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Mechanism and Kinetic Parameters of the Thermal Decomposition of Gibbsite Al(OH)₃ by Thermogravimetric Analysis

100%
Redaoui D., Sahnoune F., Heraiz M., Raghdi A.
Acta Physica Polonica A
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2017
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vol. 131
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issue 3
562-565
EN
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 20°C min¯¹. The 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}: g(x)=[-ln(1-x)]^{2/3}) also called Avrami-Erofeev equation of order 2/3. The values of the activation energy E_{A} and of the pre-exponential factor K are 157 kJ mol¯¹ and 7.58×10¹⁵ 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 E_{A} and pre-exponential factor K correspond to 243 kJ mol¯¹ and 3.73×10²² 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 E_{A} and K were determined to be around 296 kJ mol¯¹ and 1.82×10¹⁹ 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.
2
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The Kinetics of Spinel Formation of Algerian Halloysite by Differential Thermal Analysis

88%
Heraiz M., Sahnoune F., Belhouchet H., Raghdi A., Ouali A.
Acta Physica Polonica A
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2017
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vol. 131
|
issue 1
139-142
EN
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 20°C min¯¹. The 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.
3
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Preparation and Phase Transformation of Mullite-Zirconia from Boehmite and Algerian Halloysite

88%
Raghdi A., Heraiz M., Sahnoune F., Ouali A., Redaoui D.
Acta Physica Polonica A
|
2017
|
vol. 131
|
issue 1
136-138
EN
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°C. The cristobalite started to form at 1350°C, and disappeared at 1500°C in the samples of mullite, and at 1450°C when ZrO₂ was added. The zircon compound ZrSiO₄ started to form at 1350°C and completely disappeared at 1550°C. The increase in ZrO₂ ratio promoted the formation of grains with a spherical shape.
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