基于奇特焓弛豫特征过冷液体结构异质的研究

When cooling a liquid from a high temperature via an enough rapid cooling rate towards the glass state, the dynamic slowdown is a universal phenomenon of glassy dynamics. However, the structural origin of the dynamic slowdown is complex and not yet fully understood. In this project, the hyperquenching-sub-Tg annealing-calorimetric scan approach will be used to study the structural heterogeneity in a supercooled liquid aimed to improve our understanding of the structural origin of the dynamic slowdown. Firstly, we hyperquench the xSiO2-yMgO-(100-x-y)CaO liquid at different cooling rate to obtain the ternary silicate glass fibers with low glass forming ability. We will intensively study the influences of the composition, cooling rate, thermal history including both the annealing and dynamic heating process on the peculiar enthalpy relaxation, i.e., on the two successive exothermic peaks below Tg in the temperature dependence isobaric heat capacity (Cp) curve. This will provide valuable information on the mechanism and form of the structural heterogeneity, and further help us to understand the mixed effect and size effect of different local structural domains in the supercooled liquid. We will also explore and then reveal the physical origins of the fragile-to-strong transition and the liquid-liquid transition in terms of the structural heterogeneity in the studied supercooled liquid via studying the high- and low- temperature viscosities as well as the high temperature Cp. In addition, in order to clarify the influence of the structural heterogeneity on the glass forming ability (GFA) of the studied glass system and further to find a corresponding parameter describing the GFA, we will investigate the evolvement of the crystallization behavior with different thermal histories. The outcome of the project will contribute to clarify the course and mechanism of the structural heterogeneity on the dynamic slowdown near the glass transition. From experimental aspects, this work will certainly enhance the understanding of the overall dynamics of glasses and liquids.

玻璃液体由高温快速冷却，玻璃转变附近的动力学慢化是一种普遍存在的现象，但是对动力学慢化的结构根源仍然缺乏认识。本项目拟利用急冷-退火-热扫描方法对过冷液体中结构异质行为进行研究，探索引起动力学慢化的结构根源。首先设计并制备出具有较低玻璃成形能力的三元硅钙镁玻璃纤维，主要研究其化学组成，冷却速率以及动、静态热历史对其奇特焓弛豫行为（Tg以下热容曲线有两个放热峰）的影响规律，以此阐明过冷液体中结构异质的存在形式和机理，明确结构异质的耦合混合效应和尺寸效应；通过对体系高、低温黏度，高温热容的研究，证实强弱转变和液液分相的存在，并研究它们与结构异质的物理关联；研究析晶行为随动、静态热历史的变化，以此探索结构异质对玻璃成形能力的影响，寻找利用结构异质描述玻璃成形能力的参数。本项目的研究为阐明结构异质对玻璃转变动力学慢化行为的影响过程和机制奠定基础，这对玻璃和液体整个动力学过程的理解具有重要的意义。

玻璃是一种具有其过冷液体结构的固体物质。玻璃转变是过冷液体中微观结构多级弛豫的宏观体现，是一个复杂的动力学不均匀过程，而过冷液态的结构异质是动力学异质的结构根源。本项目围绕探索玻璃过冷液体中结构异质的存在，以及其对焓弛豫，相变和性能的影响等方面开展了相关的研究工作。.主要研究内容和取得的主要成果如下：（1）以三元硅酸盐玻璃纤维结为研究对象，研究了20.8CaO–15.6MgO–62.2SiO2玻璃纤维的析晶动力学行为，以及Tg以下退火对其析晶峰的影响，结果表明，该玻璃纤维存在着结构异质，且是化学异质，富硅区对应着强玻璃相，富碱土区对应着脆玻璃相。（2）以二元硅酸盐玻璃纤维为研究对象，研究了61SiO2–39Al2O3玻璃纤维中焓弛豫，玻璃转变，相变及结构演化的关系，主要研究了焓弛豫过程中中程结构的变化。随着温度的升高，该体系经历了一系列的热力学响应，揭示了不稳定玻璃体系中相变，玻璃转变和玻璃稳定性之间的关系。随着Tg以下退火的进行，5，6配位的Al3+离子逐渐减小，4配位的Al3+离子逐渐增加，Al3+配位数的变化促使了3配位氧离子的形成，且形成纳米有序微区，研究结果表明该玻璃体系极易析晶的根源主要是较高的5配位Al的存在和极易形成的3配位氧离子。本工作揭示了不稳定玻璃体系的结构根源，并深刻理解结构异质和极差玻璃成形体系的稳定性之间的关系，对相变和焓弛豫的理解有利于优化玻璃纤维的生产并提高物理性能。（3）以多元硅酸盐玻璃纤维为研究对象，研究了火焰喷吹法（F）得到的纤维制备的无纺毡比旋转法（R）得到的纤维制备的无纺毡拉伸强度高的原因。结果表明F纤维比R纤维具有较高的假想温度，冷却速率和在纤维表面具有较高浓度的羟基含量，这是引起力学性能差异的三个主要原因。该工作对于优化玻璃纤维的生产，提高玻璃纤维的强度具有重要的意义。