Study On Preparation And Performances Of Al₂O₃/Al₂O₃-SiO₂ Aerogels By Atmospheric Drying

Al₂O₃ aerogel is a kind of three-dimensional nano-material with high specific surface area,low density and high porosity.It has broad application prospects in thermal insulation,catalysts,adsorbent and aerospace.The performance of Al₂O₃ aerogel is very stable at 900-1000℃.With the increase of temperature,α-Al₂O₃ will appear,companying with porous structure being destroyed and its excellent performance being lost.Therefore,it is very important to find an effective method to improve high temperature performances of Al₂O₃ aerogel at more than1000℃and prevent sintering and phase transition at high temperature,maintaining its high specific surface area.At present,Al₂O₃ aerogels are mainly prepared by supercritical drying.However,the cost and risk of aerogels prepared by supercritical drying are high.At the same time,thermal insulation of Al₂O₃ aerogel is greatly reduced after it is in damp atmosphere,which will have a great impact on transportation,storage and use.In the paper,Al₂O₃ aerogel and Al₂O₃-SiO₂ aerogel is prepared by sol-gel method and atmospheric drying at more than1000℃.The addition of silicon can enhance the skeleton and improve thermal performances.Al₂O₃-SiO₂ aerogel is hydrophobic/superhydrophobic with methyltriethoxysilane as silicon precursor,improving its water-resistance,which is beneficial to transportation,storage and usage.By using aluminum chloride hexahydrate（AlCl₃·6H₂O）as the precursor,water and ethanol as the solvent,and propylene oxide（PO）as gelation initiator,monolithic Al₂O₃aerogel is prepared via sol-gel method and atmospheric drying（APD）.The structures and performances of Al₂O₃aerogel with different PO addition and different heat treatment temperatures are systematically studied.The formation of Al₂O₃aerogel and its mechanism of thermal insulation are discussed.AlCl₃·6H₂O is dissolved in the mixture of distilled water and ethanol,n（AlCl₃·6H₂O）:n（EtOH）:n（H₂O）:n（PO）=1:2.69:13.06:5.04,monolithic Al₂O₃ aerogel is prepared under the optimal conditions.The monolithic Al₂O₃ aerogel is with specific surface area of 642.54 m²/g,average pore diameter of 5.12 nm and thermal conductivity of 0.028W/（m.K）,and it is with narrow and normal pore diameter distribution.The Al₂O₃aerogel is present inα-Al₂O₃ andθ-Al₂O₃ crystalline phase with specific surface area of 22.09 m²/g,average pore diameter of 42.72 nm and thermal conductivity of 0.059 W/（m.K）after heat-treated at 1200℃.It reports a facile approach to synthesize Al₂O₃-SiO₂ aerogel through sol-gel method followed by atmospheric drying（APD）,in which aluminum chloride hexahydrate（AlCl₃·6H₂O）and methyltriethoxysilane（MTES）as alumina source and silica source,respectively.The formation of Al₂O₃-SiO₂aerogel and its mechanism of thermal insulation are discussed.n（AlCl₃·6H₂O）:n（EtOH）:n（H₂O）:n（PO）=1:2.69:13.06:2.52,aluminum-sol is prepared in the proportion.n（MTES）:n（EtOH）:n（H₂O）=1:3:10.5,silicon-sol is prepared in the proportion.n（Al）:n（Si）=1:3,Al₂O₃-SiO₂ aerogel is prepared under the condition.The specific surface area of Al₂O₃-SiO₂ aerogel is 681.32 m²/g,with average pore diameter of 3.46 nm,thermal conductivity of 0.024 W/（m.K）and hydrophobic angle of 141°,and it is with narrow and normal pore diameter distribution.Comparing with Al₂O₃ aerogel,Al₂O₃-SiO₂ aerogels have smaller particle size and are full of methyl groups on the skeleton which makes it hydrophobic/super-hydrophobic.After heat-treated at 1200℃,mullite phase is the main crystalline of Al₂O₃-SiO₂ aerogel.At this temperature,the specific surface area of Al₂O₃-SiO₂aerogel is 114.52 m²/g,thermal conductivity of 0.051 W/（m.K）and average pore size of 23.57nm,which is also a kind of mesopore material.The phase which it is toα-Al₂O₃ is inhibited by Sielement,improving high temperature stability of aerogel.Methyl group of MTES can also prevent sharp increase of thermal conductivity about Al₂O₃-SiO₂aerogel at moisture atmosphere,which is conducive to storage,transportation and usage.