Preparation Of Bulk High Strength SiO₂ Aerogel And The Study On Hydrophobic Properties

SiO₂ aerogel because of its low density and low thermal conductivity,which comes from its nano-scale porous structure,is widely used in the field of thermal insulation.However,the conventional SiO₂ aerogel has the disadvantages of high brittleness and easy moisture cracking,which makes it impossible to maintain a relatively complete block structure in practical applications.In this paper,bulk SiO₂aerogels with super-mechanical properties and superhydrophobic aerogels were prepared by sol-gel method,and the properties and structure of the samples were analyzed.First,the bulk SiO₂ aerogel was prepared by supercritical drying of liquid CO₂using tetramethoxysilane?TMOS?and 3-aminopropyltriethoxysilane?APTES?as mixed precursors.The best preparation conditions were discussed by analyzing the influence of factors such as content and silicon source concentration on the physical properties,microstructure and pore structure of bulk SiO₂ aerogel.When the molar ratio of TMOS to APTES is 9.5:0.5,the molar ratio of silicon source to water is 1:4,and the molar ratio of silicon source to acetonitrile is 1:20,the prepared SiO₂ aerogel has the best comprehensive performance,the density is 0.137 g/cm³,and the specific surface area is 565.65 m²/g.Adding methyltrimethoxysilane?MTMS?to the silicon source,it is possible to obtain a uniform mesh size SiO₂ aerogel with a larger pore size structure,and the hydrophobicity is also enhanced.Secondly,in order to improve the mechanical properties of SiO₂ aerogel,TMOS,APTES and MTMS were used as mixed precursors,and hexamethylene diisocyanate?HDI?and its trimer?N3300?were used as modifiers to reinforce isocyanate-based SiO₂aerogel,the effect of the type and concentration of the modified solution on the properties of the composite aerogel was studied.The results show that when the molar ratio of APTES,TMOS and MTMS is 2:6:2,and the modifier is N3300,the composite aerogel prepared by 5%mass fraction has better effect.At this time,the density is0.248 g/cm³,the specific surface area is 206.39 m²/g,the mechanics of the composite aerogel is increased by more than 40 times,and the compressive strength is 12.426MPa.When HDI is used as the modifier,the composite aerogel prepared by 15%mass fraction can obtain the best composite effect,the material density is 0.268 g/cm³,the specific surface area is 270.69 m²/g,and the compressive strength reaches 20.688 MPa.Finally,in order to improve the hydrophobicity of SiO₂ aerogel and reduce the manufacturing cost,a lower-priced tetraethoxysilane?TEOS?is used as the precursor,and methyltriethoxysilane?MTES?was used as the surface modifier,superhydrophobic SiO₂ aerogel was prepared by acid-base two-step method.When the molar ratio of TEOS to MTES is 1:0.75,the prepared aerogel has the best hydrophobicity,and the hydrophobic angle is as high as 136°,which maintained a relatively complete block structure in a humid environment.