Doping silica glass with fluorine in equilibrium conditions at a temperature of more than 2000 °C

The process of the silica glass doping with fluorine during its long-term high-temperature treatment at 2100-2300 degrees C in an oxygen atmosphere containing from 1 to 5 mol% SiF4 has been studied. With an increase in its concentration in the gas, the rate of gas-phase glass etching increases. Fluorine concentration in glass was estimated by refractometry and X-ray microanalyzer. It has been established that under conditions of chemical equilibrium of the silica glass surface with SiF4, an increase in temperature leads to a decrease in the fluorine content in the glass. The process of dissolving fluorine in glass is exothermic. The dependence of its doping degree on SiF4 pressure up to 5000 Pa is close to linear. Under equilibrium conditions, the fluorine content in silica glass is 2 times higher than under the conditions of a nonequilibrium MCVD process.