Fabrication of alumina supports for membrane applications by freeze-casting

Full text: The freeze-casting technique is an attractive forming process for preparing porous materials with tailored pore networks. In addition, it is an environmentally friendly, cost effective, and easy scale-up method. It consists of preparing a stable suspension, pouring it into a mould, freezing the suspension, sublimating the dispersing medium, and sintering the green body [1]. Samples obtained by freeze-casting exhibit ordered pore networks which are accessible diffusion pathways for gas permeation. In this work alumina-based membrane supports were fabricated by the unidirectional freeze-casting process and coated with microporous silica film. First, tert-butyl alcohol, a dispersant, and alumina powder were mixed in the milling container at 30 °C. Next they were kept under sonication for 10 minutes to inhibit the presence of agglomerates. The prepared slurries were poured into the polymethylmethacrylate mold, and were subsequently frozen from the bottom to top face. After the solidification and elimination of the solvent, the green bodies were sintered at 1500 deg C during 1 h. The microstructure of the sintered bodies was observed with scanning electron microscopy and X-ray microtomography (μ-CT). The three-point flexural strength of the freeze-cast samples was assessed using a mechanical testing stage available in the μ-CT equipment. Based on the results, the asymmetric membrane was processed by dipping in a silica sol the freeze-cast supports fabricated from the slurry containing a 25 vol% of alumina loading. The structural properties of non-supported membranes were examined by nitrogen sorption tests. In order to assess the permeation behavior of the membranes prepared, single-gas permeation experiments were performed at room temperature with nitrogen, helium, and carbon dioxide. The gas flow through the membrane was measured with a soap film flow meter. (author)