Properties of Na2O–Al2O3–B2O3 glasses

The density and transformation temperature of (30−y)Na2O·yAl2O3·70B2O3 glasses show a minimum at Al2O3/Na2O≈1. The molar volume increases with increasing Al2O3 concentration and it seems to be constant for Al2O3/Na2O≥1. There is a slight increase in the thermal expansion coefficient up to Al2O3/Na2O≈0.9 followed by a fast decrease. It is assumed that up to Al2O3/Na2O=1, Na2O associates preferentially with Al2O3 to form AlO4 tetrahedra. Infrared spectra are consistent with this assumption. There is a linear dependence between the electric resistivity and the activation energy. Both of these quantities increase with increasing Al2O3 content. It is concluded that the conduction parameters are determined by the Na+–Na+ distance.