Preparation Of Silica Aerogel By Surfactant-free Microemulsion System

With the country’s proposal on carbon peaking and carbon neutrality strategies,the development of emerging materials will surely become a scientific research central issue in the future.Silica aerogel is a new type of material composed of Si O₂nanoparticles aggregated to form a three-dimensional network structure.It has lower density（～0.003 g/cm³）,high poriness（80%～99%）,large specific surface area(～1200m²/g and low heat conductivity coefficient（0.01～0.02 W/（m?K））.It has a wide range of application prospects in the fields of building energy saving and thermal insulation,aerospace,catalyst carrier,high energy physics and sound noise reduction by absorption.In the great project of my country’s"dual carbon"goal,the related industry of silica aerogel will make a huge contribut on.In order to widen the preparation process system of silica aerogel and reduce the preparation cost of silica aerogel,and obtain large-size silica aerogel under normal pressure drying conditions,this paper proposes a water/ethanol/n-hexane without surfactant-free microemulsion（SFME）is a method for preparing silica aerogel from a solvent system.The composite silica source is hydrolyzed by the SFME system,and the silica aerogel is obtained by the two-step acid-base sol-gel method,aging treatment,and drying at atmospheric pressure.SFME system is the formula and process steps for preparing silica aerogel with solvent.The performance indicators of the samples were characterized and analyzed while eliminating the surface modification and tedious solvent exchange process in the preparation of silica aerogels by conventional atmospheric drying.At the same time,a SFME system bismethyl-enhanced silica aerogel product was prepared by adding bismethyl silicon source,and the addition of bismethyl silicon source significantly improved the hydrophobic properties of the silica aerogel.Specifically,the following three aspects have been studied and the relevant experimental results have been obtained.（1）Preparation and representation of water/ethanol/n-hexane SFME system.The phase diagram of the SFME system was drawn by visual titration,and the conductivity of the water/ethanol/n-hexane SFME system was measured by conductivity method.The results showed that:with an increase of n-hexane in the oil phase,the conductivity of the system presents three changing stages:the initial rapid rise stage,the middle slow decline stage,the final rapid decline stage,accordingly,the single-phase region of the water/ethanol/n-hexane SFME system can be divided into three sub-microphase regions of O/W,B.C.and W/O.At the same time,the UV-visible light absorption spectroscopy wasused totest the change of the maximum absorption wavelength of methyl orange with the mass fraction of n-hexane in the water/ethanol/n-hexane SFME system,and the results showed that the maximum absorption wavelength of methyl orange showed the initial rapid decline stage,the intermediate level stabilization stage and the final rapid decline stage with the increase of n-hexane massfraction,according to which the correctness of the conductivity method for the sub-microphase zone division of the SFME system could be verified,and a complete ternary phase diagram of the water/ethanol/n-hexane SFME system is obtained.（2）Using water/ethanol/n-hexane SFME system as solvent,because methyltrie-thoxysilane（MTES）is insoluble in water,and O/W type microemulsion is suitable for dissolving water-insoluble substances,this experiment all reaction processes were carried out in an O/W type SFME system formed by water/ethanol/n-hexane.Tetraethyl orthosilicate（TEOS）and MTES as precursors,hydrochloric acid（HCl）and ammonia water（NH₃·H₂O）as acid-base catalysts,and triethoxyoctysilane as in-situ modifier.Silica aerogel were obtained by a two-step acid-base sol-gel method through ethanol aging and step drying at atmospheric pressure.The change of the addition amount of the relevant raw materials is analyzed to get the optimal craftsmanship ratio.The optimal process ratio is obtained by analyzing the change of the addition amount of the relevant raw materials.Taking TEOS:MTES:Et OH:H₂O:n-hexane:triethoxyoctylsilane=0.25:0.75:12:10:0.1:0.07 as the best molar ratio of raw materials,the hydrolysis time is 30 min,and the hydrolysis p H is 2-3,the p H of the gel is 6-7,the aging solution is an ethanol solution with p H=8,the aging time is24 h and the aging temperature is 60°C,and the drying method is step drying at80°C,100°C and 120°C.The density of silica aerogel is 0.127 g/cm³,the porosity is 94.4%,the BJH average pore size is 6.35 nm,the average of the particle size is19.86 nm,the specific surface area is 341.56 g/cm³,the contact angle is 132.6°,the heat conductivity coefficient is 0.0503 W/（m?K）,the pore size between 2 and 20 nm account for 83.39%of the total pore volume,amorphous structure.It has good microscopic morphology and obvious infrared characteristic peaks of Si-O-Si and Si-CH₃.（3）In order to further hoist the capability of silica aerogel prepared by using SFME system as solvent,this paper added silane dimethyldiethoxylsilane containing dimethyl as reinforcement phase to prepare a SFME system of dimethyl-enhanced silica aerogel.When the amount of DEDMS is 0.08 mol,the density of silica aerogel is 0.157 g/cm³,the porosity is 93%,the average pore size of BJH is 3.92 nm,the heat conductivity coefficient is 0.0522 W/（m?K）,the average particle size is 15.78 nm,which is 20.54%lower than that without DEDMS,and the contact angle is 152.5°.It is an amorphous structure composed of mesoporous Si O₂,with good microstructure and obvious infrared characteristic peaks of Si-O-Si and Si-CH₃.The content of-OH only accounts for 0.32%of the total mass,and the pore sizes of 2-20 nm account for84.31%of the total pore volume.The hydrophobicity of the sample remains good when the temperature is below 240.5℃.When the temperature exceeds 510℃,the hydrophobicity of the sample completely disappears.