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  • Author or Editor: A. Marotta x
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The importance of the choice of a suitable kinetic equation in the DTA study of the silica gel to cristobalite transformation is pointed out. It is shown that isothermal and non-isothermal data reported in the literature are better fitted by a Johnson-Mehl-Avrami equation withn=3, rather than a “first-order” one.

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A kinetic analysis of the applicability of the Piloyan method to the process of glass devitrification for evaluation of the exponentn in the Johnson-Mehl-Avrami equation is carried out.

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The effect of the addition of Ag2O on the mechanism of non-isothermal devitrification of Li2O · 2 SiO2 glass has been studied by differential thermal analysis. In both bulk and powdered samples, the presence of heterogeneous nuclei lowers the crystallization temperatures but not the value of the crystal growth activation energy.

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The non-isothermal devitrification kinetics of lithium, sodium and barium disilicate gelglasses have been investigated by differential thermal analysis. Using methods proposed by the present authors, the kinetic parameters and the mechanism of crystal growth in the studied glasses were evaluated from DTA curves. The results were compared with those obtained for the glasses of the same composition prepared using a mixture of oxides as starting materials and quenching the melts.

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Abstract  

The non-isothermal devitrification of lithium germanate glasses, examined by DTA and XRD, is reported and discussed. The glass compositions are expressed by the general formula:xLi2O(1−x)GeO2 withx=0.050, 0.125, 0.167, 0.200 and 0.250. All the glasses studied, unlike GeO2 glass, exhibit internal crystal nucleation without the addition of any nucleating agent. The devitrification processes occur in one or more steps. Phases which crystallized at each step are identified and crystallization mechanisms proposed. These crystallization mechanisms are related to structures of the crystallizing phases. Activation energy values as well as those for glass transition temperatures, do not vary linearly with increase in Li2O content but pass through a maximum atx=0.200.

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