Fabrication Of Alumina-based Aerogels

With the development of the new ultra-high speed aircraft,military thermal batteries,industrial furnaces and so on,higher and higher requirements have been put for heat-insulation materials.However,the widely used silica aerogel materials have been unable to meet the application requirements.Therefore,there is an urgent need in designing and synthesizing heat insulation material with higher temperature resistant,lighter weight and higher efficiency.We report a facile approach to synthesize alumina aerogels through sol-gel reaction controlled by fractional hydrolysis and fractional hydrolysis,followed by supercritical drying,in which Aluminum sec-butoxide(ASB)as precursor,water(H2O)as the reagent,ethanol(EtOH)as solvent,ethyl acetoacetate(Etac)as chelating reagent,acetic acid(HAc)as catalysts.We have study the effects of parameters such as concentration of H2O,EtOH and Etac on the alumina aerogel and the optimized reaction ratio was determined as n(ASB):n(H2O):n(EtOH):n(Etac):n(Hac)=1:3:30:0.15:0.1.The as-prepared alumina aerogel by the optimized condition was polycrystalline boehmite phase connected by leaf-shape porous structure,the special surface area was 744.52 m/g,the pore volume was 2.23 cm/g,the average pore size was 12.76 nm and the average particle sizes was 8.05 nm.Meanwhile,we had studied the thermal stability of the alumina aerogel under different temperatures through TG-DTA?SEM?XRD?FT-IR?N2 adsorption-desorption.The results indicated that the leaf-shape porous structure and pores size of the alumina aerogel did not change obviously.However,when the temperature rise to 1200?,the special surface area reduced to 153.45 m2/g,the crystal type of ?-Al2O3 structure transformed to ?-Al2O3 structure,and the volume density also shrinked from 0.078 g/cm3 to 0.504 g/cm3.We report a facile approach to synthesize alumina-silica composite aerogel through sol-gel reaction followed by supercritical drying,in which aluminum sec-butoxide(ASB)and tetraethoxysilane(TEOS)as alumina source and silica source,respectively.We prepared alumina sol and silica sol by hydrolysis reaction,and then mixed each other in different mole ratios to get the composite gel.We find that as the increase of the silica content of the alumina-silica composite aerogels,the transparency was increase,the leaf-shape porous structure transformed into particle mesh three-dimensional structure gradually.The average pore size and particle size increased first and then decreased with the increase of silica content.We made some thermal stability analysis about high-alumina type composite aerogels and high-silica type composite aerogels,respectively.The results showed that the high-alumina type composite aerogels(S1A4)have obvious temperature tolerance contrast with high-silica type composite aerogels(S4A1).The samples of S1A4 can keep the high special surface area of around 400 m/g and the average pore size of around 25nm,while the samples of S4A1 just keep the low special surface area of 100 m2/g and the average pore size of around 60nm.Simultaneously,the composite aerogels(S1A4)can still keep the basic characteristic and form the mullite crystal at 1200 ? for 2 h,but the high-silica typecomposite aerogels(S4A1)had been ceramic densification.The alumina-silica composite aerogels belongs to the low-k dielectrics materials,and the dielectric constant is less than 3.2 and the dielectric loss is the minimum at around 10.5 GHz,the dielectric constant and dielectric loss decrease with the increase of silica content.