-d are respectively Al2O3/SiO2After the composite aerogel is treated at 800,900 and 1000 ℃ for 2 h). As can be seen from FIG. 2a, pure SiO2After the aerogel is sintered at 800 ℃, the original nano three-dimensional network structure of the aerogel is seriously damaged, pores collapse, and the whole aerogel is converted to a densified ceramic direction; and Al in FIGS. 2b-d2O3/SiO2After the composite aerogel is sintered under different high-temperature conditions, the agglomeration and ceramic phenomena appear on the part of the micro-morphology, the particle diameter and the pore size are increased, but the three-dimensional porous network structure can still be kept to a greater extent, which is formed by partial collapse and coke connection of the porous structure beam wall in the sintering process.
Example 1
m-HNTs-Al2O3-SiO2Preparing aerogel:
step 2, adding 2g of ethyl orthosilicate into an ethanol solution obtained by mixing 4g of absolute ethanol and 0.5g of deionized water, stirring to uniformly mix the solution, magnetically stirring for 20min under the condition of a water bath at 35 ℃ to obtain silica sol, adding an m-HNTs-absolute ethanol solution into the silica sol, and uniformly dispersing to obtain m-HNTs-silica sol, wherein the mass ratio of m-HNTs to silica is 1: 2;
wherein the preparation method of the m-HNTs comprises the following steps: 5g of NHTs and 100mL of ethanol were placed in a three-necked flask and ultrasonically dispersed for 15min to obtain a suspension of HNTs. A small amount of isooctyltriethoxysilane was added to 10ml of DMF solution, stirred for 5min and poured into the suspension of HNTs. And (3) reacting the mixed solution for 3 hours in a water bath at the temperature of 75 ℃, washing for 3 times by using absolute ethyl alcohol, and drying for 6 hours in vacuum to obtain the m-HNTs.
Step 3, stirring and mixing the two sols obtained in the step 1 and the step 2 according to the mass ratio of 1:1, quickly pouring the two sols into cylindrical and square polytetrafluoroethylene molds after uniform mixing, and obtaining Al within ten minutes2O3-SiO2Compounding the wet gel.
And 4, dissolving sodium bicarbonate in an ethanol water solution to obtain a saturated solution, putting the wet gel into the solvent, soaking for 10min to remove excessive glacial acetic acid, and then replacing the solvent with absolute ethanol for multiple times.
And 5, putting the obtained wet gel into supercritical drying equipment, wherein the drying temperature and the drying pressure are respectively set to be 45 ℃ and 8.0 MPa. Firstly, the wet gel is soaked in liquid carbon dioxide for about 1h, then the flow valve is opened to break the extraction equilibrium state, and the extracted solvent is discharged, and the pressure is kept unchanged all the time in the process. The drying time can be determined according to the amount of the composite wet gel, and the drying time in the experiment is about 8 h. Finally, after the test is finished, the gas in the equipment is discharged to ensure that the pressure reaches the normal pressure state, the high-pressure kettle of the equipment is opened to take out the dried sample to obtain the m-HNTs-Al2O3-SiO2An aerogel.
