Research On Preparation And Modification Of Alumina Aerogels

Alumina aerogel is a lightweight nano material with three-dimensional network structure.Alumina aerogel is widely used in heat insulation,catalysis and so on,due to its excellent properties such as low density,high specific surface area,low thermal conductivity,etc..However,alumina aerogel is prone to sintering and phase transformation at high temperature,resulting in the destruction of its three-dimensional network structure and the sudden decrease of its specific surface area.Therefore,it is of great practical significance to study the preparation technique of alumina aerogel with low cost and high thermal stability.Firstly,the gel precursors were prepared using different raw materials by sol-gel method and powder dispersion method.Then,the obtained precursors were dried by supercritical drying method with ethanol as drying medium.It was found that the alumina aerogel prepared from AlCl₃·6H₂O was block and had good strength properties.The optimum process conditions for the preparation of gels are as follows:n?Al?:n?PO?:n?abs.EtOH?:n?H₂O?:n?CH₃NO?=1:8:8:15:1.8,reaction temperature is 30°C.XRD diffraction and infrared spectroscopy analysis show that alumina aerogel is?-AlOOH morphology.The results of N₂adsorption-desorption test show that the aerogel belongs to mesoporous materials and the pore size is concentrated in 18-50 nm.The specific surface area,average pore size and pore volume are 498 m²/g,21.8 nm and 1.95 cm³/g,respectively.Secondly,La-doped alumina aerogel was prepared with LaCl₃·7H₂O as additive.The structure and morphology of the monolithic alumina aerogels doped with different La₂O₃concentration were investigated.The results indicate that the addition of La₂O₃ can increase the specific surface area of alumina aerogels and improve their thermal stability at high calcination temperature.After calcination at 1000°C,alumina aerogel doped with 10 wt.%La₂O₃ still maintains a three-dimensional network structure and its specific surface area can reach 282 m²/g.and increase 69.8%compared with the pure alumina aerogel.Finally,Sr-doped alumina aerogel was also prepared with Sr?NO₃?₂ as additive.The results show that Sr doping can inhibit sintering and phase transition of alumina aerogels at high temperature.At higher temperature,the presence of SrO can react with alumina aerogel to form hexaaluminate?SrO·6Al₂O₃?,which inhibit the phase transition of alumina aerogel.The specific surface area of alumina aerogel doped with 5 wt.%SrO still reach 122 m²/g after calcination at 1200°C.