Relationship between Average Coordination Number and Fragility of Sodium Borate Glasses

Abstract

Viscosities and thermal properties of glasses in the Na₂O-B₂O₃ system were investigated in the temperature region of glass transformation. The values of ΔC_p and Eη/T_g, where ΔC_p is the heat capacity change during glass transition and Eη is the activation energy for viscous flow at around the glass transition temperature T_g, were maximum at 30mol% Na₂O, indicating that the fragility of liquid was maximized at the composition where the BO₄ tetrahedral unit was maximum in the corresponding glass. The relationship between the measures of liquid fragility and the average coordination number ‹r› of the elements comprising the glasses was discussed on the basis of Phillips' idea concerning the degree of bond constraint and glass-forming ability.