Kinetics of thermal dehydroxylation of aluminous goethite
01 May 1996-Journal of Thermal Analysis and Calorimetry (Kluwer Academic Publishers)-Vol. 46, Iss: 5, pp 1223-1238
TL;DR: In this paper, the Johnson-Mehl equation was used to describe the complete isothermal dehydroxylation of synthetic aluminous goethite using isothermal and non-isothermal thermogravimetry.
Abstract: The kinetics of dehydroxylation of synthetic aluminous goethite was studied using isothermal and non-isothermal thermogravimetry. The complete isothermal dehydroxylation can be described by the Johnson-Mehl equation with up to three linear regions in plots of lnln [1/(1−y)]vs. Int Kinetics for the initial stage of dehydroxylation changed from diffusion to first-order through the temperature range 190 to 260°C. The rate of dehydroxylation was reduced by Al-substitution and increased with temperature. Activation energy for dehydroxylation, calculated from the time to achieve a given dehydroxylation extent, varied depending on the extent of dehydroxylation and Al-substitution. Non-stoichiometric OH existed in goethite and some remained in hematite after the complete crystallographic transition.
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TL;DR: In this paper, it was shown that a structural continuity could exist between wustite Fe(1−x)O and hematite via ferrihydrite via topotactic transformation.
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TL;DR: Aluminium substitutedGoethite is thermally more stable than non-substituted goethite and is in harmony with the results of XRD and DTGA.
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TL;DR: In this article, the thermal dehydration of Ca-montmorillonite has been studied by in situ X-ray diffraction at temperatures between 60› 1208C and 90°C.
Abstract: The thermal dehydration of naturally occurring Ca-montmorillonite has been studied by in situ X-ray diffraction at temperatures between 60› 1208C. The time-temperature-dependence of the position of the basal (001) reflection reveals that interlayer water loss on isothermal dehydration occurs in two stages. After an initial rapid decrease in interlayer spacing (on shock heating to an isothermal soak temperature) the reaction proceeds towards equilibrium more slowly. Furthermore, the width of the (001) reflection changes with time, reflecting transformation-dependent changes in homogeneity perpendicular to (001) with a maximum in peak width at the point where the rate of the reaction appears to change. This suggests that, as the interlayer spacing collapses, a local change is induced in the structure, affecting the means of movement of the water from the interlayer.
84 citations
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TL;DR: In this article, a method of comparing the kinetics of isothermal solid-state reactions based on the classical equation for analysis of nucleation-and growth processes is described, where plots of In In (1-α) vs In (time), where α is the fraction reacted, are used to distinguish reaction mechanisms.
Abstract: A method of comparing the kinetics of isothermal solid-state reactions based on the classical equation for analysis of nucleation-and-growth processes is described. In this method, plots of In In (1-α) vs In (time), where α is the fraction reacted, are used to distinguish reaction mechanisms. Even nonintegral slopes obtained for values of the fraction reacted from 0.15 to 0.50 may indicate whether the reaction rate is diffusion- or phase-boundary-controlled. The problems of ascertaining zero time and self-cooling (or heating) of the reacting sample can be observed in the analysis but do not cause severe difficulties in interpretation, as they can for analyses based on reduced-time plots. The analysis is applied to the dehydroxylation of kaolinite and of brucite and the decomposition of BaCO3.
636 citations
TL;DR: A survey of the reaction series of metakaolin can be found in this paper, where the authors provide the necessary historical background for a new approach to their solution, and the present survey brings these problems into sharp focus.
Abstract: This reaction series has been a subject of extensive investigation since the work of Le Chatelier in 1887. Nevertheless, major problems have remained concerned with the nature of metakaolin, the manner in which it transforms into a spinel-type phase and mullite, and the relation of this spinel-type phase to mullite. The present survey brings these problems into sharp focus and provides the necessary historical background fora new approach to their solution.
275 citations
TL;DR: In this article, the properties of goethites were characterized by a combination of Al substitution and structural defects, with Δa being a measure of the defects, and the effect of structural defects was opposite to that of Al substitutions.
Abstract: Fifty-seven goethites, synthesized by a variety of procedures and with Al substitutions of 0–33 mole%, were characterized by XRD, IR, DTA, TEM and chemical techniques. Most of the properties measured showed significant intercorrelations. Mole% Al substitution (measured chemically) did not explain all the relationships among variables, but the inclusion of Δa, defined as the observed a dimension minus the a dimension predicted by the Vegard rule, explained much of the variation not explained by Al substitution. OH stretching frequency, in particular, was better correlated with Δa than the Al substitution or other properties. The properties of the goethites could best be explained by a combination of Al substitution and structural defects, with Δa being a measure of the defects. In general, the effect of structural defects was opposite to that of Al substitution. Increase in Al substitution led to a decrease in all three unit-cell dimensions and OH stretching frequency and to an increase in the distance between the two OH bending vibrations (δOH-γOH) and the temperature of dehydroxylation. Increase in structural defects, on the other hand, caused the a dimension, Δa, and OH stretching frequency to increase and δOH-γOH and the average temperature of dehydroxylation to decrease. Crystal size tended to decrease with increases in both Al substitution and structural defects. Surface area was significantly correlated with the reciprocal of the mean crystal thickness in the a direction. Comparison of XRD and TEM data showed that many samples consisted of crystals with several coherently scattering domains. The nature of the defects, i.e. whether they occur primarily in the interdomain areas or whether they are also distributed throughout the coherently diffracting domains, could not be determined.
250 citations