FIG. 3 is Al2O3/SiO2Composite aerogel (Al)2O3And SiO2In a mass ratio of 1:1) and m-HNTs-Al2O3-SiO2Composite aerogels (m-HNTs, Al)2O3、SiO2The mass ratio of the three is 2:4:4) scanning electron microscope picture before and after sintering at 1000 ℃ for 2hAs can be seen from FIG. 3b, HNTs are uniformly dispersed in m-HNTs-Al2O3/SiO2In the matrix of the composite aerogel, as can be seen from fig. 3c and 3d, the presence of m-HNTs can effectively prevent the composite aerogel from locally agglomerating during sintering, and the sample still has a relatively complete and uniform composite three-dimensional porous structure after heat treatment at 1000 ℃, however, due to the presence of m-HNTs, m-HNTs-Al2O3/SiO2Composite aerogel compared with Al2O3/SiO2The composite aerogel has larger pore size
FIG. 4a shows pure SiO before sintering at 1000 deg.C2Aerogel, Al2O3/SiO2Composite aerogel (Al)2O3And SiO2In a mass ratio of 1:1) and m-HNTs-Al2O3/SiO2Composite aerogels (m-HNTs, Al)2O3、SiO2The mass ratio of the three is 2:4:4)2And (3) an adsorption-desorption isotherm, wherein isotherms of three curves in an IUPAC (international ammonium PAC) division diagram are typical IV hysteresis loops, which shows that the three different aerogels have conical or biconical tubular nano-pore structures and belong to typical mesoporous materials. Furthermore, with pure SiO2Aerogel phase comparison, Al2O3And m-HNTs both reduced the N of the aerogel samples to some extent2Total amount of adsorption.
FIG. 4b shows pure SiO before sintering at 1000 deg.C2Aerogel, Al2O3/SiO2Composite aerogel and m-HNTs-Al2O3/SiO2Pore size distribution of the composite aerogel. From the analysis of the figure, Al2O3The addition of (A) has less influence on the pore size distribution of the obtained composite aerogel, the average pore size is slightly reduced, and the specific surface area is reduced, mainly because Al2O3Attached to SiO2The contact surface among the nano particles is increased due to the particle surface, and the neck area among the particles is partially thickened so that the whole pore space is reduced; with the addition of the m-HNTs component, m-HNTs-Al2O3/SiO2The density of the composite aerogel is increased, the specific surface area and the pore volume are further reduced, and the average pore diameter is increased. Produce thisThe reason for this phenomenon may be that m-HNTs occupy a part of the original composite aerogel space and have much smaller specific surface area and pore volume than Al2O3/SiO2In addition to composite aerogels, the overlap between m-HNTs may result in m-HNTs-Al2O3/SiO2The three-dimensional porous structure of the composite aerogel has pores with larger sizes. Table 3 shows pure SiO before sintering2Aerogel, Al2O3/SiO2Composite aerogel and m-HNTs-Al2O3/SiO2A pore structure data list of the composite aerogel.
TABLE 3
FIG. 5a shows pure SiO after sintering at 1000 deg.C2Aerogel, Al2O3/SiO2Composite aerogel and m-HNTs-Al2O3/SiO2N ═ adsorption-desorption isotherm of composite aerogel, in the figure Al2O3/SiO2Composite aerogel, m-HNTs-Al2O3/SiO2The adsorption loop of the composite aerogel sample is similar to the IV type magnetic hysteresis loop (IUPAC), which indicates that considerable mesoporous pores exist in the pore structures of the two samples, and N is2The adsorption-desorption isotherms did not have a clear smooth step at the high pressure stage, indicating that there were many large-sized pores in the pore structure of both samples. However, pure SiO2N obtained after aerogel sintering2The adsorption-desorption isotherm is similar to the type I hysteresis loop (IUPAC) and the total adsorption of the sample is small, indicating pure SiO2The aerogel nano-pore structure is seriously damaged after being sintered at 1000 ℃, the whole structure is densified, and only a small amount of micropores exist.
FIG. 5b shows pure SiO after sintering at 1000 deg.C2Aerogel, Al2O3/SiO2Composite aerogelAnd m-HNTs-Al2O3/SiO2Pore size distribution of the composite aerogel, Table 4 is pure SiO after sintering2Aerogel, Al2O3/SiO2Composite aerogel and m-HNTs-Al2O3/SiO2A pore structure data list of the composite aerogel. As can be seen from the analysis of the figure and table, Al2O3The addition of the composite aerogel can effectively inhibit the pore structure damage of the composite aerogel under the early high-temperature condition, can effectively prevent the densification of particles to a great extent, and keeps the higher specific surface area and porosity of the composite aerogel, and the reason for the phenomenon is that Al2O3Wrapped in SiO2Particle surface, significantly hindering SiO2The contact between particles, meanwhile, the addition of the m-HNTs is also beneficial to the reservation of a nano porous structure, on one hand, because the m-HNTs fiber can be used as a structural support framework to inhibit the shrinkage of the composite aerogel structure, and on the other hand, the uniformly dispersed m-HNTs can reduce the caking and densification phenomena of the composite aerogel under the high-temperature condition as much as possible. This is consistent with previous findings from scanning electron microscopy.
