Preparation Of Silica Aerogels With Sol-Gel Method By Ambient-dried Technique And The Study Of Their Properties

Aerogels are advanced man-made nanophase materials which have high porosity and specific surface area, varied chemical structures, fascinating and useful properties.In this thesis, silica aerogels were prepared with sol-gel method by ambient drying technique. The alcogels were made from tetraethoxysilane(TEOS), water and ethanol in a definite molar ratio. If acid or base catalysts are not added to the formulation, the reaction is impractically slow at room temperature, often requiring several days to each completion. The effects of the pH value on the gelation time were studied by using HC1 and NILjOH catalysts. The amount of water also affects gelation time. The more the water, the shorter the gelation time. But if the gelation time is much shorter, the reaction will give much precipitation of hydrated silica. This is disadvantage for formation of silica network structure. The ethanol does not react with TEOS, it fills the free space. So the density of silica aerogel was controlled by the amount of ethanol. The optimal technological parameters for silica gelation were obtained by orthogonal experiment. The pH value is 8, the molar ratio of TEOS to ethanol is 5, and the molar ratio of TEOS to water is 6 .Condensations in the wet gel was prevented by developing non-reactive -CHs species by means of surface modification with silane compound TMCS (trimethylchlorosilane). With Fourier-transform spectroscopy (FTIR) and differential thermal analysis (DTA), the mechanism of surface modification was studied. By detecting -CrtyC-H) substituted for-OH species, surface modification was identified.The chemical components of aerogel products are almost pure silica. The content of silica in aerogel samples is greater than 99.6%. By means of X-ray diffraction (XRD), scanning electron microscopy (SEM), specific surface area and pore size analyzers, small angle X-ray scattering (SAXS) , FTIR and some other experimental methods, the structure and other physical properties of these aerogels were primarily investigated. The results show that silica aerogels were coherent, non-crystalline, nanosized porous solids. The BET specific surface areas measured by nitrogen adsorption were 583-61 Im2/g. The Pore size determined by nitrogen desorption were 20-150nm. However, the majority of the pores fall in 30-50nm, i.e. the mesopore regime. The diameters of silica particles ranged from 4 to 60nm. The mean diameter ofregime. The diameters of silica particles ranged from 4 to 60nm. The mean diameter of particles was 9.39nm. For some silica aerogels undried at 105C, FTIR spectra showed a strong, broad absorbance band at 3500 cm-1, due to O-H stretching vibrations. A weaker O-H bending vibration band is seen at 1600 cm-1. Both adsorbed water and surface -OH groups contribute to these bands.With theory of surface chemistry , the model has been developed for predicting the shrinkage that occurs during drying. The model can predict the final density of the dried gels using only data that can be obtained from the wet gel.Thermal properties and adsorptive ability were also studied. At ambient pressure and vacuum, the thermal conductivity of slica aerogel is 0.019 W.m-1.K-1 and 0.008 W.m-1.K-1 , respectively. The adsorption-removal of volatile organic compounds , such as benzene, toluene, trichloromethane and acetone , was effective with silica aerogels. At the same time, the experimental study indicated that SO2, NO and H2S were also effectively adsorbed with silica aerogels.