THE DENSITY OF SODIUM BOROSILICATE GLASSES IN RELATION TO THE MICROSTRUCTURE

The density and microhardness of Na2O-B2O3-SiO2 glasses have been studied for R less-than-or-equal-to 0.5. (R = molar ratio of Na2O to B2O3). Compared with SiO2 and sodium borate glasses, the sodium borosilicate glasses are compressed. The compression is partially attributed to a reduction in the volume of the SiO4 tetrahedra but mainly to a decrease in the volume of the BO4 tetrahedra. The microhardness increases linearly with the density in two regions; with a higher rate for R < 0.2 and with a lower rate for R > 0.2. The results are discussed based on the change in the volumes of the SiO4, BO3 and BO4 units.