Advances in understanding the structure of borosilicate glasses: A Raman spectroscopy study

This study is focused on the behavior of ternary SiO2-Na2O-B2O3 borosilicate glasses at temperatures between 298 and 1800 K. Unpolarized Raman spectra were measured up to high temperature. SiO2-Na2O-B2O3 glass samples were prepared with different values of the ratio R = [Na2O]/[B2O3], while the ratio K = [SiO2]/[B2O3] was kept constant and equal 2.12. Spectra were measured at room temperature in samples with 0.43 <= R <= 1.68, and the effect of the modifier content was clearly observed in these glasses, only in partial agreement with previous literature results. In particular, the formation in the glass of sodium-danburite units Na2O-B2O3-2SiO(2) was postulated. This feature led to a new assessment of R*, the critical value of R above which every new alkali atom added to the system breaks a Fo-O-Fo (Fo=glass former) bridge causing depolymerization of the glass. A revised formula is proposed to obtain the value of R* as a function of K. Raman spectra measured at high temperature yielded important information about the temperature-dependent evolution of the borosilicate system. In particular, borate and borosilicate units including tetra-coordinated boron seem to be unstable at high temperature, where the formation of metaborate chains or rings is fostered. Above 1500 degrees C, evaporation of borate compounds is clearly observed, stemming from the small sample size.