Thermal Desorption Process of Water in Amorphous AlO(OH)·αH2O Fibres Prepared by an Electrochemical Method

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Abstract
Amorphous fibres of similar to 30 mu m average diameter of AlO(OH).alpha H2O, alpha less than or equal to 0.25, have been prepared by a spontaneous surface oxidation of AI-metal in air by an electrochemical reaction method. These fibres present similar to 5.5 times lower value of the specific density (rho) than for the usual bulk sample obtained by the wet methods. The low rho = 0.17 Mg.m(-3) value is primarily the result of peculiarly modified interatomic and/or intermolecular distances (0.1 to 0.3 nm) in the fibres. On heating in a DTA (differential thermal analysis) machine, the fibres desorb off the water, 2[AlO(OH).alpha H2O] --> Al2O3 + (2 alpha + 1)H2O, in two successive steps in two irreversible endothermic signals. The two signals appear at T-p similar to 340 and 400 K in the DTA thermogram in heating the sample with the heating rate of beta = 0.33 K.s(-1). Two distinct desorption signals also occur in the in-situ measurements of the thermograms in the thermogravimetric analysis (TGA) or differential thermogravimetric analysis (DTG). It has been found that the T-p varies with beta following a linear ln(T-p(2)/beta) versus T-p(-1) Kissinger plot, whose slope Delta E/R (with R = 8.314 J.mol(-1).K-1 the gas constant) determines the value of the activation energy Delta E of the desorption process. A value of Delta E = 33.7 kJ.mol(-1) thus has been calculated from the plot for the prominent (the first signal) DTA thermogram. The results are discussed in correlation with the microstructure and the macroscopic and/or microscopic properties.