Glass transition kinetics and fragility index of chalcogenides from Ag–As–S–Se system

The glass transition temperature dependence on a heating rate was investigated by differential scanning calorimetry for chalcogenide glasses from the Agx(As2(S0.5Se0.5)3)100−x system for x = 0, 0.5, 2 and 3 at.%. According to this dependence, apparent glass transition activation energies Eg were calculated for differently defined glass transition temperatures (onset Tg1, midpoint Tg2, endpoint Tg3 and endset Tg4). Also, Eg was analysed using isoconversional (model free) method. The results showed that the activation energy is not constant during the investigated process and vary with the extent of transformation from the glassy to the supercooled phase, pointing out that a complicated, multi-step process occurs. Eg decreases from about 340 to 200 kJ mol−1 in the composition with 2 at.% of silver. The values of the fragility index showed that these glasses do not exhibit large configuration changes during the glass transition process. Influence of silver content on investigated parameters was discussed. Apparent glass transition activation energy, variability parameter and fragility index showed a significant change in their values for the composition with 3 at.% of silver. Application of the isoconversion model for determination of apparent glass transition activation energy has showed that this parameter (at extent of conversion of 0.5) decreases from about 260 kJ mol−1 for x = 0, 0.5, 2 at.% to about 185 kJ mol−1 for x = 3 at.% of silver content. Likewise, fragility parameter drops from ~ 28.5 for x = 0, 0.5, 2 at.% to 21.5 x = 3 at.% of silver.