| The structural relaxation times of silica glass, both bulk glass and fiber glass with various fictive temperatures, were estimated at selected temperatures. The structural relaxation time of a glass is needed to estimate the rate of change of various glass properties in the glass transition temperature range. Traditionally, the Tool-Narayanaswamy model, in which activation energy of the relaxation time is divided into two parts, one representing fictive temperature effect and the other representing the temperature effect, has been used. The model can explain the property changes of glass in the glass transition temperature range well when the change of the fictive temperature is small, but the model fails when the change of the fictive temperature is large, as is the case when a fiber sample was heat-treated near the glass transition temperature. The structural relaxation times estimated in the present work exhibited activation energies which varied with fictive temperature, unlike the assumption in the Tool-Narayanaswamy model. On the other hand, the relaxation time for both bulk glass and fiber glass exhibited the same temperature and fictive temperature dependence within experimental error. From these observations, one can see the source of discrepancy between the Tool-Narayanaswamy model and experimental data when the change in fictive temperature is large. |
