In this study, the mechanism and the kinetic parameters of the thermal decomposition of gibbsite Al(OH)3 were studied by differential thermogravimetry technique under non-isothermal conditions, between room temperature and 1200 K at heating rates of 5, 10, 15 and 20The obtained differential thermogravimetry curves show clearly three distinct peaks. The first peak is due to the partial dehydroxylation of gibbsite. Among the 32 types of differential equations of non-isothermal kinetics, we have found that the most suitable mechanism is (A 3/2 :2/3 ) also called Avrami-Erofeev equation of order 2/3. The values of the activation energy EA and of the pre-exponential factor K are 157 kJ mol −1 and 7.58 × 10 15 s −1 , respectively. The second peak corresponds to the decomposition of gibbsite to boehmite. Decomposition is controlled by the rate of second-order reaction (F2: g(x) = (1 − x) −1 − 1), under the applied conditions. The activation energy EA and pre-exponential factor K correspond to 243 kJ mol −1 and 3.73 × 10 22 s −1 , respectively. The third peak is due to transformation of boehmite to alumina. However the mechanism for such transformation is better described by the 3/2 rate order reaction (F 3/2 : g(x) = (1 − x) −1/2 − 1). In addition, the values of EA and K were determined to be around 296 kJ mol −1 and 1.82 × 10 19 s −1 , respectively. The results of differential thermogravimetry were supplemented by the differential thermal analysis. X-ray powder diffraction analysis was carried out for samples of gibbsite treated at different temperatures between 200 and 1200• C in 200• C steps.

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The kinetics of spinel (Al-Si) crystallization from Algerian halloysite (DD1) was investigated using differential thermal analysis. Experiments were carried out on samples between room temperature and 1400• C with constant heating rate from 2 to 20The activation energies measured from isothermal and non-isothermal treatments were 1054.85 and 1140 kJ mol −1 , respectively, for the spinel (Al-Si) formation. The Avrami constant n obtained by the Ligero method and the m parameter obtained by the Matusita method were about 2 for spinel crystallization. This value indicates that the crystallization mechanism of Al-Si spinel phase proceeds by bulk nucleation of the phase formation with a constant number of nuclei and that the three-dimensional growth of crystals is controlled by diffusion.

In this study, the dehydroxylation of Algerian halloysite (DD1) was studied using differential thermal analysis. Measurements of samples were performed between room temperature and 1400 • C, at different heating rates of 10, 20, 30, 40 and 50 • Cmin −1. The activation energy calculated from isothermal (using Johnson-Mehl-Avrami and Ligero methods) and non-isothermal treatments (using Kissinger-Akahira-Sunose) were 135 kJ/mol and 153 kJ/mol, respectively. The Avrami parameter obtained by method of Ligero and the m parameter (a numerical factor) obtained by Matusita method were 1.55 and 1.59, respectively. The results show that bulk nucleation was dominant in halloysite transformation, followed by three-dimensional growth of meta-halloysite. This growth is controlled by diffusion from a constant number of nuclei. The frequency factor calculated by the isothermal treatment is equal to 4.16 × 10 6 s −1 .

In this work, mullite-zirconia composite were fabricated by reaction sintering of Algerian halloysite Al2Si2O5(OH)4, boehmite Al(OOH), and zirconia (ZrO2) powder using conventional heating. The appropriate amount of the three raw powders was ball milled for 5 h and sintered between 1250 and 1650• C for 2 h. A scanning electron microscope was used to characterize the microstructure of sintered samples. A dilatometer and X-ray diffractometer were used to analyze the formation and transformation of phases. It is found that for the addition of zirconia up to 20wt.% the zirconia phase retains its tetragonal structure. The formation of primary mullite in all samples was complete at 1220•