Separating the effects of composition and fictive temperature on Al and B coordination in Ca, La, Y aluminosilicate, aluminoborosilicate and aluminoborate glasses

摘要

Glass transition temperatures often change significantly (100's of degrees) as the boron oxide to silica ratio, or the field strength of the modifier oxide, is varied in aluminoborosilicate glasses. At the same time, fictive temperature itself can strongly affect the glass structure, most notably in the coordination environments of the network cations boron and aluminum. The latter are especially significant when proportions of Al-[5] (AlO5 groups) and Al-[6] (AlO6 groups) are high, as in glasses with high field-strength (small, highly charged) modifier cations. To obtain a clearer picture of how composition and temperature independently control structure, we present new data from differential scanning calorimetric (DSC) and high-field B-11 and Al-27 NMR on a series of Ca, La, and Y aluminosilicate, aluminoborosilicate and aluminoborate glasses prepared with different cooling rates, and compare results in detail with other recently published data on the same compositional joins. We find that previous, apparently disparate results on effects of temperature on Al coordination are in fact related to B/Si ratios: average Al coordination number increased with increased T in aluminosilicates and low-B aluminoborosilicates, but decreased with increased Tin glasses with high B/Si ratios, indicating some interaction of the two network cations. We find that effects of T on boron coordination (always lower at higher T) are consistent with a simple thermodynamic view, but may be difficult to detect in some compositions simply because of relative proportions of species. We confirm as well that in boron-rich aluminoborosilicates, B and Al coordination number decreases with increasing temperature are quantitatively linked when calculated per formula unit, again suggesting a strong coupling, and that Al coordination systematically increases with modifier field strength and B2O3/SiO2 ratio. (C) 2015 Elsevier B.V. All rights reserved.