Physicochemical Properties of Glasses of the Na2O-B2O3-SiO2-Fe2O3 System with a Varying SiO2 Content

Glasses with compositions 6Na(2)O.xB(2)O(3).(86 - x)SiO2.8Fe(2)O(3), where x varies from 16 to 21 mol %, are preliminarily heat treated at a temperature of 550 degrees C for 8-144 h and studied by scanning electron microscopy and dilatometry. The density and Vickers microhardness are measured, and the molar volume is calculated. It is found that in the course of heat treatment, a phase-separated structure with interpenetrating phases is formed in all studied glasses. As the SiO2 content increases the diameter of the liquation channels decreases. It is shown that after 8 h, the coexisting liquid glass phases reach their equilibrium compositions, as indicated by the invariability of the glass transition temperature with an increase in the duration of heat treatment. With the maximum duration of heat treatment of 144 h for all the studied glasses, there is a decrease in the glass transition temperature of the low-viscosity phase and an increase in the density of glasses; this is accompanied by crystallization of the tridymite and cristobalite silica phases. The microhardness of the studied glasses does not depend on the SiO2 content, and is about 2.5 GPa.