In situ formation and imaging of cellular alumina in a cryo - SEM environment

Although freeze-drying or lyophilization techniques are used extensively in the pharmaceutical and food industries, few reports are available on freezing and freeze-drying of ceramic gels for making cellular materials. It has been reported that freeze-thawing of polysilicic acid results in the formation of an insoluble polymer in the form of fibers, flakes with parallel ridges and even honeycomb structures. In the present work, the formation of the cellular material was attempted in a cryogenic scanning electron microscope (cryo-SEM) environment with the objective of in situ lyophilization and imaging of the freeze-dried gels. Moreover, a low temperature behavior of alcohol rich gels derived from alkoxides is compared with that of colloidally processed alumina gels. Freeze-drying techniques may be successfully applied to ceramic gels to synthesize cellular materials. Sol concentration, aging time and freezing rates are critical processing parameters in the formation of cellular alumina. Cellular structures in reversible systems like alumina can be retained by using freeze-drying procedures rather than freeze-thawing. Use of excess alcohols prevents nucleation of ice crystals by playing the role of a cryoprotectant. As a result, no cellular structures are observed in alcohol rich gels. It is believed that controlled amounts of cryoprotectant in the system can be helpful in tailoring the cell structures of freeze-dried gels.