Long Term Aging of Selenide Glasses: Evidence of Sub-T_g Endotherms and Pre-T_g Exotherms

Authors

Ping Chen, Boise State UniversityFollow
P. Boolchand, University of Toledo
D. G. Georgiev, University of Toledo

Document Type

Article

Publication Date

2-17-2010

DOI

http://dx.doi.org/10.1088/0953-8984/22/6/065104

Abstract

Long term aging, extending from months to several years, is studied on several families of chalcogenide glasses including the Ge–Se, As–Se, and Ge–As–Se systems. Special attention is given to the As–Se binary, a system that displays a rich variety of aging behavior intimately tied to sample synthesis conditions and the ambient environment in which samples are aged. Calorimetric (modulated DSC) and Raman scattering experiments are undertaken. Our results show all samples display a sub-T_g endotherm typically 10–70 °C below T_g in glassy networks possessing a mean coordination number r in the 2.25 < r < 2.45 range. Two sets of As_xSe_100−x samples aged for eight years were compared, set A consisted of slow cooled samples aged in the dark, and set B consisted of melt-quenched samples aged at laboratory environment. Samples of set B in the As concentration range, 35% < x < 60%, display a pre-T_g exotherm, but the feature is not observed in samples of set A. The aging behavior of set A presumably represents intrinsic aging in these glasses, while that of set B is extrinsic due to the presence of light. The reversibility window persists in both sets of samples, but is less well defined in set B. These findings contrast with a recent study by Golovchak et al (2008 Phys. Rev. B 78 014202), which finds the onset of the reversibility window moved up to the stoichiometric composition (x = 40%). Here we show that the up-shifted window is better understood as resulting due to demixing of As₄Se₄ and As₄Se₃ molecules from the backbone, i.e., nanoscale phase separation (NSPS). We attribute sub-T_g endotherms to compaction of the flexible part of the networks upon long term aging, while the pre-T_g exotherm is to NSPS. The narrowing and sharpening of the reversibility window upon aging is interpreted as the slow 'self-organizing' stress relaxation of the phases just outside the intermediate phase, which itself is stress free and displays little aging.

Publication Information

Chen, Ping; Boolchand, P.; and Georgiev, D. G.. (2010). "Long Term Aging of Selenide Glasses: Evidence of Sub-T_g Endotherms and Pre-T_g Exotherms". Journal of Physics: Condensed Matter, 22(6), 065104-1 - 065104-16.