Low-temperature heat capacity of sodium borosilicate glasses at temperatures from 13 K to 300 K

Thermodynamic properties of sodium borosilicate glasses {56.7 SiO2- (43.7-x ) B2O3, xNa(2)O} where x = 14.4, 22.9, and 32.5, have been studied. The hea t capacity was measured using an adiabatic calorimeter at temperatures betw een 13 K and 300 K. The thermodynamic functions were calculated from the sm oothed values of C-p,C-m. The results differ from an additive model with pu re glassy SiO2, B2O3, and crystalline Na2O as components. A model based on the assumption that the contribution of structural units of glasses to the heat capacity is equal to those of glasses with the same molecular formula is proposed. (C) 2001 Academic Press.