An anomaly in the glass transition width trends of alkali borate glasses at low modifier loadings

Abstract

Previous studies conducted at Coe College have examined the glass transition width (ΔTg) among alkali borate glasses. Munhollon et al and Starkenburg et al discovered anomalies in the widths of the glass transition of alkali borate glasses at low alkali contents. Masao Kodama has also studied these thermal properties; Kodama’s published Tg data exhibit an identical, albeit unmentioned, anomaly. The anomaly has been verified, and the dataset widely expanded to more fully cover the compositional range. Onset definitions were used for Tg and Te. ΔTg is defined as Te−Tg. This study describes the anomaly and provides an explanation based on Shakhmatkin et al’s thermodynamic model of glass structure. The basis of the Shakhmatkin et al’s approach is that the glass is composed of structural groupings from various associated crystalline stoichiometries. Knowledge of borate crystal structures were taken from Wright et al. The concept of the fictive temperature (the temperature at which the glass structure is frozen in) along with a modified view of constraint theory was also employed in which the chemical groupings used were the superstructural units rather than the short range structures. We use the superstructural groupings rather than the short range trigonal and tetrahedral borons proposed by Mauro et al since there is no difference in the abundances of the short range units between any of the alkali borate series in this range of composition.