| Aerogel is a kind of three-dimensional network skeleton structure solid material with high porosity and high specific surface area,prepared by sol-gel technology and appropriate drying process using organic or inorganic salt as precursor.The special physical and chemical properties of aerogel materials,such as ultra-low density,high specific surface area and excellent thermal insulation,give them a wide range of applications in different fields,such as thermal insulation,adsorption,purification,explosion,muffling,catalysis,construction.Silica aerogel is one of the earliest prepared and applied aerogels,and its preparation technique is also one of the most mature aerogel preparation techniques.However,due to the fragile skeleton structure,silica aerogel tends to suffer from a collapse of the pore structure and lost the original high porosity feature,which limits its application in various fields.Through the introduction of alumina,higher-light transmittance and better-heat resistance of Al2O3/SiO2 aerogels were successfully prepared.Heat treatment of Al2O3/SiO2 aerogels was also performed to investigate the effects on their microstructure.The mechanical properties of aerogels are further improved by introducing chopped mullite fibers,which greatly extends the range of applications for oxide aerogels.In this paper,transparent bulk Al2O3/SiO2 aerogels were successful prepared by supercritical drying technology using tetraethyl orthosilicate as the silicon source,aluminum chloride hex hydrate as the aluminum source,deionized water and ethanol mixture as the solvent,and propylene oxide as the coagulant aids.Then heat treatment was performed to improve the intensity of its framework structure.The results showed that when the volume ratio of deionized water/ethanol was 1:1,the molar ratio of precursor aluminum salt/organic silicon was 4:1-6:1,the molar ratio of deionized water/tetraethyl orthosilicate was 123.5:1,and the heat treatment temperature was 800 ?,the prepared Al2O3/SiO2 aerogels exhibited the highest light transmittance,small linear shrinkage rate(15%),and low weight loss rate(15%-20%)as well as high specific surface area(732 m2/g).Even,after 1200 ? high temperature processing,the specific surface area was still up to 266 m2/g.In the heat-treatment process,the light transmittance increased first and then decreased with increasing the temperature.Notable,the light transmittance reached the highest after heat treatment at 800 ?,which is about 87%when the wavelength is 800 nm.By using ethyl orthosilicate as silicon source,aluminum chloride hex hydrate as aluminum source with the addition of quantitative chopped mullite fiber in the sol-gel process,the oxide composite fiber aerogel samples were obtained.Mechanical test results showed:the composite aerogel doped with chopped fiber has significantly enhanced the compressive strength,and the slope of the elastic region in the stress-strain curve becomes smaller.It was found that toughness of the oxide composite aerogel can continue to enhanced with increasing the amount of doping fiber.The compressive strength reduced continually,and the highest mechanical strength is 0.35 MPa at the doping amount of 0.01 g/cm3,which is about 1.5?2 times of Al2O3/SiO2aerogels.Through further exploration,the mechanical properties of the composite aerogels increased first and then decreased with the heat treatment process,and the best mechanical properties can be obtained at 800 ?.In addition,the high-temperature linear shrinkage of composite aerogels was significantly lower(only about 5%after treatment at 1000 ?)than that of Al2O3/SiO2 aerogels,indicated that the introduction of fibers enhances the shrinkage resistance of the composite aerogel skeleton. |
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