Energetics of Silica-Poor Glasses in the Systems MgO-SiO_2 and Mg_(0.5)Ca_(0.5)O-SiO_2

摘要

A series of alkaline-earth silicate glasses, with compositions ranging from the metasilicate to the ortho- and suborthosili-cate, have been synthesized by aerodynamic levitation and CO_2 laser melting. They have been studied by high-temperature oxide melt solution calorimetry with 2PbO·B_2O_3 as solvent. The enthalpies of formation from the oxides at room temperature (ΔH_(298K, oxides)~(0,gl) have been calculated from the solution enthalpies. Glasses in the Ca_(0.5)Mg_(0.5)O-SiO_2 system show greater energetic stability than those in the MgO-SiO_2 system, with a more pronounced negative enthalpy of mixing near the orthosilicate composition. This stabilization may explain why it is possible to prepare glasses poorer in silica (suborthosilicate) in the Ca_(0.5)Mg_(0.5)O-SiO_2 system but not in the MgO-SiO_2 system. The thermodynamic observations support earlier structural studies in these systems.