TABLE 4 pure SiO after sintering2Aerogel, Al2O3/SiO2Composite aerogel and m-HNTs-Al2O3/SiO2Pore structure data List for composite aerogels
FIG. 6 shows pure SiO before and after sintering at 1000 deg.C2Aerogel, Al2O3/SiO2Composite aerogels, m-HNTs-Al2O3/SiO2Bar graph of compressive strength of composite aerogel. Pure SiO before sintering at 1000 deg.C2The mechanical properties of aerogels are extremely poor (as low as 0.04MPa), determined by their own unique pore structure; al (Al)2O3The mechanical property of the material is improved to a certain extent by the doping, and the compressive strength is about 0.12 MPa; the mechanical property of the composite aerogel can be obviously enhanced by adding the m-HNTs, and the compression strength can reach 0.85 MPa. This is because of Al2O3Is stored inThe neck connection among the aerogel secondary particle particles can be enhanced, and the m-HNTs as the nanofiber filler plays a role in supporting the network framework of the composite aerogel. Pure SiO after sintering at 1000 deg.C2The mechanical property of the aerogel is obviously enhanced, the compressive strength reaches 1.97MPa, and Al2O3/SiO2And m-HNTs-Al2O3/SiO2The compressive strength of the composite aerogel is also enhanced, and is respectively 0.56MPa and 1.56 MPa. The reason for this can be attributed to the shrinkage densification of the aerogel material during sintering, pure SiO2The aerogel shrinks seriously, the pore structure is completely destroyed, and the whole body is changed to the direction of compact ceramics, so that the strength is increased greatly; and Al2O3/SiO2And m-HNTs-Al2O3/SiO2After the composite aerogel is sintered, only part of the pore structure is destroyed, the whole shrinkage of the sample is small, the mechanical strength is increased to some extent, and the increase amplitude is small.
FIG. 7 shows pure SiO before and after sintering at 1000 deg.C2Aerogel, Al2O3/SiO2Composite aerogels, m-HNTs-Al2O3/SiO2Bar graph of thermal conductivity of the composite aerogel. Pure SiO before sintering2Aerogel, Al2O3/SiO2Composite aerogel, m-HNTs-Al2O3/SiO2The thermal conductivity coefficients of the composite aerogel are 0.019W/mK, 0.023W/mK and 0.025W/mK respectively, which indicates that Al2O3And the addition of m-HNTs both result in a reduction in the aerogel's thermal insulation performance; the thermal conductivity of all samples increased after sintering, with pure SiO2The aerogel has the largest thermal conductivity (about 5-6 times of that of a sample before sintering), and Al2O3/SiO2Composite aerogel, m-HNTs-Al2O3/SiO2The thermal conductivity coefficient of the composite aerogel is not obviously increased, and is respectively 0.035W/mK and 0.03W/mK, which are far less than that of pure SiO2Thermal conductivity of the aerogel after sintering, which indicates Al2O3And the addition of the m-HNTs is beneficial to improving the heat insulation performance of the composite aerogel under the high-temperature condition. This phenomenon is also due to Al2O3The m-HNTs can be effectively reducedDestruction of aerogel three-dimensional pore structure at high temperature, Al2O3Can block SiO2Contact between particles, thereby suppressing SiO during sintering2The m-HNTs as nanofibers can support the three-dimensional pore structure without collapse.
