Synthesis Of Silica Aerogel/Glass Fiber Composites With Improved Insulation And Mechanical Performance

Silica aerogel is a kind of amorphous mesoporous material with high specific surface area,low density,and high porosity,and is the best thermal insulation material to date.It has broad applications,such as in the thermal insulation material,sound insulation,dust removal,optics,catalysis,chromatographic separation,and drug delivery system.However,the mechanical properties of the silica aerogel are poor due to its highly porous structure,making it hard to use directly.Glass fiber is often used to reinforce the aerogel,but its thermal insulation performance would be impaired.Moreover,the aerogel dust releases easily because of weak bonding between the glass fiber and the aerogel,which limits its application.Therefore,it is still a challenging issue for researchers to solve in making the silica aerogel/glass fiber composite with excellent mechanical performance nowadays.In this thesis,a new strategy was proposed to improve the mechanical properties of the composite without deteriorating its thermal insulation property.Given the problems of existing big void space in the composite of silica aerogel/glass fiber and the weak combination between them,and long drying period,the composite was prepared by using an acid-base catalyzed sol-gel and supercritical CO2 drying method,and characterized and analyzed by SEM,TEM,BET,FTIR,constant thermal analyzer,mechanical compressing tester,etc.The effects of adding silica gel and gas-phase silica on the properties of the composites were studied,and the effects of temperature and pressure on the solubility of ethanol in Supercritical CO2 were studied too.The mechanisms of improving the performance of the composite were explored.The main results and conclusions are as follows:A dynamic extraction method was applied to measure the solubility of ethanol in supercritical CO2.The influences of experimental parameters such as pressure and temperature on the solubility were investigated,respectively.When the temperature was kept at 35°C,the solubility of ethanol increased from 54.3 to 87.9(g ethanol/kg CO2)when the pressure increased from 10 MPa to 25MPa.When the pressure was fixed at 25 MPa,the solubility increased from 87.9 to 123.6(g ethanol/kg CO2)when the temperature increased from 35°C to 40°C.However,the solubility of ethanol decreased from 123.6 to 102.3(g ethanol/kg CO2)when the temperature increased from 40?to 50?.The solubility data were correlated with Chrastil and Del Valle-Aguilera models,and the absolute average relative deviations were 6.21%and 3.19%,respectively.The results provide an experimental basis for selecting suitable supercritical CO2 drying process parameters.Based on the results of the study of the solubility of ethanol in supercritical CO2,the influences of pressure and temperature on drying time during supercritical CO2 drying were investigated.At a fixed temperature of 40°C,the drying time was shortened significantly with increasing pressure.When the temperature 40°C and pressure were 25 MPa,respectively,the 60min of drying time was enough to completely remove the ethanol from the composite while the good structure of silica aerogel remained intact.Extending drying time from 60 min to 120 min did not make any difference in terms of the silica aerogel textural properties.The optimum drying conditions in the subsequently Chapters were selected based on these results.The effects of adding 200-450 mesh size silica gel on the properties of the silica aerogel/glass fiber composite were investigated.Different amount of adding silica gel from 1%,3%,5%,7%and9%were examined.Results showed that the thermal conductivity of the composites decreased first and then increased with an increasing further amount.With adding 5 wt.%of silica gel,the thermal conductivity of the composite reached 0.0179 W·(m·K)^-1 at room temperature(25?),and its elastic modulus and density were 1530 KPa and 0.246 g/cm³,respectively.SEM images showed that the added silica gel filled the void spaces between the glass fiber and silica aerogel,acted as a bridge between them through the Si-O-Si network.The added silica gel not only provides a skeleton structure for the aerogel but also enhances the strength of the composite.Moreover,it reduced thermal conductivity by the reduction of the routeways of heat transportation via decreasing the spaces between the fibers largely.The results provide a new strategy for improving the performances of the thermal insulating and mechanical strength of the composites.The preparation of the hydrophobic silica aerogel/glass fiber felt was studied by using tetraethylorthosilicate(TEOS)and methyltrimethoxysilane(MTMS)as co-precursors with adding silica gel.The influences of the ratios of TEOS:MTMS(1:0,1:0.25,1:0.5,1:0.75,1:1)on the performance of the felt were investigated.Results showed that the thermal conductivity of the felt was 0.0258 W·(m·K)^-11 at room temperature when the ratio of TEOS:MTMS and silica gel addition were 1:1 and 5%,respectively.The water contact angle and the density of the felt were 150°and0.281g/cm³,respectively.The combination between the fiber and the aerogel of the felt was strong without a dust-release issue and had good thermal stability(664?).This study broadens the application of the felt.The effects of the amount of adding fumed silica(1%,3%,5%,7%,9%,and 12%)on the thermal insulation and mechanical properties of the composite were investigated.The thermal conductivity of the obtained composite reached 0.0194 W·(m·K)^-11 at 25?when the added amount of the fumed silica was 7%.Also,it had a high fracture strength of 0.58 MPa and a low bulk density of 0.239 g/cm³.SEM and TEM images showed that the fumed silica interconnected with the silica aerogel,and the big pores of the former reduced to the mesoporous and became a part similar to the latter,forming a mesoporous network.Therefore,the introduction of the fumed silica to the composite not only reinforced the-Si-O-Si-network on the surface of the fiber but also improved the pore structure.Thus,thermal insulation and mechanical performance were improved greatly.The research results provide a new method for the preparation of excellent thermal insulation materials.