On the Elastic behavior of Sodium Borate Glasses

Alkali Borates are industrial glasses and their physical properties are of general interest. We have made a special effort to synthesize dry (Na$_{2}$O)$_{x}$(B$_{2}$O$_{3})_{100-x}$ glasses over a wide composition range, 0 $<$ x $<$ 70{\%}, and have examined them in modulated-DSC, Raman scattering, FTIR, and molar volume experiments. The enthalpy of relaxation at T$_{g}$ shows a global minimum in the 20{\%} $<$ x $<$ 40{\%} range, which we identify with the rigid but stress-free Intermediate Phase (IP). The Boroxyl ring vibrational mode near 808 cm$^{-1}$ in B$_{2}$O$_{3}$, steadily softens by about 4 cm$^{-1}$ as the soda content increases to about 20{\%}. A vibrational mode of mixed rings\footnote{Kamitsos et al., Jour. Mol. Struct 247, 1 (1996).} (containing 3-fold and 4-fold B) is also observed near 775 cm$^{-1}$ at low x, and it also steadily softens by nearly 10 cm$^{-1}$ as x increases in the 20{\%} $<$ x $<$ 40{\%} soda range (IP). We are examining the underlying optical elasticity power-laws to ascertain the nature of the elastic phases. IR reflectance experiments provide the 4-fold coordinated B fraction to increase from 0.17 near x = 20{\%} to 0.44 near x = 40{\%} in broad agreement with NMR results. Evolution of physical properties of these glasses with soda content will be reviewed.