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.
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.
Authors:A. Marotta, P. Pernice, A. Aronne, and A. Buri
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.
Authors:F. Branda, A. Costantini, A. Buri, and A. Tomasi
In this paper a thermoanalytical study of the kinetic parameters and mechanism of the devitrification process of CaO·SiO2, 1.6CaO·0.4MgO·2SiO2 and 1.4CaO·(0.6/3)Y2O3·2SiO2 is reported. The experimental results suggest that, in the studied glasses, a surface nucleation process is operative; however,
in finely powdered samples, that soften and efficiently sinter before devitrifying, surface nuclei behave as bulk nuclei.
In this case lamellar crystalline structures are obtained.
Authors:A. Buri, A. Costantini, G. Laudisio, R. Fresa, M. Catauro, and F. Branda
Glasses of composition expressed by the following general formula xCaO·(1−x)SiO2 (0.30≤x≤0.50) can be prepared by means of the sol-gel route starting from tetramethylorthosilicate and calcium nitrate tetrahydrate
They are all difficult to prepare by means of the traditional technique of quenching the melt, because of the high liquidus
temperature,T1t that in the case ofx=0.3 glass isT1=1650°C. The DTA apparatus appears a valuable tool for defining the procedure necessary to obtain the glass through the sol-gel
route. The glassx=0.3 is bioactive. The experimental results suggest that the gel structures, such as obtained at room temperature, are very
similar; only at high temperature do the reactions of hydrolysis and polycondensation go to completion and the structural
units characteristic of each glass are obtained.