Physical aging of heavy metal fluoride glasses-sub-Tg enthalpy relaxation in a ZrF4-BaF2-LaF3-AlF3 glass

The enthalpy changes during structural relaxation or physical aging of a 58ZrF4‐33BaF2‐5LaF3‐4AlF3 (ZBLA) glass during annealing well below the glass‐transition temperature have been studied using differential‐scanning calorimetry at several sub‐Tg temperatures. Substantial relaxation within the course of several hours was detected at temperatures as low as sixty degrees below Tg (585K). The relaxation process is extremely nonlinear and self‐retarding. The time dependence of the enthalpy during the initial stages of annealing was modeled approximately using the Narayanaswamy‐Tool approach. The structural‐relaxation parameters obtained from this fit were used to predict rates of physical aging for rapidly cooled ZBLA glass at temperatures close to ambient.