ANOMALOUS EXPANSION OF SODIUM TRIBORATE MELT AND ITS EFFECT ON GLASS PROPERTIES

Melt conditions were observed to affect the properties of sodium triborate glass. To maintain identical cooling rate, the glass samples were prepared from melt which was kept at 1400-degrees-C for 2 h and then maintained at 850-degrees-C for varying times. The activation energy for electrical conductivity, density and IR absorptivity of the glass decrease with annealing time of the melt at 850-degrees-C. This is an anomalous behavior which is also very slow by comparison with viscosity relaxation time. The origin of anomalous expansion of the glass structure is in the transformation of tetrahedrally coordinated boron to less compact triangularly coordinated boron structural units.