NONISOTHERMAL CALORIMETRIC STUDIES OF THE CRYSTALLIZATION OF LITHIUM DISILICATE GLASS

The influence of preannealing treatments on the polymorphic crystalliz ation of lithium disilicate glasses is examined. As expected, glasses heated at different rates through the temperature range where there is significant nucleation develop widely different numbers of nuclei. Th is can dramatically influence the stability and transformation charact eristics of the annealed glass. Non-isothermal differential scanning c alorimetry (DSC) and differential thermal analysis (DTA) measurements are demonstrated to be useful to probe the nucleation behavior. The fi rst systematic investigations of particle size effects on the non-isot hermal transformation behavior are presented and discussed. Based on D TA and microscopy experiments, we show that small particles of lithium disilicate glasses crystallize primarily by surface crystallization. The relative importance of surface versus volume crystallization is ex amined by varying particle size, by introducing nucleating agents and by exposing glasses to atmospheres of different water content. These d ata are analyzed quantitatively using a numerical model developed in a second paper following in this volume.