RELATIONSHIP BETWEEN AVERAGE COORDINATION-NUMBER AND FRAGILITY OF SODIUM-BORATE GLASSES

Viscosities and thermal properties of glasses in the Na2O-B2O3 system were investigated in the temperature region of glass transformation. The values of Delta C-p and En/T-g, where Delta C-p is the heat capacity change during glass transition and En is the activation energy for viscous flow at around the glass transition temperature T-g, were maximum at 30 mol% 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.