Sintering and crystallization of volume- and surface-modified cordierite glass powders

The sintering and crystallization behaviour of a series of cordierite-type glass powders with varying substitution of Na2O for MgO was compared with that of glass powders subjected to an Mg2+2Na+ ion-exchange treatment. The ion-exchange modifies only the properties of a 3–4 μm thick surface layer on the glass grains. Whereas the bulk substitution results in a decrease in the glass transition temperature, Tg, the surface modification shows only a small influence on the overall Tg of the ion-exchanged glass. Both glass powder series show, however, a temperature decrease of the start of sintering with increasing Na2O content which demonstrates that only the viscosity of a surface layer is decisive for the start of sintering. The sintering range is enlarged to higher temperatures through the suppression of the crystallization of β-quartz solid solution. The surface-modified glass powders show strong cordierite crystallization at constant temperatures around 1000 °C, whereas in the bulk-modified glass powders, the temperature of cordierite crystallization increases and the total amount of cordierite decreases with increasing Na2O content.