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The Research On Preparation Technology Of Silica Aerogels By Ambient Pressure Drying

Posted on:2014-01-30Degree:MasterType:Thesis
Country:ChinaCandidate:X R ZuoFull Text:PDF
GTID:2231330398456387Subject:Forest Chemical Processing Engineering
Abstract/Summary:PDF Full Text Request
The research status and developments of silica aerogels prepared by ambient pressure drying were summarized in detail in this thesis. Considering the disadvantages of supercritical drying method such as complex technology, expensive equipments, low yield, high safety risks and difficult to achieve the industrial production in traditional preparation of silica aerogels, hydrophobic silica aerogels with large block size were prepared using TEOS as silicon source by the optimization of preparation technologies of sol-gel method, ageing process, surface modification and solvent exchange with ambient pressure drying in this thesis. The influences of block size and physical properties of silica aerogels with surface modifers and ambient pressure drying mediums were discussed mainly. The silica aerogels were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), fourier infrared spectroscopy (FT-IR) and BET adsorption method to study the phase analysis, microstructure, surface chemical groups and pore structure respectively.By the discussions of single-factor experiments and orthogonal test, as well as the analysis that surface modifers impact on block size and physical properties of silica aerogels, blocks of silica aerogels using HMDZ as surface modifer get better greatly. The results indicate that NH3·H2O dosage has the greatest effect on the gelation time of silica aerogels and EtOH dosage has the greatest effect on the porosity of silica aerogels. The optimum molar ratio is determined by analysis that TEOS:EtOH:H2O: HCl:NH3·H2O=1:5:5:2.4×10-3:3.6×10-3.On this basis, silica aerogels were modified respectively using TMCS, HMDZ and HMDSO as surface modifiers. The sample of silica aerogels modified by TMCS has the optimal properties with the apparent density of0.1209g/cm3, the contact angle of142°, the specific surface area of884.0m2/g; the sample of silica aerogels modified by HMDZ followed, the apparent density is0.1873g/cm3, the contact angle is128°and the specific surface area is838.1m2/g; the sample of silica aerogels modified by HMDSO turns up the severe contraction crack, the contact angle is only68°.However, the block size of silica aerogels modified by TMCS is only1.2cm3and modified by HMDZ is6.4cm3. Therefore, hydrophobic silica aerogels with large block size were prepared using TMCS as surface hydrophobic modifier.By the analysis of heat and mass transfer mechanism of ambient pressure drying process, as ambient pressure drying medium, n-heptane can ameliorate the shortcoming of fast evaporation rate of other solvents with low surface tension, and achieve the dual control between reducing capillary forces within the gel and solvent evaporation rate. Thus silica aerogels with large block size, low density and high specific surface area can be prepared. The results indicate that when n-hexane, n-heptane, benzene and toluene were used for drying mediums, the sample of silica aerogels using n-heptane as drying medium has the optimal properties and the larger block size with the apparent density of0.1225g/cm3, the contact angle of148°, the specific surface area of928.2m2/g, the pore volume of2.823cm3/g and the block size is7.2cm3.
Keywords/Search Tags:silica aerogels, surface modification, solvent exchange, ambientpressure drying
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