Abstract
Assuming the structural units in Na2O-B2O3-GeO2 glass being composed of structural units of the Na2O-B2O3 and Na2O-GeO2 systems (sub-systems), glass transition temperature Tg was estimated from the viscosity-temperature relationship based on the sub-system concept. The distribution of Na2O component among the B2O3 and GeO2 components is determined according to the assumption that Na2O activity in the Na2O-B2O3 system is equal to that in the Na2O-GeO2 system and the ratio of the Na2O activity coefficients γ1 in the Na2O-B2O3 system over that γ2 in the Na2O-GeO2 system is constant throughout the composition of the Na2O-B2O3-GeO2 system. The calculated and the observed Tg was in reasonable agreement within the experimental scatters.
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