과냉각 액체상의 혼합 알칼리 효과 발생 메커니즘에 대한 고찰

Abstract

Mixed-alkali metaphosphate glasses and supercooled liquids are investigated using a combination of calorimetric, electrical impedance spectroscopic, and rheological measurements to explore the atomic nature of the mixed-alkali effect (MAE) on the glass transition temperature (Tg) and viscous flow. While Tg and the fragility index of single-alkali liquids are influenced by the segmental motion of phosphate chains, mixed-alkali liquids exhibit a secondary relaxation process that is slower than chain motion and characterized by lower activation energy. This process is attributed to the scission and renewal of P-O bonds, which alleviate accumulated stress in the network caused by matrix-mediated coupling with pairwise hopping of different alkalis. The viscous flow and glass transition in mixed-alkali phosphate liquids seem to be governed by this P-O bond scission-renewal process, which aligns with a reduction in their fragility index and a decrease in the stretching exponent of structural relaxation compared to single-alkali liquids. The fundamental nature of the mixed-alkali effect (MAE) on viscosity, fragility, and glass transition in supercooled borate and phosphate liquids is analyzed using dynamic and thermodynamic measurements. The largest negative deviation of Tg from linearity in mixed- alkali borates is observed when two different alkalis are present in equal proportions, consistent with the matrix-mediated coupling model. However, this deviation varies non- monotonically with the size difference of the alkalis, attributed to a balance between the strength and reorientation probability of mechanical dipoles associated with alkali hopping to unlike sites. On the other hand, heat capacity measurements of mixed-alkali phosphate glasses and supercooled liquids indicate that the negative deviation of the fragility index (m) from linearity may not be related to the configurational entropy of alkali mixing. It is proposed that the bond scission-renewal dynamics of the network responsible for viscous flow are catalyzed by the reorientation of alkali mechanical dipoles in mixed-alkali liquids, resulting in a reduction of the activation energy. This reduction in activation energy leads to a significant decrease in the fragility index (m) of mixed-alkali liquids compared to their single-alkali counterparts lacking this catalytic feedback. The relative significance of this catalytic effect diminishes at higher temperatures as the thermal contribution to the activation barrier crossing becomes dominant, causing the MAE on viscosity to eventually vanish above a critical temperature.