[论文解读] Fragility, Configurational Entropy and the Potential Energy Landscape of Glass Forming Liquids

Srikanth Sastry|arXiv (Cornell University)|Nov 19, 2000

Material Dynamics and Properties被引用 2

一句话总结

本文通过将势能景观结构与玻璃形成液体的脆性联系起来，提出了一个热力学解释。研究表明，脆性源于能量势阱内振动特性的变化，而不仅仅是势阱数量或能量分布范围；并推导出一个热力学表达式，其定量结果与模型液体体系中由自扩散系数数据获得的动力学脆性一致。

ABSTRACT

Glass is a microscopically disordered, solid form of matter that results when a fluid is cooled or compressed in such a fashion that it does not crystallise. Almost all types of materials are capable of glass formation -- polymers, metal alloys, and molten salts, to name a few. Given such diversity, organising principles which systematise data concerning glass formation are invaluable. One such principle is the classification of glass formers according to their fragility\cite{fragility}. Fragility measures the rapidity with which a liquid's properties such as viscosity change as the glassy state is approached. Although the relationship between features of the energy landscape of a glass former, its configurational entropy and fragility have been analysed previously (e. g.,\cite{speedyfr}), an understanding of the origins of fragility in these features is far from being well established. Results for a model liquid, whose fragility depends on its bulk density, are presented in this letter. Analysis of the relationship between fragility and quantitative measures of the energy landscape (the complicated dependence of energy on configuration) reveal that the fragility depends on changes in the vibrational properties of individual energy basins, in addition to the total number of such basins present, and their spread in energy. A thermodynamic expression for fragility is derived, which is in quantitative agreement with {\it kinetic} fragilities obtained from the liquid's diffusivity.

研究动机与目标

• 理解玻璃形成液体脆性的起源，超越经验相关性。
• 研究构型熵与势能景观特征如何与脆性相关。
• 确定热力学测量的脆性是否能与从自扩散系数推导出的动力学脆性定量匹配。
• 分析势阱内振动特性在决定脆性中的作用。
• 建立一个统一框架，连接热力学与动力学的脆性度量。

提出的方法

• 研究使用一种可调体密度的模型液体体系，以改变脆性。
• 通过分析能量对构型的依赖关系，研究势能景观。
• 将势阱的数量及其能量分布范围量化为关键景观特征。
• 计算单个势阱内的振动特性，并将其与脆性相关联。
• 基于构型熵与景观特征，推导出脆性的热力学表达式。
• 从自扩散系数数据计算动力学脆性，并与热力学表达式进行比较。

实验结果

研究问题

• RQ1 势阱内的振动特性如何影响玻璃形成液体的脆性？
• RQ2 势阱的总数及其能量分布范围在多大程度上决定脆性？
• RQ3 能否推导出一个与从自扩散系数获得的动力学脆性相匹配的热力学脆性表达式？
• RQ4 构型熵与振动动力学对脆性的相对贡献是什么？
• RQ5 在模型液体中改变体密度如何影响其势能景观及由此产生的脆性？

主要发现

• 脆性显著依赖于单个势阱内振动特性的变化，而不仅仅是势阱数量或能量分布范围。
• 推导出的脆性热力学表达式与从自扩散系数获得的动力学脆性表现出定量一致。
• 构型熵与脆性之间的关系受景观的结构与动力学特征的调制。
• 随着密度变化，势阱内振动软化或硬化与脆性变化密切相关。
• 该模型体系表明，脆性并非仅取决于势阱数量或能量分散度，还取决于局部动力学响应。
• 本研究建立了热力学景观特征与玻璃形成体动力学行为之间的直接联系。