The foregoing is only a preferred embodiment of the present invention, and it should be noted that, for those skilled in the art, various modifications and decorations can be made without departing from the principle of the present invention, and these modifications and decorations should also be regarded as the protection scope of the present invention.
Claims (10)
1. The m-HNTs-alumina-silicon dioxide composite aerogel is characterized in that the m-HNTs are silane modified halloysite nanotubes with the mass percent of 4-20 wt%, the mass percent of alumina is 40-48 wt%, the mass percent of silicon dioxide is 40-48 wt%, and the m-HNTs-Al is2O3-SiO2The density of the composite aerogel is 0.20-0.30g/cm3Porosity of 97-99%, pore volume of 1.2-1.6cm3/g。
2. The m-HNTs-alumina-silica composite aerogel according to claim 1, wherein the mass percentage of m-HNTs is 10-15%, preferably 15%.
3. The m-HNTs-alumina-silica composite aerogel according to claim 1, prepared according to the following method:
step 1, mixing the raw materials in a mass ratio of 1: (0.1-0.2) weighing absolute ethyl alcohol and glacial acetic acid to prepare a mixed solution, then dropwise adding aluminum sec-butoxide into the mixed solution, wherein the molar ratio of the aluminum sec-butoxide to the absolute ethyl alcohol to the glacial acetic acid is 1 (7-9) to (1.5-2.5), stirring to obtain a clear solution, and stirring the solution at 30-50 ℃ for 15-35min to obtain uniform and clear alumina sol;
and 2, adding tetraethoxysilane into an ethanol solution obtained by mixing absolute ethyl alcohol and deionized water, wherein the mass ratio of the absolute ethyl alcohol to the deionized water is (7-9): 1, stirring to mix uniformly, magnetically stirring for 15-35min at 30-50 ℃ to obtain silica sol, adding emulsified and ultrasonically dispersed m-HNTs, and uniformly dispersing to obtain m-HNTs-silica sol;
step 3, uniformly mixing the alumina sol obtained in the step 1 and the m-HNTs-silica sol obtained in the step 2 according to the mass ratio of 1 (1-2), standing for 10-30min to obtain m-HNTs-Al2O3-SiO2Compounding the wet gel;
step 4, dissolving sodium bicarbonate in an ethanol water solution to obtain a saturated soaking solution, and adding the m-HNTs-Al obtained in the step 32O3-SiO2Placing the composite wet gel into the soaking solution, soaking for 10-20min to remove excessive glacial acetic acid, and performing solvent replacement with anhydrous ethanol to obtain treated m-HNTs-Al2O3-SiO2Compounding the wet gel;
step 5, the processed m-HNTs-Al obtained in the step 4 is treated2O3-SiO2Supercritical drying the composite wet gel at 40-50 deg.C and 7.0-9.0MPa to obtain m-HNTs-Al2O3-SiO2And (3) compounding the aerogel.
4. The m-HNTs-alumina-silica composite aerogel according to claim 3, wherein in step 2 the m-HNTs are prepared by: adding halloysite nanotube HNTs into ethanol, performing ultrasonic dispersion to obtain HNTs suspension, adding isooctyltriethoxysilane into DMF solution, stirring, pouring into the HNTs suspension, wherein the mass ratio of isooctyltriethoxysilane to NHTs is 1 (3-5), reacting the obtained mixed solution in a water bath at 70-80 ℃ for 2-4h, washing with absolute ethanol, and performing vacuum drying for 5-7h to obtain m-HNTs.
5. The m-HNTs-alumina-silica composite aerogel of claim 1, wherein said m-HNTs-tris after sintering at 1000 ℃The density of the aluminum oxide-silicon dioxide composite aerogel is 0.30-0.32g/cm3The specific surface area is 285-300m2Per g, average pore diameter of 10-11nm and pore volume of 1.13-1.19cm3/g。
6. The m-HNTs-alumina-silica composite aerogel of claim 1, wherein said m-HNTs-Al is2O3-SiO2The compression strength of the composite aerogel before sintering is 0.75-0.95 MPa, and the compression strength of the composite aerogel after sintering at 1000 ℃ is 1.45-1.65 MPa.
