Positron annihilation lifetime spectroscopy (PALS) is used to investigate the detailed structure of amorphous alumina tubes prepared via thermohydrolysis of anhydrous AlCl 3 . The results demonstrate that PALS is a sensitive probe of the structure of amorphous materials, capable of distinguishing between the structures of amorphous alumina tubes and amorphous alumina prepared by a standard procedure, which are shown to differ slightly in the concentration and size of free-volume elements. The observed distinctions can be understood in terms of the anisotropic aggregation of primary alumina particles during hydrolysis in aqueous media or thermohydrolysis in water vapor.
Thermal hydrolysis of aluminum chloride was studied using 36 Cl and 3 H tracers in combination with energy-dispersive and morphological (scanning electron microscopy) analyses. Pure partially hydrated AlCl 3 or mixtures of AlCl 3 and AlCl 3 . 6H 2 O were used as initial materials. It was found that Al 7 O 10
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