With increasing calcination temperature increases the

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Unformatted text preview: e spinel phase. Due to the fact that the well crystallised spinel phase does not reconstruct the ‘memory effect’ diminishes with increasing temperature [25, 55, 56]. It has also been shown that the crystallinity of the M(II)O phase increases with increasing calcination temperature resulting in larger M(II)O particles [8, 60]. Above 850°C: The HT is transformed into its final product a mixed metal oxide consisting of well-crystallised stoichiometric spinel and divalent metal oxide phases [25]. Indeed all divalent and trivalent cations that were found to be able to form HTs also give rise to spinel phases. At this final stage of the thermal evolution process, the solids have completely lost the ‘memory effect’. They do not reconstruct anymore, due to the stability of the spinel phases [25, 54, 55]. The exact temperatures for the transitions depend qualitatively and quantitatively on several factors. The most important ones are the nature and relative amount of the 1. Introduction 17 ytrtrghrhrhhrhrhhffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff metal cations, the nature of the anions and the crystallinity of the HT. For instance results the substitution of Mg2+ or Al3+ ions by transition metal ions such as Co2+, Cu2+, Ni2+ or Fe3+ in a decrease in the thermal stability of the HT [61, 62, 63]. Another example is that the temperature of the reversible loss of the intersistal water in Mg1-xAlx HTs decreases with decreasing amount of Al from 200°C for x = 0.33 to 160°C for x = 0.2 as determined by in situ high temperature XRD [57]. The type of anions can have an even greater effect, e.g. the thermal destruction of bromate in HTs occurs at around 200°C whereas sulphate in the interlayer region decomposes only at temperatures as high as 930-970°C [25]. HT materials with a lower degree of crystallinity show less defined temperature profiles in TG analyses, especially for the first weight loss, which is due to the loss of intersistal water [1, 25, 60]. The ability to reconstruct also depends on the me...
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This document was uploaded on 10/07/2013.

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