No abstract

Novelty of the synthesis method is the use of alumina hydroxides with very low initial density ~0.02 g/cm 3 . This porous alumina monolith (PAM) is obtained first by active oxidation of aluminum in a humid atmosphere through a liquid mercury layer. Then, in the second step, a thermal treatment of PAM between 1000 and 1700 °C allows sintering of porous alumina with the density varying between 0.04 and 3 g/cm 3 consisting of -or ¡ -alumina crystallites with submicron or micron size and with specific surface area up to ~100 m 2 /g. Fibrous structures have a high surface to volume ratio and transport is possible only along fibers -this determines the kinetics of structure formation and transformations during synthesis. Due to very high porosity, these materials can have different applications: high temperature filters, substrate for the catalysts and nuclear waste storage. Porous aluminas can be impregnated by liquid or gaseous species and after drying and solidification can be used as precursors for subsequent synthesis of porous compounds. Impregnation by magnesium, nickel or cobalt nitrate solutions followed by thermal treatment leads to spinels; synthesis of mullite can be provided using silicon species. The obtained porous spinels have finer microstructures and higher specific surface areas than those formed with ¡ -alumina. However, the nanometric size of Al 2 O 3 allows reaction at lower temperatures.Materials, having low density or high porosity, high specific surface area and fine grain structure, are needed for catalysts, sensors, filtration membranes, composites, etc. The range of technological applications of these materials depends on possibilities to obtain and control the required microstructure or/and chemical composition. This paper describes a new approach to the problem of synthesis of porous materials based on alumina and alumina compounds.