Physicochemical properties of glasses of the Na2O-B2O3-SiO2-Fe2O3 system in the 8Na2O/70SiO2 section

Glasses of the 8Na2O-(22-x)B2O3-70SiO2-xFe2O3 system, where x varies from 0.3 to 10 mol %, have been studied by dilatometry, X-ray powder diffractometry (XRPD), and transmission electron microscopy (TEM). The glasses were heat treated at temperatures of 550, 650, and 700°C for 144, 10, and 2 h, respectively. When comparing the data of dilatometry and electron microscopy, we conclude that phase-separated glasses have a difference between the onset deformation temperature and the glass transition temperature of more than 100°C, while for single-phase glasses, the difference is less than 65°C. According to XRPD, it is shown that in the glasses containing from 4 to 8 mol % of Fe2O3, cristobalite is formed after exposure at 700°C for 2 hours, whose reversible α-β-transition is fixed in the subsequent dilatometry as a sharp jump in the thermal expansion in the temperature interval of 190–210°C. Starting from 3 mol % of Fe2O3, magnetite is formed in the glasses whose content increases with an increase in the content of iron oxide.