Effect of composition on the properties of glasses in the K2O–BaO–MgO–SiO2–Al2O3–B2O3–MgF2 system

Twenty five glasses in the(1−Z)BaO:ZK2O:(6−X)MgO:XMgF2:(3−Q)Al2O3:QB2O3:8SiO2 system have been prepared and the effect of systematic changes in composition (Z=0, 0.25, 0.5, 0.75 and 1.0, X=2, 2.5 and 3.0 and Q=0, 0.5 and 1) on molar volume (MV), fractional glass compactness (C), microhardness (μHV), glass transition temperature (Tg mid) and coefficient of thermal expansion (α) determined. As potassium is substituted for barium (increasing Z), increases in MV and α, and decreases in C, μHV and Tg mid arise which are attributed to the replacement of ONB–Ba–ONB ionic bridges by two ONB–K terminations (ONB = non-bridging oxygen). When fluorine is substituted for oxygen, reductions in μHv and Tg mid and increases in MV and α values occur. These property changes occur because of reduced crosslink densities associated with the replacement of ≡Al–O–Si≡ crosslinks by ≡Al–F terminations. In general, the substitution of aluminium by boron results in decreases in MV, μHV and Tg mid values and increases in α values. These effects are attributed to boron assuming a tri-coordinated state in the glass, giving rise to reduced crosslink densities, together with the release of modifier cations (Mg, K and Ba) from their charge balancing role for four-coordinated aluminiums leaving them free to cause greater network disruption.