Cryogenic-Scanning Electron Microscopy as a Technique to Image Sol-to-Gel Transformation in Chelated Alkoxide Systems

摘要

Direct imaging of wet sol-to-gel microstructures in chelated alkoxide systems is reported for the first time by using fast-freeze and freeze-fracture techniques for sample fixing. The system chosen for microstructural imaging is that of a chelated alkoxide, in an effort to observe sol-to-gel transformation. The batch composition corresponds to a 1:1 molar ratio of aluminum sec-butoxide and ethyl acetoacetate made using isopropyl alcohol as a mutual solvent. About 100 μl of samples in varying stages of sonogelation, at a water addition rate of 0.4 ml per hour per gram of alkoxide, were injected between two 300-mesh TEM grids mounted on 0.1 mm copper plates and plunged into melting Freon to bring about a rapid solidification. The sample was cryo-transferred into an SEM, equipped with a cold stage maintained at -170℃, using liquid N_2. The specimen was fractured and imaged for microstructural detail under the electron beam. The direct imaging techniques developed here have been applied to chelated alkoxides for the first time, showing the morphology and microstructure in unhydrolysed as well as fully hydrolysed sols, which were found to indicate complex shapes that were not previously known to exist in such systems. Thus, more microstructural information could be directly gained as a result of this cryo-SEM technique.