Experimental Study On The Sol-gel Reaction Conditions During The Synthesis Of Silica Aerogel

Silica aerogel is a promising next-generation thermal insulation material.Its skeleton consists of three-dimensional hierarchy nana-pore structure,with a size on the order of nanometers.The hierarchy nano-structure leads to many excellent properties including superhigh specific surface area,high porosity,high surface activity and extremely low thermal conductivity.Yet the application of silica aerogels is limited because of the complication of preparation process,the low yield ratio of the product,and the high cost for producing silica aerogels.The latter is mainly due to the insufficient stability and ductility of silica aerogels.Therefore,for large scale application it is critical to reduce the cost of silica aerogel production,to simplify the preparation process of silica aerogel,and to improve the thermal stability and mechanical properties of silica aerogel.To pursue these goals,this master’s degree candidate conducted a comprehensive study,which is outlined as follows:A complete and crack-free SiO2 aerogel was prepared after processing common silicon source tetraethyl orthosilicate(TEOS)by related processes.Adjust the molar ratio when preparing silica aerogel.A hydrophobic silica aerogel with the highest porosity of 94.8%,density of 0.11 g/cm3,average pore diameter of 15.35nm,specific surface area of 498 m2/g and thermal conductivity of 0.028 W/(m·K)was obtained.Using cheap water glass as the silicon source,a new preparation process has been developed,and a massive,crack-free silica aerogel has been obtained.To study the influence factors of silica aerogel prepared by single-step acid catalysis method and multi-step acid catalysis method on the performance of silica aerogel by comparing different reference groups.The influence of the solution pH,the water glass dilution ratio,the aging temperature and the acid concentration on the aerogel gel time and the resulting aerogel are studied by controlling a single variable.The influence of glue density is characterized by SEM,specific surface area analysis,thermal constant analysis,thermogravimetric differential thermal analysis,Raman spectroscopy analysis,N2 adsorption-desorption analysis,and hydrophobic angle test.The experiment shows that the aerogel prepared by the single-step acid catalysis method has a porosity of 83.9%,a thermal conductivity of 0.041 W/(m·K),an average pore diameter of 13.1 nm,a density of 0.33 g/cm3,and a specific surface area of 425 m2/g,Silica aerogel with a hydrophobic angle of 127°.The aerogel prepared by the two-step acid catalysis method has a porosity of 81.5%,a thermal conductivity of 0.0439 W/(m·K),an average pore diameter of 14.3 nm a density of 0.38 g/cm3,a specific surface area of 430 m2/g,and hydrophobic Silica aerogel with an angle of 148°.The hydrophobic silica aerogel prepared by the three-step acid catalysis method has a porosity of 90.7%,a thermal conductivity of 0.031 W/(m·K),an average pore diameter of 15.3 um,and a density of 0.19 g/cm3.The surface area is 480 m2/g and the hydrophobic angle is 139°.When the alkaline catalyst is introduced,the prepared aerogel has a porosity of 93.4%,a thermal conductivity of 0.033 W/(m·K),an average pore diameter of 15.8 nm,a density of 0.26 g/cm3,and a specific surface area of 485 m2/g,silica aerogel with a hydrophobic angle of 143°.The experimental results show that using different silicon sources,namely ethyl orthosilicate and water glass,silica aerogel samples with complete skeleton structure and uniform particles can be obtained,and the samples prepared by the two silicon sources have abundant three-dimensional networks.The structure,pores are well-developed and evenly distributed,and both surfaces have strong thermal stability after sample testing.However,the samples obtained from the two silicon sources show a certain degree of difference in some properties.The performance of the samples prepared with ethyl orthosilicate will be more prominent,mainly because the sodium ions in the water glass(NaSiO3)are impurity ions.The presence of sodium ions will have a certain impact on the transparency and density of the aerogel.In addition,experiments have proved that in the preparation of silica aerogels using water glass as a source,the use of a multi-step acid catalysis method and the introduction of alkaline catalysts can effectively improve the practical performance of silica aerogels.