Thermal desorption process of water in amorphous AlO(OH)·αH2O fibres prepared by an electrochemical method

Abstract

Amorphous fibres of ∼30 µm average diameter of AlO(OH)·αH2O, α≤0.25, have been prepared by a spontaneous surface oxidation of Al-metal in air by an electrochemical reaction method. These fibres present ∼5.5 times lower value of the specific density (ρ) than for the usual bulk sample obtained by the wet methods. The low ρ=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)·αH2O]→Al2O3+(2α+1)H2O, in two successive steps in two irreversible endothermic signals. The two signals appear at Tp∼340 and 400 K in the DTA thermogram in heating the sample with the heating rate of β=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 Tp varies with β following a linear ln (T2p/β) versus Tp-1 Kissinger plot, whose slope ΔE/R (with R=8.314 J·mol-1·K-1 the gas constant) determines the value of the activation energy ΔE of the desorption process. A value of ΔE=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.