Egami Takeshi, Ryu Chae Woo
Department of Materials Science and Engineering, Shull-Wollan Center - Joint Institute for Neutron Sciences, University of Tennessee, Knoxville, Knoxville, TN, United States.
Department of Physics and Astronomy, University of Tennessee, Knoxville, Knoxville, TN, United States.
Front Chem. 2020 Sep 29;8:579169. doi: 10.3389/fchem.2020.579169. eCollection 2020.
The viscosity and the relaxation time of a glass-forming liquid vary over 15 orders of magnitude before the liquid freezes into a glass. The rate of the change with temperature is characterized by liquid fragility. The mechanism of such a spectacular behavior and the origin of fragility have long been discussed, but it remains unresolved because of the difficulty of carrying out experiments and constructing theories that bridge over a wide timescale from atomic (ps) to bulk (minutes). Through the x-ray diffraction measurement and molecular dynamics simulation for metallic liquids we suggest that large changes in viscosity can be caused by relatively small changes in the structural coherence which characterizes the medium-range order. Here the structural coherence does not imply that of atomic-scale structure, but it relates to the coarse-grained density fluctuations represented by the peaks in the pair-distribution function (PDF) beyond the nearest neighbors. The coherence length is related to fragility and increases with decreasing temperature, and it diverges only at a negative temperature. This analysis is compared with several current theories which predict a phase transition near the glass transition temperature.
在玻璃形成液体冻结成玻璃之前，其粘度和弛豫时间会在超过15个数量级的范围内变化。这种变化随温度的速率由液体的脆性来表征。这种显著行为的机制以及脆性的起源长期以来一直被讨论，但由于难以开展跨越从原子尺度（皮秒）到宏观尺度（分钟）的广泛时间尺度的实验并构建相关理论，该问题仍未得到解决。通过对金属液体进行X射线衍射测量和分子动力学模拟，我们认为粘度的大幅变化可能由表征中程有序的结构相干性的相对较小变化引起。这里的结构相干性并非指原子尺度结构的相干性，而是与由对分布函数（PDF）中超出最近邻原子的峰所代表的粗粒度密度涨落相关。相干长度与脆性相关，并随温度降低而增加，且仅在负温度下发散。该分析与几种预测在玻璃转变温度附近存在相变的当前理论进行了比较。
