Effect of aging on alumina gels rheology and aerogels surface area

Alumina gels were synthesized by catalyzed gelation of aluminum sec-butoxid e (ASB) via the Yoldas process. The gels were aged for up to 6 months and t hen supercritically dried (SCD) with CO2. The molar ratio of acid to ASB wa s in the range of 0.01-0.6. Viscosity measurements of the gels showed a she ar thinning and plastic behavior with no response up to a limiting yield st ress. The gel rheology obeys the Casson model. Analysis of the viscosity as a function of the acid to alkoxide molar ratio, showed that the average mo lecular weight of the gels is inversely proportional to the acid to alkoxid e molar ratio. The viscosity of all the gels increased with aging time for a period of abo ut 6 months reaching an asymptotic value after 1-2 weeks. The viscosity is shown to correlate with the microstructure of these nanomaterials during ag ing. Aging gives rise to a nearly constant surface area of similar to 350 m (2)/g regardless of acid to alkoxide ratio in an aging period of about 6 mo nths.