Sol-gel preparation and crystallisation of 2.5CaO-2SiO(2) glassy powders

2.5CaO . 2SiO(2) gel was synthesised by hydrolytic polycondensation of tetr amethyl orthosilicate (TMOS) with calcium nitrate in alcoholic medium. Ther mal treatments were carried out to convert the gel into the glass. However, a completely amorphous sample was not obtained. The gel-derived material structure was examined by FTIR spectroscopy. The I R spectra suggested that the gel-derived material has a different structure than the corresponding melt-quenched glass, for having a more uniform dist ribution of nonbridging oxygens among the SiO4 tetrahedra. Crystallisation of the gel-derived material, examined by differential therm al analysis is reported and discussed. The activation energy for crystal gr owth, E-c = 782 kJ/mol, and the Avrami parameter, n = 1, which indicates a surface nucleation mechanism, were evaluated by non-isothermal methods. The results were compared with those obtained for a conventionally prepared glass of the same composition. (C) 1998 Elsevier Science B.V.