Characterization of structural relaxation in As_2Se_3 for analysis of lens shape change in glass press mold cooling and post-process annealing

摘要

This study explores the structural relaxation behavior of As_2Se_3 by thermo mechanical analysis in order to characterize and eventually predict volume change in As_2Se_3 upon relaxation during cooling after precision glass molding (PGM) and annealing. A vertical beam of As_2Se_3 was placed in a thermo mechanical analyzer (TMA) and fully relaxed at a given temperature. The temperature was then quickly changed a given amount and the 1 -D relaxation of the beam was measured until it reached equilibrium at the new temperature. The resultant curve was then fit with a Prony series which captured the relaxation data. The mathematical representation of the relaxation is then analyzed as a function of time, temperature, and quench rate and can be used to predict one dimensional (1-D) length change upon relaxation. A maximum of three terms is needed to describe the relaxation behavior and that number declines with an increase in temperature. This decay of the number of Prony terms needed to describe relaxation points to a structure that relaxes with less complexity as it approaches T_g. These trends can be converted to 3-D due to the amorphous and therefore typically isotropic nature of As_2Se_3 glass. This volume change information as a function of vital processing parameters can then be used to predict the change in shape of a work piece during cooling or post process annealing within a precision molding cycle. The mathematical representation of volume relaxation can then be applied to finite element models (FEM) of As_2Se_3 lenses or other optical elements.