Role of modifiers in the structural interpretation of the glass transition behavior in MgO/BaO-Al2O3-P2O5 glasses

Abstract

Thermomechanical analysis, differential scanning calorimetry and Raman spectroscopy were used to study the role of the modifying oxides on the glass transition behavior of the MgO/BaO-Al2O3-P2O5 glasses. In case of the volumetric changes associated with the glass-liquid transformation, a complex description combining the effects of viscosity, thermal expansion and structural relaxation was successfully applied to the thermomechanical data based on non-linear optimization of the involved variables. For the enthalpy relaxation data, a novel simulation-comparative method was developed to accurately determine the kinetic parameters of the Tool-Narayanaswamy-Moynihan relaxation model, overcoming the difficulties associated with the commonly encountered instrumental distortions of the experimental data. In most cases, the modifying oxides had the dominant role, as compared to the primarily cross-linking effect of the Al2O3 oxide, in determination of the physico-chemical properties of the studied glasses. Differences between the magnesium and barium phosphates were consistent with the concept of larger size Ba2+ ions leading to the higher disorganization and diversification of the phosphate glass matrix. Compositional trends in the measured quantities were correlated with the representation of the particular structural units occurring in the MgO/BaO-Al2O3-P2O5 glasses.