Influence of hydroxyl on glass transformation kinetics in lithium disilicate melt and a re-evaluation of structural relaxation in NBS 710 and 711

Differential scanning calorimetry (DSC) experiments are presented that document the strong effect that ppm-level hydroxyl concentrations have on glass transformation kinetics in silicate melt. Tn particular, relaxation times decrease by an order of magnitude and limiting fictive temperatures (T(f)x) decrease similar to 20 K with an increase in hydroxyl content from 30 to similar to 1400 ppmw in the lithium disilicate system. Activation energies derived from Arrhenius plots of reciprocal Tr values versus log(quench rate) show no dependence on hydroxyl content. Heat capacity scans of samples with various hydroxyl contents quenched at the same rate collapse to single curves when scans are plotted using (T - T(f)x). Such behavior can be incorporated most simply into the Tool-Narayanaswamy (TN) model of structural relaxation by allowing only the pre-exponential term in the relaxation function to be a function of hydroxyl content. Fits of our data to the TN model were obtained using three different activation energy values: (1) 757 kJ/mol (from cooling rate dependence of T(f)x); (2) 599 kJ/mol (from published viscosity data); and (3) 526 kJ/mol (4-parameter fit). The fit using the activation energy from viscosity measurements provided the best overall fit. In addition, structural relaxation phenomena of NBS 711 and NBS 710, lead silicate and soda-lime silicate compositions, respectively, were re-examined and a significant discrepancy in activation energies for NBS 710 and 711 between this study and earlier work was documented. (C) 1999 Elsevier Science B.V. All rights reserved.