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Preparation, Characterization And Properties Of Silica Aerogel Via Ambient Pressure Drying

Posted on:2008-04-07Degree:DoctorType:Dissertation
Country:ChinaCandidate:F ShiFull Text:PDF
GTID:1101360218953579Subject:Materials science
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Silica aerogels are a class of light mesoporous materials with low density, high porosity, high surface area and low thermal conductivity. Because of their unique properties, silica aerogels have great potential application future in many fields. Conventionally silica aerogels have been made by a supercritical drying process. However, supercritical drying process is complicated and expensive, and it is unsafe to a certain extent. In order to actualize the production of silica aerogels on a large scale and practical applications in many fields, research on preparation of silica aerogels via ambient pressure drying at a reasonable cost is very necessary. In this study, using industrial silica sols, water glass and waste residue fly ash as raw materials respectively, hydrophobic mesoporous silica aerogels have been prepared via ambient pressure drying. The routes of one-step solvent exchange-surface modification of the hydrogels in the ambient pressure drying process have been investigated and determined. The surface group, thermal stability, morphology and microstructure of silica aerogels have been investigated by FT-IR, DTA-TG. XRD, SEM, TEM and BET adsorption method. Silica aerogel films have been prepared by ambient pressure drying using tetraethoxysilane (TEOS) as raw materials. The properties of adsorbing methyl orange in the water and organic solvent such as chloroform, as well as loading and releasing of gentamicin sulfate for silica aerogels have been analyzed primarily by UV/VIS spectrometer.Using industrial silica sols and water glass as silica sources, hydrophobic silica aerogels have been synthesized via ambient pressure drying by using ethanol/trimethylchlorosilane/ heptane(hexane) (EtOH/TMCS/Heptane(Hexane)) solution for modification of the hydrogel. The mechanism of one-step solvent exchange-surface modification for the hydrogel by EtOH/TMCS/Heptane(Hexane) solution was analyzed, and the comparative study of the properties of silica aerogels prepared by two modification process using EtOH/TMCS /Heptane(Hexane) and TMCS/HMDSO(hexamethyldisiloxane) respectively were conducted. Additionaly, the effects of heat-treatment on the morphology, hydrophobicity, specific surface area and pore size distribution of silica aerogels were discussed. The results indicate that modification treatment of hydrogel using EtOH/TMCS/Heptane(Hexane) is favorable for obtaining monolithic low density silica aerogels. The synthesized silica aerogel is a light-weight mesoporous solid, exhibiting a sponge structure, with the density of 0.128~0.197g/cm~3 and pore diameter 7~18nm. The specific surface area of silica aerogels are 559~699m~2/g, with 91%-94% porosity and uniform pore size distribution. It was found that the specific surface area, pore volume and pore size increased with increasing of heat-treatment temperature at 150-450℃. After heat treatment at 500℃, the pore volume decreased, but specific surface area increased to the highest, and hydrophobicity of silica aerogel turned into hydrophilia completely.Using tetraethoxysilane (TEOS) as silica sources, silica aerogel films were prepared via three different ambient pressure drying process by sol-gel method. The influences of different process on the structure and properties of silica aerogel films were analyzed. The results indicate that silica aerogels films with good hydrophobicity could be obtained by three ambient pressure drying process. However, the silica aerogels films prepared by first coating and then solvent exchange-surface modification have better optical transmission.Using industrial waste residue fly ash as raw materials, preparation of porous light-weight silica aerogels and ultra-fine powders was studied. Two different processes of preparing silica aerogels from fly ash were investigated. The processⅠis as follows: the obtained waterglass from fly ash was first catalyzed with H2SO4 so as to promote gelation, and then silica aerogel ultra-fine powders could be formed by immersing the hydrogel in the deionized water for a period and succedent modification with EtOH/TMCS/Hexane and ambient pressure drying. With the increasing of immersing time in the deionized water, the purity of silica aerogel increased and particle size distribution became narrow. The processⅡis as follows: the obtained waterglass from fly ash was first ion exchanged with cation exchange resin, and then silica aerogels could be obtained by treating the gel with EtOH/TMCS/Hexane and drying at ambient pressure. The morphology and pore size distribution of the obtained silica aerogels are similar to that of the aerogels prepared from industrial waterglass, with specific surface area 635.19 m~2/g and pore diameter 6.67nm.The adsorbability of methyl orange in the water and organic solvent chloroform for silica aerogels were investigated. Hydrophilic silica aerogels heat-treated at 450℃have higher adsorbability of methyl orange. However, the adsorbability of hydrophilic aerogels decreased a little after they were laid in the air for a period. Hydrophobic silica aerogels are capable of adsorbing organic solvents chloroform etc effectually, and the characteristics of high specific surface area and porosity of aerogels almost not changed after adsorption. Thus hydrophobic silica aerogels can be used repetitiously, and the application future is good in the aspect of waste water treatment and organic substance adsorption.The adsorption and release of gentamicin sulfate for silica aerogels were investigated. The hydrophilia or hydrophobicity of silica aerogels had important effects on the adsorption and release of drug. In the range of 450-500℃, the adsorbability and drug loading amount of gentamicin sulfate for silica aerogels increased with the increasing of heat treatment temperature. Silica aerogels heat-treated at 500℃could adsorb drug uniformly with a larger drug loading percent of 122.42%, and have a uniform drug releasing rate.
Keywords/Search Tags:Silica Aerogels, Ambient Pressure Drying, One Step Solvent Exchange-Surface Modification, Fly Ash, Adsorption
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