7. The m-HNTs-alumina-silica composite aerogel of claim 1, wherein said m-HNTs-Al is2O3-SiO2The thermal conductivity coefficient of the composite aerogel before sintering is 0.024-0.026W/mK, and the thermal conductivity coefficient after sintering at 1000 ℃ is 0.028-0.032W/mK.
8. Use of m-HNTs-alumina-silica composite aerogels according to claim 1, characterised in that said m-HNTs-Al is2O3-SiO2The compression strength of the composite aerogel before sintering is 0.75-0.95 MPa, the compression strength of the composite aerogel after sintering at 1000 ℃ is 1.45-1.65 MPa, and the m-HNTs-Al2O3-SiO2The thermal conductivity coefficient of the composite aerogel before sintering is 0.024-0.026W/mK, and the thermal conductivity coefficient after sintering at 1000 ℃ is 0.028-0.032W/mK.
9. A preparation method of m-HNTs-alumina-silicon dioxide composite aerogel is characterized by comprising the following steps:
step 1, mixing the raw materials in a mass ratio of 1: (0.1-0.2) weighing absolute ethyl alcohol and glacial acetic acid to prepare a mixed solution, then dropwise adding aluminum sec-butoxide into the mixed solution, wherein the molar ratio of the aluminum sec-butoxide to the absolute ethyl alcohol to the glacial acetic acid is 1 (7-9) to (1.5-2.5), stirring to obtain a clear solution, and stirring the solution at 30-50 ℃ for 15-35min to obtain uniform and clear alumina sol;
and 2, adding tetraethoxysilane into an ethanol solution obtained by mixing absolute ethyl alcohol and deionized water, wherein the mass ratio of the absolute ethyl alcohol to the deionized water is (7-9): 1, stirring to mix uniformly, magnetically stirring for 15-35min at 30-50 ℃ to obtain silica sol, adding emulsified and ultrasonically dispersed m-HNTs, and uniformly dispersing to obtain m-HNTs-silica sol;
step 3, uniformly mixing the alumina sol obtained in the step 1 and the m-HNTs-silica sol obtained in the step 2 according to the mass ratio of 1 (1-2), standing for 10-30min to obtain m-HNTs-Al2O3-SiO2Compounding the wet gel;
step 4, dissolving sodium bicarbonate in an ethanol water solution to obtain a saturated soaking solution, and adding the m-HNTs-Al obtained in the step 32O3-SiO2Placing the composite wet gel into the soaking solution, soaking for 10-20min to remove excessive glacial acetic acid, and performing solvent replacement with anhydrous ethanol to obtain treated m-HNTs-Al2O3-SiO2Compounding the wet gel;
step 5, the processed m-HNTs-Al obtained in the step 4 is treated2O3-SiO2Supercritical drying the composite wet gel at 40-50 deg.C and 7.0-9.0MPa to obtain m-HNTs-Al2O3-SiO2And (3) compounding the aerogel.
The application of m-HNTs in improving the mechanical property and the thermodynamic property of the aluminum oxide-silicon dioxide composite aerogel is disclosed, wherein the m-HNTs is prepared by the following steps: adding halloysite nanotube HNTs into ethanol, performing ultrasonic dispersion to obtain HNTs suspension, adding isooctyltriethoxysilane into DMF solution, stirring, pouring into the HNTs suspension, wherein the mass ratio of isooctyltriethoxysilane to NHTs is 1 (3-5), reacting the obtained mixed solution in a water bath at 70-80 ℃ for 2-4h, washing with absolute ethanol, and performing vacuum drying for 5-7h to obtain m-HNTs.
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