Regina Monteiro

The changes occurring during the heating of a borosilicate glass have been investigated by differential thermal analysis, dilatometric analysis and thermomechanical analysis. The thermal properties of this glass, such as glass transition temperature, dilatometric softening temperature and linear thermal expansion coefficient, have been determined. Viscosity measurements in the temperature range 898-1048 K were performed in a thermomechanical analyser equipped with a penetration attachment for isothermal measurement, and from the temperature dependence of viscosity a value of 290 kJ mol-1 was obtained for the activation energy for viscous flow. Devitrification of the glass was observed, specifically in finely powdered glass samples, where the precipitation of cristobalite was identified by X-ray diffraction. Glass powder compacts sintered by viscous flow and cristobalite precipitation reduced strongly the shrinkage rate. © 2001 Elsevier Science B.V.

Coal fly ashes produced by an extinguished power plant in the north of Portugal have been melted with addition of CaCO3 and Na2CO3 to obtain glasses. One of the formulated compositions was selected for further studies and it was possible to manufacture glass-ceramics by crystallizing the parent glass through adequate time-temperature schedules. The macroscopic appearance, microstructure, mechanical, thermal and chemical properties indicated that these materials are quite attractive for cladding applications, exhibiting in some cases better performances than the conventional ceramic tiles.

A range of CaO-Al2O3-SiO2 glasses have been prepared by fusion of pure starting materials in platinum crucibles. Compositions containing large amounts on network formers, Al2O3 and SiO2, are difficult to crystallize. If the amount of network former is reduced, glasses will self-nucleate and crystallize more readily, but the products of crystallization tend to react with water. This conflict has been partly resolved by adding MgO and ZnO and tailoring compositions so as to produce a phase, variously designated "Q" or "pleochroite", ideally Ca20Al32-2 vMg vSivO68, with v close to 4. Pleochroite crystallizes with a typically fibrous morphology. Preliminary experiments on fragments and melt-cast glass rods indicate that these compositions can be heat treated without deformation to yield highly crystalline, transparent ceramics. © 1989 Chapman and Hall Ltd.