Electron Microscopy of Modified Aluminum Alkoxide Microstructures on Freeze-Drying

Direct imaging of microstructures in chelated aluminum alkoxide systems subjected to freeze-drying and freeze-thaw-drying is reported using cryo-scanning electron microscopy (SEM) and SEM techniques. Aluminum sec-butoxide was chelated with a beta-diketoester in different molar ratios. This system was gelled by hydrolysis under intense sonication. The sonogels formed were subjected to cryo-SEM. Samples for cryo-SEM were made by freezing the chelated alkoxide samples in liquid nitrogen and freeze-fracturing them inside the cryo-SEM and monitoring the apparent morphological changes on exposure to the cryo-SEM environment including intentional sample heating. Microstructures of unchelated, 1:1 chelated and 1:2 chelated (alkoxide:chelate) sonogels after freezing, thawing, and drying were also imaged by SEM techniques. Heterogeneity introduced by chelation of alkoxide is proposed as the cause for the formation of flakes in freeze-thaw-dried chelated alkoxide gels. Microstructures of 1:1 chelated freeze dried alkoxide were compared with the freeze-thaw-dried gels. The absence of flakes in the chelated samples when subjected to in situ freeze-drying indicated reversible nature of the gels.