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Preparation And Characterization Of Monolithic Silica Aerogels And TiO2/SiO2 Composite Aerogels By Ambient Pressure Drying

Posted on:2017-05-14Degree:DoctorType:Dissertation
Country:ChinaCandidate:F Y SunFull Text:PDF
GTID:1221330488963642Subject:Mineral materials science
Abstract/Summary:PDF Full Text Request
Silica aerogels are gel materials which have continuous random network structure filling with gaseous dispersive medium. Silica aerogels have high porosity, high specific surface area, low thermal conductivity and low density which have broad application prospects in the field of aerospace, military, medical, environmental protection and insulation. 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. So, research on preparation of silica aerogels by ambient pressure drying is very necessary. Due to poor monolithicity and large shrinkage, in this study, using tetraethoxysilane and rice husk ash as silicon source respectively, monolithic silica aerogels have been prepared by and ambient pressure drying. The routes of sol-gel method and ambient pressure drying technology have been investigated and determined. At the same time, the thermal conductivity increases sharply at higher temperature, this paper selected nano-titanium dioxide powder as main opacifier and prepared Ti O2/Si O2 composite aerogels by ambient pressure drying. Effect of doped content on structure and property of composite aerogels has been investigated. The phase, thermal stability, specific surface area, morphology microstructure, pore structure, and infrared transmittance of silica aerogels and Ti O2/Si O2 composite aerogels have been investigated by XRD, SEM, TG-DTA, FTIR and Nitrogen gas adsorption method.(1)In the course of sol-gel, acid-base catalyzed monolithic silica aerogels were prepared by using of tetraethoxysilane(TEOS) as precursor, absolute alcohol as solvent, DMF as DCCA, 0.1mol/L hydrochloric acid and ammonia as acid base catalyzer respectively. The results showed that the optimal molar ratio of H2O(base catalysis)/ HCl / NH3·H2O /TEOS was 4﹕2.4×10-4﹕1.8×10-2﹕1. Meanwhile, researches showed that the best hydrolysis manner was hydrolyzing for 24 hours at 35℃, aging temperature was 50℃, molar ratio of TMCS/TEOS was 1, modified time is 24 hours. Silica aerogels obtained by this process were light blue, transparent, spongy, flawless and monolithic with small shrinkage, low density of 0.1118g/ cm3, high surface area of 923.75 m2/g, average pore diameter of 38.15 nm and hydrophobic angle of 149°. It was a typical hydrophobic mesoporous material.(2)Using sol-gel method monolithic silica aerogels have been prepared by using low-cost rice husk ash as silica source at ambient pressure. The p H value, ethanol exchange times, modified times and other parameters influencing the structures and properties of silica aerogels were investigated, respectively. The most optimal process to prepare monolithic silica aerogels by ambient pressure drying was obtained when p H value was 5.5, ethanol exchange times was 4, modified times was 3. Silica aerogels obtained by this process were light white and semitransparent with low density of 0.0789g/ cm3,high surface area of 902.56m2/g, average pore diameter of 39.18 nm, pore volume of 2.37cm3/g and hydrophobic angle of 146°.(3)Using sol-gel method, monolithic Ti O2/Si O2 composite aerogels doped nano-titanium dioxide powder were prepared via selecting TEOS as silica source by ambient pressure drying. In this experiment, the molar ratio of H2O(acid catalysis)/H2O(base catalysis)/ Et OH / HCl / NH3·H2O/TEOS was 1﹕4﹕7﹕4.8×10-4﹕9.0×10-2﹕1. The results indicated that, Ti O2/Si O2 composite aerogels obtained by this process were white, non-transparent, and the phase composition were anatase Ti O2 and amorphous Si O2. Ti O2 just was embedded in Si O2 with physical form. The microstructure of Ti O2/Si O2 composite aerogels was similar with the silica aerogels with out of doped Ti O2. Researches showed that doping content of Ti O2 had a significant impact on the density, specific surface area, pore size and infrared transmittance. The density of composite aerogels decreased first and then increases with the doped Ti O2 increasing. When the doped Ti O2 was 5wt%, composite aerogels had the lowest density of 0.1642 g/cm3, and then had the maximum density of 0.1831 g/cm3 when the doped Ti O2 was 10wt%. The specific surface area decreased and the mean pore size increased in general with the doped Ti O2 increasing. The specific surface area was in excess of 700m2/g for the doped Ti O2 varies from 1wt% to 5wt%, and the average pore size was about 40 nm. The specific surface area of composite aerogels reduced to only of 620.25 m2/g for the doped Ti O2 was 10wt%, and the average pore size increased to 51.34 nm. The effect of doped Ti O2 on the infrared transmittance was obvious over a range of wavelengths between 2-8μm for the doped Ti O2 was less than 5wt% and then the downtrend became slower. The infrared transmittance of Ti O2/Si O2 composite aerogels was about 37%, which is lower than it was of silica aerogels with about 63%. This indicates that the composite aerogels doped Ti O2 can effectively scatter and adsorb the infrared ray.(4)Using sol-gel method, monolithic Ti O2/Si O2 composite aerogels doped nano-titanium dioxide powder were prepared via selecting rice husk as silica source by ambient pressure drying. Research showed that the appearance, phase composition and microstructure of Ti O2/Si O2 composite aerogels made by rice husk and TEOS had no obvious change. Doping content of Ti O2 had a significant impact on the density, specific surface area, pore size and infrared transmittance as well. The density of composite aerogels decreased first and increases then with the doped Ti O2 increasing. When the doped Ti O2 was 7wt%, composite aerogels had the lowest density of 0.1194 g/cm3, and then had the maximum density of 0.1308g/cm3 when the doped Ti O2 was 10wt%. The specific surface area decreased and the mean pore size increased in general with the doped Ti O2 increasing. The specific surface area was in excess of 700m2/g for the doped Ti O2 varies from 1wt% to 7wt%, and the average pore size was about 40 nm. The specific surface area of composite aerogels reduced to only of 698.64 m2/g for the doped Ti O2 was 10wt%, and the average pore size increased to 48.76 nm. The effect of doped Ti O2 on the infrared transmittance was obvious over a range of wavelengths between 2-8μm for the doped Ti O2 was less than 7wt% and then the downtrend became slower. The infrared transmittance of Ti O2/Si O2 composite aerogels was about 38%, which is lower than it was of silica aerogels with about 65%. The composite aerogels doped Ti O2 can effectively scatter and adsorb the infrared electromagnetic wave effectively, which resulted in the reducing infrared transmittance.
Keywords/Search Tags:Monolithic Silica aerogels, TiO2/Si O2 composite aerogels, Sol-gel method, Ambient pressure drying, Infrared transmittance
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