Crystallization in Glasses of the Na2O–B2O3–SiO2–Fe2O3 System with a Different SiO2 Content

Abstract

Glasses of the 6Na2O · хB2O3 · (86 – x)SiO2 · 8Fe2O3 compositions, where x varies from 16 to 31 mol %, are studied by X-ray powder diffractometry and dilatometry. The glasses were preliminarily held at a temperature of 550°C for 8–144 h. It is determined that there are crystalline phases of magnetite and hematite in the studied glasses in addition to the amorphous component. The hematite content gradually decreases as the amount of SiO2 increases and it completely disappears when the SiO2 concentration is higher than 65 mol %. As the duration of the heat treatment increases, the amount of magnetite does not change. At the maximum duration of the heat treatment (144 h), crystallization of the silica phases—tridymite and cristobalite—is observed for all the studied glasses. The stability of the glass transition temperature of all glasses with an increase of the duration of the heat treatment indicates that the coexisting phases of phase-separated glass reach their equilibrium compositions after 8 hours. The crystallization of silica in the glasses is accompanied by a decrease in the glass transition temperature of the low-viscosity phase.