CONFIGURATIONAL HEAT-CAPACITY AND ENTROPY OF BOROSILICATE MELTS

The heat capacities of a soda-lime silicate and four borosilicate glas ses and liquids have been determined from drop-calorimetry measurement s made between 450 and about 1700 K. Between 850 and 1000 K, the glass transition takes place when the glass specific heat, C-p, approaches the Dulong-Petit limit. At the glass transition, the heat capacity of liquids is from 17 to 31% higher than that of glasses, the increase be ing greater for melts with smaller SiO2 contents. The heat capacity of borosilicate liquids has a markedly non-linear composition dependence and shows either positive or negative temperature dependences. The ex treme case is that of a melt with 5 wt% Na2O and 16 wt% B2O3, whose co nfigurational heat capacity decreases by 50% between 800 and 1800 K. T hese anomalous variations do not correlate with temperature-induced co ordination changes of boron above the glass transition and could indic ate instead some mixing of SiO2 and BO4 units in the liquid whose rate would decrease progressively at higher temperatures.