Variation of Glass-Forming Regions with Cooling Rate in the GeO2-Na2O System

• Published in: Journal of the Ceramic Association, Japan Vol. 94; no. 1086; pp. 249 - 254
• Main Authors: OTA, Rikuo, SOGA, Naohiro, TASHIRO, Hiroyuki
• Format: Journal Article
• Language: Japanese
• Published: The Ceramic Society of Japan 01.02.1986
• Subjects: Critical cooling rate; GeO2-Na2O system; Glass-forming region; Liquidus temperature; Liquidus viscosity
• ISSN: 0009-0255; 1884-2127
• DOI: 10.2109/jcersj1950.94.1086_249

The glass-forming regions have been determined in the GeO2-Na2O system by quenching the melts in the compositional range of GeO2=100-25mol% from above the liquidus to below Tg at various cooling rates, Q1=1.0×10-1-Q7=105K/s. Three glass-forming regions were found but the boundary compositions varied with cooling rate. The reciprocal of the critical cooling rate, -logQ, versus GeO2 concentration curve shows maxima at concentrations around GeO2=90, 72 and 35mol%, and minima at GeO2=85 and 50mol%. The liquidus viscosity, which is highest at GeO2=100mol%, is maximum at GeO2=92 and 65mol%, and minimum at GeO2=78mol%. The slope parameter, Eη/TL, is also largest at GeO2=100mol% and becomes maximum at GeO2=92 and 65mol%, and minimum at 78mol%. The ratio of the constant T0 in the Fulcher type equation to the liquidus temperature TL, T0/TL, remains small at 0-0.1 in the composition of GeO2=100-98mol%, but constant in the range 0.5±0.1 in the composition of GeO2<98mol%. A comparison was made between the compositional dependence of the viscosity parameters (liquidus viscosity, slope parameter and T0/TL) and that of the critical cooling rate. From these data, the magnitude of the structural parameter ΔSf (the fusion entropy per flow unit) was estimated, and the size of the flow unit was elucidated. Extraordinarily high crystallization tendency was observed in the composition of GeO2=100-99mol%. It was suspected that this is due to an inhomogeneous nucleation resulting from the nonstoichiometry of GeO2 including oxygen deficiency.