Sol-gel preparation and crystallisation of 2.5CaO-2SiO2 glassy powders

2.5CaO . 2SiO(2) gel was synthesised by hydrolytic polycondensation of tetramethyl orthosilicate (TMOS) with calcium nitrate in alcoholic medium. Thermal 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 IR spectra suggested that the gel-derived material has a different structure than the corresponding melt-quenched glass, for having a more uniform distribution of nonbridging oxygens among the SiO4 tetrahedra. Crystallisation of the gel-derived material, examined by differential thermal analysis is reported and discussed. The activation energy for crystal growth, 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.