Glass Transition and Heat Capacities of Inorganic Glasses: Diminishing Change in the Heat Capacity at T g for xNa2S + (1 − x)B2S3 Glasses

Abstract

The Tg's and heat capacity functions have been measured for a series of Na2S + B2S3 glasses for the first time. Unlike the alkali borates, Tg decreases rapidly as Na2S is added to B2S3. This effect, even in the presence of a rapidly increasing fraction of tetrahedrally coordinated borons, has been associated with the “over crosslinking” effect of the sulfide ion. Unlike the borate glasses where each added oxygen produces two tetrahedral borons, the conversion rate for the thioborates is between four and six. This behavior is suggested to result in the formation of local tightly-bonded molecular-like structures that exhibit less long-range network bonding than the alkali borate glasses. As a result, Tg decreases with added alkali in alkali thioborates rather than increases as in the alkali borate glasses. The change in heat capacity at Tg, ΔCp(Tg) has been carefully measured and is found to also decrease dramatically as alkali sulfide is added to the glass. Again this effect is opposite to the trends observed for the alkali borate glasses. The decreasing ΔCp(Tg) occurs even in the presence of a decreasing Tg. We have tentatively associated the diminishing ΔCp(Tg) values to the decreasing density of the configurational states above Tg. This is attributed to the high coordination number and site specificity caused by the added alkali sulfide. The glassy state heat capacities were analyzed and found to reach ˜90% of the classical limiting DuLong-Petit value just below Tg for all glasses. This was used to suggest that the diminishing ΔCp(Tg) values are associated with a unique behavior in the system to become a liquid with very little change in the density of configurational states.