Synthesis and thermogravimetric analysis of amorphous boehmite fibres

Abstract

Amorphous fibres of boehmite Al=O [GRAPHICS] are synthesised using an electrochemical surface oxidation reaction of an amorphous Al plate in open air at room temperature. The microstructure es of the samples, measured at selected periods of time of reaction, demonstrate a very rapid growth in the separated fibres with a highly exothermic reaction. Two broad haloes occur in the X-ray diffractogram of the sample, at scattering vectors k(1) = 25.5 nm(-1) and k(2) = 6.3 nm(-1), intrinsic to amorphous solids. Thermogravimetric analysis together with infrared studies determine the chemical composition of the sample to be AlO(OH) . xH(2)O, with x approximate to 0.25. The water in this example is hydrogen bonded with the AlO(OH) molecule in a particular fashion so that they form a correlated structure at molecular scale. As a result, its controlled thermal desorption at 320 to 350 K modifies the 2AlO(OH) --> Al2O3 + H2O thermal desorption transformation, which occurs at subsequently higher temperatures. The presence of the hydrogen bonding is clearly evidenced by a significantly modified (lowered) value for the symmetric or asymmetric stretching vibration frequency of the [GRAPHICS] group in the IR spectrum at 1022 or 1125 cm(-1) in comparison to 1070 or 1150 cm(-1) found for a gel derived boehmite sample. The OH functional group in the AlO(OH) molecule exhibits three characteristic vibration bands at approximate to 600, 1640 and 3460 cm(-1).