Congruent Evaporation of Borosilicate Glass Strengthening Silica Fiber

The work aims to study the evaporation process of borosilicate glass used for strengthening of silica fiber light guides. The composition of glass after high-temperature treatment was determined based on refractive index measurement. It was found that at temperature 2100 °C two-component SiO₂-B₂O₃ glass vaporizes without changing its composition at a boron oxide content of ≈ 4 mol% in both oxygen and nitrogen atmospheres. The IR absorption spectra of borosilicate glass indicate the absence of boroxol rings of B₂O₃, which can explain the low partial pressure of its vapors compared to the ideal melt state. It is experimentally shown that the content of only 1 mol % B₂O₃ in the outer cladding of silica fiber increases its strength by 10% when the drawing temperature is reduced to 1900 °C. Such a small addition of boron oxide to silica glass decreases its viscosity, ensuring the occurrence of compressive stresses in the borosilicate cladding.