Thermodynamic Quantities of Alkali Silicates in the Temperature Range from 25°C to Melting Point

The heat contents of the alkali silicate glasses and crystals have been measured from 25°C to melting temperature with a continuous high-temperature calorimeter.The entropy in the above temperature range is determined for the glasses and crystals of the composition R2O⋅2SiO2 (R=Li, Na, K). The free-energy change is calculated for the reaction of forming the glasses from the crystals. The free-energy values thus obtained are compared with those calculated from two approximate equations, ΔG=ΔT⋅ΔHf/Tm and ΔG=(ΔT⋅ΔHf/Tm)⋅(T/Tm). This comparison indicates that the free-energy values from these equations are inapplicable to the discussion on the kinetic process except for the case in the narrow temperature range near the melting point.The relation between the heat capacity per mole of SiO2 and composition expressed by the mole ratio R2O/SiO2 at 300° and 600°C is also investigated. In the system Na2O-SiO2 crystals, the heat capacity increases linearly with Na2O/SiO2 ratio. The slope of this straight line and the intersection at zero Na2O composition approximately correspond to the heat capacity of crystalline Na2O and that of crystalline SiO2, respectively. From this result, it can be assumed that the additivity approximately holds for the heat capacity of Na2O-SiO2 crystals.For Na2O-SiO2 and K2O-SiO2 glasses and supercooled liquids, the composition dependence of heat capacity shows an inflection at the composition of R2O/SiO2=0.5. These results are discussed from the view point of structure of glass and liquid.