Structures and phase changes of three mixed hydroxide systems and their thermal dehydration products: Effect of compaction

Abstract

Differential thermal analysis (DTA) and thermogravimetric analysis (TGA) were carried out for various tested samples, namely the parent hydroxides of aluminium, magnesium, calcium and cadmium, the mechanically mixed powders of Al-Mg, Al-Ca and Al-Cd hydroxides, and the compacted form of the mixed powders. X-ray analysis was carried out on the samples and on their dehydration products between 200 and 800 °C. From DTA and TGA it is noticed that the peaks of the Al-Mg and Al-Cd hydroxides correspond to the differential peaks of the parent hydroxides. By contrast, Al-Ca mixed hydroxide exhibits two peaks at 245 °C and 280 °C for aluminium and calcium hydroxides respectively whereas parent Al(OH) 3 and Ca(OH) 2 exhibit peaks at 200 °C and 450 °C respectively. This indicates that the mass loss observed from TGA data is not due to dehydration of Ca(OH) 2 but due to dehydration of another entity formed from the mixing of Al(OH) 3 and Ca(OH) 2. X-ray analysis indicates that a spinel species of chemical formula Ca 3Al 2(OH) 12 formed even at 110 °C. Therefore the differential effect centred at 285 °C may be correlated with the dehydration of this spinel. The parent hydroxides of aluminium, magnesium, calcium and cadmium reveal the main characteristic diffraction lines of β-Al 2O 3, brucite, portlandite and cadmium hydroxide respectively. Thermal treatment led to partial disappearance of the main diffraction pattern of the hydroxides and at the same time the appearance of other lines corresponding to the formation of both oxides and spinels except for Ca(OH) 2 with which no characteristic lines were observed for the corresponding oxide up to 600 °C. Compaction before thermal treatment led to a marked decrease in the relative intensities of the X-ray diffraction lines of individual hydroxides, oxides and spinels of Al-Mg, and the spinel of Al-Ca which formed up to 200 °C, compared with the loose powders. By contrast, the crystallization of the Al-Ca spinel that formed at 400 °C was enhanced markedly and also, slightly, was that of Al-Cd spinel. The dimunution in the relative intensities of the X-ray diffraction lines by compaction may be tentatively correlated with a lower density of the crystalline forms than of the amorphous forms in the solid matrix.