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 to 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·B2O3·2SiO2 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 °C, evaporation of borate compounds is clearly observed, stemming from the small sample size.